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High-Temperature Profile Monitoring in Gas Turbine Exhaust-Gas Diffusors with Six-Point Fiber-Optic Sensor Array

High-Temperature Profile Monitoring in Gas Turbine Exhaust-Gas Diffusors with Six-Point... International Journal of Turbomachinery Propulsion and Power Article High-Temperature Profile Monitoring in Gas Turbine Exhaust-Gas Di usors with Six-Point Fiber-Optic Sensor Array 1 , 2 2 3 1 , Franz J. Dutz * , Sven Boje , Ulrich Orth , Alexander W. Koch and Johannes Roths * Photonics Laboratory, Munich University of Applied Sciences, Lothstrasse 34, 80335 Munich, Germany MAN Energy Solutions SE, Steinbrinkstrasse 1, 46145 Oberhausen, Germany; sven.boje@man-es.com (S.B.); ulrich.orth@man-es.com (U.O.) Institute for Measurement Systems and Sensor Technology, Technical University of Munich, Theresienstrasse 90, 80333 Munich, Germany; a.w.koch@tum.de * Correspondence: franz_josef.dutz@hm.edu (F.J.D.); johannes.roths@hm.edu (J.R.); Tel.: +49-(0)89-1265-3654 (F.J.D.); +49-(0)89-1265-1658 (J.R.) Received: 26 March 2020; Accepted: 21 September 2020; Published: 24 September 2020 Abstract: In this paper, the deployment of a newly developed, multipoint, fiber-optic temperature- sensor system for temperature distribution measurements in a 6 MW gas turbine is demonstrated. The optical sensor fiber was integrated in a stainless steel protection cable with a 1.6 mm outside diameter. It included six measurement points, distributed over a length of 110 mm. The sensor cable was mounted in a temperature probe and was positioned radially in the exhaust-gas di usor of the turbine. With this temperature probe, the radial temperature profiles in the exhaust-gas di usor were measured with high spatial and temporal resolution. During a test run of the turbine, characteristic temperature gradients were observed when the machine operated at di erent loads. Keywords: fiber Bragg grating; multipoint; high temperature; regenerated grating; gas turbine 1. Introduction Due to its improved maturity and reliability, fiber-optic temperature sensing based on fiber Bragg gratings (FBGs) is becoming more and more interesting for real-world applications. FBGs are chemically inert and immune to electromagnetic interference and exhibit a small size. Probably, the most important feature for industrial applications relates to their multiplexing capability. This enables the realization of several tens of temperature measurement points consecutively located in a single fiber and brings down the cabling e orts and the associated obstructions of the air-gas streams due to sensor wiring. Therefore, multipoint FBG technology allows an unprecedented density of temperature measurement points when compared to conventional approaches based on electrical sensors, e.g., thermocouples. In some of the industrial applications that would benefit from using multipoint FBG sensors, temperatures exceed 400 C. Here, common type I FBGs are not suitable because they strongly degrade at elevated temperatures. Instead, FBG types capable of resisting high temperatures, such as femtosecond laser-inscribed FBGs [1,2], chemical composition gratings [3,4], and regenerated gratings [5,6], come into operation. Packaging techniques, calibration procedures and temperature drifts strongly influence the functionality and performance of FBG sensors. For the measurements presented here, we used regenerated FBGs (RFBGs), which emerge from type I FBGs in H2-loaded fibers when subjected to a special annealing process. RFBGs can be used at temperatures up to 1200 C [7]. Applications of multipoint sensing based on high-temperature FBGs have been reported for combustor systems [8], nuclear reactors [9], chemical reactors [10,11], and gas turbines [10,12,13]. Int. J. Turbomach. Propuls. Power 2020, 5, 25; doi:10.3390/ijtpp5040025 www.mdpi.com/journal/ijtpp Int. J. Turbomach. Propuls. Power 2020, 5, 25 2 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 2 of 6 In gas turbines, temperature measurements in the hot exhaust gas are used to control and protect In gas turbines, temperature measurements in the hot exhaust gas are used to control and protect the turbine. It is of paramount importance that these measurements are accurate and reliable for the turbine. It is of paramount importance that these measurements are accurate and reliable for years, years, even under rough site conditions (e.g., deserts or offshore). To date, state-of-the-art even under rough site conditions (e.g., deserts or o shore). To date, state-of-the-art thermocouples thermocouples with tolerance class 1 and a resulting temperature uncertainty of about 2 K at 500 °C with tolerance class 1 and a resulting temperature uncertainty of about 2 K at 500 C have been used to have been used to measure temperatures in the gas turbine flow path. However, during R&D tests at measure temperatures in the gas turbine flow path. However, during R&D tests at a test bed, a shorter a test bed, a shorter lifetime (in the range of months) with a defined accuracy and a limited drift is lifetime (in the range of months) with a defined accuracy and a limited drift is acceptable. Here, acceptable. Here, the recording of temperature profiles in the exhaust gas with high local resolution the recording of temperature profiles in the exhaust gas with high local resolution under di erent load under different load conditions is of great interest. For this task, multipoint fiber-optic sensors based conditions is of great interest. For this task, multipoint fiber-optic sensors based on RFBGs can be used on RFBGs can be used at locations where the installation of a multitude of conventional sensors is not at locations where the installation of a multitude of conventional sensors is not possible due to size possible due to size and cabling efforts. Additionally, less influence on gas flows is expected due to and cabling e orts. Additionally, less influence on gas flows is expected due to the small diameter of the small diameter of the fibers. MAN Energy Solutions produces, amongst other turbomachines, the the fibers. MAN Energy Solutions produces, amongst other turbomachines, the MGT6000 gas turbine MGT6000 gas turbine in the power class of 6 MW. The twin-shaft version is mainly used to drive in the power class of 6 MW. The twin-shaft version is mainly used to drive natural gas compressors natural gas compressors or pumps in the oil and gas industry. The single-shaft version is best suited or pumps in the oil and gas industry. The single-shaft version is best suited for combined heat and for combined heat and electricity generation (so-called CHP processes) in smaller installations such electricity generation (so-called CHP processes) in smaller installations such as paper mills or car as paper mills or car factories. As both heat and power are produced by the