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Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot
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 8 ) 1 e1 0
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Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot Shengbing Zhou a, Hu Ma a,*, Shuai Li b, Changsheng Zhou a, Daokun Liu c a
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Guiyang Aero Engine Design Research Institute, Guiyang 550081, China c Shanghai Space Propulsion Technology Research Institute, Shanghai 201109, China b
article info
abstract
Article history:
Rotating detonation engines have attracted considerable attentions in recent years. In this
Received 7 September 2017
study, the experiments of initiating rotating detonation waves were performed on a H2/air
Received in revised form
rotating detonation wave with the variable air-inlet slot. The results showed that the
11 April 2018
stability of detonation-wave pressure and velocity both initially increased and then
Accepted 4 May 2018
decreased with the increase of slot width, and it could improve the stability of detonation-
Available online xxx
wave velocity via increasing the equivalence ratio. The intensity of reflected wave was strong for the tests of d ¼ 0.5 mm, which leaded to the advance ignition of fresh mixture
Keywords:
and a velocity deficit reaching up to 20%. The strong interaction between air plenum and
Rotating detonation engine
combustor and bad mixing effect may be the reasons of forming unstable detonation wave
Rotating detonation wave
for the tests of large-scale slots. The air-inlet slot of d ¼ 1 mm, which got a best experiment
Air-inlet slot
results relative to other tests, had a wide equivalence-ratio scope to produce stable deto-
Stability
nation wave. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction At present, the power and propulsion systems based on detonation mainly are pulse detonation engines (PDEs) [1,2], standing detonation engines (SDEs) [3] and rotating detonation engines (RDEs) [4,5]. Comparing with the PDEs, the RDEs have advantages of a high operation frequency, stable thrust, small axial size, and the ability to produce continuous detonation waves with the only one-time ignition, i.e., RDEs rely on the ignition systems a little. Comparing with the SDEs, the RDEs can work in the condition of wide flight mach numbers, and have hardly any limits of mach numbers. Besides, RDEs can perform thrust vector control via adjusting the mass flow rate of reactants in different azimuthal locations. Therefore,
the RDEs have a promising application in the field of aerospace and weaponry. RDEs have attracted considerable attentions in recent years, and the related researches have been carried out in many countries, such as Russia, America, Poland, France, China, Japan, Singapore and South Korea [6,7]. Research on RDEs mainly includes fuel and oxidiser compositions [8e10], the structures of the rotating detonation wave (RDW), the working modes and self-sustained mechanism [11e15], the ignition methods [16e19], the effects of the reactant-injection conditions on the RDW propagation characteristics [20,21], the effects of the RDW on the plenumworking characteristics [22], the structures of rotating detonation combustor (RDC) [10,23], and the thrust performance [20,24,25] and application [26e29]. The premise to ensure RDEs operate steadily is obtaining stable RDWs; however, the
* Corresponding author. E-mail address: [email protected] (H. Ma). https://doi.org/10.1016/j.ijhydene.2018.05.033 0360-3199/© 2018 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 engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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conditions are strict. The stability of RDWs easily has a switcheroo to produce an unstable phenomenon, even extinguishing. There are many instabilities of RDWs. Anand et al. [12] revealed four fundamentally different instabilitiesdchaotic instability, waxing and waning instability, mode switching and longitudinal pulsed detonation instabilitydvisa a large number of experiments. Peng et al. [17] found a phenomenon that detonation-wave intensity appeared strong weak alternately in experiments. Liu et al. [30] found the detonation-wave peak values changed periodically and thought this unstable phenomenon was related to the variation of propellant's mass flow rate. The injection structure of RDEs has a direct influence on the detonation-wave propagation stability. Anand et al. [13] investigated the operation performance with three fuelinjection structure and the results showed that the fuel plate with the highest length-to-diameter ratio exhibited lean operation devoid of transitional or pop-out events; and further testing showed that the increase of air injection area increased the number of pop-out events. Frolov et al. [20] found that the number of detonation wave would be decreased with the increase of air-inlet slit width. Rankin et al. [14] revealed that the wave front was more concave as the air injection area was increased from low to intermediate values via a representative optically accessible RDE. In this study, the experiments of initiating RDW were performed on a H2/air RDE. The aim of this work is to study the RDW propagation characteristics via gradually changing the width of air-inlet slot, and analyze the reasons of producing the stable or unstable detonation wave. Finally, the experiment got an optimum slot with stable detonation wave in combustor. This work hoped to provide a reference for the research of RDEs.
annular combustor, with an inner diameter of 124 mm and an outer diameter of 136 mm, is used in this experiment. The RDW can be successfully initiated by the predetonator, thermal-jet tube, low-energy or high-energy spark plug, etc. The initiation way of detonation wave only influences on its formation process, and the operation characteristics of the engine model are independent of the ignition device [16]. Peng [17] obtained the ignition success rate of 94% using ordinary spark-plug device in experiments. Besides, the spark-plug device has advantages of easy to installation and simple control. The ignition energy of spark-up device is 50 mJ using in this experiment, and the working time of spark plug is 30 ms. The experiments also got a high success rate of ignition. Three piezoresistive pressure sensors, with the sensitivity of 0.5%FS, are used to measure the pressure inside the RDC (PC), the hydrogen plenum (PH2) and the air plenum (PAir). The dynamic piezoelectric pressure sensors (PCB, 113B24), with the sensitivity of 10%, are installed on the outside wall of the RDC to measure the pressure trend of the RDW. The response time of piezoelectric transducer is lower than 1 ms, and the inherent frequency is more than 500 kHz. The location of sensors (p1, p2, p3 &p4) is shown in Fig. 1. Besides, a dynamic piezoelectric pressure sensor is mounted on the wall of air plenum, where closed to air-injection slot, to measure the high-frequency pressure wave which comes from combustor. 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.
