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Combustion modes of hydrogen jet combustion in a cavity-based supersonic combustor
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Combustion modes of hydrogen jet combustion in a cavity-based supersonic combustor Hongbo Wang, Zhenguo Wang*, Mingbo Sun, Haiyan Wu Science and Technology on Scramjet Laboratory, National University of Defense Technology, Changsha 410073, China
article info
abstract
Article history:
Optical diagnosis-based combustion experiments were conducted to investigate the
Received 9 April 2013
characteristics of cavity assisted hydrogen jet combustion in a supersonic flow with a total
Received in revised form
pressure of 1.6 MPa, a total temperature of 1486 K, and a Mach number of 2.52, simulating
27 June 2013
flight Mach 6 conditions. A supersonic combustor with a constant cross-sectional area was
Accepted 29 June 2013
employed with several cavity configurations, fueling schemes and equivalence ratios. It
Available online 26 July 2013
was found that stable combustion could not be obtained without a cavity, indicating that pure jet-wake stabilized combustion could not be achieved and the cavity acted as a
Keywords:
flameholder. Three combustion modes were observed for the cavity assisted hydrogen jet
Cavity
combustion: cavity assisted jet-wake stabilized combustion, cavity shear-layer stabilized
Supersonic combustion
combustion, and combined cavity shear-layer/recirculation stabilized combustion. The
Combustion mode
cavity assisted jet-wake stabilized combustion was observed to be the most unstable mode, accompanied by intermittent blowoff under the present conditions, while the combined cavity shear-layer/recirculation stabilized combustion mode seemed to be the most robust one. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Scramjet engines are the most promising candidates for future air-breathing systems in hypersonic flight and receive more and more attention, where hydrogen is widely used as fuel [1e7] due to its high energy release, high reactivity, wide flammability limits and high diffusivity [8]. Scramjet engine lets the air stream enter into the combustor supersonically and organizes combustion within supersonic flow, where robust flameholding schemes are necessary due to the short combustor residence time. One promising candidate for such a flameholder is the wall cavity which has been shown to be effective in stabilizing the flame without excessively decreasing total pressure [9]. When used as an integrated fuel
injection/flameholding approach [10], cavity flameholders have become even more attractive in supersonic combustors. There has been a large amount of research related to supersonic cavity flow and combustion. Only the studies most relevant to the present work are highlighted here. Ben-Yakar and Hanson [11] used high-speed framing schlieren and OH-Planar Laser-Induced Fluorescence (PLIF) to investigate hydrogen transverse jet injected upstream of a cavity in air cross-flow simulating flight Mach 10 conditions, where auto-ignition was achieved and OH fluorescence appeared first in the recirculation upstream of the jet and extended along outer edge of the jet plume. O’Byrne et al. [12] used OH-PLIF to investigate the supersonic combustion with horizontal injection from the aft wall of a cavity and found
* Corresponding author. Tel.: þ86 13787207654. E-mail addresses: [email protected], [email protected] (Z. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.132
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combustion occurred in the shear layer above the cavity rather than in the recirculating cavity flow. However, Jeong et al. [13] found the cavity acted as a flameholder for the case of upstream injection. Gruber et al. [14] studied the mixing and combustion in a supersonic flow using cavity flameholders. It was found that an imposed shock train had a significant impact on the mixing and chemical reaction processes that occurred in the cavity flameholder. Rasmussen et al. [15,16] investigated stability limits and flameholding mechanism of cavity-stabilized flames with direct cavity injection. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. Sun et al. [17] studied the combustion in a supersonic combustor with hydrogen injection upstream of cavity flameholders using OH-PLIF and hybrid Reynolds-Averaged NaviereStokes (RANS)/Large Eddy Simulation (LES). It was shown that an approximately steady flame existed in the cavity shear layer and hot combustion products were transported into the injection jet by the vortex interactions of the jet-with-cavity shear layer, where the counter-rotating vortex induced by the jet and the cavity shear layer played an important role. Wang et al. [3] investigated combustion characteristics in a supersonic combustor with hydrogen injection upstream of a cavity flameholder. It was found that the combustion spreading from the cavity shear layer to the main stream seemed to be dominated not only by the traditional diffusion process but also by the convection process associated with the extended recirculation flows resulting from the heat release and the interactions between the jet and the cavity shear layer. The cavitystabilized combustion appeared to be a strongly coupled process of flow and heat release around the cavity flameholder. Lin et al. [18] studied the performance and operating limits of an ethylene-fueled recessed cavity flameholder with various cavity lengths (L/D ¼ 4, 5, and 6) both experimentally and numerically. It was found that the same cavity flameholder might perform quite differently under different injection conditions. Micka et al. [2,19] investigated the combustion characteristics of a dual-mode scramjet combustor with cavity flameholder, and found that the combustion was anchored at the cavity leading edge at low stagnation temperature and stabilized a short distance downstream of the fuel injection jet in the jet wake at high stagnation temperature. The cavity stabilized combustion was consistent with the theory of a premixed flame spreading controlled reaction and the flame spread into the main flow as continuous or shredded reaction layers, while the jet-wake stabilized combustion was consistent with a lifted jet flame description (premixed flame base with diffusion flame downstream) and the reaction zone was generally highly shredded and continuous layers were not observed. Tuncer [20] numerically studied the combustion characteristics in a ramjet combustor with cavity flameholder for an entrance Mach number of 1.4 and stagnation temperature of 702 K. It was observed that flame anchored at the
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leading edge of the cavity, and the flame was stabilized in the cavity mode rather than the jet-wake mode. Masumoto et al. [21] experimentally studied combustion modes in a supersonic combustor. Four combustion modes were observed: nonignition, weak combustion (with little pressure rise), supersonic combustion, and dual-mode combustion, which were organized in terms of combustor length and total temperature at different equivalence ratios. In the present work, optical diagnosis-based combustion experiments are conducted to provide further insights into the characteristics of cavity assisted jet combustion in supersonic flows. The focus is the identification and analysis of possible combustion modes in terms of cavity configurations, fueling schemes and equivalence ratios.
2.
Experimental apparatus
2.1.
Test apparatus and conditions
The experiments are carried out at National University of Defense Technology in a recently developed direct-connected rig [3]. The model scramjet combustor is installed behind the nozzle of the air heater, which heats the air by means of air/ O2/alcohol combustion to simulate flight Mach 6 conditions, resulting in a Mach 2.52 and mass flow 1 kg/s stream in the combustor entrance. The run time is 6e7 s for the air heater and 1.8e2 s for the fuel injection, and the non-cooling supersonic combustor walls are directly exposed to the atmosphere. Detailed flow conditions at the nozzle exit and fuel jet exit are listed in Table 1, where the fuel is 99.5e99.8% pure H2. Schematic of the test section is shown in Fig. 1. The combustor has a constant width of 50 mm and height of 40 mm. A cavity with depth D ¼ 8 mm, length-to-depth ratio L/ D ¼ 4 or 7 and aft angle A ¼ 22.5 , 45 or 90 can be mounted on the bottom side. A main fuel injector with orifice diameter 2 mm can be installed 10 or 30 mm upstream of the cavity leading edge. For simplicity, SsLlAaPp is used to denote the cavity with main fuel injection located s mm upstream of the cavity leading edge, L/D ¼ l, A ¼ a and Pjet ¼ p MPa from now on. Pilot fuel can be injected directly into the cavity through 3, 0.5 mm diameter spanwise ports in the cavity rear wall. A spark plug is fixed on the cavity floor to assist the ignition. Quartz glass windows can be mounted on the top and side walls to allow optical access. In order to evaluate the temperature within the cavity, two type B thermocouples with a
Table 1 e Test conditions. Parameter T 0, K P0, MPa Ma YO 2 ; % YH 2 O ; % YCO2 ; % YN 2 ; % YH 2 ; %
Air
Jet
1486 1.6 2.52 23.38 6.22 10.16 60.24 0.0
300 0.6/1.2/1.8 1.0 0.0 0.0 0.0 0.0 100.0
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wake stabilized combustion, cavity shear-layer stabilized combustion, and combined cavity shear-layer/recirculation stabilized combustion. Representative cases of these combustion modes are shown in Fig. 2, characteristics of which will be presented in more detail below.
3.1.
