Experimental research on initiation characteristics of a rotating detonation engine

Experimental Thermal and Fluid Science 71 (2016) 154–163

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Experimental research on initiation characteristics of a rotating detonation engine Yang Chenglong, Wu Xiaosong, Ma Hu ⇑, Peng Lei, Gao Jian School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

a r t i c l e

i n f o

Article history: Received 17 June 2015 Received in revised form 19 October 2015 Accepted 19 October 2015 Available online 23 October 2015 Keywords: Rotating detonation engine Ignition method Ignition energy Detonation formation time Operation mode

a b s t r a c t An experimental study on a rotating detonation engine model using hydrogen/air mixture as propellant was conducted to analyze the initiation characteristics of detonation wave. Three ignition methods, including the ordinary spark plug, high-energy spark plug and thermal-jet tube, were used in the tests. The initiation process of rotating detonation wave was recorded and analyzed by a high frequency pressure measurement system and high-speed photographs. Operating range of the model engine was determined by series of experiments. Results indicate that the model engine could be successfully run by all the three ignition methods. Although the detonation formation time using spark plugs is quite stochastic, increasing the ignition energy can reduce the formation time of the detonation wave. Among the three ignition methods, thermal-jet provides the shortest detonation formation time. It is also found that the operating characteristic of the model engine is independent of the ignition device in the same operating condition. Three operation modes of the engine model have been observed, which are failed initiation of the detonation, unstable detonation and continuous rotating detonation. The experimental results show that the stable operating range can be extended by increasing the fuel mass flow rate. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The primary form of chemical energy conversion for most airbreathing and rocket propulsion devices comes from deflagration of fuel and oxidant. In the past several decades, the propulsive efficiency of constant-pressure combustion engines has been significantly improved, and further improvement of the propulsive efficiency becomes more and more difficult. Due to its rapid energy release rate, the theoretically thermal cycle efficiency of detonation combustion is much greater than the performance of conventional deflagration-based propulsion systems such as the gas turbine engine (GTE) [1–3]. For example, the ideal detonation combustion efficiency of acetylene/air mixture with an initial compression ratio of 5 is 61.4%, while the isobaric cycle with the same condition is only about 36.9%. The significant increase in cycle efficiency has motivated many of the studies on detonative combustion, and the concept of detonation-based propulsion has also attracted widespread attention in recent years. The rotating detonation engine (RDE) produces thrust by utilizing one or several detonation waves continuously propagating in the azimuthal direction around the annular combustion chamber. Comparing ⇑ Corresponding author. Tel.: +86 15005169412. E-mail address: [email protected] (H. Ma). http://dx.doi.org/10.1016/j.expthermflusci.2015.10.019 0894-1777/Ó 2015 Elsevier Inc. All rights reserved.

with the pulse detonation engine (PDE), the operation frequency of a RDE is up to several thousand hertz. The higher operating frequency makes the RDE more similar to the continuously operating engines, so its propulsive performance is higher and more stable than PDE. Another feature of RDE is the detonation wave can be established by only igniting once during the operation. Once a fully developed detonation wave is established, it will sustain itself and propagate continuously as long as sufficient reactants are supplied. Moreover, the deflagration to detonation transition (DDT) could be achieved through the circumferentially traveling combustion wave, so that the complex ignition device and specialized enhancement device of the DDT are not necessary in the RDE. In the 1960s, Voitsekhovskii et al. [4,5] presented the feasibility of rotating detonation wave for propulsive applications at the Lavrent’ev Institute of Hydrodynamics (LIH). Experiments were conducted for stoichiometric acetylene/oxygen mixture in a diskshaped chamber, and the detonation was initiated by an electrical spark. In 1966, Nicholls et al. [6] presented the RDE experiments for gaseous hydrogen/oxygen and methane/oxygen in a smallscale annular rocket motor. A baffle plate was added in the chamber to prevent denotation waves propagating in two directions. Unfortunately, continuous operation of the model motor was not achieved for the limited mass flow and heterogeneous mixtures. Since 1970s, systematic experimental research of the RDE was