Experimental facilities
Experimental results and analysis
The experimental system is composed of a hydrogen supply system, an air supply system, an RDE, an ignition device, a data-acquisition system, and a timing-sequence control system, as shown in Fig. 1. The injection modes of oxidiser and fuel are slot-slot type, slot-orifice type and orifice-orifice type; among them, the slotorifice type is generally used in experiments. A slot-orifice injection configuration is used in experiments for this study, as shown in Fig. 1. Hydrogen, which is used as fuel, is injected into chamber through 120 orifices uniformly distributed in front of combustor, and air, which is functioned as an oxidiser, is injected into combustion chamber through an annular slot. The width of air-inlet slot (d) could be changed by the slot spacer at the front end of center body. Six widths of air-inlet slot (d ¼ 0.5, 1, 1.5, 2, 2.5, 3 mm) are selected for experiments to investigate the detonation-wave propagation stability. The structure of combustion chamber in RDE is different with other propulsion system. It is demonstrated that the structure patterns, which can be used as combustion chamber, are disk-shaped combustor [31], annular combustor and hollow combustor [32]; among them, the annular combustor is used the most and has a good operation performance. An
RDE operation process Multiple experiments were carried out to verify the repeatability of results for every width of air-inlet slot. Three equivalence ratios (Ф ¼ 0.88, 1.08, 1.27) are used to study the effects on the stability of RDW. To ensure the air and hydrogen mass flow rate remains relatively stable before the operation of the RDC, the supply systems operate 0.8 s in advance of the RDC. The dynamic piezoelectric pressure sensor is unable to function for a long time in the experiment because of the high temperature in combustor; thus, the operation time of RDC is set at 0.4 s, which is controlled by the operation time of supply systems. Two sonic nozzles are respectively set at the manifold of the hydrogen and air supply system to control the mass flow rate of hydrogen and air. The air mass flow rate keeps constant value of 153 g/s in experiments, and the equivalence ratios are controlled by the hydrogen mass flow rate. Fig. 2a shows the static pressure curves during the operation of RDE. Fig. 2b shows the results of pressure wave which acquired via dynamic piezoelectric pressure sensors. The value of p1 is obviously greater than other sensors, this is because the p1 is located in the scope of detonation wave height, and others may close to the region of expansion.
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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 8 ) 1 e1 0
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Fig. 1 e Schematic diagram of the experiment system.
Fig. 2 e Pressure trace for the operation process.
Fig. 3 shows the experimental results (p1) with different widths of air-inlet slot. The experimental results of d ¼ 0.5 mm (Fig. 3a) show the unstable state of detonation wave which easily produces a secondary-peak-pressure phenomenon; however, the trend of pressure for d ¼ 1 mm (Fig. 3d) is generally stable which indicates the experiments get a good result. When d ¼ 1.5 mm (Fig. 3c), it could get a stable pressure of detonation wave with Ф ¼ 1.08 and 1.27, besides Ф ¼ 1.08 which produces an unstable detonation phenomenon. Further increase of the width of slot (d ¼ 2.0 mm), the detonation wave becomes unstable when Ф ¼ 1.08 and 1.27, as shown in Fig. 3d. Finally, it can't get a stable detonation wave with d ¼ 2.5 mm
and 3 mm, especially the trend of pressure is different to the trend of detonation wavedsimilar to the pressure of oscillation combustion. Therefore, the width of air-inlet slot has significant effects on detonation-wave pressure, and the equivalence ratio has a particular function for the stability of detonation wave. The reasons of existing different pressure characteristics of detonation wave will be discussed later.
RDW stability analysis The propagation velocity of detonation wave will keep stable if the pressure has a stable trend. The detonation-wave
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Fig. 3 e Pressure trace of detonation wave.
instantaneous velocity (Vi) distribution can be obtained from the two time intervals of two adjacent detonation-wave pressure peaks; this way is widely used to study the propagation characteristics of detonation wave [12,28]. Fig. 4 shows the instantaneous velocity distribution of detonation wave. There are some differences of detonation-wave velocity distribution for each air-inlet slot, and the distribution trend of Vi
is also largely effected by the equivalence ratio. As shown in Fig. 4a (Ф ¼ 0.88), the velocity of detonation wave for all airinlet width tests has a fluctuation to some extent, and the distribution is most chaotic for the tests of d ¼ 2.5 mm and 3 mm. The peaks of oscillation pressure produced by unstable deflagration combustion would confound the calculating of detonation-wave instantaneous velocity [12], which is the
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Fig. 4 e Detonation-wave velocity distribution: a) Ф ¼ 0.88. b)Ф ¼ 1.08. c) Ф ¼ 1.27.
