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Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control

Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control applied sciences Article Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control Dongyu Wu, Kun Yin, Qilei Yin *, Xinxin Zhang, Jingqing Cheng, Dong Ge and Pengfei Zhang College of Construction Engineering, Jilin University, Changchun 130026, China; wudy14@mails.jlu.edu.cn (D.W.); yinkun@jlu.edu.cn (K.Y.); zxx@mails.jlu.edu.cn (X.Z.); chengjq16@mails.jlu.edu.cn (J.C.); jitk14@mails.jlu.edu.cn (D.G.); yangdd@mails.jlu.edu.cn (P.Z.) * Correspondence: yinqilei@jlu.edu.cn; Tel.: 138-4498-3100 Academic Editor: Giuseppe Lacidogna Received: 5 November 2016; Accepted: 19 December 2016; Published: 27 December 2016 Abstract: To reduce dust generated from drilling processes, a reverse circulation drilling method based on a supersonic nozzle is proposed. The suction performance is evaluated by the entrainment ratio. A series of preliminary laboratory experiments based on orthogonal experimental design were conducted to test the suction performance and reveal the main factors. Computational fluid dynamics (CFD) were conducted to thoroughly understand the interaction mechanism of the flows. The Schlieren technique was further carried out to reveal the flow characteristic of the nozzle. The results show that the supersonic nozzle can significantly improve the reverse circulation effect. A high entrainment ratio up to 0.76 was achieved, which implied strong suction performance. The CFD results agreed well with experimental data with a maximum difference of 17%. This work presents the great potential for supersonic nozzles and reverse circulation in dust control, which is significant to protect the envrionment and people’s health. Keywords: supersonic nozzle; reverse circulation; dust control; entrainment ratio; shock strain 1. Introduction Air drilling technology is a basic and important technique which is widely used in mineral production and foundation construction. The heavy dust due to drilling processes has always been a key issue. The adverse effects of fugitive dust emissions to the local environment and human health, including workers and the general public, has attracted attention from researchers [1–3]. Hence, we proposed a reverse-circulation drilling method to solve this problem. This method utilizes the nozzle suction effect to draw in and pump a significant amount of air with the cuttings and dust to the ground for collection. It is a highly efficient and cleaner drilling technique aimed to protect the environment and human health [4]. A high penetration rate and good quality of borehole are also important advantages to shorten time and save cost. Satisfying dust control performance was achieved after drilling to a certain depth, however, heavy dust can still be observed in the initial drilling phase because of lower annular space pressure and deficient suction capability. Additionally, leakage loss of air at fractured formations is another problem due to deficient suction performance. The combination of a supersonic nozzle and reverse-circulation drilling method is likely to be a potential solution to this issue. A supersonic nozzle is a minor and simple mechanical component utilizing the conversion of pressure energy and kinetic energy of the motive or primary stream to entrain and pump flows. It is widely used in aerospace [5], fuel cell [6], refrigeration [7], waste heat recovery [8], and solar power technologies [9]. Considering its simplicity in construction and high efficiency, the supersonic nozzle seems to be available to the reverse-circulation drilling technique for better dust control. How to make the best of the supersonic nozzle to maximize the entrainment ratio has attracted significant attention from researchers. Some of them focused on the design of the nozzle Appl. Sci. 2017, 7, 5; doi:10.3390/app7010005 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 5 2 of 15 structure. A lobed supersonic nozzle was proposed by Tillman et al. [10] to improve entrainment and mixing within a rectangular ejector. Fanshi et al. [11] compared the performance of the Chevron nozzle and a conventional nozzle, and stated that the Chevron nozzle has a positive effect on the supersonic ejector performance. Srisha et al. [12] developed two supersonic nozzles named the Tip Ring nozzle and Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle, and the results show that both nozzles achieved a 30% increase in entrainment of the secondary flow. Nevertheless, the relative complexity of these nozzle geometries is such that it is not easy to fabricate them. A group of studies focused on geometry parameter matching. Yan et al. [13] investigated the optimum area ratios of the nozzle/mixing chamber and operating conditions with an experimental setup. Eames et al. [14] confirmed that the nozzle exit position (NXP) and primary nozzle geometry had a strong influence on the entrainment performance. Varga et al. [15] and Rusly et al. [16] also observed that the position of the nozzle was an important design parameter, and the nozzle exit position affected the critical back pressure considerably. For the reverse-circulation method, the effect of area ratio can be regarded as the relationship between the nozzle and the center passage area. The difference is that the supersonic nozzle is not coaxial with the “mixing chamber” (center passage). The supersonic nozzle was designed inclined with an unequal-length divergent part which was different from the axial symmetry nozzle in previous studies. Furthermore, multiple supersonic nozzles were distributed in the radial direction instead of a single nozzle. The flows from nozzles in adjacent or opposite positions may interact and, hence, change the back pressure in the center passage. Therefore, it is necessary to thoroughly understand the flow characteristic and mixture mechanism for suction performance improvement. In the past, significant effort was made to investigate the entrainment performance of a supersonic nozzle based on one-dimensional gas dynamics theory. Keenan et al. [17] proposed a one-dimensional model called as constant area model. Later, they modified the model with the concept of constant pressure mixing [18]. Huang et al. [19] further proposed a model that could calculate the entrainment ratio. Zhu et al. [20] proposed a shock circle model to predict the velocity distribution at the entrance of the constant-area mixing chamber. The one-dimensional method is effective and helpful, however, it seems harder to accurately predict the internal flow field in a complex three-dimensional structure. Visualized analysis, including computational fluid dynamics (CFD) and the Schlieren technique, constitute incontestably powerful routes to enable accurate predictions and visualizations of aerodynamics in complex structures. Bartosiewicz et al. [21] investigated ejector flow characteristics using CFD techniques. They compared different models and proposed that the RNG k-epsilon model was suitable to represent the entrainment ratio of the ejector. Pianthong et al. [22] carried out a CFD method to investigate the effect of operating conditions, NXP, and throat length on ejector performance. Hakkaki-Fard et al. [23] conducted a computational methodology for ejector design and performance maximization based on CFD. Chong et al. [24] studied the performance and flow field inside ejectors numerically and experimentally. They considered that there existed an optimal NXP corresponding to a maximum entrainment ratio and the CFD results were in agreement with the experimental data. To visualize the shock structures and yield better understanding of complex flow phenomena and the performance characteristics, Dvorak and Safarik [25] investigated the transonic instability in the mixing chamber of a high-speed ejector using the Schlieren technique. They found that the instability caused the movement of the position of the boundary layer separation, the structure of shock waves changed, and all flow structures oscillated. Sugiyama et al. [26] further studied the shock wave and turbulent boundary layer interactions in supersonic rectangular ducts using color Schlieren photographs. More recently, Zhu et al. [27,28] investigated the entrainment performance and the shock wave structures in a three-dimensional ejector by CFD and Schlieren flow visualization. They drew a conclusion that the entrainment ratio of a convergent–divergent nozzle was greater than that of the convergent nozzle for the same first shock wave length. Appl. Sci. 2017, 7, 5 3 of 15 Appl. Sci. 2017, 7, 5   3 of 15  In In this this work, work, CFD CFD and and Schlier Schlieren en methods methods were were al also so a applied pplied to to thoroughly thoroughly under understand stand the the flo flow w  characteristics characteristics an and d  entrainment entrainment mecha mechanism. nism. Orthogo Orthogonal nal design design wa was s al also so cond conducted ucted to to re reduce duce  test test  times times and and study study the the suction suction performance performance mor more e systematically systematically..  2. Description of the Suction Process and Nozzle Structure 2. Description of the Suction Process and Nozzle Structure  The reverse-circulation drilling method aims at dust control, which utilizes the suction and The  reverse‐circulation  drilling  method  aims  at  dust  control,  which  utilizes  the  suction  and  entrainment performance of a supersonic nozzle with no need for an additional precipitator. As shown entrainment  performance  of  a  supersonic  nozzle  with  no  need  for  an  additional  precipitator.  As  in Figure 1, compressed air flows into the supersonic nozzle and the flushing nozzle, which are located shown in Figure 1, compressed air flows into the supersonic nozzle and the flushing nozzle, which  on the drill bit body towards the center passage and the borehole bottom, respectively. The flushing are located on the drill bit body towards the center passage and the borehole bottom, respectively.  nozzle is designed to cool the working face and suspend rock debris. The motive air from the The flushing nozzle is designed to cool the working face and suspend rock debris. The motive air  supersonic nozzle induces ambient air upward, and converts the pressure energy to kinetic energy, from the supersonic nozzle induces ambient air upward, and converts the pressure energy to kinetic  resulting in a decrease of pressure in the center passage. The difference of pressure between the center energy, resulting in a decrease of pressure in the center passage. The difference of pressure between  passage and the annulus space causes the suction of a lot of air into the center passage. By this process, the center passage and the annulus space causes the suction of a lot of air into the center passage. By  the cuttings and dust are carried into the center passage by the air and collected by a dust bag at this process, the cuttings and dust are carried into the center passage by the air and collected by a  passage outlet. dust bag at passage outlet.  Figure 1. Schematic of the reverse‐circulation drilling method.  Figure 1. Schematic of the reverse-circulation drilling method. 3. Experimental  3. Experimental 3.1. Orthogonal Experimental Design and Experimental Setup  3.1. Orthogonal Experimental Design and Experimental Setup As shown in Figure 2, the experimental reverse‐circulation apparatus consists of three parts, a  As shown in Figure 2, the experimental reverse-circulation apparatus consists of three parts, a supporting  pedestal,  a  simulated  reverse‐circulation  bit,  and  an  upper  discharge  passage,  supporting pedestal, a simulated reverse-circulation bit, and an upper discharge passage, respectively. respectively. For easier facility parameter adjustment, a modular design method was applied to the  For easier facility parameter adjustment, a modular design method was applied to the design of the design of the simulated bit in the laboratory. The simulated reverse‐circulation bit is divided into two  simulated bit in the laboratory. The simulated reverse-circulation bit is divided into two modules; modules;  namely,  the  center  passage  with  supersonic  nozzles  and  flushing  nozzle  modules.  The  namely, the center passage with supersonic nozzles and flushing nozzle modules. The center passage center passage is connected to the upper discharge passage so that the flow rate of the mixing air can  is connected to the upper discharge passage so that the flow rate of the mixing air can be measured by be measured by mass flow meter. There exists a certain gap between the supporting pedestal and bit  mass flow meter. There exists a certain gap between the supporting pedestal and bit to simulate the to simulate the annular space in borehole where the outside air can be inhaled into the bit through  annular space in borehole where the outside air can be inhaled into the bit through the air vent on the air vent on the supporting pedestal. An air compressor is used as the air source, and a surge tank  the supporting pedestal. An air compressor is used as the air source, and a surge tank is utilized to is utilized to provide a steady pressure in experimental system.  