same installation, the factories. As both heat and power are produced by the same installation, the overall eciency and overall efficiency and economy are maximized compared to those of separate generation. economy are maximized compared to those of separate generation. Here, we demonstrate the application of a six-point fiber-optic sensor array based on RFBGs for Here, we demonstrate the application of a six-point fiber-optic sensor array based on RFBGs monitoring the radial temperature profile in the exhaust-gas diffusor of a MGT6000 gas turbine (see for monitoring the radial temperature profile in the exhaust-gas di usor of a MGT6000 gas turbine Figure 1). For the measurements, the RFBG array was mounted in a probe made of Inconel. (see Figure 1). For the measurements, the RFBG array was mounted in a probe made of Inconel. Characteristic temperature profiles were measured when the gas turbine was operated at different Characteristic temperature profiles were measured when the gas turbine was operated at di erent loads. loads. Figure 1. Gas turbine MGT6000 (MAN Energy Solutions SE, Oberhausen, Germany). Figure 1. Gas turbine MGT6000 (MAN Energy Solutions SE, Oberhausen, Germany). 2. Methodology and Measurement Setup 2. Methodology and Measurement Setup As mentioned in Section 1, the gas turbine was instrumented with a fiber-optic temperature-sensing As mentioned in Section 1, the gas turbine was instrumented with a fiber-optic temperature- system based on wavelength-multiplexed RFBGs. The functional principle is shown in Figure 2. sensing system based on wavelength-multiplexed RFBGs. The functional principle is shown in Figure 2. Six RFBGs were consecutively inscribed in a standard single-mode fiber SMF28 (Corning, Corning, NY, Six RFBGs were consecutively inscribed in a standard single-mode fiber SMF28 (Corning, Corning, USA) with individual spacings of about 20 mm. An FBG interrogator SM125 (Micron Optics, Atlanta, NY, USA) with individual spacings of about 20 mm. An FBG interrogator SM125 (Micron Optics, GA USA) measured the spectral data. Each RFBG caused a specific peak in the spectrum, and the spectral Atlanta, GA USA) measured the spectral data. Each RFBG caused a specific peak in the spectrum, separation of the peaks was about 5 nm (Figure 2). The individual positions of the peak wavelengths and the spectral separation of the peaks was about 5 nm (Figure 2). The individual positions of the depend on the temperatures of the respective fiber segments. Using a wavelength–temperature peak wavelengths depend on the temperatures of the respective fiber segments. Using a wavelength– calibration function [10,11,14], the local temperatures can be calculated from the corresponding temperature calibration function [10,11,14], the local temperatures can be calculated from the wavelength shifts. It has been shown that RFBG sensor elements, which were fabricated in the same corresponding wavelength shifts. It has been shown that RFBG sensor elements, which were type of optical fiber and which were produced with the same parameters, share the same calibration fabricated in the same type of optical fiber and which were produced with the same parameters, share function [14]. By applying this calibration procedure, a temperature uncertainty of 4.5 K has been the same calibration function [14]. By applying this calibration procedure, a temperature uncertainty estimated in a temperature range up to 500 C [11]. In previous investigations with three-point RFBG of 4.5 K has been estimated in a temperature range up to 500 °C [11]. In previous investigations with temperature sensors in a gas turbine of the same kind, RFBG-based temperature data were compared three-point RFBG temperature sensors in a gas turbine of the same kind, RFBG-based temperature data with thermocouple data, and the comparison showed deviations of less than 3 K [10], which was in were compared with thermocouple data, and the comparison showed deviations of less than 3 K [10], good accordance with the estimated temperature uncertainty. It is worth mentioning, here, that the which was in good accordance with the estimated temperature uncertainty. It is worth mentioning, Int. J. Turbomach. Propuls. Power 2020, 5, 25 3 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 6 maximum number of RFBGs is not generally limited to six; several tens of temperature measurement here, that the maximum number of RFBGs is not generally limited to six; several tens of temperature here, that the maximum number of RFBGs is not generally limited to six; several tens of temperature points are possible. measurement points are possible. measurement points are possible. Figure 2. Principle of fiber Bragg grating (FBG) multiplexing. Figure 2. Principle of fiber Bragg grating (FBG) multiplexing. Figure 2. Principle of fiber Bragg grating (FBG) multiplexing. In order to protect the sensor fiber from mechanical damage, it was installed in a stainless steel In order to protect the sensor fiber from mechanical damage, it was installed in a stainless steel In order to protect the sensor fiber from mechanical damage, it was installed in a stainless steel capillary with an outer diameter of 1.6 mm. To instrument the gas turbine, the packaged RFBG array capillary with an outer diameter of 1.6 mm. To instrument the gas turbine, the packaged RFBG array capillary with an outer diameter of 1.6 mm. To instrument the gas turbine, the packaged RFBG array was mounted in a temperature probe prototype made by additive manufacturing (see Figure 3a). was mounted in a temperature probe prototype made by additive manufacturing (see Figure 3a). It was mounted in a temperature probe prototype made by additive manufacturing (see Figure 3a). It It consisted of a cylindrical Inconel body with an axial through-hole of 2 mm in diameter that extended consisted of a cylindrical Inconel body with an axial through-hole of 2 mm in diameter that extended consisted of a cylindrical Inconel body with an axial through-hole of 2 mm in diameter that extended from the tip to the flange, covering a length of 160 mm. The stainless steel capillary with the RFBG from the tip to the flange, covering a length of 160 mm. The stainless steel capillary with the RFBG from the tip to the flange, covering a length of 160 mm. The stainless steel capillary with the RFBG array was inserted into this through-hole. The upper end of the sensor capillary was welded to a disc, array was inserted into this through-hole. The upper end of the sensor capillary was welded to a disc, array was inserted into this through-hole. The upper end of the sensor capillary was welded to a disc, which was clamped inside the assembly to seal the sample towards the flange. At the locations of the six which was clamped inside the assembly to seal the sample towards the flange. At the locations of the which was clamped inside