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reason of producing a chaotic detonation-wave distribution. In general, the test of d ¼ 1 mm has a stable detonation-wave velocity for Ф ¼ 0.88. With Ф ¼ 1.08 for all widths (Fig. 4b), it shows a better stability of detonation-wave velocity for d ¼ 1 mm and 1.5 mm, and other tests don't appear large fluctuation of detonation-wave velocity and keep relative stable. The tests of d ¼ 1 mm, 1.5 mm and 2 mm all have a better stability of detonation-wave velocity with Ф ¼ 1.27, and the scope of steady state is larger than the tests of Ф ¼ 0.88 and 1.08. In generally, the width of air-inlet slot has large effects on the propagation velocity of detonation wave, corresponding to the similar trend of detonation-wave pressure; and, it could improve the stability of detonation-wave velocity via increasing the equivalence ratio to some extent. Fig. 5 shows the average velocity (Vave) of detonation wave obtained by averaging the Vi for all experiments. For the experiments of Ф ¼ 0.88, it gets a largest average velocity of detonation wave (1689 m/s) with d ¼ 1 mm. With Ф ¼ 1.08, the tests of d ¼ 1 mm and d ¼ 1.5 mm get large detonation-wavevelocity values of 1795 m/sand 1700 m/s respectively. With Ф ¼ 1.27, the tests of d ¼ 1 mm, d ¼ 1.5 mm and d ¼ 2 mm get large detonation-wave-velocity values of 1826 m/s, 1755 m/s and 1750 m/s respectively. In the aforementioned experiments, the Vave initially increases and then decreases with the increase of slot width for the same equivalence ratio. Corresponding to the stability of detonation-wave pressure and propagation velocity, it indicates that the detonation wave is more stable, the more large propagation velocity can be got. To quantitatively describe the propagation stability of detonation wave, the equation of calculating the standard deviation (S) and the relative standard deviation (D) are used: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi #, u" n u X 2 t ðVi Vave Þ ðn 1Þ; S¼
(1)
1
D ¼ S=Vave ;
(2)
where n is the number of RDW propagation periods. The connection of relative standard deviation with slot width and equivalence ratio is shown in Fig. 6. The relative standard
Fig. 5 e Detonation-wave average velocity.
Fig. 6 e Detonation-wave velocity relative standard deviation.
deviation gets a relative small value about 2% for the tests of d ¼ 1 mm and which is hardly effected by equivalence ratios, which indicates its good stability. The relative standard deviation is largely effected by equivalence ratios with d 1 mm; for example, the equivalence ratio is more high, and the relative standard deviation is more small and the detonation wave is more stable for the tests of d ¼ 2 mm. In the tests of d ¼ 3 mm, the relative standard deviation is 12.7% with Ф ¼ 0.88, which is far larger than the value of other tests, resulting in the chaotic pressure (Fig. 3f) and unstable velocity distribution (Fig. 4a). For the tests of d ¼ 0.5 mm, which has a smallest width of air-inlet slot, but its relative standard deviation is larger than the tests of d ¼ 1 mm; this indicates that there is a other factor, which is different from the reasons existing in large-scale slot tests, to influence on the stability of detonation wave.
Effects of air-inlet slot on RDW propagation characteristics The overall performance of detonation wave would be affected predominately by pressure ratio [33]. The air pressure ratio (PR) hydrogen pressure ratio (PF) and are defined as follows: PR ¼ PAir =PC ;
(3)
PF ¼ PH2 PC;
(4)
The statistics of PR for the experiments are shown in Fig. 7. There is unblocked at the combustor exit, so the pressure of combustor is relative lowdwhich is a little more than ambient pressure. Because the blockage ratio of injection section improves with the increase of slot width, the PR has a maximum value for the tests of d ¼ 0.5 mm, and the air pressure ratio decrease with the increase of d. With air as oxidiser and hydrogen as fuel, the fuel influences very little on the fill rate of reactants [13]. Anand et al. [13] thought that the fill height of recovering reactant exceeding the detonation height could cause auto ignition of fresh mixture, thereby leading to the formation of second detonation wave. The oblique waves
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Fig. 7 e Air-injection pressure ratio.
would be induced during the propagation process of detonation waves, which could enter into the plenums via the injection channel [22]; meanwhile, part of oblique waves may be reflected back to combustor by the wall of injection structure, and it has a high intensity of pressure for the small airinjection slot width. Fig. 8 shows the tests of d ¼ 0.5 mm have strong reflected waves, and it has a significant velocity loss of detonation wave in these tests (more than 20%). The intensity of pressure waves which come from combustor is different with the different slots by the structure. Fig. 9 shows the results of high-frequency pressure waves in air plenum, and the intensity is increased with the increase of the width of slot. The plenum pressure pays an important role to ensure the reactants to be injected into combustor and defense the backpressure. The Fig. 7 shows that the air-injection pressure ratio is only 1.2e1.3, which is weak to defense the backpressure of detonation, so that the backpressure would slow down the gas feeding, resulting in reducing the intensity of detonation wave; and then, the backpressure would decrease and the gas feeding can be recovered. Therefore, an interaction between air plenum and combustor is formed [30]. As shown in Fig. 10, the velocity of
Fig. 9 e High-frequency pressure in air plenum (Ф ¼ 1.08).
