- Email: [email protected]
Effect of aerodynamic valve on backflow in pulsed detonation tube
Aerospace Science and Technology 25 (2013) 1–15
Contents lists available at SciVerse ScienceDirect
Aerospace Science and Technology www.elsevier.com/locate/aescte
Effect of aerodynamic valve on backflow in pulsed detonation tube Qiu Hua ∗ , Xiong Cha, Yan Chuan-jun, Zheng Long-xi School of Power and Energy, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 November 2010 Received in revised form 23 November 2011 Accepted 1 December 2011 Available online 7 December 2011
To research the operating characteristics of aerodynamic valves, which are applied for pulse detonation engine with adaptive control for fuel, seven aero-valves were designed and tested in a square pulse detonation tube. Shadowgraph images, self-luminous photos and pressure measurement were used to investigate the abilities of the aerodynamic valves to decrease the backward pressure for valveless pulsed detonation tube. The experimental results show that residual fuel droplets moved upstream with backflow followed by flame, and flame could propagate across the valves. The heat release at upstream of the valve weakened the ability of the valve to damp the back pressure. Run-free sheet steel was introduced into the perforated aerodynamic valve and the inference is verified. Influences of hollow region and venturi on the throat-type aerodynamic valves were also investigated. At last, performance of each aerodynamic valve was analyzed by the total-pressure recovery coefficients of positive and reverse direction. Crown Copyright © 2011 Published by Elsevier Masson SAS. All rights reserved.
Keywords: Pulsed detonation engine Aerodynamic valve Adaptive control Shadowgraph
1. Introduction Pulsed detonation engines (PDEs) are unsteady propulsion devices that produce thrust by using repetitive detonation. Pulse detonation engines exhibit several performance advantages in comparison with current steady-deflagration jets. The inherent mechanical design simplicity of the PDE results in smaller packaging volumes and lower part counts, aiding in integration and maintenance. One of the key technologies for a practical detonation based engine is its valving scheme, which is used to prevent the combustor flow from traveling into the inlet during certain periods of the operation cycle. Based on how this function is realized, PDEs can be classified as either valved or valveless/aerovalve, as summarized by Roy et al. [10]. Use of mechanical valves, such as disk-shaped rotary valve [1,5], to control incoming flow rate of fresh air into detonation tube can prevent detonations or shocks from moving outwards from detonation tube through inlet and provide a sufficient time for mixing of fuel with air. But mechanical valve will add engine weight and increase complexity of PDE. Valveless PDE concepts imply continuous or intermittent supply of propellants to the detonation tube without using mechanical valves, which means mechanically simpler. And several aerodynamic configurations were designed, such as supersonic isolator inlet [3], circumferential seam configuration [12], naturally aspirated fluidic control device [13] and so on. Ma [6] simulates PDE inlet aerodynamics and its response to downstream disturbances. The results indicate
*
Corresponding author. Fax: +86 029 88492748. E-mail address: [email protected] (H. Qiu).
1270-9638/$ – see front matter Crown Copyright doi:10.1016/j.ast.2011.12.003
© 2011
that strong pressure disturbance may propagate upstream to the inlet and make the device more vulnerable to inlet instability. Piton et al. [9] take shadowgraph images at entrance of a valveless detonation tube and it’s shown that shock wave was discharged from inlets. Until recently, most studies have focused on the flow dynamics and propulsive performance of valved PDEs, with only limited efforts on valveless PDEs. Few references are associated with the research of aerodynamic valve for PDEs, especially for PDEs with adaptive control for fuel. The purpose of this research is to investigate the performance of two types of aerodynamic valves for damping backflow. By the use of high-speed shadowgraph images, self-luminous photos and pressure measurement, the present work has produced a good understanding of the operating mechanism of aerodynamic valve and provides guidelines for design and optimization of aerodynamic valve. 2. Experimental setup The experiments were carried out in a 2.63-m-long-square transparent detonation tube with a 60 mm × 60 mm cross section as shown in Fig. 1. Air duct passes the air in pressure tank to detonation tube through a switching section that is ∅70 mm circular cross section to 60 mm × 60 mm square section transition. The detonation tube comprises 0.715-m-long mixing section and 1.915-m-long detonation section. A staggered array of thirty-three 10 mm × 10 mm × 60 mm strip obstacles is inserted and fixed to the top and bottom inner wall of the detonation section and the spacing between the adjacent obstacles is 50 mm. And there are sixteen strip obstacles attached
Published by Elsevier Masson SAS. All rights reserved.
2
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 1. Schematic of experimental setup and the position of pressure transducers.
Fig. 2. Schematic of the improved twin-fluid air-assist atomizer.
to the top inner wall and the clearance between the adjacent obstacles is 100 mm. An improved twin-fluid air-assist atomizer is used for gasoline injection and the configuration is shown in Fig. 2 where fuel is injected at top of tube and the pressure-air is injected at bottom of tube. A cone is used to be an inflow diversion. The liquid gasoline, pressure-air and main air flow are introduced to detonation tube by adaptive control without valve and an air flow meter and a gear meter are used to measure the average flow rate of air and fuel respectively. The atomizer is mounted at the position of 330 mm from spark ignition. The spark is produced via a standard automotive inductive ignition system and supplied approximately 50 mJ of energy. Aerodynamic valves can be assembled at four positions as shown in Fig. 1, that is Pos. 1–Pos. 4. The measurements of propagation velocity and pressure records are performed with pressure transducers, which are embedded in side walls, namely P1, P2, P3, P4, P5, P6, P7 as shown in Fig. 1. P1–P5 mounted at mixing section are piezoresistive pressure transducers with 500 kHz resonant frequency and 1 μs rise time. P6–P7 mounted at detonation section are piezoelectric pressure transducers with 200 kHz resonant frequency and 2 μs rise time to diagnose whether detonation wave form. The advantage of piezoresistive pressure transducer is excellent response to zero-frequency,
much approach real pressure alteration, and there is no overshoot of pressure. But the piezoresistive pressure transducer is made of diffuse silicone material and is much sensitive to temperature. Since all of experiments were operated under working frequency lower than 4 Hz during one to two seconds, and the residual time of the hot backward flow in mixing section lasted only several milliseconds, effects of temperature on the records of pressure transducer could be neglected. Gasoline and air propellants at an average equivalence ratio of 1.2 (rates of air flow and fuel were approximately 124 kg/h and 5.2 ml/s respectively) were used because it is relatively easy to achieve detonation combustion at initial pressure of 101.3 kPa. A high-speed shadowgraph visual system was used to record flame and flow field structures. The diameter of spherical mirror is 300 mm and the focal distance is 3000 mm. A xenon lamp with 500 W was used as light source. The maximum sampling frame in a second of PhantomV7.2 high-speed camera is 200 000 frames and the minimum exposure time is 2 μs. An N/D light filter was fixed behind the camera to adjust light exposure. The sample rates and exposure time of the camera were set to 50 000 fps and 4 μs respectively for the experiments. PhantomV7.2 high-speed camera was also used for recording self-luminous process in mixing
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
3
Fig. 3. Shadowgraph of flow field ahead of detonation tube with 64 × ∅5 mm perforated plate and T was set to be the triggering time.
section. And the sample rates and exposure time were set to 20 000 fps and 40 μs respectively. 3. Experimental results for perforated aerodynamic valve Perforated plates can be used as a blast wave shielding technique [4]. Medvedev [7] reported the blast front velocity could be decreased by increasing the blockage ratio (BR), which is an area ratio between the blocked area and the total area. It is implied that back pressure could be damped by the perforated plates for the pulse detonation engine. 3.1. Experiment on 64 × ∅5 mm perforated plate A 60 mm × 60 mm square perforated plate with 64 × ∅5 mm hole (BR is 0.651) was mounted at Pos. 2 between P3 and P4. Fig. 3
is a shadowgraph and the character T is set to be the triggering time. The mixing section (upstream) is on the left and detonation tube (downstream) is on the right in the picture. A black gas mass (fuel droplets) appeared at 13.7 ms after ignition and moved to mixing section at speed 50 m/s. It passed through the perforated plate at 16.8 ms, and a bright flame appeared at 17.3 ms. The bright flame moved upstream at 140 m/s and was accelerated to 250 m/s after passing through the perforated plate, at the meantime, the brightness of flame became much stronger (at 18.8 ms) and diffused to the whole observation field. After 22.8 ms, the gas flow from inlet moved downstream and was accelerated by pressure difference (at 25 ms). A special phenomenon was shown that fuel droplets were ejected from downstream to upstream and a gray gas mass flowed to upstream during very short time (at 47.4 ms). It is possible that a compression wave was formed because of overexpansion during exhaust of combustion products.