C. Yang et al. / Experimental Thermal and Fluid Science 71 (2016) 154–163

carried out by Bykovskii et al. [7–10]. Extremely stable detonation wave was observed within a wide range of propellant components, mass flow rate, geometric combustion chambers and ambient pressure. And different ignition methods were utilized including the spark plug, electric detonator, glowing wire and predetonator. Kindracki et al. [11] found that the automotive spark plug had only 40% repeatability rate for a methane/oxygen mixture in the combustion annulus. However, in the same engine, the repeatability could be increased to 95% by using a pre-detonator and breakable diaphragm. But the diaphragm should be replaced before the next ignition, and it was not convenient to start the engine again. Thomas [12], Russo [13] and Shank [14] studied the operating range and propagation characteristic of rotating detonation wave experimentally. Miller et al. [15] performed some tests to evaluate the development of detonation wave with the schlieren apparatus. They discovered that there was no a strong relation between the inclination of the pre-detonator tube entering the channel and the successful detonation transition. The wave would decouple immediately after coming from the predetonator, and the reflected wave off the bottom plate could reinitiate the detonation wave. Liu et al. [16,17] successfully ignited the engine model by using a tangentially injected hydrogen/oxygen hotshot jet. The hotshot tube contained two convergent sections and had a Shchelkin spiral for the DDT. They found that rotating detonation waves could propagate in three modes: single wave mode, single/dual-wave hybrid mode, and dual-wave mode. Presently, the RDE has been extensively studied experimentally, numerically and theoretically [18–21]. Although a variety of ways to initiate the rotating detonation wave have been attempted and considerable success has been achieved, the initiation condition and formation mechanism of rotating detonation wave are still not completely understood. Using a pre-detonator tube can have a higher success and repetition rate, but it needs a long straight pipe to ensure that the DDT process is accomplished in the tube. Other initiation methods such as those using the electric detonator, glowing wire and explosive mass have disadvantages in the engine restart. To better understand the formation of rotating detonation wave, series of experiments were carried out with different ignition devices, which will be presented in this paper. The propellant applied in this research is hydrogen/air, and the ignition devices used are the ordinary automotive spark plug, high-energy spark plug and thermal-jet tube respectively. The initiation characteristics of the rotating detonation wave are recorded and analyzed by a high frequency pressure measurement system and highspeed photographs.

2. Experimental facility and methodology Fig. 1 shows the schematic diagram of the experimental facility, the whole test rig includes a propellant feed system, an engine model, a control system, an ignition system and a measurement system. The propellant feed system consists of a high-pressure gas source, a pressure reducing valve, a solenoid valve, a Venturi tube, a check valve and several pipes. The Venturi tube was validated by a gas vortex flow meter to ensure the accurate control of the propellant mass flow rate. Fig. 2 shows the schematic of the detonation chamber and the injection structure of fuel and oxidant. The engine has an annular combustion chamber with the inner and outer diameters of 70 mm and 80 mm, and the length of chamber is 40 mm, the combustor outlet is connected directly to the atmosphere. Fuel and oxidant are injected separately into the chamber. Air is injected through a convergence-divergence annulus from the oxidant plenum, and the width of the throat is 0.5 mm. Hydrogen enters the chamber through 90 orifices which

