Access the full text.
Sign up today, get DeepDyve free for 14 days.
applied sciences Article Development of Hybrid Airlift-Jet Pump with Its Performance Analysis 1 2 2 2 , ID 3 Dan Yao , Kwongi Lee , Minho Ha , Cheolung Cheong * and Inhiug Lee School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; yaodan_sjtu@foxmail.com School of Mechanical Engineering, Pusan National University, Busan 46241, Korea; dlrnjsrl93@pusan.ac.kr (K.L.); hmh@pusan.ac.kr (M.H.) Dongjin Motor, Busan 47534, Korea; ihlee@dongjinmotor.com * Correspondence: ccheong@pusan.ac.kr; Tel.: +82-(0)51-510-2311 Received: 8 July 2018; Accepted: 17 August 2018; Published: 21 August 2018 Abstract: A new pump, called the hybrid airlift-jet pump, is developed by reinforcing the advantages and minimizing the demerits of airlift and jet pumps. First, a basic design of the hybrid airlift-jet pump is schematically presented. Subsequently, its performance characteristics are numerically investigated by varying the operating conditions of the airlift and jet parts in the hybrid pump. The compressible unsteady Reynolds-averaged Navier-Stokes equations, combined with the homogeneous mixture model for multiphase ﬂow, are used as the governing equations for the two-phase ﬂow in the hybrid pump. The pressure-based methods combined with the Pressure-Implicit with Splitting of Operators (PISO) algorithm are used as the computational ﬂuid dynamics techniques. The validity of the present numerical methods is conﬁrmed by comparing the predicted mass ﬂow rate with the measured ones. In total, 18 simulation cases that are designed to represent the various operating conditions of the hybrid pump are investigated: eight of these cases belong to the operating conditions of only the jet part with different air and water inlet boundary conditions, and the remaining ten cases belong to the operating conditions of both the airlift and jet parts with different air and water inlet boundary conditions. The mass ﬂow rate and the efﬁciency are compared for each case. For further investigation into the detailed ﬂow characteristics, the pressure and velocity distributions of the mixture in a primary pipe are compared. Furthermore, a periodic ﬂuctuation of the water ﬂow in the mass ﬂow rate is found and analyzed. Our results show that the performance of the jet or airlift pump can be enhanced by combining the operating principles of two pumps into the hybrid airlift-jet pump, newly proposed in the present study. Keywords: jet pump; airlift pump; hybrid pump; two-phase ﬂow 1. Introduction A pump is a device that actuates liquids by mechanical actions. Numerous types of pumps exist according to methods used to actuate the ﬂuid. Both the airlift and jet pumps can be classiﬁed into indirect types, where the direct contact of mechanical parts is not required to drive the ﬂuid. Airlift pumps operate on the air and are inserted into vertical pipes, which lowers the density of the ﬂuid mixture and lifts it. Jet pumps (or educator-jet pumps) use a jet, often of steam, to create a low pressure. This lower pressure draws in the target ﬂuid and propels it into a higher-pressure region. The primary merit of these types of pumps is their simple operating mechanism due to their simple structure, and they can operate on ill-conditioned target liquids and are thus durable. However, the demerit of these pumps is their low efﬁciency. Therefore, our aim is to develop a new pump concept by combining Appl. Sci. 2018, 8, 1413; doi:10.3390/app8091413 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1413 2 of 18 the operating mechanisms of the airlift and jet pumps, which supplements their weakness of low efﬁciency, and reinforces their advantages simultaneously. The operating mechanism of a jet pump is based on the principle of kinetic energy exchange of a primary ﬂuid with a secondary ﬂuid in a mixture chamber, as well as drawing the secondary ﬂuid by the static pressure difference. Jet pumps can be characterized according to the types of primary and secondary ﬂuids as gas-gas, gas-liquid, and liquid-liquid. It has been more than a century since the basic design of a jet pump was ﬁrst introduced [1]. Initially, one-dimensional methods were used to predict the pump performance [2–4]. Although these studies facilitated the basic understanding of the simple operating mechanism of a jet pump, they could not investigate the local ﬂow phenomenon that generally strongly depends on three-dimensional ﬂow characteristics. Therefore, studies using computational ﬂuid dynamics (CFD) [5–11] and experimental [12–16] methods were performed. To obtain more reliable and accurate designs of jet pumps in different applications, CFD techniques are widely used owing to its reasonable cost. Some studies have focused on the performance of ejectors for various types of refrigerants used in heat pumps [5] and ejector refrigeration systems [6]. Zhu [7] investigated the inﬂuence of the primary nozzle exit position and the mixing section converging angle on the ejectors’ performance. Li [8] investigated gas–liquid ejectors by varying the geometry, ﬂuid property, and operating condition, and compared them with a single-phase ejector. Fan [9] performed a global optimization to improve the jet pump efﬁciency. Shah et al. [10,11] investigated the characteristics of the steam jet pump using saturated steam and the direct-contact condensation model. Additionally, studies using experimental and numerical approaches have been performed. Narabayashi et al. [12,13] performed visualization experiments and developed a two-phase ﬂow model for the performance prediction of the next-generation nuclear reactors. Bartosiewcz et al. [14] compared six different turbulence models and proposed the Shear-Stress Transport (SST) k-! model as the most appropriate one for the numerical method to investigate supersonic ejectors. Yan et al. [15] performed experimental studies to measure the performance of vapor–water ejectors in different temperature conditions. Chong et al. [16] performed the optimization of the ejector shape using analytical and experimental approaches. The operating principle of the airlift pump is relatively simple compared to that of the jet pump. It is a type of device that raises liquids or mixtures of liquids and solids through a vertical pipe, partially submerged in the liquid, using the compressed air introduced into the pipe near the lower end [17]. Many studies have been performed to investigate the factors that affect the performance of airlift pumps. The typical ﬂow pattern in airlift pumps changes with the mass quantity [18], and the best efﬁciency is likely to occur in the slug or slug-churn ﬂow pattern [17,19]. Further, the effects of liquid properties [20], pipe geometry [17] or gas-injection methods [21,22] on the performance of airlift pumps were investigated. Indirect pumps, such as the jet and airlift pumps, have not been used frequently due to their less efﬁciency than the direct type of pumps. However, the efﬁciency of the direct types of pumps strongly depends on the gap distance between an impeller and its housing. As the gap distance decreases, the efﬁciency of direct pumps also increases. Modern direct types of pumps keep the distance very small due to the advanced modern manufacturing technology, but the smaller distance increases the possibility that some particles in liquid block the gap and eventually cause the failure of the pump. The indirect pump can operate on such ill-conditioned liquids without suffering from such problems. In this respect, the indirect types of pumps are more durable than the direct pumps. To develop more efﬁcient designs of indirect pumps and thus to increase their applicability in water pumping applications, the concept of hybrid airlift/jet pump is newly proposed in the preceding patent [23]. Our aim is to develop the baseline design of the hybrid airlift-jet pump and to assess its validity in terms of its performance and efﬁciency. The performance and efﬁciency characteristics of hybrid airlift-jet pumps are investigated by numerically solving the compressible unsteady Reynolds-averaged Navier-Stokes equations with a cavitation model. For simplicity, the operating condition of the airlift Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 18 Appl. Sci. 2018, 8, 1413 3 of 18 hybrid pump are investigated: the former eight cases belong to the operating condition of only the jet pump with different air inlet boundary conditions and the latter ten cases belong to the operating part is considered using the inlet boundary conditions of water. Altogether, eighteen simulation cases conditions of both the airlift and jet parts in the hybrid pump with different air and water inlet that are designed to represent the various operating conditions of the hybrid pump are investigated: boundary conditions. The performance and efficiency curves of the hybrid pump are compared for the former eight cases belong to the operating condition of only the jet pump with different air inlet each case. For further detailed investigations into the flow characteristics, the pressure and velocity boundary conditions and the latter ten cases belong to the operating conditions of both the airlift and jet distributions of the mixture in a primary pipe are compared. The periodic fluctuation of the parts in the hybrid pump with different air and water inlet boundary conditions. The performance and secondary flow in the mass flow rate is found and analyzed. efﬁciency curves of the hybrid pump are compared for each case. For further detailed investigations In Section 2, the baseline design of a hybrid airlift-jet pump is schematically presented with its into the ﬂow characteristics, the pressure and velocity distributions of the mixture in a primary pipe are relevant operating principle. In Section 3, the governing equations and the numerical methods are compared. The periodic ﬂuctuation of the secondary ﬂow in the mass ﬂow rate is found and analyzed. described. In Section 4, the detailed geometry of the pump and its computational mesh are presented In Section 2, the baseline design of a hybrid airlift-jet pump is schematically presented with its with the detailed boundary conditions. In Section 5, the validity of the numerical method is relevant operating principle. In Section 3, the governing equations and the numerical methods are confirmed by the comparison of the prediction results with the measured ones. Subsequently, the described. In Section 4, the detailed geometry of the pump and its computational mesh are presented with the detailed boundary conditions. In Section 5, the validity of the numerical method is conﬁrmed numerical results and discussions are presented to assess its performance and efficiency by by the comparison of the prediction results with the measured ones. Subsequently, the numerical investigating the detailed flow characteristics. The conclusion is presented in the final section. results and discussions are presented to assess its performance and efﬁciency by investigating the detailed ﬂow characteristics. The conclusion is presented in the ﬁnal section. 