Fig. 1 e Schematic of the supersonic combustor.
response time of about 1 s and a maximum measure of 2000 K are installed on the cavity floor, the sensors of which are located in the recirculation region, 3 mm above the cavity floor and 3 mm away from the cavity walls. Moreover, a pressure scanner is also introduced to obtain the pressure distributions along the bottom wall.
2.2.
Optical measurements
OH-PLIF, OH spontaneous emission, high-speed framing of flame luminosity and schlieren are introduced to characterize the combustion flow. The exposure time of PLIF and spontaneous emission are 50 ns and 50 ms, respectively. The exposure time and frame rate of flame luminosity and schlieren are 0.25 ms and 4000 frame/s, respectively. The Q1(8) Ultraviolet (UV) spectrum is chosen for OH-PLIF based on the consideration that the Q1(8) UV spectrum is not sensible to the temperature variation between 1000 and 2600 K so that the fluorescence intensity is approximately proportional to the OH mole concentration. The PLIF system includes an Nd:YAG laser, a dye laser, a frequency doubler and an intensified charged-coupled device (ICCD) system. The output beam from Nd:YAG laser is at 532 nm. It pumps the dye laser to generate the laser beam at 567.106 nm. Then the output from the dye laser is transferred into the doubler, which transforms it into an Ultraviolet (UV) laser beam at 283.553 nm and 18 mJ/pulse. The laser energy fluctuation is about 5%. The UV beam is adjusted to a laser sheet by a group of lenses. The thickness of the plane beam is about 0.2 mm. The PLIF image is photographed by the ICCD camera through a filter (UG11 and WG305), allowing the pass of lights at 300e400 nm.
3.
Results and discussion
Three injection conditions Pjet ¼ 0.6, 1.2 and 1.8 MPa, resulting in equivalence ratios of 0.038, 0.076 and 0.11 for the main fuel injection alone, are considered. It is found in the experiment that stable combustion cannot be obtained without a cavity, which means that pure jet-wake stabilized combustion cannot be achieved and the cavity acts as a flameholder under the present conditions. Based on the cavity configurations, fueling schemes and equivalence ratios considered here, three combustion modes are observed: cavity assisted jet-
Cavity assisted jet-wake stabilized combustion
Though pure jet-wake stabilized combustion cannot be achieved, it is found that the jet combustion can be obtained when coupled with a downstream cavity. For the short cavity with L/D ¼ 4, the cavity length is however not long enough for the cavity shear layer to evolve to have strong interactions with the injection jet, the fuel transport into the cavity shear layer or into the cavity recirculation is thus suppressed and the combustion basically occurs above the cavity shear layer, as illustrated in Fig. 2 for cavity S10L4A45P1.8. Technically speaking, the reaction zone is stabilized in the wake of the fuel injection jet and no obvious reaction occurs in the cavity mode. However, the downstream cavity is believed to have positive effects on the jet-wake stabilized combustion since pure jet-wake stabilized combustion cannot be achieved under present conditions as tested in the experiment. Notably, the jet combustion coupled with a further downstream cavity is always stabilized in the cavity shear-layer mode and never stabilized in the jet-wake mode, as illustrated in Fig. 2 for cavity S30L7A45P1.8, which again indicates that the cavity installed shortly downstream of the fuel injector assists the jet-wake stabilized combustion. Once the combustion is stabilized in the jet wake, the intense heat release around the cavity may greatly compress the incoming flow, inducing the separation of the incoming boundary layer approaching the cavity leading edge and the extension of the jet downstream recirculation. As a result, the cavity recirculation and the jet downstream recirculation may coalesce into an integrated one, where the hot products in the cavity recirculation may be transported into the jet downstream recirculation intermittently, supporting the stabilization of jet-wake combustion. When the distance between the fuel injector and the cavity is increased, these two recirculation regions can never coalesce into a single one, and then the cavity can no longer promote the stabilization of jet-wake combustion as for cavity S30L7A45P1.8. Fig. 3 shows the processes of jet ignition and transition from cavity stabilized combustion to cavity assisted jet-wake stabilized combustion. It is observed that the cavity assisted jet-wake stabilized combustion always originates from the cavity stabilized combustion. Once the reaction zone is stabilized in the cavity mode, the heat release leads to boundarylayer separation shortly and the reaction zone leading edge propagates into the jet wake, leading to conditions favorable for combustion stabilization in the jet wake. Subsequently, the heavy separation of the incoming boundary layer and the establishment of the jet-wake combustion suppress the fuel transport into the cavity shear layer or into the cavity recirculation, resulting in extinction of the reaction in the cavity mode. However, the interactions between the cavity shear layer and the injection jet can still transport a portion of hot products from the fuel jet reaction zone to the cavity recirculation and even into the jet downstream recirculation due
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Fig. 2 e Representative cases of the three combustion modes. Top: cavity assisted jet-wake stabilized combustion (S10L4A45P1.8); Middle: cavity shear-layer stabilized combustion (S30L7A45P1.8); Bottom: combined cavity shear-layer/recirculation stabilized combustion (S10L7A45P1.8). Left: OH spontaneous emission images; Right: flame luminosity images.
to the interactions of these two recirculation regions via boundary-layer separation, supporting the jet-wake stabilized combustion. It is also observed that, however, the cavity assisted jetwake stabilized combustion experiences strong oscillations
and is not very robust. During a test lasting hundreds of milliseconds, the combustion may be blown off for several times. If the spark plug is on during the entire fueling duration, the jet can be reignited several milliseconds after the blowoff. A representative process of the blowoff is shown in Fig. 4. In
Fig. 3 e Ignition and transition to cavity assisted jet-wake stabilized combustion.
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Fig. 4 e Representative flame luminosity images during the blowoff.
contrast to its establishment, the blowoff of the cavity assisted jet-wake stabilized combustion is a much shorter process, where the combustion is directly blown off without an inbetween state of cavity stabilized combustion. During a single test, the ignition/transition/blowoff cycle occurs several times when the fuel flow rate has already achieved a very stable state. Therefore, the fuel flow ramp up time does not have significant effects on the ignition and transition to jet-wake mode. It has been tested that the ignition cannot occur without the assistance of the spark plug. Thus, the ignition mechanism is inferred as below. The premixed or partially premixed mixture within the cavity is heated by the spark plug when passing by. The ignition location is determined by the competition between the ignition delay and the residence time of the heated mixture. If the residence time within the cavity is long enough compared to the ignition delay, the ignition occurs in the cavity. Otherwise, the ignition occurs downstream of the cavity. Actually, both cases are observed in our previous experiments under different conditions. Since the flame appearing downstream of the cavity has to enter into the cavity before transition to the jet-wake mode, the ignition location does not have substantial effects on the transition process.
3.2.
Cavity stabilized combustion
In the previous studies, the combustion is identified either as jet-wake stabilized mode or cavity stabilized mode [2,20]. In the present study, however, it is observed that there exist two distinct combustion modes for the cavity stabilized combustion: cavity shear-layer stabilized combustion and combined cavity shear-layer/recirculation stabilized combustion. First, let us still take the cavities S30L7A45P1.8 and S10L7A45P1.8 for example to show these two combustion modes. As shown in Fig. 2, the combustion is stabilized in the cavity shear layer for cavity S30L7A45P1.8 but stabilized in both the cavity shear layer and recirculation within the cavity
for S10L7A45P1.8. OH-PLIF images shown in Fig. 5 give additional detail of these two combustion modes, where the images are displayed with a false color scale ranging from 100 to 800 counts but normalized from 0 to 1. For the cavity shearlayer stabilized combustion, the combustion is always anchored at the cavity leading edge, and the reaction zones only appear in the cavity shear layer and around the fuel jet in the main stream. There rarely exists intensive reaction in the cavity recirculation region since the concentration of OH radical is much lower in the cavity recirculation compared to that in the cavity shear layer. Thus, the heat is mainly released in the regions out of the cavity. For the combined cavity shearlayer/recirculation stabilized combustion, however, there seems to be a strong reaction in both the cavity shear layer and the cavity recirculation region since the concentration of OH radical is equivalently high in these regions. Accordingly, a large portion of the heat is released in and around the cavity. The horizontal and vertical plots in Fig. 6 are the OH-signal distributions along the streamwise and transverse directions, respectively. First, the four instantaneous images shown in Fig. 5 are averaged for each combustion mode. Then the total signal is obtained by integrating the normalized signal along the streamwise and transverse directions, as processed by Jeong et al. [13]. The mean signal is the line-averaged signal of the pixels where the normalized signal is positive. These plots allow clearer comparisons of the combustion regions where OH generation occurs in the flow between the two combustion modes. For the combined cavity shear-layer/recirculation stabilized combustion, the OH signal intensity is much higher in the cavity especially in the font portion of the cavity. Moreover, two interesting phenomena are observed from the statistic results. One is the presence of OH peaks in the streamwise direction, similar to those detected by O’Byrne et al. [12] and Jeong et al. [13]. This intermittent occurrence of high OH signal may be caused by the excitation of acoustic modes within the cavity [12,13]. The other is that there seem to be two major combustion layers in the vertical plots for
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Fig. 5 e Representative OH-PLIF images in centerplane for cavity stabilized combustion. Left: cavity shear-layer stabilized combustion (S30L7A45P1.8); Right: combined cavity shear-layer/recirculation stabilized combustion (S10L7A45P1.8).