the assembly to seal the sample towards the flange. At the locations of the RFBG sensor elements, the probe had 6 mm-wide and 8 mm-long transverse channels, through which six RFBG sensor elements, the probe had 6 mm-wide and 8 mm-long transverse channels, through six RFBG sensor elements, the probe had 6 mm-wide and 8 mm-long transverse channels, through the gas flow was partially guided and where the gas flow was in direct thermal contact with the sensor which the gas flow was partially guided and where the gas flow was in direct thermal contact with which the gas flow was partially guided and where the gas flow was in direct thermal contact with capillary (see the inset of Figure 3a). The probe was installed in the exhaust-gas di usor in such a way the sensor capillary (see the inset of Figure 3a). The probe was installed in the exhaust-gas diffusor the sensor capillary (see the inset of Figure 3a). The probe was installed in the exhaust-gas diffusor that the radial positions of RFBGs #1 to #6 ranged from 5 to 115.3 mm (as measured from the outer in such a way that the radial positions of RFBGs #1 to #6 ranged from 5 to 115.3 mm (as measured in such a way that the radial positions of RFBGs #1 to #6 ranged from 5 to 115.3 mm (as measured casing), and the total clearance was 132 mm (see Figure 3b). from the outer casing), and the total clearance was 132 mm (see Figure 3b). from the outer casing), and the total clearance was 132 mm (see Figure 3b). (a) (b) (a) (b) Figure 3. (a) Temperature probe with six-point regenerated FBG (RFBG) array. The inset provides an Figure 3. (a) Temperature probe with six-point regenerated FBG (RFBG) array. The inset provides an Figure 3. (a) Temperature probe with six-point regenerated FBG (RFBG) array. The inset provides an enlarged view of one of the six flow channels of the assembly. (b) Installation of the RFBG-sensor probe enlarged view of one of the six flow channels of the assembly. (b) Installation of the RFBG-sensor enlarged view of one of the six flow channels of the assembly. (b) Installation of the RFBG-sensor in the exhaust-gas di usor of the gas turbine (flow direction perpendicular to plane). probe in the exhaust-gas diffusor of the gas turbine (flow direction perpendicular to plane). probe in the exhaust-gas diffusor of the gas turbine (flow direction perpendicular to plane). Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 4 of 6 3. Results of High-Temperature Measurements Figure 4 depicts the measurement data taken during a typical performance run of the gas turbine. In the beginning, an electric motor started the gas turbine and then the turbine ran idle for Int. J. Turbomach. Propuls. Power 2020, 5, 25 4 of 6 about 20 min. During the period from 40 to 200 min, the mechanical load on the drive shaft was gradually increased, with a time of twenty minutes between the steps. Afterwards, the applied load was removed, and the machine was switched off and coasted down to standstill. 3. Results of High-Temperature Measurements Figure 4 shows the corresponding time series of the temperatures, as measured by the RFBGs in Figure 4 depicts the measurement data taken during a typical performance run of the gas turbine. the exhaust-gas diffusor with a frequency of about 0.5 Hz. After ignition, the temperatures rose fast In the beginning, an electric motor started the gas turbine and then the turbine ran idle for about 20 min. and stabilized in idle mode. With increasing power of the gas turbine, the temperatures further During the period from 40 to 200 min, the mechanical load on the drive shaft was gradually increased, increased. At a certain power level, the maximum exhaust-gas temperature was reached and was with a time of twenty minutes between the steps. Afterwards, the applied load was removed, and the then kept nearly constant up to maximum power. After the machine was turned off, the temperature machine was switched o and coasted down to standstill. of the turbine casing still remained at a higher level (Figure 4, 225 to 240 min). Figure 4. Time series of the temperatures measured with the six-point RFBG sensor array. Figure 4. Time series of the temperatures measured with the six-point RFBG sensor array. Figure 4 shows the corresponding time series of the temperatures, as measured by the RFBGs 4. Discussion in the exhaust-gas di usor with a frequency of about 0.5 Hz. After ignition, the temperatures rose fast and stabilized in idle mode. With increasing power of the gas turbine, the temperatures further Figure 5 provides a more detailed view of the progress of the radial temperature profiles in the increased. At a certain power level, the maximum exhaust-gas temperature was reached and was then exhaust-gas diffusor during the gradual increase in the turbine power. In Figure 5a, a contour plot of kept nearly constant up to maximum power. After the machine was turned o , the temperature of the the temperatures in terms of their dependence on the radial position and time is depicted. Again, the turbine casing still remained at a higher level (Figure 4, 225 to 240 min). zero position refers to the outer side of the diffusor channel (compare with Figure 3b). The dashed lines mark the positions of the RFBG temperature sensors, and the temperatures between two RFBG 4. Discussion positions were linearly interpolated. Figure 5b shows temperature distributions selected from the Figure 5 provides a more detailed view of the progress of the radial temperature profiles in the data at 35, 55, 150, and 200 min. exhaust-gas di usor during the gradual increase in the turbine power. In Figure 5a, a contour plot As already mentioned, the temperatures considerably rose when the power gradually grew of the temperatures in terms of their dependence on the radial position and time is depicted. Again, (Figure 5a, between 20 and 100 min). In this time, the boundaries of the diffusor channel were hotter the zero position refers to the outer side of the di usor channel (compare with Figure 3b). The dashed than the center. Instead, after about 100 min, the temperature differences between the radial positions lines mark the positions of the RFBG temperature sensors, and the temperatures between two RFBG decreased and the temperature profile became almost uniform at nominal power (see Figure 5b), positions were linearly interpolated. Figure 5b shows temperature distributions selected from the data indicating a homogenous temperature distribution in the exhaust-gas stream. With regard to the at 35, 55, 150, and 200 min. probe design, considerable systematic uncertainties such as convective heat transfer inside the As already mentioned, the temperatures considerably rose when the power gradually grew assembly are not expected. Following this assumption, the measurement accuracy is mainly (Figure 5a, between 20 and 100 min). In this time, the boundaries