Fig. 10 e Air-injection pressure and detonation-wave velocity (d ¼ 2.5 mm &Ф ¼ 1.07).
Fig. 8 e Detonation-wave pressure curves with d ¼ 0.5 mm &Ф ¼ 1.08.
detonation wave has a same oscillation trend with the pressure of air plenum during the RDE operation process. It indicates that the strong interaction between air plenum and combustor is formed with the large-scale slot, resulting in an unstable detonation wave. Based on the injection structure of RDE in this study, the distance between hydrogen-injection exit surface and center body would be increased with the increase of the air-inlet slot width, i.e., the length that hydrogen can verticality inject to center body would be increased, which is defined as hydrogen-injection distance. For the non-premixed injection structure, the hydrogen concentration is never uniformly distributed in combustor [21]. The hydrogen-concentration
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Fig. 11 e Hydrogen-injection pressure ratio. distribution would be effected with the increase of hydrogeninjection distance. The concentration of hydrogen near the inner wall of combustor is less than outer wall. If the hydrogen-injection distance is relative large and the hydrogen-injection pressure is relative low, the hydrogen concentration near the inner wall could less than the limit of initiation. It indicates that the poor mixing effect of reactant could be the reason of producing unstable detonation wave for the tests of d ¼ 2.5 mm and 3 mm. Fig. 11 shows the hydrogen-injection pressure ratios, the equivalence ratios are controlled by the hydrogen mass flow rate. Not all hydrogen can penetrate the air flayer to arrive the center body because of air flow. The injection pressure
increases with the increase the hydrogen mass flow rate, so the power to penetrate the air becomes strong, resulting in forming a good mixing effect. As shown in Figs. 3 and 4, for the tests of d ¼ 1.5 mm with Ф ¼ 0.88, and d ¼ 2 mm with Ф ¼ 0.88 and 1.08, they all have a low hydrogen-injection pressure ratio, resulting in a poor mixing effect of reactant and a unstable detonation wave; however, for the tests of d ¼ 1.5 mm with Ф ¼ 1.08 and 1.27, and d ¼ 2 mm with Ф ¼ 1.27, they all have a relative high hydrogen-injection pressure ratio, so the power to penetrate the air is strong, resulting in a good mixing effect of reactant and a stable detonation wave. The air-inlet slot of d ¼ 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave. The stability of rotating detonation is dependent on the mixing level, the total pressure of the air and hydrogen, and also the chamber back pressure. For the six slots, the chamber back pressure keeps constant (1 atm). As shown in Fig. 11, the total pressure of hydrogen has a minor difference for the six slots and mainly depends on the mass flow rate. The total pressure of air decreases with the increase of slot width, as shown in Fig. 7. The numerical results of Ref. [21], which has a similar geometry, shows the mixing performance is directly related to the air injection area (width of air slot). With the decrease of slot width, the majority of air flow deflects the fuel jet towards the inner wall. The experimental results of Ref. [20] show the narrow slot tends to produce multiple detonation waves, and the injection velocity of air may be the primary factor. As shown in Fig. 12, the detonation wave propagates steadily in combustor for all experimental equivalence ratios, it indicates that the slot of d ¼ 1 mm can reach an optimum performance for detonation combustor.
Fig. 12 e High-frequency pressure for d ¼ 1 mm with different equivalence ratios. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Conclusions [5]
The experiments were performed on a H2/air RDE with variable air-inlet slot to investigate the stability of RDW. Six widths of air-inlet slot (d ¼ 0.5, 1, 1.5, 2, 2.5, 3 mm) were selected for experiments, and all were successfully initiated, the conclusions are summarized as follows:
[6]
[7] [8]
(1) The stability of detonation-wave pressure and velocity had a similar trend which initially increased and then decreased with the increase of slot width, and it could improve the stability of detonation-wave velocity via increasing the equivalence ratio to some extent because of the mixing effect of reactant. (2) Through analyzing the average velocity of detonation wave for all the tests, it showed that the velocity initially increased and then decreased with the increase of slot width, and the detonation wave was more stable, the more large propagation velocity could be got. The intensity of reflected wave was strong for the tests of d ¼ 0.5 mm. The strong interaction between air plenum and combustor and bad mixing effect may be the reasons of forming unstable detonation wave for the tests of large-scale slots. (3) The air-inlet slot of d ¼ 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave. This slot reaches an optimum performance for detonation combustor and can give a guidance for the design of RDE.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgment 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), the Fundamental Research Funds for the Central Universities of China (No. 30915118836), and the Aerospace science and technology innovation foundation of China Aerospace Science and Technology Corporation for funding this study.