Fig. 6 e OH-signal distributions along the streamwise and transverse directions.
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Fig. 7 e Schlieren for cavity stabilized combustion. Left: cavity shear-layer stabilized combustion (S30L7A45P1.8); Right: combined cavity shear-layer/recirculation stabilized combustion (S10L7A45P1.8).
cavity S30L7A45P1.8. One appears around the cavity shear layer and the other around the upper boundary of the jet. The first one may result from the shear-layer stabilized flame and the second one should be caused by the auto-ignition process associated with the formation of the large-scale, hairpin-like structures generated in the windward portion of the jet boundary. The present conditions are similar to that adopted by Micka [19], where the combustion is found to be an autoignition assisted flame. For s ¼ 30 mm, the probability of auto-ignition in the regions above the cavity should be higher due to a longer ignition delay distance, resulting in thus stronger reaction around the upper boundary of the fuel jet. Compared with the cavity shear-layer stabilized combustion, the combined cavity shear-layer/recirculation stabilized combustion is more robust and induces heavier separation of the upper wall boundary layer as shown in Fig. 7. Also can be seen is that the incoming flow is well decelerated by the intense heat release around the cavity by the combined cavity shear-layer/recirculation stabilized combustion, leading to vanishing of the shock wave originating from the cavity leading edge. Effects of cavity configurations and equivalence ratios on the combustion mode are also addressed. Figs. 8e10 show the images of flame luminosity, OH spontaneous emission and schlieren for cavity S10L7 with different aft angles and equivalence ratios. For low equivalence ratio and small cavity aft angle, the combustion is always stabilized in the cavity shear layer and starts some distance downstream of the cavity leading edge, which means that the combustion is dominated by the cavity shear-layer stabilized mode. With increasing equivalence ratio or increasing cavity aft angle, the combustion zone may appear in both the cavity shear layer and cavity recirculation region, indicating a transition to the combined cavity shear-layer/recirculation stabilized combustion; meanwhile, the fuel jet in the main stream is ignited earlier and the intense heat release is pushed upstream, leading to heavier separation of the upper wall boundary layer. With increasing equivalence ratio, i.e. increasing injection pressure, both the horseshoe vortex formed around the jet exit and the counter-rotating vortices around the jet become stronger, which tends to promote the fuel transport into the cavity shear layer as well as into the cavity recirculation region via boundary-layer separation at the cavity leading edge and vortex interactions of the jet-with-cavity shear layer, consistent with that observed by Thakur and
Segal [22]. The increase of cavity aft angle may lead to two important characteristics of the flowfield. First, the cavity with large aft angle induces stronger oscillations in the flowfield around it [23], which promotes the jet instabilities and results in a larger jet plume at the same axial positions. Second, a compression wave forms around the cavity leading edge when the cavity aft angle is large enough while expansion waves are generated for cavities with small aft angle [23]. This indicates that the cavity shear layer inclines toward the main stream for large aft angle but dives into the cavity volume for small aft angle. Consequently, the increase of cavity aft angle tends to shorten the distance between the jet plume and the cavity shear layer and thus enhance the interactions between the jet and cavity shear layer, as a result of which more fuels can be transported into the cavity shear layer as well as into the cavity recirculation region. Hence, it is inferred that the fuel transport process may have significant influences on the subsequent combustion mode; further analyses will be given below. It is also observed that, with high injection pressure or large cavity aft angle, the flame base may flash back into the jet wake upstream of the cavity leading edge intermittently; however, a larger portion of the reaction still occurs in the cavity mode, which is quite different from that of the short cavity S10L4A45P1.8 where the entire reaction zone is stabilized in the jet wake. The possible reason is that the longer cavities have a better developed shear layer which is beneficial to the fuel transport into the cavity shear layer and cavity recirculation region, supporting the cavity stabilized combustion.
3.3.
Discussion
Based on the abundant experimental observations, it is evident that the combustion becomes more and more stable and robust, accompanied by the transition of combustion mode from the cavity assisted jet-wake stabilized mode to the cavity shear-layer stabilized mode and to the combined cavity shear-layer/recirculation stabilized mode. Also clearly observed is that the combustion tends to shift to more robust mode with increasing cavity length or equivalence ratio, or decreasing distance between the fuel injection and the cavity. Obviously, the combustion can be stabilized in the cavity mode only when the fluid within the cavity shear layer or recirculation region is of proper equivalence ratio. The study
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Fig. 8 e Representative flame luminosity images for cavities with different aft angles and equivalence ratios.
of Thakur and Segal [22] showed that the local equivalence ratio within the recirculation region might be even lower than the global one for light injectant (helium or hydrogen) injected upstream in the non-reacting cases. In their combustion experiments, a stable flame could not be established at the conditions adopted for upstream hydrogen injection, which again indicated insufficient fuel entrained into the recirculation region as pointed out by the authors. Considering the larger injection hole used in the present study compared to those adopted by Thakur and Segal [22], it may be inferred that the cavity is fuel-lean with upstream injection only under the present conditions. Thus, any means that promotes the fuel transport into the cavity shear layer and cavity recirculation region can support the transition of combustion to the cavity stabilized modes. In order to verify this guess, additional tests are carried out. Fig. 11 shows the reaction zone of cavity L7A45 with direct cavity injection. Hydrogen is injected through 3, 0.5 mm diameter spanwise ports in the cavity rear wall. The injection pressure is Pjet ¼ 1.8 MPa, which results in an overall equivalence of 0.021. It is found that the combustion is stabilized in the combined cavity shear-layer/recirculation mode. Fig. 12
shows the reaction zone of cavity L7A45 with both upstream and cavity injections. Hydrogen is injected through both an injector with orifice diameter 2 mm installed 10 mm upstream of the cavity leading edge and 3, 0.5 mm diameter spanwise ports in the cavity rear wall. The injection pressure is Pjet ¼ 1.2 MPa, which results in an overall equivalence of 0.09. It is also found that the combustion is stabilized in the combined cavity shear-layer/recirculation mode. It is notable that the combustion is basically stabilized in the cavity shear-layer mode when the direct injection is turned off, as shown in Figs. 8 and 9 for upstream injection of Pjet ¼ 1.2 MPa. Notably, in the study of O’Byrne et al. [12] where direct cavity fueling was also used, it is found that combustion occurred in the shear layer above the cavity rather than in the cavity recirculation region. The possible reason is that the high equivalence ratios adopted led to too fuel-rich regions in the cavity as pointed out by the authors, inhibiting the reaction within the cavity recirculation. Moreover, the high Mach number also tended to suppress the growth of the cavity shear layer, prohibiting the transport of fresh air into the cavity. Thus, the cavity recirculation combustion was difficult to obtain.
Fig. 9 e Representative OH spontaneous emission images for cavities with different aft angles and equivalence ratios.
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Fig. 10 e Representative schlieren images for cavities with different aft angles and equivalence ratios.
Fig. 11 e Cavity L7A45 with cavity injection.
Fig. 12 e Cavity L7A45 with both upstream and cavity injections.
