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Experimental investigation on the operating characteristics in a multi-tube two-phase valveless air-breathing pulse detonation engine
Applied Thermal Engineering 73 (2014) 21e29
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation on the operating characteristics in a multi-tube two-phase valveless air-breathing pulse detonation engine Zhiwu Wang a, *, Jie Lu a, Jingjing Huang a, Changxin Peng b, Longxi Zheng a a b
School of Power and Energy, Northwestern Polytechnical University, Xi'an 710072, PR China China Aviation Powerplant Research Institute, Aviation Industry Corporation of China, Zhuzhou, Hunan 412002, PR China
h i g h l i g h t s The four-tube two-phase PDE was performed with maximum frequency of 30 Hz per tube. Four different firing patterns were successfully carried out in the four-tube PDE. The synchronicity of detonation waves under four firing patterns was quantified. The synchronicity of back-propagation waves under four firing patterns was compared.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2013 Accepted 7 July 2014 Available online 17 July 2014
A four-tube two-phase gasoline/air valveless air-breathing pulse detonation engine (PDE) was designed and tested to investigate the operating characteristics of multi-tube air-breathing PDE. The PDE was operated at a maximum frequency of 30 Hz per tube with a total maximum frequency of 120 Hz. Four different firing patterns were successfully carried out: single tube firing, dual tubes firing simultaneously, three tubes firing simultaneously, and four tubes firing simultaneously. The synchronicity of the detonation waves and back-propagation pressure waves under multi-tube firing patterns were quantified and compared. The averaged peak pressure of back-propagation pressure waves in the air plenum chamber was discussed. The experimental results indicated that the four-tube PDE could be operated successfully under the four firing patterns and transformed from one firing pattern to another. The time of arrival differences of detonation waves showed up in all multi-tubes firing patterns, which led to greater arrival differences of back-propagation pressure waves. The averaged peak pressure of back-propagation pressure waves in the air plenum chamber had the same change trend under multi-tube firing patterns. Single tube firing pattern resulted in the smallest pressure peak in the air plenum chamber and the pressure peak increased at the increased fired tubes. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Two-phase detonation Multi-tube pulse detonation engine Valveless Air-breathing Experimental
1. Introduction Pulse detonation engines (PDEs) are exciting propulsion devices which obtain thrust by generating detonations intermittently. The pressure rise associated with a detonation makes it an efficient combustion process when compared to a constant pressure deflagration in traditional gas turbine combustor. Due to the potential higher thermodynamic efficiency and mechanical simplicity, extensive efforts have been applied to develop practical PDEs since the early 1940's [1e3]. Considerable progress had been made in the
* Corresponding author. Tel.: þ86 029 88460756. E-mail addresses: [email protected], [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2014.07.018 1359-4311/© 2014 Elsevier Ltd. All rights reserved.
past few decades: reliable initiation of repeatable detonation waves had been achieved either by a initiator or a low energy spark which ignited the initial flame kernel and experienced a deflagration-todetonation transition (DDT) process [2,4]; different obstacles were extensively studied to accelerate the DDT [5e8]; many experimental studies were performed in order to improve the understanding of the thrust augmentation performance of nozzles or ejectors [9,10]; detonation initiation in more practical applications utilizing liquid fuels such as kerosene or JP10 attracted much attention and additional researches on two-phase detonation to aid in further development of liquid-fueled detonation engines were still needed [11e16]. More advanced concepts integrating a pulse detonation combustor into a traditional gas turbine engine had been proposed and performance calculations indicated potential
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Nomenclature PDE DDT ID
pulse detonation engine deflagration-to-detonation transition inner diameter
improvements on specific thrust and specific fuel consumption [17e19]. In these concepts, a traditional deflagration combustor is replaced by multiple pulsed detonation combustors. Many proposed applications of pulsed detonation into propulsion systems involve multi-tube operation, no matter in a simple ‘pure’ pulse detonation engine where the detonation products exhaust directly into the ambient through nozzles or in a hybrid pulse detonation turbine engine. The multi-tube system could offer many advantages. Firstly, in air-breathing applications, multi-tube configuration allows continual flow delivered from a common inlet and reduces the inlet loss associated with inlet airflow stagnation. Secondly, feeding the exhausts from multiple detonation tubes into a common nozzle reduces the degree of unsteadiness in the thrust and provides a more stable nozzle flow by increasing the nozzle exit pressure. Moreover, the benefits of multi-tube system also include higher overall engine frequency and the possibility of thrust vectoring. In the past years, there were many multi-tube PDE studies. Ebrahimi et al. [20e22] conducted a series of twodimensional simulation of multi-tube PDE. They found that the pressure induced by the detonation in adjacent tubes was nearly as large as that produced by the detonation itself. It was conjectured that pressure spike would adversely affect the fill process of the adjacent tube. A more comprehensive two-dimensional simulation of multi-tube PDE was carried out by Ma et al. [23]. They modeled the multi-cycle operation of PDE and revealed the benefits of precompression of refilled fresh reactants by shock waves originating from other tubes. Experimental investigations on multi-tube PDE have been reported by many research institutions. Hinkey at al [24] reported a dual-tube PDE system with a common exit nozzle which was successfully operated at frequencies of up to 12 Hz per tube. The system was primarily used to test the operability of a patented rotary valve inlet. Schauer et al. [4] designed a four-tube PDE system based on an automotive engine block. The engine served as a testbed for research of detonation initiation and DDT minimization, heat transfer, noise levels, pulsed ejector concepts, and multi-tube interactions. Yatsufusa et al. [25] conducted experimental investigation of wave interactions in a four-tube PDE firing into a common converging nozzle. The maximum operational frequency of the PDE was 30 Hz per tube with hydrogeneair mixture. They found that as the operating frequency increased, the multi-tube interaction became remarkable and DDT process was interfered by the waves reflected from adjacent tubes. Glaser et al. [26,27] conducted experimental investigation of thrust performance of a valved multitube PDE with different nozzle geometries and the pressure rise in the PDE was quantified. In order to investigate the performance of a hybrid propulsion system, Rasheed et al. [28e31] integrated a multi-tube pulse detonation combustor turbine system. This hybrid system consisted of eight tubes and operated with three firing patterns using ethylene-air mixtures. Their results showed that complex wave interactions with significant downstream tube-totube interactions affected its operability when using the sequential firing pattern. The air-valveless configuration allowed significant upstream propagation of pressure waves and backflow of detonation products with the possibility of upstream tube-to-tube interactions affecting operability. Caldwell et al. [32,33] also developed a similar system consisted of six-tubes PDE and a small axial turbine with ethylene and nitrogenediluted oxygen as the fuel
