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Experimental study of a hydrogen-air rotating detonation combustor
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
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Experimental study of a hydrogen-air rotating detonation combustor Shengbing Zhou a, Hu Ma a,*, Daokun Liu b, Yu Yan c, Shuai Li a, Changsheng Zhou a a
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Shanghai Space Propulsion Technology Research Institute, Shanghai 201109, China c Laboratory of Science and Technology on Liquid Rocket Engine, Xi'an Aerospace Propulsion Institute, Xi'an 710100, China b
article info
abstract
Article history:
The rotating detonation engine can generate continuous thrust via one or more detonation
Received 6 January 2017
waves. In this study, rotating detonation experiments were performed on a combined
Received in revised form
structure which included a rotating detonation combustor (RDC) and a centrifugal
12 April 2017
compressor. Air, which functioned as an oxidiser, was obtained from the environment by
Accepted 21 April 2017
the compressor, and hydrogen, which was used as fuel, was provided by the supply sys-
Available online xxx
tem. The propagation velocity of the rotating detonation wave (RDW) reached 81% of the ChapmaneJouguet value in experiments. With the increase of the air-injection area, the
Keywords:
detonation-wave pressure increased, but the stability decreased. An air-injection area of
Rotating detonation combustor
495 mm2 was selected for long-duration experiments, and the frequency of the RDW
Rotating detonation wave
ranged from 3 to 3.5 kHz. Through the self-adjustment of the combined structure, the air
Compressor
pressure ultimately reached a stable state after a certain period of time, and a stable
Air-injection area
detonation wave was formed in the RDC.
Stability
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Combustion is an important process in propulsion systems and can be divided into two main types: deflagration and detonation. In the past several decades, the propulsive efficiency of constant-pressure engines has significantly improved, and further improvement of the propulsive efficiency is becoming increasingly difficult. Detonation is a nearly constant-volume process and theoretically has a higher thermodynamic efficiency than deflagration. Rotating detonation engines (RDEs) can generate continuous thrust via one or more detonation waves. RDEs not only have all the
advantages of detonationdsuch as the high thermodynamic efficiency and energy-release ratedbut also the advantages of a compact structure, a high operating frequency, stable thrust, and the ability to perform thrust vector control [1,2]. Therefore, RDEs have attracted considerable attention in detonation propulsion research. Research has shown that RDEs can operate in the rocketengine mode and the air-breathing engine mode. In the rocket-engine mode, the injection locations of the fuel and oxidant are at the front of the combustion chamber, which is similar to the configuration of a traditional rocket engine. At present, research on RDEs is mainly focused on the rocket-
* Corresponding author. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.ijhydene.2017.04.214 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
engine mode, and most studies have used air or oxygen at a normal temperature as the oxidiser. Research on RDEs mainly includes the structures of the rotating detonation wave (RDW), the working modes and self-sustained mechanism [3e7], the ignition methods [8e11], the effects of the injection conditions of the oxidiser on the RDW propagation characteristics [12e14], the effects of the RDW on the working characteristics of the plenum [15], and the thrust performance [12,16]. Studies on air-breathing RDEs are underway. In this mode, only fuel injection is needed to sustain the engine operation. Compressed air flows into the combustion chamber and mixes with the fuel, initiating a continuous RDW. For an airbreathing RDE, the rotating detonation turbine engine can produce a larger thrust with lower pressure ratios. This can reduce the number of compressors compared to those in conventional turbine engines and therefore can reduce the requirements of the turbine manufacturing process. The combustion chamber of rotating detonation turbine engines is smaller and simpler but has a higher thrust/weight ratio [1]. Debarmore [17] and Welsh et al. [18] tested a T63 gas turbine driven by H2/air continuous rotation detonation. They found that the static pressure of the detonation products decreased by a certain extent after the products passed through the turbine guide vanes. The frequency of the pressure oscillation was equal to that of the detonation wave. However, the temperature of the detonation products exceeded the turbine inlet limit temperature. Naples et al. [19] investigated the RDE interactions with an annular ejector using H2/air. The results showed that adding air through the ejector greatly reduced the pressure oscillation ahead of the turbine inlet, and temperature measurement results showed that the process of mixing detonation combustion products with the ejector flow is rapid. In experiments and numerical simulations, Wolanski et al. [20] replaced the conventional pressure chamber with a rotating detonation combustor (RDC), improving the efficiency of the turboshaft engine (GTD-350). At present, research on rotating detonation turbine engines mainly focuses on the structures combining the turbine and the RDC. In this study, rotating detonation experiments were performed on a structure combining the RDC and a centrifugal compressor to examine the operation of the RDE with an H2/air mixture. The initiation and propagation characteristics of the RDW were experimentally examined by
changing the air-injection area, and an air-injection area with high performance was selected to verify the stably working feasibility of the combined structure over a long period.
Experimental facilities The experimental system is shown in Fig. 1. The system includes an RDC, a centrifugal compressor with a radial turbine, a fuel supply system, a high-pressure gas supply system, an ignition system, a lubricating system, a data-acquisition system, and a timing-sequence control system. The hydrogen and air are separately injected into the combustor using the slot-orifice injection configuration. The width of the air slot is changed using the slot spacer. The air-injection area is controlled by changing the width of the slot spacer. The RDC is an annular combustor with an inner diameter of 124 mm and an outer diameter of 136 mm. The length of the combustor is 80 mm. An NI X Series multifunction data-acquisition device (DAQ) is used in the experiments for data acquisition. The dataacquisition card (USB-6366) based on NI-STC3 synchronization technology has eight channels of simultaneous analog input and a 16-bit ADC resolution. The single-channel sampling frequency is up to 2 MS/s, which is high enough to ensure the authenticity and stability of the pressure signal. The centrifugal compressor is a part of the turbocharger. The turbocharger mainly includes a centrifugal compressor, a radial turbine, and a bearing box. The compressor is driven by the turbine and compresses air, which then acts as the oxidiser for the RDC. An automotive spark plug is applied to ignite the RDE, and its ignition energy is 50 mJ. The ambient temperature and pressure are 280 K and 1 atm respectively, and the relative humidity in the air is 40%.