In order to further clarify the combustion modes, surface pressure distributions along the bottom wall and temperature in the cavity recirculation region are also measured for cavity L7A45 under some typical conditions, as shown in Figs. 13 and 14. Each experiment is executed three times for repeatability; the uncertainties are estimated to be 3e5% for the pressure measurements and 6e9% for the temperature measurements. One can see that the cases in the combined cavity shear-layer/ recirculation combustion mode always induce higher pressure rise around the cavity, which is believed to be beneficial to the combustion stabilization due to the close coupling of
flow and combustion [3]. In the far downstream regions, the pressure rise of s ¼ 30 mm, Pjet ¼ 1.8 MPa which operates in the cavity shear-layer combustion mode tends to exceed that of s ¼ 10 mm, Pjet ¼ 1.8 MPa which operates in the combined cavity shear-layer/recirculation combustion mode. This indicates that intense heat release would take place in further downstream regions if the combustion is stabilized in the cavity shear-layer mode. That is, the required combustor length would be shorter if the combustion is stabilized in the combined cavity shear-layer/recirculation mode. With respect to the temperature in the cavity recirculation region, it is
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Fig. 13 e Surface pressure distributions along the bottom wall.
found that the recirculation temperature for the cases in the combined cavity shear-layer/recirculation combustion mode is always hundreds of Kelvin higher than that for the cases in the cavity shear-layer combustion mode. Moreover, the temperature distribution is far from uniform in the streamwise direction. Basically, the temperature in the aft region is hundreds of Kelvin higher than that in the front region. The possible reason is that the hot products or reacting mixture are mainly entrained into the cavity near the aft wall via the interactions between the cavity shear layer and the aft wall, and then transported to the front region by the recirculation. Thus, the temperature tends to be higher in the aft region. Furthermore, the region near the front corner is a kind of “dead zone” with relatively weak mass and energy exchange
Fig. 14 e Temperature in the cavity recirculation region.
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with other regions in the recirculation so that the heat loss to the solid walls cannot be compensated in time, which also tends to result in lower temperature in this region. However, the combined cavity shear-layer/recirculation combustion mode can well elevate the temperature in the whole recirculation region to enhance the robustness of the combustion stabilization. Fig. 15 shows the schematic of the reaction zone location and plausible combustion stabilization mechanisms for the three combustion modes as inferred from the optical observations. Fig. 15(a) depicts the reaction zone structure and stabilization mechanism for the cavity assisted jet-wake combustion. Though no combustion takes place in the cavity shear layer or in the cavity recirculation region, it is believed that the cavity has positive effects on combustion stabilized in the jet wake since pure jet-wake combustion cannot be obtained without the downstream coupled cavity. It is inferred that the vortex interactions of the jet-with-cavity shear layer tend to transport some hot products from the reactive jet into the cavity shear layer and cavity recirculation region, maintaining the hot environment within the cavity. Compared to the cavity stabilized combustion, the jet-wake combustion appears to ignite the fuel jet earlier and release more heat around the cavity leading edge, leading to heavier separation of the incoming boundary layer approaching the cavity leading edge. Therefore, the hot products in the cavity recirculation region can be transported into the jet wake readily, promoting the ignition and combustion in the jet wake. However, only a small portion of the hot products can go
Fig. 15 e Schematic of combustion stabilization mechanisms for different combustion modes as inferred from the optical observations.
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through the cavity shear layer and enter into the cavity recirculation region, so this stabilization mode is not very stable and is susceptible to flow disturbances. The intermittent blowoff observed in the test supports these analyses. Fig. 15(b) depicts the reaction zone structure and stabilization mechanism for the cavity shear-layer combustion. The reaction zone is always stabilized in the cavity shear layer, some distance downstream of the cavity leading edge. The extension of the reaction zone appears to be significantly influenced by the growth of the cavity shear layer. The hot products can be transported into the cavity recirculation region directly by the vortex interactions of the cavity recirculation-with-cavity shear layer and interactions between the cavity shear and the cavity aft wall. Therefore, more hot products may be transported into the cavity recirculation and the combustion becomes more robust compared to the cavity assisted jet-wake combustion. Notably, the reaction zone is narrow in the transverse direction for quite a long distance because the fuel jet is not completely ignited very soon. Fig. 15(c) depicts the reaction zone structure and stabilization mechanism for the combined cavity shear-layer/ recirculation combustion. The reaction zone appears in both the cavity shear layer and cavity recirculation. It is inferred that more fuel is transported into the cavity shear layer, thus the fuel is not totally consumed by the shear-layer combustion and a portion of the fuel is then entrained into the cavity recirculation region and continues to react there. The reaction zone moves with the cavity recirculating flow and usually combines with the cavity shear-layer combustion, coalescing into a large reaction zone. Since the residence time in the cavity recirculation region is much larger than that in the cavity shear layer, the combustion stabilized in the combined cavity shear-layer/recirculation mode is more robust than that only stabilized in the cavity shear-layer mode. Furthermore, the combined cavity shear-layer/recirculation combustion may release more heat around the cavity and enhance the near-field combustion, leading to earlier ignition of the entire fuel jet. A schematic of velocity profiles in the jet wake and cavity shear layer displayed in Fig. 16 can also help to understand the stabilization characteristics. When the fuel jet is transversely injected into the crossflow, a jet wake is formed. There exists a recirculation in the near-field region of the jet wake, where the streamwise velocity around the centerplane is negative (uc < 0). With increasing downstream distance, the wake
spreads and decays soon with uNeuc decreasing toward zero. The flame base could be possibly stabilized only in a limited region where the flow velocity and flame speed satisfy uc sf. As a result, the flame base will be readily blown off when it is perturbed downstream by a large flow disturbance since uc increases soon with increasing downstream distance while sf basically keeps constant. Thus, the flame stabilized in the jet wake is not very robust. On the other hand, since the flow around the cavity shear layer is basically self-similar, lowspeed regions are always available for the flame base to be stabilized with increasing downstream distance. Once a disturbance destroys the velocity balance, the flame base can adjust its position to achieve a new balance readily. Hence, the cavity-stabilized combustion modes are much more robust compared to the jet-wake stabilized combustion mode.
4.
Conclusions
Combustion experiments were conducted to investigate the characteristics of cavity assisted hydrogen jet combustion in a supersonic flow with a total pressure of 1.6 MPa, a total temperature of 1486 K, and a Mach number of 2.52, simulating flight Mach 6 conditions. Optical diagnosis-based measurements, including OH-PLIF, OH spontaneous emission, highspeed framing of flame luminosity and schlieren were introduced to characterize the combustion flow. Pressure and temperature measurements were also carried out for some typical cases to supply more quantitative information. A supersonic combustor with a constant cross-sectional area was employed with several cavity configurations, fueling schemes and equivalence ratios. It was found that stable combustion could not be obtained without a cavity, indicating that pure jet-wake stabilized combustion cannot be achieved and the cavity acts as a flameholder. Three combustion modes were observed: cavity assisted jet-wake stabilized combustion, cavity shear-layer stabilized combustion, and combined cavity shear-layer/ recirculation stabilized combustion. The cavity assisted jetwake stabilized combustion was observed to be the most unstable mode, accompanied by intermittent blowoff under the present conditions, while the combined cavity shearlayer/recirculation stabilized combustion mode seemed to be the most robust one. It was also observed that the combustion tended to shift to more robust mode with increasing cavity length or equivalence ratio, or decreasing distance between the fuel injection and the cavity. The fluid within the cavity was usually fuellean mixture under the present conditions with upstream transverse injection. Thus, any means that promoted the fuel transport into the cavity shear layer and cavity recirculation region could support the transition of combustion to the more robust cavity stabilized modes.
Acknowledgments Fig. 16 e Schematic of velocity profiles in the jet wake (left) and cavity shear layer (right).
This work is supported by the National Natural Science Foundation of China under Grant Nos. 50906098 and 91016028,
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and Fok Ying Tung Education Foundation under Grant No. 131055. The authors would like to thank Dr. Zhouqin Fan for the help in conducting the experiments.