and oxidizer. They investigated the interaction between two combustor tubes through detonation wave speed measurements and the shadowgraph flow visualization. It was found that when two tubes were fired simultaneously, their decaying leading shocks coalesced into a larger shock front resembling a single shock emanating from the midpoint of the two detonation tubes. When a small time delay was introduced, the blowdown jet of one tube appeared to degrade the leading shock of the adjacent tube. The above mentioned numerical simulations had been focused on the wave interaction and detonation wave was directly initiated from the head end by driver gas with high temperature and pressure. Though detonation wave could be initiated by an initiator, this demanded additional oxidizer carried aboard. The simulation results could deviate from the characteristics of the real multi-tube PDE flow field. In experimental investigations cited above, gaseous fuels were used. However, in practical air-breathing applications, liquid hydrocarbon fuels were more preferred. It will significantly increase the DDT run-up time and distance by using liquid hydrocarbon fuel instead of gaseous fuel. Additional issues such as atomization, droplet breakup, partial vaporization, and incomplete mixing of fuel and air should be considered. Parameters such as droplet size and temporal and spatial distribution of droplets will play a major role [34], and how to efficiently inject liquid fuel and rapidly initiate two-phase detonation in a practical distance is still a challenge. Up to now, successful operation of multi-tube two-phase pulse detonation engine at high frequency has little been reported in the published literature. Only Peng et al. [35] reported an experimental study on a dual-tube two-phase PDE, and the operating frequency was only 12 Hz per tube. Li [36] conducted experimental investigation on Kerosene/Air Tripletube Aero-valve pulse detonation engine and found that the three tubes operated poorly under simultaneously firing pattern. This paper addressed a successful operation of a four-tube twophase valveless air-breathing PDE under different firing patterns. Gasoline and air were used as fuel and oxidizer respectively. The PDE system was operated at a maximum frequency of 30 Hz per tube with a total maximum frequency of 120 Hz in the system. Firstly, each tube was test individually. Then, three multi-tube firing patterns were performed to investigate the operability of the fourtube PDE: dual tubes firing simultaneously, three tubes firing simultaneously, and all the tubes firing simultaneously. The operating frequencies were varied from 5 Hz to 25 Hz. For all the tubes firing simultaneously, the maximum frequency was 30 Hz per tube. The synchronicities of detonation waves and back-propagation pressure waves under multi-tube firing patterns were quantitatively investigated. Additionally, the peak pressures of the backpropagation pressure waves in the common air inlet were compared and analyzed under the different multi-tube firing patterns. All of the above will provide some theoretical and experimental bases to improve the design level and accelerate the application of PDE. 2. Experimental setup A schematic of the experimental setup is shown in Fig. 1. The engine consisted of a circular array (0.155 m) of four detonation tubes. The tubes were mounted on a common air plenum chamber by flanges. A transition section was designed to match the inner diameter (ID，0.09 m) from the air supply exit plane to the larger ID (0.24 m) of the air plenum chamber inlet plane. Air was delivered into the common air plenum chamber through the transition section and divided into each tube uniformly guided by a big entrance cone placed on the center of the air plenum chamber. The length of the transition section and the air plenum chamber were 0.16 m and 0.4 m, respectively. Each tube consisted of four different sections:
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Fig. 1. Schematic of PDE experimental setup.
single tube air inlet, fuel supply and mixing chamber, ignition chamber, and detonation chamber. The lengths of each section were 0.3 m, 0.147 m, 0.276 m, and 1.0 m respectively. The inner diameters of the first two sections were 0.08 m and the others were 0.06 m. All sections were held together by flanges. An intake cone was placed in the middle of each tube fuel supply and mixing chamber to guide the flow and reduce the loss due to airflow stagnation. The end side of the cone partially closed the flow field and acted as thrust wall. Twin-fluid air-assist atomizers were used for gasoline injection. The gasoline/air mixture was initiated by an automotive spark plug located in middle of the ignition chamber. The ignition energy was less than 50 mJ. An in-house designed ignition system was used to control the firing patterns and frequency. In order to accelerate the deflagration-to-detonation transition process, the Shchelkin spiral with a length of 1 m was used in each of the detonation chambers. The diameter of the spiral was 0.008 m and the pitch was 0.05 m. A photo of the experimental apparatus is shown in Fig. 2(a). High frequency pressure transducers were installed along the flow path of each tube. At the end of each detonation chamber, high frequency piezoelectric pressure transducers（Type: CY-YD-205） (P1~P8) were installed with a spacing of 0.05 m to verify the successful initiation of detonation wave in each tube (Fig. 1). Piezoresistive pressure transducers (F1~F4) were placed in each single tube inlet to capture the back-propagation pressure wave dynamics during PDE firing operation. Another piezoresistive pressure transducer (F0) was located in top of the air plenum chamber to capture the wave interactions of back-propagation pressure waves in the common air inlet. F0 was directly in-line with tube 1. The locations of the pressure measurement points are shown in Figs. 1
and 2(b). All the data were monitored and collected through DEWE3020 high-speed data acquisition system for a total of 16 channels and the sampling rates was 200 K samples/s. The output signals from the pressure transducer were sampled with the Kistler data acquisition and processing system, in which the voltage value and the pressure value had the relationship of 1 V equaled 1 MPa. The calibration of pressure transducers was applied to convert the raw values to pressure values. These values had an error of ±72.5 mV/MPa. Therefore, the pressure measurement uncertainty was approximately ±7.25%. 3. Result and discussion First of all, each tube was tested individually. Then, in order to test the operability of the four-tube PDE, three different multi-tube firing patterns were performed: dual tubes firing simultaneously, three tubes firing simultaneously, and all the tubes firing simultaneously. Experimental test conditions are shown in Table 1. The PDE was operated without any cooling system except for the high frequency pressure transducers in the tube end. So when the PDE was operated at the frequency of 30 Hz, the maximum run time was limited to 2 min. Experimental results of four different firing patterns were presented in this part. Fig. 3 shows the tubes involved under different firing patterns. All the experimental results were divided into three sections. The first section contained analyzing of the operability of the engine when all the tubes fired simultaneously at operating frequencies from 5 Hz to 30 Hz. The second section showed the results of single tube firing pattern and discussed the propagation of back-propagation pressure waves into the air plenum chamber. The last section quantified and compared
Fig. 2. (a) Photograph of four-tube PDE (back view) (b) schematic of pressure sensors location (front view).
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Table 1 The flow rate at different operating conditions. Frequency (Hz)
Fuel flow rate (kg/s 103)
Air flow rate (kg/s)
Single tube
Dual tubes
Three tubes
Four tubes
Single tube
Dual tubes
Three tubes
Four tubes
5 10 15 20 25 30
3.26 4.13 5.87 8.92 10.66
6.67 8.27 11.60 15.73 18.27
9.72 14.50 18.27 22.33 28.06
11.75 22.62 25.74 35.60 42.63 48.00
0.184 0.262 0.374 0.502 0.656
0.190 0.239 0.344 0.449 0.593
0.164 0.272 0.344 0.443 0.656
0.141 0.269 0.370 0.459 0.623 0.688
the synchronicity of the detonation waves and back-propagation pressure waves in the single tube inlet under multi-tube firing patterns. The peak value of back-propagation pressure waves in the air plenum chamber under different firing patterns were investigated and compared. The effects of synchronicity of the detonation waves and back-propagation pressure waves on the wave interactions in the air plenum chamber were discussed respectively. 3.1. Four-tube PDE operation verification
Fig. 4. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 5 Hz.