4
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 4. The pressure histories of P1–P7 with 64 × ∅5 mm perforated plate.
Fig. 5. High-speed photos of self-luminous field at mixing section.
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 6. The pressure histories of P1–P7 with 64 × ∅3 mm perforated valve.
Fig. 7. Shadowgraph of flow field ahead of detonation tube with 64 × ∅3 mm + 64 × ∅4 mm double perforated plates.
5
6
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 8. The pressure histories of P1–P7 with 64 × ∅3 mm + 64 × ∅4 mm double perforated plates.
The pressure histories of P1–P7 are shown in Fig. 4. The peak pressure of the last pressure transducer P7 was up to 7 MPa and the rise time was 10 μs, which indicated a fully developed detonation wave was formed. According to Fig. 4, forward (illustrated by dashed line 2, to downstream) and backward (illustrated by dashed line 1, to upstream) compression waves were generated when gasoline/air mixture was ignited, which induced the pressure perturbation in the mixing and detonation sections. The forward compression wave was accelerated by obstacles and heat release, and then became detonation wave. Local explosion occurred at some place between P4 and P5, and the backward explosion wave propagated back up the length of mixing section and induced the pressure increase further, which is illustrated by the third dashed line (line 3). And the backward flow generated by the explosion wave was clearly displayed at 17.3 ms in Fig. 3. The backward explosion wave propagated to upstream further and mostly reflected at bend section of air duct, which formed the propagation of pressure wave as shown by dashed line 4 and caused the peak pressure at P1 up to 0.42 MPa after 18.5 ms. And the partially forward explosion wave passing through bend section was reflected at upstream of the air duct and propagated downstream again (illustrated by dashed line 5). Fig. 5 is a high-speed photo of self-luminous field at mixing section. Local explosion was found nearby fuel atomizer. A blue flame was formed and followed by a bright red flame. The blue field corresponded with oxidation procedure of big molecule at about 1100 K and the bright field corresponded with high temperature reaction to produce CO2 and H2 O. (For interpretation of the references to color in Fig. 5, the reader is referred to the web version of this article.) According to Figs. 3–5, a description of flow field at mixing section after ignition was followed: locally explosion produced a forward and a backward combustion wave after ignition. The forward pressure wave propagated to the open end of detonation tube and transformed to detonation wave between P6 and P7. The backward pressure propagated along the dashed line 1 shown in Fig. 4. But the pressure perturbation at this moment cannot be distinguished by high-speed shadowgraph camera. Then the increasing pressure in detonation tube induced gas flow mixing with fuel droplets and moving upstream. A secondary explosion was formed nearby the fuel injector. A blue flame propagated upstream and accelerated diffusion of fuel fog, as shown by the ascent of pressure between dashed line 1 and dashed line 3 in Fig. 4. More heat was released during this process and the blue flame transformed
Fig. 9. The configuration of 64 × ∅5 mm plus sheet steel valve.
Fig. 10. The flow characteristic of 64 × ∅5 mm plus sheet steel valve.
to bright flame. The resultant backward explosion wave propagating upstream was mostly reflected at bend section of air duct and the reflected compression wave propagated along path of dashed line 4, while partial of that was reflected at upstream of the air duct and propagated downstream along dashed line 5. When the detonation wave was exhausted from the open end of the detonation tube, a reflection rarefaction wave moved back and induced the pressure to decrease while the products began to exhaust the tube, as shown by the pressure decrease between dashed line 4 and dashed line 5. 3.2. Experiment on 64 × ∅3 mm perforated plate A 60 mm × 60 mm square perforated plate with 64 × ∅3 mm hole (BR is 0.874) was mounted at Pos. 2 between P3 and P4. The pressure histories (Fig. 6) and shadowgraph photos were recorded and the results were similar to that of 64 × ∅5 mm perforated plate. However, there were differences between these two plates. According to the dashed line 3 along which the compression wave caused by local explosion propagated, travel time of the wave
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
7
Fig. 11. High-speed photo of self-luminous field at mixing section with sheet steel valve (success).
from P4 to P3 became longer. And the peak pressure difference between P3 and P4 also became larger. This meant that the increasing blockage ratio decreased backward flow flux and black fuel droplets through the perforated plate, which caused the flame weaken. For the 64 × ∅3 mm perforated plate, pressure peaks at
P3 and P4 were caused by backward explosion wave which was induced by local explosion at some place between P4 and P5, while those of the 64 ×∅5 mm perforated plate were caused by reflected compression wave of the backward explosion wave at bend section of air duct.
8
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 12. High-speed photo of self-luminous field at mixing section with sheet steel valve (failure).
3.3. Experiment on 64 × ∅3 mm + 64 × ∅4 mm double perforated plates In order to prevent fuel droplets from penetrating into inlet, a double perforated valve was researched. A 64 × ∅3 mm perforated plate was mounted at Pos. 4 between P4 and fuel injector, and another 64 × ∅4 mm perforated plate was fixed at Pos. 2 between P3 and P4, as shown in Fig. 7. Fig. 7 showed that the 64 × ∅3 mm perforated plate holds some fuel droplets back, but still some fuel moved upstream. So the flame still could propagate from fuel injector to upstream through perforated aerodynamic valve and a pressure wave was formed as dashed line 3 in Fig. 8. Direction of the wave propagation between P5 to P4 was different from those of perforated aerodynamic valves before. The double perforated plates reduced the speed of flame, but the heat release of reactants was aggravated which caused by hot jet, as shown by the spherical pressure wave at 54.1 ms in Fig. 7. The spherical pressure wave propagated through 64 × ∅4 mm perforated plate and chased the preceding flame, which led to acceleration of the heat release of combustion process and the increase of pressure between P1–P5. The compression wave mostly reflected at bend section of air duct and formed the propagation of pressure wave as shown by dashed line 4. The pressure wave (shock) corresponded to cylindrical bright zone at 55.82 ms and 55.86 ms in shadowgraph, induced the pressure at P1 up to 0.41 MPa. The cylindrical bright zone cannot be ob-
Fig. 13. The configuration of center cone valve.
served in shadowgraph for single perforated valve because the flux of backflow was much more and formed a strong shadowgraph. 3.4. Experiment on perforated aerodynamic valve with sheet steel To research the influences of the additional heat release at upstream of the perforated plates on the characteristics of valve, a kind of sheet steel valve was used and mounted at Pos. 2 between P3 and P4. The configuration of sheet steel valve is shown in Fig. 9: a 64 × ∅5 mm perforated plate is fixed at left side and is connected with a 36 × ∅5 mm perforated plate at its right side by 4 supporting rod. The distance between these two perforated plates is 6 mm and a 0.5-mm-thickness run-free sheet steel (reed) with a 42 mm × 42 mm cross section is inserted between plates. When the fresh air is induced into the detonation tube, the air passes through 64 × ∅5 mm perforated plate first. Then the sheet
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
9
Fig. 14. Shadowgraph of flow field ahead of detonation tube with center cone valve.
steel moves to 36 × ∅5 mm perforated plate caused by pressure difference and the air flows around the edge of 36 × ∅5 mm perforated plate, as shown in Fig. 10(a). The forward blockage ratio is 0.651 based on 64 × ∅5 mm perforated plate. When the pressure at downstream of the valve is larger than that at upstream of the
valve, the sheet steel will move to upstream and stop behind of 64 × ∅5 mm perforated plate, as shown in Fig. 10(b). Some backward flow will pass through the holes at the brim of 64 × ∅5 mm perforated plate and the backward blockage ratio is 0.88 which is very close to that of 64 × ∅3 mm perforated plate (0.874).