Nomenclature
A D L Ma P s t T Y
cavity aft angle, degree depth of cavity, mm length of cavity, mm Mach number pressure, Pa upstream injection distance, mm time, ms temperature, K mass fraction of species
Subscript 0 stagnation parameters i index of species jet fuel jet parameters
references
[1] Lu S, Fan J, Luo K. High-fidelity resolution of the characteristic structures of a supersonic hydrogen jet flame with heated co-flow air. Int J Hydrogen Energy 2012;7:3528e39. [2] Micka DJ, Driscoll JF. Combustion characteristics of a dualmode scramjet combustor with cavity flameholder. Proc Combust Inst 2009;32:2397e404. [3] Wang HB, Wang ZG, Sun MB, Qin N. Combustion characteristics in a supersonic combustor with hydrogen injection upstream of cavity flameholder. Proc Combust Inst 2013;34:2073e82. [4] Bao W, Qin J, Zhou WX, Yu DR. Parametric performance analysis of multiple re-cooled cycle for hydrogen fueled scramjet. Int J Hydrogen Energy 2009;34:7334e41. [5] Bao W, Qin J, Zhou WX, Yu DR. Effect of cooling channel geometry on re-cooled cycle performance for hydrogen fueled scramjet. Int J Hydrogen Energy 2010;35:7002e11. [6] Qin J, Bao W, Zhou WX, Yu DR. Flow and heat transfer characteristics in fuel cooling channels of a recooling cycle. Int J Hydrogen Energy 2010;35:10589e98. [7] Qin J, Zhou WX, Bao W, Yu DR. Thermodynamic analysis and parametric study of a closed Brayton cycle thermal management system for scramjet. Int J Hydrogen Energy 2010;35:356e64.
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[8] Cecere D, Ingenito A, Giacomazzi E, Romagnos L, Bruno C. Hydrogen/air supersonic combustion for future hypersonic vehicles. Int J Hydrogen Energy 2011;36:11969e84. [9] Mathur T, Gruber M, Jackson K, Donbar J, Donaldson W, Jackson T, et al. Supersonic combustion experiments with a cavity-based fuel injector. J Propul Power 2001;17:1305e12. [10] Ben-Yakar A, Hanson RK. Cavity flame-holders for ignition and flame stabilization in scramjets: an overview. J Propul Power 2001;17:869e77. [11] Ben-Yakar A, Hanson RK. Supersonic combustion of crossflow jets and the influence of cavity flame-holders. In: 37th AIAA aerospace sciences meeting and exhibit 11e14 January 1999, Reno, Nevada 1999. AIAA 99-0484. [12] O’Byrne S, Stotz I, Neely AJ, Boyce RR, Mudford NR, Houwing AFP. OH PLIF imaging of supersonic combustion using cavity injection. In: 13th international space planes and hypersonic systems and technology 16e20 May 2005, Italy 2005. AIAA 2005-3357. [13] Jeong E, O’Byrne S, Jeung I-S, Houwing AFP. Investigation of supersonic combustion with angled injection in a cavitybased combustor. J Propul Power 2008;24:1258e68. [14] Gruber MR, Nejda AS, Chen TH, Dutton JC. Mixing and penetration studies of sonic jets in a Mach 2 freestream. J Propul Power 1995;11:315e23. [15] Rasmussen CC, Driscoll JF, Hsub K-Y. Stability limits of cavity-stabilized flames in supersonic flow. Proc Combust Inst 2005;30:2825e34. [16] Rasmussen CC, Dhanuka SK, Driscoll JF. Visualization of flameholding mechanisms in a supersonic combustor using PLIF. Proc Combust Inst 2007;31:2505e12. [17] Sun MB, Wang ZG, Liang JH, Geng H. Flame characteristics in a supersonic combustor with hydrogen injection upstream of a cavity flameholder. J Propul Power 2008;24:688e96. [18] Lin K-C, Tam C-J, Jackson K. Study on the operability of cavity flameholders inside a scramjet combustor. In: 45th AIAA/ASME/SAE/ASEE joint propulsion conference and exhibit 2e5 August 2009, Denver, Colorado 2009. AIAA 20095028. [19] Micka DJ. Combustion stabilization, structure, and spreading in a laboratory dual-mode scramjet combustor [PhD thesis]. Michigan, USA: The University of Michigan; 2010. [20] Tuncer O. Combustion in a ramjet combustor with cavity flame holder. In: 48th AIAA aerospace sciences meeting including the new horizons forum and aerospace exposition, 4e7 January 2010, Orlando, Florida 2010. AIAA 2010-1532. [21] Masumoto R, Tomioka S, Kudo K, Murakami A, Kato K, Yamasaki H. Experimental study on combustion modes in a supersonic combustor. J Propul Power 2011;27:346e55. [22] Thakur A, Segal C. Concentration distribution in a supersonic flow recirculation region. J Propul Power 2008;24:64e73. [23] Gruber MR, Baurle RA, Mathur T, Hsu K-Y. Fundamental studies of cavity-based flameholder concepts for supersonic combustors. J Propul Power 2001;17:146e53.
Available online at www.sciencedirect.com
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Combustion modes of hydrogen jet combustion in a cavity-based supersonic combustor Hongbo Wang, Zhenguo Wang*, Mingbo Sun, Haiyan Wu Science and Technology on Scramjet Laboratory, National University of Defense Technology, Changsha 410073, China
article info
abstract
Article history:
Optical diagnosis-based combustion experiments were conducted to investigate the
Received 9 April 2013
characteristics of cavity assisted hydrogen jet combustion in a supersonic flow with a total
Received in revised form
pressure of 1.6 MPa, a total temperature of 1486 K, and a Mach number of 2.52, simulating
27 June 2013
flight Mach 6 conditions. A supersonic combustor with a constant cross-sectional area was
Accepted 29 June 2013
employed with several cavity configurations, fueling schemes and equivalence ratios. It
Available online 26 July 2013
was found that stable combustion could not be obtained without a cavity, indicating that pure jet-wake stabilized combustion could not be achieved and the cavity acted as a
Keywords:
flameholder. Three combustion modes were observed for the cavity assisted hydrogen jet
Cavity
combustion: cavity assisted jet-wake stabilized combustion, cavity shear-layer stabilized
Supersonic combustion
combustion, and combined cavity shear-layer/recirculation stabilized combustion. The
Combustion mode
cavity assisted jet-wake stabilized combustion was observed to be the most unstable mode, accompanied by intermittent blowoff under the present conditions, while the combined cavity shear-layer/recirculation stabilized combustion mode seemed to be the most robust one. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Scramjet engines are the most promising candidates for future air-breathing systems in hypersonic flight and receive more and more attention, where hydrogen is widely used as fuel [1e7] due to its high energy release, high reactivity, wide flammability limits and high diffusivity [8]. Scramjet engine lets the air stream enter into the combustor supersonically and organizes combustion within supersonic flow, where robust flameholding schemes are necessary due to the short combustor residence time. One promising candidate for such a flameholder is the wall cavity which has been shown to be effective in stabilizing the flame without excessively decreasing total pressure [9]. When used as an integrated fuel
injection/flameholding approach [10], cavity flameholders have become even more attractive in supersonic combustors. There has been a large amount of research related to supersonic cavity flow and combustion. Only the studies most relevant to the present work are highlighted here. Ben-Yakar and Hanson [11] used high-speed framing schlieren and OH-Planar Laser-Induced Fluorescence (PLIF) to investigate hydrogen transverse jet injected upstream of a cavity in air cross-flow simulating flight Mach 10 conditions, where auto-ignition was achieved and OH fluorescence appeared first in the recirculation upstream of the jet and extended along outer edge of the jet plume. O’Byrne et al. [12] used OH-PLIF to investigate the supersonic combustion with horizontal injection from the aft wall of a cavity and found
* Corresponding author. Tel.: þ86 13787207654. E-mail addresses: [email protected], [email protected] (Z. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.132
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combustion occurred in the shear layer above the cavity rather than in the recirculating cavity flow. However, Jeong et al. [13] found the cavity acted as a flameholder for the case of upstream injection. Gruber et al. [14] studied the mixing and combustion in a supersonic flow using cavity flameholders. It was found that an imposed shock train had a significant impact on the mixing and chemical reaction processes that occurred in the cavity flameholder. Rasmussen et al. [15,16] investigated stability limits and flameholding mechanism of cavity-stabilized flames with direct cavity injection. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. Sun et al. [17] studied the combustion in a supersonic combustor with hydrogen injection upstream of cavity flameholders using OH-PLIF and hybrid Reynolds-Averaged NaviereStokes (RANS)/Large Eddy Simulation (LES). It was shown that an approximately steady flame existed in the cavity shear layer and hot combustion products were transported into the injection jet by the vortex interactions of the jet-with-cavity shear layer, where the counter-rotating vortex induced by the jet and the cavity shear layer played an important role. Wang et al. [3] investigated combustion characteristics in a supersonic combustor with hydrogen injection upstream of a cavity flameholder. It was found that the combustion spreading from the cavity shear layer to the main stream seemed to be dominated not only by the traditional diffusion process but also by the convection process associated with the extended recirculation flows resulting from the heat release and the interactions between the jet and the cavity shear layer. The cavitystabilized combustion appeared to be a strongly coupled process of flow and heat release around the cavity flameholder. Lin et al. [18] studied the performance and operating limits of an ethylene-fueled recessed cavity flameholder with various cavity lengths (L/D ¼ 4, 5, and 6) both experimentally and numerically. It was found that the same cavity flameholder might perform quite differently under different injection conditions. Micka et al. [2,19] investigated the combustion characteristics of a dual-mode scramjet combustor with cavity flameholder, and found that the combustion was anchored at the cavity leading edge at low stagnation temperature and stabilized a short distance downstream of the fuel injection jet in the jet wake at high stagnation temperature. The cavity stabilized combustion was consistent with the theory of a premixed flame spreading controlled reaction and the flame spread into the main flow as continuous or shredded reaction layers, while the jet-wake stabilized combustion was consistent with a lifted jet flame description (premixed flame base with diffusion flame downstream) and the reaction zone was generally highly shredded and continuous layers were not observed. Tuncer [20] numerically studied the combustion characteristics in a ramjet combustor with cavity flameholder for an entrance Mach number of 1.4 and stagnation temperature of 702 K. It was observed that flame anchored at the
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leading edge of the cavity, and the flame was stabilized in the cavity mode rather than the jet-wake mode. Masumoto et al. [21] experimentally studied combustion modes in a supersonic combustor. Four combustion modes were observed: nonignition, weak combustion (with little pressure rise), supersonic combustion, and dual-mode combustion, which were organized in terms of combustor length and total temperature at different equivalence ratios. In the present work, optical diagnosis-based combustion experiments are conducted to provide further insights into the characteristics of cavity assisted jet combustion in supersonic flows. The focus is the identification and analysis of possible combustion modes in terms of cavity configurations, fueling schemes and equivalence ratios.