Figs. 4e9 show the raw pressure traces from the last piezoelectric pressure transducers of each tube (P2, P4, P6, P8) when all tubes were fired simultaneously at operating frequency from 5 Hz to 30 Hz. What's more, by measuring the elapsed time that a detonation wave traveled across two successive pressure transducer ports with a fixed distance, the velocity of the leading shock wave was determined. The averaged wave velocities between the two transducers at tube 1 to tube 4 were 1264.2 ± 169.9 m/s, 1128.3 ± 91.0 m/s, 1157.2 ± 138.8 m/s, 1236.5 ± 143.2 m/s at current work. For detonation in gaseous fuels, the forming of the detonation wave can be determined by the velocity of the leading shock wave, the peak pressure and the rise time of the peak pressure. But in two-phase detonation, the detonation velocity is lower than the CeJ detonation velocity. Wall losses, incomplete combustion before the CeJ plane, and energy loss due to rearward propagating waves have all been proposed as reasons for the observed velocity deficits [15]. So the peak pressure is the main criteria to determine the formation of detonation wave. In all tests, the averaged peak pressure of P2 to P8 were higher than 1.9 MPa which was the theoretical value of C8H18 and air mixture at experimental conditions calculated by the NASA CEA400 code [34], which indicated that the detonation wave were successfully obtained. Fig. 10 shows the photograph of the engine exhaust plume when four tubes fired simultaneously at operating frequency of 30 Hz. What's more, all the tubes could stably operate under other firing patterns. Although the pressure profiles are not given here, four-tube PDE was still carried out successfully under other firing patterns and could be
operated under one firing pattern to another firing pattern by controlling the supply of fuel. Therefore the engine would have a wide range of thrust through controlling the number of fired tubes. 3.2. Single tube firing pattern A DDT event usually arises from a sufficiently strong microexplosion or the interaction of multiple explosions. Then leftrunning retonation wave and right-running detonation wave were formed [13]. For valveless PDE, the retonation wave will propagate into the common air inlet and cause pressure oscillation [30]. Unlike the valved PDE in which all of the combustion product is forced to exhaust through the open exit because the valve interface is completely closed during the certain period of each cycle, part of the combustion product can propagate back into the upstream inlet in valveless PDE because the flow path is only partly closed. The interaction between combustor and inlet would incur significant performance losses and possibly lead to an inlet unstart [37]. Fig. 11 shows the enlargement of raw pressure traces of one detonation cycle in tube 1 at the operating frequency of 15 Hz. The dotted lines indicate directions of the upstream-propagating pressure waves. First of all, the back-propagation pressure waves reached the single air inlet of tube 1 and caused a large pressure spike at F1 (Line 1). The largest peak pressure could reach about 0.22 MPa. Then the back-propagation pressure waves propagated
Fig. 3. Schematic of different firing patterns.
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Fig. 5. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 10 Hz.
Fig. 7. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 20 Hz.
into a large space (the air plenum chamber), and the amplitude of the pressure spike was greatly reduced to 0.064 MPa (Line 5). Additionally, the back-propagation pressure waves could diffracted into the adjacent tube when it propagated into the air plenum chamber at the same time due to the valveless configuration at the head of PDE. Then the direction of the pressure waves changed and was in line with the inlet flow direction. These were the tube-totube interactions at the head end which could be observed by the pressure rise at F2, F3 and F4 (Line 2~Line 4).
3.3.1. Synchronism of detonation waves Fig. 12 shows the enlargement of raw pressure traces of one detonation cycle for all the tubes fired simultaneously at frequency
of 30 Hz. Since the transducers P2, P4, P6 and P8 were placed symmetrically, the detonation waves initiated in the four tubes should exhaust the four tubes at exactly the same time for the simultaneous firing patterns. However, the detonation waves showed a time of arrival difference. The maximum time arrival difference between detonation waves in Fig. 12 is approximately 1.7 ms. Additionally, the time of arrival difference showed up in all multi-tubes firing patterns. This was likely due to the slight difference in the deflagration-to-detonation process in each of the tubes resulted from minor tube-to-tube geometric differences, and perhaps more significantly, flow differences due to airflow differences to each tube, fuel injection flow rate and wall wetting. Fig. 13 summarizes the averaged time interval (△t1) between peak pressure P2 and P4 over 20 cycles under multi-tube firing patterns, in which the error bars represent the standard deviation. For dual tubes firing pattern, the time interval decreased with the increasing
Fig. 6. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 15 Hz.
Fig. 8. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 25 Hz.
3.3. Multi-tube firing patterns
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Fig. 9. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 30 Hz.
frequency which was consistent with previous work of dual tube PDE experiments [35]. However, when more tubes were fired, this relation was not applicable any more. The wave interactions between different tubes became more significant when more tubes were fired and tube-to-tube flow difference became more obvious which caused larger variation on the synchronism of detonation waves. The averaged time interval obtained in all the tests was greater than the experimental result obtained by Rasheed [30] in which they found that the time of arrival difference was approximately 0.2 ms relative to one another. In our tests, liquid gasoline was used as fuel instead of ethylene in Rasheed's experiments. For two-phase detonation, atomization, vaporization, and the mixing of fuel and oxidizer played major roles in the DDT process. It was impossible to ensure that they were exactly the same in each tube, and little differences would lead to large effect on the DDT process which may cause larger time intervals. Fig. 14 shows comparisons of the averaged time intervals of P2eP6 and P2eP4 under multi-tube firing patterns. It seems that the time interval of P2eP6 shows a similar variation trend with that of P2eP4 when the operating frequency increased. The synchronism of detonation waves was closely related to the synchronism of back-propagation pressure waves. Therefore the time of arrival difference of detonation waves would lead to the arrival difference of back-propagation waves which may be even more greater. A comparison between the time intervals of
Fig. 10. Photograph of the engine fired simultaneously at operating frequency of 30 Hz.
Fig. 11. Pressure time history for tube 1 fired alone at frequency of 15 Hz.
detonation waves (△t1) and the intervals of peak values between back-propagation waves at F1 and F2 (△t2) are shown in Fig. 15. The data was also averaged over 20 operation cycles when all tubes fired simultaneously. It was obvious that △t1 and △t2 had the same variation trend and △t2 was bigger than △t1 at each operating frequency. 3.3.2. Wave Interactions in the air plenum chamber One important feature of the four-tube PDE was valveless configuration. No mechanical valves were placed between inlet and combustor. The system was mechanically simpler and circumvents
Fig. 12. Pressure time history when all tubes firing simultaneously at operating frequency of 30 Hz.
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Fig. 13. Degree of synchronism of the detonation waves in tube 1 and tube2 at different frequencies under (a) dual tubes firing pattern, (b) three tubes firing pattern, (c) four tubes firing pattern and (d) multitube firing patterns.
Fig. 14. Comparison of the degree of synchronism of detonation waves (P2eP4) and (P2eP6) at different frequencies under dual tube firing pattern.
Fig. 15. Comparsion of time intervals between detonation waves and back-propagation waves under four tubes firing pattern.