2. New Concept of Hybrid Airlift-Jet Pump 2. New Concept of Hybrid Airlift-Jet Pump In this section, the concept of the hybrid airlift-jet pump is schematically described by presenting its design with the relevant operating principle. In this section, the concept of the hybrid airlift-jet pump is schematically described by presenting its design with the relevant operating principle. 2.1. Operating Principle of Jet Pump 2.1. Operating Principle of Jet Pump The schematic design of a jet pump is described in Figure 1. The law of quasi-one-dimensional The schematic design of a jet pump is described in Figure 1. The law of quasi-one-dimensional mass conservation leads to: mass conservation leads to: (1) v A = = v A ,, (1) 1 1 2 2 where v is the axial ﬂow velocity, and A is the cross-sectional area. Equation (1) and the condition where v is the axial flow velocity, and A is the cross-sectional area. Equation (1) and the condition A > A result in the unequal equation v < v . For an incompressible, inviscid, and irrotational 1 2 1 2 result in the unequal equation . For an incompressible, inviscid, and irrotational ﬂow, the Bernoulli’s equation is satisﬁed in the form: flow, the Bernoulli’s equation is satisfied in the form: 11 1 1 2 2 p + rv = p + rv (2) 1+ = 2+ (2) 1 2 2 2 2 2 where p is the static pressure and r is the density. Equation (2) and the condition v < v lead to where p is the static pressure and ρ is the density. Equation (2) and the condition lead to 1 2 p > p . Subsequently, the pressure p is lower than the atmospheric pressure p , which is the water 1 2 2 0 . Subsequently, the pressure is lower than the atmospheric pressure , which is the inlet pressure. The pressure difference pumps the water upwards. water inlet pressure. The pressure difference pumps the water upwards. Figure 1. Schematics of a jet pump. Figure 1. Schematics of a jet pump. 2.2. Operating Principle of Airlift Pump The schematic design of an airlift pump is described in Figure 2. Based on the dotted line in Figure 2, the pressure of water, and that of the water/air mixture are the same, which can be written as: (3) ℎ = ℎ Appl. Sci. 2018, 8, 1413 4 of 18 2.2. Operating Principle of Airlift Pump The schematic design of an airlift pump is described in Figure 2. Based on the dotted line in Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 18 Figure 2, the pressure of water, and that of the water/air mixture are the same, which can be written as: Because the mixture density is lower than the pure water density, Equation (3) results in ℎ r gh = r gh (3) water 1 mix 2 ℎ . This difference causes water to be pumped up. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 18 Because the mixture density is lower than the pure water density, Equation (3) results in ℎ ℎ . This difference causes water to be pumped up. Figure 2. Schematics of an airlift pump. Figure 2. Schematics of an airlift pump. 2.3. Operating Principle of Hybrid Pump Because the mixture density is lower than the pure water density, Equation (3) results in h > h . 2 1 This difference causes water to be Figure 2. pumped Sche up. matics of an airlift pump. A schematic diagram of the hybrid airlift-jet pump is shown in Figure 3. As described above, the driving force of the jet pump is the pressure difference induced by the nozzle (ejector), while that 2.3. Operating Principle of Hybrid Pump 2.3. Operating Principle of Hybrid Pump of the airlift pump is the buoyancy induced by the lower density of the mixture. The basic idea of the A schematic diagram of the hybrid airlift-jet pump is shown in Figure 3. As described above, A schematic diagram of the hybrid airlift-jet pump is shown in Figure 3. As described above, hybrid airlift-jet pump, as schematically depicted in Figure 3, is to increase its efficiency by the driving force of the jet pump is the pressure difference induced by the nozzle (ejector), while that the driving force of the jet pump is the pressure difference induced by the nozzle (ejector), while that arranging these two driving forces to act complementarily. The upper and lower passages guide the of the airlift pump is the buoyancy induced by the lower density of the mixture. The basic idea of the of the airlift pump is the buoyancy induced by the lower density of the mixture. The basic idea of the air flow from an air compressor to drive the jet and airlift parts, respectively. The additional hybrid airlift-jet pump, as schematically depicted in Figure 3, is to increase its efficiency by hybrid airlift-jet pump, as schematically depicted in Figure 3, is to increase its efﬁciency by arranging advantage of the hybrid pump is its versatile applicability to various operating environments, while arranging these two driving forces to act complementarily. The upper and lower passages guide the these two driving forces to act complementarily. The upper and lower passages guide the air ﬂow from retaining its high efficiency by controlling the flow rate entering each passage. For example, when air flow from an air compressor to drive the jet and airlift parts, respectively. The additional an air compressor to drive the jet and airlift parts, respectively. The additional advantage of the hybrid the suction head is high, all the air from the compressor is initially supplied to the airlift part to advantage of the hybrid pump is its versatile applicability to various operating environments, while pump is its versatile applicability to various operating environments, while retaining its high efﬁciency overcome such a high head height. As the water column approaches the jet part, some of the air is retaining its high efficiency by controlling the flow rate entering each passage. For example, when by controlling the ﬂow rate entering each passage. For example, when the suction head is high, all the diverted to the jet part to drive the water to the exit of the outlet pipe. After this transient state, the the suction head is high, all the air from the compressor is initially supplied to the airlift part to air from the compressor is initially supplied to the airlift part to overcome such a high head height. operating condition of the hybrid pump is set to be in the best efficiency or performance condition. A overcome such a high head height. As the water column approaches the jet part, some of the air is As the water column approaches the jet part, some of the air is diverted to the jet part to drive the primary goal of the present study is to investigate the performance and efficiency of the hybrid diverted to the jet part to drive the water to the exit of the outlet pipe. After this transient state, the water to the exit of the outlet pipe. After this transient state, the operating condition of the hybrid pump in various operating conditions. operating condition of the hybrid pump is set to be in the best efficiency or performance condition. A pump is set to be in the best efﬁciency or performance condition. A primary goal of the present study primary goal of the present study is to investigate the performance and efficiency of the hybrid is to investigate the performance and efﬁciency of the hybrid pump in various operating conditions. pump in various operating conditions. Figure 3. Schematic design of a hybrid pump. Figure 3. Schematic design of a hybrid pump. Figure 3. Schematic design of a hybrid pump. 3. Governing Equations and Numerical Methods 3. Governing Equations and Numerical Methods The compressible unsteady Reynolds-averaged Navier-Stokes equations are used as the governing equations for the flow in the hybrid pump. The homogeneous mixture model, which The compressible unsteady Reynolds-averaged Navier-Stokes equations are used as the assumes that each phase traverses at different velocities but with a local equilibrium over short governing equations for the flow in the hybrid pump. The homogeneous mixture model, which spat assiu ames t l lengt hat h sc each pha ales, is u se setd raverse for ths at e tw dif o-fphas erent e ve flolo wcit . The mixture ies but with a loc model consists of the al equilibrium ove continuity r short , spatial length scales, is used for the two-phase flow. The mixture model consists of the continuity, Appl. Sci. 2018, 8, 1413 5 of 18 3. Governing Equations and Numerical Methods The compressible unsteady Reynolds-averaged Navier-Stokes equations are used as the governing equations for the ﬂow in the hybrid pump. The homogeneous mixture model, which assumes that each phase traverses at different velocities but with a local equilibrium over short spatial length scales, is used for the two-phase ﬂow. The mixture model consists of the continuity, momentum, and energy equations for the mixture, and the volume fraction equations for the secondary phase. The continuity equation for the mixture can be described as: ¶ ¶ (r ) + (r u ) = 0 (4) m m m,i ¶t ¶x where u is the mass-averaged mixture velocity vector and r is the mixture density. The momentum m m equation for the mixture can be obtained by adding the individual momentum