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are distributed uniformly at the inner wall of the combustion chamber. Each of the orifices has a diameter of 0.8 mm. Piezoresistive pressure transducer and dynamic piezoelectric transducer (CY-YD-205) are used to measure the pressure inside the hydrogen/air plenums and the combustion chamber respectively. The response time of piezoelectric transducer is lower than 2 ls and the inherent frequency is more than 100 kHz. Fig. 2 shows the locations of the chamber pressure transducers and the ignition devices, the distance between these two transducers and the combustion chamber inlet is 8 mm, and the installation pores (P1, P2) have a circumferential angle of 120°. The spark plug is mounted vertically at the outer surface of the engine, and the ignition energy of the ordinary automotive spark plug and high-energy spark plug are 30 mJ and 3 J respectively. The thermal-jet tube is filled with a mixture of hydrogen and oxygen and started by an ordinary spark plug. The hot jet is injected into the chamber tangentially. The hydrogen and oxygen are injected separately into the tube and their mass flow rates are 0.14 g/s and 1.12 g/s respectively, corresponding to an equivalence ratio of 1. The injecting time of the propellants is 0.7 s, and the spark plug is triggered 0.1 s after finishing the injection process. The whole length of the tube is about 100 mm. And there is a convergent section in it to accelerate the hot jet. NI X series multifunction DAQ is used in the experiments for data acquisition. The data acquisition card (USB-6366) based on NI-STC3 synchronization technology has 8 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. A CCD camera (Phantom v210) is used to allow intuitively observing the initiation and propagation of the detonation wave. The maximum frame rate of this camera is up to 300,000 fps and the minimum exposure time is about 2 ls. The time sequence of propellant supply and ignition timing is controlled by a self-developed program. In this work, the operation time is set to 0.4 s and the sampling frequency is 1 MS/s. 3. Experimental results and discussions In order to compare the formation process of the rotating detonation wave, same operating condition was chosen in Sections 3.1–3.3. The hydrogen mass flow was 4.52 g/s, the air mass flow rate was 102.5 g/s, and the equivalence ratio / was 1.51. 3.1. Initiation process using automotive spark plug Fig. 3 shows the combustion chamber and manifold pressure histories during a typical test process. Fig. 4 is the enlargement of pressure profiles at P1 position of Fig. 3. It can be seen from Fig. 4 that the ignition signal was triggered at 1601.086 ms, due to the low energy of automotive spark plug, the reactant was not formed into detonation wave directly. So at the preliminary stage of the ignition process, the propellant was burning in the deflagration form, and the pressure had little change in the combustion chamber. As the flame area and energy release rate were increased rapidly by the turbulence caused by the channel surface roughness, curvature and the injected gas flow, the flame was accelerating gradually. The accelerating flame in the circumferential direction delivered compression waves to the anterior combustible gas. Small pressure spikes caused by the compression waves appeared soon in the figure. The amplitude of pressure variation became larger with the continual development of the flame. Finally, the pressure value rose up suddenly at 1602.862 ms forming the first pressure peak. The entire time from ignition to detonation wave established, tdet, was 1.776 ms. With the rapid increase of the

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(a) Schematic diagram of the experimental system

(b) Cross-section of the detonation chamber Fig. 1. The experimental system and combustion chamber.

combustion chamber pressure, the pressure near the detonation wave was higher than the injection pressure and a portion of orifices and annulus in this area were blocked. The volume of propellant source tanks upstream were large enough to keep the mass flow rates stable during the whole test process, so the pressure in the hydrogen and air plenums (ph2, pair) rose up with the decrease of injection area. It was more clearly for the increase of ph2, because ph2 was much lower than pair, the chamber pressure had a greater influence on the hydrogen injection. The solenoid valve was shut off at about 2007 ms, then the manifold pressure began to go down. Since the upstream pipelines between the plenums and the solenoid valves still had some explosive gases, the rotating detonation wave could propagate for a while, and extinguished gradually from the time of 2060 ms. In this case, the rotating detonation wave had propagated continuously for about 0.46 s. It was slightly larger than the set value of 0.4 s for the influence of the extra propellant. Fig. 5(a) shows the pressure profiles versus time at stable propagation stage. As shown in the figure, the value of pressure peaks changed from 0.71 MPa to 0.99 MPa, and all the pressure peaks of p2 were located behind p1. Same phenomenon could also be seen in the other periods of time, indicating that the detonation wave was propagating around the circumference continuously in one direction during the whole working time. The average propagation