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Fig. 16. Raw pressure traces of F0 for different firing patterns at operating frequency of 15 Hz.
the disadvantages associated with airflow stagnation during the valve-closed period [38]. However, one disadvantage was that the strong retonation wave may propagate freely into the upstream plenum and have an adverse impact on the performance. Fig. 16 shows the raw pressure traces at F0 under different firing patterns at operating frequency of 15 Hz. As expected, single tube firing pattern resulted in the smallest pressure peak and the pressure peak increased with the increasing of the fired tubes number. It could be interpreted as follows. First of all, when single tube was fired, the diffraction of the back-propagation pressure waves would bring part of the back flow into the non-detonate tubes, thus reduced the peak pressure in the air plenum chamber. When the number of fired tubes increased, the non-detonate tubes for mitigating the pressure oscillation in the air plenum chamber decreased, which would lead to an increase of peak pressure in the air plenum chamber. Secondly, the wave interactions in the air plenum chamber became more intensive with the increasing of the fired tubes number and the constructive interference between back-propagation pressure waves would lead to greater pressure peak under simultaneous firing patterns. It's worth noting that there existed an arrival difference of back-propagation pressure waves caused by the asynchronism of detonation waves (Fig. 15).
The effect of this difference allowed the constructive and destructive interference which would result in the large variation in peak pressure. Fig. 17 shows the averaged peak pressure at F0 under different firing patterns at operating frequencies from 5 Hz to 25 Hz. All curves had the same change trend. The averaged peak pressure increased with the increasing of frequency on the whole but showed oscillation at certain frequencies. When the operating frequency increased, the mass flow rate of supplied air increased which would raise the total pressure of the coming air. Multi-tube numerical simulation conducted by Ripley et al. [39] also revealed that the amplitude of the pressure oscillations at the plenum inlet was a function of mass flow rate through the engine. Moreover, the purging time of each cycle would reduce when the frequency increased, so the pressure in the engine had no sufficient time to decay. However, due to the arrival difference of back-propagation pressure waves caused by the asynchronism of detonation waves, the peak pressure in the air plenum chamber was greatly affected by the constructive and destructive interference and showed large variation. For larger arrival difference of back-propagation waves (Fig. 15 at frequency of 20 Hz), destructive interference became obvious and caused a drop in averaged peak pressure (Fig. 17 at frequency of 20 Hz). This was the main reason why the averaged peak pressure showed oscillation at certain frequencies. If the arrival difference of detonation wave increased such as firing all tubes sequentially, the peak pressure would be greatly reduced, which would be investigated in the future. The reduction of pressure oscillation is important for the practical application, like hybrid pulse detonation turbine engine. Large pressure oscillation in the air plenum chamber could adversely affect compressor performance, especially lead to stall. 4. Conclusions
Fig. 17. Averaged peak pressures in common air inlet for different operating patterns.
A four-tube two-phase valveless air-breathing PDE was designed and tested. Gasoline and air were used as fuel and oxidizer respectively. The PDE system was operated at a maximum frequency of 30 Hz per tube with a total maximum frequency of 120 Hz. Four different firing patterns were carried out: single tube firing, dual tubes firing simultaneously, three tubes firing simultaneously, and all the tubes firing simultaneously. The operability of the engine was firstly discussed. Then, the propagation of backpropagation pressure waves at single tube firing pattern were discussed. The synchronicity of the detonation waves and backpropagation pressure waves under multi-tube firing patterns were quantified and compared. The averaged peak values of back-
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propagation pressure waves in the air plenum chamber under different firing patterns were compared. The effect of synchronicity of the detonation waves and back-propagation pressure waves on peak pressure in the air plenum chamber was also discussed. The experimental results showed that the four-tube PDE could be successfully operated under the four firing patterns and transformed from one firing pattern to another. Hence, the four-tube PDE would have a wide range of thrust by controlling the number of fired tubes. Investigation of propagation of back-propagation pressure waves showed that the largest pressure peak was about 0.22 MPa in the single tube inlet for single tube firing pattern and the amplitude of pressure spike was greatly reduced to 0.064 MPa when the pressure propagated into the air plenum chamber. A time of arrival difference of detonation waves showed up in all multitubes firing patterns. For dual tubes firing, the time interval decreased with the increasing frequency which was consistent with previous work of dual tube PDE experiments. However, when more tubes were fired, this relation was not applicable any more. The wave interactions between different tubes became more significant when more tubes were fired and the randomness of the detonation waves became more obvious. The synchronism of detonation waves were closely related to the synchronism of backpropagation pressure waves and the time of arrival difference of detonation waves led to greater arrival difference of backpropagation pressure waves. The averaged peak pressures in the air plenum chamber had the same change trend under multi-tubes firing patterns. Single tube firing pattern resulted in the smallest pressure peak in the air plenum chamber and the pressure peak increased at the increased fired tubes. All of the above results will provide some theoretical and experimental bases to improve the design level and accelerate the application of PDE.
Acknowledgements This work was supported by the National Natural Science Foundation of China through Grant No. (51306153), the Natural Science Foundation of Shanxi Province of China through Grant No. (2014JM7258), Doctoral Fund of Ministry of Education of China (grant number 20116102120027) and Northwestern Polytechnical University Foundation for Fundamental Research (grant number NPU-FFR-JCY20130129).