equations for all phases and be expressed as: ¶ ¶ r u + r u u ( ) m m,i m m,i m,j ¶t ¶x h i ¶u ¶u ¶u ¶ p m,j ¶ m,i 2 m,k = + m + d + r g + F (5) m i j m ¶x ¶x ¶x ¶x 3 ¶x i j j i k h i ¶u ¶u ¶u m,j ¶ m,i 2 m,k + m + r k + d t m i j ¶x ¶x ¶x ¶x j j i k where p is the static pressure, m is the viscosity of the mixture, m is the turbulence viscosity, g is the m t gravity, and F is the body force. The energy equation for the mixture assumes the following form: n n ¶ ! (a r E ) +r a v (r E + p) = r k rT (6) å k k k å k k k k e f f ¶t k=1 k=1 where k is the effective conductivity, and T is the temperature. From the continuity equation for the eff secondary phase p, the volume fraction equation for the secondary phase p can be obtained: ¶ ¶ a r + a r u = 0 (7) p p p p m,i ¶t ¶x The standard k-# model is used for the turbulence model and the standard wall function was employed for the near-wall treatment. The effects of turbulence models on two phase numerical solutions were intensively investigated in the preceding study [24]. The standard k-# turbulent model is based on the transport equations for the turbulent kinetic energy k and its dissipation rate #, respectively. The transport equations can be written as: " # ¶ ¶ ¶ m ¶k (r k) + (r ku ) = m + + G + G r # Y . (8) m m m m m m,i k b ¶t ¶x ¶x s ¶x i j k j " # ¶ ¶ ¶ m ¶# # # (r #) + (r #u ) = m + + C (G + C G ) C r (9) m m m m m,i 1# k 3# b 2# ¶t ¶x ¶x s ¶x k k i j j In these equations, G represents the generation of turbulence kinetic energy due to the mass-averaged mixture velocity gradients. G is the generation of turbulence kinetic energy due to buoyancy. Y represents the contribution of the ﬂuctuating dilatation in compressible turbulence to the overall dissipation rate. C , C and C are constants. s and s are the turbulent Prandtl numbers # # # 1 2 3# k for k and #, respectively. The pressure-based solver in ANSYS Fluent (version 18.2) is used and the pressure–velocity coupling algorithm is chosen as the PISO to obtain a high computational efﬁciency. The coupling between these variables is achieved via velocity and pressure corrections to enforce both global Appl. Sci. 2018, 8, 1413 6 of 18 and local mass conservation. The PRessure STaggering Option (PRESTO!) scheme is used for pressure. First-order upwind schemes are used for density, momentum, energy, volume fraction, turbulence kinetic energy and turbulent dissipation rate. The ﬁrst-order implicit scheme is used for transient formulation. For the transient calculation, the time step size was set as 0.001 s to obtain an acceptable CFL number. The converged criteria are set as the residual of 10 and the maximum iteration number is set as 20. Monitors of the mass ﬂow rate at the inlets and outlet are set up to assess the performance Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 18 and efﬁciency of the pump, and to observe any potential unsteady characteristics. 4. Models of Target Pump 4. Models of Target Pump 4.1. Geometry Ejector 4.1. Geometry Ejector The baseline design of the hybrid airlift-jet pump is shown in Figure 4, which presents the detailed The baseline design of the hybrid airlift-jet pump is shown in Figure 4, which presents the view of the ejector in the jet part. The overall geometric shape and its detailed dimensions of the pump detailed view of the ejector in the jet part. The overall geometric shape and its detailed dimensions of are determined by following the result presented in the previous study. The length of the water suction the pump are determined by following the result presented in the previous study. The length of the pipe is set as 1 m. The dimensions and names of the geometric parameters are also listed in Table 1. water suction pipe is set as 1 m. The dimensions and names of the geometric parameters are also Note that the air nozzle inlet diameter is set to be the same as the outer diameter of the air compressor listed in Table 1. Note that the air nozzle inlet diameter is set to be the same as the outer diameter of used in the experiment. the air compressor used in the experiment. Figure 4. Schematics of a hybrid airlift-jet pump (units: mm; red-colored numbers and symbols Figure 4. Schematics of a hybrid airlift-jet pump (units: mm; red-colored numbers and symbols present present dimensions of a nozzle). dimensions of a nozzle). Table 1. Geometry dimensions of a hybrid airlift-jet pump (units: mm). Table 1. Geometry dimensions of a hybrid airlift-jet pump (units: mm). Air nozzle inlet diameter 6.5 Air nozzle inlet diameter 6.5 Air nozzle diameter 24 Air nozzle diameter 24 Air nozzle exit diameter 8 Air nozzle exit diameter 8 Water nozzle diameter 50 Water nozzle diameter 50 Distance between an air nozzle exit and a mixing section 25 Distance between an air nozzle exit and a mixing section 25 Mixing section diameter 24 Mixing section diameter 24 Diffuser outlet diameter 34 Diffuser outlet diameter 34 Mixing section and diffuser length 200 Mixing section and diffuser length 200 4.2. Mesh and Boundary Conditions 4.2. Mesh and Boundary Conditions To evaluate the independence of numerical solutions to the mesh used, a grid reﬁnement study is To evaluate the independence of numerical solutions to the mesh used, a grid refinement study carried out. The boundary conditions of Case No. 7 in Table 2 are applied. Figure 5 shows the result is carried out. The boundary conditions of Case No. 7 in Table 2 are applied. Figure 5 shows the in terms of the water mass ﬂow rate at the outlet and the number of grids used. Based on this result, result in terms of the water mass flow rate at the outlet and the number of grids used. Based on this a mesh of 1.5 million tetrahedron elements, as shown in Figure 6a, is used in the computation domain in result, a mesh of 1.5 million tetrahedron elements, as shown in Figure 6a, is used in the computation all of following computations. The mesh geometries at the air inlet and the air nozzle exit are densiﬁed domain in all of following computations. The mesh geometries at the air inlet and the air nozzle exit to obtain a more precise simulation of the local detailed ﬂow characteristics, as shown in Figure 6b,c. are densified to obtain a more precise simulation of the local detailed flow characteristics, as shown in Figure 6b,c. The mesh size is set to be around 0.25 mm for the air inlet and the air nozzle, and 2.5 mm for the other parts. The maximum y-plus (y ) values are around 500 in the nozzle and 800 in the diffuser. Appl. Sci. 2018, 8, 1413 7 of 18 The mesh size is set to be around 0.25 mm for the air inlet and the air nozzle, and 2.5 mm for the other parts. The maximum y-plus (y ) values are around 500 in the nozzle and 800 in the diffuser. Table 2. Cases and their boundary conditions. Total Pressure at Air Inlet Total Pressure at Water Inlet Volume Fraction of Water at Case No. (kPa)-Air Compressor (kPa)-Water Height Water Inlet-Airlift Pump 1 100 2 200 1 1 3 300 4 400 5 100 6 200 6 1 7 300 8 400 9 100 10 200 1 0.5 11 300 12 400 13 300 6 0.5 14 0.05 15 0.1 16 0.2 300 1 17 0.7 18 0.9 The total pressure boundary conditions were applied at the inlet section of the air nozzle and the vertical pipe. The range of total pressure at the air inlet is determined according to the capacity of air compressor used in the validation experiment shown in Figure 7. The total pressure at the water inlet is determined by including the effects of suction pipe length submerged in water; the total pressures of 1 kPa and 6 kPa correspond to the submerged suction pipe height h (shown in Figure 7g) of 0.1 m and 0.6 m, respectively. However, the volume fraction of water at the water inlet, which depends on the capacity of air compressor, the generated cavitation types and the water ﬂow velocity (water mass ﬂow rate) are relatively difﬁcult to determine. Therefore, the range of water volume fraction at the water inlet is set to investigate the effects of airlift pump on the overall performance of hybrid pump in its wide operation range. Altogether, eighteen cases are investigated including the operating conditions of only the jet part, and both the airlift and jet parts. For the jet-pump-only operation, the total pressure at the air inlet is varied along with specifying the volume fraction of air as one. At the water inlet, the volume fraction of air is speciﬁed as zero. In the hybrid mode, for simplicity, the volume fraction of air, as well as the total pressure at the water inlet, is varied to simulate the effects of the airlift pump and the submerged suction pipe height. The total pressure equation can be written as: g/(g1) g 1 p = p 1 + M (10) T s where p is the total pressure, p is the static pressure, M is the Mach number and g is the ratio of T S speciﬁc heats. The boundary conditions of each case are listed in Table 2. The total pressure boundary conditions are set at the air and water inlets. At the outlet, the static pressure condition is applied and set as zero gauge pressure. The total pressure range at the air inlet is varied from 100 kPa to 400 kPa. The cases denoted by No. 1–8 are set to simulate the inﬂuence of the air inlet pressure for different water inlet pressures, which are determined to represent different suction lengths (interpreted as the water suction pipe being buried into water or reducing the length of the suction pipe). Cases No. 9–12 are set Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 18 Appl. Sci. 2018, 8, 1413 8 of 18 to investigate the performance of the hybrid airlift-jet pump in different air inlet pressures; Cases Figure 5. The grid refinement study. No. 14–18 are set to different volume fractions of water at the water inlet. Case No. 13 is one example where both the abovementioned conditions are applied simultaneously. The hybrid initialization Table 2. Cases and their boundary conditions. method that solves the Laplace equation is used. Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 18 Case Total Pressure at Air Inlet Total Pressure at Water Inlet Volume Fraction of Water at Water No. (kPa)-Air Compressor (kPa)-Water Height Inlet-Airlift Pump 1 100 2 200 1 1 3 300 4 400 5 100 6 200 6 1 7 300 8 400 9 100 10 200 1 0.5 11 300 12 400 13 300 6 0.5 14 0.05 15 0.1 16 300 1 0.2 17 0.7 18 0.9 Figure 5. The grid refinement study. Figure 5. The grid reﬁnement study. Table 2. Cases and their boundary conditions. Case Total Pressure at Air Inlet Total Pressure at Water Inlet Volume Fraction of Water at Water No. (kPa)-Air Compressor (kPa)-Water Height Inlet-Airlift Pump 1 100 2 200 1 1 3 300 4 400 5 100 6 200 6 1 7 300 8 400 9 100 10 200 Appl. Sci. 2018, 8, x FOR PEER REVIEW 1 0.5 8 of 18 11 300 12 400 (a) 13 300 6 0.5 14 0.05 15 0.1 16 300 1 0.2 17 0.7 18 0.9 (b) (c) Figure 6. Mesh geometries of a pump model: (a) volume meshes of total domain; (b) volume meshes Figure 6. Mesh geometries of a pump model: (a) volume meshes of total domain; (b) volume meshes with the zoomed nozzle part; (c) volume meshes with the inlet part. with the zoomed nozzle part; (c) volume meshes with the inlet part. The total pressure boundary conditions were applied at the inlet section of the air nozzle and (a) the vertical pipe. The range of total pressure at the air inlet is determined according to the capacity of air compressor used in the validation experiment shown in Figure 7. The total pressure at the water inlet is determined by including the effects of suction pipe length submerged in water; the total pressures of 1 kPa and 6 kPa correspond to the submerged suction pipe height ℎ (shown in Figure 7g) of 0.1 m and 0.6 m, respectively. However, the volume fraction of water at the water inlet, which depends on the capacity of air compressor, the generated cavitation types and the water flow velocity (water mass flow rate) are relatively difficult to determine. Therefore, the range of water volume fraction at the water inlet is set to investigate the effects of airlift pump on the overall performance of hybrid pump in its wide operation range. Altogether, eighteen cases are investigated including the operating conditions of only the jet part, and both the airlift and jet parts. For the jet-pump-only operation, the total pressure at the air inlet is varied along with specifying the volume fraction of air as one. At the water inlet, the volume fraction of air is specified as zero. In the hybrid mode, for simplicity, the volume fraction of air, as well as the total pressure at the water inlet, is varied to simulate the effects of the airlift pump and the submerged suction pipe height. The total pressure equation can be written as: −1 (10) = 1+ where pT is the total pressure, pS is the static pressure, M is the Mach number and γ is the ratio of specific heats. (a) (b) Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 18 (b) (c) Figure 6. Mesh geometries of a pump model: (a) volume meshes of total domain; (b) volume meshes with the zoomed nozzle part; (c) volume meshes with the inlet part. The total pressure boundary conditions were applied at the inlet section of the air nozzle and the vertical pipe. The range of total pressure at the air inlet is determined according to the capacity of air compressor used in the validation experiment shown in Figure 7. The total pressure at the water inlet is determined by including the effects of suction pipe length submerged in water; the total pressures of 1 kPa and 6 kPa correspond to the submerged suction pipe height ℎ (shown in Figure 7g) of 0.1 m and 0.6 m, respectively. However, the volume fraction of water at the water inlet, which depends on the capacity of air compressor, the generated cavitation types and the water flow velocity (water mass flow rate) are relatively difficult to determine. Therefore, the range of water volume fraction at the water inlet is set to investigate the effects of airlift pump on the overall performance of hybrid pump in its wide operation range. Altogether, eighteen cases are investigated including the operating conditions of only the jet part, and both the airlift and jet parts. For the jet-pump-only operation, the total pressure at the air inlet is varied along with specifying the volume fraction of air as one. At the water inlet, the volume fraction of air is specified as zero. In the hybrid mode, for simplicity, the volume fraction of air, as well as the total pressure at the water inlet, is varied to simulate the effects of the airlift pump and the submerged suction pipe height. The total pressure equation can be written as: −1 (10) = 1+ Appl. Sci. 2018, 8, 1413 9 of 18 where pT is the total pressure, pS is the static pressure, M is the Mach number and γ is the ratio of specific heats. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 18 (a) (b) (c) (d) (e) (f) (g) Figure 7. Experiment using the prototype hybrid pump: (a) prototype of hybrid pump; (b) jet pump Figure 7. Experiment using the prototype hybrid pump: (a) prototype of hybrid pump; (b) jet pump part; (c) airlift pump part; (d) water tank; (e) air compressor; (f) prototype hybrid pump connected part; (c) airlift pump part; (d) water tank; (e) air compressor; (f) prototype hybrid pump connected with an air compressor in water tank; and (g) schematics of an experimental setup. with an air compressor in water tank; and (g) schematics of an experimental setup. The boundary conditions of each case are listed in Table 2. The total pressure boundary conditions are set at the air and water inlets. At the outlet, the static pressure condition is applied and set as zero gauge pressure. The total pressure range at the air inlet is varied from 100 kPa to 400 kPa. The cases denoted by No. 1–8 are set to simulate the influence of the air inlet pressure for different water inlet pressures, which are determined to represent different suction lengths (interpreted as the water suction pipe being buried into water or reducing the length of the suction pipe). Cases No. 9–12 are set to investigate the performance of the hybrid airlift-jet pump in different air inlet pressures; Cases No. 14–18 are set to different volume fractions of water at the water inlet. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 18 Appl. Sci. 2018, 8, 1413 10 of 18 Case No. 13 is one example where both the abovementioned conditions are applied simultaneously. The hybrid initialization method that solves the Laplace equation is used. 5. Results and Discussion 5. Results and Discussion 5.1. Validation of Numerical Methods T 5o .1validate . Validation of Numerical the current numerical Methods methods, experiments using the prototype hybrid pump shown in Figure 7 are performed. The air compressor of SP5-150-5 [25] is used and can compresses air up To validate the current numerical methods, experiments using the prototype hybrid pump to 9.0 bar. The prototype hybrid pump is connected to the air compressor to provide the air source shown in Figure 7 are performed. The air compressor of SP5-150-5 [25] is used and can compresses to theai jet r up and to airlift 9.0 bar. The prot parts. Among otype hybrid the cases pump is listed connected to the in Table 2, Cases air compressor to provi of No. 5–8 that reprd esent e the the air sole operation source to the of the jet jet a part nd for airli the ft pa submer rts. Among the ca ged heightses of 0.6 listed m iar n Ta e selected ble 2, Cases of for comparison. No. 5–8 thaFigur t represent e 7 shows the sole operation of the jet part for the submerged height of 0.6 m are selected for comparison. the water tank, the air compressor and the prototype hybrid pump connected to the air compressor, Figure 7 shows the water tank, the air compressor and the prototype hybrid pump connected to the together with the schematics of the experimental setup. The static pressure at the inlet of the jet pump air compressor, together with the schematics of the experimental setup. The static pressure at the is measured by using the manometer of P110 [26], which can be measured up to 1 MPa, and the water inlet of the jet pump is measured by using the manometer of P110 [26], which can be measured up to mass ﬂow rate is measured by weighing the water expelled from the pump during 10 s. 1 MPa, and the water mass flow rate is measured by weighing the water expelled from the pump Figure 8 compares the variation in static pressure at the inlet of the airﬂow versus the water during 10 s. ﬂow rate between the experimental and computation results. Although the difference between the Figure 8 compares the variation in static pressure at the inlet of the airflow versus the water two results increases as the static pressure increases, a good agreement between the two results is flow rate between the experimental and computation results. Although the difference between the found in terms of the increasing trend of water mass ﬂow rate at the outlet, as the air inlet pressure two results increases as the static pressure increases, a good agreement between the two results is found in terms of the increasing trend of water mass flow rate at the outlet, as the air inlet pressure increases. The larger difference at the higher air inlet pressure seems to be due to the fact that the increases. The larger difference at the higher air inlet pressure seems to be due to the fact that the numerical simulation assumes the constant total pressure at the water inlet, while the total pressure numerical simulation assumes the constant total pressure at the water inlet, while the total pressure in the experiment decreases with a reduction in the submerged suction pipe height when water in in the experiment decreases with a reduction in the submerged suction pipe height when water in the tank is pumped out. The more realistic boundary condition may be needed to reproduce the the tank is pumped out. The more realistic boundary condition may be needed to reproduce the experimental condition more closely. However, since the aim of present study is to assess the relative experimental condition more closely. However, since the aim of present study is to assess the performance and efﬁciency of the hybrid pump in various operation conditions, the validation of the relative performance and efficiency of the hybrid pump in various operation conditions, the present numerical method is sufﬁcient in this respect. validation of the present numerical method is sufficient in this respect. Figure 8. Comparison of predicted water flow rate with measured data according to air inlet pressure. Figure 8. Comparison of predicted water ﬂow rate with measured data according to air inlet pressure. 