of the di usor channel were hotter determined by the uncertainty of the calibration procedure of 4.5 K (see Section 2). After the than the center. Instead, after about 100 min, the temperature di erences between the radial positions deployment, the spectra of the RFBGs were compared to the pristine data at the reference decreased and the temperature profile became almost uniform at nominal power (see Figure 5b), temperature. No degradations in terms of temperature drift, signal strength or mechanical fragility indicating a homogenous temperature distribution in the exhaust-gas stream. With regard to the probe occurred. design, considerable systematic uncertainties such as convective heat transfer inside the assembly are not expected. Following this assumption, the measurement accuracy is mainly determined by the uncertainty of the calibration procedure of 4.5 K (see Section 2). After the deployment, the spectra of the RFBGs were compared to the pristine data at the reference temperature. No degradations in terms of temperature drift, signal strength or mechanical fragility occurred. Int. J. Turbomach. Propuls. Power 2020, 5, 25 5 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 5 of 6 (a) (b) Figure 5. Figure 5. ( (a a)) C Contour ontour plot of plot of the temporal p the temporal pr rogress of ogress of th the e t temperatur emperatures in the es in the exhaust-gas exhaust-gas diffusor; di usor; ( (b b) ) Rad Radial ial t temperatur emperature distri e distributions butions ex extracted tracted from the from the contour contour diagram. diagram. 5. Conclusions 5. Conclusions By employing a newly developed fiber-optic temperature probe based on regenerated fiber Bragg By employing a newly developed fiber-optic temperature probe based on regenerated fiber gratings, the radial temperature distributions in the exhaust-gas di usor of a MGT6000 gas turbine Bragg gratings, the radial temperature distributions in the exhaust-gas diffusor of a MGT6000 gas were observed with high spatial and temporal resolutions. The sensor cable consisted of an optical turbine were observed with high spatial and temporal resolutions. The sensor cable consisted of an fiber that was installed in a stainless steel capillary with a 1.6 mm outside diameter. Over a length optical fiber that was installed in a stainless steel capillary with a 1.6 mm outside diameter. Over a of 110 mm, six measurement points were integrated into the sensor cable. When the machine was length of 110 mm, six measurement points were integrated into the sensor cable. When the machine operated at di erent loads, characteristic temperature levels and temperature profiles were observed. was operated at different loads, characteristic temperature levels and temperature profiles were Upon approaching the nominal power level, the temperature profile became almost uniform. This study observed. Upon approaching the nominal power level, the temperature profile became almost demonstrates the capabilities of RFBG-based multipoint sensing for R&D applications in gas turbines. uniform. This study demonstrates the capabilities of RFBG-based multipoint sensing for R&D Upcoming investigations will focus on the verification of long-term reliability and an increase in the applications in gas turbines. Upcoming investigations will focus on the verification of long-term number of measurement points in order to achieve an even higher spatial resolution. reliability and an increase in the number of measurement points in order to achieve an even higher spatial resolution. Author Contributions: Conceptualization, F.J.D. and J.R.; methodology, F.J.D. and S.B.; formal analysis, F.J.D.; investigation, F.J.D.; resources, S.B. and U.O.; writing—original draft preparation, F.J.D.; writing—review and Autho editing, r Co S.B., ntributio U.O., A.W ns: .K. Conce andp J.R.; tualization, F.J.D. and J visualization, F.J.D.;.R.; m supervision, ethodolo A.W gy,.K. F.J. and D. and J.R.; S. pr B. oject ; formal administration, analysis, F.J. J.R.; D.; funding acquisition, U.O. and J.R. All authors have read and agreed to the published version of the manuscript. investigation, F.J.D.; resources, S.B. and U.O.; writing—original draft preparation, F.J.D.; writing—review and editing, S.B., U.O., A.W.K. and J.R.; visualization, F.J.D.; supervision, A.W.K. and J.R.; project administration, Funding: This research was funded by Bayerische Forschungsstiftung (BFS), grant number AZ-1146-14. J.R.; funding acquisition, U.O. and J.R. All authors have read and agreed to the published version of the Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the manuscript. study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. Funding: This research was funded by Bayerische Forschungsstiftung (BFS), grant number AZ-1146-14. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the References study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to 1. Mihailov, S.J.; Hnatovsky, C.; Grobnic, D. Novel type II Bragg grating structures in silica fibers using publish the results. femtosecond lasers and phase masks. J. Lightwave Technol. 2019, 37, 2549–2556. [CrossRef] 2. Mihailov, S.J. Fiber Bragg grating sensors for harsh environments. Sensors 2012, 12, 1898–1918. [CrossRef] References [PubMed] 1. Mihailov, S.J.; Hnatovsky, C.; Grobnic, D. Novel type II Bragg grating structures in silica fibers using 3. Fokine, M. Thermal stability of oxygen-modulated chemical-composition gratings in standard femtosecond lasers and phase masks. J. Lightwave Technol. 2019, 37, 2549–2556, telecommunication fiber. 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A study of regenerated gratings gratings in standard optical fibers. Opt. Exp. 2015, 23, 27520–27535. produced in germanosilicate fibers by high temperature annealing. Opt. Exp. 2011, 19, 1198–1206. [CrossRef] Int. J. Turbomach. Propuls. Power 2020, 5, 25 6 of 6 7. Canning, J.; Stevenson, M.; Bandyopadhyay, S.; Cook, K. Extreme silica optical fibre gratings. Sensors 2008, 8, 6448–6452. [CrossRef] [PubMed] 8. Mihailov, S.J.; Grobnic, D.; Hnatovsky, C.; Walker, R.B.; Lu, P.; Coulas, D.; Ding, H. Extreme environment sensing using femtosecond laser-inscribed fiber Bragg gratings. Sensors 2017, 17, 2909. [CrossRef] [PubMed] 9. La ont, G.; Cotillard, R.; Roussel, N.; Desmarchelier, R.; Rougeault, S. Temperature resistant fiber Bragg gratings for on-line and structural health monitoring of the next-generation of nuclear reactors. Sensors 2018, 18, 1791. [CrossRef] [PubMed] 10. Dutz, F.J.; Lindner, M.; Heinrich, A.; Seydel, C.G.; Bosselmann, T.; Koch, A.W.; Roths, J. Multipoint high temperature sensing with regenerated fiber Bragg gratings. Proc. SPIE 2018, 10654, 1065407. [CrossRef] 11. Dutz, F.J.; Heinrich, A.; Bank, R.; Koch, A.W.; Roths, J. Fiber-optic multipoint sensor system with low drift for the long-term monitoring of high-temperature distributions in chemical reactors. Sensors 2019, 19, 5476. [CrossRef] [PubMed] 12. Willsch, M.; Bosselmann, T.; Flohr, P.; Kull, R.; Ecke, W.; Latka, I.; Fischer, D.; Thiel, T. Design of fiber optical high temperature sensors for gas turbine monitoring. Proc. SPIE 2009, 7503, 75037R. [CrossRef] 13. Xia, H.; Byrd, D.; Dekate, S.; Lee, B. High-density fiber optical sensor and instrumentation for gas turbine operation condition monitoring. J. Sens. 2013, 2013, 1–10. [CrossRef] 14. Lindner, M.; Tunc, E.; Weraneck, K.; Heilmeier, F.; Volk, W.; Jakobi, M.; Koch, A.W.; Roths, J. Regenerated Bragg grating sensor array for temperature measurements during an aluminum casting process. IEEE Sens. J. 2018, 18, 5352–5360. [CrossRef] © 2020 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-NC-ND) license (http://creativecommons.org/licenses/by-nc-nd/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Turbomachinery, Propulsion and Power Multidisciplinary Digital Publishing Institute