provide a steady pressure in experimental system. As  the  analysis  using  conventional  experimental  methods  is  inefficient  and  expensive,  the  As the analysis using conventional experimental methods is inefficient and expensive, the orthogonal design is applied in this paper. By application of this method, less experimental work is  orthogonal design is applied in this paper. By application of this method, less experimental work is required to study multiple levels of all input parameters and some effects due to statistical variations  required to study multiple levels of all input parameters and some effects due to statistical variations can be filtered out. The quantity of the supersonic nozzle (Ns), the flushing nozzle diameter (Df),  can be filtered out. The quantity of the supersonic nozzle (N ), the flushing nozzle diameter (D ), divergent part length (the longer side) of the supersonic nozzle (Ls), the spray angle of the supersonic  nozzle (θ), the supersonic nozzle height (H), and the center passage diameter (Dc) are determined as  Appl. Sci. 2017, 7, 5   4 of 15  six factors of the orthogonal experiment and each factor has three levels. For each factor, three levels  Appl. Sci. 2017, 7, 5 4 of 15 are  selected  to  eliminate  the  influence  and  validate  the  experimental  results.  To  thoroughly  understand the interrelationships between the factors, the interaction terms (Ns × Df and Ns × Ls) are  investigated. The considered factors and levels are listed in Table 1.  divergent part length (the longer side) of the supersonic nozzle (L ), the spray angle of the supersonic nozzle (), the supersonic nozzle height (H), and the center passage diameter (D ) are determined as six factors of the orthogonal experiment and each factor has three levels. For each factor, three levels are selected to eliminate the influence and validate the experimental results. To thoroughly understand the interrelationships between the factors, the interaction terms (N  D and N  L ) are investigated. s s s The considered factors and levels are listed in Table 1. Figure 2. Reverse-circulation drilling apparatus based on supersonic nozzle. 1. Air outlet; 2. Top joint; 3. Center passage; 4. Seal ring; 5. Supersonic nozzle; 6. Outer shell; 7.Flushing nozzle; 8. Bit crown; 9. Air vent; 10. Work bench; 11. Air inlete. Table 1. Factors and levels for L (3 ) orthogonal array. Factor Level 1 Level 2 Level 3 Figure 2. Reverse‐circulation drilling apparatus based on supersonic nozzle. 1. Air outlet; 2. Top joint;  N (mm) 4 5 6 3. Center passage; 4.Seal ring; 5. Supersonic nozzle; 6. Outer shell; 7.Flushing nozzle; 8. Bit crown;  D (mm) 4 6 8 9.Air vent; 10. Work bench; 11. Air inlete.  L (mm) 13 25 37 ( ) 24 32 40 H Table (mm) 1. Factors and 186 levels for L27 (3 )198  orthogonal array. 210 D (mm) 56 44 32 Factor Level 1 Level 2 Level 3 Ns (mm)  4  5  6  The nozzle diameters of the entrance and throat were 6.6 mm and 2.8 mm, respectively. Df (mm)  4  6  8  Twenty-seven tests derived from the L (3 ) orthogonal array design are shown in Table 2. The blank Ls (mm)  13  25  37  columns are not shown in this table. The entrainment ratio was defined as ! = m /m . K represented S P i θ (°)  24  32  40  the sum of the experimental values with the same level in any column. Further analysis was conducted Hc (mm)  186  198  210  by the method of analysis of variance (ANOVA). Based on the results given in Table 2, the total sum of Dc (mm)  56  44  32  squares of deviation (SS ) was calculated by the following expression: The nozzle diameters of the entrance and throat were 6.6 mm and 2.8 mm, respectively. Twenty‐ SS = Q R (1) seven tests derived from the L27 (3 ) orthogonal array design are shown in Table 2. The blank columns  are not shown in this table. The entrainment ratio was defined as ω = mS/mP. Ki represented the sum  where Q and R were defined as: of the experimental values with the same level in any column. Further analysis was conducted by the  Q = w (2) method of analysis of variance (ANOVA). Based on the i  results given in Table 2, the total sum of  i=1 squares of deviation (SST) was calculated by the following expression:  SST = Q − R  (1)  ( w ) i=1 R = (3) where Q and R were defined as:  Similarly, the sum of squares of deviation of each factor was expressed as: Q   (2)   i i1 2 2 2 K + K + K 1 2 3 SS = R (4) 9 Appl. Sci. 2017, 7, 5 5 of 15 Then F-test was carried out to gain a better understand of mentioned parameters. The mean square and the value of F of each factor can be calculated from the following equations: SS MS = (5) d f MS F = (6) MS where df represented the degree of freedom and MS represented the mean square of error term, respectively. Table 2. Design matrix based on the L (3 ) array and the experimental results. Exp. Number N D (N  D ) (N  D ) L (N  L ) (N  L ) q H D ! s f s f 1 s f 2 s s s 1 s s 1 c c 1 1 1 1 1 1 1 1 1 1 1 0.7551 2 1 1 1 1 2 2 2 2 2 2 0.6238 3 1 1 1 1 3 3 3 3 3 3 0.0238 4 1 2 2 2 1 1 1 2 2 3 0.3554 5 1 2 2 2 2 2 2 3 3 1 0.4100 6 1 2 2 2 3 3 3 1 1 2 0.5123 7 1 3 3 3 1 1 1 3 3 2 0.2500 8 1 3 3 3 2 2 2 1 1 3 0.0762 9 1 3 3 3 3 3 3 2 2 1 -0.0146 10 2 1 2 3 1 2 3 2 3 2 0.2611 11 2 1 2 3 2 3 1 3 1 3 0.1700 12 2 1 2 3 3 1 2 1 2 1 0.6146 13 2 2 3 1 1 2 3 3 1 1 0.7123 14 2 2 3 1 2 3 1 1 2 2 0.4338 15 2 2 3 1 3 1 2 2 3 3 0.0454 16 2 3 1 2 1 2 3 1 2 3 0.1792 17 2 3 1 2 2 3 1 2 3 1 0.3569 18 2 3 1 2 3 1 2 3 1 2 0.0815 19 3 1 3 2 1 3 2 3 2 3 0.4054 20 3 1 3 2 2 1 3 1 3 1 0.5654 21 3 1 3 2 3 2 1 2 1 2 0.2985 22 3 2 1 3 1 3 2 1 3 2 0.5369 23 3 2 1 3 2 1 3 2 1 3 0.0115 24 3 2 1 3 3 2 1 3 2 1 0.2446 25 3 3 2 1 1 3 2 2 1 1 0.3823 26 3 3 2 1 2 1 3 3 2 2 0.0538 27 3 3 2 1 3 2 1 1 3 3 0.1523 K 2.9923.718 2.650 2.787 3.838 2.479 2.712 2.230 2.837 4.027 K 2.6013.172 2.607 3.002 2.532 2.653 2.922 3.521 2.896 2.889 K 2.3461.050 2.682 2.150 1.40 2.807 2.305 2.188 2.206 1.024 3.2. Schlieren Experiment As shown in Figure 3, Schlieren method is based on the light deflection by the refractive index gradient due to the variation of flow density. In this test, a tungsten halogen lamp was chosen as the light source. The light passed through a condenser lens and was reflected to a parabolic mirror by a reflector. A collimated light beam was generated from the parabolic mirror and then passed through the flow zone where the supersonic nozzle was placed. After that, a similar light path was accomplished by another parabolic mirror and reflector. Finally, the converged light spot is cut by a knife edge and imaged in a camera. The Schlieren system is designed in such a way that the phase difference can be eliminated and saves limited space. The inlet pressure was kept at 0.7 MPa. Appl. Sci. 2017, 7, 5   6 of 15  accomplished by another parabolic mirror and reflector. Finally, the converged light spot is cut by a  knife edge and imaged in a camera. The Schlieren system is designed in such a way that the phase  Appl. Sci. 2017, 7, 5 6 of 15 difference can be eliminated and saves limited space. The inlet pressure was kept at 0.7 MPa.  Figure 3. Schematic of the Schlieren system.  Figure 3. Schematic of the Schlieren system. 4. CFD Method  4. CFD Method Although  the  model  of  solid  and  gas  dual‐phase  flows  (particles  and  gas)  can  evaluate  the  Although the model of solid and gas dual-phase flows (particles and gas) can evaluate the suction performance more directly [29], considering that compared with the Schlieren results and a  suction performance more directly [29], considering that compared with the Schlieren results and tremendous  amount  of  simulation it  is  more  convenient  and  reasonable  to  use  a  fluid  model.  a tremendous amount of simulation it is more convenient and reasonable to use a fluid model. Mathematical modeling of the experiment was accomplished using the commercial CFD software  Mathematical modeling of the experiment was accomplished using the commercial CFD software package FLUENT14 (ANSYS Inc., Pittsburgh, PA, USA). The detailed three‐dimensional geometry is  package FLUENT14 (ANSYS Inc., Pittsburgh, PA, USA). The detailed three-dimensional geometry built  up  using  SolidWorks  software  and  meshed  by  Hyper  Mesh  software.  In  order  to  obtain  is built up using SolidWorks software and meshed by Hyper Mesh software. In order to obtain sufficient accuracy, the simulation domain, including supersonic nozzles and a partial center passage  sufficient accuracy, the simulation domain, including supersonic nozzles and a partial center passage in the vicinity of the nozzle exit, adopt the grid refining. The mesh profile is established with 4.5  in the vicinity of the nozzle exit, adopt the grid refining. The mesh profile is established with million  elements,  which  has  been  validated  by  grid‐independence  analysis  and  proven  to  be  4.5 million elements, which has been validated by grid-independence analysis and proven to be reasonable. A segregated implicit solver is chosen to solve the non‐linear governing equations. The  reasonable. A segregated implicit solver is chosen to solve the non-linear governing equations. The controlling equations of mass conservation, momentum conservation, and energy conservation are  controlling equations of mass conservation, momentum conservation, and energy conservation are set set to steady‐state forms, which neglects time derivatives. It can be written as:   to steady-state forms, which neglects time derivatives. It can be written as: div( - U grad) = S (8)  div(U' G grad ) = S (8) ' ' ' where  , U, Γ, and S represent the dependent variable, velocity vector, diffusion coefficient, and  wher source e ' term, , U, G respectively , and S repr [2 esent 3]. As the  the dependent  concernedvariable,  fluid is hi velocity gh‐speed vector  and ,compressib diffusion coef le (M ficient, ach > 0. and 3),  sour the  COUPLED ce term, respectively   algorithm [23  was ]. As  applie the concerned d  to  obtain fluid  velocity is high-speed   and  pr and essu compr re  fields. essible   The (Mach   second > 0.3),   order the  COUPLED upwind sche algorithm me was cho wassen applied  as the to int obtain erpolat velocity ion scheme and pr to essur  disceretize fields. the The  con second vective or terms der upwind  for its  scheme second‐order was chosen   accura ascy. the  Th inte e  disc rpolation ussionscheme   about to turb discr ulence etize  models the convective   has  been terms   con for tinits ued second-or .  Zhu  et der al.  accuracy rendered.  The that discussion the  RNG  about k‐epsilon turbulence   turbulen models ce  viscosity has been   model continued.   agreed Zhu best etwit al.hr  ender the  shoc ed that k  wave the  RNG structure k-epsilon s [27], turbulence while Besag viscosity ni compar model ed seven agreed  models best with and show the shock ed tha wave t the str k–uctur ω SST es prese [27],n while ted a  Besagni better performa comparnce ed seven [30]. C models onsiderand ing the showed  calcul that ating the time k–! and SST con prv esented ergencea ra better te, the performance  standard model [30].  Considering was applied to the sicalculating mulate the time high and speed conver  flowgence  in this rate,  work. the Add standar itionally, d model  thewas  predictions applied of to th simulate e mass  the flow high  ratespeed  were nearly flow in the this sa work. me for A these dditionally  two models , the pr when edictions  the in ofle the t pressu massre flow  reached rate wer  0.6 eMP nearly a [27] the .  sameThe for these ideal gas two model models wa when s used the as inlet  the pr air essur  defin eirtion eached  due0.6  to MPa the norm [27].al temperature condition. In  all caThe ses, ideal the walls gas are model  cons was idered used  to be as the smooth air definition  and no‐slip due  adito abthe atic normal boundatemperatur ries. Numerous e condition.  studies  In about all cases  the inf , the luence walls of ar eoper consider ating ed conditions to be smooth  (temperature, and no-slip in adiabatic let pressur boundaries. e, and outl Numer et pressure ous studies ) have  about been conduct the influence ed. Accor of d operating ing to their conditions  results, the (temperatur  variation trends e, inlet of pr the essur  entrainm e, and e outlet nt ratio pr tha essur t ch e)an have ged  been with conducted. pressure are Accor  simila ding r [2to 2,2 thei 7]. rHenc results, e, thi the s conten variation t will trends  not of be the disc entrainment ussed in thiratio s paper. that Th changed e inlet  with pressu prre essur  wase set are to similar  a const [a 22 nt ,27 value ]. Hence,  of 0.7 this MPa. content   will not be discussed in this paper. The inlet pressure was set to a constant value of 0.7 MPa. 5. Results and Discussion  Appl. Sci. 2017, 7, 5 7 of 15 5. Results and Discussion 5.1. Parameter Analysis Appl. Sci. 2017, 7, 5   7 of 15  Factors should be recalculated into the error term if the mean square of some factor is less than 5.1. Parameter Analysis  that of the error term. As the MS values of the N N  D , N  L H are all less than the MS , these s, s s s, c e factors have Factors little sho influence uld be recon alcul the ateentrainment d into the error ratio  termand  if the the  mea interaction n square of ef some fect factor can be is negligible. less than  It that of the error term. As the MS values of the Ns, Ns × Df, Ns × Ls, Hc are all less than the MSe, these  can be inferred that the quantity of four nozzles was sufficient to achieve good suction performance. factors have little influence on the entrainment ratio and the interaction effect can be negligible. It can  By readjustment of the error term, the analysis results are shown in Table 2. be inferred that the quantity of four nozzles was sufficient to achieve good suction performance. By  As there exist some pressure fluctuation when the compressor starts and stops, the average mass readjustment of the error term, the analysis results are shown in Table 2.  