- Email: [email protected]
Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process
Energy xxx (2014) 1e10
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process Ke Wang, Wei Fan*, Wei Lu, Fan Chen, Qibin Zhang, Chuanjun Yan School of Power and Energy, Northwestern Polytechnical University, Mail Box 209, 127 Youyi Xilu, Xi’an 710072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2013 Received in revised form 25 April 2014 Accepted 2 May 2014 Available online xxx
In traditional PDRE (pulse detonation rocket engines), mechanical valves are used for periodic supply control and a purge process is widely used to form a buffer zone to prevent fresh fuel-oxidizer mixture from pre-ignition. However, both of them increase hardware complexity and limit increase of operating frequency. To eliminate mechanical valves and the purge process, a valveless mode without the purge process has been proposed. Multi-cycle detonations are able to create periodic pressure oscillations inside a detonation tube, which are capable to interrupt fuel and oxidizer supply. In the present study, liquid gasoline was used because vaporization of the liquid fuel would cool hot combustion products, which acted as a buffer zone. Therefore, a liquid-fueled and valveless PDRE without the purge process became possible. When oxygen-enriched air with 25% w 45% oxygen by volume was employed, a maximum operating frequency of 110 Hz was achieved. It was observed that supply pressures of fuel and oxidizer were of great importance for such an operating mode. Exhaust plumes at different operating frequencies were also investigated. The results indicated that it was feasible for the valveless PDRE to run steadily without the purge process when liquid gasoline was utilized. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Detonation Rocket engine Frequency Oxidizer Exhaust plume
1. Introduction The PDE (pulse detonation engine) has received considerable interest over the past few decades due to its potential for increased performance and hardware simplicity. There have been many studies on PDEs worldwide and several reviews have been published. For example, Kailasanath made a review on the propulsion applications of detonation waves [1]; Nettleton reviewed the work on gaseous detonations [2]; then Kailasanath summarized the developments on PDEs [3]; Roy et al. [4] published a literature on the pulse detonation propulsions including the challenges, current status and future perspective; Kailasanath concluded the work on liquid-fueled detonations in tubes [5] and the research on pulse detonation combustion systems [6]; more recently, Wolanski gave a conclusion and discussion on detonative propulsions including various detonation engines [7]. With oxidizer on board, the PDE works in a rocket mode. Like traditional PDE, the PDRE (pulse detonation rocket engines) operates by repeatedly generating detonation waves that propagate through fuel-oxidizer mixture and produce high chamber pressure intermittently which resulting
* Corresponding author. E-mail address: [email protected] (W. Fan).
in discrete impulses [8]. The basic structure of the PDE is a straight tube, also known as a detonation tube or chamber, with one end closed and the other open. A basic conventional detonation cycle consists of the following processes: (a) filling process, i.e., the detonation tube is filled with fresh detonable fuel-oxidizer mixture; (b) the mixture is ignited near the tube closed end and detonation is initiated directly or indirectly through a DDT (deflagration-to-detonation transition) process; (c) a self-sustained detonation wave, which compresses the fuel-oxidizer mixture by shock waves and initiates combustion of reactants, propagates toward the open end; (d) the combustion products exhaust out of the detonation tube through a blow-down process; (e) purge process, i.e., the purge gas is injected to form a buffer zone for isolating the hot combustion products from contact with the fresh detonable fuel-oxidizer mixture to prevent pre-ignition. The purge process was always needed to ensure stable operation of the multi-cycle PDRE in previous studies. For example, Lu et al. [9] used air as purge gas; Li et al. [10] utilized nitrogen as purge gas; helium was adopted as purge gas by Kasahara et al. [11] in their studies on the PDRE; Matsuoka et al. employed helium as the purge gas to produce a buffer zone between adjacent cycles in their work on an inflow-driven system of a PDE [12] and thrust measurement of a pulse detonation combustor [13] as well. However, it takes certain time to accomplish injection of purge gas into the
http://dx.doi.org/10.1016/j.energy.2014.05.002 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
2
K. Wang et al. / Energy xxx (2014) 1e10
detonation tube during the purge process, which occupies considerable proportion of a single detonation cycle. Therefore, together with an injection process, a DDT process, a propagation process, and a blow-down process, it limits the overall cycle time and puts a ceiling on the upper limit of the operating frequency. In addition, it requires independent supply system that increases the hardware complexity. Thus, if the purge process was eliminated, great benefits can be derived as the complicated injection systems will be greatly simplified, e.g., complexity of the rotary valve for gaseous fuel in Ref. [14] and the rotary valves for liquid fuels in Ref. [15] and another valve system for the PDE in Ref. [16] can be reduced. In fact, operation of the PDE results in periodic pressure oscillations inside the detonation tube, which will interrupt the injection of fuel and oxidizer periodically. Thus mechanical valves such as solenoid valves and rotary valves, for supply control but restricting the operating frequency of the practical PDE due to their own responding frequencies, can be removed. Eliminating the purge process and utilizing periodic pressure oscillations inside the detonation tube for supply control can reduce the hardware complexity and, meanwhile, increase the operating frequency of a PDRE, which is highly attractive. However, residual hot combustion products in previous cycle will ignite the fresh charge without the buffer zone created by the purge process. Valaev et al. [17] proposed that cooled combustion products through special designed cooling means would form a buffer zone and ensure stable operation of multi-cycle detonations in their patent. DeRoche [18] patented another somewhat similar idea, which suggested that the pressure oscillations inside the detonation chamber was capable of intermittent supply control and the combustion products cooled in the feeding lines could prevent uncontrolled ignition of the fresh detonable mixture. Practical implementation of the above schemes was experimentally performed by Baklanove et al. [19], and they pointed out that “gasdynamic valves” were produced by the pressure oscillations. A maximum operating frequency of 92 Hz was obtained with forced cooling and gaseous fuel-oxidizer mixture in their experiments. This study presents a successful attempt to achieve stable operation of a valveless PDRE without the purge process. The difference, between the current study and previous ideas and implementation, lies in liquid fuel instead of gaseous fuel was used and forced cooling by special designed means was removed because vaporization of the liquid fuel would cool the hot combustion products. Oxygen-enriched air was utilized as oxidizer in this work. Different oxygen percentages by volume were also tested to determine the upper and lower limits for stable operation without the purge process. In addition, exhaust plumes under different operating frequencies were observed and the operational mechanisms were analyzed.
Fig. 1 shows the schematic diagram of this valveless operation sequences in actual operations. As illustrated in Fig. 1, fuel and oxidizer are separately injected into the detonation tube where they are mixed to form a detonable mixture. After initiation of combustion, a detonation wave is formed often by the DDT process and then transverses through the tube. Now pressure inside the tube increases higher than fuel and oxidizer supply pressures, thus a gasdynamic valve forms interrupting the flow of fuel and oxidizer; meanwhile, some combustion products penetrate into the oxidizer feeding line. As the pressure inside the tube diminishes due to outflow of detonation wave into the ambience, fuel and oxidizer supply resumes when pressure inside the tube drops below supply pressures (the gasdynamic valve disappear). Vaporization of the liquid fuel will cool hot combustion products to form a buffer zone even though no purge gas is employed. Then the detonation tube is filled with fresh detonable mixture. At this point in time, the system has undergone a complete operation cycle and then the cycle is repeated. The pressure trace measured at the closed end of a detonation tube is shown in Fig. 2, which is similar to that in Ref. [20]. As shown in Fig. 2, different processes corresponding to Fig. 1 are presented. If supply pressures of fuel and oxidizer (pf and po) are appropriate, e.g., lower than 1.0 MPa as shown in Fig. 2, fuel and oxidizer supply will be stopped due to higher pressure inside the detonation tube. Therefore, based on previous analysis, a liquid-fueled PDRE without the purge process should be operated steadily in this valveless mode.
3. Experimental investigation 3.1. Experimental setup To ensure implementation of this idea, complex mechanical valves, such as solenoid valves and rotary valves, were removed as intermittent supply control was realized by “gasdynamic valves”.
2. Description of the principle To isolate fresh detonable mixture from direct contact with hot combustion products when eliminating the purge process, a buffer zone of cooled combustion products is also required. However, cooling of hot combustion products is accomplished by neither taking the methods of forced cooling in Refs. [17,19] nor through the feeding lines in Ref. [18] but mainly depending on the liquid fuel. When liquid fuel is injected into the detonation tube, it will be vaporized by contact with hot combustion products, which cools them down to a temperature at which the explosion induction period of the fresh combustible mixture exceeds the time of contact with cooled combustion products [19]. Furthermore, oxygenenriched air, instead of pure oxygen, was employed as oxidizer to increase the explosion induction period in this work.
Fig. 1. Schematic of the valveless operation sequences.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
3
Fig. 4. Schematic of the detonation tube.
Fig. 2. Pressure trace at the closed end of a detonation tube.
Only electrically operated valves were needed to control on/off before and after each run. Fuel and oxidizer should be injected into the detonation tube separately to avoid their premixing; otherwise, premixed fuel-oxidizer mixture would be ignited by contact with hot combustion products leading to a deflagration. The TTL (transistoretransistor logic) control signals for supply and ignition are shown in Fig. 3. As illustrated in the figure, supply of fuel and oxidizer was not interrupted by mechanical valves during the operation, and ignition was initiated by a phase delay after the fuel and oxidizer supply to activate pressure oscillations inside the detonation tube. The detonation tube utilized is shown in Fig. 4. It comprised three sections, such as an injection and ignition section, a DDT section and a measurement section. An ordinary automobile spark plug with an ignition energy of about 50 mJ was used to initiate combustion. A typical Shchelkin spiral, which could accelerate the DDT process, was installed in the DDT section. Three detonation tubes with different dimensions, i.e., large model with long DDT section (Tube A), large model with short DDT section (Tube B), and small model (Tube C), were utilized in this work. The dimensions of these tubes are shown in Table 1. For Tubes A and B, the pitch and wire diameter of the Shchelkin spiral were 30 mm and 4 mm creating a blockage ratio of 0.46. For Tube C, a Shchelkin spiral, whose pitch and wire diameter were 24 mm and 3 mm creating a blockage ratio of 0.43, was installed in the DDT section. Three pressure transducers (p1, p2, and p3) were employed to record the pressure history along the tube. Oxygen-enriched air and liquid gasoline were utilized. A series of gas tanks with initial pressure higher than 10 MPa were
Fig. 3. TTL control signals for supply and ignition.