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hydrogen-oxygen continuously rotating detonation engine. Chin J Aeronaut 2015;28:669e75. [31] Nakagami S, Matsuoka K, Kasahara J, Matsuo A, Funaki I. Experimental study of the structure of forward-tilting rotating detonation waves and highly maintained combustion chamber pressure in a disk-shaped combustor. Proc Combust Inst 2016;36:2673e80. [32] Lin W, Zhou J, Liu S, Lin Z. An experimental study on CH4/O2 continuously rotating detonation wave in a hollow combustion chamber. Exp Therm Fluid Sci 2015;62:122e30. [33] Schwer D, Kailasanath K. Numerical investigation of the physics of rotating-detonation-engines. Proc Combust Inst 2011;33:2195e202.
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Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot Shengbing Zhou a, Hu Ma a,*, Shuai Li b, Changsheng Zhou a, Daokun Liu c a
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Guiyang Aero Engine Design Research Institute, Guiyang 550081, China c Shanghai Space Propulsion Technology Research Institute, Shanghai 201109, China b
article info
abstract
Article history:
Rotating detonation engines have attracted considerable attentions in recent years. In this
Received 7 September 2017
study, the experiments of initiating rotating detonation waves were performed on a H2/air
Received in revised form
rotating detonation wave with the variable air-inlet slot. The results showed that the
11 April 2018
stability of detonation-wave pressure and velocity both initially increased and then
Accepted 4 May 2018
decreased with the increase of slot width, and it could improve the stability of detonation-
Available online xxx
wave velocity via increasing the equivalence ratio. The intensity of reflected wave was strong for the tests of d ¼ 0.5 mm, which leaded to the advance ignition of fresh mixture
Keywords:
and a velocity deficit reaching up to 20%. The strong interaction between air plenum and
Rotating detonation engine
combustor and bad mixing effect may be the reasons of forming unstable detonation wave
Rotating detonation wave
for the tests of large-scale slots. The air-inlet slot of d ¼ 1 mm, which got a best experiment
Air-inlet slot
results relative to other tests, had a wide equivalence-ratio scope to produce stable deto-
Stability
nation wave. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction At present, the power and propulsion systems based on detonation mainly are pulse detonation engines (PDEs) [1,2], standing detonation engines (SDEs) [3] and rotating detonation engines (RDEs) [4,5]. Comparing with the PDEs, the RDEs have advantages of a high operation frequency, stable thrust, small axial size, and the ability to produce continuous detonation waves with the only one-time ignition, i.e., RDEs rely on the ignition systems a little. Comparing with the SDEs, the RDEs can work in the condition of wide flight mach numbers, and have hardly any limits of mach numbers. Besides, RDEs can perform thrust vector control via adjusting the mass flow rate of reactants in different azimuthal locations. Therefore,
the RDEs have a promising application in the field of aerospace and weaponry. RDEs have attracted considerable attentions in recent years, and the related researches have been carried out in many countries, such as Russia, America, Poland, France, China, Japan, Singapore and South Korea [6,7]. Research on RDEs mainly includes fuel and oxidiser compositions [8e10], the structures of the rotating detonation wave (RDW), the working modes and self-sustained mechanism [11e15], the ignition methods [16e19], the effects of the reactant-injection conditions on the RDW propagation characteristics [20,21], the effects of the RDW on the plenumworking characteristics [22], the structures of rotating detonation combustor (RDC) [10,23], and the thrust performance [20,24,25] and application [26e29]. The premise to ensure RDEs operate steadily is obtaining stable RDWs; however, the
* Corresponding author. E-mail address: [email protected] (H. Ma). https://doi.org/10.1016/j.ijhydene.2018.05.033 0360-3199/© 2018 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 engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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conditions are strict. The stability of RDWs easily has a switcheroo to produce an unstable phenomenon, even extinguishing. There are many instabilities of RDWs. Anand et al. [12] revealed four fundamentally different instabilitiesdchaotic instability, waxing and waning instability, mode switching and longitudinal pulsed detonation instabilitydvisa a large number of experiments. Peng et al. [17] found a phenomenon that detonation-wave intensity appeared strong weak alternately in experiments. Liu et al. [30] found the detonation-wave peak values changed periodically and thought this unstable phenomenon was related to the variation of propellant's mass flow rate. The injection structure of RDEs has a direct influence on the detonation-wave propagation stability. Anand et al. [13] investigated the operation performance with three fuelinjection structure and the results showed that the fuel plate with the highest length-to-diameter ratio exhibited lean operation devoid of transitional or pop-out events; and further testing showed that the increase of air injection area increased the number of pop-out events. Frolov et al. [20] found that the number of detonation wave would be decreased with the increase of air-inlet slit width. Rankin et al. [14] revealed that the wave front was more concave as the air injection area was increased from low to intermediate values via a representative optically accessible RDE. In this study, the experiments of initiating RDW were performed on a H2/air RDE. The aim of this work is to study the RDW propagation characteristics via gradually changing the width of air-inlet slot, and analyze the reasons of producing the stable or unstable detonation wave. Finally, the experiment got an optimum slot with stable detonation wave in combustor. This work hoped to provide a reference for the research of RDEs.