10
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 15. The pressure histories of P1–P7 with center cone valve.
Fig. 11 shows high-speed photos of self-luminous field at mixing section with sheet steel valve. The results showed that backward flame was choked by the run-free sheet. Although some hot backward flow and fuel droplets still passed upstream through the valve, the passage form of the valve made it difficult for the formation of hot jet. So the heat release of reactants at upstream of valve was stopped. Pressure histories at P3 and P4 were also shown in Fig. 11 and the peak pressure (0.065 MPa) at P3 was far smaller in contrast to those of forgoing perforated plates. However, when the pressure peak value of P4 was too high, the formation of hot jet across orifices was reinforced and the residual reactants at upstream of valve were ignited. More reaction heat of gas was released and the pressure value at P3 increased further, as shown by the sudden formation of flame at upstream of valve and the rapid rise of pressure history at P3 in Fig. 12. 4. Experimental results for throat-type aerodynamic valve Perforated plates can be used as a blast wave disrupter. However, when it is placed in the path of deflagration/detonation to quench the chemical reactions, a variety of nonlinear combustion phenomena can be generated depending on the amount of blockage. For the interaction of a detonation with a perforated plate, if the temperature of the combustion products discharged through the plate is sufficiently high, ignition can occur downstream at the interface that separates the combustion products from the unreacted mixture [8]. Perforated/orifice plates can also be used as flame jet ignition system [11] to reduce the distance and time of deflagration to detonation transition (DDT), and it means that the passage form of the valve should be changed. 4.1. Experiment on center cone aerodynamic valve A center cone with blockage ratio 0.667 was screwed at Pos. 1 and Pos. 3 between P3 and P4, as shown in Fig. 13. The hollow region of center cone was used as plenum chamber for backflow and residual fuel droplets. Fig. 14 was a shadowgraph, and a black gas mass (fuel droplets) appeared at 26 ms after ignition and moved to mixing section at speed of 40 m/s. Since the plenum chamber had only one vent exposing to downstream, the pressure of the chamber increased more quickly than the surrounding area. When the fuel droplets, which were followed by a bright flame (at 28.4 ms in Fig. 14), flowed back to the downstream of the cone, it firstly passed through the throat of the cone at speed of 150 m/s for
the lower pressure contrasted with that of plenum chamber. After short time, the pressure difference between them changed and small part of it streamed into the hollow region at speed of 35 m/s (at 28.4 ms in Fig. 14). The strength of ‘hot-shot’ for the chamber was so small that heat of residual fuel droplets in it wasn’t released until 30 ms. After 34 ms, the gas flow from inlet moved downstream through throat and was accelerated by pressure difference. And at this moment, the pressure in plenum chamber was higher for the delay of heat release. Gases were exhausted from it and mixed with the flow through throat, as shown by photo at 37 ms in Fig. 14. As the decrease of pressure in chamber and outflow from it, streamline of flow through throat appeared (at 42 ms in Fig. 14) and coherent vortical structure appeared at downstream of center cone. Due to the low pressure difference, the residual combustion products in chamber were exhausted and mixed with fresh air through the throat at last. The pressure histories (Fig. 15) were also recorded and the results were similar to those of perforated plates. 4.2. Experiment on center cone aerodynamic valve with venturi To impel more backflow into the hollow region of the center cone and prolong the time for backflow passing through the valve, a venturi throat with blockage ratio 0.533 was fixed at Pos. 3. Fig. 16 was high-speed photos of self-luminous field with center cone and venturi. The flame was accelerated from 110 m/s up to 140 m/s after passing through the venture throat. The center cone decelerated the speed of flame a bit, and the filling process of backflow into hollow body of cone became easier because of the venturi throat. For this working cycle (Fig. 16), this method had few effects on the backflow and the delay of heat release in the plenum chamber of the cone could be clearly seen from the figure. Fig. 17 was a result of self-luminous in another working cycle. When back flowing phenomena occurred, the propagation speed of the flame was approximately 80 m/s at downstream of the valve. And the valve cut off the propagation of flame to the upstream. It meant that the introduction of venturi had potential effects to backflow of the fuel droplets. 4.3. Experiment on flared venturi aerodynamic valve A flared venturi aerodynamic valve was screwed at Pos. 1 and Pos. 3 between P3 and P4 as shown in Fig. 18. The flared venturi is very similar to the Bertin rectifier [2], which was first appeared in
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
11
Fig. 16. High-speed photo of self-luminous field with center cone and venture (failure).
design of aerodynamic valve for pulsed jet engines. The structure of flared venturi has a venturi shape and its walls contain multiple sharp corners, each inducing flow separation. The size of flared venturi is 60 mm × 60 mm × 120 mm with the throat distance 21 mm. And the blockage ratio is 0.65. Fig. 19 is a shadowgraph of flared venturi. A black gas mass (fuel droplets) appeared at 25 ms after ignition and moved upstream to the valve at speed of 30 m/s. After the moment of 26.5 ms, high-bright flame emerged from the right of the vision and started to chase the foregoing backflow of fuel droplets at the speed of 95 m/s. When the flame passed through the throat of
valve, it was accelerated to 271 m/s. During the first stage of back flowing process (before 28.5 ms), the pressures in chambers between corners were little higher than that in the passage of the valve, therefore many detached vortexes were formed near the end of corners and a nearly smooth channel was shaped by the streamlines of the backflow. With more heat released by combustion, the pressure of backflow increased and flame diffused into the space between corners. After 33 ms, the gas flow from upstream moved to downstream through throat and was quickly accelerated to 110 m/s at upstream of the valve by pressure difference. And speed of gas flow at throat was approximately 380 m/s (lower than
12
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 17. High-speed photo of self-luminous field with center cone and venture (success).
local sound speed). When the high-speed gas flow passed through the valve, coherent vortical structures were formed at downstream of each sharp corners, especially the left three sharp corners of the valve in Fig. 19. The instantaneous delivery of gas passing through the throat was greater than that passing around the fuel injector. Therefore, more and more gases accumulated at upper and bottom wall between valve and fuel injector. Due to these influences, the coherent vortical structures at both sides of divergent section of the venturi were pushed to centerline of the device nearer and nearer. The scale of coherent vortical structures also became larger and larger. After certain moment (39 ms for this working cycle), the distance between the vortices at each sides was so short that main gas flow was choked and shocks were formed and propagated from downstream to upstream. Gas flow at upstream was decelerated by the shocks. Since the vortices moved downstream,
Fig. 18. The configuration of flared venturi valve.
streamline of the gas flow changed with time and shock waves were very irregular. 5. Performance of aerodynamic valves Aerodynamic valves of PDE have the difficult task of damping the backflow or back pressure when instantaneously high pres-
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
13
Fig. 19. Shadowgraph of flow field ahead of detonation tube with flared venturi valve.
sure is produced in detonation chamber. And this task must be
The forward total-pressure recovery coefficient (σ f ) as a func-
accomplished with the minimum loss in pressure during the fill-
tion of incoming air flow (mair ) for the aerodynamic valves is
ing process of working cycle.
shown in Fig. 20, which was measured during steady flow op-
14
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 20. Forward total-pressure recovery coefficients as a function of inflow air flux for the seven valves.