Table 1 Dimensions (in mm) of the detonation tube. Tube
d
l
l1
l2
l3
l4
l5
l6
l7
A B C
30 30 24
880 780 660
160 160 110
360 260 230
360 360 320
100 100 60
600 500 330
670 570 400
740 640 470
employed for oxidizer supply. One regulator was adopted in the oxidizer feeding passage to ensure required supply pressure. Liquid gasoline was stored in an independent tank and it was forced out by nitrogen with certain pressure. To reduce the fuel depositing on the tube wall, liquid gasoline was sprayed into the detonation tube through a pressure swirl atomizer axially while oxidizer was injected through an annular gap surrounding the atomizer. Dynamic piezoelectric pressure transducers (SINOCERA CY-YD-205, natural frequency larger than 200 kHz with the measurement precision of 3%) were utilized. The pressure transducers were connected to a multi-channel data acquisition system through a signal conditioner module. The pressure transducers were sampled at 200 kS/s. In addition, the pressure transducers were recessed in mounting ports and water-cooled to prevent them from being damaged by extremely high temperature. 3.2. Results and discussion 3.2.1. Tube A operation Experiments were conducted to validate whether or not the PDRE could reach stable operation under such an operating mode. Tube A was used in the preliminary experiments. Supply pressures of fuel and oxidizer were set to be identical to ensure that fuel and oxidizer would stop supplying simultaneously during the period when the pressure inside the detonation tube was higher than supply pressure. It was observed that when supply pressures were lower than 0.70 MPa and oxidizer with a volume percentage of 35% oxygen was used, stable operations could be obtained and a maximum operating frequency of 50 Hz was achieved. Oxidizer with different oxygen percentages by volume were also tried to determine the upper and lower limits of oxygen percentage for stable operations. Experimental results indicated that, with liquid gasoline as fuel, stable operations could be achieved when oxygen percentages varied from 25% to 45% in the present study. Lower oxygen percentage required larger flow rate for adequate oxidizer filling under the same operating frequency to achieve fullydeveloped detonations. Whereas, higher oxygen percentage in oxidizer would induce pre-ignition of fresh fuel-oxidizer mixture by contacting with cooled combustion products of the same temperature, as in this case the explosion induction period of the fresh combustible mixture might be shorter than the time of contact
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
4
K. Wang et al. / Energy xxx (2014) 1e10
with cooled combustion products. This device may not be able to replace traditional PDREs in this case. However, this is just a preliminary try on such an operating mode. After further attempts, other types of liquid fuels may be available under such an operating scheme leading to the application of an oxidizer with higher percentage of oxygen. Thus, the liquid-fueled and valveless PDRE has the potential to replace the traditional one. Measured peak pressure decreased when operating frequency increased above 50 Hz, which was believed due to pressure decay along the tube caused by lower fill fractions. Fill fraction was defined as the volume ratio of oxidizer and fuel injected into the detonation tube during each cycle to the detonation tube. As the supply was pulsating, it was hard to measure the flow rate accurately by flow meters. In the current study, the flow rate of oxidizer was determined indirectly by pressure changes of the oxidizer source, while the flow rate of fuel was also indirectly determined but by mass changes of the supply system. 3.2.2. Tube B operation In accordance with the fact that smaller volume may exhibit higher fill fraction, Tube A was replaced by Tube B, i.e., the DDT section was replaced by a shorter one with a length of 260 mm, which was also proved capable of facilitating the DDT process successfully. For the operation of Tube B, a maximum detonation frequency of 60 Hz was obtained, and the measured pressure profiles are shown in Fig. 5. As illustrated in Fig. 6, the fill fraction decreased with operating frequency (less than 50 Hz); it slightly increased at 50 Hz when the tube length changed, but then it decreased again when the operating frequency was increased. This was helpful to understand the variations of measured peak pressure and wave velocity with operating frequency shown below. Comparisons of measured peak pressure and wave velocity with CeJ (Chapman-Jouguet) values are shown in Figs. 7 and 8. The CeJ pressure and velocity calculated by CEA (Chemical Equilibrium with Applications) codes (analysis on the codes [21] and users manual and program description [22] were used), were 2.69 MPa and 2034.1 m/s. It should be noted that the actual pressure and temperature of fuel-oxidizer mixture were hard to be determined as the flow and heat transfer processes inside the detonation tube were very complicated. Yet, it was believed that the pressure of fresh charge before detonation was close to ambient pressure and its temperature varied little compared to the initial state. Ambient pressure and temperature (also the fuel and oxidizer supply temperature), 1.0 bar and 282 K, were employed in the calculation. An
Fig. 5. Pressure profiles measured at 60 Hz for Tube B operation.
Fig. 6. Variation of fill fraction with frequency.
Fig. 7. Comparison of measured peak pressure with CeJ pressure.
equivalence ratio of 1.71 was determined using the measured fuel and oxidizer flow rates. As shown in Fig. 7, measured peak pressures by the transducers were higher than CeJ pressure when the operating frequency below 35 Hz, which, probably, due to that the initial pressure was greater than the ambient pressure because of large filling amount during each cycle. When the operating frequency increased to 40 Hz and 50 Hz, the peak pressures decreased to lower than CeJ value due to decay of detonation wave when propagating toward the open end caused by inadequate filling. This was proved by the fact that the peak pressures increased distinctly at 50 Hz by shortening the tube length, resulting in a greater fill fraction. However, the peak pressures decreased with operating frequency once again and only a maximum frequency of 60 Hz was achieved under current supply condition. In addition, fuel and oxidizer might not be uniformly filled from the closed end to the open end; the space around the tube wall probably not be filled with the fuel-oxidizer mixture, once the filling amount decreased, due to the influence of boundary layer. That was why all three pressures were affected at 40 Hz compared to 30 Hz instead of p3 to be affected first, then p2, and finally p1. Besides, detonation wave velocity could be obtained by the time-of-flight method as introduced in Ref. [23] and employed in Ref. [15]. Here, v12 andv23 denoted average velocities between transducers p1 and p2, p2 and
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
5
Table 2 Average peak pressure and wave velocity at 100 Hz for different oxidizer supply pressures. Pressure (MPa)
p1 (MPa)
p2 (MPa)
p3 (MPa)
v12 (m/s)
v23 (m/s)
0.60 0.65 0.75
2.39 2.46 2.76
1.90 2.03 2.33
1.55 1.79 2.07
1813 1875 2000
1609 1783 2000
Fig. 8. Comparison of measured wave velocity with CeJ velocity.
p3. The calculated wave velocities are shown in Fig. 8. As illustrated in Fig. 8, measured wave velocities were slightly lower than but approached above 90% of the CeJ velocity at different operating frequencies. The velocities are averaged ones between adjacent transducers, thus there might be higher velocities not captured. In addition, measured wave velocities slightly increased when shortening the tube length at 50 Hz, which was produced by increase of the fill fraction. Considering liquid gasoline was used in this study and liquid-fuels were assumed in gas-phase in CEA codes as well as the fact that friction of the tube wall would also reduce the propagation velocity, fully-developed detonations were believed to be obtained. There would be some possibilities of unvaporized liquid gasoline portion in the initial stage of one operation, which might reduce the actual equivalence ratio. However, with the operation progressing, the detonation tube was heated and the unvaporized liquid gasoline would be vaporized completely as no residual fuel was observed inside the detonation tube after each run. Higher operating frequencies were also tried, but fullydeveloped detonations could not be produced at all when fill fractions were lower than that of 60 Hz. However, it was believed that higher frequency detonations could be achieved in a detonation tube with smaller inner diameter or by employing electronically operated valves with larger flow rates for oxidizer, both producing a larger fill fraction. 3.2.3. Tube C operation To increase the operating frequency and validate the surmise, Tube C was employed in the subsequent experiments. In the following experiments, oxygen-enriched air with 45% oxygen by volume was used. Supply pressures of fuel and oxidizer were set at 0.60 MPa. A maximum operating frequency of 100 Hz was tried. The results indicated that measured peak pressure decreased with the increase of frequency, which was believed due to drop of the fill fraction. Therefore, supply pressure of oxidizer was increased to 0.75 MPa but the fuel supply pressure was fixed at 0.60 MPa. In this case, supply of oxidizer would start refilling before fuel. Table 2 lists measured peak pressures and wave velocities at 100 Hz under different oxidizer supply pressures. It can be seen that both the peak pressure and the wave velocity increased with oxidizer supply pressure. The maximum operating frequency achieved was 110 Hz with an oxidizer supply pressure of 0.75 MPa. Pressure profiles obtained at 100 Hz and 110 Hz are shown in Figs. 9 and 10. Operating frequencies higher than 110 Hz were also attempted but the measured peak pressures decreased evidently caused by decrease of the fill fraction.
Fig. 9. Pressure profiles measured at 100 Hz.
As shown in Fig. 11, the fill fraction decreased with operating frequency when supply pressure of oxidizer was fixed at 0.60 MPa; at 100 Hz, the fill fraction increased with supply pressure; however, once again it dropped with the increase of frequency when the operating frequency exceeded 100 Hz under a supply pressure of 0.75 MPa. To verify detonations were obtained, measured peak pressure and wave velocity were also compared with CeJ values, shown in Figs. 12 and 13. Increase of oxidizer supply pressure led to change of the oxidizer flow rate that affected the equivalence ratio because the fuel flow rate was unchanged. Then the calculated CeJ pressure and velocity were different and, therefore, there were two y-coordinates for p/pCJ and v/vCJ in Figs. 12 and 13. The ambient pressure and temperature, employed in the calculation, were also 1.0 bar and 282 K but with the equivalence ratio changed. As illustrated in Fig. 12, measured peak pressures decreased with operating frequency and then increased correspondingly when
Fig. 10. Pressure profiles measured at 110 Hz.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
6
K. Wang et al. / Energy xxx (2014) 1e10
Fig. 11. Variation of fill fraction with frequency under different supply pressures of oxidizer (&100 Hz, 0.60 MPa; S100 Hz, 0.75 MPa).