Experimental methodology The centrifugal compressor must start up before the RDC operation. The startup process of the compressor in these experiments is described as follows. First, the high-pressure gas provided by the high-pressure gas supply system flows into the radial turbine, and then it is discharged into the
Fig. 1 e Schematic of the experimental system. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
3
Fig. 2 e Time sequence of experiments.
atmosphere, resulting in the rapid rotation of the turbine. Simultaneously, the compressor is driven by the turbine and intakes air from the environment to subsequently perform compression. Next, the compressed air flows into the equaldiameter section. Finally, the air is injected into the RDC through the slot at the end of the equal-diameter section. After the compressor operation, the lubricating system starts to lubricate and cool the bearing box. To ensure that the air mass flow rate remains relatively stable before the operation of the RDC, the centrifugal compressor operates 1.5 s in advance of the RDC. Fig. 2 shows a sequence diagram of the experimental process. A sonic nozzle is set at the manifold of the hydrogen supply system to control the mass flow rate of hydrogen. Three dynamic piezoelectric pressure sensors (p1, p2, and p3) are installed on the outside wall of the RDC, at the locations shown in Fig. 3, to measure the pressure trend of the RDW. The response time of piezoelectric transducer is lower than 2 ms, and the inherent frequency is more than 100 kHz. The ion probe (I1) has the same circumferential position as p3 and detects whether the shock wave is coupled with the flame. Three piezoresistive pressure sensors are used to measure the pressure inside the RDC (pC), the hydrogen plenum (pH2) and the equal-diameter section (pAir). A temperature sensor (T) and an air mass flowmeter (mAir) are used to measure the temperature and mass flow rate of the compressed air, respectively. A revolution speed sensor (n) is used to measure the rotational speed of the compressor.
To investigate the propagation characteristics of the RDW and the operating stability of the combined structure with different injection areas and operation times, three airinjection areas and four RDC operation times are selected, and different equivalence ratios are used in the experiments, as listed in Table 1. The operation time of the RDC is controlled by changing the hydrogen supplying time and the RDC ignition time.
Experimental results and analysis RDC operation process analysis Fig. 4 shows the experimental results of the RDW for test #2. After the flow of the mixture stabilises, the revolution speed of the compressor is 11,000 rpm; the temperature is 287 K at the compressor exit, with an increase of 7 K; the air mass flow rate is 194 g/s. The hydrogen mass flow rate is 4 g/s, and the equivalence ratio is 0.7. Fig. 4a shows that the air pressure is 1.57 bar before ignition and the combustor pressure is close to the environment pressure. The air pressure is used as the injection pressure to ensure that the air smoothly flows into the combustion chamber. The starting time of the RDC operation is considered as 0 s. After ignition, the RDC pressure (pC) and hydrogen plenum pressure (pH2) increase to a relatively stable magnitude. However, the air-injection pressure (pAir) is unstable. At 0.2 s, the RDC ceases operation because the fuel
Fig. 3 e Locations of the sensors on the RDC outside wall. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Table 1 e Experimental conditions. Test #1 #2 #3 #4 #5 #6
Air-injection area (mm2)
RDC operation time Dt (s)
Equivalence ratios
330 495 659 495 495 495
0.2 0.2 0.2 2 4 10
0.6e1.4 0.6e1.4 0.6e1.4 0.95 0.95 0.95
supply system is turned off; then, the fuel pressure decreases. The RDC operating time is approximately 0.2 s, which is enough to verify the initiation characteristics of the combined structure.
Fig. 4b and c shows the signal curves of p1 and I1 respectively, during the RDW propagation process, which exhibit a similar trend. The one-dimensional ZND model is proposed to explain the detonation as a combination of a shock wave, an induction zone, and a reaction zone. The shock wave couples with a flame in the experiment; thus, the RDW is formed in the RDC during the operation. The dynamic piezoelectric pressure sensor is unable to function for a long time in the experiments, because of the high temperature. However, the ion probe can adapt to the conditions, and the signals can reflect the propagation characteristics of the detonation wave to some extent. The advantages of the ion probe are more prominent under long-term working conditions. In this study, these advantages are exploited to measure the signals of the RDW in the RDC long-duration experiments.
Fig. 4 e Experimental results for test #2: a) Pressure trace of operation process. b) Pressure trace of p1. c) Ion-concentration trace of I1. d) FFT plot of p1. e) FFT plot of I1. f) Velocityetime distribution. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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The results obtained through fast Fourier transform (FFT) analysis are shown in Fig. 4d (p1) and 4e (I1); the domain frequency is Gave ¼ 3.39 kHz in both cases. The RDW pressure and ionization signal exhibit similar trends. The average propagation velocity (Vave) of the detonation wave can be determined as follows: Vave ¼ pDout fave N;
(1)
where the Dout ¼ 136 mm is the annular combustion chamber diameter, and N is the number of RDWs. In the experiment, there is only one RDW in the annular combustion chamber; therefore, the average propagation velocity is calculated using Eq. (1), and the result is 1.45 km/s. The ideal ChapmaneJouguet (CeJ) detonation-wave velocity calculated using the software NASA Chemical Equilibrium and Analysis is 1.79 km/s. The experimental value reaches 81% of CeJ value. In this study, the air-injection condition could be the main reason for the RDW velocity deficit during the RDE operation process. The detonation-wave propagation velocity distribution can be obtained from the two time intervals of two adjacent detonation-wave pressure peaks, as shown in Fig. 4f. The main distribution range of the detonation-wave velocity is 1.38e1.53 km/s during the propagation process, and the propagation velocity is stable.