2.
Experimental apparatus
2.1.
Test apparatus and conditions
The experiments are carried out at National University of Defense Technology in a recently developed direct-connected rig [3]. The model scramjet combustor is installed behind the nozzle of the air heater, which heats the air by means of air/ O2/alcohol combustion to simulate flight Mach 6 conditions, resulting in a Mach 2.52 and mass flow 1 kg/s stream in the combustor entrance. The run time is 6e7 s for the air heater and 1.8e2 s for the fuel injection, and the non-cooling supersonic combustor walls are directly exposed to the atmosphere. Detailed flow conditions at the nozzle exit and fuel jet exit are listed in Table 1, where the fuel is 99.5e99.8% pure H2. Schematic of the test section is shown in Fig. 1. The combustor has a constant width of 50 mm and height of 40 mm. A cavity with depth D ¼ 8 mm, length-to-depth ratio L/ D ¼ 4 or 7 and aft angle A ¼ 22.5 , 45 or 90 can be mounted on the bottom side. A main fuel injector with orifice diameter 2 mm can be installed 10 or 30 mm upstream of the cavity leading edge. For simplicity, SsLlAaPp is used to denote the cavity with main fuel injection located s mm upstream of the cavity leading edge, L/D ¼ l, A ¼ a and Pjet ¼ p MPa from now on. Pilot fuel can be injected directly into the cavity through 3, 0.5 mm diameter spanwise ports in the cavity rear wall. A spark plug is fixed on the cavity floor to assist the ignition. Quartz glass windows can be mounted on the top and side walls to allow optical access. In order to evaluate the temperature within the cavity, two type B thermocouples with a
Table 1 e Test conditions. Parameter T 0, K P0, MPa Ma YO 2 ; % YH 2 O ; % YCO2 ; % YN 2 ; % YH 2 ; %
Air
Jet
1486 1.6 2.52 23.38 6.22 10.16 60.24 0.0
300 0.6/1.2/1.8 1.0 0.0 0.0 0.0 0.0 100.0
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wake stabilized combustion, cavity shear-layer stabilized combustion, and combined cavity shear-layer/recirculation stabilized combustion. Representative cases of these combustion modes are shown in Fig. 2, characteristics of which will be presented in more detail below.
3.1.
Fig. 1 e Schematic of the supersonic combustor.
response time of about 1 s and a maximum measure of 2000 K are installed on the cavity floor, the sensors of which are located in the recirculation region, 3 mm above the cavity floor and 3 mm away from the cavity walls. Moreover, a pressure scanner is also introduced to obtain the pressure distributions along the bottom wall.
2.2.
Optical measurements
OH-PLIF, OH spontaneous emission, high-speed framing of flame luminosity and schlieren are introduced to characterize the combustion flow. The exposure time of PLIF and spontaneous emission are 50 ns and 50 ms, respectively. The exposure time and frame rate of flame luminosity and schlieren are 0.25 ms and 4000 frame/s, respectively. The Q1(8) Ultraviolet (UV) spectrum is chosen for OH-PLIF based on the consideration that the Q1(8) UV spectrum is not sensible to the temperature variation between 1000 and 2600 K so that the fluorescence intensity is approximately proportional to the OH mole concentration. The PLIF system includes an Nd:YAG laser, a dye laser, a frequency doubler and an intensified charged-coupled device (ICCD) system. The output beam from Nd:YAG laser is at 532 nm. It pumps the dye laser to generate the laser beam at 567.106 nm. Then the output from the dye laser is transferred into the doubler, which transforms it into an Ultraviolet (UV) laser beam at 283.553 nm and 18 mJ/pulse. The laser energy fluctuation is about 5%. The UV beam is adjusted to a laser sheet by a group of lenses. The thickness of the plane beam is about 0.2 mm. The PLIF image is photographed by the ICCD camera through a filter (UG11 and WG305), allowing the pass of lights at 300e400 nm.
3.
Results and discussion
Three injection conditions Pjet ¼ 0.6, 1.2 and 1.8 MPa, resulting in equivalence ratios of 0.038, 0.076 and 0.11 for the main fuel injection alone, are considered. It is found in the experiment that stable combustion cannot be obtained without a cavity, which means that pure jet-wake stabilized combustion cannot be achieved and the cavity acts as a flameholder under the present conditions. Based on the cavity configurations, fueling schemes and equivalence ratios considered here, three combustion modes are observed: cavity assisted jet-
Cavity assisted jet-wake stabilized combustion
Though pure jet-wake stabilized combustion cannot be achieved, it is found that the jet combustion can be obtained when coupled with a downstream cavity. For the short cavity with L/D ¼ 4, the cavity length is however not long enough for the cavity shear layer to evolve to have strong interactions with the injection jet, the fuel transport into the cavity shear layer or into the cavity recirculation is thus suppressed and the combustion basically occurs above the cavity shear layer, as illustrated in Fig. 2 for cavity S10L4A45P1.8. Technically speaking, the reaction zone is stabilized in the wake of the fuel injection jet and no obvious reaction occurs in the cavity mode. However, the downstream cavity is believed to have positive effects on the jet-wake stabilized combustion since pure jet-wake stabilized combustion cannot be achieved under present conditions as tested in the experiment. Notably, the jet combustion coupled with a further downstream cavity is always stabilized in the cavity shear-layer mode and never stabilized in the jet-wake mode, as illustrated in Fig. 2 for cavity S30L7A45P1.8, which again indicates that the cavity installed shortly downstream of the fuel injector assists the jet-wake stabilized combustion. Once the combustion is stabilized in the jet wake, the intense heat release around the cavity may greatly compress the incoming flow, inducing the separation of the incoming boundary layer approaching the cavity leading edge and the extension of the jet downstream recirculation. As a result, the cavity recirculation and the jet downstream recirculation may coalesce into an integrated one, where the hot products in the cavity recirculation may be transported into the jet downstream recirculation intermittently, supporting the stabilization of jet-wake combustion. When the distance between the fuel injector and the cavity is increased, these two recirculation regions can never coalesce into a single one, and then the cavity can no longer promote the stabilization of jet-wake combustion as for cavity S30L7A45P1.8. Fig. 3 shows the processes of jet ignition and transition from cavity stabilized combustion to cavity assisted jet-wake stabilized combustion. It is observed that the cavity assisted jet-wake stabilized combustion always originates from the cavity stabilized combustion. Once the reaction zone is stabilized in the cavity mode, the heat release leads to boundarylayer separation shortly and the reaction zone leading edge propagates into the jet wake, leading to conditions favorable for combustion stabilization in the jet wake. Subsequently, the heavy separation of the incoming boundary layer and the establishment of the jet-wake combustion suppress the fuel transport into the cavity shear layer or into the cavity recirculation, resulting in extinction of the reaction in the cavity mode. However, the interactions between the cavity shear layer and the injection jet can still transport a portion of hot products from the fuel jet reaction zone to the cavity recirculation and even into the jet downstream recirculation due
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Fig. 2 e Representative cases of the three combustion modes. Top: cavity assisted jet-wake stabilized combustion (S10L4A45P1.8); Middle: cavity shear-layer stabilized combustion (S30L7A45P1.8); Bottom: combined cavity shear-layer/recirculation stabilized combustion (S10L7A45P1.8). Left: OH spontaneous emission images; Right: flame luminosity images.