velocity with CeJ value, shown in Fig. 13, indicated that variation of measured wave velocity with frequency was similar to that of peak pressure. Both phenomena were due to variation of the fill fraction. As illustrated in Figs. 12 and 13, peak pressures measured by p1 and p2 approached above 80% and 70% of CeJ pressure, and measured wave velocity approached above 85% of CeJ velocity when the operating frequency was below 110 Hz. Yet, when the operating frequency increased to 120 Hz, measured peak pressure and wave velocity dropped evidently. For instance, peak pressure measured by p1 was about 70% of the CeJ value while the wave velocity between p1 and p2 reached about 85% of the CeJ value, and both the peak pressure and the wave velocity decreased distinctly along the tube. For frequencies higher than 80 Hz, the pressure and velocity dropped with distance. As fully-developed detonations were obtained at frequencies below 80 Hz, the mixture cell size should not account for such a trend. It was believed that decrease of the fill fraction was responsible for this since detonation waves decayed when propagating inside the tube not fully filled with the fueloxidizer mixture. In a word, the maximum operating frequency obtained was 110 Hz in this study. However, since no other restrictions in increasing the operating frequency were observed in the present research, it was possible to increase the operating frequency further if greater fill fractions could be achieved by replacing the detonation tube with a smaller volume or employing electronically operated valves with larger flow rates. The oxidizer supply pressure was increased to 0.75 MPa and fuel supply pressure was fixed at 0.60 MPa when the operating frequency exceeded 100 Hz. In this case, the buffer zone consisted of a portion of oxidizer fed before liquid fuel and the cooled combustion products. However, with operating frequency below 100 Hz, the PDRE could not operate in this valveless mode when the oxidizer supply pressure was above 0.65 MPa. It was believed that greater pressure oscillations were produced when the operating frequency exceeded 100 Hz, which was able to interrupt the supply with a pressure of 0.75 MPa. This indicated that appropriate supply pressures of fuel and oxidizer were crucial for implementation of the valveless operating mode without the purge process. 4. Exhaust plumes
Fig. 12. Comparison of measured peak pressure with CeJ pressure.
oxidizer supply pressure was increased at 100 Hz but dropped again with the operating frequency exceeding 100 Hz. Anyway, variation of the measured peak pressures with frequency was consistent with that of fill fraction. Comparison of measured wave
Fig. 13. Comparison of measured wave velocity with CeJ velocity.
During the experiments, exhaust plumes were recorded when operations of the PDRE were carried out until tube-wall ignition happened. As discussed in Ref. [24], the tube-wall ignition indicated that fresh detonable fuel-oxidizer mixture was ignited by hot tube wall after a long run duration as no cooling measure was adopted. Each operation was divided into three stages (such as initial stage, middle stage and terminal stage) equally with an equivalent time span according to the overall run time. Different exhaust plumes, varying with the operating frequency and the operation stage, were observed. According to the operating frequency, they were divided into low frequency cases (below 60 Hz) and high frequency cases (above 60 Hz). Additionally, exhaust plumes of the startup process and the deflagration mode caused by the tube-wall ignition were also observed. Since a straight tube without exit nozzles were used in the present study, an underexpanded flow, instead of an over-expanded one, with sonic speed would be produced at the exit plane. Thus, Mach disks (also known as Mach diamonds, shock diamonds, Mach rings, doughnut tails or thrust diamonds) might form in the exhaust flow. Wave structures creating Mach disks in an under-expanded flow are given in Fig. 14. Detailed explanations on the wave structures and flows can be found in Ref. [25]. For sonic jet flows, the distance between the first Mach disk and the nozzle exit plane, xM, and the diameter of the nozzle exit, de, had the following relation, proposed by Crist et al. [26],
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
Fig. 14. Wave structures in an under-expanded flow.
xM ¼ 0:6455 de
rffiffiffiffiffi pe pa
(1)
where pe and pa were the pressure of the gases exiting the nozzle and the ambient pressure. Equation (1) was obtained from steady flow experiments. When the operating frequency of a PDRE was high enough, the exhaust flow approached steady flow especially during a relatively small time span. Therefore, according to the exhaust plumes observed, the pressure of the gases exiting the nozzle could be obtained approximately. 4.1. Low frequency cases In low frequency cases, Tube A was utilized with the same equivalence ratio of 1.71. During each operation, exhaust plumes varied with the run time. Two photos of the exhaust plumes for initial stage and middle and terminal stages at an operating frequency of 46 Hz were captured. As shown in Fig. 15a, when combustion products exhausted out of the detonation tube, one Mach disk pattern with clear jet boundary was formed close to the tube open end and then the plume expanded suddenly. There existed one clear boundary perpendicular to the flow between the Mach disk pattern and the suddenly expanded jet, which might be a normal shock wave. Since the detonable mixture inside the tube was fuel-rich, passing through this normal shock wave caused the
Fig. 15. Exhaust plumes at 46 Hz ((a) Initial stage; (b) Middle and terminal stages).
7
temperature of the flow to increase, igniting any excess fuel present in the exhaust. It was this burning fuel that made the expanded jet glow. Combustion products were under-expanded when they arrived at the tube open end. Thus, a Mach disk similar to Ref. [27] formed in the exhaust flow. Then the combustion products suddenly expanded into the atmosphere, which was investigated in the study of Ma et al. [28]. Unlike at the initial stage of the operation, only thicker plume without evident boundaries was produced at the middle and terminal stages (Fig. 15b) because the tube wall was heated up after some period of operation. Then the initial temperature of the detonable mixture was increased by contact with the tube wall, which would lead to a lower detonation pressure. This was actually observed in this study, e.g., peak pressure recorded by p2 decreased from about 3.0 MPa to 2.3 MPa after a run of 18 s. Lower detonation pressure led to drop of the time-averaged pressure inside the detonation tube. Therefore, no Mach disk with evident boundaries was produced at the middle and terminal stages. Based on Fig. 15a, the value of xM/de, which was 1.493, could be obtained as the exit diameter was known. Thus, substituting this value in Eq. (1), pe ¼ 5.35pa was obtained, which indicated that the flow was under-expanded. As illustrated in Fig. 16, the exhaust plumes at an operating frequency of 56 Hz had some difference with that at 46 Hz. First, two Mach disks, instead of only one, were produced close to the tube exit at the initial stage of the operation. Second, a middle stage with one evident Mach disk was observed. Third, one Mach disk still existed at the terminal stage except that it was not that legible as before, but was still similar to that of the initial stage at 46 Hz. This, probably, was because the time-averaged pressure inside the detonation tube increased due to higher frequency detonations
Fig. 16. Exhaust plumes at 56 Hz ((a) Initial stage; (b) Middle stage; (c) Terminal stage).
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
8
K. Wang et al. / Energy xxx (2014) 1e10
burned more fuel-oxidizer mixture in detonation mode. Based on Eq. (1), the pressures of the gases exiting the nozzle corresponding to the three stages pictured in Fig. 16 were 6.34pa, 5.10pa, and 4.34pa, respectively. Comparison of the exhaust plumes of 46 Hz and 56 Hz indicated that the time-averaged pressure inside the detonation tube increased with detonation frequency. 4.2. High frequency cases In high frequency cases, the detonation frequency was greater than 60 Hz and Tube C was utilized with an equivalence ratio of 1.09. The time-averaged pressure inside the tube increased distinctly compared with the low frequency cases, which resulted in different exhaust plumes. As shown in Fig. 17, the exhaust plumes at 100 Hz exhibited some differences from those of the low frequency cases. For instance, two Mach disk patterns with clear boundaries existed at the initial stage, but the second jet pattern had a greater length than the first one. Similar to the low frequency cases, the exhaust plumes varied with operation time. The second jet pattern died away with the operation progressing and it almost disappeared at the middle stage; finally, only the first one could be distinguished at the terminal stage, as shown in Fig. 17. The reason was the same as what discussed in low frequency cases, i.e., heat transferred from the tube wall increased initial temperature of the fresh detonable mixture. In the same way, it could be obtained that the pressures of the gases exiting the nozzle corresponding to the three stages in Fig. 17 were 6.55pa, 6.01pa, and 5.43pa, respectively, which were greater
than that at 56 Hz but the trend that dropped with operation time was consistent. Therefore, the time-averaged pressure, at 100 Hz, inside the detonation tube was higher than that of 56 Hz. In addition, the nondimensional distances, defined as the distance between the tube exit and the tail of the visible exhaust flames divided by inner diameter of the detonation tube, at three different stages were around 18.7, 17.8, and 18.2. However, at 56 Hz, as shown in Fig. 16, they were about 14.0, 13.6, and 12.5, respectively. This indicated that the exhaust flames in high frequency cases had greater penetrability, which implied that higher velocities of the exhaust flow were obtained in high frequency cases because of higher time-averaged pressure. The exhaust plumes at high frequency presented snow-white and blue color due to higher temperature with the equivalence ratio close to unity; yet, color of the exhaust plumes in low frequency cases were orange-yellow with lower temperature because the fuel-oxidizer mixture was fuel-rich, inducing incomplete combustion. 4.3. Startup process As shown in Fig. 3, ignition was initiated by a phase delay after the fuel and oxidizer supply to ensure activation of pressure oscillations inside the detonation tube. Actually, the phase delay was managed manually by initiating the fuel and oxidizer supply some time (about 0.2 s) before ignition in the present work. Then some special phenomena were obtained during the startup process. As illustrated in Fig. 18, the exhaust plumes varied greatly during the short period for the startup process. The time span between Fig. 18a and f was 0.21 s and the time interval between two adjacent pictures was 42 ms. The exhaust plumes from Fig. 18aef were captured in a chronological order. After ignition, detonation waves were obtained but the exhaust plumes were quite different from those discussed previously. As shown in Fig. 18a, a bright and long flame was formed. Because fuel and oxidizer were injected into the detonation tube and some of them flowed outside with some fuel deposited on the tube wall at the same time before the first ignition. Then the first detonation wave formed and meanwhile the fuel-oxidizer mixture flowed outside the tube was ignited. With the operation carrying on, detonation waves blew the flame off the tube, as shown in Fig. 18b. Then, as shown in Fig. 18c, only a slim flame existed close to the tube exit as the fuel deposited on the tube wall was blown out producing too rich a mixture zone to be ignited. Then the detonation waves cleaned the fuel deposited on the tube wall and exhaust plumes, similar to Fig. 17a, formed and maintained for a while before transforming to an exhaust plume consistent with that of Fig. 17a. The flame expanded some distance away from the second jet pattern (see Fig. 18d and e), which was different from that in Fig. 18f, because of larger flow rate of oxidizer. Since the oxidizer supply passage was full with certain pressure before the start of supply, the oxidizer flow rate would decrease to a balance after some time. Temperature increase of the fuel-oxidizer mixture was not considered because the time span was only around 0.2 s, which was too short for the tube to be heated up distinctly. It could be seen that some fuel-oxidizer mixture injected ahead too much before the first ignition was wasted when burned outside the detonation tube, as illustrated in Fig. 18a. One feasible way to avoid this was to determine the phase delay of ignition than the fuel and oxidizer supply accurately. A phase delay of about 7/12T (T in Fig. 3) in each cycle was believed to be appropriate. 4.4. Deflagration mode
Fig. 17. Exhaust plumes at 100 Hz ((a) Initial stage; (b) Middle stage; (c) Terminal stage).