Experimental results with different air-injection areas The automotive spark plug is applied to ignite the RDC, which cannot directly initiate the RDW, because the ignition energy is small; therefore, a deflation-to-detonation transition occurs. Fig. 5 shows the signal trends of the shock wave pressure and ion concentration during the initial process of the detonation-wave formation in test #2. The distance between the pressure monitoring point (p3) and the combustion chamber head is 32 mm. The average peak pressure is lower (1 bar) because of the further distance. The p3 and I1 are in the same circumferential position; thus, the coupling of the shock wave and flame can be directly observed through the trends of the pressure and ion-concentration variation. Fig. 5 shows that the spark plug ignites the mixture at 0 s, and the ion probe collects a weak ion signal. Then, the mixture forms a chemical
Fig. 5 e Initial process of detonation-wave formation (test #2).
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reaction and gradually produces a step pressure signal; the ion concentration does not produce a step response, as the chemical reaction rate is low. The step signals of the pressure and ion concentration almost simultaneously appear at 2.23 ms and then are continuously monitored over time, which indicates that the pressure is coupled with the reaction zone and that the RDW formation time is 2.23 ms (tD). The hydrogen and air are injected into the annular combustion chamber separately; thus, the injection conditions directly affect the RDW formation time and propagation characteristics. Fig. 6 shows the statistics of the detonationwave formation time (tD) for different equivalence ratios and different air-injection areas. As shown, the equivalence ratio has a small influence on the detonation-wave formation time in tests #1 and #2, and the average value of tD is only 2.43 ms in test #1, which is far smaller than that in test #3. The formation time of the detonation wave can be affected by the detonation-cell size. Ciccarell [21] and Anand [5] examined the relationship between the equivalence ratios and the cell size for a hydrogeneair mixture and reported that with the increase of the equivalence ratio, the detonation-cell size first decreases and then increases. The detonation-wave formation times for tests #3 and #2 exhibit the similar trends. As the magnitude of the formation time for test #1 is smaller, the trend is not obvious. The fill height of the fresh mixture differs among the three air-injection areas, which may have influenced the detonation-wave formation time. Fig. 7 shows the experimental results for three air-injection areas and a mixture equivalence ratio of 0.95. As shown in the left figure, the average peak pressure of the RDW is not the same for the three injection areas and increases with the injection area. The pressure is approximately 9 bar in test #3. The right figure shows the propagation frequency distribution of the detonation wave. The average frequency is close to 3.4e3.5 kHz for the three tests, but the dispersion degree of the frequency distribution differs. The propagation frequency distribution of the detonation wave is relatively concentrated for test #1, which indicates the stability of the detonation wave. The propagation frequency distribution of the detonation wave is between 3.0 and 3.7 kHz for test #3. This indicates that as the air-injection area increases, the peak pressure of
Fig. 6 e Statistical graph of tD.
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Fig. 7 e High-frequency pressure (left) and frequency distribution (right) for three air-injection areas.
the detonation wave increases, whereas the propagation stability decreases. With the increase of the air-injection area, the stability of the RDW decreases and an intermittence detonation phenomenon is easily produced. Fig. 8 shows the experimental results for test #3; the equivalence ratio is 1.05. According to this figure, the RDW produces an intermittence detonation phenomenon multiple times during the operation of the RDC, while the pressure signal is gradually weakened and the ion concentration has no strong signals during the intermittence part. This indicates that the pressure is decoupled from the mixture reaction zone. The pressure and concentration signals are still reproduced after a certain period of time; thus, the detonation wave is regenerated.
Fig. 8 e Intermittence detonation phenomenon (test #3).
Long-duration experimental results and analysis In the aforementioned experiments, the combined engine operated stably for a short time. However, the air-injection pressure (pAir) is unstable during the short-time working process; thus, the long-term operating stability of the engine must be verified, and a stable pAir must be experimentally achieved. Among the three air-injection areas, the value of 495 mm2 is selected for the long-duration experiments. The operation times of the RDC in tests #4, #5, and #6 are 2, 4, and 10 s respectively. Figs. 9a, 10a and 11a show the results of the experiments, i.e., the data collected by the piezoresistive pressure sensors. The hydrogen chamber pressure, the air-injection pressure, and the combustion-chamber pressure increase and remain stable during the RDC operation process. The ion-concentration signal of the detonation wave is measured during the long-duration experiments, and the results are shown in Figs. 9b, 10b and 11b. All the ionconcentration signals initially decrease and then increase to a relatively stable value. The reason for this trend could be the changes in the resistance of the ion probe at a high temperature, which results in a greater variety of the ion probe signal. The ion signal exhibits no gaps, indicating that the RDC has a continuously propagating detonation wave during the operation. Figs. 9c, 10c and 11c show the local amplification signals of the ion probe, which exhibit a similar trend to the general RDW pressure. Thus, the ion probe is feasible and effective for measuring the detonation-wave signal in the long-duration experiments. Figs. 9d, 10d and 11d show spectrogram plots of the ionprobe signal obtained via short-time Fourier transform (STFT)
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Fig. 9 e Experimental results for test #4: a) Pressure trace for the operation process. b) Ion-concentration trace for I1. c) Enlarged ion-concentration trace for I1. d) Spectrogram analysis results for I1.
analysis. The spectrogram reveals highly time-varying fluctuations in the frequency, and the RDW propagation frequency ranges from 3 to 3.5 kHz in these tests. The relatively stable frequency distribution indicates that the combined structure can operate stably for a long time.