5.2. Performance and Efficiency of Hybrid Pump 5.2. Performance and Efﬁciency of Hybrid Pump Figure 9a,b shows the average mass flow rate of the water versus the average mass flow rate of the air for all of the cases and the average mass flow rate of the water versus the water volume Figure 9a,b shows the average mass ﬂow rate of the water versus the average mass ﬂow rate of the fraction at the water inlet for the hybrid cases, respectively. As shown in Case No. 1, the mass flow air for all of the cases and the average mass ﬂow rate of the water versus the water volume fraction at rate of water is zero, implying that the airflow is not sufficiently strong to pump the water to 0.9 m the water inlet for the hybrid cases, respectively. As shown in Case No. 1, the mass ﬂow rate of water high. For the other cases, the water mass flow rate increases with the air mass flow rate. In Cases No. is zero, implying that the airﬂow is not sufﬁciently strong to pump the water to 0.9 m high. For the 5–8, which are set to simulate a shorter suction height of 0.4 m, the air mass flow rate does not other cases, the water mass ﬂow rate increases with the air mass ﬂow rate. In Cases No. 5–8, which are set to simulate a shorter suction height of 0.4 m, the air mass ﬂow rate does not change enormously in the same air inlet boundary condition, while the water mass ﬂow rate is increased signiﬁcantly in comparison with Cases No. 1–4. This relatively larger mass ﬂow rate is due to the less work required to obtain gravitational potential energy with almost the same energy from the air ﬂow. In the hybrid Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 18 change enormously in the same air inlet boundary condition, while the water mass flow rate is increased significantly in comparison with Cases No. 1–4. This relatively larger mass flow rate is due Appl. Sci. 2018, 8, 1413 11 of 18 to the less work required to obtain gravitational potential energy with almost the same energy from the air flow. In the hybrid pump, Cases No. 9–18 where the jet and airlift parts operate together are pump, Cases No. 9–18 where the jet and airlift parts operate together are also shown in Figure 9a. also shown in Figure 9a. When the volume fraction of water is set to be 0.5 at the water inlet (Cases When the volume fraction of water is set to be 0.5 at the water inlet (Cases No. 9–12), it shows a similar No. 9–12), it shows a similar mass flow rate in Cases No. 5–8, which represent a shorter pipe suction mass ﬂow rate in Cases No. 5–8, which represent a shorter pipe suction length. The similarity here is length. The similarity here is considered to be related to less work required in the suction pipe owing considered to be related to less work required in the suction pipe owing to the lower mixture density to the lower mixture density or shorter suction height. When the hybrid pump case is performed or shorter suction height. When the hybrid pump case is performed with a shorter suction pipe length with a shorter suction pipe length (Case No. 13), a more significant increase in the water mass flow (Case No. 13), a more signiﬁcant increase in the water mass ﬂow rate is achieved. For the hybrid pump rate is achieved. For the hybrid pump cases with different volume fractions of water (Cases No. 14– cases with different volume fractions of water (Cases No. 14–18), their water mass ﬂow rates increase 18), their water mass flow rates increase as the water volume fraction decrease to the value of 0.1; as the water volume fraction decrease to the value of 0.1; when the water volume fraction is further when the water volume fraction is further reduced to below 0.1, the water mass flow rate decreases, reduced to below 0.1, the water mass ﬂow rate decreases, that is, the water mass ﬂow rate in Case that is, the water mass flow rate in Case No. 14 is less than that in Case No. 15. The maximum water No. 14 is less than that in Case No. 15. The maximum water mass ﬂow rate is achieved through Case mass flow rate is achieved through Case No. 13, with a value of 0.21 kg/s. The effects of the water inlet No. 13, with a value of 0.21 kg/s. The effects of the water inlet volume fraction and the water suction volume fraction and the water suction height on the water mass flow rate can be seen in Figure 9b. height on the water mass ﬂow rate can be seen in Figure 9b. (a) (b) Figure 9. Comparison of mass flow rates of hybrid airlift-jet pump in various operating conditions: Figure 9. Comparison of mass ﬂow rates of hybrid airlift-jet pump in various operating conditions: (a) (a) all cases; (b) airlift-jet hybrid pump cases. all cases; (b) airlift-jet hybrid pump cases. To compare the performance of the pump operating under different conditions more To compare the performance of the pump operating under different conditions more quantitatively, quantitatively, the efficiency of the hybrid airlift-jet pump is defined as: the efﬁciency of the hybrid airlift-jet pump is deﬁned as: , , = = (11) P ( H H )m − w out 2,w 3,w , , h = = (11) P ( H H )m in 1,a 2,a a where represents the mass flow rate of the fluid; the subscripts w and a represent the water and air, respectively; the subscripts 1, 2, 3 denote the monitoring locations, as shown in Figure 10; H is where m represents the mass ﬂow rate of the ﬂuid; the subscripts w and a represent the water and air, the total head of water or air in a certain location, which can be expressed as: respectively; the subscripts 1, 2, 3 denote the monitoring locations, as shown in Figure 10; H is the total head of water or air in a certain location, which can be expressed as: = + +ℎ (12) P v H = + + h (12) rg 2g Equation (11) is typically used for assessing the performance of the jet pump; however, it is also suitable for the hybrid pump. With only normal-pressure air being released at the water inlet, the additional input power is sufﬁciently small to be ignored owing to its low kinetic energy and small mass ﬂow rate at the water inlet. Therefore, the equation is still valid provided that the state for each phase is correctly monitored at every boundary. However, note that Equation (11) needs to be corrected if the airlift part is modeled in more realistic ways: for example, the water volume fraction boundary condition is replaced with air-injection with higher pressure. Appl. Sci. 2018, 8, x FOR PEER REVIEW 12 of 18 Appl. Sci. 2018, 8, 1413 12 of 18 Figure 10. Monitoring points in the computational domain. Equation (11) is typically used for assessing the performance of the jet pump; however, it is also The predicted efﬁciency, the air mass ﬂow rate, and the water mass ﬂow rate in each case are suitable for the hybrid pump. With only normal-pressure air being released at the water inlet, the shown in Figure 11. Figure 11a shows a similar relationship to that in Figure 9a. A higher efﬁciency is additional input power is sufficiently small to be ignored owing to its low kinetic energy and small generally achieved when more air is injected as the air inﬂow in Cases No. 1–8, while the efﬁciency mass flow rate at the water inlet. Therefore, the equation is still valid provided that the state for each decreases in higher airﬂow rates for Cases No. 10–12. The efﬁciency for each air inlet boundary phase is correctly monitored at every boundary. However, note that Equation (11) needs to be condition improves in the cases of a shorter suction length (Cases No. 5–8), a hybrid pump setting corrected if the airlift part is modeled in more realistic ways: for example, the water volume fraction (Cases boundar No. 9–12), y condit orion both is rep (Case laceNo. d wit13) h aiin r-inject comparison ion with higher with Cases pressure No. . 1–4. The maximum efﬁciency The predicted efficiency, the air mass flow rate, and the water mass flow rate in each case are (13.3%) is achieved through Case No. 13, as indicated in Figure 11b, because, more water is suctioned shown in Figure 11. Figure 11a shows a similar relationship to that in Figure 9a. A higher efficiency forAppl. a favorable Sci. 2018, 8,case x FOR P setting EER RE at VIEW the same efﬁciency. 12 of 18 is generally achieved when more air is injected as the air inflow in Cases No. 1–8, while the efficiency decreases in higher airflow rates for Cases No. 10–12. The efficiency for each air inlet boundary condition improves in the cases of a shorter suction length (Cases No. 5–8), a hybrid pump setting (Cases No. 9–12), or both (Case No. 13) in comparison with Cases No. 1–4. The maximum efficiency (13.3%) is achieved through Case No. 13, as indicated in Figure 11b, because, more water is suctioned for a favorable case setting at the same efficiency. The variations in the water mass flow rate and efficiency in terms of varied volume fractions in Cases No. 3, No. 11, and No. 14–18 are shown in Figure 11c. Both variables initially increase when the hybrid pump setting is activated by injecting air into the water column, while a relative low volume fraction of water restricts the increase in mass flow rate when a great deal of air is injected. The maximum point for the mass flow rate is achieved at the volume fraction of 0.1. However, the efficiency curve continues to increase without a maximum point. The velocity of the outlet water increases continually even when the volume fraction is small, resulting in a high efficiency. Figure 10. Monitoring points in the computational domain. Figure 10. Monitoring points in the computational domain. Equation (11) is typically used for assessing the performance of the jet pump; however, it is also suitable for the hybrid pump. With only normal-pressure air being released at the water inlet, the additional input power is sufficiently small to be