Since the PDRE was operated without active cooling, the detonation tube was heated up during each run. The hot tube wall
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
Fig. 18. Exhaust plumes of the startup process at 100 Hz.
would ignite the fresh fuel-oxidizer mixture once its temperature exceeded the self-ignition temperature of fresh detonable mixture, which would lead to deflagration due to inadequate filling of reactants and inappropriate ignition position. The long duration operations were carried out until the tube-wall ignition causing deflagration to happen in the present study. Fig. 19 shows exhaust plumes at the deflagration mode for different operating frequencies. Obviously, the exhaust plumes were quite different from those obtained in a detonation mode. There was almost no pressure increase for deflagration, so only slim exhaust plumes, with weaker penetrability and without sudden expansion and Mach disk, were produced. Besides, the exhaust plumes at a deflagration mode were consistent with those obtained in premixed combustion as fuel and oxidizer might be probably well completely mixed in the detonation tube. As shown in Fig. 19a and b, the exhaust plumes had a yellow cone-shaped core (in web version), where the temperature was relatively low due to incomplete combustion in fuel-rich condition, and a blue outer flame (in web version) whose temperature was higher due to complete combustion. However, the exhaust plume had a snow-white inner flame implying complete combustion and a higher temperature, as illustrated in Fig. 19c, because the equivalence ratio of fuel-oxidizer mixture was close to unity. In this study, detonation mode only lasted for a few seconds, no more than 20 s, before the tube wall ignition inducing deflagration happened. Therefore, active cooling [23] should be employed to ensure stable operation during long duration runs, but active cooling based on liquid fuel preheating utilized to enhance the performance of PDREs [29] or for thermal management in advanced aeroengines [30] is impractical as liquid fuel at a low temperature is crucial in such an operating mode. Although heat exchange will cause cracking of the liquid hydrocarbon fuel inside
Fig. 19. Exhaust plumes at deflagration mode ((a) 46 Hz; (b) 56 Hz; (c) 100 Hz).
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
9
10
K. Wang et al. / Energy xxx (2014) 1e10
cooling channels [31], which is beneficial for the DDT process. Therefore, cooling methods employed in gas turbine engines [32] and waste heat recovery in diesel engines [33] may provide other considerations for effective cooling under such an operating mode. In addition, the impact of cooling measures on the thermodynamic performance should be paid special attention to because they may lead to a decrease of the combined cycle performance [34]. 5. Conclusions To increase the operating frequency and meanwhile reduce the hardware complexity, a valveless mode without the purge process was proposed and experimentally performed. Liquid gasoline was used as fuel because vaporization of the liquid fuel, after being injected into the tube, would consume a lot of heat that inevitably resulted in a temperature drop of the combustion products. Oxygen-enriched air was utilized as oxidizer. Experimental results indicated that stable operation of a PDRE without the purge process could be achieved and a maximum operating frequency of 60 Hz was obtained due to restriction of the flow rate. It was observed that the PDRE was able to run steadily when oxidizer with 25% w 45% oxygen by volume was employed. Appropriate supply pressures for fuel and oxidizer were also found to be important for stable operations in this operating mode. When utilizing a detonation tube with smaller volume, a maximum operating frequency of 110 Hz was achieved. No other restrictions except for the flow rate in increasing the operating frequency were observed in this study. Therefore, higher operating frequency was believed to be attainable if the detonation tube with a smaller volume or larger flow rates were available. Additionally, different exhaust plumes were observed under different operating frequencies. The analysis about them showed that higher operating frequency resulted in larger exhaust flow velocity due to higher time-averaged pressure inside the detonation tube. In conclusion, the experimental results revealed that it was feasible for the valveless PDRE to be operated stably without the purge process when liquid gasoline was utilized to cool hot combustion products to form a buffer zone. In this valveless mode, high frequency two-phase detonations were achieved and the hardware complexity of the practical PDRE could be greatly reduced. Acknowledgments This work is supported by National Natural Science Foundation of China (51176158, 51376151), Doctoral Program Foundation of Education Ministry of China (20126102110029), Doctorate Foundation of Northwestern Polytechnical University (CX201112) and Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China. The authors wish to thank Le Jin, Qiang Xiao, Haoyi Song, and Yongjian Zhang for their assistance in carrying out the experiments. References [1] Kailasanath K. Review of propulsion applications of detonation waves. AIAA J 2000;38(9):1698e708. [2] Nettleton MA. Recent work on gaseous detonations. Shock Waves 2002;12(1): 3e12. [3] Kailasanath K. Recent developments in the research on pulse detonation engines. AIAA J 2003;41(2):145e59.
[4] Roy GD, Frolov SM, Borisov AA, Netzer DW. Pulse detonation propulsion: challenges, current status and future perspective. Prog Energy Combust Sci 2004;30(6):545e672. [5] Kailasanath K. Liquid-fueled detonations in tubes. J Propul Power 2006;22(6): 1261e8. [6] Kailasanath K. Research on pulse detonation combustion systems e a status report. Orlando, Florida, USA; 2009. AIAA Paper. p. 2009e2631. [7] Wolanski P. Detonative propulsion. Proc Combust Inst 2013;34:125e58. [8] Brophy CM, Hanson RK. Fuel distribution effects on pulse detonation engine operation and performance. J Propul Power 2006;22(5):1155e61. [9] Lu FK, Meyers JM, Wilson DW. Experimental study of a propane-fueled pulsed detonation rocket. Norfolk, Virginia, USA; 2003. AIAA Paper. p. 2003e6974. [10] Li JL, Fan W, Chen W, Wang K, Yan CJ. Propulsive performance of a liquid kerosene/oxygen pulse detonation rocket engine. Exp Therm Fluid Sci 2011;35:265e71. [11] Kasahara J, Hirano M, Matsuo A, Daimon Y, Endo T. Thrust measurement of a multicycle partially filled pulse detonation rocket engine. J Propul Power 2009;25(6):1281e90. [12] Matsuoka K, Yageta J, Nakamichi T, Kasahara J, Yajima T, Kojima T. Inflowdriven valve system for pulse detonation engines. J Propul Power 2011;27(3): 597e607. [13] Matsuoka K, Esumi M, Ikeguchi KB, Kasahara J, Matsuo A, Funaki I. Optical and thrust measurement of a pulse detonation combustor with a coaxial rotary valve. Combust Flame 2012;159:1321e38. [14] Lo PP, Gonzalez DE. Development of a fuel injection system for a high frequency pulse detonation engine. Seattle, Washington, USA; 1997. AIAA Paper. p. 1997e2744. [15] Wang K, Fan W, Zhu XD, Jin L, Chen F. Experimental studies on rotary valves for single-tube pulse detonation rocket engines. Proc IMechE Part G-J Aerosp Eng 2014;228:262e70. [16] Meholic GV. Rotary valve for pulse detonation engines 2001. United States Patent 6505462B2. [17] Valaev AA, Zhimerin DG, Mironov EA, Popov VA. Method and apparatus for intermittent combustion 1976. United States Patent 3954380. [18] DeRoche M. Repetitive detonation Generator 1998. United States Patent 5800153. [19] Baklanove DI, Gvozdeva LG, Scherbak NB. Pulsed detonation combustion chamber for PDE. In: High-speed deflagration and detonation: fundamentals and control. Moscow: Elex-KM Publ; 2001. pp. 239e50. [20] Cooper M, Jackson S, Austin J, Wintenberger E, Shepherd JE. Direct experimental impulse measurements for detonations and deflagrations. J Propul Power 2002;18(5):1033e41. [21] Gordon S, McBride BJ. Computer program for calculation of complex chemical equilibrium compositions and applications I: analysis. Cleveland, Ohio, USA; 1994. NASA RP 1311. [22] McBride BJ, Gordon S. Computer program for calculation of complex chemical equilibrium compositions and applications II: user manual and program description. Cleveland, Ohio, USA; 1996. NASA RP 1311. [23] Panicker PK, Wilson DR, Lu FK. Operational Issues affecting the Practical Implementation of Pulsed Detonation Engines. Canberra, Australia; 2006. AIAA Paper. p. 2006e7959. [24] Wang K, Fan W, Zhu XD, Yan Y, Gao Z. Experimental investigations on effects of wall-temperature on performance of a pulse detonation rocket engine. Exp Therm Fluid Sci 2013;48:230e7. [25] Scott J. www.aerospaceweb.org/question/propulsion/q0224.shtml; April 25, 2014. [26] Crist S, Sherman PM, Glass DR. Study of highly underexpanded sonic jet. AIAA J 1966;4(1):68e71. [27] Courant R, Friedrichs KO. Supersonic flow and shock waves. Reprint. Beijing: World Book Inc; China; 2006. [28] Ma F, Choi JY, Yang V. Thrust chamber dynamics and propulsive performance of single-tube pulse detonation engines. J Propul Power 2005;21(3):512e26. [29] Li JL, Fan W, Yan CJ, Tu HY, Xie KC. Performance enhancement of a pulse detonation rocket engine. Proc Combust Inst 2011;33:2243e54. [30] Qin J, Zhang S, Bao W, Zhou W, Yu D. Thermal management method of fuel in advanced aeroengines. Energy 2013;49:459e68. [31] Bao W, Zhang S, Qin J, Zhou W, Xie K. Numerical analysis of flowing cracked hydrocarbon fuel inside cooling channels in view of thermal management. Energy 2014;67:149e61. [32] Farzaneh-Gord M, Deymi-Dashtebayaz M. Effect of various inlet air cooling methods on gas turbine performance. Energy 2011;36:1196e205. [33] Gao W, Zhai J, Li G, Bian Q, Feng L. Performance evaluation and experiment system for waste heat recovery of diesel engine. Energy 2013;55:226e35. [34] Mohapatra AK, Sanjay. Thermodynamic assessment of impact of inlet air cooling techniques on gas turbine and combined cycle performance. Energy 2014;68:191e203.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process Ke Wang, Wei Fan*, Wei Lu, Fan Chen, Qibin Zhang, Chuanjun Yan School of Power and Energy, Northwestern Polytechnical University, Mail Box 209, 127 Youyi Xilu, Xi’an 710072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2013 Received in revised form 25 April 2014 Accepted 2 May 2014 Available online xxx
In traditional PDRE (pulse detonation rocket engines), mechanical valves are used for periodic supply control and a purge process is widely used to form a buffer zone to prevent fresh fuel-oxidizer mixture from pre-ignition. However, both of them increase hardware complexity and limit increase of operating frequency. To eliminate mechanical valves and the purge process, a valveless mode without the purge process has been proposed. Multi-cycle detonations are able to create periodic pressure oscillations inside a detonation tube, which are capable to interrupt fuel and oxidizer supply. In the present study, liquid gasoline was used because vaporization of the liquid fuel would cool hot combustion products, which acted as a buffer zone. Therefore, a liquid-fueled and valveless PDRE without the purge process became possible. When oxygen-enriched air with 25% w 45% oxygen by volume was employed, a maximum operating frequency of 110 Hz was achieved. It was observed that supply pressures of fuel and oxidizer were of great importance for such an operating mode. Exhaust plumes at different operating frequencies were also investigated. The results indicated that it was feasible for the valveless PDRE to run steadily without the purge process when liquid gasoline was utilized. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Detonation Rocket engine Frequency Oxidizer Exhaust plume
1. Introduction The PDE (pulse detonation engine) has received considerable interest over the past few decades due to its potential for increased performance and hardware simplicity. There have been many studies on PDEs worldwide and several reviews have been published. For example, Kailasanath made a review on the propulsion applications of detonation waves [1]; Nettleton reviewed the work on gaseous detonations [2]; then Kailasanath summarized the developments on PDEs [3]; Roy et al. [4] published a literature on the pulse detonation propulsions including the challenges, current status and future perspective; Kailasanath concluded the work on liquid-fueled detonations in tubes [5] and the research on pulse detonation combustion systems [6]; more recently, Wolanski gave a conclusion and discussion on detonative propulsions including various detonation engines [7]. With oxidizer on board, the PDE works in a rocket mode. Like traditional PDE, the PDRE (pulse detonation rocket engines) operates by repeatedly generating detonation waves that propagate through fuel-oxidizer mixture and produce high chamber pressure intermittently which resulting
* Corresponding author. E-mail address: [email protected] (W. Fan).