After the initiation of the RDW, the RDC head is blocked by a part of the area, which increases the injection pressure of hydrogen and air [2], as shown in Figs. 9a, 10a and 11a. The hydrogen is directly provided by the supply system. The sonic nozzle in the hydrogen manifold ensures a constant hydrogen
Fig. 10 e Experimental results for test #5: a) Pressure trace of the operation process. b) Ion-concentration trace for I1. c) Enlarged ion-concentration trace for I1. d) Spectrogram analysis results for I1. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Fig. 11 e Experimental results for test #6: a) Pressure trace of the operation process. b) Ion-concentration trace for I1. c) Enlarged ion-concentration trace for I1. d) Spectrogram analysis results for I1.
Fig. 12 e Detonation-wave velocity: a) Initial process. b) Stable detonation-wave velocity distribution.
mass flow. Therefore, the hydrogen injection pressure is increased to ensure a constant mass flow rate of the hydrogen injected into the combustion chamber. However, the airinjection pressure is obtained by the compressor, which compresses air from the environment. With the high-pressure gas supply system providing stable gas to the impact turbine, the compressor revolution speed remains constant. According to the revolution-speed characteristic curve of the compressor, the air mass flow rate decreases with the increase of the pressure at the same revolution speed. Therefore, the air mass flow rate decreases after the RDW initiation, which increases the mixture equivalence ratio. Then, the detonation-wave intensity increases, increasing the air-blockage area, and the air pressure continues to increase. If the air flow rate decreases, the detonation-wave stability may decrease, reducing the
detonation-wave intensity. Consequently, when the air pressure reaches a certain value, the blockage area stops increasing. As shown in Figs. 9a, 10a and 11a, after the RDC operates for 0.5 s, the air-injection pressure reaches a relatively stable state (1.65 bar), and a stable detonation wave is obtained. Fig. 12a shows that the detonation-wave velocity gradually increases during the initial process. As shown in Fig. 12b, the detonation velocity reaches a very stable state. The RDC can operate stably via the self-adjustment of its combined structure.
Conclusions Hydrogen-air rotating detonation experiments were performed on a structure combining an RDC and a centrifugal
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
compressor, and the operation of the RDC was examined. The air entered the annular combustion chamber after being compressed by the compressor and then reacted with hydrogen. The increased pressure of the air, i.e., the injection pressure, was intended to ensure the stability of the air mass flow rate. The propagation velocity of the RDW reached 81% of the CeJ value in experiments. The experimental results showed that the injection area significantly affected the initiation and propagation characteristics of the detonation wave. The RDW was successfully initiated with three different air-injection areas, and the average formation time was only 2.43 ms in test #1. With the increase of the air-injection area, the detonation-wave pressure increased, but the stability decreased, and it was easier to produce an intermittence detonation phenomenon with a large air-injection area. According to the experimental results for the three air-injection areas, the area of 495 mm2 was selected for long-duration experiments, in which an ion probe was used. The frequency of the RDW ranged from 3 to 3.5 kHz during the operation. The relatively stable frequency distribution revealed by STFT analysis indicates that the combined structure can operate stably for a long time. According to the operating characteristics of the compressor and the propagation characteristics of the detonation wave, the air mass flow rate was decreased by the detonation wave. Through the self-adjustment of the combined structure, the air pressure ultimately reached a stable state after a certain period of time, and a stable detonation wave was formed in the RDC.
Acknowledgement The authors express sincere gratitude to those that aided this study and to the National Natural Science Foundation of China (No. 51606100), the Natural Science Foundation of Jiangsu Province of China (No. BK20150782), and the Fundamental Research Funds for the Central Universities of China (No. 30915118836) for funding this study. The authors also thank the Laboratory of Science and Technology on Liquid Rocket Engine of the Xi'an Aerospace Propulsion Institute for the guidance and support, as well as Dr. Song Chen of Nanyang Technological University for the guidance regarding English grammar.
references
[1] Zhou R, Wu D, Wang J. Progress of continuously rotating detonation engines. Chin J Aeronautics 2016;29(1):15e29. ski P. Detonative propulsion. Proc Combust Inst [2] Wolan 2013;34(1):125e58. [3] Anand V, St. George A, Driscoll R, Gutmark E. Longitudinal pulsed detonation instability in a rotating detonation combustor. Exp Therm Fluid Sci 2016;77:212e25. [4] Anand V, St. George A, Driscoll R, Gutmark E. Characterization of instabilities in a rotating detonation combustor. Int J Hydrogen Energy 2015;40(46):16649e59.