flow rate in the stable stage is selected as the experimental value. As shown in Table 3, the parameter As there exist some pressure fluctuation when the compressor starts and stops, the average mass  order between the primary and secondary is D > D > L >  according to the value of F. Furthermore, c s flow rate in the stable stage is selected as the experimental value. As shown in Table 3, the parameter  by comparison to F (2, 18) = 3.55 and F (2, 18) = 6.01, the q has significant effects on entrainment 0.05 0.01 order between the primary and secondary is Dc > Df > Ls> θ according to the value of F. Furthermore,  and suction performance, and L , D , and D have very significant effects. s c by comparison to F0.05 (2, 18) = 3.55 f and F0.01 (2, 18) = 6.01, the θ has significant effects on entrainment  and suction performance, and Ls, Df, and Dc have very significant effects.  Table 3. Results of variance analysis. Table 3. Results of variance analysis.  Source of Variance SS df MS F Source of Variance SS  d f MS  F  D 0.441 2 0.221 14.73 f Df  0.441  2  0.221  14.73  L 0.232 2 0.116 7.733 Ls  0.232  2  0.116  7.733  0.128 2 0.064 4.267 θ  0.128  2  0.064  4.267  D 0.511 2 0.255 17.00 Dc  0.511  2  0.255  17.00  Error 0.273 18 0.015 - Error  0.273  18  0.015 ‐  The result show that the supersonic nozzle can obviously improve the suction performance.  The result show that the supersonic nozzle can obviously improve the suction performance. Obvious  suction  phenomena  can  be  observed  during  the  test,  as  shown  in  Figure  4.  A  high  Obvious suction phenomena can be observed during the test, as shown in Figure 4. A high entrainment entrainment ratio up to 0.76 was obtained, which made an approximate doubling improvement of  ratio up to 0.76 was obtained, which made an approximate doubling improvement of the entrainment the entrainment ratio compared with 0.3 of the previous reverse‐circulation drilling method [4]. The  ratio compared with 0.3 of the previous reverse-circulation drilling method [4]. The results obtained results obtained by orthogonal design and ANOVA reveal the primary and secondary sequence of  by orthogonal design and ANOVA reveal the primary and secondary sequence of the factors, however, the  factors,  however,  how  these  factors  work  on  the  suction  performance  need  to  be  further  how these factors work on the suction performance need to be further investigated. investigated.  Figure 4. Suction performance of the reverse circulation apparatus.  Figure 4. Suction performance of the reverse circulation apparatus. 5.2. Influence of the Dominant Effective Parameters  5.2. Influence of the Dominant Effective Parameters Single factor analysis was carried out and the parameter combination of group 1 in Table 2 was  Single selected factor . The static analysis  pressure was near carried  the wal outl and along the  the parameter  axial directcombination ion of the center of pa grssa oup ge1 isin shTown able in 2  was Figure 5. The pressures were all below 1 bar below the nozzle and there existed a sudden pressure  selected. The static pressure near the wall along the axial direction of the center passage is shown in drop near the nozzle exit which resulted in a pressure difference for suction. The drop of pressure  Appl. Sci. 2017, 7, 5 8 of 15 Figure 5. The pressures were all below 1 bar below the nozzle and there existed a sudden pressure drop Appl. Sci. 2017, 7, 5   8 of 15  near the nozzle exit which resulted in a pressure difference for suction. The drop of pressure was due to the conversion of pressure energy to kinetic energy according to energy conservation. The increase was due to the conversion of pressure energy to kinetic energy according to energy conservation. The  of pressure may be due to the effect of the shock wave. Appl. Sci. 2017, 7, 5   8 of 15  increase of pressure may be due to the effect of the shock wave.  was due to the conversion of pressure energy to kinetic energy according to energy conservation. The  increase of pressure may be due to the effect of the shock wave.  Figure 5. Distribution of static pressure near the wall along the axial direction of the center passage.  Figure 5. Distribution of static pressure near the wall along the axial direction of the center passage. The impact of divergent section length on the entrainment ratio is presented in Figure 6. The  The impact Figure 5. Distribution of divergent  of stasection tic pressulength re near the on wall the along entrainment  the axial direct ratio ion ofis the pr cente esented r passag in e.  Figure 6. entrainment ratio decreased with the length of divergent part monotonously when the Ls increased  from 13 mm to 37 mm. Figure 7 shows the streamlines obtained for different values of Ls. According  The entrainment ratio decreased with the length of divergent part monotonously when the L increased The impact of divergent section length on the entrainment ratio is presented in Figure 6. The  to  previous  studies,  the  flow  can  be  accelerated  in  the  divergent  part.  The  longer  the  divergent  from 13 mm to 37 mm. Figure 7 shows the streamlines obtained for different values of L . According entrainment ratio decreased with the length of divergent part monotonously when the Ls increased  section, the higher speed of air at the nozzle exit can be obtained [31]. The result showed that a longer  to previous studies, the flow can be accelerated in the divergent part. The longer the divergent from 13 mm to 37 mm. Figure 7 shows the streamlines obtained for different values of Ls. According  divergent part did obtain a higher speed in the nozzle; however, the velocity decreased at the nozzle  section, the higher speed of air at the nozzle exit can be obtained [31]. The result showed that a longer to  previous  studies,  the  flow  can  be  accelerated  in  the  divergent  part.  The  longer  the  divergent  exit, especially for the zone near the wall. As shown in Figure 7a, the high‐speed flow was distributed  divergent part did obtain a higher speed in the nozzle; however, the velocity decreased at the nozzle section, the higher speed of air at the nozzle exit can be obtained [31]. The result showed that a longer  to the longer side of diffuser zone. Expansion waves and shock waves can be seen at the nozzle outlet.  exit, especially for the zone near the wall. As shown in Figure 7a, the high-speed flow was distributed divergent part did obtain a higher speed in the nozzle; however, the velocity decreased at the nozzle  This phenomenon seemed not to benefit to entrianment and mixing process, however, the high‐speed  to theexit, longer  especia side llyof for dif the fuser  zone zone.  near the Expansion  wall. As shown waves in and  Figshock ure 7a, waves the high can ‐speed be seen flow was at the distr nozzle ibuted outlet.   flow distributed near the wall can reduce the conflict of the primary flows. A larger zone with higher  This phenomenon to the longer side seemed  of diffu not ser to zone. benefit  Expan tosion entrianment  waves and and shock mixing  waves can process,  be seen however  at the nozzle , the high-speed outlet.  upward velocity can also be observed compared with Figure 7b,c, which was easier to shear and mix  This phenomenon seemed not to benefit to entrianment and mixing process, however, the high‐speed  flow distributed with the seconear nd flow the dr wall awncan  in. On reduce  the contrary, the confli  alt ctho of ugh the the primary  flow fieflows. ld was more A lar ger uniform, zone the with  flow higher   flow distributed near the wall can reduce the conflict of the primary flows. A larger zone with higher  seemed “choked” with an obvious velocity decrease before the nozzle exit, which can be seen in  upward velocity can also be observed compared with Figure 7b,c, which was easier to shear and upward velocity can also be observed compared with Figure 7b,c, which was easier to shear and mix  Figure  7c.  Hence,  the  divergent  part  length  should  be  shorter  for  multiple  and  oblique  crossing  mix with the second flow drawn in. On the contrary, although the flow field was more uniform, the with the second flow drawn in. On the contrary, although the flow field was more uniform, the flow  supersonic nozzles.  flow seemed “choked” with an obvious velocity decrease before the nozzle exit, which can be seen seemed “choked” with an obvious velocity decrease before the nozzle exit, which can be seen in  in Figure 7c. Hence, the divergent part length should be shorter for multiple and oblique crossing Figure  7c.  Hence,  the  divergent  part  length  should  be  shorter  for  multiple  and  oblique  crossing  supersonic nozzles. supersonic nozzles.  Figure 6. Comparison of predicted results and experimental data for ω with different Ls.  Figure 6. Comparison of predicted results and experimental data for ω with different Ls.  Figure 6. Comparison of predicted results and experimental data for ! with different L . s Appl. Sci. 2017, 7, 5 9 of 15 Appl. Sci. 2017, 7, 5   9 of 15  Figure 7. Visualization of the streamlines inside the reverse-circulation bit: (a) L = 13 mm; Figure 7. Visualization of the streamlines inside the reverse‐circulation bit: (a) Ls = 13 mm; s  (b) Ls = 25  (b) L = 25 mm; and (c) L = 37 mm. mm; sand (c) Ls = 37 mm. s The relation between the entrainment ratio and the spray angle of supersonic nozzle is presented  The relation between the entrainment ratio and the spray angle of supersonic nozzle is presented in Figure 8. The curves show that ω increased with θ at first and then turned to constantly decrease.  in Figure 8. The curves show that w increased with  at first and then turned to constantly It is indicated that there existed an optimum value of θ in the range of 32° to 40° for the entrainment  decrease. It is indicated that there existed an optimum value of  in the range of 32 to 40 for ratio.  the entrainment ratio. Appl. Sci. 2017, 7, 5 10 of 15 Appl. Sci. 2017, 7, 5   10 of 15  Appl. Sci. 2017, 7, 5   10 of 15  Figure 8. Comparison of the predicted results and experimental data for ω with different θ.  Figure 8. Comparison of the predicted results and experimental data for w with different . Figure 8. Comparison of the predicted results and experimental data for ω with different θ.  Figure 9 compares the Mach number distribution with different value of θ. Although θ changed,  Figure 9 compares the Mach number distribution with different value of . Although  changed, Figure 9 compares the Mach number distribution with different value of θ. Although θ changed,  the difference of the largest Mach number between each group seemed quite small when Ls was kept  the difference of the largest Mach number between each group seemed quite small when L was kept the difference of the largest Mach number between each group seemed quite small when Ls was kept  constant. This result further proved that Ls made an obvious effect on the flow. An obvious and large  constant. This result further proved that L made an obvious effect on the flow. An obvious and large constant. This result further proved that Ls made an obvious effect on the flow. An obvious and large  low‐speed zone in the center can be observed in Figure 9a, and a narrow high‐speed distribution near  low-speed zone in the center can be observed in Figure 9a, and a narrow high-speed distribution near low‐speed zone in the center can be observed in Figure 9a, and a narrow high‐speed distribution near  the wall can be clearly seen at the upper outlet. Figure 9b shows a much better mixing uniformity of  the wall can be clearly seen at the upper outlet. Figure 9b shows a much better mixing uniformity the wall can be clearly seen at the upper outlet. Figure 9b shows a much better mixing uniformity of  the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary  of the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary  flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together.  flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together. flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together.  As shown in Figure 9c, the non‐uniformity zone was also presented at the upper outlet boundary of  As shown in Figure 9c, the non-uniformity zone was also presented at the upper outlet boundary of As shown in Figure 9c, the non‐uniformity zone was also presented at the upper outlet boundary of  the center passage and the zone near the wall. One possible explanation is that the flow too close to  the center passage and the zone near the wall. One possible explanation is that the flow too close to the center passage and the zone near the wall. One possible explanation is that the flow too close to  the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage  the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage  diameter is larger. Yet, with the increase of the spray angle θ, unnecessary energy consumption was  diameter is larger. Yet, with the increase of the spray angle , unnecessary energy consumption was diameter is larger. Yet, with the increase of the spray angle θ, unnecessary energy consumption was  increased due to the increase of multiple fluid collisions in the radial direction.  increased due to the increase of multiple fluid collisions in the radial direction. increased due to the increase of multiple fluid collisions in the radial direction.  Figure 9. Cont. Appl. Sci. 2017, 7, 5 11 of 15 Appl. Sci. 2017, 7, 5   11 of 15  Appl. Sci. 2017, 7, 5   11 of 15  Figure 9. Schematic view within the reverse‐circulation bit based on the contours of the Mach number: (a)  Figure 9. Schematic view within the reverse‐circulation bit based on the contours of the Mach number: (a)  Figure 9. Schematic view within the reverse-circulation bit based on the contours of the Mach number: θ = 24°; (b) θ = 32°; and (c) θ = 48°.  θ = 24°; (b) θ = 32°; and (c) θ = 48°.  (a)  = 24 ; (b)  = 32 ; and (c)  = 48 . Figures 10 and 11 present the impacts of flushing nozzle diameter and center‐passage diameter  Figures 10 and 11 present the impacts of flushing nozzle diameter and center‐passage diameter  Figures 10 and 11 present the impacts of flushing nozzle diameter and center-passage diameter on on the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center‐ on the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center‐ the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center-passage passage diameter, and then the curve tended to be smoother. It can be inferred that the interaction of  passage diameter, and then the curve tended to be smoother. It can be inferred that the interaction of  diameter, and then the curve tended to be smoother. It can be inferred that the interaction of nozzles was nozzles was obvious and the back pressure at the supersonic nozzle outlet would be increased when  nozzles was obvious and the back pressure at the supersonic nozzle outlet would be increased when  obvious and the back pressure at the supersonic nozzle outlet would be increased when the expansion the expansion process was restricted in a smaller space. The pressure difference between the center‐ the expansion process was restricted in a smaller space. The pressure difference between the center‐ process was restricted in a smaller space. The pressure difference between the center-passage and passage  and  the  air  in  the  annular  space  was  likely  to  decrease,  which  resulted  in  poor  suction  passage  and  the  air  in  the  annular  space  was  likely  to  decrease,  which  resulted  in  poor  suction  the air in the annular space was likely to decrease, which resulted in poor suction performance. performance. However, the effect gradually decreased when the gas expansion was accomplished  performance. However, the effect gradually decreased when the gas expansion was accomplished  However, the effect gradually decreased when the gas expansion was accomplished and insufficiently and insufficiently supplemented the flow. The flushing nozzle diameter, by contrast, presented a  and insufficiently supplemented the flow. The flushing nozzle diameter, by contrast, presented a  supplemented the flow. The flushing nozzle diameter, by contrast, presented a negative relation with negative relation with the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to  negative relation with the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to  the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to suspend the particles in suspend the particles in the flow for better suction. However, the backflow caused by the flushing  suspend the particles in the flow for better suction. However, the backflow caused by the flushing  the flow for better suction. However, the backflow caused by the flushing nozzle disturbed the suction nozzle disturbed the suction process of the annular air, which can be seen in Figure 7. Moreover, it is  nozzle disturbed the suction process of the annular air, which can be seen in Figure 7. Moreover, it is  process of the annular air, which can be seen in Figure 7. Moreover, it is likely that the total inlet flow likely  that  the  total  inlet  flow  was  shunted  by  the  flushing  nozzles,  which  caused  the  flow  rate  likely  that  the  total  inlet  flow  was  shunted  by  the  flushing  nozzles,  which  caused  the  flow  rate  was shunted by the flushing nozzles, which caused the flow rate decrease of the supersonic nozzle. decrease of the supersonic nozzle. Thus, the velocity at the nozzle outlet decreased and poor suction  decrease of the supersonic nozzle. Thus, the velocity at the nozzle outlet decreased and poor suction  Thus, the velocity at the nozzle outlet decreased and poor suction performance was achieved. performance was achieved.  performance was achieved.  Figure 10. Comparison of predicted results and experimental data for ω with different Dc.  Figure 10. Comparison of predicted results and experimental data for ω with different Dc.  Figure 10. Comparison of predicted results and experimental data for ! with different D . c Appl. Sci. 2017, 7, 5 12 of 15 Appl. Sci. 2017, 7, 5   12 of 15  Appl. Sci. 2017, 7, 5   12 of 15  Figure 11. Comparison of predicted results and experimental data for ω with different Df.  Figure 11. Comparison of predicted results and experimental data for ! with different D . According to the analysis above, the CFD method is realiable to estimate the overall suction  According Figure to the  11. Comparison analysis above,  of predict the ed CFD results method and experimental is realiable  data for to ωestimate  with different the Doverall f.  suction performance and flow field charactristic. The deviations of calculated results were no more than 17%.  performance and flow field charactristic. The deviations of calculated results were no more than 17%. The result indicates that the standard k‐e model was not sufficient to accurately present the whole  According to the analysis above, the CFD method is realiable to estimate the overall suction  The result indicates that the standard k-e model was not sufficient to accurately present the whole details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic,  performance and flow field charactristic. The deviations of calculated results were no more than 17%.  details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic, and it is reliable to calculate ω. It was also found that the calculated values were generally less than  The result indicates that the standard k‐e model was not sufficient to accurately present the whole  and it is reliable to calculate w. It was also found that the calculated values were generally less than the experimental values. Undiscovered leakage may be a reason for the difference.  details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic,  the experimental values. Undiscovered leakage may be a reason for the difference. and it is reliable to calculate ω. It was also found that the calculated values were generally less than  5.3. Observation of Shock Wave Structure  the experimental values. Undiscovered leakage may be a reason for the difference.  5.3. Observation of Shock Wave Structure Figure 12 show the experimental Schlieren pictures of the flows after the nozzle exit. With a  5.3. Observation of Shock Wave Structure  Figure 12 show the experimental Schlieren pictures of the flows after the nozzle exit. With a constant value of θ (32°), a clear under‐expanded flow with shock wave can be observed in Figure  constant 12a, anvalue d Fig then ureof  the 12 sh (32 shock ow), the  awave clear  experimental  gr under adually -expanded   Sblurred chlieren and  flow pictures  th with e shock  ofshock  the train  flows wave  lengt  aft can ehr  decr the be  observed nozzle eased  as ex it. the in With  Figur increa  ae se 12  a, constant value of θ (32°), a clear under‐expanded flow with shock wave can be observed in Figure  and of then Ls. The the phenomenon shock wave was gradually  basically blurr  consistent ed and with the the shock  CFD train  result. length  Although decreased  higheras pressure the incr at ease  the of 12a, and then the shock wave gradually blurred and the shock train length decreased as the increase  nozzle outlet is not beneficial to suction performance, and the formation of strong shock waves may  L . The phenomenon was basically consistent with the CFD result. Although higher pressure at the of Ls. The phenomenon was basically consistent with the CFD result. Although higher pressure at the  result in irreversible energy losses and a nonuniform field [31], the higher flow speed at the nozzle  nozzle outlet is not beneficial to suction performance, and the formation of strong shock waves may nozzle outlet is not beneficial to suction performance, and the formation of strong shock waves may  exit  presented  stronger  shear  and  entrainment  processes,  which  achieved  a  better  performance,  result in irreversible energy losses and a nonuniform field [31], the higher flow speed at the nozzle result in irreversible energy losses and a nonuniform field [31], the higher flow speed at the nozzle  finally. It seems that the effect of flow shear and entrainment is more important than that of pressure  exit presented stronger shear and entrainment processes, which achieved a better performance, finally. exit  presented  stronger  shear  and  entrainment  processes,  which  achieved  a  better  performance,  for the multiple‐inclined supersonic nozzle structure.  It seems that the effect of flow shear and entrainment is more important than that of pressure for the finally. It seems that the effect of flow shear and entrainment is more important than that of pressure  multiple-inclined supersonic nozzle structure. for the multiple‐inclined supersonic nozzle structure.  Figure Figure 12. 12. Sc Schlier hliereen n photographs photographs  of the of  th sho ecshock k structu strres uctur  at nozzle es at outlet nozzle : (aoutlet: ) Ls = 13 (mm; a) L (b=) L13 s = 25 mm;   Figure 12. Schlieren photographs of the shock structures at nozzle outlet: (a) Ls = 13 mm; (b) Ls = 25  mm; (c) Ls = 37 mm; (d) θ = 24°; (e) θ = 36°; and (f) θ = 48°.  (b) L = 25 mm; (c) L = 37 mm; (d)  = 24 ; (e)  = 36 ; and (f)  = 48 . s s mm; (c) Ls = 37 mm; (d) θ = 24°; (e) θ = 36°; and (f) θ = 48°.  Appl. Sci. 2017, 7, 5 13 of 15 Clear images of the flow from nozzles with different  give a comparative picture from which qualitative inferences on the possible mechanisms can be inferred. As can be seen in Figure 12d–f, the length of the shock wave increased with the value of  and the shock train direction deviated from the nozzle axial. The flow in the unequal-length diffuser zone tended to be close to the wall of the longer side and made a deviation at the nozzle exit due to Coanda effect. When the deflection between the direction of supersonic nozzle exit and center passage wall was in a proper curvature range, the jet stream was easier to flow along the surface resulting in a shorter length of shock wave. It can also be inferred that the very long shock waves from multiple nozzles were likely to collide in the center passage, which led to a lower entrainment ratio. Nevertheless, too much deflection decreased the entrainment and mixture zone in center passage so that the entrainment ratio decreased as well. In general, the shock trian length did not present a clear relationship with the entrainment ratio. The possible reason is that the design of multiple-inclined supersonic nozzles need a synthetic consideration of the flow shear and mixture, flow collision, and pressure. 6. Conclusions The suction performance of an optimized reverse circulation drilling method was investigated. The primary and secondary relations of various factors and flow characteristics were systematically discussed. The main findings are summarized as follows: (1) The application of a supersonic nozzle can significantly improve the suction performance of reverse-circulation drilling. A high entrainment ratio up to 0.76 was obtained, which implied a strong suction performance for dust control. (2) The length of the divergent section (L ), spray angle (), flushing nozzle diameter (D ), and center s f passage diameter (D ) had significant effects on entrainment and suction performance, while the effects of nozzle quantity and height can be ignored. The entrainment ratio decreased with L monotonously. There existed an optimum value of  in the range of 32 –40 for better suction performance. The entrainment ratio increased with the center-passage diameter while presenting a negative relation with D . (3) The CFD result agreed well with experimental data with a maximum difference of 17%. The Standard k-e was not proper to describe the details of shockwave, however, it is reliable to predict the entrainment ratio to save time for numerous and complex calculations and is helpful to assisting analysis. (4) For the unequal-length divergent part, the flow with high speed tended to distribute along the longer side, which was easier to deflect. The shock train length presented a negtive correlation with L and a positve correlation with . A synthetic consideration of the effects of flow shear and mixture, flow collision, pressure, and the distributary situation is necessary. The combination of a supersonic nozzle and reverse circulation drilling method is likely to be a solution to the heavy dust problems caused by drilling processes. The findings above can also provide a reference to the design of other suction structures. Acknowledgments: The authors gratefully acknowledge the support of China Geological Survey Project (No. 12120113096900) for this work. The authors are also grateful to the reviewers for their helpful advices. Author Contributions: Qilei Yin and Kun Yin proposed this study. Dongyu Wu and Xinxin Zhang performed the numerical simulations. Dongyu Wu, Jingqing Cheng, Dong Ge and Pengfei Zhang conducted the experiments. Dongyu Wu wrote the paper. Kun Yin reviewed and edited the manuscript. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2017, 7, 5 14 of 15 Nomenclature N quantity of supersonic nozzle L length of divergent section, mm the spray angle, D flushing nozzle diameter, mm D center passage diameter, mm H supersonic nozzle height, mm ! entrainment ratio K sum of the results SS sum of squares of deviations df degree of freedom M mean square References 1. Neuman, C.M.; Boulton, J.W.; Sanderson, S. Wind tunnel simulation of environmental controls on fugitive dust emissions from mine tailings. Atmos. Environ. 2009, 43, 520–529. [CrossRef] 2. National Institute of Occupational Safety and Health. NIOSH Alert: Request for Assistance in Preventing Silicosis and Deaths in Rock Drillers; DHHS Publication No. (NIOSH); US Department of Health and Human Services, Public Health Service, Service, CDC, NIOSH: Cincinnati, OH, USA, 1992; pp. 92–107. 3. 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Numerical simulation and experimental validation of powder flux distribution in coaxial laser cladding. J. Mater. Process. Technol. 2010, 210, 2125–2134. [CrossRef] 30. Besagni, G.; Mereu, R.; Chiesa, P.; Inzoli, F. An Integrated Lumped Parameter-CFD approach for off-design ejector performance evaluation. Energy Convers. Manag. 2015, 105, 697–715. [CrossRef] 31. Sung, H.J.; Sung, H.P.; Jeong, W.H.; Chang, H.L.; Hyung, J.K. Factors of Nozzle Design Affecting on Supersonic Flow in Cold Spray Process. Mater. Sci. Forum 2006, 510, 1046–1049. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control