in discrete impulses [8]. The basic structure of the PDE is a straight tube, also known as a detonation tube or chamber, with one end closed and the other open. A basic conventional detonation cycle consists of the following processes: (a) filling process, i.e., the detonation tube is filled with fresh detonable fuel-oxidizer mixture; (b) the mixture is ignited near the tube closed end and detonation is initiated directly or indirectly through a DDT (deflagration-to-detonation transition) process; (c) a self-sustained detonation wave, which compresses the fuel-oxidizer mixture by shock waves and initiates combustion of reactants, propagates toward the open end; (d) the combustion products exhaust out of the detonation tube through a blow-down process; (e) purge process, i.e., the purge gas is injected to form a buffer zone for isolating the hot combustion products from contact with the fresh detonable fuel-oxidizer mixture to prevent pre-ignition. The purge process was always needed to ensure stable operation of the multi-cycle PDRE in previous studies. For example, Lu et al. [9] used air as purge gas; Li et al. [10] utilized nitrogen as purge gas; helium was adopted as purge gas by Kasahara et al. [11] in their studies on the PDRE; Matsuoka et al. employed helium as the purge gas to produce a buffer zone between adjacent cycles in their work on an inflow-driven system of a PDE [12] and thrust measurement of a pulse detonation combustor [13] as well. However, it takes certain time to accomplish injection of purge gas into the
http://dx.doi.org/10.1016/j.energy.2014.05.002 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
2
K. Wang et al. / Energy xxx (2014) 1e10
detonation tube during the purge process, which occupies considerable proportion of a single detonation cycle. Therefore, together with an injection process, a DDT process, a propagation process, and a blow-down process, it limits the overall cycle time and puts a ceiling on the upper limit of the operating frequency. In addition, it requires independent supply system that increases the hardware complexity. Thus, if the purge process was eliminated, great benefits can be derived as the complicated injection systems will be greatly simplified, e.g., complexity of the rotary valve for gaseous fuel in Ref. [14] and the rotary valves for liquid fuels in Ref. [15] and another valve system for the PDE in Ref. [16] can be reduced. In fact, operation of the PDE results in periodic pressure oscillations inside the detonation tube, which will interrupt the injection of fuel and oxidizer periodically. Thus mechanical valves such as solenoid valves and rotary valves, for supply control but restricting the operating frequency of the practical PDE due to their own responding frequencies, can be removed. Eliminating the purge process and utilizing periodic pressure oscillations inside the detonation tube for supply control can reduce the hardware complexity and, meanwhile, increase the operating frequency of a PDRE, which is highly attractive. However, residual hot combustion products in previous cycle will ignite the fresh charge without the buffer zone created by the purge process. Valaev et al. [17] proposed that cooled combustion products through special designed cooling means would form a buffer zone and ensure stable operation of multi-cycle detonations in their patent. DeRoche [18] patented another somewhat similar idea, which suggested that the pressure oscillations inside the detonation chamber was capable of intermittent supply control and the combustion products cooled in the feeding lines could prevent uncontrolled ignition of the fresh detonable mixture. Practical implementation of the above schemes was experimentally performed by Baklanove et al. [19], and they pointed out that “gasdynamic valves” were produced by the pressure oscillations. A maximum operating frequency of 92 Hz was obtained with forced cooling and gaseous fuel-oxidizer mixture in their experiments. This study presents a successful attempt to achieve stable operation of a valveless PDRE without the purge process. The difference, between the current study and previous ideas and implementation, lies in liquid fuel instead of gaseous fuel was used and forced cooling by special designed means was removed because vaporization of the liquid fuel would cool the hot combustion products. Oxygen-enriched air was utilized as oxidizer in this work. Different oxygen percentages by volume were also tested to determine the upper and lower limits for stable operation without the purge process. In addition, exhaust plumes under different operating frequencies were observed and the operational mechanisms were analyzed.
Fig. 1 shows the schematic diagram of this valveless operation sequences in actual operations. As illustrated in Fig. 1, fuel and oxidizer are separately injected into the detonation tube where they are mixed to form a detonable mixture. After initiation of combustion, a detonation wave is formed often by the DDT process and then transverses through the tube. Now pressure inside the tube increases higher than fuel and oxidizer supply pressures, thus a gasdynamic valve forms interrupting the flow of fuel and oxidizer; meanwhile, some combustion products penetrate into the oxidizer feeding line. As the pressure inside the tube diminishes due to outflow of detonation wave into the ambience, fuel and oxidizer supply resumes when pressure inside the tube drops below supply pressures (the gasdynamic valve disappear). Vaporization of the liquid fuel will cool hot combustion products to form a buffer zone even though no purge gas is employed. Then the detonation tube is filled with fresh detonable mixture. At this point in time, the system has undergone a complete operation cycle and then the cycle is repeated. The pressure trace measured at the closed end of a detonation tube is shown in Fig. 2, which is similar to that in Ref. [20]. As shown in Fig. 2, different processes corresponding to Fig. 1 are presented. If supply pressures of fuel and oxidizer (pf and po) are appropriate, e.g., lower than 1.0 MPa as shown in Fig. 2, fuel and oxidizer supply will be stopped due to higher pressure inside the detonation tube. Therefore, based on previous analysis, a liquid-fueled PDRE without the purge process should be operated steadily in this valveless mode.
3. Experimental investigation 3.1. Experimental setup To ensure implementation of this idea, complex mechanical valves, such as solenoid valves and rotary valves, were removed as intermittent supply control was realized by “gasdynamic valves”.
2. Description of the principle To isolate fresh detonable mixture from direct contact with hot combustion products when eliminating the purge process, a buffer zone of cooled combustion products is also required. However, cooling of hot combustion products is accomplished by neither taking the methods of forced cooling in Refs. [17,19] nor through the feeding lines in Ref. [18] but mainly depending on the liquid fuel. When liquid fuel is injected into the detonation tube, it will be vaporized by contact with hot combustion products, which cools them down to a temperature at which the explosion induction period of the fresh combustible mixture exceeds the time of contact with cooled combustion products [19]. Furthermore, oxygenenriched air, instead of pure oxygen, was employed as oxidizer to increase the explosion induction period in this work.
Fig. 1. Schematic of the valveless operation sequences.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
3
Fig. 4. Schematic of the detonation tube.
Fig. 2. Pressure trace at the closed end of a detonation tube.
Only electrically operated valves were needed to control on/off before and after each run. Fuel and oxidizer should be injected into the detonation tube separately to avoid their premixing; otherwise, premixed fuel-oxidizer mixture would be ignited by contact with hot combustion products leading to a deflagration. The TTL (transistoretransistor logic) control signals for supply and ignition are shown in Fig. 3. As illustrated in the figure, supply of fuel and oxidizer was not interrupted by mechanical valves during the operation, and ignition was initiated by a phase delay after the fuel and oxidizer supply to activate pressure oscillations inside the detonation tube. The detonation tube utilized is shown in Fig. 4. It comprised three sections, such as an injection and ignition section, a DDT section and a measurement section. An ordinary automobile spark plug with an ignition energy of about 50 mJ was used to initiate combustion. A typical Shchelkin spiral, which could accelerate the DDT process, was installed in the DDT section. Three detonation tubes with different dimensions, i.e., large model with long DDT section (Tube A), large model with short DDT section (Tube B), and small model (Tube C), were utilized in this work. The dimensions of these tubes are shown in Table 1. For Tubes A and B, the pitch and wire diameter of the Shchelkin spiral were 30 mm and 4 mm creating a blockage ratio of 0.46. For Tube C, a Shchelkin spiral, whose pitch and wire diameter were 24 mm and 3 mm creating a blockage ratio of 0.43, was installed in the DDT section. Three pressure transducers (p1, p2, and p3) were employed to record the pressure history along the tube. Oxygen-enriched air and liquid gasoline were utilized. A series of gas tanks with initial pressure higher than 10 MPa were
Fig. 3. TTL control signals for supply and ignition.