velocity of the detonation wave in one circle, which can be regarded as the instantaneous velocity, can be calculated by the interval time of the adjacent peaks Dt and the outer diameter of the combustion chamber Dout. The corresponding velocity curves of Fig. 5(a) is shown in (b), the instantaneous velocity of the detonation wave within 1663–1666 ms ranged from 1514 m/s to 1611 m/s. The propagation direction could also be judged from these two curves and the location of the pressure transducers. The interval time of adjacent pressure spikes of p1 and p2 Dt0  1=3Dt, indicating that the propagation direction was counter-clockwise. Fast Fourier Transform (FFT) analysis based on measured data of pressure transducers is illustrated in Fig. 6. The result revealed a dominant frequency f at 6263 Hz, and the average propagation velocity V could be calculated by V ¼ pDout f =n. In this formula, n represented the number of transverse detonation waves. Suppose that there was only one rotating detonation wave, n = 1, and the average propagation velocity of the detonation wave in the whole propagation process was estimated to be 1574 m/s, proving that there was only one detonation wave in the combustion chamber. All these above evidences indicated that the propagation of rotating detonation wave was quite stable in this case. Fig. 7 shows the images of detonation formation and stable propagation process. The frame rate of the CCD camera in this test

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(a) Schematic diagram of the combustion chamber

and circumferential direction and consumed the reactants. The combustion wave propagating along the axial direction was emitted out from the combustion chamber with the combustion products. But in the circumferential direction, the combustion wave could develop sequentially due to the continuous injection of combustible gas. With the acceleration of the flame, two flame fronts traveling in opposite directions were observed at 1601.97 ms. Then the flame fronts disappeared after the collision with each other and there was no flame in the chamber again. At 1602.803 ms, the detonation wave spreading continuously in one direction was captured by the camera for the first time. The building-up time of rotating detonation wave was in accord with the result of pressure curve, and the error may be caused by the installation location of transducers. Eight photographs of adjacent moment in stable propagation stage are shown in Fig. 7(b). The images are shown in order from left to right, top to bottom. It can be seen clearly that there was only one detonation wave traveling counter-clockwise in the channel. The rotation angle was about 250° during the whole interval time, based on the outer diameter of combustion chamber and the frame rate, the propagation velocity of rotating detonation wave was estimated to be 1496 m/s, which was consistent with the value calculated by the dominant frequency and the pressure curves. 3.2. Initiation process using high-energy spark plug

(b) View of the setup

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was 60,000 fps and the exposure time was 15 ls. The light spot in the red1 circle was the spark generated by the automotive spark plug. But soon there was no light in the photographs because the brightness of deflagration flame was too weak to capture (see t = 1601.353 ms). The deflagration wave spread out both in the axial

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Fig. 2. The research stand: 1 and 2 – piezoelectric transducers, 3 – piezoresistive transducer, 4 – high-energy spark plug, 5 – thermal-jet tube, 6 – RDE model, and 7 – feed lines.

Although the continuous rotating detonation can be obtained by an ordinary spark plug for the mixture of hydrogen and air, the light of combustion wave was too weak to determine the development of the flame from the high-speed camera experiment. In order to get the whole initiation process clearly, a high-energy spark plug and a hydrogen/oxygen thermal-jet tube were used as the ignition devices in the optical observation test. Moreover, the frame rate of CCD camera was set to 48,000 fps, and the exposure time was 19.8 ls so that the camera could improve the ability to capture the light in the chamber. Fig. 8(a) shows the pressure history of the initiation process using a high-energy spark plug as the ignition device. In this test, the formation of detonation took about 1.41 ms. The pressure signal fluctuated in a small range at the earlier stage of initiation process, which was similar to the results in Fig. 4, indicating that the high-energy spark plug also failed to achieve the detonation wave directly. The dominant frequency of pressure signal was 6296 Hz in this case, and the average propagation velocity of rotating detonation wave was about 1582 m/s. Fig. 7(b) shows the images of detonation formation process. The development of detonation wave for high-energy spark plug test

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Fig. 4. Enlarged graph of initiation process.