High-Temperature Profile Monitoring in Gas Turbine Exhaust-Gas Diffusors with Six-Point Fiber-Optic Sensor Array

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International Journal of Turbomachinery Propulsion and Power Article High-Temperature Profile Monitoring in Gas Turbine Exhaust-Gas Di usors with Six-Point Fiber-Optic Sensor Array 1 , 2 2 3 1 , Franz J. Dutz * , Sven Boje , Ulrich Orth , Alexander W. Koch and Johannes Roths * Photonics Laboratory, Munich University of Applied Sciences, Lothstrasse 34, 80335 Munich, Germany MAN Energy Solutions SE, Steinbrinkstrasse 1, 46145 Oberhausen, Germany; sven.boje@man-es.com (S.B.); ulrich.orth@man-es.com (U.O.) Institute for Measurement Systems and Sensor Technology, Technical University of Munich, Theresienstrasse 90, 80333 Munich, Germany; a.w.koch@tum.de * Correspondence: franz_josef.dutz@hm.edu (F.J.D.); johannes.roths@hm.edu (J.R.); Tel.: +49-(0)89-1265-3654 (F.J.D.); +49-(0)89-1265-1658 (J.R.) Received: 26 March 2020; Accepted: 21 September 2020; Published: 24 September 2020 Abstract: In this paper, the deployment of a newly developed, multipoint, fiber-optic temperature- sensor system for temperature distribution measurements in a 6 MW gas turbine is demonstrated. The optical sensor fiber was integrated in a stainless steel protection cable with a 1.6 mm outside diameter. It included six measurement points, distributed over a length of 110 mm. The sensor cable was mounted in a temperature probe and was positioned radially in the exhaust-gas di usor of the turbine. With this temperature probe, the radial temperature profiles in the exhaust-gas di usor were measured with high spatial and temporal resolution. During a test run of the turbine, characteristic temperature gradients were observed when the machine operated at di erent loads. Keywords: fiber Bragg grating; multipoint; high temperature; regenerated grating; gas turbine 1. Introduction Due to its improved maturity and reliability, fiber-optic temperature sensing based on fiber Bragg gratings (FBGs) is becoming more and more interesting for real-world applications. FBGs are chemically inert and immune to electromagnetic interference and exhibit a small size. Probably, the most important feature for industrial applications relates to their multiplexing capability. This enables the realization of several tens of temperature measurement points consecutively located in a single fiber and brings down the cabling e orts and the associated obstructions of the air-gas streams due to sensor wiring. Therefore, multipoint FBG technology allows an unprecedented density of temperature measurement points when compared to conventional approaches based on electrical sensors, e.g., thermocouples. In some of the industrial applications that would benefit from using multipoint FBG sensors, temperatures exceed 400 C. Here, common type I FBGs are not suitable because they strongly degrade at elevated temperatures. Instead, FBG types capable of resisting high temperatures, such as femtosecond laser-inscribed FBGs [1,2], chemical composition gratings [3,4], and regenerated gratings [5,6], come into operation. Packaging techniques, calibration procedures and temperature drifts strongly influence the functionality and performance of FBG sensors. For the measurements presented here, we used regenerated FBGs (RFBGs), which emerge from type I FBGs in H2-loaded fibers when subjected to a special annealing process. RFBGs can be used at temperatures up to 1200 C [7]. Applications of multipoint sensing based on high-temperature FBGs have been reported for combustor systems [8], nuclear reactors [9], chemical reactors [10,11], and gas turbines [10,12,13]. Int. J. Turbomach. Propuls. Power 2020, 5, 25; doi:10.3390/ijtpp5040025 www.mdpi.com/journal/ijtpp Int. J. Turbomach. Propuls. Power 2020, 5, 25 2 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 2 of 6 In gas turbines, temperature measurements in the hot exhaust gas are used to control and protect In gas turbines, temperature measurements in the hot exhaust gas are used to control and protect the turbine. It is of paramount importance that these measurements are accurate and reliable for the turbine. It is of paramount importance that these measurements are accurate and reliable for years, years, even under rough site conditions (e.g., deserts or offshore). To date, state-of-the-art even under rough site conditions (e.g., deserts or o shore). To date, state-of-the-art thermocouples thermocouples with tolerance class 1 and a resulting temperature uncertainty of about 2 K at 500 °C with tolerance class 1 and a resulting temperature uncertainty of about 2 K at 500 C have been used to have been used to measure temperatures in the gas turbine flow path. However, during R&D tests at measure temperatures in the gas turbine flow path. However, during R&D tests at a test bed, a shorter a test bed, a shorter lifetime (in the range of months) with a defined accuracy and a limited drift is lifetime (in the range of months) with a defined accuracy and a limited drift is acceptable. Here, acceptable. Here, the recording of temperature profiles in the exhaust gas with high local resolution the recording of temperature profiles in the exhaust gas with high local resolution under di erent load under different load conditions is of great interest. For this task, multipoint fiber-optic sensors based conditions is of great interest. For this task, multipoint fiber-optic sensors based on RFBGs can be used on RFBGs can be used at locations where the installation of a multitude of conventional sensors is not at locations where the installation of a multitude of conventional sensors is not possible due to size possible due to size and cabling efforts. Additionally, less influence on gas flows is expected due to and cabling e orts. Additionally, less influence on gas flows is expected due