Table 1 Dimensions (in mm) of the detonation tube. Tube
d
l
l1
l2
l3
l4
l5
l6
l7
A B C
30 30 24
880 780 660
160 160 110
360 260 230
360 360 320
100 100 60
600 500 330
670 570 400
740 640 470
employed for oxidizer supply. One regulator was adopted in the oxidizer feeding passage to ensure required supply pressure. Liquid gasoline was stored in an independent tank and it was forced out by nitrogen with certain pressure. To reduce the fuel depositing on the tube wall, liquid gasoline was sprayed into the detonation tube through a pressure swirl atomizer axially while oxidizer was injected through an annular gap surrounding the atomizer. Dynamic piezoelectric pressure transducers (SINOCERA CY-YD-205, natural frequency larger than 200 kHz with the measurement precision of 3%) were utilized. The pressure transducers were connected to a multi-channel data acquisition system through a signal conditioner module. The pressure transducers were sampled at 200 kS/s. In addition, the pressure transducers were recessed in mounting ports and water-cooled to prevent them from being damaged by extremely high temperature. 3.2. Results and discussion 3.2.1. Tube A operation Experiments were conducted to validate whether or not the PDRE could reach stable operation under such an operating mode. Tube A was used in the preliminary experiments. Supply pressures of fuel and oxidizer were set to be identical to ensure that fuel and oxidizer would stop supplying simultaneously during the period when the pressure inside the detonation tube was higher than supply pressure. It was observed that when supply pressures were lower than 0.70 MPa and oxidizer with a volume percentage of 35% oxygen was used, stable operations could be obtained and a maximum operating frequency of 50 Hz was achieved. Oxidizer with different oxygen percentages by volume were also tried to determine the upper and lower limits of oxygen percentage for stable operations. Experimental results indicated that, with liquid gasoline as fuel, stable operations could be achieved when oxygen percentages varied from 25% to 45% in the present study. Lower oxygen percentage required larger flow rate for adequate oxidizer filling under the same operating frequency to achieve fullydeveloped detonations. Whereas, higher oxygen percentage in oxidizer would induce pre-ignition of fresh fuel-oxidizer mixture by contacting with cooled combustion products of the same temperature, as in this case the explosion induction period of the fresh combustible mixture might be shorter than the time of contact
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
4
K. Wang et al. / Energy xxx (2014) 1e10
with cooled combustion products. This device may not be able to replace traditional PDREs in this case. However, this is just a preliminary try on such an operating mode. After further attempts, other types of liquid fuels may be available under such an operating scheme leading to the application of an oxidizer with higher percentage of oxygen. Thus, the liquid-fueled and valveless PDRE has the potential to replace the traditional one. Measured peak pressure decreased when operating frequency increased above 50 Hz, which was believed due to pressure decay along the tube caused by lower fill fractions. Fill fraction was defined as the volume ratio of oxidizer and fuel injected into the detonation tube during each cycle to the detonation tube. As the supply was pulsating, it was hard to measure the flow rate accurately by flow meters. In the current study, the flow rate of oxidizer was determined indirectly by pressure changes of the oxidizer source, while the flow rate of fuel was also indirectly determined but by mass changes of the supply system. 3.2.2. Tube B operation In accordance with the fact that smaller volume may exhibit higher fill fraction, Tube A was replaced by Tube B, i.e., the DDT section was replaced by a shorter one with a length of 260 mm, which was also proved capable of facilitating the DDT process successfully. For the operation of Tube B, a maximum detonation frequency of 60 Hz was obtained, and the measured pressure profiles are shown in Fig. 5. As illustrated in Fig. 6, the fill fraction decreased with operating frequency (less than 50 Hz); it slightly increased at 50 Hz when the tube length changed, but then it decreased again when the operating frequency was increased. This was helpful to understand the variations of measured peak pressure and wave velocity with operating frequency shown below. Comparisons of measured peak pressure and wave velocity with CeJ (Chapman-Jouguet) values are shown in Figs. 7 and 8. The CeJ pressure and velocity calculated by CEA (Chemical Equilibrium with Applications) codes (analysis on the codes [21] and users manual and program description [22] were used), were 2.69 MPa and 2034.1 m/s. It should be noted that the actual pressure and temperature of fuel-oxidizer mixture were hard to be determined as the flow and heat transfer processes inside the detonation tube were very complicated. Yet, it was believed that the pressure of fresh charge before detonation was close to ambient pressure and its temperature varied little compared to the initial state. Ambient pressure and temperature (also the fuel and oxidizer supply temperature), 1.0 bar and 282 K, were employed in the calculation. An
Fig. 5. Pressure profiles measured at 60 Hz for Tube B operation.
Fig. 6. Variation of fill fraction with frequency.
Fig. 7. Comparison of measured peak pressure with CeJ pressure.
equivalence ratio of 1.71 was determined using the measured fuel and oxidizer flow rates. As shown in Fig. 7, measured peak pressures by the transducers were higher than CeJ pressure when the operating frequency below 35 Hz, which, probably, due to that the initial pressure was greater than the ambient pressure because of large filling amount during each cycle. When the operating frequency increased to 40 Hz and 50 Hz, the peak pressures decreased to lower than CeJ value due to decay of detonation wave when propagating toward the open end caused by inadequate filling. This was proved by the fact that the peak pressures increased distinctly at 50 Hz by shortening the tube length, resulting in a greater fill fraction. However, the peak pressures decreased with operating frequency once again and only a maximum frequency of 60 Hz was achieved under current supply condition. In addition, fuel and oxidizer might not be uniformly filled from the closed end to the open end; the space around the tube wall probably not be filled with the fuel-oxidizer mixture, once the filling amount decreased, due to the influence of boundary layer. That was why all three pressures were affected at 40 Hz compared to 30 Hz instead of p3 to be affected first, then p2, and finally p1. Besides, detonation wave velocity could be obtained by the time-of-flight method as introduced in Ref. [23] and employed in Ref. [15]. Here, v12 andv23 denoted average velocities between transducers p1 and p2, p2 and
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
5
Table 2 Average peak pressure and wave velocity at 100 Hz for different oxidizer supply pressures. Pressure (MPa)
p1 (MPa)
p2 (MPa)
p3 (MPa)
v12 (m/s)
v23 (m/s)
0.60 0.65 0.75
2.39 2.46 2.76
1.90 2.03 2.33
1.55 1.79 2.07
1813 1875 2000
1609 1783 2000
Fig. 8. Comparison of measured wave velocity with CeJ velocity.
p3. The calculated wave velocities are shown in Fig. 8. As illustrated in Fig. 8, measured wave velocities were slightly lower than but approached above 90% of the CeJ velocity at different operating frequencies. The velocities are averaged ones between adjacent transducers, thus there might be higher velocities not captured. In addition, measured wave velocities slightly increased when shortening the tube length at 50 Hz, which was produced by increase of the fill fraction. Considering liquid gasoline was used in this study and liquid-fuels were assumed in gas-phase in CEA codes as well as the fact that friction of the tube wall would also reduce the propagation velocity, fully-developed detonations were believed to be obtained. There would be some possibilities of unvaporized liquid gasoline portion in the initial stage of one operation, which might reduce the actual equivalence ratio. However, with the operation progressing, the detonation tube was heated and the unvaporized liquid gasoline would be vaporized completely as no residual fuel was observed inside the detonation tube after each run. Higher operating frequencies were also tried, but fullydeveloped detonations could not be produced at all when fill fractions were lower than that of 60 Hz. However, it was believed that higher frequency detonations could be achieved in a detonation tube with smaller inner diameter or by employing electronically operated valves with larger flow rates for oxidizer, both producing a larger fill fraction. 3.2.3. Tube C operation To increase the operating frequency and validate the surmise, Tube C was employed in the subsequent experiments. In the following experiments, oxygen-enriched air with 45% oxygen by volume was used. Supply pressures of fuel and oxidizer were set at 0.60 MPa. A maximum operating frequency of 100 Hz was tried. The results indicated that measured peak pressure decreased with the increase of frequency, which was believed due to drop of the fill fraction. Therefore, supply pressure of oxidizer was increased to 0.75 MPa but the fuel supply pressure was fixed at 0.60 MPa. In this case, supply of oxidizer would start refilling before fuel. Table 2 lists measured peak pressures and wave velocities at 100 Hz under different oxidizer supply pressures. It can be seen that both the peak pressure and the wave velocity increased with oxidizer supply pressure. The maximum operating frequency achieved was 110 Hz with an oxidizer supply pressure of 0.75 MPa. Pressure profiles obtained at 100 Hz and 110 Hz are shown in Figs. 9 and 10. Operating frequencies higher than 110 Hz were also attempted but the measured peak pressures decreased evidently caused by decrease of the fill fraction.
Fig. 9. Pressure profiles measured at 100 Hz.
As shown in Fig. 11, the fill fraction decreased with operating frequency when supply pressure of oxidizer was fixed at 0.60 MPa; at 100 Hz, the fill fraction increased with supply pressure; however, once again it dropped with the increase of frequency when the operating frequency exceeded 100 Hz under a supply pressure of 0.75 MPa. To verify detonations were obtained, measured peak pressure and wave velocity were also compared with CeJ values, shown in Figs. 12 and 13. Increase of oxidizer supply pressure led to change of the oxidizer flow rate that affected the equivalence ratio because the fuel flow rate was unchanged. Then the calculated CeJ pressure and velocity were different and, therefore, there were two y-coordinates for p/pCJ and v/vCJ in Figs. 12 and 13. The ambient pressure and temperature, employed in the calculation, were also 1.0 bar and 282 K but with the equivalence ratio changed. As illustrated in Fig. 12, measured peak pressures decreased with operating frequency and then increased correspondingly when
Fig. 10. Pressure profiles measured at 110 Hz.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
6
K. Wang et al. / Energy xxx (2014) 1e10
Fig. 11. Variation of fill fraction with frequency under different supply pressures of oxidizer (&100 Hz, 0.60 MPa; S100 Hz, 0.75 MPa).