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wave would become weaker when they collided with each other. After a few times of collision, the weaker combustion wave vanished and there was only one flame front propagating in the chamber. Then the combustion wave continued to develop and became faster and brighter. The rotating detonation wave was established successfully till 1441.44 ms and began to propagate continuously in clockwise direction.

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Fig. 9 shows the initiation process of the rotating detonation wave using a thermal-jet tube as the ignition device. The dominant frequency of pressure signal and average propagation velocity were 6340 Hz and 1593 m/s, respectively. One wave head was observed in the channel during operation, and the result shows that ignition device has little effect on the number of wave heads and the average propagation velocity. As illustrated in Fig. 9(a), the initial stage of the pressure history was quite different from the results of Figs. 4 and 8. After initiating the RDE, there was a pressure peak which created by the combustion products from the thermal-jet tube, and detonation was formed after a period of irregular oscillation. It took just 1.118 ms to initiate the continuous rotating detonation wave, slightly lower than the tdet of highenergy spark plug. After the hot jet was ejected tangentially into the combustion chamber (from top to bottom in the image), the flame spread rapidly along the original direction, but the hot jet would also induce another flame front in the opposite direction. Because there was still sensitive H2/O2 mixture coming from the tube in a relatively short period of time, the chemical reaction rate and energy release rate near the exit of thermal-jet tube were great enough to induce new flame front in a while, and the flame of here was brighter than other place (Fig. 9(b), t = 1373.36 ms). In this stage, the number and distribution even the propagation direction of these flame fronts were changing erratically, but multi-flame fronts consumed more fresh hydrogen/air mixture, and the supply of combustible gas could not catch up with the amount burned in the combustion chamber. Combined with the interaction of combustion waves and wall, the number of flame fronts reduced to one gradually, then the flame was accelerating in counterclockwise direction and forming a continuously traveling rotating detonation wave.

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can be divided into three stages. Since the spark plug was installed vertically, two opposite direction of flame fronts were formed at the ignition moment. And the light of flame was weakened by the axial flow of propellants, which was similar to the phenomenon in Fig. 7(a). But it did not affect the development of the combustion wave in circumferential direction. After about 0.1 ms, two opposite direction of flame fronts appeared again in the images. The intensity and velocity of these two flame fronts was different, which can be seen clearly in Fig. 8(b) (from 1440.5 ms to 1440.54 ms). The velocity of flame 1 propagating clockwise was much higher than flame 2, and the strength of combustion

Repetitive experiments were conducted to analyze the formation time tdet for all the ignition devices. The experimental conditions were kept consistent with the last three sections. The statistical result is shown in Fig. 10, where cases 1–3 represent the ordinary spark plug, high-energy spark plug and thermal-jet respectively. Each case was repeated for 10 times, and the results indicated that the success rate of detonation initiation in this condition achieved 100%. As can be seen from Fig. 10, the distribution of tdet for cases 1 and 2 was quite stochastic in the same condition, but the average tdet for case 2 was shorter overall. The formation time of detonation for case 3 was relatively shorter and more concentrated than the other two cases. It seemed that tdet was concerned with the status of the initial flame. The initial flame produced by the spark plugs was a deflagration flame and the flame would propagate in the opposite direction. But the initial flame of case 2 was stronger for the higher ignition energy. However, the initiation process of case 3 was different from cases 1 and 2 because the initial flame produced by thermal-jet was a high-speed hot jet combined with a leading shock wave. The pressure and temperature of combustible gas were improved significantly with the compression effect of the leading shock wave, so