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Merkle, Modeling of Multi-tube Pulse Detonation Engine Operation, 2001. AIAA Paper 2001-3813. [21] H.B. Ebrahimi, F.J. Malo-Molina, Simulation of 2-D and 3-D Multitube Pulse Detonation Engines with Conical Nozzle and Different Splitter Plates, 2005. AIAA Paper 2005-1305. [22] H.B. Ebrahimi, C.L. Merkle, Wave reverberations in multitube pulse detonation engines, J. Propul. Power 24 (2) (2008) 345e352. [23] F. Ma, J.-Y. Choi, V. Yang, Thrust chamber dynamics and propulsive performance of multitube pulse detonation engines, J. Propul. Power 21 (4) (2005) 681e691. [24] J.B. Hinkey, J.T. Williams, S.E. Henderson, T.R. Bussing, Rotary-valved, Multiple-cycle, Pulse Detonation Engine Experimental Demonstration, 1997. AIAA Paper 1997-2746. [25] T. Yatsufusa, K. Nishimura, T. Yamaguchi, et al., Experiments on inter-tube interference in a four-tube pulse detonation engine, in: Proceedings of the 20th ICDERS, McGill University, Montreal, Canada [CD-ROM], 31 Julye5 August 2005. [26] A. Glaser, A. Rasheed, R. Dunton, V. Tangirala, Investigations of Thrust Generated by a Valved, Multi-tube PDE with Exit Nozzles, 2008. AIAA Paper 2008-4692. [27] A.J. Glaser, A. Rasheed, R. Dunton, V. Tangirala, Measurement of Pressure-rise in a Pulse Detonation Engine, 2009. AIAA Paper 2009-857. [28] A. Rasheed, A. Furman, A.J. Dean, Experimental Investigations of an Axial Turbine Driven by a Multi-tube Pulsed Detonation Combustor System, 2005. AIAA Paper 2005-4209. [29] A. Rasheed, A. Furman, A.J. Dean, Wave Interactions in a Multi-tube Pulsed Detonation Combustor-turbine Hybrid System, 2006. AIAA Paper 2006-4447. [30] A. Rasheed, A. Furman, A.J. Dean, Pressure measurements and attenuation in a hybrid multitube pulse detonation turbine system, J. Propul. Power 25 (1) (2009) 148e161. [31] A. Rasheed, A. Furman, A.J. Dean, Experimental investigations of the performance of a multitube pulse detonation turbine system, J. Propul. Power 27 (3) (2011) 586e596. [32] N. Caldwell, A. Glaser, E. Gutmark, Acoustic interactions of a pulse detonation engine array with a gas turbine, in: 44th AIAA Aerospace Sciences Meeting & Exhibit, Reno, NV, 2006. AIAA Paper 2006-1233. [33] N. Caldwell, R. Brunet, A. Antoine, et al., Experimental analysis of a hybrid pulse detonation combustor/gas turbine engine, in: 46th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 2008. AIAA Paper 2008-121. [34] S. Gordon, B.J. McBride, Computer program for calculation of complex chemical equilibrium compositions and applications, in: National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Program, 1996. [35] C. Peng, W. Fan, L. Zheng, Z. Wang, C. Yuan, Experimental investigation on valveless air-breathing dual-tube pulse detonation engines, Appl. Therm. Eng. 51 (2013) 1116e1123. [36] J.-Z. Li, J.-H. Wang, H. Tang, L. Yuan, Kerosene/Air triple-tube aero-valve pulse detonation engine, Acta Aeronout. Astronaut. Sin. 30 (11) (2009) 2052e2058 (in Chinese). [37] F. Ma, J.-Y. Choi, V. Yang, Internal flow dynamics in a valveless airbreathing pulse detonation engine, J. Propul. Power 24 (3) (2008) 479e490. [38] J. Gustavsson, V. Nori, C. Segal, Inlet/engine interactions in an axisymmetric pulse detonation engine system, J. Propul. Power 19 (2) (2003) 282e286. [39] R. Ripley, P. Harris, R. Farinaccio, Multi-tube two-dimensional evaluation of a pulse detonation engine as a ramjet replacement, in: 40th AIAA/ASME/SAE/ ASEE Joint Propulsion Conference, 2004. AIAA 2004-3745.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation on the operating characteristics in a multi-tube two-phase valveless air-breathing pulse detonation engine Zhiwu Wang a, *, Jie Lu a, Jingjing Huang a, Changxin Peng b, Longxi Zheng a a b
School of Power and Energy, Northwestern Polytechnical University, Xi'an 710072, PR China China Aviation Powerplant Research Institute, Aviation Industry Corporation of China, Zhuzhou, Hunan 412002, PR China
h i g h l i g h t s The four-tube two-phase PDE was performed with maximum frequency of 30 Hz per tube. Four different firing patterns were successfully carried out in the four-tube PDE. The synchronicity of detonation waves under four firing patterns was quantified. The synchronicity of back-propagation waves under four firing patterns was compared.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2013 Accepted 7 July 2014 Available online 17 July 2014
A four-tube two-phase gasoline/air valveless air-breathing pulse detonation engine (PDE) was designed and tested to investigate the operating characteristics of multi-tube air-breathing PDE. The PDE was operated at a maximum frequency of 30 Hz per tube with a total maximum frequency of 120 Hz. Four different firing patterns were successfully carried out: single tube firing, dual tubes firing simultaneously, three tubes firing simultaneously, and four tubes firing simultaneously. The synchronicity of the detonation waves and back-propagation pressure waves under multi-tube firing patterns were quantified and compared. The averaged peak pressure of back-propagation pressure waves in the air plenum chamber was discussed. The experimental results indicated that the four-tube PDE could be operated successfully under the four firing patterns and transformed from one firing pattern to another. The time of arrival differences of detonation waves showed up in all multi-tubes firing patterns, which led to greater arrival differences of back-propagation pressure waves. The averaged peak pressure of back-propagation pressure waves in the air plenum chamber had the same change trend under multi-tube firing patterns. Single tube firing pattern resulted in the smallest pressure peak in the air plenum chamber and the pressure peak increased at the increased fired tubes. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Two-phase detonation Multi-tube pulse detonation engine Valveless Air-breathing Experimental
1. Introduction Pulse detonation engines (PDEs) are exciting propulsion devices which obtain thrust by generating detonations intermittently. The pressure rise associated with a detonation makes it an efficient combustion process when compared to a constant pressure deflagration in traditional gas turbine combustor. Due to the potential higher thermodynamic efficiency and mechanical simplicity, extensive efforts have been applied to develop practical PDEs since the early 1940's [1e3]. Considerable progress had been made in the
* Corresponding author. Tel.: þ86 029 88460756. E-mail addresses: [email protected], [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2014.07.018 1359-4311/© 2014 Elsevier Ltd. All rights reserved.
past few decades: reliable initiation of repeatable detonation waves had been achieved either by a initiator or a low energy spark which ignited the initial flame kernel and experienced a deflagration-todetonation transition (DDT) process [2,4]; different obstacles were extensively studied to accelerate the DDT [5e8]; many experimental studies were performed in order to improve the understanding of the thrust augmentation performance of nozzles or ejectors [9,10]; detonation initiation in more practical applications utilizing liquid fuels such as kerosene or JP10 attracted much attention and additional researches on two-phase detonation to aid in further development of liquid-fueled detonation engines were still needed [11e16]. More advanced concepts integrating a pulse detonation combustor into a traditional gas turbine engine had been proposed and performance calculations indicated potential
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Nomenclature PDE DDT ID
pulse detonation engine deflagration-to-detonation transition inner diameter
improvements on specific thrust and specific fuel consumption [17e19]. In these concepts, a traditional deflagration combustor is replaced by multiple pulsed detonation combustors. Many proposed applications of pulsed detonation into propulsion systems involve multi-tube operation, no matter in a simple ‘pure’ pulse detonation engine where the detonation products exhaust directly into the ambient through nozzles or in a hybrid pulse detonation turbine engine. The multi-tube system could offer many advantages. Firstly, in air-breathing applications, multi-tube configuration allows continual flow delivered from a common inlet and reduces the inlet loss associated with inlet airflow stagnation. Secondly, feeding the exhausts from multiple detonation tubes into a common nozzle reduces the degree of unsteadiness in the thrust and provides a more stable nozzle flow by increasing the nozzle exit pressure. Moreover, the benefits of multi-tube system also include higher overall engine frequency and the possibility of thrust vectoring. In the past years, there were many multi-tube PDE studies. Ebrahimi et al. [20e22] conducted a series of twodimensional simulation of multi-tube PDE. They found that the pressure induced by the detonation in adjacent tubes was nearly as large as that produced by the detonation itself. It was conjectured that pressure spike would adversely affect the fill process of the adjacent tube. A more comprehensive two-dimensional simulation of multi-tube PDE was carried out by Ma et al. [23]. They modeled the multi-cycle operation of PDE and revealed the benefits of precompression of refilled fresh reactants by shock waves originating from other tubes. Experimental investigations on multi-tube PDE have been reported by many research institutions. Hinkey at al [24] reported a dual-tube PDE system with a common exit nozzle which was