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[5] Anand V, St. George A, Driscoll R, Gutmark E. Investigation of rotating detonation combustor operation with H2-Air mixtures. Int J Hydrogen Energy 2016;41(2):1281e92. [6] Rankin BA, Richardson DR, Caswell AW, Naples AG, Hoke JL, Schauer FR. Chemiluminescence imaging of an optically accessible non-premixed rotating detonation engine. Combust Flame 2017;176:12e22. [7] Zheng Q, Weng C, Bai Q. Experimental research on the propagation process of continuous rotating detonation wave. Def Technol 2013;9(4):201e7. [8] Yang C, Wu X, Ma H, Peng L, Gao J. Experimental research on initiation characteristics of a rotating detonation engine. Exp Therm Fluid Sci 2016;71:154e63. [9] Peng L, Wang D, Wu X, Ma H, Yang C. Ignition experiment with automotive spark on rotating detonation engine. Int J Hydrogen Energy 2015;40(26):8465e74. [10] St. George A, Randall S, Anand V, Driscoll R, Gutmark E. Characterization of initiator dynamics in a rotating detonation combustor. Exp Therm Fluid Sci 2016;72:171e81. [11] Goto K, Kato Y, Ishihara K, Matsuoka K, Kasahara J. Experimental study of effects of injector configurations on rotating detonation engine performance. Salt Lake City, UT: Aiaa/sae/asee Joint Propulsion Conference; 25-27July, 2016. 2016e5100. [12] Frolov SM, Aksenov VS, Ivanov VS, Shamshin IO. Large-scale hydrogeneair continuous detonation combustor. Int J Hydrogen Energy 2015;40(3):1616e23. [13] Rankin BA, Richardson DR, Caswell AW, Naples A, Hoke J, Schauer F. Imaging of OH chemiluminescence in an optically accessible nonpremixed rotating detonation engine. In: 53rd AIAA Aerospace sciences meeting. Kissimmee, Florida; 5-9 January 2015. 2015e1604. [14] Driscoll R, Aghasi P, St George A, Gutmark EJ. Three-dimensional, numerical investigation of reactant injection variation in a H2/air rotating detonation engine. Int J Hydrogen Energy 2016;41(9):5162e75. [15] Anand V, St. George A, Driscoll R, Gutmark E. Analysis of air inlet and fuel plenum behavior in a rotating detonation combustor. Exp Therm Fluid Sci 2016;70:408e16. [16] Fotia ML, Schauer F, Kaemming T, Hoke J. Experimental study of the performance of a rotating detonation engine with nozzle. In: 53rd AIAA Aerospace sciences meeting, Kissimmee; 5e9 January 2015. 2015e0631. [17] Debarmore N, King P, Schauer F, Hoke J. Nozzle guide vane integration into rotating detonation engine. In: AIAA Aerospace sciences meeting including the N... Grapevine (Dallas/Ft. Worth region), Texas; 07e10 January 2013. 2013-1030. [18] Welsh DJ, King P, Schauer F, Hoke J. RDE integration with T63 turboshaft engine components. In: Aerospace sciences meeting. National Harbor, Maryland; 13e17 January 2014. 2014e1316. [19] Naples A, Hoke J, Schauer F. Rotating detonation engine interaction with an annular ejector. In: Aerospace sciences meeting. National Harbor, Maryland; 13e17 January 2014. 2014e0287. ski P. Application of the continuous [20] Wolan rotating detonation to gas turbine. Appl Mech Mater 2015;782:3e12. [21] Ciccarelli G, Ginsberg T, Boccio J. Detonation cell size measurements and predictions in hydrogen-air- steam mixtures at elevated temperatures. Combust Flame 1994;99:212e20.
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Experimental study of a hydrogen-air rotating detonation combustor Shengbing Zhou a, Hu Ma a,*, Daokun Liu b, Yu Yan c, Shuai Li a, Changsheng Zhou a a
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Shanghai Space Propulsion Technology Research Institute, Shanghai 201109, China c Laboratory of Science and Technology on Liquid Rocket Engine, Xi'an Aerospace Propulsion Institute, Xi'an 710100, China b
article info
abstract
Article history:
The rotating detonation engine can generate continuous thrust via one or more detonation
Received 6 January 2017
waves. In this study, rotating detonation experiments were performed on a combined
Received in revised form
structure which included a rotating detonation combustor (RDC) and a centrifugal
12 April 2017
compressor. Air, which functioned as an oxidiser, was obtained from the environment by
Accepted 21 April 2017
the compressor, and hydrogen, which was used as fuel, was provided by the supply sys-
Available online xxx
tem. The propagation velocity of the rotating detonation wave (RDW) reached 81% of the ChapmaneJouguet value in experiments. With the increase of the air-injection area, the
Keywords:
detonation-wave pressure increased, but the stability decreased. An air-injection area of
Rotating detonation combustor
495 mm2 was selected for long-duration experiments, and the frequency of the RDW
Rotating detonation wave
ranged from 3 to 3.5 kHz. Through the self-adjustment of the combined structure, the air
Compressor
pressure ultimately reached a stable state after a certain period of time, and a stable
Air-injection area
detonation wave was formed in the RDC.
Stability
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Combustion is an important process in propulsion systems and can be divided into two main types: deflagration and detonation. In the past several decades, the propulsive efficiency of constant-pressure engines has significantly improved, and further improvement of the propulsive efficiency is becoming increasingly difficult. Detonation is a nearly constant-volume process and theoretically has a higher thermodynamic efficiency than deflagration. Rotating detonation engines (RDEs) can generate continuous thrust via one or more detonation waves. RDEs not only have all the
advantages of detonationdsuch as the high thermodynamic efficiency and energy-release ratedbut also the advantages of a compact structure, a high operating frequency, stable thrust, and the ability to perform thrust vector control [1,2]. Therefore, RDEs have attracted considerable attention in detonation propulsion research. Research has shown that RDEs can operate in the rocketengine mode and the air-breathing engine mode. In the rocket-engine mode, the injection locations of the fuel and oxidant are at the front of the combustion chamber, which is similar to the configuration of a traditional rocket engine. At present, research on RDEs is mainly focused on the rocket-
* Corresponding author. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.ijhydene.2017.04.214 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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engine mode, and most studies have used air or oxygen at a normal temperature as the oxidiser. Research on RDEs mainly includes the structures of the rotating detonation wave (RDW), the working modes and self-sustained mechanism [3e7], the ignition methods [8e11], the effects of the injection conditions of the oxidiser on the RDW propagation characteristics [12e14], the effects of the RDW on the working characteristics of the plenum [15], and the thrust performance [12,16]. Studies on air-breathing RDEs are underway. In this mode, only fuel injection is needed to sustain the engine operation. Compressed air flows into the combustion chamber and mixes with the fuel, initiating a continuous RDW. For an airbreathing RDE, the rotating detonation turbine engine can produce a larger thrust with lower pressure ratios. This can reduce the number of compressors compared to those in conventional turbine engines and therefore can reduce the requirements of the turbine manufacturing process. The combustion chamber of rotating detonation turbine engines is smaller and simpler but has a higher thrust/weight ratio [1]. Debarmore [17] and Welsh et al. [18] tested a T63 gas turbine driven by H2/air continuous rotation detonation. They found that the static pressure of the detonation products decreased by a certain extent after the products passed through the turbine guide vanes. The frequency of the pressure oscillation was equal to that of the detonation wave. However, the temperature of the detonation products exceeded the turbine inlet limit temperature. Naples et al. [19] investigated the RDE interactions with an annular ejector using H2/air. The results showed that adding air through the ejector greatly reduced the pressure oscillation ahead of the turbine inlet, and temperature measurement results showed that the process of mixing detonation combustion products with the ejector flow is rapid. In experiments and numerical simulations, Wolanski et al. [20] replaced the conventional pressure chamber with a rotating detonation combustor (RDC), improving the efficiency of the turboshaft engine (GTD-350). At present, research on rotating detonation turbine engines mainly focuses on the structures combining the turbine and the RDC. In this study, rotating detonation experiments were performed on a structure combining the RDC and a centrifugal compressor to examine the operation of the RDE with an H2/air mixture. The initiation and propagation characteristics of the RDW were experimentally examined by
changing the air-injection area, and an air-injection area with high performance was selected to verify the stably working feasibility of the combined structure over a long period.