to the small diameter of the small diameter of the fibers. MAN Energy Solutions produces, amongst other turbomachines, the the fibers. MAN Energy Solutions produces, amongst other turbomachines, the MGT6000 gas turbine MGT6000 gas turbine in the power class of 6 MW. The twin-shaft version is mainly used to drive in the power class of 6 MW. The twin-shaft version is mainly used to drive natural gas compressors natural gas compressors or pumps in the oil and gas industry. The single-shaft version is best suited or pumps in the oil and gas industry. The single-shaft version is best suited for combined heat and for combined heat and electricity generation (so-called CHP processes) in smaller installations such electricity generation (so-called CHP processes) in smaller installations such as paper mills or car as paper mills or car factories. As both heat and power are produced by the same installation, the factories. As both heat and power are produced by the same installation, the overall eciency and overall efficiency and economy are maximized compared to those of separate generation. economy are maximized compared to those of separate generation. Here, we demonstrate the application of a six-point fiber-optic sensor array based on RFBGs for Here, we demonstrate the application of a six-point fiber-optic sensor array based on RFBGs monitoring the radial temperature profile in the exhaust-gas diffusor of a MGT6000 gas turbine (see for monitoring the radial temperature profile in the exhaust-gas di usor of a MGT6000 gas turbine Figure 1). For the measurements, the RFBG array was mounted in a probe made of Inconel. (see Figure 1). For the measurements, the RFBG array was mounted in a probe made of Inconel. Characteristic temperature profiles were measured when the gas turbine was operated at different Characteristic temperature profiles were measured when the gas turbine was operated at di erent loads. loads. Figure 1. Gas turbine MGT6000 (MAN Energy Solutions SE, Oberhausen, Germany). Figure 1. Gas turbine MGT6000 (MAN Energy Solutions SE, Oberhausen, Germany). 2. Methodology and Measurement Setup 2. Methodology and Measurement Setup As mentioned in Section 1, the gas turbine was instrumented with a fiber-optic temperature-sensing As mentioned in Section 1, the gas turbine was instrumented with a fiber-optic temperature- system based on wavelength-multiplexed RFBGs. The functional principle is shown in Figure 2. sensing system based on wavelength-multiplexed RFBGs. The functional principle is shown in Figure 2. Six RFBGs were consecutively inscribed in a standard single-mode fiber SMF28 (Corning, Corning, NY, Six RFBGs were consecutively inscribed in a standard single-mode fiber SMF28 (Corning, Corning, USA) with individual spacings of about 20 mm. An FBG interrogator SM125 (Micron Optics, Atlanta, NY, USA) with individual spacings of about 20 mm. An FBG interrogator SM125 (Micron Optics, GA USA) measured the spectral data. Each RFBG caused a specific peak in the spectrum, and the spectral Atlanta, GA USA) measured the spectral data. Each RFBG caused a specific peak in the spectrum, separation of the peaks was about 5 nm (Figure 2). The individual positions of the peak wavelengths and the spectral separation of the peaks was about 5 nm (Figure 2). The individual positions of the depend on the temperatures of the respective fiber segments. Using a wavelength–temperature peak wavelengths depend on the temperatures of the respective fiber segments. Using a wavelength– calibration function [10,11,14], the local temperatures can be calculated from the corresponding temperature calibration function [10,11,14], the local temperatures can be calculated from the wavelength shifts. It has been shown that RFBG sensor elements, which were fabricated in the same corresponding wavelength shifts. It has been shown that RFBG sensor elements, which were type of optical fiber and which were produced with the same parameters, share the same calibration fabricated in the same type of optical fiber and which were produced with the same parameters, share function [14]. By applying this calibration procedure, a temperature uncertainty of 4.5 K has been the same calibration function [14]. By applying this calibration procedure, a temperature uncertainty estimated in a temperature range up to 500 C [11]. In previous investigations with three-point RFBG of 4.5 K has been estimated in a temperature range up to 500 °C [11]. In previous investigations with temperature sensors in a gas turbine of the same kind, RFBG-based temperature data were compared three-point RFBG temperature sensors in a gas turbine of the same kind, RFBG-based temperature data with thermocouple data, and the comparison showed deviations of less than 3 K [10], which was in were compared with thermocouple data, and the comparison showed deviations of less than 3 K [10], good accordance with the estimated temperature uncertainty. It is worth mentioning, here, that the which was in good accordance with the estimated temperature uncertainty. It is worth mentioning, Int. J. Turbomach. Propuls. Power 2020, 5, 25 3 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 6 maximum number of RFBGs is not generally limited to six; several tens of temperature measurement here, that the maximum number of RFBGs is not generally limited to six; several tens of temperature here, that the maximum number of RFBGs is not generally limited to six; several tens of temperature points are possible. measurement points are possible. measurement points are possible. Figure 2. Principle of fiber Bragg grating (FBG) multiplexing. Figure 2. Principle of fiber Bragg grating (FBG) multiplexing. Figure 2. Principle of fiber Bragg grating (FBG) multiplexing. In order to protect the sensor fiber from mechanical damage, it was installed in a stainless steel In order to protect the sensor fiber from mechanical damage, it was installed in a stainless steel In order to protect the sensor fiber from mechanical damage, it was installed in a stainless steel capillary with an outer diameter of 1.6 mm. To instrument the gas turbine, the packaged RFBG array capillary with an outer diameter of 1.6 mm. To instrument the gas turbine, the packaged RFBG array capillary with an outer diameter of 1.6 mm. To instrument the gas turbine, the packaged RFBG array was mounted in a temperature probe prototype made by additive manufacturing (see Figure 3a). was mounted in a temperature probe prototype made by additive manufacturing (see Figure 3a). It was mounted in a temperature probe prototype made by additive manufacturing (see Figure 3a). It It consisted of a cylindrical Inconel body with an axial through-hole of 2 mm in diameter that extended consisted of a cylindrical Inconel body with an axial through-hole of 2 mm in diameter that extended consisted of a cylindrical Inconel body with