to the interactions of these two recirculation regions via boundary-layer separation, supporting the jet-wake stabilized combustion. It is also observed that, however, the cavity assisted jetwake stabilized combustion experiences strong oscillations
and is not very robust. During a test lasting hundreds of milliseconds, the combustion may be blown off for several times. If the spark plug is on during the entire fueling duration, the jet can be reignited several milliseconds after the blowoff. A representative process of the blowoff is shown in Fig. 4. In
Fig. 3 e Ignition and transition to cavity assisted jet-wake stabilized combustion.
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Fig. 4 e Representative flame luminosity images during the blowoff.
contrast to its establishment, the blowoff of the cavity assisted jet-wake stabilized combustion is a much shorter process, where the combustion is directly blown off without an inbetween state of cavity stabilized combustion. During a single test, the ignition/transition/blowoff cycle occurs several times when the fuel flow rate has already achieved a very stable state. Therefore, the fuel flow ramp up time does not have significant effects on the ignition and transition to jet-wake mode. It has been tested that the ignition cannot occur without the assistance of the spark plug. Thus, the ignition mechanism is inferred as below. The premixed or partially premixed mixture within the cavity is heated by the spark plug when passing by. The ignition location is determined by the competition between the ignition delay and the residence time of the heated mixture. If the residence time within the cavity is long enough compared to the ignition delay, the ignition occurs in the cavity. Otherwise, the ignition occurs downstream of the cavity. Actually, both cases are observed in our previous experiments under different conditions. Since the flame appearing downstream of the cavity has to enter into the cavity before transition to the jet-wake mode, the ignition location does not have substantial effects on the transition process.
3.2.
Cavity stabilized combustion
In the previous studies, the combustion is identified either as jet-wake stabilized mode or cavity stabilized mode [2,20]. In the present study, however, it is observed that there exist two distinct combustion modes for the cavity stabilized combustion: cavity shear-layer stabilized combustion and combined cavity shear-layer/recirculation stabilized combustion. First, let us still take the cavities S30L7A45P1.8 and S10L7A45P1.8 for example to show these two combustion modes. As shown in Fig. 2, the combustion is stabilized in the cavity shear layer for cavity S30L7A45P1.8 but stabilized in both the cavity shear layer and recirculation within the cavity
for S10L7A45P1.8. OH-PLIF images shown in Fig. 5 give additional detail of these two combustion modes, where the images are displayed with a false color scale ranging from 100 to 800 counts but normalized from 0 to 1. For the cavity shearlayer stabilized combustion, the combustion is always anchored at the cavity leading edge, and the reaction zones only appear in the cavity shear layer and around the fuel jet in the main stream. There rarely exists intensive reaction in the cavity recirculation region since the concentration of OH radical is much lower in the cavity recirculation compared to that in the cavity shear layer. Thus, the heat is mainly released in the regions out of the cavity. For the combined cavity shearlayer/recirculation stabilized combustion, however, there seems to be a strong reaction in both the cavity shear layer and the cavity recirculation region since the concentration of OH radical is equivalently high in these regions. Accordingly, a large portion of the heat is released in and around the cavity. The horizontal and vertical plots in Fig. 6 are the OH-signal distributions along the streamwise and transverse directions, respectively. First, the four instantaneous images shown in Fig. 5 are averaged for each combustion mode. Then the total signal is obtained by integrating the normalized signal along the streamwise and transverse directions, as processed by Jeong et al. [13]. The mean signal is the line-averaged signal of the pixels where the normalized signal is positive. These plots allow clearer comparisons of the combustion regions where OH generation occurs in the flow between the two combustion modes. For the combined cavity shear-layer/recirculation stabilized combustion, the OH signal intensity is much higher in the cavity especially in the font portion of the cavity. Moreover, two interesting phenomena are observed from the statistic results. One is the presence of OH peaks in the streamwise direction, similar to those detected by O’Byrne et al. [12] and Jeong et al. [13]. This intermittent occurrence of high OH signal may be caused by the excitation of acoustic modes within the cavity [12,13]. The other is that there seem to be two major combustion layers in the vertical plots for
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Fig. 5 e Representative OH-PLIF images in centerplane for cavity stabilized combustion. Left: cavity shear-layer stabilized combustion (S30L7A45P1.8); Right: combined cavity shear-layer/recirculation stabilized combustion (S10L7A45P1.8).
Fig. 6 e OH-signal distributions along the streamwise and transverse directions.
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Fig. 7 e Schlieren for cavity stabilized combustion. Left: cavity shear-layer stabilized combustion (S30L7A45P1.8); Right: combined cavity shear-layer/recirculation stabilized combustion (S10L7A45P1.8).
cavity S30L7A45P1.8. One appears around the cavity shear layer and the other around the upper boundary of the jet. The first one may result from the shear-layer stabilized flame and the second one should be caused by the auto-ignition process associated with the formation of the large-scale, hairpin-like structures generated in the windward portion of the jet boundary. The present conditions are similar to that adopted by Micka [19], where the combustion is found to be an autoignition assisted flame. For s ¼ 30 mm, the probability of auto-ignition in the regions above the cavity should be higher due to a longer ignition delay distance, resulting in thus stronger reaction around the upper boundary of the fuel jet. Compared with the cavity shear-layer stabilized combustion, the combined cavity shear-layer/recirculation stabilized combustion is more robust and induces heavier separation of the upper wall boundary layer as shown in Fig. 7. Also can be seen is that the incoming flow is well decelerated by the intense heat release around the cavity by the combined cavity shear-layer/recirculation stabilized combustion, leading to vanishing of the shock wave originating from the cavity leading edge. Effects of cavity configurations and equivalence ratios on the combustion mode are also addressed. Figs. 8e10 show the images of flame luminosity, OH spontaneous emission and schlieren for cavity S10L7 with different aft angles and equivalence ratios. For low equivalence ratio and small cavity aft angle, the combustion is always stabilized in the cavity shear layer and starts some distance downstream of the cavity leading edge, which means that the combustion is dominated by the cavity shear-layer stabilized mode. With increasing equivalence ratio or increasing cavity aft angle, the combustion zone may appear in both the cavity shear layer and cavity recirculation region, indicating a transition to the combined cavity shear-layer/recirculation stabilized combustion; meanwhile, the fuel jet in the main stream is ignited earlier and the intense heat release is pushed upstream, leading to heavier separation of the upper wall boundary layer. With increasing equivalence ratio, i.e. increasing injection pressure, both the horseshoe vortex formed around the jet exit and the counter-rotating vortices around the jet become stronger, which tends to promote the fuel transport into the cavity shear layer as well as into the cavity recirculation region via boundary-layer separation at the cavity leading edge and vortex interactions of the jet-with-cavity shear layer, consistent with that observed by Thakur and
Segal [22]. The increase of cavity aft angle may lead to two important characteristics of the flowfield. First, the cavity with large aft angle induces stronger oscillations in the flowfield around it [23], which promotes the jet instabilities and results in a larger jet plume at the same axial positions. Second, a compression wave forms around the cavity leading edge when the cavity aft angle is large enough while expansion waves are generated for cavities with small aft angle [23]. This indicates that the cavity shear layer inclines toward the main stream for large aft angle but dives into the cavity volume for small aft angle. Consequently, the increase of cavity aft angle tends to shorten the distance between the jet plume and the cavity shear layer and thus enhance the interactions between the jet and cavity shear layer, as a result of which more fuels can be transported into the cavity shear layer as well as into the cavity recirculation region. Hence, it is inferred that the fuel transport process may have significant influences on the subsequent combustion mode; further analyses will be given below. It is also observed that, with high injection pressure or large cavity aft angle, the flame base may flash back into the jet wake upstream of the cavity leading edge intermittently; however, a larger portion of the reaction still occurs in the cavity mode, which is quite different from that of the short cavity S10L4A45P1.8 where the entire reaction zone is stabilized in the jet wake. The possible reason is that the longer cavities have a better developed shear layer which is beneficial to the fuel transport into the cavity shear layer and cavity recirculation region, supporting the cavity stabilized combustion.