Fig. 21. Backward total-pressure recovery coefficients as a function of inflow air flux for the seven valves.
eration. Total-pressure recovery coefficient is defined as the ratio of total pressure at downstream to upstream of the valve. Under the same forward flow conditions, it can be seen that the recovery coefficient of the simple valves increasing with the decrease of blockage ratio. Any changes to the simple valve, such as adding a perforated plate, reed or venturi, would decrease the recovery coefficient of the valve. The steady flow direction was also changed to measure the backward total-pressure recovery coefficient (σrev ) of the valve, as shown in Fig. 21. Many different aero-valves available could be compared in performance by considering their diodicity, which is defined as the ratio of forward to backward total-pressure recovery coefficient (σ f /σrev ) with a given mass flow. Diodicity of the different valves is shown in Fig. 22. The diodicity of single perforated plate equaled 1 and wasn’t shown in Fig. 20. If the region between σ f and σrev curve of a valve lies within that of another valve, the latter one has higher diodicity and works better for a blast wave disrupter. Therefore, the performance of the flared venturi is better than that of 64 × ∅5 mm perforated plate and 64 × ∅5 mm perforated plate plus reed works better than 64 × ∅3 mm perforated plate and double perforated plates. Under the other conditions, choice of valves should depend upon the working conditions of the valve. A model was established to compute the performance of a single-cycle detonation tube open at one end and closed at the other [14]. Results show that the impulse of a single-cycle detonation tube is proportional to the initial pressure before ignition. When a valve is introduced into PDE, the initial pressure in detonation tube is increasing in proportion to the forward total-pressure recovery coefficient of the valve. This means that the thrust produced by detonation tube is proportional to the σ f of the valve. When backflow occurs during the detonation process, thrust pro-
Fig. 22. Diodicity (σ f /σrev ) as a function of inflow air flux during steady flow operation.
Fig. 23. Absolute peak pressure ratios between P3 and P4 as a function of absolute peak pressure at P4 during each working cycle.
duced by detonation tube will decrease. For a straight detonation tube, the single-cycle impulse is generated by the pressure differential at the thrust surface. When a valve is introduced into PDE, the valve is the thrust surface. That is
I=A
∞
∞
P d (t ) − P u (t ) dt = A
0
P d (t ) 1 − σrev (t ) dt 0
∞ = A (1 − σrev )
P d (t ) dt
(1)
0
The thrust generated by detonation tube would be inverse proportional to the σrev of the valve. Assumed that the σrev of valve was the value at mair equaling 700 kg/h, when the filling airflow flux was 400 kg/h, thrust of the detonation tube with 64 × ∅3 mm perforated plate would be 10.9% higher than that with 64 × ∅5 mm perforated plate. However, when the filling airflow flux was 900 kg/h, thrust of the detonation tube with 64 × ∅3 mm perforated plate would be 7.3% lower than that with 64 × ∅5 mm perforated plate. The practical σrev (t ) of the backflow should be measured during the detonation process. The P 3,max / P 4,max (σrev ) of backflow of different valves during each working cycle are shown in Fig. 23. P 3,max and P 4,max are the absolute peak pressures at P3 and P4 between dashed lines 3 and 4. The P 3,max / P 4,max of the 64 × ∅5 mm perforated plate, double perforated plates and center cone were generally higher than 0.8, and some results were even higher than 1.0. The σrev of valves in Fig. 21 were measured under the reverse absolute total pressure lower than 1.5 atm. Heat release of chemical reaction across the valves weakens the damping ability of the valve and it can be clearly indicated by the results of the
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
center cone plus venturi. For the valves of the 64 × ∅5 mm perforated plate plus reed and 64 × ∅3 mm perforated plate, which have similar backflow blockage ratio, the σrev of the two valves are closer as long as flame propagating across the valve. When back flame across the valve doesn’t occur, σrev measured during steady flow operation can be used as a parameter to evaluate the performance of valve for damping backflow during hot-firing test, which were shown by the results of 64 × ∅5 mm perforated plate plus reed and center cone plus venturi. 6. Conclusions The perforated aerodynamic valve can be used as a blast wave disputer. However, when it is applied to pulse detonation tube with adaptive control for fuel, fuel droplets will move upstream with the backflow and pass through the valve, followed by bright flame. Hot jet is easily induced by the perforated plate in combustible mixture, which accelerates the speed of flame. The influences of heat release at upstream of the valve weaken the damping ability for backflow of the valve. Improvement could be achieved by the introduction of a sheet steel (reed) to the perforated plate and the same phenomenon of back flame would also occur if the back pressure of backflow is too high. The throat-type aerodynamic valves work better than the perforated aerodynamic valves with the same blockage ratio. Flame could also propagate across the valves. Occurring probability of this phenomenon could be reduced by the introductions of plenum chamber and venturi. Performance of different aerodynamic valves could be compared, to a certain extent, by diodicity, which is measured during steady flow operation. The optimal choice of valves also depends on the working condition of pulse detonation tube. The practical performance of a valve, which is correlated with time, should be measured during the detonation process. If no flame passes through the valve, the performance of anti-backflow during hot-firing test of a valve can be evaluated by the results measured during steady flow operation. To analyze the experimental data, more attention should be paid to whether flame is passing through aerodynamic valves of pulsed detonation tube with adaptive control for fuel.
15
Acknowledgements The work was supported by National Natural Science Foundation of China 50906072. References [1] S.D. Anderson, J.H. Tonouchi, et al., Performance trends for a product scale pulse detonation engine, in: 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, AIAA 2004-3402. [2] J.H. Bertin, et al., Aerodynamic valve, U.S. Patent No. 2670011. [3] C.M. Brophy, R.K. Hanson, Fuel distribution effects on pulse detonation engine operation and performance, Journal of Propulsion and Power 22 (6) (2006) 1155–1161. [4] G.S. Langdon, G.N. Nurick, N.J. du Plessis, The influence of separation distance on the performance of perforated plates as a blast wave shielding technique, Engineering Structures 33 (2011) 3537–3545. [5] Fuhua Ma, Jeong-Yeol Choi, Vigor Yang, Propulsive performance of airbreathing pulse detonation engines, Journal of Propulsion and Power 22 (6) (2006) 1188– 1203. [6] Fuhua Ma, Jeong-Yeol Choi, Vigor Yang, Internal flow dynamics in a valveless airbreathing pulse detonation engine, Journal of Propulsion and Power 24 (3) (2008) 479–490. [7] S.P. Medvedev, S.V. Khomik, H. Olivier, Experimental set-up, measurement technique and test condition for explosions with active/passive additives, EXPO Deliverable Report D5, Contract EVG1-CT-2001-00042, 2003. [8] Hoi Dick Ng, Jenny Chao, Yiguang Ju, et al., Combustion regimes subsequent to the reflection of a detonation from a perforated plate, Communications in Nonlinear Science and Numerical Simulation 13 (2008) 243–247. [9] D. Piton, A. Prigent, L. Serre, C. Monjaret, Performance of a valveless air breathing pulse detonation engine, in: 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, AIAA 2004-3749. [10] G.D. Roy, S.M. Frolov, A.A. Boriov, D.W. Netzer, Pulse detonation propulsion: Challenges, current states, and future perspective, Proceeding in Energy and Combustion Science 30 (2004) 545–672. [11] Hidetoshi Shimada, Yasuhiro Kenmonku, Hiroyuki Sato, et al., A new ignition system for pulse detonation engine, in: 42nd AIAA Aerospace Sciences Meeting & Exhibit, Reno, NV, AIAA 2004-308. [12] Z. Wang, C. Yan, M. Li, W. Fan, Experimental investigation on a two-phase valveless air-breathing pulse detonation engine, in: 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Sacramento, AIAA 2006-4310. [13] James Fredric Wiedenhoefer, Naturally aspirated fluidic control for diverting strong pressure waves, U.S. Patent No. 2007/0015099A1, January 18, 2007. [14] E. Wintenberger, J.M. Austin, M. Cooper, et al., Analytical model for the impulse of single-cycle pulse detonation tube, Journal of Propulsion and Power 19 (1) (2003) 22–38.