an axial through-hole of 2 mm in diameter that extended from the tip to the flange, covering a length of 160 mm. The stainless steel capillary with the RFBG from the tip to the flange, covering a length of 160 mm. The stainless steel capillary with the RFBG from the tip to the flange, covering a length of 160 mm. The stainless steel capillary with the RFBG array was inserted into this through-hole. The upper end of the sensor capillary was welded to a disc, array was inserted into this through-hole. The upper end of the sensor capillary was welded to a disc, array was inserted into this through-hole. The upper end of the sensor capillary was welded to a disc, which was clamped inside the assembly to seal the sample towards the flange. At the locations of the six which was clamped inside the assembly to seal the sample towards the flange. At the locations of the which was clamped inside the assembly to seal the sample towards the flange. At the locations of the RFBG sensor elements, the probe had 6 mm-wide and 8 mm-long transverse channels, through which six RFBG sensor elements, the probe had 6 mm-wide and 8 mm-long transverse channels, through six RFBG sensor elements, the probe had 6 mm-wide and 8 mm-long transverse channels, through the gas flow was partially guided and where the gas flow was in direct thermal contact with the sensor which the gas flow was partially guided and where the gas flow was in direct thermal contact with which the gas flow was partially guided and where the gas flow was in direct thermal contact with capillary (see the inset of Figure 3a). The probe was installed in the exhaust-gas di usor in such a way the sensor capillary (see the inset of Figure 3a). The probe was installed in the exhaust-gas diffusor the sensor capillary (see the inset of Figure 3a). The probe was installed in the exhaust-gas diffusor that the radial positions of RFBGs #1 to #6 ranged from 5 to 115.3 mm (as measured from the outer in such a way that the radial positions of RFBGs #1 to #6 ranged from 5 to 115.3 mm (as measured in such a way that the radial positions of RFBGs #1 to #6 ranged from 5 to 115.3 mm (as measured casing), and the total clearance was 132 mm (see Figure 3b). from the outer casing), and the total clearance was 132 mm (see Figure 3b). from the outer casing), and the total clearance was 132 mm (see Figure 3b). (a) (b) (a) (b) Figure 3. (a) Temperature probe with six-point regenerated FBG (RFBG) array. The inset provides an Figure 3. (a) Temperature probe with six-point regenerated FBG (RFBG) array. The inset provides an Figure 3. (a) Temperature probe with six-point regenerated FBG (RFBG) array. The inset provides an enlarged view of one of the six flow channels of the assembly. (b) Installation of the RFBG-sensor probe enlarged view of one of the six flow channels of the assembly. (b) Installation of the RFBG-sensor enlarged view of one of the six flow channels of the assembly. (b) Installation of the RFBG-sensor in the exhaust-gas di usor of the gas turbine (flow direction perpendicular to plane). probe in the exhaust-gas diffusor of the gas turbine (flow direction perpendicular to plane). probe in the exhaust-gas diffusor of the gas turbine (flow direction perpendicular to plane). Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 4 of 6 3. Results of High-Temperature Measurements Figure 4 depicts the measurement data taken during a typical performance run of the gas turbine. In the beginning, an electric motor started the gas turbine and then the turbine ran idle for Int. J. Turbomach. Propuls. Power 2020, 5, 25 4 of 6 about 20 min. During the period from 40 to 200 min, the mechanical load on the drive shaft was gradually increased, with a time of twenty minutes between the steps. Afterwards, the applied load was removed, and the machine was switched off and coasted down to standstill. 3. Results of High-Temperature Measurements Figure 4 shows the corresponding time series of the temperatures, as measured by the RFBGs in Figure 4 depicts the measurement data taken during a typical performance run of the gas turbine. the exhaust-gas diffusor with a frequency of about 0.5 Hz. After ignition, the temperatures rose fast In the beginning, an electric motor started the gas turbine and then the turbine ran idle for about 20 min. and stabilized in idle mode. With increasing power of the gas turbine, the temperatures further During the period from 40 to 200 min, the mechanical load on the drive shaft was gradually increased, increased. At a certain power level, the maximum exhaust-gas temperature was reached and was with a time of twenty minutes between the steps. Afterwards, the applied load was removed, and the then kept nearly constant up to maximum power. After the machine was turned off, the temperature machine was switched o and coasted down to standstill. of the turbine casing still remained at a higher level (Figure 4, 225 to 240 min). Figure 4. Time series of the temperatures measured with the six-point RFBG sensor array. Figure 4. Time series of the temperatures measured with the six-point RFBG sensor array. Figure 4 shows the corresponding time series of the temperatures, as measured by the RFBGs 4. Discussion in the exhaust-gas di usor with a frequency of about 0.5 Hz. After ignition, the temperatures rose fast and stabilized in idle mode. With increasing power of the gas turbine, the temperatures further Figure 5 provides a more detailed view of the progress of the radial temperature profiles in the increased. At a certain power level, the maximum exhaust-gas temperature was reached and was then exhaust-gas diffusor during the gradual increase in the turbine power. In Figure 5a, a contour plot of kept nearly constant up to maximum power. After the machine was turned o , the temperature of the the temperatures in terms of their dependence on the radial position and time is depicted. Again, the turbine casing still remained at a higher level (Figure 4, 225 to 240 min). zero position refers to the outer side of the diffusor channel (compare with Figure 3b). The dashed lines mark the positions of the RFBG temperature sensors, and the temperatures between two RFBG 4. Discussion positions were linearly interpolated. Figure 5b shows temperature distributions selected from the Figure 5 provides a more detailed view of the progress of the radial temperature profiles in the data at 35, 55, 150, and 200 min. exhaust-gas di usor during the gradual increase in the turbine power. In Figure 5a, a contour plot As already mentioned, the temperatures considerably rose when the power gradually grew of the temperatures in terms of their dependence on the radial position and time is depicted. Again, (Figure 5a, between 20 and 100 min). In this time, the boundaries of the diffusor channel were hotter the zero position refers to the outer side of the di usor channel (compare with Figure 3b). The dashed than the center. Instead, after about 100 min, the temperature differences between the radial positions lines mark the positions of the RFBG temperature sensors, and the temperatures between two RFBG decreased and the temperature profile became almost uniform at nominal power (see Figure 5b), positions were linearly interpolated. Figure 5b shows temperature distributions selected from the data indicating a homogenous temperature distribution in the exhaust-gas stream. With regard to the at 35, 55, 150, and 200 min. probe design, considerable systematic uncertainties such as convective heat transfer inside the As already mentioned, the temperatures considerably rose when the power gradually grew assembly are not expected. Following this assumption, the measurement accuracy is mainly (Figure 5a, between 20 and 100 min). In this time, the boundaries of the di usor channel were hotter determined by the uncertainty of the calibration procedure of 4.5 K (see Section 2). After the than the center. Instead, after about 100 min, the temperature di erences between the radial positions deployment, the spectra of the RFBGs were compared to the pristine data at the reference decreased and the temperature profile became almost uniform at nominal power (see Figure 5b), temperature. No degradations in terms of temperature drift, signal strength or mechanical fragility indicating a homogenous temperature distribution in the exhaust-gas stream. With regard to the probe occurred. design, considerable systematic uncertainties such as convective heat transfer inside the assembly are not expected. Following this assumption, the measurement accuracy is mainly determined by the uncertainty of the calibration procedure of 4.5 K (see Section 2). After the deployment, the spectra of the RFBGs were compared to the pristine data at the reference temperature. No degradations in terms of temperature drift, signal strength or mechanical fragility occurred. Int. J. Turbomach. Propuls. Power 2020, 5, 25 5 of 6 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 5 of 6 (a) (b) Figure 5. Figure 5. ( (a a)) C Contour ontour plot of plot of the temporal p the temporal pr rogress of ogress of th the e t temperatur emperatures in the es in the exhaust-gas exhaust-gas diffusor; di usor; ( (b b) ) Rad Radial ial t temperatur emperature distri e distributions butions ex extracted tracted from the from the contour contour diagram. diagram. 5. Conclusions 5. Conclusions By employing a newly developed fiber-optic temperature probe based on regenerated fiber Bragg By employing a newly developed fiber-optic temperature probe based on regenerated fiber gratings, the radial temperature distributions in the exhaust-gas di usor of a MGT6000 gas turbine Bragg gratings, the radial temperature distributions in the exhaust-gas diffusor of a MGT6000 gas were observed with high spatial and temporal resolutions. The sensor cable consisted of an optical turbine were observed with high spatial and temporal resolutions. The sensor cable consisted of an fiber that was installed in a stainless steel capillary with a 1.6 mm outside diameter. Over a length optical fiber that was installed in a stainless steel capillary with a 1.6 mm outside diameter. Over a of 110 mm, six measurement points were integrated into the sensor cable. When the machine was length of 110 mm, six measurement points were integrated into the sensor cable. When the machine operated at di erent loads, characteristic temperature levels and temperature profiles were observed. was operated at different loads, characteristic temperature levels and temperature profiles were Upon approaching the nominal power level, the temperature profile became almost uniform. This study observed. Upon approaching the nominal power level, the temperature profile became almost demonstrates the capabilities of RFBG-based multipoint sensing for R&D applications in gas turbines. uniform. This study demonstrates the capabilities of RFBG-based multipoint sensing for R&D Upcoming investigations will focus on the verification of long-term reliability and an increase in the applications in gas turbines. Upcoming investigations will focus on the verification of long-term number of measurement points in order to achieve an even higher spatial resolution. reliability and an increase in the number of measurement points in order to achieve an even higher spatial resolution. Author Contributions: Conceptualization, F.J.D. and J.R.; methodology, F.J.D. and S.B.; formal analysis, F.J.D.; investigation, F.J.D.; resources, S.B. and U.O.; writing—original draft preparation, F.J.D.; writing—review and Autho editing, r Co S.B., ntributio U.O., A.W ns: .K. Conce andp J.R.; tualization, F.J.D. and J visualization, F.J.D.;.R.; m supervision, ethodolo A.W gy,.K. F.J. and D. and J.R.; S. pr B. oject ; formal administration, analysis, F.J. J.R.; D.; funding acquisition, U.O. and J.R. All authors have read and agreed to the published version of the manuscript. investigation, F.J.D.; resources, S.B. and U.O.; writing—original draft preparation, F.J.D.; writing—review and editing, S.B., U.O., A.W.K. and J.R.; visualization, F.J.D.; supervision, A.W.K. and J.R.; project administration, Funding: This research was funded by Bayerische Forschungsstiftung (BFS), grant number AZ-1146-14. J.R.; funding acquisition, U.O. and J.R. All authors have read and agreed to the published version of the Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the manuscript. study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. Funding: This research was funded by Bayerische Forschungsstiftung (BFS), grant number AZ-1146-14. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the References study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to 1. Mihailov, S.J.; Hnatovsky, C.; Grobnic, D. Novel type II Bragg grating structures in silica fibers using publish the results. femtosecond lasers and phase masks. J. Lightwave Technol. 2019, 37, 2549–2556. [CrossRef] 2. Mihailov, S.J. Fiber Bragg grating sensors for harsh environments. Sensors 2012, 12, 1898–1918. [CrossRef] References [PubMed] 1. Mihailov, S.J.; Hnatovsky, C.; Grobnic, D. Novel type II Bragg grating structures in silica fibers using 3. Fokine, M. Thermal stability of oxygen-modulated chemical-composition gratings in standard femtosecond lasers and phase masks. J. Lightwave Technol. 2019, 37, 2549–2556, telecommunication fiber. 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J. 2018, 18, 5352–5360. [CrossRef] © 2020 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-NC-ND) license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Journal

International Journal of Turbomachinery, Propulsion and PowerMultidisciplinary Digital Publishing Institute

Published: Sep 24, 2020

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