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applied sciences Article Reverse Circulation Drilling Method Based on a Supersonic Nozzle for Dust Control Dongyu Wu, Kun Yin, Qilei Yin *, Xinxin Zhang, Jingqing Cheng, Dong Ge and Pengfei Zhang College of Construction Engineering, Jilin University, Changchun 130026, China; wudy14@mails.jlu.edu.cn (D.W.); yinkun@jlu.edu.cn (K.Y.); zxx@mails.jlu.edu.cn (X.Z.); chengjq16@mails.jlu.edu.cn (J.C.); jitk14@mails.jlu.edu.cn (D.G.); yangdd@mails.jlu.edu.cn (P.Z.) * Correspondence: yinqilei@jlu.edu.cn; Tel.: 138-4498-3100 Academic Editor: Giuseppe Lacidogna Received: 5 November 2016; Accepted: 19 December 2016; Published: 27 December 2016 Abstract: To reduce dust generated from drilling processes, a reverse circulation drilling method based on a supersonic nozzle is proposed. The suction performance is evaluated by the entrainment ratio. A series of preliminary laboratory experiments based on orthogonal experimental design were conducted to test the suction performance and reveal the main factors. Computational fluid dynamics (CFD) were conducted to thoroughly understand the interaction mechanism of the flows. The Schlieren technique was further carried out to reveal the flow characteristic of the nozzle. The results show that the supersonic nozzle can significantly improve the reverse circulation effect. A high entrainment ratio up to 0.76 was achieved, which implied strong suction performance. The CFD results agreed well with experimental data with a maximum difference of 17%. This work presents the great potential for supersonic nozzles and reverse circulation in dust control, which is significant to protect the envrionment and people’s health. Keywords: supersonic nozzle; reverse circulation; dust control; entrainment ratio; shock strain 1. Introduction Air drilling technology is a basic and important technique which is widely used in mineral production and foundation construction. The heavy dust due to drilling processes has always been a key issue. The adverse effects of fugitive dust emissions to the local environment and human health, including workers and the general public, has attracted attention from researchers [1–3]. Hence, we proposed a reverse-circulation drilling method to solve this problem. This method utilizes the nozzle suction effect to draw in and pump a significant amount of air with the cuttings and dust to the ground for collection. It is a highly efficient and cleaner drilling technique aimed to protect the environment and human health [4]. A high penetration rate and good quality of borehole are also important advantages to shorten time and save cost. Satisfying dust control performance was achieved after drilling to a certain depth, however, heavy dust can still be observed in the initial drilling phase because of lower annular space pressure and deficient suction capability. Additionally, leakage loss of air at fractured formations is another problem due to deficient suction performance. The combination of a supersonic nozzle and reverse-circulation drilling method is likely to be a potential solution to this issue. A supersonic nozzle is a minor and simple mechanical component utilizing the conversion of pressure energy and kinetic energy of the motive or primary stream to entrain and pump flows. It is widely used in aerospace [5], fuel cell [6], refrigeration [7], waste heat recovery [8], and solar power technologies [9]. Considering its simplicity in construction and high efficiency, the supersonic nozzle seems to be available to the reverse-circulation drilling technique for better dust control. How to make the best of the supersonic nozzle to maximize the entrainment ratio has attracted significant attention from researchers. Some of them focused on the design of the nozzle Appl. Sci. 2017, 7, 5; doi:10.3390/app7010005 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 5 2 of 15 structure. A lobed supersonic nozzle was proposed by Tillman et al. [10] to improve entrainment and mixing within a rectangular ejector. Fanshi et al. [11] compared the performance of the Chevron nozzle and a conventional nozzle, and stated that the Chevron nozzle has a positive effect on the supersonic ejector performance. Srisha et al. [12] developed two supersonic nozzles named the Tip Ring nozzle and Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle, and the results show that both nozzles achieved a 30% increase in entrainment of the secondary flow. Nevertheless, the relative complexity of these nozzle geometries is such that it is not easy to fabricate them. A group of studies focused on geometry parameter matching. Yan et al. [13] investigated the optimum area ratios of the nozzle/mixing chamber and operating conditions with an experimental setup. Eames et al. [14] confirmed that the nozzle exit position (NXP) and primary nozzle geometry had a strong influence on the entrainment performance. Varga et al. [15] and Rusly et al. [16] also observed that the position of the nozzle was an important design parameter, and the nozzle exit position affected the critical back pressure considerably. For the reverse-circulation method, the effect of area ratio can be regarded as the relationship between the nozzle and the center passage area. The difference is that the supersonic nozzle is not coaxial with the “mixing chamber” (center passage). The supersonic nozzle was designed inclined with an unequal-length divergent part which was different from the axial symmetry nozzle in previous studies. Furthermore, multiple supersonic nozzles were distributed in the radial direction instead of a single nozzle. The flows from nozzles in adjacent or opposite positions may interact and, hence, change the back pressure in the center passage. Therefore, it is necessary to thoroughly understand the flow characteristic and mixture mechanism for suction performance improvement. In the past, significant effort was made to investigate the entrainment performance of a supersonic nozzle based on one-dimensional gas dynamics theory. Keenan et al. [17] proposed a one-dimensional model called as constant area model. Later, they modified the model with the concept of constant pressure mixing [18]. Huang et al. [19] further proposed a model that could calculate the entrainment ratio. Zhu et al. [20] proposed a shock circle model to predict the velocity distribution at the entrance of the constant-area mixing chamber. The one-dimensional method is effective and helpful, however, it seems harder to accurately predict the internal flow field in a complex three-dimensional structure. Visualized analysis, including computational fluid dynamics (CFD) and the Schlieren technique, constitute incontestably powerful routes to enable accurate predictions and visualizations of aerodynamics in complex structures. Bartosiewicz et al. [21] investigated ejector flow characteristics using CFD techniques. They compared different models and proposed that the RNG k-epsilon model was suitable to represent the entrainment ratio of the ejector. Pianthong et al. [22] carried out a CFD method to investigate the effect of operating conditions, NXP, and throat length on ejector performance. Hakkaki-Fard et al. [23] conducted a computational methodology for ejector design and performance maximization based on CFD. Chong et al. [24] studied the performance and flow field inside ejectors numerically and experimentally. They considered that there existed an optimal NXP corresponding to a maximum entrainment ratio and the CFD results were in agreement with the experimental data. To visualize the shock structures and yield better understanding of complex flow phenomena and the performance characteristics, Dvorak and Safarik [25] investigated the transonic instability in the mixing chamber of a high-speed ejector using the Schlieren technique. They found that the instability caused the movement of the position of the boundary layer separation, the structure of shock waves changed, and all flow structures oscillated. Sugiyama et al. [26] further studied the shock wave and turbulent boundary layer interactions in supersonic rectangular ducts using color Schlieren photographs. More recently, Zhu et al. [27,28] investigated the entrainment performance and the shock wave structures in a three-dimensional ejector by CFD and Schlieren flow visualization. They drew a conclusion that the entrainment ratio of a convergent–divergent nozzle was greater than that of the convergent nozzle for the same first shock wave length. Appl. Sci. 2017, 7, 5 3 of 15 Appl. Sci. 2017, 7, 5   3 of 15  In In this this work, work, CFD CFD and and Schlier Schlieren en methods methods were were al also so a applied pplied to to thoroughly thoroughly under understand stand the the flo flow w  characteristics characteristics an and d  entrainment entrainment mecha mechanism. nism. Orthogo Orthogonal nal design design wa was s al also so cond conducted ucted to to re reduce duce  test test  times times and and study study the the suction suction performance performance mor more e systematically systematically..  2. Description of the Suction Process and Nozzle Structure 2. Description of the Suction Process and Nozzle Structure  The reverse-circulation drilling method aims at dust control, which utilizes the suction and The  reverse‐circulation  drilling  method  aims  at  dust  control,  which  utilizes  the  suction  and  entrainment performance of a supersonic nozzle with no need for an additional precipitator. As shown entrainment  performance  of  a  supersonic  nozzle  with  no  need  for  an  additional  precipitator.  As  in Figure 1, compressed air flows into the supersonic nozzle and the flushing nozzle, which are located shown in Figure 1, compressed air flows into the supersonic nozzle and the flushing nozzle, which  on the drill bit body towards the center passage and the borehole bottom, respectively. The flushing are located on the drill bit body towards the center passage and the borehole bottom, respectively.  nozzle is designed to cool the working face and suspend rock debris. The motive air from the The flushing nozzle is designed to cool the working face and suspend rock debris. The motive air  supersonic nozzle induces ambient air upward, and converts the pressure energy to kinetic energy, from the supersonic nozzle induces ambient air upward, and converts the pressure energy to kinetic  resulting in a decrease of pressure in the center passage. The difference of pressure between the center energy, resulting in a decrease of pressure in the center passage. The difference of pressure between  passage and the annulus space causes the suction of a lot of air into the center passage. By this process, the center passage and the annulus space causes the suction of a lot of air into the center passage. By  the cuttings and dust are carried into the center passage by the air and collected by a dust bag at this process, the cuttings and dust are carried into the center passage by the air and collected by a  passage outlet. dust bag at passage outlet.  Figure 1. Schematic of the reverse‐circulation drilling method.  Figure 1. Schematic of the reverse-circulation drilling method. 3. Experimental  3. Experimental 3.1. Orthogonal Experimental Design and Experimental Setup  3.1. Orthogonal Experimental Design and Experimental Setup As shown in Figure 2, the experimental reverse‐circulation apparatus consists of three parts, a  As shown in Figure 2, the experimental reverse-circulation apparatus consists of three parts, a supporting  pedestal,  a  simulated  reverse‐circulation  bit,  and  an  upper  discharge  passage,  supporting pedestal, a simulated reverse-circulation bit, and an upper discharge passage, respectively. respectively. For easier facility parameter adjustment, a modular design method was applied to the  For easier facility parameter adjustment, a modular design method was applied to the design of the design of the simulated bit in the laboratory. The simulated reverse‐circulation bit is divided into two  simulated bit in the laboratory. The simulated reverse-circulation bit is divided into two modules; modules;  namely,  the  center  passage  with  supersonic  nozzles  and  flushing  nozzle  modules.  The  namely, the center passage with supersonic nozzles and flushing nozzle modules. The center passage center passage is connected to the upper discharge passage so that the flow rate of the mixing air can  is connected to the upper discharge passage so that the flow rate of the mixing air can be measured by be measured by mass flow meter. There exists a certain gap between the supporting pedestal and bit  mass flow meter. There exists a certain gap between the supporting pedestal and bit to simulate the to simulate the annular space in borehole where the outside air can be inhaled into the bit through  annular space in borehole where the outside air can be inhaled into the bit through the air vent on the air vent on the supporting pedestal. An air compressor is used as the air source, and a surge tank  the supporting pedestal. An air compressor is used as the air source, and a surge tank is utilized to is utilized to provide a steady pressure in experimental system.  