annular combustor, with an inner diameter of 124 mm and an outer diameter of 136 mm, is used in this experiment. The RDW can be successfully initiated by the predetonator, thermal-jet tube, low-energy or high-energy spark plug, etc. The initiation way of detonation wave only influences on its formation process, and the operation characteristics of the engine model are independent of the ignition device [16]. Peng [17] obtained the ignition success rate of 94% using ordinary spark-plug device in experiments. Besides, the spark-plug device has advantages of easy to installation and simple control. The ignition energy of spark-up device is 50 mJ using in this experiment, and the working time of spark plug is 30 ms. The experiments also got a high success rate of ignition. Three piezoresistive pressure sensors, with the sensitivity of 0.5%FS, are used to measure the pressure inside the RDC (PC), the hydrogen plenum (PH2) and the air plenum (PAir). The dynamic piezoelectric pressure sensors (PCB, 113B24), with the sensitivity of 10%, are installed on the outside wall of the RDC to measure the pressure trend of the RDW. The response time of piezoelectric transducer is lower than 1 ms, and the inherent frequency is more than 500 kHz. The location of sensors (p1, p2, p3 &p4) is shown in Fig. 1. Besides, a dynamic piezoelectric pressure sensor is mounted on the wall of air plenum, where closed to air-injection slot, to measure the high-frequency pressure wave which comes from combustor. 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.
Experimental facilities
Experimental results and analysis
The experimental system is composed of a hydrogen supply system, an air supply system, an RDE, an ignition device, a data-acquisition system, and a timing-sequence control system, as shown in Fig. 1. The injection modes of oxidiser and fuel are slot-slot type, slot-orifice type and orifice-orifice type; among them, the slotorifice type is generally used in experiments. A slot-orifice injection configuration is used in experiments for this study, as shown in Fig. 1. Hydrogen, which is used as fuel, is injected into chamber through 120 orifices uniformly distributed in front of combustor, and air, which is functioned as an oxidiser, is injected into combustion chamber through an annular slot. The width of air-inlet slot (d) could be changed by the slot spacer at the front end of center body. Six widths of air-inlet slot (d ¼ 0.5, 1, 1.5, 2, 2.5, 3 mm) are selected for experiments to investigate the detonation-wave propagation stability. The structure of combustion chamber in RDE is different with other propulsion system. It is demonstrated that the structure patterns, which can be used as combustion chamber, are disk-shaped combustor [31], annular combustor and hollow combustor [32]; among them, the annular combustor is used the most and has a good operation performance. An
RDE operation process Multiple experiments were carried out to verify the repeatability of results for every width of air-inlet slot. Three equivalence ratios (Ф ¼ 0.88, 1.08, 1.27) are used to study the effects on the stability of RDW. To ensure the air and hydrogen mass flow rate remains relatively stable before the operation of the RDC, the supply systems operate 0.8 s in advance of the RDC. The dynamic piezoelectric pressure sensor is unable to function for a long time in the experiment because of the high temperature in combustor; thus, the operation time of RDC is set at 0.4 s, which is controlled by the operation time of supply systems. Two sonic nozzles are respectively set at the manifold of the hydrogen and air supply system to control the mass flow rate of hydrogen and air. The air mass flow rate keeps constant value of 153 g/s in experiments, and the equivalence ratios are controlled by the hydrogen mass flow rate. Fig. 2a shows the static pressure curves during the operation of RDE. Fig. 2b shows the results of pressure wave which acquired via dynamic piezoelectric pressure sensors. The value of p1 is obviously greater than other sensors, this is because the p1 is located in the scope of detonation wave height, and others may close to the region of expansion.
Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Fig. 1 e Schematic diagram of the experiment system.
Fig. 2 e Pressure trace for the operation process.
Fig. 3 shows the experimental results (p1) with different widths of air-inlet slot. The experimental results of d ¼ 0.5 mm (Fig. 3a) show the unstable state of detonation wave which easily produces a secondary-peak-pressure phenomenon; however, the trend of pressure for d ¼ 1 mm (Fig. 3d) is generally stable which indicates the experiments get a good result. When d ¼ 1.5 mm (Fig. 3c), it could get a stable pressure of detonation wave with Ф ¼ 1.08 and 1.27, besides Ф ¼ 1.08 which produces an unstable detonation phenomenon. Further increase of the width of slot (d ¼ 2.0 mm), the detonation wave becomes unstable when Ф ¼ 1.08 and 1.27, as shown in Fig. 3d. Finally, it can't get a stable detonation wave with d ¼ 2.5 mm
and 3 mm, especially the trend of pressure is different to the trend of detonation wavedsimilar to the pressure of oscillation combustion. Therefore, the width of air-inlet slot has significant effects on detonation-wave pressure, and the equivalence ratio has a particular function for the stability of detonation wave. The reasons of existing different pressure characteristics of detonation wave will be discussed later.
RDW stability analysis The propagation velocity of detonation wave will keep stable if the pressure has a stable trend. The detonation-wave
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Fig. 3 e Pressure trace of detonation wave.
instantaneous velocity (Vi) distribution can be obtained from the two time intervals of two adjacent detonation-wave pressure peaks; this way is widely used to study the propagation characteristics of detonation wave [12,28]. Fig. 4 shows the instantaneous velocity distribution of detonation wave. There are some differences of detonation-wave velocity distribution for each air-inlet slot, and the distribution trend of Vi
is also largely effected by the equivalence ratio. As shown in Fig. 4a (Ф ¼ 0.88), the velocity of detonation wave for all airinlet width tests has a fluctuation to some extent, and the distribution is most chaotic for the tests of d ¼ 2.5 mm and 3 mm. The peaks of oscillation pressure produced by unstable deflagration combustion would confound the calculating of detonation-wave instantaneous velocity [12], which is the
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Fig. 4 e Detonation-wave velocity distribution: a) Ф ¼ 0.88. b)Ф ¼ 1.08. c) Ф ¼ 1.27.