C. Yang et al. / Experimental Thermal and Fluid Science 71 (2016) 154–163

1601.086 ms

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the velocity and energy release rate of the initial flame was faster than the deflagration flame. As a result, it took less time to form the rotating detonation wave. Fig. 11 shows the statistical result of dominant frequency in all the thirty tests. The dominant frequency kept fairly stable in the same operating condition, ranging from 5793 Hz to 6364 Hz. So the average propagation velocity of detonation wave was 1456– 1600 m/s. Indicating that the experiments was repeatable, and the operating characteristic of the engine model was independent of the ignition device in the same operating condition. 3.5. Initiation range analysis In order to investigate the operating range of the experimental engine model, series of ignition tests were carried out by fixing the hydrogen mass flow rate at 3.23 g/s, 4.52 g/s and 5.81 g/s. All these tests used high-energy spark plug as the ignition device, and the air mass flow rate and equivalence ratio were controlled by changing the supply pressure of air upstream the engine. Fig. 12 shows the statistical result of ignition tests, it can be seen that the equivalence ratio and the propellant mass flux (ratio of the total mass flow rate and the combustion chamber cross sectional area) have a crucial effect on the engine operation mode. As the hydrogen mass flow rate increased from 3.23 g/s to 4.52 g/s, the operating range of equivalence ratio were widened from 0.95–1.8 to 0.78–2.06, and the stable operating range of equivalence ratio

1.08–1.80 expanded to 1.08–2.06. Three operation modes can be distinguished according to the pressure history of the combustion chamber. The first mode represents the failure of detonation initiation, and the reactant is burning by means of deflagration. The second mode is defined as unstable detonation, the detonation wave is initiated successfully in this case, but small interruption exists during the detonation wave propagation (see Fig. 13). The third mode is the successful detonation initiation and the detonation wave is traveling continuously, but the equivalence ratios of this mode are all greater than 1, rather than vicinity of the ideal stoichiometric ratio. Because the engine model uses a separate injection scheme, fuel and oxidant are mixing while burning in the combustion chamber, and hydrogen is injected through series of orifices located in the inner surface of the channel, so that the mixture of hydrogen and air is not homogeneous, especially in the area outside the air annulus exit. As a result, part of fuel is not reacted completely and the energy released by reaction is insufficient to maintain the continuous spread of detonation wave. So increasing the hydrogen mass flow rate and the equivalence ratio of reactant can obtain a higher energy release rate and reactant activity. Meanwhile, the inhomogeneous mixing of reactants can also lead to the failure of detonation initiation and unstable detonation in the low equivalence ratio cases. The pressure curves of unstable detonation at different equivalence ratio are illustrated in Fig. 13, both of the hydrogen mass flow rates in these two experiments were 4.52 g/s, and the equivalence

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ratios were 0.98 and 0.78 respectively. During the operation of engine, the value of pressure spikes had a remarkable change. With the decrease of the equivalence ratio, the average pressure in combustion chamber was reduced for the decrease of the reactants activity. The maximum peak pressure in Fig. 13(a) was up to 2.0 MPa, and (b) gave a maximum value of about 0.75 MPa. The remarkable oscillation of pressure value was another important reason for the detonation interruption. Typical phenomenon of the pressure oscillation is shown in Fig. 14, the instantaneous velocity of detonation wave within 1790–1795 ms was between 1200 and 1532 m/s. The velocity fluctuation in different cycles was caused by the change of fresh gaseous mixture height before the detonation wave. The axial height of the fresh reactants had a significant influence on the mixing quality [22]. If the mixture height was higher, there would be more time and more space for the gas diffusion to improve the mixing quality, and the larger heat release of detonation combustion gave rise to a stronger rotating detonation wave. The propagation velocity of detonation wave was faster when the detonation intensity improved, so the accumulation time of combustible gas in the next cycle was reduced. Meanwhile, a stronger detonation wave also brought a larger pressure in the combustion

chamber. The blocked area of annulus and orifice was larger for a larger chamber pressure, and a longer time was needed for the pressure of combustion product behind detonation wave to fall to the critical injection condition. Especially for the fuel plenum, the chamber pressure had a more obvious effect on its injection process. Sometimes, the pressure oscillation was observed in the continuous detonation mode for a while. But the pressure oscillation was regular and the range of pressure was not large. So the detonation wave could maintain the dynamic balance and propagating continuously. However, if the pressure oscillation was remarkable and irregular, like that of Fig. 13, the large pressure peak would lead to the mixture height in the next cycle to be too low, so that the energy would not be enough to maintain the detonation wave due to the inhomogeneous mixture, resulting in the decoupling of detonation wave. The average chamber pressure would decrease after the extinguishment of detonation wave, and the reduced pressure was more readily for the following injection, so the injection area of propellant would recover, and the mixing quality of fuel and oxidant would be improved. Since there was still combustion wave traveling circumferentially in the chamber, the rotating detonation wave could be formed again through the development of the flame.