successfully operated at frequencies of up to 12 Hz per tube. The system was primarily used to test the operability of a patented rotary valve inlet. Schauer et al. [4] designed a four-tube PDE system based on an automotive engine block. The engine served as a testbed for research of detonation initiation and DDT minimization, heat transfer, noise levels, pulsed ejector concepts, and multi-tube interactions. Yatsufusa et al. [25] conducted experimental investigation of wave interactions in a four-tube PDE firing into a common converging nozzle. The maximum operational frequency of the PDE was 30 Hz per tube with hydrogeneair mixture. They found that as the operating frequency increased, the multi-tube interaction became remarkable and DDT process was interfered by the waves reflected from adjacent tubes. Glaser et al. [26,27] conducted experimental investigation of thrust performance of a valved multitube PDE with different nozzle geometries and the pressure rise in the PDE was quantified. In order to investigate the performance of a hybrid propulsion system, Rasheed et al. [28e31] integrated a multi-tube pulse detonation combustor turbine system. This hybrid system consisted of eight tubes and operated with three firing patterns using ethylene-air mixtures. Their results showed that complex wave interactions with significant downstream tube-totube interactions affected its operability when using the sequential firing pattern. The air-valveless configuration allowed significant upstream propagation of pressure waves and backflow of detonation products with the possibility of upstream tube-to-tube interactions affecting operability. Caldwell et al. [32,33] also developed a similar system consisted of six-tubes PDE and a small axial turbine with ethylene and nitrogenediluted oxygen as the fuel
and oxidizer. They investigated the interaction between two combustor tubes through detonation wave speed measurements and the shadowgraph flow visualization. It was found that when two tubes were fired simultaneously, their decaying leading shocks coalesced into a larger shock front resembling a single shock emanating from the midpoint of the two detonation tubes. When a small time delay was introduced, the blowdown jet of one tube appeared to degrade the leading shock of the adjacent tube. The above mentioned numerical simulations had been focused on the wave interaction and detonation wave was directly initiated from the head end by driver gas with high temperature and pressure. Though detonation wave could be initiated by an initiator, this demanded additional oxidizer carried aboard. The simulation results could deviate from the characteristics of the real multi-tube PDE flow field. In experimental investigations cited above, gaseous fuels were used. However, in practical air-breathing applications, liquid hydrocarbon fuels were more preferred. It will significantly increase the DDT run-up time and distance by using liquid hydrocarbon fuel instead of gaseous fuel. Additional issues such as atomization, droplet breakup, partial vaporization, and incomplete mixing of fuel and air should be considered. Parameters such as droplet size and temporal and spatial distribution of droplets will play a major role [34], and how to efficiently inject liquid fuel and rapidly initiate two-phase detonation in a practical distance is still a challenge. Up to now, successful operation of multi-tube two-phase pulse detonation engine at high frequency has little been reported in the published literature. Only Peng et al. [35] reported an experimental study on a dual-tube two-phase PDE, and the operating frequency was only 12 Hz per tube. Li [36] conducted experimental investigation on Kerosene/Air Tripletube Aero-valve pulse detonation engine and found that the three tubes operated poorly under simultaneously firing pattern. This paper addressed a successful operation of a four-tube twophase valveless air-breathing PDE under different firing patterns. Gasoline and air were used as fuel and oxidizer respectively. The PDE system was operated at a maximum frequency of 30 Hz per tube with a total maximum frequency of 120 Hz in the system. Firstly, each tube was test individually. Then, three multi-tube firing patterns were performed to investigate the operability of the fourtube PDE: dual tubes firing simultaneously, three tubes firing simultaneously, and all the tubes firing simultaneously. The operating frequencies were varied from 5 Hz to 25 Hz. For all the tubes firing simultaneously, the maximum frequency was 30 Hz per tube. The synchronicities of detonation waves and back-propagation pressure waves under multi-tube firing patterns were quantitatively investigated. Additionally, the peak pressures of the backpropagation pressure waves in the common air inlet were compared and analyzed under the different multi-tube firing patterns. All of the above will provide some theoretical and experimental bases to improve the design level and accelerate the application of PDE. 2. Experimental setup A schematic of the experimental setup is shown in Fig. 1. The engine consisted of a circular array (0.155 m) of four detonation tubes. The tubes were mounted on a common air plenum chamber by flanges. A transition section was designed to match the inner diameter (ID，0.09 m) from the air supply exit plane to the larger ID (0.24 m) of the air plenum chamber inlet plane. Air was delivered into the common air plenum chamber through the transition section and divided into each tube uniformly guided by a big entrance cone placed on the center of the air plenum chamber. The length of the transition section and the air plenum chamber were 0.16 m and 0.4 m, respectively. Each tube consisted of four different sections:
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Fig. 1. Schematic of PDE experimental setup.
single tube air inlet, fuel supply and mixing chamber, ignition chamber, and detonation chamber. The lengths of each section were 0.3 m, 0.147 m, 0.276 m, and 1.0 m respectively. The inner diameters of the first two sections were 0.08 m and the others were 0.06 m. All sections were held together by flanges. An intake cone was placed in the middle of each tube fuel supply and mixing chamber to guide the flow and reduce the loss due to airflow stagnation. The end side of the cone partially closed the flow field and acted as thrust wall. Twin-fluid air-assist atomizers were used for gasoline injection. The gasoline/air mixture was initiated by an automotive spark plug located in middle of the ignition chamber. The ignition energy was less than 50 mJ. An in-house designed ignition system was used to control the firing patterns and frequency. In order to accelerate the deflagration-to-detonation transition process, the Shchelkin spiral with a length of 1 m was used in each of the detonation chambers. The diameter of the spiral was 0.008 m and the pitch was 0.05 m. A photo of the experimental apparatus is shown in Fig. 2(a). High frequency pressure transducers were installed along the flow path of each tube. At the end of each detonation chamber, high frequency piezoelectric pressure transducers（Type: CY-YD-205） (P1~P8) were installed with a spacing of 0.05 m to verify the successful initiation of detonation wave in each tube (Fig. 1). Piezoresistive pressure transducers (F1~F4) were placed in each single tube inlet to capture the back-propagation pressure wave dynamics during PDE firing operation. Another piezoresistive pressure transducer (F0) was located in top of the air plenum chamber to capture the wave interactions of back-propagation pressure waves in the common air inlet. F0 was directly in-line with tube 1. The locations of the pressure measurement points are shown in Figs. 1
and 2(b). All the data were monitored and collected through DEWE3020 high-speed data acquisition system for a total of 16 channels and the sampling rates was 200 K samples/s. The output signals from the pressure transducer were sampled with the Kistler data acquisition and processing system, in which the voltage value and the pressure value had the relationship of 1 V equaled 1 MPa. The calibration of pressure transducers was applied to convert the raw values to pressure values. These values had an error of ±72.5 mV/MPa. Therefore, the pressure measurement uncertainty was approximately ±7.25%. 3. Result and discussion First of all, each tube was tested individually. Then, in order to test the operability of the four-tube PDE, three different multi-tube firing patterns were performed: dual tubes firing simultaneously, three tubes firing simultaneously, and all the tubes firing simultaneously. Experimental test conditions are shown in Table 1. The PDE was operated without any cooling system except for the high frequency pressure transducers in the tube end. So when the PDE was operated at the frequency of 30 Hz, the maximum run time was limited to 2 min. Experimental results of four different firing patterns were presented in this part. Fig. 3 shows the tubes involved under different firing patterns. All the experimental results were divided into three sections. The first section contained analyzing of the operability of the engine when all the tubes fired simultaneously at operating frequencies from 5 Hz to 30 Hz. The second section showed the results of single tube firing pattern and discussed the propagation of back-propagation pressure waves into the air plenum chamber. The last section quantified and compared
Fig. 2. (a) Photograph of four-tube PDE (back view) (b) schematic of pressure sensors location (front view).