ignored owing to its low kinetic energy and small mass flow rate at the water inlet. Therefore, the equation is still valid provided that the state for each phase is correctly monitored at every boundary. However, note that Equation (11) needs to be corrected if the airlift part is modeled in more realistic ways: for example, the water volume fraction boundary condition is replaced with air-injection with higher pressure. The predicted efficiency, the air mass flow rate, and the water mass flow rate in each case are shown in Figure 11. Figure 11a shows a similar relationship to that in Figure 9a. A higher efficiency is generally achieved when more air is injected as the air inflow in Cases No. 1–8, while the efficiency decreases in higher airflow rates for Cases No. 10–12. The efficiency for each air inlet boundary condit Appl. Sci. ion im 2018, proves in t 8, x FOR PEER h RE e case VIEW s of a shorter suction length (Cases No. 5–8), a hybrid pump 13 of set 18 ting (Cases No. 9–12), or both (Case No. 13) in compa rison with Cases No. 1–4. The maximum efficienc y (a) (b) (13.3%) is achieved through Case No. 13, as indicated in Figure 11b, because, more water is suctioned for a favorable case setting at the same efficiency. The variations in the water mass flow rate and efficiency in terms of varied volume fractions in Cases No. 3, No. 11, and No. 14–18 are shown in Figure 11c. Both variables initially increase when the hybrid pump setting is activated by injecting air into the water column, while a relative low volume fraction of water restricts the increase in mass flow rate when a great deal of air is injected. The maximum point for the mass flow rate is achieved at the volume fraction of 0.1. However, the efficiency curve continues to increase without a maximum point. The velocity of the outlet water increases continually even when the volume fraction is small, resulting in a high efficiency. (c) Figure 11. Comparison of efﬁciencies in terms of the mass ﬂow rate and volume fraction for each case: Figure 11. Comparison of efficiencies in terms of the mass flow rate and volume fraction for each (a) efcase ﬁciency : (a) evs. fficiair encmass y vs. ai ﬂow r mass rate; flow (b ra ) ef teﬁciency ; (b) effici vs. ency wate vs. w r mass ater m ﬂow ass flow rate; rate; and and (c) (ef c) ef ﬁciency ficiency and and water mass water mass f ﬂow rate versus low rate versus v volume fraction olume fraction of water of water. . 5.3. Pressure and Velocity Distributions inside Hybrid Pump Case No. 3 is chosen to illustrate the characteristics of the two-phase flow field inside the hybrid pump by investigating the detailed distributions of static gauge pressure and fluid velocity. As shown in Figure 12a, the air pressure decreases rapidly when it arrives at the air nozzle part. Just outside of the air nozzle (ejector), a negative pressure area forms. In Figure 12b, the maximal axial velocity is observed in the exit of the air nozzle, although the flow path is enlarged. This is due to the (a) (b) decrease in pressure that causes the air to obtain a significant expansion. (a) (b) Figure 12. Cross-sectional static pressure and velocity distributions: (a) static pressure; (b) axial velocity. The static gauge pressures and the Mach numbers of the air along the center line of the air nozzle and the primary exit pipe are shown in Figure 13, where Cases No. 1–4 are chosen to illustrate the relationship between these variables and the air inlet total pressure. It is noteworthy that the Appl. Sci. 2018, 8, 1413 13 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 18 The variations in the water mass ﬂow rate and efﬁciency in terms of varied volume fractions in Cases No. 3, No. 11, and No. 14–18 are shown in Figure 11c. Both variables initially increase when the hybrid pump setting is activated by injecting air into the water column, while a relative low volume fraction of water restricts the increase in mass ﬂow rate when a great deal of air is injected. The maximum point for the mass ﬂow rate is achieved at the volume fraction of 0.1. However, the efﬁciency curve continues to increase without a maximum point. The velocity of the outlet water increases continually even when the volume fraction is small, resulting in a high efﬁciency. 5.3. Pressure and Velocity Distributions inside Hybrid Pump Case No. 3 is chosen to illustrate the characteristics of the two-phase ﬂow ﬁeld inside the hybrid pump by investigating the detailed distributions of static gauge pressure and ﬂuid velocity. As shown in Figure 12a, the air pressure decreases rapidly when it arrives at the air nozzle part. Just outside of the air nozzle (ejector), a negative pressure area forms. In Figure 12b, the maximal axial velocity is observed in the exit of the air nozzle, although the ﬂow path is enlarged. This is due to the decrease in (c) pressure that causes the air to obtain a signiﬁcant expansion. The static gauge pressures and the Mach numbers of the air along the center line of the air nozzle Figure 11. Comparison of efficiencies in terms of the mass flow rate and volume fraction for each and the primary exit pipe are shown in Figure 13, where Cases No. 1–4 are chosen to illustrate the case: (a) efficiency vs. air mass flow rate; (b) efficiency vs. water mass flow rate; and (c) efficiency and relationship between these variables and the air inlet total pressure. It is noteworthy that the remaining water mass flow rate versus volume fraction of water. cases share the similar trend. In each case, the negative gauge pressure is formed at the air nozzle exit due 5.3. to Pressu the expansion re and Velocity Di of air. When stributions the static inside H pressur ybrid Pu e dif mp ference between the inlet and the outlet of pipe ﬂow increases, the air ﬂow is accelerated along the converging nozzle, and a choked ﬂow is formed, Case No. 3 is chosen to illustrate the characteristics of the two-phase flow field inside the hybrid in which the Mach number is maintained to be one in the nozzle. However, if the pressure difference pump by investigating the detailed distributions of static gauge pressure and fluid velocity. As continues to increase, the Mach number at the nozzle is maintained at 1, but the downstream ﬂow shown in Figure 12a, the air pressure decreases rapidly when it arrives at the air nozzle part. Just is more accelerated to be supersonic in the diffusion region [27]. Note that the increase of the static outside of the air nozzle (ejector), a negative pressure area forms. In Figure 12b, the maximal axial pressure and the decrease of Mach number near the exit of main pipe are due to the diverging diffuser, velocity is observed in the exit of the air nozzle, although the flow path is enlarged. This is due to the as shown in Figure 12. decrease in pressure that causes the air to obtain a significant expansion. (a) (b) Figure 12. Cross-sectional static pressure and velocity distributions: (a) static pressure; (b) axial velocity. Figure 12. Cross-sectional static pressure and velocity distributions: (a) static pressure; (b) axial velocity. The static gauge pressures and the Mach numbers of the air along the center line of the air nozzle and the primary exit pipe are shown in Figure 13, where Cases No. 1–4 are chosen to illustrate the relationship between these variables and the air inlet total pressure. It is noteworthy that the Appl. Sci. 2018, 8, x FOR PEER REVIEW 14 of 18 remaining cases share the similar trend. In each case, the negative gauge pressure is formed at the air nozzle exit due to the expansion of air. When the static pressure difference between the inlet and the outlet of pipe flow increases, the air flow is accelerated along the converging nozzle, and a choked flow is formed, in which the Mach number is maintained to be one in the nozzle. However, if the pressure difference continues to increase, the Mach number at the nozzle is maintained at 1, but the downstream flow is more accelerated to be supersonic in the diffusion region [27]. Note that the Appl. Sci. 2018, 8, 1413 14 of 18 increase of the static pressure and the decrease of Mach number near the exit of main pipe are due to the diverging diffuser, as shown in Figure 12. Table 3 shows the inlet static pressures and the maximum Mach numbers along the center line of Table 3 shows the inlet static pressures and the maximum Mach numbers along the center line the air nozzle and the primary exit pipe. The static pressure at the air inlet increases and the minimum of the air nozzle and the primary exit pipe. The static pressure at the air inlet increases and the value decreases as a higher total pressure boundary condition is set at the air inlet. Meanwhile, minimum value decreases as a higher total pressure boundary condition is set at the air inlet. a negative gauge pressure in the mixing section appears to be more obvious with a higher inlet total Meanwhile, a negative gauge pressure in the mixing section appears to be more obvious with a pressure, causing the pressure in the diffuser to recover more quickly. As shown in Figure 13b, despite higher inlet total pressure, causing the pressure in the diffuser to recover more quickly. As shown in the different inlet pressure distributions, the velocity in the air nozzle is almost the same at the inlet Figure 13b, despite the different inlet pressure distributions, the velocity in the air nozzle is almost as that at the middle section of the air nozzle. At the air nozzle exit, air with a higher inlet pressure the same at the inlet as that at the middle section of the air nozzle. At the air nozzle exit, air with a boundary condition increases signiﬁcantly in velocity; the velocity difference persists until the air higher inlet pressure boundary condition increases significantly in velocity; the velocity difference reaches persists until the outlet the a of ir rea the dif ches fuser the out . let of the diffuser. (a) (b) Figure 13. Static gauge pressure and Mach number along the center line of air nozzle: (a) static Figure 13. Static gauge pressure and Mach number along the