Contents lists available at SciVerse ScienceDirect
Aerospace Science and Technology www.elsevier.com/locate/aescte
Effect of aerodynamic valve on backflow in pulsed detonation tube Qiu Hua ∗ , Xiong Cha, Yan Chuan-jun, Zheng Long-xi School of Power and Energy, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 November 2010 Received in revised form 23 November 2011 Accepted 1 December 2011 Available online 7 December 2011
To research the operating characteristics of aerodynamic valves, which are applied for pulse detonation engine with adaptive control for fuel, seven aero-valves were designed and tested in a square pulse detonation tube. Shadowgraph images, self-luminous photos and pressure measurement were used to investigate the abilities of the aerodynamic valves to decrease the backward pressure for valveless pulsed detonation tube. The experimental results show that residual fuel droplets moved upstream with backflow followed by flame, and flame could propagate across the valves. The heat release at upstream of the valve weakened the ability of the valve to damp the back pressure. Run-free sheet steel was introduced into the perforated aerodynamic valve and the inference is verified. Influences of hollow region and venturi on the throat-type aerodynamic valves were also investigated. At last, performance of each aerodynamic valve was analyzed by the total-pressure recovery coefficients of positive and reverse direction. Crown Copyright © 2011 Published by Elsevier Masson SAS. All rights reserved.
Keywords: Pulsed detonation engine Aerodynamic valve Adaptive control Shadowgraph
1. Introduction Pulsed detonation engines (PDEs) are unsteady propulsion devices that produce thrust by using repetitive detonation. Pulse detonation engines exhibit several performance advantages in comparison with current steady-deflagration jets. The inherent mechanical design simplicity of the PDE results in smaller packaging volumes and lower part counts, aiding in integration and maintenance. One of the key technologies for a practical detonation based engine is its valving scheme, which is used to prevent the combustor flow from traveling into the inlet during certain periods of the operation cycle. Based on how this function is realized, PDEs can be classified as either valved or valveless/aerovalve, as summarized by Roy et al. [10]. Use of mechanical valves, such as disk-shaped rotary valve [1,5], to control incoming flow rate of fresh air into detonation tube can prevent detonations or shocks from moving outwards from detonation tube through inlet and provide a sufficient time for mixing of fuel with air. But mechanical valve will add engine weight and increase complexity of PDE. Valveless PDE concepts imply continuous or intermittent supply of propellants to the detonation tube without using mechanical valves, which means mechanically simpler. And several aerodynamic configurations were designed, such as supersonic isolator inlet [3], circumferential seam configuration [12], naturally aspirated fluidic control device [13] and so on. Ma [6] simulates PDE inlet aerodynamics and its response to downstream disturbances. The results indicate
*
Corresponding author. Fax: +86 029 88492748. E-mail address: [email protected] (H. Qiu).
1270-9638/$ – see front matter Crown Copyright doi:10.1016/j.ast.2011.12.003
© 2011
that strong pressure disturbance may propagate upstream to the inlet and make the device more vulnerable to inlet instability. Piton et al. [9] take shadowgraph images at entrance of a valveless detonation tube and it’s shown that shock wave was discharged from inlets. Until recently, most studies have focused on the flow dynamics and propulsive performance of valved PDEs, with only limited efforts on valveless PDEs. Few references are associated with the research of aerodynamic valve for PDEs, especially for PDEs with adaptive control for fuel. The purpose of this research is to investigate the performance of two types of aerodynamic valves for damping backflow. By the use of high-speed shadowgraph images, self-luminous photos and pressure measurement, the present work has produced a good understanding of the operating mechanism of aerodynamic valve and provides guidelines for design and optimization of aerodynamic valve. 2. Experimental setup The experiments were carried out in a 2.63-m-long-square transparent detonation tube with a 60 mm × 60 mm cross section as shown in Fig. 1. Air duct passes the air in pressure tank to detonation tube through a switching section that is ∅70 mm circular cross section to 60 mm × 60 mm square section transition. The detonation tube comprises 0.715-m-long mixing section and 1.915-m-long detonation section. A staggered array of thirty-three 10 mm × 10 mm × 60 mm strip obstacles is inserted and fixed to the top and bottom inner wall of the detonation section and the spacing between the adjacent obstacles is 50 mm. And there are sixteen strip obstacles attached
Published by Elsevier Masson SAS. All rights reserved.
2
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 1. Schematic of experimental setup and the position of pressure transducers.
Fig. 2. Schematic of the improved twin-fluid air-assist atomizer.
to the top inner wall and the clearance between the adjacent obstacles is 100 mm. An improved twin-fluid air-assist atomizer is used for gasoline injection and the configuration is shown in Fig. 2 where fuel is injected at top of tube and the pressure-air is injected at bottom of tube. A cone is used to be an inflow diversion. The liquid gasoline, pressure-air and main air flow are introduced to detonation tube by adaptive control without valve and an air flow meter and a gear meter are used to measure the average flow rate of air and fuel respectively. The atomizer is mounted at the position of 330 mm from spark ignition. The spark is produced via a standard automotive inductive ignition system and supplied approximately 50 mJ of energy. Aerodynamic valves can be assembled at four positions as shown in Fig. 1, that is Pos. 1–Pos. 4. The measurements of propagation velocity and pressure records are performed with pressure transducers, which are embedded in side walls, namely P1, P2, P3, P4, P5, P6, P7 as shown in Fig. 1. P1–P5 mounted at mixing section are piezoresistive pressure transducers with 500 kHz resonant frequency and 1 μs rise time. P6–P7 mounted at detonation section are piezoelectric pressure transducers with 200 kHz resonant frequency and 2 μs rise time to diagnose whether detonation wave form. The advantage of piezoresistive pressure transducer is excellent response to zero-frequency,
much approach real pressure alteration, and there is no overshoot of pressure. But the piezoresistive pressure transducer is made of diffuse silicone material and is much sensitive to temperature. Since all of experiments were operated under working frequency lower than 4 Hz during one to two seconds, and the residual time of the hot backward flow in mixing section lasted only several milliseconds, effects of temperature on the records of pressure transducer could be neglected. Gasoline and air propellants at an average equivalence ratio of 1.2 (rates of air flow and fuel were approximately 124 kg/h and 5.2 ml/s respectively) were used because it is relatively easy to achieve detonation combustion at initial pressure of 101.3 kPa. A high-speed shadowgraph visual system was used to record flame and flow field structures. The diameter of spherical mirror is 300 mm and the focal distance is 3000 mm. A xenon lamp with 500 W was used as light source. The maximum sampling frame in a second of PhantomV7.2 high-speed camera is 200 000 frames and the minimum exposure time is 2 μs. An N/D light filter was fixed behind the camera to adjust light exposure. The sample rates and exposure time of the camera were set to 50 000 fps and 4 μs respectively for the experiments. PhantomV7.2 high-speed camera was also used for recording self-luminous process in mixing
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
3
Fig. 3. Shadowgraph of flow field ahead of detonation tube with 64 × ∅5 mm perforated plate and T was set to be the triggering time.
section. And the sample rates and exposure time were set to 20 000 fps and 40 μs respectively. 3. Experimental results for perforated aerodynamic valve Perforated plates can be used as a blast wave shielding technique [4]. Medvedev [7] reported the blast front velocity could be decreased by increasing the blockage ratio (BR), which is an area ratio between the blocked area and the total area. It is implied that back pressure could be damped by the perforated plates for the pulse detonation engine. 3.1. Experiment on 64 × ∅5 mm perforated plate A 60 mm × 60 mm square perforated plate with 64 × ∅5 mm hole (BR is 0.651) was mounted at Pos. 2 between P3 and P4. Fig. 3
is a shadowgraph and the character T is set to be the triggering time. The mixing section (upstream) is on the left and detonation tube (downstream) is on the right in the picture. A black gas mass (fuel droplets) appeared at 13.7 ms after ignition and moved to mixing section at speed 50 m/s. It passed through the perforated plate at 16.8 ms, and a bright flame appeared at 17.3 ms. The bright flame moved upstream at 140 m/s and was accelerated to 250 m/s after passing through the perforated plate, at the meantime, the brightness of flame became much stronger (at 18.8 ms) and diffused to the whole observation field. After 22.8 ms, the gas flow from inlet moved downstream and was accelerated by pressure difference (at 25 ms). A special phenomenon was shown that fuel droplets were ejected from downstream to upstream and a gray gas mass flowed to upstream during very short time (at 47.4 ms). It is possible that a compression wave was formed because of overexpansion during exhaust of combustion products.