velocity with CeJ value, shown in Fig. 13, indicated that variation of measured wave velocity with frequency was similar to that of peak pressure. Both phenomena were due to variation of the fill fraction. As illustrated in Figs. 12 and 13, peak pressures measured by p1 and p2 approached above 80% and 70% of CeJ pressure, and measured wave velocity approached above 85% of CeJ velocity when the operating frequency was below 110 Hz. Yet, when the operating frequency increased to 120 Hz, measured peak pressure and wave velocity dropped evidently. For instance, peak pressure measured by p1 was about 70% of the CeJ value while the wave velocity between p1 and p2 reached about 85% of the CeJ value, and both the peak pressure and the wave velocity decreased distinctly along the tube. For frequencies higher than 80 Hz, the pressure and velocity dropped with distance. As fully-developed detonations were obtained at frequencies below 80 Hz, the mixture cell size should not account for such a trend. It was believed that decrease of the fill fraction was responsible for this since detonation waves decayed when propagating inside the tube not fully filled with the fueloxidizer mixture. In a word, the maximum operating frequency obtained was 110 Hz in this study. However, since no other restrictions in increasing the operating frequency were observed in the present research, it was possible to increase the operating frequency further if greater fill fractions could be achieved by replacing the detonation tube with a smaller volume or employing electronically operated valves with larger flow rates. The oxidizer supply pressure was increased to 0.75 MPa and fuel supply pressure was fixed at 0.60 MPa when the operating frequency exceeded 100 Hz. In this case, the buffer zone consisted of a portion of oxidizer fed before liquid fuel and the cooled combustion products. However, with operating frequency below 100 Hz, the PDRE could not operate in this valveless mode when the oxidizer supply pressure was above 0.65 MPa. It was believed that greater pressure oscillations were produced when the operating frequency exceeded 100 Hz, which was able to interrupt the supply with a pressure of 0.75 MPa. This indicated that appropriate supply pressures of fuel and oxidizer were crucial for implementation of the valveless operating mode without the purge process. 4. Exhaust plumes
Fig. 12. Comparison of measured peak pressure with CeJ pressure.
oxidizer supply pressure was increased at 100 Hz but dropped again with the operating frequency exceeding 100 Hz. Anyway, variation of the measured peak pressures with frequency was consistent with that of fill fraction. Comparison of measured wave
Fig. 13. Comparison of measured wave velocity with CeJ velocity.
During the experiments, exhaust plumes were recorded when operations of the PDRE were carried out until tube-wall ignition happened. As discussed in Ref. [24], the tube-wall ignition indicated that fresh detonable fuel-oxidizer mixture was ignited by hot tube wall after a long run duration as no cooling measure was adopted. Each operation was divided into three stages (such as initial stage, middle stage and terminal stage) equally with an equivalent time span according to the overall run time. Different exhaust plumes, varying with the operating frequency and the operation stage, were observed. According to the operating frequency, they were divided into low frequency cases (below 60 Hz) and high frequency cases (above 60 Hz). Additionally, exhaust plumes of the startup process and the deflagration mode caused by the tube-wall ignition were also observed. Since a straight tube without exit nozzles were used in the present study, an underexpanded flow, instead of an over-expanded one, with sonic speed would be produced at the exit plane. Thus, Mach disks (also known as Mach diamonds, shock diamonds, Mach rings, doughnut tails or thrust diamonds) might form in the exhaust flow. Wave structures creating Mach disks in an under-expanded flow are given in Fig. 14. Detailed explanations on the wave structures and flows can be found in Ref. [25]. For sonic jet flows, the distance between the first Mach disk and the nozzle exit plane, xM, and the diameter of the nozzle exit, de, had the following relation, proposed by Crist et al. [26],
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
Fig. 14. Wave structures in an under-expanded flow.
xM ¼ 0:6455 de
rffiffiffiffiffi pe pa
(1)
where pe and pa were the pressure of the gases exiting the nozzle and the ambient pressure. Equation (1) was obtained from steady flow experiments. When the operating frequency of a PDRE was high enough, the exhaust flow approached steady flow especially during a relatively small time span. Therefore, according to the exhaust plumes observed, the pressure of the gases exiting the nozzle could be obtained approximately. 4.1. Low frequency cases In low frequency cases, Tube A was utilized with the same equivalence ratio of 1.71. During each operation, exhaust plumes varied with the run time. Two photos of the exhaust plumes for initial stage and middle and terminal stages at an operating frequency of 46 Hz were captured. As shown in Fig. 15a, when combustion products exhausted out of the detonation tube, one Mach disk pattern with clear jet boundary was formed close to the tube open end and then the plume expanded suddenly. There existed one clear boundary perpendicular to the flow between the Mach disk pattern and the suddenly expanded jet, which might be a normal shock wave. Since the detonable mixture inside the tube was fuel-rich, passing through this normal shock wave caused the
Fig. 15. Exhaust plumes at 46 Hz ((a) Initial stage; (b) Middle and terminal stages).
7
temperature of the flow to increase, igniting any excess fuel present in the exhaust. It was this burning fuel that made the expanded jet glow. Combustion products were under-expanded when they arrived at the tube open end. Thus, a Mach disk similar to Ref. [27] formed in the exhaust flow. Then the combustion products suddenly expanded into the atmosphere, which was investigated in the study of Ma et al. [28]. Unlike at the initial stage of the operation, only thicker plume without evident boundaries was produced at the middle and terminal stages (Fig. 15b) because the tube wall was heated up after some period of operation. Then the initial temperature of the detonable mixture was increased by contact with the tube wall, which would lead to a lower detonation pressure. This was actually observed in this study, e.g., peak pressure recorded by p2 decreased from about 3.0 MPa to 2.3 MPa after a run of 18 s. Lower detonation pressure led to drop of the time-averaged pressure inside the detonation tube. Therefore, no Mach disk with evident boundaries was produced at the middle and terminal stages. Based on Fig. 15a, the value of xM/de, which was 1.493, could be obtained as the exit diameter was known. Thus, substituting this value in Eq. (1), pe ¼ 5.35pa was obtained, which indicated that the flow was under-expanded. As illustrated in Fig. 16, the exhaust plumes at an operating frequency of 56 Hz had some difference with that at 46 Hz. First, two Mach disks, instead of only one, were produced close to the tube exit at the initial stage of the operation. Second, a middle stage with one evident Mach disk was observed. Third, one Mach disk still existed at the terminal stage except that it was not that legible as before, but was still similar to that of the initial stage at 46 Hz. This, probably, was because the time-averaged pressure inside the detonation tube increased due to higher frequency detonations
Fig. 16. Exhaust plumes at 56 Hz ((a) Initial stage; (b) Middle stage; (c) Terminal stage).
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
8
K. Wang et al. / Energy xxx (2014) 1e10
burned more fuel-oxidizer mixture in detonation mode. Based on Eq. (1), the pressures of the gases exiting the nozzle corresponding to the three stages pictured in Fig. 16 were 6.34pa, 5.10pa, and 4.34pa, respectively. Comparison of the exhaust plumes of 46 Hz and 56 Hz indicated that the time-averaged pressure inside the detonation tube increased with detonation frequency. 4.2. High frequency cases In high frequency cases, the detonation frequency was greater than 60 Hz and Tube C was utilized with an equivalence ratio of 1.09. The time-averaged pressure inside the tube increased distinctly compared with the low frequency cases, which resulted in different exhaust plumes. As shown in Fig. 17, the exhaust plumes at 100 Hz exhibited some differences from those of the low frequency cases. For instance, two Mach disk patterns with clear boundaries existed at the initial stage, but the second jet pattern had a greater length than the first one. Similar to the low frequency cases, the exhaust plumes varied with operation time. The second jet pattern died away with the operation progressing and it almost disappeared at the middle stage; finally, only the first one could be distinguished at the terminal stage, as shown in Fig. 17. The reason was the same as what discussed in low frequency cases, i.e., heat transferred from the tube wall increased initial temperature of the fresh detonable mixture. In the same way, it could be obtained that the pressures of the gases exiting the nozzle corresponding to the three stages in Fig. 17 were 6.55pa, 6.01pa, and 5.43pa, respectively, which were greater
than that at 56 Hz but the trend that dropped with operation time was consistent. Therefore, the time-averaged pressure, at 100 Hz, inside the detonation tube was higher than that of 56 Hz. In addition, the nondimensional distances, defined as the distance between the tube exit and the tail of the visible exhaust flames divided by inner diameter of the detonation tube, at three different stages were around 18.7, 17.8, and 18.2. However, at 56 Hz, as shown in Fig. 16, they were about 14.0, 13.6, and 12.5, respectively. This indicated that the exhaust flames in high frequency cases had greater penetrability, which implied that higher velocities of the exhaust flow were obtained in high frequency cases because of higher time-averaged pressure. The exhaust plumes at high frequency presented snow-white and blue color due to higher temperature with the equivalence ratio close to unity; yet, color of the exhaust plumes in low frequency cases were orange-yellow with lower temperature because the fuel-oxidizer mixture was fuel-rich, inducing incomplete combustion. 4.3. Startup process As shown in Fig. 3, ignition was initiated by a phase delay after the fuel and oxidizer supply to ensure activation of pressure oscillations inside the detonation tube. Actually, the phase delay was managed manually by initiating the fuel and oxidizer supply some time (about 0.2 s) before ignition in the present work. Then some special phenomena were obtained during the startup process. As illustrated in Fig. 18, the exhaust plumes varied greatly during the short period for the startup process. The time span between Fig. 18a and f was 0.21 s and the time interval between two adjacent pictures was 42 ms. The exhaust plumes from Fig. 18aef were captured in a chronological order. After ignition, detonation waves were obtained but the exhaust plumes were quite different from those discussed previously. As shown in Fig. 18a, a bright and long flame was formed. Because fuel and oxidizer were injected into the detonation tube and some of them flowed outside with some fuel deposited on the tube wall at the same time before the first ignition. Then the first detonation wave formed and meanwhile the fuel-oxidizer mixture flowed outside the tube was ignited. With the operation carrying on, detonation waves blew the flame off the tube, as shown in Fig. 18b. Then, as shown in Fig. 18c, only a slim flame existed close to the tube exit as the fuel deposited on the tube wall was blown out producing too rich a mixture zone to be ignited. Then the detonation waves cleaned the fuel deposited on the tube wall and exhaust plumes, similar to Fig. 17a, formed and maintained for a while before transforming to an exhaust plume consistent with that of Fig. 17a. The flame expanded some distance away from the second jet pattern (see Fig. 18d and e), which was different from that in Fig. 18f, because of larger flow rate of oxidizer. Since the oxidizer supply passage was full with certain pressure before the start of supply, the oxidizer flow rate would decrease to a balance after some time. Temperature increase of the fuel-oxidizer mixture was not considered because the time span was only around 0.2 s, which was too short for the tube to be heated up distinctly. It could be seen that some fuel-oxidizer mixture injected ahead too much before the first ignition was wasted when burned outside the detonation tube, as illustrated in Fig. 18a. One feasible way to avoid this was to determine the phase delay of ignition than the fuel and oxidizer supply accurately. A phase delay of about 7/12T (T in Fig. 3) in each cycle was believed to be appropriate. 4.4. Deflagration mode
Fig. 17. Exhaust plumes at 100 Hz ((a) Initial stage; (b) Middle stage; (c) Terminal stage).