Experimental facilities The experimental system is shown in Fig. 1. The system includes an RDC, a centrifugal compressor with a radial turbine, a fuel supply system, a high-pressure gas supply system, an ignition system, a lubricating system, a data-acquisition system, and a timing-sequence control system. The hydrogen and air are separately injected into the combustor using the slot-orifice injection configuration. The width of the air slot is changed using the slot spacer. The air-injection area is controlled by changing the width of the slot spacer. The RDC is an annular combustor with an inner diameter of 124 mm and an outer diameter of 136 mm. The length of the combustor is 80 mm. An NI X Series multifunction data-acquisition device (DAQ) is used in the experiments for data acquisition. The dataacquisition card (USB-6366) based on NI-STC3 synchronization technology has eight channels of simultaneous analog input and a 16-bit ADC resolution. The single-channel sampling frequency is up to 2 MS/s, which is high enough to ensure the authenticity and stability of the pressure signal. The centrifugal compressor is a part of the turbocharger. The turbocharger mainly includes a centrifugal compressor, a radial turbine, and a bearing box. The compressor is driven by the turbine and compresses air, which then acts as the oxidiser for the RDC. An automotive spark plug is applied to ignite the RDE, and its ignition energy is 50 mJ. The ambient temperature and pressure are 280 K and 1 atm respectively, and the relative humidity in the air is 40%.
Experimental methodology The centrifugal compressor must start up before the RDC operation. The startup process of the compressor in these experiments is described as follows. First, the high-pressure gas provided by the high-pressure gas supply system flows into the radial turbine, and then it is discharged into the
Fig. 1 e Schematic of the experimental system. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Fig. 2 e Time sequence of experiments.
atmosphere, resulting in the rapid rotation of the turbine. Simultaneously, the compressor is driven by the turbine and intakes air from the environment to subsequently perform compression. Next, the compressed air flows into the equaldiameter section. Finally, the air is injected into the RDC through the slot at the end of the equal-diameter section. After the compressor operation, the lubricating system starts to lubricate and cool the bearing box. To ensure that the air mass flow rate remains relatively stable before the operation of the RDC, the centrifugal compressor operates 1.5 s in advance of the RDC. Fig. 2 shows a sequence diagram of the experimental process. A sonic nozzle is set at the manifold of the hydrogen supply system to control the mass flow rate of hydrogen. Three dynamic piezoelectric pressure sensors (p1, p2, and p3) are installed on the outside wall of the RDC, at the locations shown in Fig. 3, to measure the pressure trend of the RDW. The response time of piezoelectric transducer is lower than 2 ms, and the inherent frequency is more than 100 kHz. The ion probe (I1) has the same circumferential position as p3 and detects whether the shock wave is coupled with the flame. Three piezoresistive pressure sensors are used to measure the pressure inside the RDC (pC), the hydrogen plenum (pH2) and the equal-diameter section (pAir). A temperature sensor (T) and an air mass flowmeter (mAir) are used to measure the temperature and mass flow rate of the compressed air, respectively. A revolution speed sensor (n) is used to measure the rotational speed of the compressor.
To investigate the propagation characteristics of the RDW and the operating stability of the combined structure with different injection areas and operation times, three airinjection areas and four RDC operation times are selected, and different equivalence ratios are used in the experiments, as listed in Table 1. The operation time of the RDC is controlled by changing the hydrogen supplying time and the RDC ignition time.
Experimental results and analysis RDC operation process analysis Fig. 4 shows the experimental results of the RDW for test #2. After the flow of the mixture stabilises, the revolution speed of the compressor is 11,000 rpm; the temperature is 287 K at the compressor exit, with an increase of 7 K; the air mass flow rate is 194 g/s. The hydrogen mass flow rate is 4 g/s, and the equivalence ratio is 0.7. Fig. 4a shows that the air pressure is 1.57 bar before ignition and the combustor pressure is close to the environment pressure. The air pressure is used as the injection pressure to ensure that the air smoothly flows into the combustion chamber. The starting time of the RDC operation is considered as 0 s. After ignition, the RDC pressure (pC) and hydrogen plenum pressure (pH2) increase to a relatively stable magnitude. However, the air-injection pressure (pAir) is unstable. At 0.2 s, the RDC ceases operation because the fuel
Fig. 3 e Locations of the sensors on the RDC outside wall. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Table 1 e Experimental conditions. Test #1 #2 #3 #4 #5 #6
Air-injection area (mm2)
RDC operation time Dt (s)
Equivalence ratios
330 495 659 495 495 495
0.2 0.2 0.2 2 4 10
0.6e1.4 0.6e1.4 0.6e1.4 0.95 0.95 0.95
supply system is turned off; then, the fuel pressure decreases. The RDC operating time is approximately 0.2 s, which is enough to verify the initiation characteristics of the combined structure.