4
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 4. The pressure histories of P1–P7 with 64 × ∅5 mm perforated plate.
Fig. 5. High-speed photos of self-luminous field at mixing section.
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 6. The pressure histories of P1–P7 with 64 × ∅3 mm perforated valve.
Fig. 7. Shadowgraph of flow field ahead of detonation tube with 64 × ∅3 mm + 64 × ∅4 mm double perforated plates.
5
6
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 8. The pressure histories of P1–P7 with 64 × ∅3 mm + 64 × ∅4 mm double perforated plates.
The pressure histories of P1–P7 are shown in Fig. 4. The peak pressure of the last pressure transducer P7 was up to 7 MPa and the rise time was 10 μs, which indicated a fully developed detonation wave was formed. According to Fig. 4, forward (illustrated by dashed line 2, to downstream) and backward (illustrated by dashed line 1, to upstream) compression waves were generated when gasoline/air mixture was ignited, which induced the pressure perturbation in the mixing and detonation sections. The forward compression wave was accelerated by obstacles and heat release, and then became detonation wave. Local explosion occurred at some place between P4 and P5, and the backward explosion wave propagated back up the length of mixing section and induced the pressure increase further, which is illustrated by the third dashed line (line 3). And the backward flow generated by the explosion wave was clearly displayed at 17.3 ms in Fig. 3. The backward explosion wave propagated to upstream further and mostly reflected at bend section of air duct, which formed the propagation of pressure wave as shown by dashed line 4 and caused the peak pressure at P1 up to 0.42 MPa after 18.5 ms. And the partially forward explosion wave passing through bend section was reflected at upstream of the air duct and propagated downstream again (illustrated by dashed line 5). Fig. 5 is a high-speed photo of self-luminous field at mixing section. Local explosion was found nearby fuel atomizer. A blue flame was formed and followed by a bright red flame. The blue field corresponded with oxidation procedure of big molecule at about 1100 K and the bright field corresponded with high temperature reaction to produce CO2 and H2 O. (For interpretation of the references to color in Fig. 5, the reader is referred to the web version of this article.) According to Figs. 3–5, a description of flow field at mixing section after ignition was followed: locally explosion produced a forward and a backward combustion wave after ignition. The forward pressure wave propagated to the open end of detonation tube and transformed to detonation wave between P6 and P7. The backward pressure propagated along the dashed line 1 shown in Fig. 4. But the pressure perturbation at this moment cannot be distinguished by high-speed shadowgraph camera. Then the increasing pressure in detonation tube induced gas flow mixing with fuel droplets and moving upstream. A secondary explosion was formed nearby the fuel injector. A blue flame propagated upstream and accelerated diffusion of fuel fog, as shown by the ascent of pressure between dashed line 1 and dashed line 3 in Fig. 4. More heat was released during this process and the blue flame transformed
Fig. 9. The configuration of 64 × ∅5 mm plus sheet steel valve.
Fig. 10. The flow characteristic of 64 × ∅5 mm plus sheet steel valve.
to bright flame. The resultant backward explosion wave propagating upstream was mostly reflected at bend section of air duct and the reflected compression wave propagated along path of dashed line 4, while partial of that was reflected at upstream of the air duct and propagated downstream along dashed line 5. When the detonation wave was exhausted from the open end of the detonation tube, a reflection rarefaction wave moved back and induced the pressure to decrease while the products began to exhaust the tube, as shown by the pressure decrease between dashed line 4 and dashed line 5. 3.2. Experiment on 64 × ∅3 mm perforated plate A 60 mm × 60 mm square perforated plate with 64 × ∅3 mm hole (BR is 0.874) was mounted at Pos. 2 between P3 and P4. The pressure histories (Fig. 6) and shadowgraph photos were recorded and the results were similar to that of 64 × ∅5 mm perforated plate. However, there were differences between these two plates. According to the dashed line 3 along which the compression wave caused by local explosion propagated, travel time of the wave
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
7
Fig. 11. High-speed photo of self-luminous field at mixing section with sheet steel valve (success).
from P4 to P3 became longer. And the peak pressure difference between P3 and P4 also became larger. This meant that the increasing blockage ratio decreased backward flow flux and black fuel droplets through the perforated plate, which caused the flame weaken. For the 64 × ∅3 mm perforated plate, pressure peaks at
P3 and P4 were caused by backward explosion wave which was induced by local explosion at some place between P4 and P5, while those of the 64 ×∅5 mm perforated plate were caused by reflected compression wave of the backward explosion wave at bend section of air duct.
8
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 12. High-speed photo of self-luminous field at mixing section with sheet steel valve (failure).
3.3. Experiment on 64 × ∅3 mm + 64 × ∅4 mm double perforated plates In order to prevent fuel droplets from penetrating into inlet, a double perforated valve was researched. A 64 × ∅3 mm perforated plate was mounted at Pos. 4 between P4 and fuel injector, and another 64 × ∅4 mm perforated plate was fixed at Pos. 2 between P3 and P4, as shown in Fig. 7. Fig. 7 showed that the 64 × ∅3 mm perforated plate holds some fuel droplets back, but still some fuel moved upstream. So the flame still could propagate from fuel injector to upstream through perforated aerodynamic valve and a pressure wave was formed as dashed line 3 in Fig. 8. Direction of the wave propagation between P5 to P4 was different from those of perforated aerodynamic valves before. The double perforated plates reduced the speed of flame, but the heat release of reactants was aggravated which caused by hot jet, as shown by the spherical pressure wave at 54.1 ms in Fig. 7. The spherical pressure wave propagated through 64 × ∅4 mm perforated plate and chased the preceding flame, which led to acceleration of the heat release of combustion process and the increase of pressure between P1–P5. The compression wave mostly reflected at bend section of air duct and formed the propagation of pressure wave as shown by dashed line 4. The pressure wave (shock) corresponded to cylindrical bright zone at 55.82 ms and 55.86 ms in shadowgraph, induced the pressure at P1 up to 0.41 MPa. The cylindrical bright zone cannot be ob-
Fig. 13. The configuration of center cone valve.
served in shadowgraph for single perforated valve because the flux of backflow was much more and formed a strong shadowgraph. 3.4. Experiment on perforated aerodynamic valve with sheet steel To research the influences of the additional heat release at upstream of the perforated plates on the characteristics of valve, a kind of sheet steel valve was used and mounted at Pos. 2 between P3 and P4. The configuration of sheet steel valve is shown in Fig. 9: a 64 × ∅5 mm perforated plate is fixed at left side and is connected with a 36 × ∅5 mm perforated plate at its right side by 4 supporting rod. The distance between these two perforated plates is 6 mm and a 0.5-mm-thickness run-free sheet steel (reed) with a 42 mm × 42 mm cross section is inserted between plates. When the fresh air is induced into the detonation tube, the air passes through 64 × ∅5 mm perforated plate first. Then the sheet
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
9
Fig. 14. Shadowgraph of flow field ahead of detonation tube with center cone valve.
steel moves to 36 × ∅5 mm perforated plate caused by pressure difference and the air flows around the edge of 36 × ∅5 mm perforated plate, as shown in Fig. 10(a). The forward blockage ratio is 0.651 based on 64 × ∅5 mm perforated plate. When the pressure at downstream of the valve is larger than that at upstream of the
valve, the sheet steel will move to upstream and stop behind of 64 × ∅5 mm perforated plate, as shown in Fig. 10(b). Some backward flow will pass through the holes at the brim of 64 × ∅5 mm perforated plate and the backward blockage ratio is 0.88 which is very close to that of 64 × ∅3 mm perforated plate (0.874).