provide a steady pressure in experimental system. As  the  analysis  using  conventional  experimental  methods  is  inefficient  and  expensive,  the  As the analysis using conventional experimental methods is inefficient and expensive, the orthogonal design is applied in this paper. By application of this method, less experimental work is  orthogonal design is applied in this paper. By application of this method, less experimental work is required to study multiple levels of all input parameters and some effects due to statistical variations  required to study multiple levels of all input parameters and some effects due to statistical variations can be filtered out. The quantity of the supersonic nozzle (Ns), the flushing nozzle diameter (Df),  can be filtered out. The quantity of the supersonic nozzle (N ), the flushing nozzle diameter (D ), divergent part length (the longer side) of the supersonic nozzle (Ls), the spray angle of the supersonic  nozzle (θ), the supersonic nozzle height (H), and the center passage diameter (Dc) are determined as  Appl. Sci. 2017, 7, 5   4 of 15  six factors of the orthogonal experiment and each factor has three levels. For each factor, three levels  Appl. Sci. 2017, 7, 5 4 of 15 are  selected  to  eliminate  the  influence  and  validate  the  experimental  results.  To  thoroughly  understand the interrelationships between the factors, the interaction terms (Ns × Df and Ns × Ls) are  investigated. The considered factors and levels are listed in Table 1.  divergent part length (the longer side) of the supersonic nozzle (L ), the spray angle of the supersonic nozzle (), the supersonic nozzle height (H), and the center passage diameter (D ) are determined as six factors of the orthogonal experiment and each factor has three levels. For each factor, three levels are selected to eliminate the influence and validate the experimental results. To thoroughly understand the interrelationships between the factors, the interaction terms (N  D and N  L ) are investigated. s s s The considered factors and levels are listed in Table 1. Figure 2. Reverse-circulation drilling apparatus based on supersonic nozzle. 1. Air outlet; 2. Top joint; 3. Center passage; 4. Seal ring; 5. Supersonic nozzle; 6. Outer shell; 7.Flushing nozzle; 8. Bit crown; 9. Air vent; 10. Work bench; 11. Air inlete. Table 1. Factors and levels for L (3 ) orthogonal array. Factor Level 1 Level 2 Level 3 Figure 2. Reverse‐circulation drilling apparatus based on supersonic nozzle. 1. Air outlet; 2. Top joint;  N (mm) 4 5 6 3. Center passage; 4.Seal ring; 5. Supersonic nozzle; 6. Outer shell; 7.Flushing nozzle; 8. Bit crown;  D (mm) 4 6 8 9.Air vent; 10. Work bench; 11. Air inlete.  L (mm) 13 25 37 ( ) 24 32 40 H Table (mm) 1. Factors and 186 levels for L27 (3 )198  orthogonal array. 210 D (mm) 56 44 32 Factor Level 1 Level 2 Level 3 Ns (mm)  4  5  6  The nozzle diameters of the entrance and throat were 6.6 mm and 2.8 mm, respectively. Df (mm)  4  6  8  Twenty-seven tests derived from the L (3 ) orthogonal array design are shown in Table 2. The blank Ls (mm)  13  25  37  columns are not shown in this table. The entrainment ratio was defined as ! = m /m . K represented S P i θ (°)  24  32  40  the sum of the experimental values with the same level in any column. Further analysis was conducted Hc (mm)  186  198  210  by the method of analysis of variance (ANOVA). Based on the results given in Table 2, the total sum of Dc (mm)  56  44  32  squares of deviation (SS ) was calculated by the following expression: The nozzle diameters of the entrance and throat were 6.6 mm and 2.8 mm, respectively. Twenty‐ SS = Q R (1) seven tests derived from the L27 (3 ) orthogonal array design are shown in Table 2. The blank columns  are not shown in this table. The entrainment ratio was defined as ω = mS/mP. Ki represented the sum  where Q and R were defined as: of the experimental values with the same level in any column. Further analysis was conducted by the  Q = w (2) method of analysis of variance (ANOVA). Based on the i  results given in Table 2, the total sum of  i=1 squares of deviation (SST) was calculated by the following expression:  SST = Q − R  (1)  ( w ) i=1 R = (3) where Q and R were defined as:  Similarly, the sum of squares of deviation of each factor was expressed as: Q   (2)   i i1 2 2 2 K + K + K 1 2 3 SS = R (4) 9 Appl. Sci. 2017, 7, 5 5 of 15 Then F-test was carried out to gain a better understand of mentioned parameters. The mean square and the value of F of each factor can be calculated from the following equations: SS MS = (5) d f MS F = (6) MS where df represented the degree of freedom and MS represented the mean square of error term, respectively. Table 2. Design matrix based on the L (3 ) array and the experimental results. Exp. Number N D (N  D ) (N  D ) L (N  L ) (N  L ) q H D ! s f s f 1 s f 2 s s s 1 s s 1 c c 1 1 1 1 1 1 1 1 1 1 1 0.7551 2 1 1 1 1 2 2 2 2 2 2 0.6238 3 1 1 1 1 3 3 3 3 3 3 0.0238 4 1 2 2 2 1 1 1 2 2 3 0.3554 5 1 2 2 2 2 2 2 3 3 1 0.4100 6 1 2 2 2 3 3 3 1 1 2 0.5123 7 1 3 3 3 1 1 1 3 3 2 0.2500 8 1 3 3 3 2 2 2 1 1 3 0.0762 9 1 3 3 3 3 3 3 2 2 1 -0.0146 10 2 1 2 3 1 2 3 2 3 2 0.2611 11 2 1 2 3 2 3 1 3 1 3 0.1700 12 2 1 2 3 3 1 2 1 2 1 0.6146 13 2 2 3 1 1 2 3 3 1 1 0.7123 14 2 2 3 1 2 3 1 1 2 2 0.4338 15 2 2 3 1 3 1 2 2 3 3 0.0454 16 2 3 1 2 1 2 3 1 2 3 0.1792 17 2 3 1 2 2 3 1 2 3 1 0.3569 18 2 3 1 2 3 1 2 3 1 2 0.0815 19 3 1 3 2 1 3 2 3 2 3 0.4054 20 3 1 3 2 2 1 3 1 3 1 0.5654 21 3 1 3 2 3 2 1 2 1 2 0.2985 22 3 2 1 3 1 3 2 1 3 2 0.5369 23 3 2 1 3 2 1 3 2 1 3 0.0115 24 3 2 1 3 3 2 1 3 2 1 0.2446 25 3 3 2 1 1 3 2 2 1 1 0.3823 26 3 3 2 1 2 1 3 3 2 2 0.0538 27 3 3 2 1 3 2 1 1 3 3 0.1523 K 2.9923.718 2.650 2.787 3.838 2.479 2.712 2.230 2.837 4.027 K 2.6013.172 2.607 3.002 2.532 2.653 2.922 3.521 2.896 2.889 K 2.3461.050 2.682 2.150 1.40 2.807 2.305 2.188 2.206 1.024 3.2. Schlieren Experiment As shown in Figure 3, Schlieren method is based on the light deflection by the refractive index gradient due to the variation of flow density. In this test, a tungsten halogen lamp was chosen as the light source. The light passed through a condenser lens and was reflected to a parabolic mirror by a reflector. A collimated light beam was generated from the parabolic mirror and then passed through the flow zone where the supersonic nozzle was placed. After that, a similar light path was accomplished by another parabolic mirror and reflector. Finally, the converged light spot is cut by a knife edge and imaged in a camera. The Schlieren system is designed in such a way that the phase difference can be eliminated and saves limited space. The inlet pressure was kept at 0.7 MPa. Appl. Sci. 2017, 7, 5   6 of 15  accomplished by another parabolic mirror and reflector. Finally, the converged light spot is cut by a  knife edge and imaged in a camera. The Schlieren system is designed in such a way that the phase  Appl. Sci. 2017, 7, 5 6 of 15 difference can be eliminated and saves limited space. The inlet pressure was kept at 0.7 MPa.  Figure 3. Schematic of the Schlieren system.  Figure 3. Schematic of the Schlieren system. 4. CFD Method  4. CFD Method Although  the  model  of  solid  and  gas  dual‐phase  flows  (particles  and  gas)  can  evaluate  the  Although the model of solid and gas dual-phase flows (particles and gas) can evaluate the suction performance more directly [29], considering that compared with the Schlieren results and a  suction performance more directly [29], considering that compared with the Schlieren results and tremendous  amount  of  simulation it  is  more  convenient  and  reasonable  to  use  a  fluid  model.  a tremendous amount of simulation it is more convenient and reasonable to use a fluid model. Mathematical modeling of the experiment was accomplished using the commercial CFD software  Mathematical modeling of the experiment was accomplished using the commercial CFD software package FLUENT14 (ANSYS Inc., Pittsburgh, PA, USA). The detailed three‐dimensional geometry is  package FLUENT14 (ANSYS Inc., Pittsburgh, PA, USA). The detailed three-dimensional geometry built  up  using  SolidWorks  software  and  meshed  by  Hyper  Mesh  software.  In  order  to  obtain  is built up using SolidWorks software and meshed by Hyper Mesh software. In order to obtain sufficient accuracy, the simulation domain, including supersonic nozzles and a partial center passage  sufficient accuracy, the simulation domain, including supersonic nozzles and a partial center passage in the vicinity of the nozzle exit, adopt the grid refining. The mesh profile is established with 4.5  in the vicinity of the nozzle exit, adopt the grid refining. The mesh profile is established with million  elements,  which  has  been  validated  by  grid‐independence  analysis  and  proven  to  be  4.5 million elements, which has been validated by grid-independence analysis and proven to be reasonable. A segregated implicit solver is chosen to solve the non‐linear governing equations. The  reasonable. A segregated implicit solver is chosen to solve the non-linear governing equations. The controlling equations of mass conservation, momentum conservation, and energy conservation are  controlling equations of mass conservation, momentum conservation, and energy conservation are set set to steady‐state forms, which neglects time derivatives. It can be written as:   to steady-state forms, which neglects time derivatives. It can be written as: div( - U grad) = S (8)  div(U' G grad ) = S (8) ' ' ' where  , U, Γ, and S represent the dependent variable, velocity vector, diffusion coefficient, and  wher source e ' term, , U, G respectively , and S repr [2 esent 3]. As the  the dependent  concernedvariable,  fluid is hi velocity gh‐speed vector  and ,compressib diffusion coef le (M ficient, ach > 0. and 3),  sour the  COUPLED ce term, respectively   algorithm [23  was ]. As  applie the concerned d  to  obtain fluid  velocity is high-speed   and  pr and essu compr re  fields. essible   The (Mach   second > 0.3),   order the  COUPLED upwind sche algorithm me was cho wassen applied  as the to int obtain erpolat velocity ion scheme and pr to essur  disceretize fields. the The  con second vective or terms der upwind  for its  scheme second‐order was chosen   accura ascy. the  Th inte e  disc rpolation ussionscheme   about to turb discr ulence etize  models the convective   has  been terms   con for tinits ued second-or .  Zhu  et der al.  accuracy rendered.  The that discussion the  RNG  about k‐epsilon turbulence   turbulen models ce  viscosity has been   model continued.   agreed Zhu best etwit al.hr  ender the  shoc ed that k  wave the  RNG structure k-epsilon s [27], turbulence while Besag viscosity ni compar model ed seven agreed  models best with and show the shock ed tha wave t the str k–uctur ω SST es prese [27],n while ted a  Besagni better performa comparnce ed seven [30]. C models onsiderand ing the showed  calcul that ating the time k–! and SST con prv esented ergencea ra better te, the performance  standard model [30].  Considering was applied to the sicalculating mulate the time high and speed conver  flowgence  in this rate,  work. the Add standar itionally, d model  thewas  predictions applied of to th simulate e mass  the flow high  ratespeed  were nearly flow in the this sa work. me for A these dditionally  two models , the pr when edictions  the in ofle the t pressu massre flow  reached rate wer  0.6 eMP nearly a [27] the .  sameThe for these ideal gas two model models wa when s used the as inlet  the pr air essur  defin eirtion eached  due0.6  to MPa the norm [27].al temperature condition. In  all caThe ses, ideal the walls gas are model  cons was idered used  to be as the smooth air definition  and no‐slip due  adito abthe atic normal boundatemperatur ries. Numerous e condition.  studies  In about all cases  the inf , the luence walls of ar eoper consider ating ed conditions to be smooth  (temperature, and no-slip in adiabatic let pressur boundaries. e, and outl Numer et pressure ous studies ) have  about been conduct the influence ed. Accor of d operating ing to their conditions  results, the (temperatur  variation trends e, inlet of pr the essur  entrainm e, and e outlet nt ratio pr tha essur t ch e)an have ged  been with conducted. pressure are Accor  simila ding r [2to 2,2 thei 7]. rHenc results, e, thi the s conten variation t will trends  not of be the disc entrainment ussed in thiratio s paper. that Th changed e inlet  with pressu prre essur  wase set are to similar  a const [a 22 nt ,27 value ]. Hence,  of 0.7 this MPa. content   will not be discussed in this paper. The inlet pressure was set to a constant value of 0.7 MPa. 5. Results and Discussion  Appl. Sci. 2017, 7, 5 7 of 15 5. Results and Discussion 5.1. Parameter Analysis Appl. Sci. 2017, 7, 5   7 of 15  Factors should be recalculated into the error term if the mean square of some factor is less than 5.1. Parameter Analysis  that of the error term. As the MS values of the N N  D , N  L H are all less than the MS , these s, s s s, c e factors have Factors little sho influence uld be recon alcul the ateentrainment d into the error ratio  termand  if the the  mea interaction n square of ef some fect factor can be is negligible. less than  It that of the error term. As the MS values of the Ns, Ns × Df, Ns × Ls, Hc are all less than the MSe, these  can be inferred that the quantity of four nozzles was sufficient to achieve good suction performance. factors have little influence on the entrainment ratio and the interaction effect can be negligible. It can  By readjustment of the error term, the analysis results are shown in Table 2. be inferred that the quantity of four nozzles was sufficient to achieve good suction performance. By  As there exist some pressure fluctuation when the compressor starts and stops, the average mass readjustment of the error term, the analysis results are shown in Table 2.  