Since the PDRE was operated without active cooling, the detonation tube was heated up during each run. The hot tube wall
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
K. Wang et al. / Energy xxx (2014) 1e10
Fig. 18. Exhaust plumes of the startup process at 100 Hz.
would ignite the fresh fuel-oxidizer mixture once its temperature exceeded the self-ignition temperature of fresh detonable mixture, which would lead to deflagration due to inadequate filling of reactants and inappropriate ignition position. The long duration operations were carried out until the tube-wall ignition causing deflagration to happen in the present study. Fig. 19 shows exhaust plumes at the deflagration mode for different operating frequencies. Obviously, the exhaust plumes were quite different from those obtained in a detonation mode. There was almost no pressure increase for deflagration, so only slim exhaust plumes, with weaker penetrability and without sudden expansion and Mach disk, were produced. Besides, the exhaust plumes at a deflagration mode were consistent with those obtained in premixed combustion as fuel and oxidizer might be probably well completely mixed in the detonation tube. As shown in Fig. 19a and b, the exhaust plumes had a yellow cone-shaped core (in web version), where the temperature was relatively low due to incomplete combustion in fuel-rich condition, and a blue outer flame (in web version) whose temperature was higher due to complete combustion. However, the exhaust plume had a snow-white inner flame implying complete combustion and a higher temperature, as illustrated in Fig. 19c, because the equivalence ratio of fuel-oxidizer mixture was close to unity. In this study, detonation mode only lasted for a few seconds, no more than 20 s, before the tube wall ignition inducing deflagration happened. Therefore, active cooling [23] should be employed to ensure stable operation during long duration runs, but active cooling based on liquid fuel preheating utilized to enhance the performance of PDREs [29] or for thermal management in advanced aeroengines [30] is impractical as liquid fuel at a low temperature is crucial in such an operating mode. Although heat exchange will cause cracking of the liquid hydrocarbon fuel inside
Fig. 19. Exhaust plumes at deflagration mode ((a) 46 Hz; (b) 56 Hz; (c) 100 Hz).
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002
9
10
K. Wang et al. / Energy xxx (2014) 1e10
cooling channels [31], which is beneficial for the DDT process. Therefore, cooling methods employed in gas turbine engines [32] and waste heat recovery in diesel engines [33] may provide other considerations for effective cooling under such an operating mode. In addition, the impact of cooling measures on the thermodynamic performance should be paid special attention to because they may lead to a decrease of the combined cycle performance [34]. 5. Conclusions To increase the operating frequency and meanwhile reduce the hardware complexity, a valveless mode without the purge process was proposed and experimentally performed. Liquid gasoline was used as fuel because vaporization of the liquid fuel, after being injected into the tube, would consume a lot of heat that inevitably resulted in a temperature drop of the combustion products. Oxygen-enriched air was utilized as oxidizer. Experimental results indicated that stable operation of a PDRE without the purge process could be achieved and a maximum operating frequency of 60 Hz was obtained due to restriction of the flow rate. It was observed that the PDRE was able to run steadily when oxidizer with 25% w 45% oxygen by volume was employed. Appropriate supply pressures for fuel and oxidizer were also found to be important for stable operations in this operating mode. When utilizing a detonation tube with smaller volume, a maximum operating frequency of 110 Hz was achieved. No other restrictions except for the flow rate in increasing the operating frequency were observed in this study. Therefore, higher operating frequency was believed to be attainable if the detonation tube with a smaller volume or larger flow rates were available. Additionally, different exhaust plumes were observed under different operating frequencies. The analysis about them showed that higher operating frequency resulted in larger exhaust flow velocity due to higher time-averaged pressure inside the detonation tube. In conclusion, the experimental results revealed that it was feasible for the valveless PDRE to be operated stably without the purge process when liquid gasoline was utilized to cool hot combustion products to form a buffer zone. In this valveless mode, high frequency two-phase detonations were achieved and the hardware complexity of the practical PDRE could be greatly reduced. Acknowledgments This work is supported by National Natural Science Foundation of China (51176158, 51376151), Doctoral Program Foundation of Education Ministry of China (20126102110029), Doctorate Foundation of Northwestern Polytechnical University (CX201112) and Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China. The authors wish to thank Le Jin, Qiang Xiao, Haoyi Song, and Yongjian Zhang for their assistance in carrying out the experiments. References [1] Kailasanath K. Review of propulsion applications of detonation waves. AIAA J 2000;38(9):1698e708. [2] Nettleton MA. Recent work on gaseous detonations. Shock Waves 2002;12(1): 3e12. [3] Kailasanath K. Recent developments in the research on pulse detonation engines. AIAA J 2003;41(2):145e59.
[4] Roy GD, Frolov SM, Borisov AA, Netzer DW. Pulse detonation propulsion: challenges, current status and future perspective. Prog Energy Combust Sci 2004;30(6):545e672. [5] Kailasanath K. Liquid-fueled detonations in tubes. J Propul Power 2006;22(6): 1261e8. [6] Kailasanath K. Research on pulse detonation combustion systems e a status report. Orlando, Florida, USA; 2009. AIAA Paper. p. 2009e2631. [7] Wolanski P. Detonative propulsion. Proc Combust Inst 2013;34:125e58. [8] Brophy CM, Hanson RK. Fuel distribution effects on pulse detonation engine operation and performance. J Propul Power 2006;22(5):1155e61. [9] Lu FK, Meyers JM, Wilson DW. Experimental study of a propane-fueled pulsed detonation rocket. Norfolk, Virginia, USA; 2003. AIAA Paper. p. 2003e6974. [10] Li JL, Fan W, Chen W, Wang K, Yan CJ. Propulsive performance of a liquid kerosene/oxygen pulse detonation rocket engine. Exp Therm Fluid Sci 2011;35:265e71. [11] Kasahara J, Hirano M, Matsuo A, Daimon Y, Endo T. Thrust measurement of a multicycle partially filled pulse detonation rocket engine. J Propul Power 2009;25(6):1281e90. [12] Matsuoka K, Yageta J, Nakamichi T, Kasahara J, Yajima T, Kojima T. Inflowdriven valve system for pulse detonation engines. J Propul Power 2011;27(3): 597e607. [13] Matsuoka K, Esumi M, Ikeguchi KB, Kasahara J, Matsuo A, Funaki I. Optical and thrust measurement of a pulse detonation combustor with a coaxial rotary valve. Combust Flame 2012;159:1321e38. [14] Lo PP, Gonzalez DE. Development of a fuel injection system for a high frequency pulse detonation engine. Seattle, Washington, USA; 1997. AIAA Paper. p. 1997e2744. [15] Wang K, Fan W, Zhu XD, Jin L, Chen F. Experimental studies on rotary valves for single-tube pulse detonation rocket engines. Proc IMechE Part G-J Aerosp Eng 2014;228:262e70. [16] Meholic GV. Rotary valve for pulse detonation engines 2001. United States Patent 6505462B2. [17] Valaev AA, Zhimerin DG, Mironov EA, Popov VA. Method and apparatus for intermittent combustion 1976. United States Patent 3954380. [18] DeRoche M. Repetitive detonation Generator 1998. United States Patent 5800153. [19] Baklanove DI, Gvozdeva LG, Scherbak NB. Pulsed detonation combustion chamber for PDE. In: High-speed deflagration and detonation: fundamentals and control. Moscow: Elex-KM Publ; 2001. pp. 239e50. [20] Cooper M, Jackson S, Austin J, Wintenberger E, Shepherd JE. Direct experimental impulse measurements for detonations and deflagrations. J Propul Power 2002;18(5):1033e41. [21] Gordon S, McBride BJ. Computer program for calculation of complex chemical equilibrium compositions and applications I: analysis. Cleveland, Ohio, USA; 1994. NASA RP 1311. [22] McBride BJ, Gordon S. Computer program for calculation of complex chemical equilibrium compositions and applications II: user manual and program description. Cleveland, Ohio, USA; 1996. NASA RP 1311. [23] Panicker PK, Wilson DR, Lu FK. Operational Issues affecting the Practical Implementation of Pulsed Detonation Engines. Canberra, Australia; 2006. AIAA Paper. p. 2006e7959. [24] Wang K, Fan W, Zhu XD, Yan Y, Gao Z. Experimental investigations on effects of wall-temperature on performance of a pulse detonation rocket engine. Exp Therm Fluid Sci 2013;48:230e7. [25] Scott J. www.aerospaceweb.org/question/propulsion/q0224.shtml; April 25, 2014. [26] Crist S, Sherman PM, Glass DR. Study of highly underexpanded sonic jet. AIAA J 1966;4(1):68e71. [27] Courant R, Friedrichs KO. Supersonic flow and shock waves. Reprint. Beijing: World Book Inc; China; 2006. [28] Ma F, Choi JY, Yang V. Thrust chamber dynamics and propulsive performance of single-tube pulse detonation engines. J Propul Power 2005;21(3):512e26. [29] Li JL, Fan W, Yan CJ, Tu HY, Xie KC. Performance enhancement of a pulse detonation rocket engine. Proc Combust Inst 2011;33:2243e54. [30] Qin J, Zhang S, Bao W, Zhou W, Yu D. Thermal management method of fuel in advanced aeroengines. Energy 2013;49:459e68. [31] Bao W, Zhang S, Qin J, Zhou W, Xie K. Numerical analysis of flowing cracked hydrocarbon fuel inside cooling channels in view of thermal management. Energy 2014;67:149e61. [32] Farzaneh-Gord M, Deymi-Dashtebayaz M. Effect of various inlet air cooling methods on gas turbine performance. Energy 2011;36:1196e205. [33] Gao W, Zhai J, Li G, Bian Q, Feng L. Performance evaluation and experiment system for waste heat recovery of diesel engine. Energy 2013;55:226e35. [34] Mohapatra AK, Sanjay. Thermodynamic assessment of impact of inlet air cooling techniques on gas turbine and combined cycle performance. Energy 2014;68:191e203.
Please cite this article in press as: Wang K, et al., Study on a liquid-fueled and valveless pulse detonation rocket engine without the purge process, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.05.002