10
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 15. The pressure histories of P1–P7 with center cone valve.
Fig. 11 shows high-speed photos of self-luminous field at mixing section with sheet steel valve. The results showed that backward flame was choked by the run-free sheet. Although some hot backward flow and fuel droplets still passed upstream through the valve, the passage form of the valve made it difficult for the formation of hot jet. So the heat release of reactants at upstream of valve was stopped. Pressure histories at P3 and P4 were also shown in Fig. 11 and the peak pressure (0.065 MPa) at P3 was far smaller in contrast to those of forgoing perforated plates. However, when the pressure peak value of P4 was too high, the formation of hot jet across orifices was reinforced and the residual reactants at upstream of valve were ignited. More reaction heat of gas was released and the pressure value at P3 increased further, as shown by the sudden formation of flame at upstream of valve and the rapid rise of pressure history at P3 in Fig. 12. 4. Experimental results for throat-type aerodynamic valve Perforated plates can be used as a blast wave disrupter. However, when it is placed in the path of deflagration/detonation to quench the chemical reactions, a variety of nonlinear combustion phenomena can be generated depending on the amount of blockage. For the interaction of a detonation with a perforated plate, if the temperature of the combustion products discharged through the plate is sufficiently high, ignition can occur downstream at the interface that separates the combustion products from the unreacted mixture [8]. Perforated/orifice plates can also be used as flame jet ignition system [11] to reduce the distance and time of deflagration to detonation transition (DDT), and it means that the passage form of the valve should be changed. 4.1. Experiment on center cone aerodynamic valve A center cone with blockage ratio 0.667 was screwed at Pos. 1 and Pos. 3 between P3 and P4, as shown in Fig. 13. The hollow region of center cone was used as plenum chamber for backflow and residual fuel droplets. Fig. 14 was a shadowgraph, and a black gas mass (fuel droplets) appeared at 26 ms after ignition and moved to mixing section at speed of 40 m/s. Since the plenum chamber had only one vent exposing to downstream, the pressure of the chamber increased more quickly than the surrounding area. When the fuel droplets, which were followed by a bright flame (at 28.4 ms in Fig. 14), flowed back to the downstream of the cone, it firstly passed through the throat of the cone at speed of 150 m/s for
the lower pressure contrasted with that of plenum chamber. After short time, the pressure difference between them changed and small part of it streamed into the hollow region at speed of 35 m/s (at 28.4 ms in Fig. 14). The strength of ‘hot-shot’ for the chamber was so small that heat of residual fuel droplets in it wasn’t released until 30 ms. After 34 ms, the gas flow from inlet moved downstream through throat and was accelerated by pressure difference. And at this moment, the pressure in plenum chamber was higher for the delay of heat release. Gases were exhausted from it and mixed with the flow through throat, as shown by photo at 37 ms in Fig. 14. As the decrease of pressure in chamber and outflow from it, streamline of flow through throat appeared (at 42 ms in Fig. 14) and coherent vortical structure appeared at downstream of center cone. Due to the low pressure difference, the residual combustion products in chamber were exhausted and mixed with fresh air through the throat at last. The pressure histories (Fig. 15) were also recorded and the results were similar to those of perforated plates. 4.2. Experiment on center cone aerodynamic valve with venturi To impel more backflow into the hollow region of the center cone and prolong the time for backflow passing through the valve, a venturi throat with blockage ratio 0.533 was fixed at Pos. 3. Fig. 16 was high-speed photos of self-luminous field with center cone and venturi. The flame was accelerated from 110 m/s up to 140 m/s after passing through the venture throat. The center cone decelerated the speed of flame a bit, and the filling process of backflow into hollow body of cone became easier because of the venturi throat. For this working cycle (Fig. 16), this method had few effects on the backflow and the delay of heat release in the plenum chamber of the cone could be clearly seen from the figure. Fig. 17 was a result of self-luminous in another working cycle. When back flowing phenomena occurred, the propagation speed of the flame was approximately 80 m/s at downstream of the valve. And the valve cut off the propagation of flame to the upstream. It meant that the introduction of venturi had potential effects to backflow of the fuel droplets. 4.3. Experiment on flared venturi aerodynamic valve A flared venturi aerodynamic valve was screwed at Pos. 1 and Pos. 3 between P3 and P4 as shown in Fig. 18. The flared venturi is very similar to the Bertin rectifier [2], which was first appeared in
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
11
Fig. 16. High-speed photo of self-luminous field with center cone and venture (failure).
design of aerodynamic valve for pulsed jet engines. The structure of flared venturi has a venturi shape and its walls contain multiple sharp corners, each inducing flow separation. The size of flared venturi is 60 mm × 60 mm × 120 mm with the throat distance 21 mm. And the blockage ratio is 0.65. Fig. 19 is a shadowgraph of flared venturi. A black gas mass (fuel droplets) appeared at 25 ms after ignition and moved upstream to the valve at speed of 30 m/s. After the moment of 26.5 ms, high-bright flame emerged from the right of the vision and started to chase the foregoing backflow of fuel droplets at the speed of 95 m/s. When the flame passed through the throat of
valve, it was accelerated to 271 m/s. During the first stage of back flowing process (before 28.5 ms), the pressures in chambers between corners were little higher than that in the passage of the valve, therefore many detached vortexes were formed near the end of corners and a nearly smooth channel was shaped by the streamlines of the backflow. With more heat released by combustion, the pressure of backflow increased and flame diffused into the space between corners. After 33 ms, the gas flow from upstream moved to downstream through throat and was quickly accelerated to 110 m/s at upstream of the valve by pressure difference. And speed of gas flow at throat was approximately 380 m/s (lower than
12
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 17. High-speed photo of self-luminous field with center cone and venture (success).
local sound speed). When the high-speed gas flow passed through the valve, coherent vortical structures were formed at downstream of each sharp corners, especially the left three sharp corners of the valve in Fig. 19. The instantaneous delivery of gas passing through the throat was greater than that passing around the fuel injector. Therefore, more and more gases accumulated at upper and bottom wall between valve and fuel injector. Due to these influences, the coherent vortical structures at both sides of divergent section of the venturi were pushed to centerline of the device nearer and nearer. The scale of coherent vortical structures also became larger and larger. After certain moment (39 ms for this working cycle), the distance between the vortices at each sides was so short that main gas flow was choked and shocks were formed and propagated from downstream to upstream. Gas flow at upstream was decelerated by the shocks. Since the vortices moved downstream,
Fig. 18. The configuration of flared venturi valve.
streamline of the gas flow changed with time and shock waves were very irregular. 5. Performance of aerodynamic valves Aerodynamic valves of PDE have the difficult task of damping the backflow or back pressure when instantaneously high pres-
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
13
Fig. 19. Shadowgraph of flow field ahead of detonation tube with flared venturi valve.
sure is produced in detonation chamber. And this task must be
The forward total-pressure recovery coefficient (σ f ) as a func-
accomplished with the minimum loss in pressure during the fill-
tion of incoming air flow (mair ) for the aerodynamic valves is
ing process of working cycle.
shown in Fig. 20, which was measured during steady flow op-
14
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
Fig. 20. Forward total-pressure recovery coefficients as a function of inflow air flux for the seven valves.