flow rate in the stable stage is selected as the experimental value. As shown in Table 3, the parameter As there exist some pressure fluctuation when the compressor starts and stops, the average mass  order between the primary and secondary is D > D > L >  according to the value of F. Furthermore, c s flow rate in the stable stage is selected as the experimental value. As shown in Table 3, the parameter  by comparison to F (2, 18) = 3.55 and F (2, 18) = 6.01, the q has significant effects on entrainment 0.05 0.01 order between the primary and secondary is Dc > Df > Ls> θ according to the value of F. Furthermore,  and suction performance, and L , D , and D have very significant effects. s c by comparison to F0.05 (2, 18) = 3.55 f and F0.01 (2, 18) = 6.01, the θ has significant effects on entrainment  and suction performance, and Ls, Df, and Dc have very significant effects.  Table 3. Results of variance analysis. Table 3. Results of variance analysis.  Source of Variance SS df MS F Source of Variance SS  d f MS  F  D 0.441 2 0.221 14.73 f Df  0.441  2  0.221  14.73  L 0.232 2 0.116 7.733 Ls  0.232  2  0.116  7.733  0.128 2 0.064 4.267 θ  0.128  2  0.064  4.267  D 0.511 2 0.255 17.00 Dc  0.511  2  0.255  17.00  Error 0.273 18 0.015 - Error  0.273  18  0.015 ‐  The result show that the supersonic nozzle can obviously improve the suction performance.  The result show that the supersonic nozzle can obviously improve the suction performance. Obvious  suction  phenomena  can  be  observed  during  the  test,  as  shown  in  Figure  4.  A  high  Obvious suction phenomena can be observed during the test, as shown in Figure 4. A high entrainment entrainment ratio up to 0.76 was obtained, which made an approximate doubling improvement of  ratio up to 0.76 was obtained, which made an approximate doubling improvement of the entrainment the entrainment ratio compared with 0.3 of the previous reverse‐circulation drilling method [4]. The  ratio compared with 0.3 of the previous reverse-circulation drilling method [4]. The results obtained results obtained by orthogonal design and ANOVA reveal the primary and secondary sequence of  by orthogonal design and ANOVA reveal the primary and secondary sequence of the factors, however, the  factors,  however,  how  these  factors  work  on  the  suction  performance  need  to  be  further  how these factors work on the suction performance need to be further investigated. investigated.  Figure 4. Suction performance of the reverse circulation apparatus.  Figure 4. Suction performance of the reverse circulation apparatus. 5.2. Influence of the Dominant Effective Parameters  5.2. Influence of the Dominant Effective Parameters Single factor analysis was carried out and the parameter combination of group 1 in Table 2 was  Single selected factor . The static analysis  pressure was near carried  the wal outl and along the  the parameter  axial directcombination ion of the center of pa grssa oup ge1 isin shTown able in 2  was Figure 5. The pressures were all below 1 bar below the nozzle and there existed a sudden pressure  selected. The static pressure near the wall along the axial direction of the center passage is shown in drop near the nozzle exit which resulted in a pressure difference for suction. The drop of pressure  Appl. Sci. 2017, 7, 5 8 of 15 Figure 5. The pressures were all below 1 bar below the nozzle and there existed a sudden pressure drop Appl. Sci. 2017, 7, 5   8 of 15  near the nozzle exit which resulted in a pressure difference for suction. The drop of pressure was due to the conversion of pressure energy to kinetic energy according to energy conservation. The increase was due to the conversion of pressure energy to kinetic energy according to energy conservation. The  of pressure may be due to the effect of the shock wave. Appl. Sci. 2017, 7, 5   8 of 15  increase of pressure may be due to the effect of the shock wave.  was due to the conversion of pressure energy to kinetic energy according to energy conservation. The  increase of pressure may be due to the effect of the shock wave.  Figure 5. Distribution of static pressure near the wall along the axial direction of the center passage.  Figure 5. Distribution of static pressure near the wall along the axial direction of the center passage. The impact of divergent section length on the entrainment ratio is presented in Figure 6. The  The impact Figure 5. Distribution of divergent  of stasection tic pressulength re near the on wall the along entrainment  the axial direct ratio ion ofis the pr cente esented r passag in e.  Figure 6. entrainment ratio decreased with the length of divergent part monotonously when the Ls increased  from 13 mm to 37 mm. Figure 7 shows the streamlines obtained for different values of Ls. According  The entrainment ratio decreased with the length of divergent part monotonously when the L increased The impact of divergent section length on the entrainment ratio is presented in Figure 6. The  to  previous  studies,  the  flow  can  be  accelerated  in  the  divergent  part.  The  longer  the  divergent  from 13 mm to 37 mm. Figure 7 shows the streamlines obtained for different values of L . According entrainment ratio decreased with the length of divergent part monotonously when the Ls increased  section, the higher speed of air at the nozzle exit can be obtained [31]. The result showed that a longer  to previous studies, the flow can be accelerated in the divergent part. The longer the divergent from 13 mm to 37 mm. Figure 7 shows the streamlines obtained for different values of Ls. According  divergent part did obtain a higher speed in the nozzle; however, the velocity decreased at the nozzle  section, the higher speed of air at the nozzle exit can be obtained [31]. The result showed that a longer to  previous  studies,  the  flow  can  be  accelerated  in  the  divergent  part.  The  longer  the  divergent  exit, especially for the zone near the wall. As shown in Figure 7a, the high‐speed flow was distributed  divergent part did obtain a higher speed in the nozzle; however, the velocity decreased at the nozzle section, the higher speed of air at the nozzle exit can be obtained [31]. The result showed that a longer  to the longer side of diffuser zone. Expansion waves and shock waves can be seen at the nozzle outlet.  exit, especially for the zone near the wall. As shown in Figure 7a, the high-speed flow was distributed divergent part did obtain a higher speed in the nozzle; however, the velocity decreased at the nozzle  This phenomenon seemed not to benefit to entrianment and mixing process, however, the high‐speed  to theexit, longer  especia side llyof for dif the fuser  zone zone.  near the Expansion  wall. As shown waves in and  Figshock ure 7a, waves the high can ‐speed be seen flow was at the distr nozzle ibuted outlet.   flow distributed near the wall can reduce the conflict of the primary flows. A larger zone with higher  This phenomenon to the longer side seemed  of diffu not ser to zone. benefit  Expan tosion entrianment  waves and and shock mixing  waves can process,  be seen however  at the nozzle , the high-speed outlet.  upward velocity can also be observed compared with Figure 7b,c, which was easier to shear and mix  This phenomenon seemed not to benefit to entrianment and mixing process, however, the high‐speed  flow distributed with the seconear nd flow the dr wall awncan  in. On reduce  the contrary, the confli  alt ctho of ugh the the primary  flow fieflows. ld was more A lar ger uniform, zone the with  flow higher   flow distributed near the wall can reduce the conflict of the primary flows. A larger zone with higher  seemed “choked” with an obvious velocity decrease before the nozzle exit, which can be seen in  upward velocity can also be observed compared with Figure 7b,c, which was easier to shear and upward velocity can also be observed compared with Figure 7b,c, which was easier to shear and mix  Figure  7c.  Hence,  the  divergent  part  length  should  be  shorter  for  multiple  and  oblique  crossing  mix with the second flow drawn in. On the contrary, although the flow field was more uniform, the with the second flow drawn in. On the contrary, although the flow field was more uniform, the flow  supersonic nozzles.  flow seemed “choked” with an obvious velocity decrease before the nozzle exit, which can be seen seemed “choked” with an obvious velocity decrease before the nozzle exit, which can be seen in  in Figure 7c. Hence, the divergent part length should be shorter for multiple and oblique crossing Figure  7c.  Hence,  the  divergent  part  length  should  be  shorter  for  multiple  and  oblique  crossing  supersonic nozzles. supersonic nozzles.  Figure 6. Comparison of predicted results and experimental data for ω with different Ls.  Figure 6. Comparison of predicted results and experimental data for ω with different Ls.  Figure 6. Comparison of predicted results and experimental data for ! with different L . s Appl. Sci. 2017, 7, 5 9 of 15 Appl. Sci. 2017, 7, 5   9 of 15  Figure 7. Visualization of the streamlines inside the reverse-circulation bit: (a) L = 13 mm; Figure 7. Visualization of the streamlines inside the reverse‐circulation bit: (a) Ls = 13 mm; s  (b) Ls = 25  (b) L = 25 mm; and (c) L = 37 mm. mm; sand (c) Ls = 37 mm. s The relation between the entrainment ratio and the spray angle of supersonic nozzle is presented  The relation between the entrainment ratio and the spray angle of supersonic nozzle is presented in Figure 8. The curves show that ω increased with θ at first and then turned to constantly decrease.  in Figure 8. The curves show that w increased with  at first and then turned to constantly It is indicated that there existed an optimum value of θ in the range of 32° to 40° for the entrainment  decrease. It is indicated that there existed an optimum value of  in the range of 32 to 40 for ratio.  the entrainment ratio. Appl. Sci. 2017, 7, 5 10 of 15 Appl. Sci. 2017, 7, 5   10 of 15  Appl. Sci. 2017, 7, 5   10 of 15  Figure 8. Comparison of the predicted results and experimental data for ω with different θ.  Figure 8. Comparison of the predicted results and experimental data for w with different . Figure 8. Comparison of the predicted results and experimental data for ω with different θ.  Figure 9 compares the Mach number distribution with different value of θ. Although θ changed,  Figure 9 compares the Mach number distribution with different value of . Although  changed, Figure 9 compares the Mach number distribution with different value of θ. Although θ changed,  the difference of the largest Mach number between each group seemed quite small when Ls was kept  the difference of the largest Mach number between each group seemed quite small when L was kept the difference of the largest Mach number between each group seemed quite small when Ls was kept  constant. This result further proved that Ls made an obvious effect on the flow. An obvious and large  constant. This result further proved that L made an obvious effect on the flow. An obvious and large constant. This result further proved that Ls made an obvious effect on the flow. An obvious and large  low‐speed zone in the center can be observed in Figure 9a, and a narrow high‐speed distribution near  low-speed zone in the center can be observed in Figure 9a, and a narrow high-speed distribution near low‐speed zone in the center can be observed in Figure 9a, and a narrow high‐speed distribution near  the wall can be clearly seen at the upper outlet. Figure 9b shows a much better mixing uniformity of  the wall can be clearly seen at the upper outlet. Figure 9b shows a much better mixing uniformity the wall can be clearly seen at the upper outlet. Figure 9b shows a much better mixing uniformity of  the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary  of the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary the flows in the center zone, the wall, and the upper outlet boundary, indicating that the primary  flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together.  flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together. flows successfully transferred the kinetic energy to the inhaled air and entrained to the exit together.  As shown in Figure 9c, the non‐uniformity zone was also presented at the upper outlet boundary of  As shown in Figure 9c, the non-uniformity zone was also presented at the upper outlet boundary of As shown in Figure 9c, the non‐uniformity zone was also presented at the upper outlet boundary of  the center passage and the zone near the wall. One possible explanation is that the flow too close to  the center passage and the zone near the wall. One possible explanation is that the flow too close to the center passage and the zone near the wall. One possible explanation is that the flow too close to  the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage  the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage the wall caused a weaker shear and entrainment to the fluid in center passage if the center passage  diameter is larger. Yet, with the increase of the spray angle θ, unnecessary energy consumption was  diameter is larger. Yet, with the increase of the spray angle , unnecessary energy consumption was diameter is larger. Yet, with the increase of the spray angle θ, unnecessary energy consumption was  increased due to the increase of multiple fluid collisions in the radial direction.  increased due to the increase of multiple fluid collisions in the radial direction. increased due to the increase of multiple fluid collisions in the radial direction.  