center line of air nozzle: (a) static pressure pressure distribution; (b) Mach number distribution (red: nozzle; blue: mixing section). distribution; (b) Mach number distribution (red: nozzle; blue: mixing section). Table 3. Static pressure and velocity at an air inlet. Table 3. Static pressure and velocity at an air inlet. Case No. Static Pressure at Air Inlet (kPa) Maximum Mach Number Case No. Static Pressure at Air Inlet (kPa) Maximum Mach Number No. 1 41.8 0.70 No. 2 104.0 1.00 No. 1 41.8 0.70 No. 2 104.0 1.00 No. 3 168.1 1.24 No. 3 168.1 1.24 No. 4 233.7 1.43 No. 4 233.7 1.43 To compare the cases with different suction heights and volume fractions that are affected by the operation To compar of the airlift e the cases with part, difdistribu ferent suction tions of st heights atic pressu and volume re and the Mach n fractions that umber in the are affected case by the s operation with the sa of m the e a airlift ir inle part, t tota distributions l pressure ofof 300 static kPa pr aessur re shown i e and the n FiMach gure 1 number 4. In the sta in the ticcases pressure with distribution, compared with Case No. 3, the other cases have a smaller negative pressure at the air the same air inlet total pressure of 300 kPa are shown in Figure 14. In the static pressure distribution, nozzle exit but a larger negative pressure in the primary exit pipe of the mixing section. A more compared with Case No. 3, the other cases have a smaller negative pressure at the air nozzle exit but obvious difference can be found in the Mach number plot, as the case with a shorter suction height a larger negative pressure in the primary exit pipe of the mixing section. A more obvious difference and a lower volume fraction of water has a higher Mach number at the mixing section. Case No. 13 can be found in the Mach number plot, as the case with a shorter suction height and a lower volume shows the highest Mach number distribution owing to the superposition of the two effects. fraction of water has a higher Mach number at the mixing section. Case No. 13 shows the highest Mach number distribution owing to the superposition of the two effects. Appl. Sci. 2018, 8, 1413 15 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 15 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 15 of 18 (a) (b) (a) (b) Figure 14. Comparison of the static gauge pressure and Mach number for the same air inlet total Figure 14. Comparison of the static gauge pressure and Mach number for the same air inlet total pressure: (a) static pressure; (b) Mach number (red: nozzle; blue: mixing section). Figure 14. Comparison of the static gauge pressure and Mach number for the same air inlet total pressure: (a) static pressure; (b) Mach number (red: nozzle; blue: mixing section). pressure: (a) static pressure; (b) Mach number (red: nozzle; blue: mixing section). 5.4. Periodic Fluctuation of Water Flow 5.4. Periodic Fluctuation of Water Flow 5.4. Periodic Fluctuation of Water Flow The time history of the mass flow rate of Case No. 3 is shown in Figure 15. Note that the positive The time history of the mass ﬂow rate of Case No. 3 is shown in Figure 15. Note that the positive sign at the boundary means the flow into the system, while the negative sign at the outlet means the The time history of the mass flow rate of Case No. 3 is shown in Figure 15. Note that the positive sign at the boundary means the ﬂow into the system, while the negative sign at the outlet means positive mass flow rate. Initially, a relatively high mass flow rate appears at the water inlet, which sign at the boundary means the flow into the system, while the negative sign at the outlet means the the positive mass ﬂow rate. Initially, a relatively high mass ﬂow rate appears at the water inlet, represents the water being pumped up from the downside as the jet pump starts to operate. When positive mass flow rate. Initially, a relatively high mass flow rate appears at the water inlet, which which represents the water being pumped up from the downside as the jet pump starts to operate. the water phase reaches the mixing section, an initial instability occurs, which leads to a strong back represents the water being pumped up from the downside as the jet pump starts to operate. When When the water phase reaches the mixing section, an initial instability occurs, which leads to a strong flow to the water inlet. Simultaneously, a large amount of water is exhausted in comparison to the the water phase reaches the mixing section, an initial instability occurs, which leads to a strong back back ﬂow to the water inlet. Simultaneously, a large amount of water is exhausted in comparison to water mass flow rate at the outlet. After this initial unstable fluctuation, the water mass flow rates at flow to the water inlet. Simultaneously, a large amount of water is exhausted in comparison to the the water mass ﬂow rate at the outlet. After this initial unstable ﬂuctuation, the water mass ﬂow rates the outlet and water inlet are developed into a periodic fluctuation with a period of approximately water mass flow rate at the outlet. After this initial unstable fluctuation, the water mass flow rates at at the outlet and water inlet are developed into a periodic ﬂuctuation with a period of approximately 0.8 s. The back flow to the water inlet occurs at every fluctuation period. the outlet and water inlet are developed into a periodic fluctuation with a period of approximately 0.8 s. The back ﬂow to the water inlet occurs at every ﬂuctuation period. 0.8 s. The back flow to the water inlet occurs at every fluctuation period. (a) (b) (a) (b) Figure 15. Mass flow rates of an inlet and an outlet: (a) total time; (b) periodic fluctuated part Figure 15. Mass flow rates of an inlet and an outlet: (a) total time; (b) periodic fluctuated part Figure 15. Mass ﬂow rates of an inlet and an outlet: (a) total time; (b) periodic ﬂuctuated part. To confirm this periodic fluctuation of water mass flow rate at the inlet, the time histories of the inlet water mass flow rate computed for the other cases are shown in Figure 16. The time span is not To confirm this periodic fluctuation of water mass flow rate at the inlet, the time histories of the To conﬁrm this periodic ﬂuctuation of water mass ﬂow rate at the inlet, the time histories of the exactly the same in every case, because the unsteady computation is halted after the converged inlet water mass flow rate computed for the other cases are shown in Figure 16. The time span is not inlet water mass ﬂow rate computed for the other cases are shown in Figure 16. The time span is not solution is obtained to save computational time. Cases No. 16–18 represent water volume fraction exactly the same in every case, because the unsteady computation is halted after the converged exactly the same in every case, because the unsteady computation is halted after the converged solution boundary conditions of 0.2, 0.7, and 0.9, respectively. As shown in the comparison in Figure 16, the solution is obtained to save computational time. Cases No. 16–18 represent water volume fraction is obtained to save computational time. Cases No. 16–18 represent water volume fraction boundary fluctuation pattern of the inlet water mass flow rate is similar to that in the jet-pump-only case (Case boundary conditions of 0.2, 0.7, and 0.9, respectively. As shown in the comparison in Figure 16, the conditions of 0.2, 0.7, and 0.9, respectively. As shown in the comparison in Figure 16, the ﬂuctuation fluctuation pattern of the inlet water mass flow rate is similar to that in the jet-pump-only case (Case Appl. Sci. 2018, 8, 1413 16 of 18 pattern of the inlet water mass ﬂow rate is similar to that in the jet-pump-only case (Case No. 3), Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 18 where the volume fraction of water is set as 0.9. An attenuated pattern of mass ﬂow rate ﬂuctuation is formed when the volume fraction of water further decreases. No. 3), where the volume fraction of water is set as 0.9. An attenuated pattern of mass flow rate The detailed distributions of volume fraction at different time steps in Case No. 3 are shown in fluctuation is formed when the volume fraction of water further decreases. Figure 17. Altogether, eight time steps are chosen according to the periodic ﬂuctuation of the water mass ﬂow rate observed in Figure 17. More water is entering the mixing section and the diffuser, respectively, as shown in Figure 17e,f; this corresponds to the time step with a local maximum value of the mass ﬂow rate. As a contrast, the volume fraction of water at the same section is much lower in Figure 17g,h. With the ﬂuctuation of the water mass ﬂow rate, a pulsating pump phenomenon occurred. This may be explained by the air being periodically blocked by the inlet water, which caused Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 18 the instability in the pump’s suction ability. Further, this may explain why this phenomenon is not remarkable in the hybrid pump cases when air is released into the water inlet and weakened the No. 3), where the volume fraction of water is set as 0.9. An attenuated pattern of mass flow rate blocking effect. This is an advantage in activating the hybrid pump phase. fluctuation is formed when the volume fraction of water further decreases. Figure 16. Mass flow rates of a water inlet in different cases. The detailed distributions of volume fraction at different time steps in Case No. 3 are shown in Figure 17. Altogether, eight time steps are chosen according to the periodic fluctuation of the water mass flow rate observed in Figure 17. More water is entering the mixing section and the diffuser, respectively, as shown in Figure 17e,f; this corresponds to the time step with a local maximum value of the mass flow rate. As a contrast, the volume fraction of water at the same section is much lower in Figure 17g,h. With the fluctuation of the water mass flow rate, a pulsating pump phenomenon occurred. This may be explained by the air being periodically blocked by the inlet water, which caused the instability in