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Fig. 10. Statistical graph of detonation formation time.

Fig. 11. Statistics of the dominant frequency with different ignition devices.

It is worth mentioning that in those unsuccessful tests which the equivalence ratios were too high, detonation phenomenon usually appeared in the chamber after the turnoff of the manifold solenoid valves (see Fig. 15). In this test, the hydrogen mass flow rate

was 4.52 g/s, the air mass flow rate was 61.5 g/s, with an equivalence ratio of 2.52. As shown in Fig. 15(a), the pressure of combustion chamber and accumulation channels had little change after ignition, the reaction was carried out based on isobaric combustion

C. Yang et al. / Experimental Thermal and Fluid Science 71 (2016) 154–163

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1700

1800

1900 Fig. 15. Restart of detonation wave.

t/ms

(b) Equivalence ratio=0.78 Fig. 13. Pressure curves of unstable detonation.

mode. The solenoid valves upstream of the engine were closed simultaneously at 1782 ms, and the mass flow rates of propellants began to decrease, but the air mass flow rate decreased more slowly, so the equivalence ratio decreased continuously, detonation was initiated when the equivalence ratio fell to the explosive range. As shown in Fig. 15(b), the detonation wave extinguished after traveling around the channel for more than 30 circles from 1851.242 ms to 1857.518 ms. The average peak pressure during

this time was about 0.51 MPa, and the propagation velocity decreased with the reduction of the reactant total mass flow. 4. Conclusion Three ignition methods were used to investigate the initiation characteristic and propagation process of rotating detonation wave experimentally. The feasibility of the use of spark plug was verified, and the operating range of the engine model was determined through a series of tests. The main conclusions are listed as follows:

C. Yang et al. / Experimental Thermal and Fluid Science 71 (2016) 154–163

(1) Successful run and stable traveling rotating detonation wave can be achieved using the ordinary automotive spark plug, high-energy spark plug and thermal-jet tube as the ignition device. The detonation formation time using spark plugs is quite stochastic. On the whole, increasing the ignition energy can reduce the formation time of detonation wave, and the detonation formation time with thermal-jet tube is shorter and more concentrated than the other two ignition methods. (2) The flow field inside the combustion chamber at initiation stage is complex, two flame fronts propagating in opposite direction are produced by the spark plugs, while the number, the distribution, and even the propagation direction of flame fronts induced by the thermal-jet tube are changing irregularly. But all of these flame fronts can be converted into a one-way combustion wave, and rotating detonation wave will be formed ultimately through flame acceleration. (3) In the same operating condition, the operating characteristic of the engine model is independent of the ignition device. The dominant frequency and average propagation velocity of detonation wave are relatively stable. (4) Three operation modes including failure of detonation initiation, unstable detonation and continuous rotating detonation are observed in the tests. The equivalence ratio and propellant mass flux have a significant impact on the engine operation mode. The continuous operating range is extended with the increase of fuel mass flow rate.

Acknowledgments This work was supported by the Natural Science Foundation of China (51376091). Special thanks to Chen Song in Nanyang Technological University and Zhao Yaqing in North Carolina State University for the advice on revising the English. References [1] P. Wolan´ski, Detonative propulsion, Proc. Combust. Inst. 34 (1) (2013) 125– 158. [2] F.K. Lu, E.M. Braun, Rotating detonation wave propulsion: experimental challenges, modeling, and engine concepts, J. Propul. Power 30 (5) (2014) 1125–1142.

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