Fig. 4b and c shows the signal curves of p1 and I1 respectively, during the RDW propagation process, which exhibit a similar trend. The one-dimensional ZND model is proposed to explain the detonation as a combination of a shock wave, an induction zone, and a reaction zone. The shock wave couples with a flame in the experiment; thus, the RDW is formed in the RDC during the operation. The dynamic piezoelectric pressure sensor is unable to function for a long time in the experiments, because of the high temperature. However, the ion probe can adapt to the conditions, and the signals can reflect the propagation characteristics of the detonation wave to some extent. The advantages of the ion probe are more prominent under long-term working conditions. In this study, these advantages are exploited to measure the signals of the RDW in the RDC long-duration experiments.
Fig. 4 e Experimental results for test #2: a) Pressure trace of operation process. b) Pressure trace of p1. c) Ion-concentration trace of I1. d) FFT plot of p1. e) FFT plot of I1. f) Velocityetime distribution. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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The results obtained through fast Fourier transform (FFT) analysis are shown in Fig. 4d (p1) and 4e (I1); the domain frequency is Gave ¼ 3.39 kHz in both cases. The RDW pressure and ionization signal exhibit similar trends. The average propagation velocity (Vave) of the detonation wave can be determined as follows: Vave ¼ pDout fave N;
(1)
where the Dout ¼ 136 mm is the annular combustion chamber diameter, and N is the number of RDWs. In the experiment, there is only one RDW in the annular combustion chamber; therefore, the average propagation velocity is calculated using Eq. (1), and the result is 1.45 km/s. The ideal ChapmaneJouguet (CeJ) detonation-wave velocity calculated using the software NASA Chemical Equilibrium and Analysis is 1.79 km/s. The experimental value reaches 81% of CeJ value. In this study, the air-injection condition could be the main reason for the RDW velocity deficit during the RDE operation process. The detonation-wave propagation velocity distribution can be obtained from the two time intervals of two adjacent detonation-wave pressure peaks, as shown in Fig. 4f. The main distribution range of the detonation-wave velocity is 1.38e1.53 km/s during the propagation process, and the propagation velocity is stable.
Experimental results with different air-injection areas The automotive spark plug is applied to ignite the RDC, which cannot directly initiate the RDW, because the ignition energy is small; therefore, a deflation-to-detonation transition occurs. Fig. 5 shows the signal trends of the shock wave pressure and ion concentration during the initial process of the detonation-wave formation in test #2. The distance between the pressure monitoring point (p3) and the combustion chamber head is 32 mm. The average peak pressure is lower (1 bar) because of the further distance. The p3 and I1 are in the same circumferential position; thus, the coupling of the shock wave and flame can be directly observed through the trends of the pressure and ion-concentration variation. Fig. 5 shows that the spark plug ignites the mixture at 0 s, and the ion probe collects a weak ion signal. Then, the mixture forms a chemical
Fig. 5 e Initial process of detonation-wave formation (test #2).
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reaction and gradually produces a step pressure signal; the ion concentration does not produce a step response, as the chemical reaction rate is low. The step signals of the pressure and ion concentration almost simultaneously appear at 2.23 ms and then are continuously monitored over time, which indicates that the pressure is coupled with the reaction zone and that the RDW formation time is 2.23 ms (tD). The hydrogen and air are injected into the annular combustion chamber separately; thus, the injection conditions directly affect the RDW formation time and propagation characteristics. Fig. 6 shows the statistics of the detonationwave formation time (tD) for different equivalence ratios and different air-injection areas. As shown, the equivalence ratio has a small influence on the detonation-wave formation time in tests #1 and #2, and the average value of tD is only 2.43 ms in test #1, which is far smaller than that in test #3. The formation time of the detonation wave can be affected by the detonation-cell size. Ciccarell [21] and Anand [5] examined the relationship between the equivalence ratios and the cell size for a hydrogeneair mixture and reported that with the increase of the equivalence ratio, the detonation-cell size first decreases and then increases. The detonation-wave formation times for tests #3 and #2 exhibit the similar trends. As the magnitude of the formation time for test #1 is smaller, the trend is not obvious. The fill height of the fresh mixture differs among the three air-injection areas, which may have influenced the detonation-wave formation time. Fig. 7 shows the experimental results for three air-injection areas and a mixture equivalence ratio of 0.95. As shown in the left figure, the average peak pressure of the RDW is not the same for the three injection areas and increases with the injection area. The pressure is approximately 9 bar in test #3. The right figure shows the propagation frequency distribution of the detonation wave. The average frequency is close to 3.4e3.5 kHz for the three tests, but the dispersion degree of the frequency distribution differs. The propagation frequency distribution of the detonation wave is relatively concentrated for test #1, which indicates the stability of the detonation wave. The propagation frequency distribution of the detonation wave is between 3.0 and 3.7 kHz for test #3. This indicates that as the air-injection area increases, the peak pressure of
Fig. 6 e Statistical graph of tD.
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Fig. 7 e High-frequency pressure (left) and frequency distribution (right) for three air-injection areas.
the detonation wave increases, whereas the propagation stability decreases. With the increase of the air-injection area, the stability of the RDW decreases and an intermittence detonation phenomenon is easily produced. Fig. 8 shows the experimental results for test #3; the equivalence ratio is 1.05. According to this figure, the RDW produces an intermittence detonation phenomenon multiple times during the operation of the RDC, while the pressure signal is gradually weakened and the ion concentration has no strong signals during the intermittence part. This indicates that the pressure is decoupled from the mixture reaction zone. The pressure and concentration signals are still reproduced after a certain period of time; thus, the detonation wave is regenerated.
Fig. 8 e Intermittence detonation phenomenon (test #3).