the pump’s suction ability. Further, this may explain why this phenomenon is not remarkable in the hybrid pump cases when air is released into the water inlet and weakened Figure 16. Mass flow rates of a water inlet in different cases. Figure 16. Mass ﬂow rates of a water inlet in different cases. the blocking effect. This is an advantage in activating the hybrid pump phase. The detailed distributions of volume fraction at different time steps in Case No. 3 are shown in Figure 17. Altogether, eight time steps are chosen according to the periodic fluctuation of the water mass flow rate observed in Figure 17. More water is entering the mixing section and the diffuser, respectively, as shown in Figure 17e,f; this corresponds to the time step with a local maximum value of the mass flow rate. As a contrast, the volume fraction of water at the same section is much lower in (a) (b) Figure 17g,h. With the fluctuation of the water mass flow rate, a pulsating pump phenomenon occurred. This may be explained by the air being periodically blocked by the inlet water, which caused the instability in the pump’s suction ability. Further, this may explain why this phenomenon is not remarkable in the hybrid pump cases when air is released into the water inlet and weakened (c) (d) the blocking effect. This is an advantage in activating the hybrid pump phase. (e) (f) (b) (a) (g) (h) Figure 17. Time-history distribution of air volume fraction: (a) t = 3.4 s; (b) t = 3.5 s; (c) t = 3.6 s; (d) t = Figure 17. Time-history (c) distribution of air volume fraction: (a) t = 3.4 s; (b) t = 3.5 s; (c) t = 3.6 s; (d) 3.7 s; (e) t = 3.8 s; (f) t = 3.9 s; (g) t = 4.0 s; (h) t = 4.1 s. (d) t = 3.7 s; (e) t = 3.8 s; (f) t = 3.9 s; (g) t = 4.0 s; (h) t = 4.1 s. (f) (e) (g) (h) Figure 17. Time-history distribution of air volume fraction: (a) t = 3.4 s; (b) t = 3.5 s; (c) t = 3.6 s; (d) t = 3.7 s; (e) t = 3.8 s; (f) t = 3.9 s; (g) t = 4.0 s; (h) t = 4.1 s. Appl. Sci. 2018, 8, 1413 17 of 18 6. Conclusions Numerical simulations of 18 cases were performed to investigate the performance of the preliminary design of a hybrid airlift-jet pump under various operational conditions. The water mass ﬂow rate increases with the higher total pressure boundary conditions at the air inlet. With the same air inlet conditions, an improvement in pumping ability can be achieved by shortening the suction height, releasing air into the water inlet to form a hybrid pump case, and applying a combined case of the abovementioned settings. Using the efﬁciency deﬁnition applicable for the current study, it is shown that the efﬁciency for each air inlet boundary condition improves when a shorter suction length is applied or a volume fraction of water is decreased in the hybrid pump phase. The best volume fraction rate of water to obtain a large mass ﬂow rate is 0.1 for the current design. The static pressure distribution shows an area of negative pressure from the outlet of the air nozzle to the diffuser in each case. The velocity distribution indicates that despite the different pressure values in the air nozzle, the air travels at almost the same speed for the different cases. The difference in velocity only occurs near the outlet of the air nozzle. The ﬂuctuating mass ﬂow rates of water in different cases, as well as the volume fraction counters, are analyzed, which shows a periodic ﬂuctuating phenomenon of the water ﬂow. The inlet water mass ﬂow rate ﬂuctuates, thus forming a pulsating pump system. This phenomenon is attributable to the air being periodically blocked by the water and weakened in the hybrid pump cases. These results conﬁrm that the performance of the jet or airlift pump can be enhanced by employing the hybrid airlift-jet pump newly proposed in the present study. In the future, this preliminary conceptual design will be improved by optimizing design factors, such as the nozzle, the mixing chamber and the suction pipe, on a basis of more intensive and extensive numerical and experimental analysis on ﬂow characteristics of the hybrid pump. The numerical methods also need to be improved by modelling the airlift part in a more realistic way: an air-injection boundary condition will be considered instead of the water volume fraction boundary condition. Author Contributions: I.L. proposed the initial concept of new hybrid pump. C.C. reﬁned the initial concept of hybrid pump and supervised the entire research. D.Y. performed the related numerical computations and prepared the ﬁrst draft of paper. K.L. carried out the related experiments. M.H. reran all of the numerical simulations in the preparation of revised manuscript. Funding: This research received no external funding. Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2016R1D1A1A09918456). Conﬂicts of Interest: The authors declare no conﬂicts of interest. References 1. Rankine, W.J.M. On the mathematical theory of combined streams. Proc. R. Soc. Lond. 1870, 19, 90–94. 2. Keenan, J.H. An investigation of ejector design by analysis and experiment. J. Appl. Mech. 1950, 17, 299–318. 3. Eames, I.W.; Aphornratana, S.; Haider, H. A theoretical and experimental study of a small-scale steam jet refrigerator. Int. J. Refrig. 1995, 18, 378–386. 4. Huang, B.J.; Chang, J.M.; Wang, C.P.; Petrenko, V.A. A 1-D analysis of ejector performance. Int. J. Refrig. 1999, 22, 354–364. 5. Riffat, S.B.; Gan, G.; Smith, S. Computational ﬂuid dynamics applied to ejector heat pumps. Appl. Therm. Eng. 1996, 16, 291–297. [CrossRef] 6. Riffat, S.B.; Omer, S.A. CFD modelling and experimental investigation of an ejector refrigeration system using methanol as the operating ﬂuid. Int. J. Energy Res. 2001, 25, 115–128. [CrossRef] 7. Zhu, Y.; Cai, W.; Wen, C.; Li, Y. Numerical investigation of geometry parameters for design of high performance ejectors. Appl. Therm. Eng. 2009, 29, 898–905. 8. Li, C.; Li, Y.Z. Investigation of entrainment behavior and characteristics of gas–liquid ejectors based on CFD simulation. Chem. Eng. Sci. 2011, 66, 405–416. Appl. Sci. 2018, 8, 1413 18 of 18 9. Fan, J.; Eves, J.; Thompson, H.M.; Toropov, V.V.; Kapur, N.; Copley, D.; Mincher, A. Computational ﬂuid dynamic analysis and design optimization of jet pumps. Comput. Fluids 2011, 46, 212–217. [CrossRef] 10. Shah, A.; Chughtai, I.R.; Inayat, M.H. Experimental and numerical analysis of steam jet pump. Int. J. Heat Fluid Flow 2011, 37, 1305–1314. [CrossRef] 11. Shah, A.; Khan, A.H.; Chughtai, I.R.; Inayat, M.H. Numerical and experimental study of steam-water two-phase ﬂow through steam jet pump. Asia Pac. J. Chem. Eng. 2013, 8, 895–905. [CrossRef] 12. Narabayashi, T.; Wataru, M.; Michitugu, M. Study on two-phase ﬂow dynamics in steam injectors. Nucl. Eng. Des. 1997, 175, 147–156. [CrossRef] 13. Narabayashi, T.; Mori, M.; Nakamaru, M.; Ohmori, S. Study on two-phase ﬂow dynamics in steam injectors: II. High-pressure tests using scale-models. Nucl. Eng. Des. 2000, 200, 261–271. [CrossRef] 14. Bartosiewicz, Y.; Aidoun, Z.; Desevaux, P.; Mercadier, Y. Numerical and experimental investigations on supersonic ejectors. Int. J. Heat Fluid Flow 2005, 26, 56–70. [CrossRef] 15. Yan, J.J.; Shao, S.F.; Liu, J.P.; Zhang, Z. Experiment and analysis on performance of steam-driven jet injector for district-heating system. Appl. Therm. Eng. 2005, 25, 1153–1167. [CrossRef] 16. Chong, D.T.; Yan, J.; Wu, G.; Liu, J. Structural optimization and experimental investigation of supersonic ejectors for boosting low pressure natural gas. Appl. Therm. Eng. 2009, 29, 2799–2807. [CrossRef] 17. Kassab, S.Z.; Kandil, H.A.; Warda, H.A.; Ahmed, W.H. Air-lift pumps characteristics under two-phase ﬂow conditions. Int. J. Heat Fluid Flow 2009, 30, 88–98. [CrossRef] 18. Taitel, Y.; Bornea, D.; Dukler, A.E. Modelling ﬂow pattern transitions for steady upward gas-liquid ﬂow in vertical tubes. AIChE J. 1980, 26, 345–354. [CrossRef] 19. De Cachard, F.; Delhaye, J.M. A slug-churn ﬂow model for small-diameter airlift pumps. Int. J. Multiph. Flow 1996, 22, 627–649. [CrossRef] 20. Furukawa, T.; Fukano, T. Effects of liquid viscosity on ﬂow patterns in vertical upward gas–liquid two-phase ﬂow. Int. J. Multiph. Flow 2001, 27, 1109–1126. [CrossRef] 21. Fujimoto, H.; Ogawa, S.; Takuda, H.; Hatta, N. Operation performance of a small air-lift pump for conveying solid particles. J. Energy Resour. Technol. 2003, 125, 17–25. [CrossRef] 22. Khalil, M.F.; Elshorbagy, K.A.; Kassab, S.Z.; Fahmy, R.I. Effect of air injection method on the performance of an air lift pump. Int. J. Heat Fluid Flow 1999, 20, 598–604. [CrossRef] 23. Cheong, C.; Lee, I. Airlift and Jet Combined Pump. Patent Number P20170450KR-01, 11 July 2018. 24. Kim, S.; Cheong, C.; Park, W.-G. Numerical investigation on cavitation ﬂow of hydrofoil and it ﬂow noise with emphasis on turbulence models. AIP Adv. 2017, 7, 065114. [CrossRef] 25. Available online: http://www.seowonco.co.kr/bbs/board.php?bo_table=product03&wr_id=12 (accessed on 19 August 2018). 26. Available online: http://www.wisecontrol.com/home/info/713 (accessed on 19 August 2018). 27. Kim, K.S.; Ku, G.R.; Lee, S.J.; Park, S.G.; Cheong, C. Wavenumber-frequency analysis of internal aerodynamic noise in constriction-expansion pipe. Appl. Sci. 2017, 7, 1137. [CrossRef] © 2018 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/).
Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Aug 21, 2018
You can share this free article with as many people as you like with the url below! We hope you enjoy this feature!
Read and print from thousands of top scholarly journals.
Already have an account? Log in
Bookmark this article. You can see your Bookmarks on your DeepDyve Library.
To save an article, log in first, or sign up for a DeepDyve account if you don’t already have one.
Copy and paste the desired citation format or use the link below to download a file formatted for EndNote
Access the full text.
Sign up today, get DeepDyve free for 14 days.
All DeepDyve websites use cookies to improve your online experience. They were placed on your computer when you launched this website. You can change your cookie settings through your browser.