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Table 1 The flow rate at different operating conditions. Frequency (Hz)
Fuel flow rate (kg/s 103)
Air flow rate (kg/s)
Single tube
Dual tubes
Three tubes
Four tubes
Single tube
Dual tubes
Three tubes
Four tubes
5 10 15 20 25 30
3.26 4.13 5.87 8.92 10.66
6.67 8.27 11.60 15.73 18.27
9.72 14.50 18.27 22.33 28.06
11.75 22.62 25.74 35.60 42.63 48.00
0.184 0.262 0.374 0.502 0.656
0.190 0.239 0.344 0.449 0.593
0.164 0.272 0.344 0.443 0.656
0.141 0.269 0.370 0.459 0.623 0.688
the synchronicity of the detonation waves and back-propagation pressure waves in the single tube inlet under multi-tube firing patterns. The peak value of back-propagation pressure waves in the air plenum chamber under different firing patterns were investigated and compared. The effects of synchronicity of the detonation waves and back-propagation pressure waves on the wave interactions in the air plenum chamber were discussed respectively. 3.1. Four-tube PDE operation verification
Fig. 4. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 5 Hz.
Figs. 4e9 show the raw pressure traces from the last piezoelectric pressure transducers of each tube (P2, P4, P6, P8) when all tubes were fired simultaneously at operating frequency from 5 Hz to 30 Hz. What's more, by measuring the elapsed time that a detonation wave traveled across two successive pressure transducer ports with a fixed distance, the velocity of the leading shock wave was determined. The averaged wave velocities between the two transducers at tube 1 to tube 4 were 1264.2 ± 169.9 m/s, 1128.3 ± 91.0 m/s, 1157.2 ± 138.8 m/s, 1236.5 ± 143.2 m/s at current work. For detonation in gaseous fuels, the forming of the detonation wave can be determined by the velocity of the leading shock wave, the peak pressure and the rise time of the peak pressure. But in two-phase detonation, the detonation velocity is lower than the CeJ detonation velocity. Wall losses, incomplete combustion before the CeJ plane, and energy loss due to rearward propagating waves have all been proposed as reasons for the observed velocity deficits [15]. So the peak pressure is the main criteria to determine the formation of detonation wave. In all tests, the averaged peak pressure of P2 to P8 were higher than 1.9 MPa which was the theoretical value of C8H18 and air mixture at experimental conditions calculated by the NASA CEA400 code [34], which indicated that the detonation wave were successfully obtained. Fig. 10 shows the photograph of the engine exhaust plume when four tubes fired simultaneously at operating frequency of 30 Hz. What's more, all the tubes could stably operate under other firing patterns. Although the pressure profiles are not given here, four-tube PDE was still carried out successfully under other firing patterns and could be
operated under one firing pattern to another firing pattern by controlling the supply of fuel. Therefore the engine would have a wide range of thrust through controlling the number of fired tubes. 3.2. Single tube firing pattern A DDT event usually arises from a sufficiently strong microexplosion or the interaction of multiple explosions. Then leftrunning retonation wave and right-running detonation wave were formed [13]. For valveless PDE, the retonation wave will propagate into the common air inlet and cause pressure oscillation [30]. Unlike the valved PDE in which all of the combustion product is forced to exhaust through the open exit because the valve interface is completely closed during the certain period of each cycle, part of the combustion product can propagate back into the upstream inlet in valveless PDE because the flow path is only partly closed. The interaction between combustor and inlet would incur significant performance losses and possibly lead to an inlet unstart [37]. Fig. 11 shows the enlargement of raw pressure traces of one detonation cycle in tube 1 at the operating frequency of 15 Hz. The dotted lines indicate directions of the upstream-propagating pressure waves. First of all, the back-propagation pressure waves reached the single air inlet of tube 1 and caused a large pressure spike at F1 (Line 1). The largest peak pressure could reach about 0.22 MPa. Then the back-propagation pressure waves propagated
Fig. 3. Schematic of different firing patterns.
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Fig. 5. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 10 Hz.
Fig. 7. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 20 Hz.
into a large space (the air plenum chamber), and the amplitude of the pressure spike was greatly reduced to 0.064 MPa (Line 5). Additionally, the back-propagation pressure waves could diffracted into the adjacent tube when it propagated into the air plenum chamber at the same time due to the valveless configuration at the head of PDE. Then the direction of the pressure waves changed and was in line with the inlet flow direction. These were the tube-totube interactions at the head end which could be observed by the pressure rise at F2, F3 and F4 (Line 2~Line 4).
3.3.1. Synchronism of detonation waves Fig. 12 shows the enlargement of raw pressure traces of one detonation cycle for all the tubes fired simultaneously at frequency
of 30 Hz. Since the transducers P2, P4, P6 and P8 were placed symmetrically, the detonation waves initiated in the four tubes should exhaust the four tubes at exactly the same time for the simultaneous firing patterns. However, the detonation waves showed a time of arrival difference. The maximum time arrival difference between detonation waves in Fig. 12 is approximately 1.7 ms. Additionally, the time of arrival difference showed up in all multi-tubes firing patterns. This was likely due to the slight difference in the deflagration-to-detonation process in each of the tubes resulted from minor tube-to-tube geometric differences, and perhaps more significantly, flow differences due to airflow differences to each tube, fuel injection flow rate and wall wetting. Fig. 13 summarizes the averaged time interval (△t1) between peak pressure P2 and P4 over 20 cycles under multi-tube firing patterns, in which the error bars represent the standard deviation. For dual tubes firing pattern, the time interval decreased with the increasing
Fig. 6. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 15 Hz.
Fig. 8. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 25 Hz.
3.3. Multi-tube firing patterns
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Fig. 9. Raw pressure traces from the last piezoelectric pressure transducers of each tube with frequency of 30 Hz.
frequency which was consistent with previous work of dual tube PDE experiments [35]. However, when more tubes were fired, this relation was not applicable any more. The wave interactions between different tubes became more significant when more tubes were fired and tube-to-tube flow difference became more obvious which caused larger variation on the synchronism of detonation waves. The averaged time interval obtained in all the tests was greater than the experimental result obtained by Rasheed [30] in which they found that the time of arrival difference was approximately 0.2 ms relative to one another. In our tests, liquid gasoline was used as fuel instead of ethylene in Rasheed's experiments. For two-phase detonation, atomization, vaporization, and the mixing of fuel and oxidizer played major roles in the DDT process. It was impossible to ensure that they were exactly the same in each tube, and little differences would lead to large effect on the DDT process which may cause larger time intervals. Fig. 14 shows comparisons of the averaged time intervals of P2eP6 and P2eP4 under multi-tube firing patterns. It seems that the time interval of P2eP6 shows a similar variation trend with that of P2eP4 when the operating frequency increased. The synchronism of detonation waves was closely related to the synchronism of back-propagation pressure waves. Therefore the time of arrival difference of detonation waves would lead to the arrival difference of back-propagation waves which may be even more greater. A comparison between the time intervals of
Fig. 10. Photograph of the engine fired simultaneously at operating frequency of 30 Hz.