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reason of producing a chaotic detonation-wave distribution. In general, the test of d ¼ 1 mm has a stable detonation-wave velocity for Ф ¼ 0.88. With Ф ¼ 1.08 for all widths (Fig. 4b), it shows a better stability of detonation-wave velocity for d ¼ 1 mm and 1.5 mm, and other tests don't appear large fluctuation of detonation-wave velocity and keep relative stable. The tests of d ¼ 1 mm, 1.5 mm and 2 mm all have a better stability of detonation-wave velocity with Ф ¼ 1.27, and the scope of steady state is larger than the tests of Ф ¼ 0.88 and 1.08. In generally, the width of air-inlet slot has large effects on the propagation velocity of detonation wave, corresponding to the similar trend of detonation-wave pressure; and, it could improve the stability of detonation-wave velocity via increasing the equivalence ratio to some extent. Fig. 5 shows the average velocity (Vave) of detonation wave obtained by averaging the Vi for all experiments. For the experiments of Ф ¼ 0.88, it gets a largest average velocity of detonation wave (1689 m/s) with d ¼ 1 mm. With Ф ¼ 1.08, the tests of d ¼ 1 mm and d ¼ 1.5 mm get large detonation-wavevelocity values of 1795 m/sand 1700 m/s respectively. With Ф ¼ 1.27, the tests of d ¼ 1 mm, d ¼ 1.5 mm and d ¼ 2 mm get large detonation-wave-velocity values of 1826 m/s, 1755 m/s and 1750 m/s respectively. In the aforementioned experiments, the Vave initially increases and then decreases with the increase of slot width for the same equivalence ratio. Corresponding to the stability of detonation-wave pressure and propagation velocity, it indicates that the detonation wave is more stable, the more large propagation velocity can be got. To quantitatively describe the propagation stability of detonation wave, the equation of calculating the standard deviation (S) and the relative standard deviation (D) are used: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi #, u" n u X 2 t ðVi Vave Þ ðn 1Þ; S¼
(1)
1
D ¼ S=Vave ;
(2)
where n is the number of RDW propagation periods. The connection of relative standard deviation with slot width and equivalence ratio is shown in Fig. 6. The relative standard
Fig. 5 e Detonation-wave average velocity.
Fig. 6 e Detonation-wave velocity relative standard deviation.
deviation gets a relative small value about 2% for the tests of d ¼ 1 mm and which is hardly effected by equivalence ratios, which indicates its good stability. The relative standard deviation is largely effected by equivalence ratios with d 1 mm; for example, the equivalence ratio is more high, and the relative standard deviation is more small and the detonation wave is more stable for the tests of d ¼ 2 mm. In the tests of d ¼ 3 mm, the relative standard deviation is 12.7% with Ф ¼ 0.88, which is far larger than the value of other tests, resulting in the chaotic pressure (Fig. 3f) and unstable velocity distribution (Fig. 4a). For the tests of d ¼ 0.5 mm, which has a smallest width of air-inlet slot, but its relative standard deviation is larger than the tests of d ¼ 1 mm; this indicates that there is a other factor, which is different from the reasons existing in large-scale slot tests, to influence on the stability of detonation wave.
Effects of air-inlet slot on RDW propagation characteristics The overall performance of detonation wave would be affected predominately by pressure ratio [33]. The air pressure ratio (PR) hydrogen pressure ratio (PF) and are defined as follows: PR ¼ PAir =PC ;
(3)
PF ¼ PH2 PC;
(4)
The statistics of PR for the experiments are shown in Fig. 7. There is unblocked at the combustor exit, so the pressure of combustor is relative lowdwhich is a little more than ambient pressure. Because the blockage ratio of injection section improves with the increase of slot width, the PR has a maximum value for the tests of d ¼ 0.5 mm, and the air pressure ratio decrease with the increase of d. With air as oxidiser and hydrogen as fuel, the fuel influences very little on the fill rate of reactants [13]. Anand et al. [13] thought that the fill height of recovering reactant exceeding the detonation height could cause auto ignition of fresh mixture, thereby leading to the formation of second detonation wave. The oblique waves
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Fig. 7 e Air-injection pressure ratio.
would be induced during the propagation process of detonation waves, which could enter into the plenums via the injection channel [22]; meanwhile, part of oblique waves may be reflected back to combustor by the wall of injection structure, and it has a high intensity of pressure for the small airinjection slot width. Fig. 8 shows the tests of d ¼ 0.5 mm have strong reflected waves, and it has a significant velocity loss of detonation wave in these tests (more than 20%). The intensity of pressure waves which come from combustor is different with the different slots by the structure. Fig. 9 shows the results of high-frequency pressure waves in air plenum, and the intensity is increased with the increase of the width of slot. The plenum pressure pays an important role to ensure the reactants to be injected into combustor and defense the backpressure. The Fig. 7 shows that the air-injection pressure ratio is only 1.2e1.3, which is weak to defense the backpressure of detonation, so that the backpressure would slow down the gas feeding, resulting in reducing the intensity of detonation wave; and then, the backpressure would decrease and the gas feeding can be recovered. Therefore, an interaction between air plenum and combustor is formed [30]. As shown in Fig. 10, the velocity of
Fig. 9 e High-frequency pressure in air plenum (Ф ¼ 1.08).