Long-duration experimental results and analysis In the aforementioned experiments, the combined engine operated stably for a short time. However, the air-injection pressure (pAir) is unstable during the short-time working process; thus, the long-term operating stability of the engine must be verified, and a stable pAir must be experimentally achieved. Among the three air-injection areas, the value of 495 mm2 is selected for the long-duration experiments. The operation times of the RDC in tests #4, #5, and #6 are 2, 4, and 10 s respectively. Figs. 9a, 10a and 11a show the results of the experiments, i.e., the data collected by the piezoresistive pressure sensors. The hydrogen chamber pressure, the air-injection pressure, and the combustion-chamber pressure increase and remain stable during the RDC operation process. The ion-concentration signal of the detonation wave is measured during the long-duration experiments, and the results are shown in Figs. 9b, 10b and 11b. All the ionconcentration signals initially decrease and then increase to a relatively stable value. The reason for this trend could be the changes in the resistance of the ion probe at a high temperature, which results in a greater variety of the ion probe signal. The ion signal exhibits no gaps, indicating that the RDC has a continuously propagating detonation wave during the operation. Figs. 9c, 10c and 11c show the local amplification signals of the ion probe, which exhibit a similar trend to the general RDW pressure. Thus, the ion probe is feasible and effective for measuring the detonation-wave signal in the long-duration experiments. Figs. 9d, 10d and 11d show spectrogram plots of the ionprobe signal obtained via short-time Fourier transform (STFT)
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Fig. 9 e Experimental results for test #4: a) Pressure trace for the operation process. b) Ion-concentration trace for I1. c) Enlarged ion-concentration trace for I1. d) Spectrogram analysis results for I1.
analysis. The spectrogram reveals highly time-varying fluctuations in the frequency, and the RDW propagation frequency ranges from 3 to 3.5 kHz in these tests. The relatively stable frequency distribution indicates that the combined structure can operate stably for a long time.
After the initiation of the RDW, the RDC head is blocked by a part of the area, which increases the injection pressure of hydrogen and air [2], as shown in Figs. 9a, 10a and 11a. The hydrogen is directly provided by the supply system. The sonic nozzle in the hydrogen manifold ensures a constant hydrogen
Fig. 10 e Experimental results for test #5: a) Pressure trace of the operation process. b) Ion-concentration trace for I1. c) Enlarged ion-concentration trace for I1. d) Spectrogram analysis results for I1. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214
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Fig. 11 e Experimental results for test #6: a) Pressure trace of the operation process. b) Ion-concentration trace for I1. c) Enlarged ion-concentration trace for I1. d) Spectrogram analysis results for I1.
Fig. 12 e Detonation-wave velocity: a) Initial process. b) Stable detonation-wave velocity distribution.
mass flow. Therefore, the hydrogen injection pressure is increased to ensure a constant mass flow rate of the hydrogen injected into the combustion chamber. However, the airinjection pressure is obtained by the compressor, which compresses air from the environment. With the high-pressure gas supply system providing stable gas to the impact turbine, the compressor revolution speed remains constant. According to the revolution-speed characteristic curve of the compressor, the air mass flow rate decreases with the increase of the pressure at the same revolution speed. Therefore, the air mass flow rate decreases after the RDW initiation, which increases the mixture equivalence ratio. Then, the detonation-wave intensity increases, increasing the air-blockage area, and the air pressure continues to increase. If the air flow rate decreases, the detonation-wave stability may decrease, reducing the
detonation-wave intensity. Consequently, when the air pressure reaches a certain value, the blockage area stops increasing. As shown in Figs. 9a, 10a and 11a, after the RDC operates for 0.5 s, the air-injection pressure reaches a relatively stable state (1.65 bar), and a stable detonation wave is obtained. Fig. 12a shows that the detonation-wave velocity gradually increases during the initial process. As shown in Fig. 12b, the detonation velocity reaches a very stable state. The RDC can operate stably via the self-adjustment of its combined structure.
Conclusions Hydrogen-air rotating detonation experiments were performed on a structure combining an RDC and a centrifugal
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compressor, and the operation of the RDC was examined. The air entered the annular combustion chamber after being compressed by the compressor and then reacted with hydrogen. The increased pressure of the air, i.e., the injection pressure, was intended to ensure the stability of the air mass flow rate. The propagation velocity of the RDW reached 81% of the CeJ value in experiments. The experimental results showed that the injection area significantly affected the initiation and propagation characteristics of the detonation wave. The RDW was successfully initiated with three different air-injection areas, and the average formation time was only 2.43 ms in test #1. With the increase of the air-injection area, the detonation-wave pressure increased, but the stability decreased, and it was easier to produce an intermittence detonation phenomenon with a large air-injection area. According to the experimental results for the three air-injection areas, the area of 495 mm2 was selected for long-duration experiments, in which an ion probe was used. The frequency of the RDW ranged from 3 to 3.5 kHz during the operation. The relatively stable frequency distribution revealed by STFT analysis indicates that the combined structure can operate stably for a long time. According to the operating characteristics of the compressor and the propagation characteristics of the detonation wave, the air mass flow rate was decreased by the detonation wave. Through the self-adjustment of the combined structure, the air pressure ultimately reached a stable state after a certain period of time, and a stable detonation wave was formed in the RDC.
Acknowledgement The authors express sincere gratitude to those that aided this study and to the National Natural Science Foundation of China (No. 51606100), the Natural Science Foundation of Jiangsu Province of China (No. BK20150782), and the Fundamental Research Funds for the Central Universities of China (No. 30915118836) for funding this study. The authors also thank the Laboratory of Science and Technology on Liquid Rocket Engine of the Xi'an Aerospace Propulsion Institute for the guidance and support, as well as Dr. Song Chen of Nanyang Technological University for the guidance regarding English grammar.
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Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.214