Figure 9. Cont. Appl. Sci. 2017, 7, 5 11 of 15 Appl. Sci. 2017, 7, 5   11 of 15  Appl. Sci. 2017, 7, 5   11 of 15  Figure 9. Schematic view within the reverse‐circulation bit based on the contours of the Mach number: (a)  Figure 9. Schematic view within the reverse‐circulation bit based on the contours of the Mach number: (a)  Figure 9. Schematic view within the reverse-circulation bit based on the contours of the Mach number: θ = 24°; (b) θ = 32°; and (c) θ = 48°.  θ = 24°; (b) θ = 32°; and (c) θ = 48°.  (a)  = 24 ; (b)  = 32 ; and (c)  = 48 . Figures 10 and 11 present the impacts of flushing nozzle diameter and center‐passage diameter  Figures 10 and 11 present the impacts of flushing nozzle diameter and center‐passage diameter  Figures 10 and 11 present the impacts of flushing nozzle diameter and center-passage diameter on on the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center‐ on the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center‐ the entrainment ratio, respectively. In Figure 10, the entrainment ratio increased with the center-passage passage diameter, and then the curve tended to be smoother. It can be inferred that the interaction of  passage diameter, and then the curve tended to be smoother. It can be inferred that the interaction of  diameter, and then the curve tended to be smoother. It can be inferred that the interaction of nozzles was nozzles was obvious and the back pressure at the supersonic nozzle outlet would be increased when  nozzles was obvious and the back pressure at the supersonic nozzle outlet would be increased when  obvious and the back pressure at the supersonic nozzle outlet would be increased when the expansion the expansion process was restricted in a smaller space. The pressure difference between the center‐ the expansion process was restricted in a smaller space. The pressure difference between the center‐ process was restricted in a smaller space. The pressure difference between the center-passage and passage  and  the  air  in  the  annular  space  was  likely  to  decrease,  which  resulted  in  poor  suction  passage  and  the  air  in  the  annular  space  was  likely  to  decrease,  which  resulted  in  poor  suction  the air in the annular space was likely to decrease, which resulted in poor suction performance. performance. However, the effect gradually decreased when the gas expansion was accomplished  performance. However, the effect gradually decreased when the gas expansion was accomplished  However, the effect gradually decreased when the gas expansion was accomplished and insufficiently and insufficiently supplemented the flow. The flushing nozzle diameter, by contrast, presented a  and insufficiently supplemented the flow. The flushing nozzle diameter, by contrast, presented a  supplemented the flow. The flushing nozzle diameter, by contrast, presented a negative relation with negative relation with the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to  negative relation with the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to  the entrainment ratio, as shown in Figure 11. The flushing nozzle was used to suspend the particles in suspend the particles in the flow for better suction. However, the backflow caused by the flushing  suspend the particles in the flow for better suction. However, the backflow caused by the flushing  the flow for better suction. However, the backflow caused by the flushing nozzle disturbed the suction nozzle disturbed the suction process of the annular air, which can be seen in Figure 7. Moreover, it is  nozzle disturbed the suction process of the annular air, which can be seen in Figure 7. Moreover, it is  process of the annular air, which can be seen in Figure 7. Moreover, it is likely that the total inlet flow likely  that  the  total  inlet  flow  was  shunted  by  the  flushing  nozzles,  which  caused  the  flow  rate  likely  that  the  total  inlet  flow  was  shunted  by  the  flushing  nozzles,  which  caused  the  flow  rate  was shunted by the flushing nozzles, which caused the flow rate decrease of the supersonic nozzle. decrease of the supersonic nozzle. Thus, the velocity at the nozzle outlet decreased and poor suction  decrease of the supersonic nozzle. Thus, the velocity at the nozzle outlet decreased and poor suction  Thus, the velocity at the nozzle outlet decreased and poor suction performance was achieved. performance was achieved.  performance was achieved.  Figure 10. Comparison of predicted results and experimental data for ω with different Dc.  Figure 10. Comparison of predicted results and experimental data for ω with different Dc.  Figure 10. Comparison of predicted results and experimental data for ! with different D . c Appl. Sci. 2017, 7, 5 12 of 15 Appl. Sci. 2017, 7, 5   12 of 15  Appl. Sci. 2017, 7, 5   12 of 15  Figure 11. Comparison of predicted results and experimental data for ω with different Df.  Figure 11. Comparison of predicted results and experimental data for ! with different D . According to the analysis above, the CFD method is realiable to estimate the overall suction  According Figure to the  11. Comparison analysis above,  of predict the ed CFD results method and experimental is realiable  data for to ωestimate  with different the Doverall f.  suction performance and flow field charactristic. The deviations of calculated results were no more than 17%.  performance and flow field charactristic. The deviations of calculated results were no more than 17%. The result indicates that the standard k‐e model was not sufficient to accurately present the whole  According to the analysis above, the CFD method is realiable to estimate the overall suction  The result indicates that the standard k-e model was not sufficient to accurately present the whole details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic,  performance and flow field charactristic. The deviations of calculated results were no more than 17%.  details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic, and it is reliable to calculate ω. It was also found that the calculated values were generally less than  The result indicates that the standard k‐e model was not sufficient to accurately present the whole  and it is reliable to calculate w. It was also found that the calculated values were generally less than the experimental values. Undiscovered leakage may be a reason for the difference.  details of shock waves; however, it is helpful and convenient to analyze the flow field charactristic,  the experimental values. Undiscovered leakage may be a reason for the difference. and it is reliable to calculate ω. It was also found that the calculated values were generally less than  5.3. Observation of Shock Wave Structure  the experimental values. Undiscovered leakage may be a reason for the difference.  5.3. Observation of Shock Wave Structure Figure 12 show the experimental Schlieren pictures of the flows after the nozzle exit. With a  5.3. Observation of Shock Wave Structure  Figure 12 show the experimental Schlieren pictures of the flows after the nozzle exit. With a constant value of θ (32°), a clear under‐expanded flow with shock wave can be observed in Figure  constant 12a, anvalue d Fig then ureof  the 12 sh (32 shock ow), the  awave clear  experimental  gr under adually -expanded   Sblurred chlieren and  flow pictures  th with e shock  ofshock  the train  flows wave  lengt  aft can ehr  decr the be  observed nozzle eased  as ex it. the in With  Figur increa  ae se 12  a, constant value of θ (32°), a clear under‐expanded flow with shock wave can be observed in Figure  and of then Ls. The the phenomenon shock wave was gradually  basically blurr  consistent ed and with the the shock  CFD train  result. length  Although decreased  higheras pressure the incr at ease  the of 12a, and then the shock wave gradually blurred and the shock train length decreased as the increase  nozzle outlet is not beneficial to suction performance, and the formation of strong shock waves may  L . The phenomenon was basically consistent with the CFD result. Although higher pressure at the of Ls. The phenomenon was basically consistent with the CFD result. Although higher pressure at the  result in irreversible energy losses and a nonuniform field [31], the higher flow speed at the nozzle  nozzle outlet is not beneficial to suction performance, and the formation of strong shock waves may nozzle outlet is not beneficial to suction performance, and the formation of strong shock waves may  exit  presented  stronger  shear  and  entrainment  processes,  which  achieved  a  better  performance,  result in irreversible energy losses and a nonuniform field [31], the higher flow speed at the nozzle result in irreversible energy losses and a nonuniform field [31], the higher flow speed at the nozzle  finally. It seems that the effect of flow shear and entrainment is more important than that of pressure  exit presented stronger shear and entrainment processes, which achieved a better performance, finally. exit  presented  stronger  shear  and  entrainment  processes,  which  achieved  a  better  performance,  for the multiple‐inclined supersonic nozzle structure.  It seems that the effect of flow shear and entrainment is more important than that of pressure for the finally. It seems that the effect of flow shear and entrainment is more important than that of pressure  multiple-inclined supersonic nozzle structure. for the multiple‐inclined supersonic nozzle structure.  Figure Figure 12. 12. Sc Schlier hliereen n photographs photographs  of the of  th sho ecshock k structu strres uctur  at nozzle es at outlet nozzle : (aoutlet: ) Ls = 13 (mm; a) L (b=) L13 s = 25 mm;   Figure 12. Schlieren photographs of the shock structures at nozzle outlet: (a) Ls = 13 mm; (b) Ls = 25  mm; (c) Ls = 37 mm; (d) θ = 24°; (e) θ = 36°; and (f) θ = 48°.  (b) L = 25 mm; (c) L = 37 mm; (d)  = 24 ; (e)  = 36 ; and (f)  = 48 . s s mm; (c) Ls = 37 mm; (d) θ = 24°; (e) θ = 36°; and (f) θ = 48°.  Appl. Sci. 2017, 7, 5 13 of 15 Clear images of the flow from nozzles with different  give a comparative picture from which qualitative inferences on the possible mechanisms can be inferred. As can be seen in Figure 12d–f, the length of the shock wave increased with the value of  and the shock train direction deviated from the nozzle axial. The flow in the unequal-length diffuser zone tended to be close to the wall of the longer side and made a deviation at the nozzle exit due to Coanda effect. When the deflection between the direction of supersonic nozzle exit and center passage wall was in a proper curvature range, the jet stream was easier to flow along the surface resulting in a shorter length of shock wave. It can also be inferred that the very long shock waves from multiple nozzles were likely to collide in the center passage, which led to a lower entrainment ratio. Nevertheless, too much deflection decreased the entrainment and mixture zone in center passage so that the entrainment ratio decreased as well. In general, the shock trian length did not present a clear relationship with the entrainment ratio. The possible reason is that the design of multiple-inclined supersonic nozzles need a synthetic consideration of the flow shear and mixture, flow collision, and pressure. 6. Conclusions The suction performance of an optimized reverse circulation drilling method was investigated. The primary and secondary relations of various factors and flow characteristics were systematically discussed. The main findings are summarized as follows: (1) The application of a supersonic nozzle can significantly improve the suction performance of reverse-circulation drilling. A high entrainment ratio up to 0.76 was obtained, which implied a strong suction performance for dust control. (2) The length of the divergent section (L ), spray angle (), flushing nozzle diameter (D ), and center s f passage diameter (D ) had significant effects on entrainment and suction performance, while the effects of nozzle quantity and height can be ignored. The entrainment ratio decreased with L monotonously. There existed an optimum value of  in the range of 32 –40 for better suction performance. The entrainment ratio increased with the center-passage diameter while presenting a negative relation with D . (3) The CFD result agreed well with experimental data with a maximum difference of 17%. The Standard k-e was not proper to describe the details of shockwave, however, it is reliable to predict the entrainment ratio to save time for numerous and complex calculations and is helpful to assisting analysis. (4) For the unequal-length divergent part, the flow with high speed tended to distribute along the longer side, which was easier to deflect. The shock train length presented a negtive correlation with L and a positve correlation with . A synthetic consideration of the effects of flow shear and mixture, flow collision, pressure, and the distributary situation is necessary. The combination of a supersonic nozzle and reverse circulation drilling method is likely to be a solution to the heavy dust problems caused by drilling processes. The findings above can also provide a reference to the design of other suction structures. Acknowledgments: The authors gratefully acknowledge the support of China Geological Survey Project (No. 12120113096900) for this work. The authors are also grateful to the reviewers for their helpful advices. Author Contributions: Qilei Yin and Kun Yin proposed this study. Dongyu Wu and Xinxin Zhang performed the numerical simulations. Dongyu Wu, Jingqing Cheng, Dong Ge and Pengfei Zhang conducted the experiments. Dongyu Wu wrote the paper. Kun Yin reviewed and edited the manuscript. 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Published: Dec 27, 2016

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