Fig. 11. Pressure time history for tube 1 fired alone at frequency of 15 Hz.
detonation waves (△t1) and the intervals of peak values between back-propagation waves at F1 and F2 (△t2) are shown in Fig. 15. The data was also averaged over 20 operation cycles when all tubes fired simultaneously. It was obvious that △t1 and △t2 had the same variation trend and △t2 was bigger than △t1 at each operating frequency. 3.3.2. Wave Interactions in the air plenum chamber One important feature of the four-tube PDE was valveless configuration. No mechanical valves were placed between inlet and combustor. The system was mechanically simpler and circumvents
Fig. 12. Pressure time history when all tubes firing simultaneously at operating frequency of 30 Hz.
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Fig. 13. Degree of synchronism of the detonation waves in tube 1 and tube2 at different frequencies under (a) dual tubes firing pattern, (b) three tubes firing pattern, (c) four tubes firing pattern and (d) multitube firing patterns.
Fig. 14. Comparison of the degree of synchronism of detonation waves (P2eP4) and (P2eP6) at different frequencies under dual tube firing pattern.
Fig. 15. Comparsion of time intervals between detonation waves and back-propagation waves under four tubes firing pattern.
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Fig. 16. Raw pressure traces of F0 for different firing patterns at operating frequency of 15 Hz.
the disadvantages associated with airflow stagnation during the valve-closed period [38]. However, one disadvantage was that the strong retonation wave may propagate freely into the upstream plenum and have an adverse impact on the performance. Fig. 16 shows the raw pressure traces at F0 under different firing patterns at operating frequency of 15 Hz. As expected, single tube firing pattern resulted in the smallest pressure peak and the pressure peak increased with the increasing of the fired tubes number. It could be interpreted as follows. First of all, when single tube was fired, the diffraction of the back-propagation pressure waves would bring part of the back flow into the non-detonate tubes, thus reduced the peak pressure in the air plenum chamber. When the number of fired tubes increased, the non-detonate tubes for mitigating the pressure oscillation in the air plenum chamber decreased, which would lead to an increase of peak pressure in the air plenum chamber. Secondly, the wave interactions in the air plenum chamber became more intensive with the increasing of the fired tubes number and the constructive interference between back-propagation pressure waves would lead to greater pressure peak under simultaneous firing patterns. It's worth noting that there existed an arrival difference of back-propagation pressure waves caused by the asynchronism of detonation waves (Fig. 15).
The effect of this difference allowed the constructive and destructive interference which would result in the large variation in peak pressure. Fig. 17 shows the averaged peak pressure at F0 under different firing patterns at operating frequencies from 5 Hz to 25 Hz. All curves had the same change trend. The averaged peak pressure increased with the increasing of frequency on the whole but showed oscillation at certain frequencies. When the operating frequency increased, the mass flow rate of supplied air increased which would raise the total pressure of the coming air. Multi-tube numerical simulation conducted by Ripley et al. [39] also revealed that the amplitude of the pressure oscillations at the plenum inlet was a function of mass flow rate through the engine. Moreover, the purging time of each cycle would reduce when the frequency increased, so the pressure in the engine had no sufficient time to decay. However, due to the arrival difference of back-propagation pressure waves caused by the asynchronism of detonation waves, the peak pressure in the air plenum chamber was greatly affected by the constructive and destructive interference and showed large variation. For larger arrival difference of back-propagation waves (Fig. 15 at frequency of 20 Hz), destructive interference became obvious and caused a drop in averaged peak pressure (Fig. 17 at frequency of 20 Hz). This was the main reason why the averaged peak pressure showed oscillation at certain frequencies. If the arrival difference of detonation wave increased such as firing all tubes sequentially, the peak pressure would be greatly reduced, which would be investigated in the future. The reduction of pressure oscillation is important for the practical application, like hybrid pulse detonation turbine engine. Large pressure oscillation in the air plenum chamber could adversely affect compressor performance, especially lead to stall. 4. Conclusions
Fig. 17. Averaged peak pressures in common air inlet for different operating patterns.
A four-tube two-phase valveless air-breathing PDE was designed and tested. Gasoline and air were used as fuel and oxidizer respectively. The PDE system was operated at a maximum frequency of 30 Hz per tube with a total maximum frequency of 120 Hz. Four different firing patterns were carried out: single tube firing, dual tubes firing simultaneously, three tubes firing simultaneously, and all the tubes firing simultaneously. The operability of the engine was firstly discussed. Then, the propagation of backpropagation pressure waves at single tube firing pattern were discussed. The synchronicity of the detonation waves and backpropagation pressure waves under multi-tube firing patterns were quantified and compared. The averaged peak values of back-
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propagation pressure waves in the air plenum chamber under different firing patterns were compared. The effect of synchronicity of the detonation waves and back-propagation pressure waves on peak pressure in the air plenum chamber was also discussed. The experimental results showed that the four-tube PDE could be successfully operated under the four firing patterns and transformed from one firing pattern to another. Hence, the four-tube PDE would have a wide range of thrust by controlling the number of fired tubes. Investigation of propagation of back-propagation pressure waves showed that the largest pressure peak was about 0.22 MPa in the single tube inlet for single tube firing pattern and the amplitude of pressure spike was greatly reduced to 0.064 MPa when the pressure propagated into the air plenum chamber. A time of arrival difference of detonation waves showed up in all multitubes firing patterns. For dual tubes firing, the time interval decreased with the increasing frequency which was consistent with previous work of dual tube PDE experiments. However, when more tubes were fired, this relation was not applicable any more. The wave interactions between different tubes became more significant when more tubes were fired and the randomness of the detonation waves became more obvious. The synchronism of detonation waves were closely related to the synchronism of backpropagation pressure waves and the time of arrival difference of detonation waves led to greater arrival difference of backpropagation pressure waves. The averaged peak pressures in the air plenum chamber had the same change trend under multi-tubes firing patterns. Single tube firing pattern resulted in the smallest pressure peak in the air plenum chamber and the pressure peak increased at the increased fired tubes. All of the above results will provide some theoretical and experimental bases to improve the design level and accelerate the application of PDE.
Acknowledgements This work was supported by the National Natural Science Foundation of China through Grant No. (51306153), the Natural Science Foundation of Shanxi Province of China through Grant No. (2014JM7258), Doctoral Fund of Ministry of Education of China (grant number 20116102120027) and Northwestern Polytechnical University Foundation for Fundamental Research (grant number NPU-FFR-JCY20130129).
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