Fig. 21. Backward total-pressure recovery coefficients as a function of inflow air flux for the seven valves.
eration. Total-pressure recovery coefficient is defined as the ratio of total pressure at downstream to upstream of the valve. Under the same forward flow conditions, it can be seen that the recovery coefficient of the simple valves increasing with the decrease of blockage ratio. Any changes to the simple valve, such as adding a perforated plate, reed or venturi, would decrease the recovery coefficient of the valve. The steady flow direction was also changed to measure the backward total-pressure recovery coefficient (σrev ) of the valve, as shown in Fig. 21. Many different aero-valves available could be compared in performance by considering their diodicity, which is defined as the ratio of forward to backward total-pressure recovery coefficient (σ f /σrev ) with a given mass flow. Diodicity of the different valves is shown in Fig. 22. The diodicity of single perforated plate equaled 1 and wasn’t shown in Fig. 20. If the region between σ f and σrev curve of a valve lies within that of another valve, the latter one has higher diodicity and works better for a blast wave disrupter. Therefore, the performance of the flared venturi is better than that of 64 × ∅5 mm perforated plate and 64 × ∅5 mm perforated plate plus reed works better than 64 × ∅3 mm perforated plate and double perforated plates. Under the other conditions, choice of valves should depend upon the working conditions of the valve. A model was established to compute the performance of a single-cycle detonation tube open at one end and closed at the other [14]. Results show that the impulse of a single-cycle detonation tube is proportional to the initial pressure before ignition. When a valve is introduced into PDE, the initial pressure in detonation tube is increasing in proportion to the forward total-pressure recovery coefficient of the valve. This means that the thrust produced by detonation tube is proportional to the σ f of the valve. When backflow occurs during the detonation process, thrust pro-
Fig. 22. Diodicity (σ f /σrev ) as a function of inflow air flux during steady flow operation.
Fig. 23. Absolute peak pressure ratios between P3 and P4 as a function of absolute peak pressure at P4 during each working cycle.
duced by detonation tube will decrease. For a straight detonation tube, the single-cycle impulse is generated by the pressure differential at the thrust surface. When a valve is introduced into PDE, the valve is the thrust surface. That is
I=A
∞
∞
P d (t ) − P u (t ) dt = A
0
P d (t ) 1 − σrev (t ) dt 0
∞ = A (1 − σrev )
P d (t ) dt
(1)
0
The thrust generated by detonation tube would be inverse proportional to the σrev of the valve. Assumed that the σrev of valve was the value at mair equaling 700 kg/h, when the filling airflow flux was 400 kg/h, thrust of the detonation tube with 64 × ∅3 mm perforated plate would be 10.9% higher than that with 64 × ∅5 mm perforated plate. However, when the filling airflow flux was 900 kg/h, thrust of the detonation tube with 64 × ∅3 mm perforated plate would be 7.3% lower than that with 64 × ∅5 mm perforated plate. The practical σrev (t ) of the backflow should be measured during the detonation process. The P 3,max / P 4,max (σrev ) of backflow of different valves during each working cycle are shown in Fig. 23. P 3,max and P 4,max are the absolute peak pressures at P3 and P4 between dashed lines 3 and 4. The P 3,max / P 4,max of the 64 × ∅5 mm perforated plate, double perforated plates and center cone were generally higher than 0.8, and some results were even higher than 1.0. The σrev of valves in Fig. 21 were measured under the reverse absolute total pressure lower than 1.5 atm. Heat release of chemical reaction across the valves weakens the damping ability of the valve and it can be clearly indicated by the results of the
H. Qiu et al. / Aerospace Science and Technology 25 (2013) 1–15
center cone plus venturi. For the valves of the 64 × ∅5 mm perforated plate plus reed and 64 × ∅3 mm perforated plate, which have similar backflow blockage ratio, the σrev of the two valves are closer as long as flame propagating across the valve. When back flame across the valve doesn’t occur, σrev measured during steady flow operation can be used as a parameter to evaluate the performance of valve for damping backflow during hot-firing test, which were shown by the results of 64 × ∅5 mm perforated plate plus reed and center cone plus venturi. 6. Conclusions The perforated aerodynamic valve can be used as a blast wave disputer. However, when it is applied to pulse detonation tube with adaptive control for fuel, fuel droplets will move upstream with the backflow and pass through the valve, followed by bright flame. Hot jet is easily induced by the perforated plate in combustible mixture, which accelerates the speed of flame. The influences of heat release at upstream of the valve weaken the damping ability for backflow of the valve. Improvement could be achieved by the introduction of a sheet steel (reed) to the perforated plate and the same phenomenon of back flame would also occur if the back pressure of backflow is too high. The throat-type aerodynamic valves work better than the perforated aerodynamic valves with the same blockage ratio. Flame could also propagate across the valves. Occurring probability of this phenomenon could be reduced by the introductions of plenum chamber and venturi. Performance of different aerodynamic valves could be compared, to a certain extent, by diodicity, which is measured during steady flow operation. The optimal choice of valves also depends on the working condition of pulse detonation tube. The practical performance of a valve, which is correlated with time, should be measured during the detonation process. If no flame passes through the valve, the performance of anti-backflow during hot-firing test of a valve can be evaluated by the results measured during steady flow operation. To analyze the experimental data, more attention should be paid to whether flame is passing through aerodynamic valves of pulsed detonation tube with adaptive control for fuel.
15
Acknowledgements The work was supported by National Natural Science Foundation of China 50906072. References [1] S.D. Anderson, J.H. Tonouchi, et al., Performance trends for a product scale pulse detonation engine, in: 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, AIAA 2004-3402. [2] J.H. Bertin, et al., Aerodynamic valve, U.S. Patent No. 2670011. [3] C.M. Brophy, R.K. Hanson, Fuel distribution effects on pulse detonation engine operation and performance, Journal of Propulsion and Power 22 (6) (2006) 1155–1161. [4] G.S. Langdon, G.N. Nurick, N.J. du Plessis, The influence of separation distance on the performance of perforated plates as a blast wave shielding technique, Engineering Structures 33 (2011) 3537–3545. [5] Fuhua Ma, Jeong-Yeol Choi, Vigor Yang, Propulsive performance of airbreathing pulse detonation engines, Journal of Propulsion and Power 22 (6) (2006) 1188– 1203. [6] Fuhua Ma, Jeong-Yeol Choi, Vigor Yang, Internal flow dynamics in a valveless airbreathing pulse detonation engine, Journal of Propulsion and Power 24 (3) (2008) 479–490. [7] S.P. Medvedev, S.V. Khomik, H. Olivier, Experimental set-up, measurement technique and test condition for explosions with active/passive additives, EXPO Deliverable Report D5, Contract EVG1-CT-2001-00042, 2003. [8] Hoi Dick Ng, Jenny Chao, Yiguang Ju, et al., Combustion regimes subsequent to the reflection of a detonation from a perforated plate, Communications in Nonlinear Science and Numerical Simulation 13 (2008) 243–247. [9] D. Piton, A. Prigent, L. Serre, C. Monjaret, Performance of a valveless air breathing pulse detonation engine, in: 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, AIAA 2004-3749. [10] G.D. Roy, S.M. Frolov, A.A. Boriov, D.W. Netzer, Pulse detonation propulsion: Challenges, current states, and future perspective, Proceeding in Energy and Combustion Science 30 (2004) 545–672. [11] Hidetoshi Shimada, Yasuhiro Kenmonku, Hiroyuki Sato, et al., A new ignition system for pulse detonation engine, in: 42nd AIAA Aerospace Sciences Meeting & Exhibit, Reno, NV, AIAA 2004-308. [12] Z. Wang, C. Yan, M. Li, W. Fan, Experimental investigation on a two-phase valveless air-breathing pulse detonation engine, in: 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Sacramento, AIAA 2006-4310. [13] James Fredric Wiedenhoefer, Naturally aspirated fluidic control for diverting strong pressure waves, U.S. Patent No. 2007/0015099A1, January 18, 2007. [14] E. Wintenberger, J.M. Austin, M. Cooper, et al., Analytical model for the impulse of single-cycle pulse detonation tube, Journal of Propulsion and Power 19 (1) (2003) 22–38.