Fig. 10 e Air-injection pressure and detonation-wave velocity (d ¼ 2.5 mm &Ф ¼ 1.07).
Fig. 8 e Detonation-wave pressure curves with d ¼ 0.5 mm &Ф ¼ 1.08.
detonation wave has a same oscillation trend with the pressure of air plenum during the RDE operation process. It indicates that the strong interaction between air plenum and combustor is formed with the large-scale slot, resulting in an unstable detonation wave. Based on the injection structure of RDE in this study, the distance between hydrogen-injection exit surface and center body would be increased with the increase of the air-inlet slot width, i.e., the length that hydrogen can verticality inject to center body would be increased, which is defined as hydrogen-injection distance. For the non-premixed injection structure, the hydrogen concentration is never uniformly distributed in combustor [21]. The hydrogen-concentration
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Fig. 11 e Hydrogen-injection pressure ratio. distribution would be effected with the increase of hydrogeninjection distance. The concentration of hydrogen near the inner wall of combustor is less than outer wall. If the hydrogen-injection distance is relative large and the hydrogen-injection pressure is relative low, the hydrogen concentration near the inner wall could less than the limit of initiation. It indicates that the poor mixing effect of reactant could be the reason of producing unstable detonation wave for the tests of d ¼ 2.5 mm and 3 mm. Fig. 11 shows the hydrogen-injection pressure ratios, the equivalence ratios are controlled by the hydrogen mass flow rate. Not all hydrogen can penetrate the air flayer to arrive the center body because of air flow. The injection pressure
increases with the increase the hydrogen mass flow rate, so the power to penetrate the air becomes strong, resulting in forming a good mixing effect. As shown in Figs. 3 and 4, for the tests of d ¼ 1.5 mm with Ф ¼ 0.88, and d ¼ 2 mm with Ф ¼ 0.88 and 1.08, they all have a low hydrogen-injection pressure ratio, resulting in a poor mixing effect of reactant and a unstable detonation wave; however, for the tests of d ¼ 1.5 mm with Ф ¼ 1.08 and 1.27, and d ¼ 2 mm with Ф ¼ 1.27, they all have a relative high hydrogen-injection pressure ratio, so the power to penetrate the air is strong, resulting in a good mixing effect of reactant and a stable detonation wave. The air-inlet slot of d ¼ 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave. The stability of rotating detonation is dependent on the mixing level, the total pressure of the air and hydrogen, and also the chamber back pressure. For the six slots, the chamber back pressure keeps constant (1 atm). As shown in Fig. 11, the total pressure of hydrogen has a minor difference for the six slots and mainly depends on the mass flow rate. The total pressure of air decreases with the increase of slot width, as shown in Fig. 7. The numerical results of Ref. [21], which has a similar geometry, shows the mixing performance is directly related to the air injection area (width of air slot). With the decrease of slot width, the majority of air flow deflects the fuel jet towards the inner wall. The experimental results of Ref. [20] show the narrow slot tends to produce multiple detonation waves, and the injection velocity of air may be the primary factor. As shown in Fig. 12, the detonation wave propagates steadily in combustor for all experimental equivalence ratios, it indicates that the slot of d ¼ 1 mm can reach an optimum performance for detonation combustor.
Fig. 12 e High-frequency pressure for d ¼ 1 mm with different equivalence ratios. Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Conclusions [5]
The experiments were performed on a H2/air RDE with variable air-inlet slot to investigate the stability of RDW. Six widths of air-inlet slot (d ¼ 0.5, 1, 1.5, 2, 2.5, 3 mm) were selected for experiments, and all were successfully initiated, the conclusions are summarized as follows:
[6]
[7] [8]
(1) The stability of detonation-wave pressure and velocity had a similar trend which initially increased and then decreased with the increase of slot width, and it could improve the stability of detonation-wave velocity via increasing the equivalence ratio to some extent because of the mixing effect of reactant. (2) Through analyzing the average velocity of detonation wave for all the tests, it showed that the velocity initially increased and then decreased with the increase of slot width, and the detonation wave was more stable, the more large propagation velocity could be got. The intensity of reflected wave was strong for the tests of d ¼ 0.5 mm. The strong interaction between air plenum and combustor and bad mixing effect may be the reasons of forming unstable detonation wave for the tests of large-scale slots. (3) The air-inlet slot of d ¼ 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave. This slot reaches an optimum performance for detonation combustor and can give a guidance for the design of RDE.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgment 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), the Fundamental Research Funds for the Central Universities of China (No. 30915118836), and the Aerospace science and technology innovation foundation of China Aerospace Science and Technology Corporation for funding this study.
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Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033
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Please cite this article in press as: Zhou S, et al., Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.033