3.3.
Discussion
Based on the abundant experimental observations, it is evident that the combustion becomes more and more stable and robust, accompanied by the transition of combustion mode from the cavity assisted jet-wake stabilized mode to the cavity shear-layer stabilized mode and to the combined cavity shear-layer/recirculation stabilized mode. Also clearly observed is that the combustion tends to shift to more robust mode with increasing cavity length or equivalence ratio, or decreasing distance between the fuel injection and the cavity. Obviously, the combustion can be stabilized in the cavity mode only when the fluid within the cavity shear layer or recirculation region is of proper equivalence ratio. The study
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Fig. 8 e Representative flame luminosity images for cavities with different aft angles and equivalence ratios.
of Thakur and Segal [22] showed that the local equivalence ratio within the recirculation region might be even lower than the global one for light injectant (helium or hydrogen) injected upstream in the non-reacting cases. In their combustion experiments, a stable flame could not be established at the conditions adopted for upstream hydrogen injection, which again indicated insufficient fuel entrained into the recirculation region as pointed out by the authors. Considering the larger injection hole used in the present study compared to those adopted by Thakur and Segal [22], it may be inferred that the cavity is fuel-lean with upstream injection only under the present conditions. Thus, any means that promotes the fuel transport into the cavity shear layer and cavity recirculation region can support the transition of combustion to the cavity stabilized modes. In order to verify this guess, additional tests are carried out. Fig. 11 shows the reaction zone of cavity L7A45 with direct cavity injection. Hydrogen is injected through 3, 0.5 mm diameter spanwise ports in the cavity rear wall. The injection pressure is Pjet ¼ 1.8 MPa, which results in an overall equivalence of 0.021. It is found that the combustion is stabilized in the combined cavity shear-layer/recirculation mode. Fig. 12
shows the reaction zone of cavity L7A45 with both upstream and cavity injections. Hydrogen is injected through both an injector with orifice diameter 2 mm installed 10 mm upstream of the cavity leading edge and 3, 0.5 mm diameter spanwise ports in the cavity rear wall. The injection pressure is Pjet ¼ 1.2 MPa, which results in an overall equivalence of 0.09. It is also found that the combustion is stabilized in the combined cavity shear-layer/recirculation mode. It is notable that the combustion is basically stabilized in the cavity shear-layer mode when the direct injection is turned off, as shown in Figs. 8 and 9 for upstream injection of Pjet ¼ 1.2 MPa. Notably, in the study of O’Byrne et al. [12] where direct cavity fueling was also used, it is found that combustion occurred in the shear layer above the cavity rather than in the cavity recirculation region. The possible reason is that the high equivalence ratios adopted led to too fuel-rich regions in the cavity as pointed out by the authors, inhibiting the reaction within the cavity recirculation. Moreover, the high Mach number also tended to suppress the growth of the cavity shear layer, prohibiting the transport of fresh air into the cavity. Thus, the cavity recirculation combustion was difficult to obtain.
Fig. 9 e Representative OH spontaneous emission images for cavities with different aft angles and equivalence ratios.
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Fig. 10 e Representative schlieren images for cavities with different aft angles and equivalence ratios.
Fig. 11 e Cavity L7A45 with cavity injection.
Fig. 12 e Cavity L7A45 with both upstream and cavity injections.
In order to further clarify the combustion modes, surface pressure distributions along the bottom wall and temperature in the cavity recirculation region are also measured for cavity L7A45 under some typical conditions, as shown in Figs. 13 and 14. Each experiment is executed three times for repeatability; the uncertainties are estimated to be 3e5% for the pressure measurements and 6e9% for the temperature measurements. One can see that the cases in the combined cavity shear-layer/ recirculation combustion mode always induce higher pressure rise around the cavity, which is believed to be beneficial to the combustion stabilization due to the close coupling of
flow and combustion [3]. In the far downstream regions, the pressure rise of s ¼ 30 mm, Pjet ¼ 1.8 MPa which operates in the cavity shear-layer combustion mode tends to exceed that of s ¼ 10 mm, Pjet ¼ 1.8 MPa which operates in the combined cavity shear-layer/recirculation combustion mode. This indicates that intense heat release would take place in further downstream regions if the combustion is stabilized in the cavity shear-layer mode. That is, the required combustor length would be shorter if the combustion is stabilized in the combined cavity shear-layer/recirculation mode. With respect to the temperature in the cavity recirculation region, it is
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Fig. 13 e Surface pressure distributions along the bottom wall.
found that the recirculation temperature for the cases in the combined cavity shear-layer/recirculation combustion mode is always hundreds of Kelvin higher than that for the cases in the cavity shear-layer combustion mode. Moreover, the temperature distribution is far from uniform in the streamwise direction. Basically, the temperature in the aft region is hundreds of Kelvin higher than that in the front region. The possible reason is that the hot products or reacting mixture are mainly entrained into the cavity near the aft wall via the interactions between the cavity shear layer and the aft wall, and then transported to the front region by the recirculation. Thus, the temperature tends to be higher in the aft region. Furthermore, the region near the front corner is a kind of “dead zone” with relatively weak mass and energy exchange
Fig. 14 e Temperature in the cavity recirculation region.
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with other regions in the recirculation so that the heat loss to the solid walls cannot be compensated in time, which also tends to result in lower temperature in this region. However, the combined cavity shear-layer/recirculation combustion mode can well elevate the temperature in the whole recirculation region to enhance the robustness of the combustion stabilization. Fig. 15 shows the schematic of the reaction zone location and plausible combustion stabilization mechanisms for the three combustion modes as inferred from the optical observations. Fig. 15(a) depicts the reaction zone structure and stabilization mechanism for the cavity assisted jet-wake combustion. Though no combustion takes place in the cavity shear layer or in the cavity recirculation region, it is believed that the cavity has positive effects on combustion stabilized in the jet wake since pure jet-wake combustion cannot be obtained without the downstream coupled cavity. It is inferred that the vortex interactions of the jet-with-cavity shear layer tend to transport some hot products from the reactive jet into the cavity shear layer and cavity recirculation region, maintaining the hot environment within the cavity. Compared to the cavity stabilized combustion, the jet-wake combustion appears to ignite the fuel jet earlier and release more heat around the cavity leading edge, leading to heavier separation of the incoming boundary layer approaching the cavity leading edge. Therefore, the hot products in the cavity recirculation region can be transported into the jet wake readily, promoting the ignition and combustion in the jet wake. However, only a small portion of the hot products can go
Fig. 15 e Schematic of combustion stabilization mechanisms for different combustion modes as inferred from the optical observations.
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through the cavity shear layer and enter into the cavity recirculation region, so this stabilization mode is not very stable and is susceptible to flow disturbances. The intermittent blowoff observed in the test supports these analyses. Fig. 15(b) depicts the reaction zone structure and stabilization mechanism for the cavity shear-layer combustion. The reaction zone is always stabilized in the cavity shear layer, some distance downstream of the cavity leading edge. The extension of the reaction zone appears to be significantly influenced by the growth of the cavity shear layer. The hot products can be transported into the cavity recirculation region directly by the vortex interactions of the cavity recirculation-with-cavity shear layer and interactions between the cavity shear and the cavity aft wall. Therefore, more hot products may be transported into the cavity recirculation and the combustion becomes more robust compared to the cavity assisted jet-wake combustion. Notably, the reaction zone is narrow in the transverse direction for quite a long distance because the fuel jet is not completely ignited very soon. Fig. 15(c) depicts the reaction zone structure and stabilization mechanism for the combined cavity shear-layer/ recirculation combustion. The reaction zone appears in both the cavity shear layer and cavity recirculation. It is inferred that more fuel is transported into the cavity shear layer, thus the fuel is not totally consumed by the shear-layer combustion and a portion of the fuel is then entrained into the cavity recirculation region and continues to react there. The reaction zone moves with the cavity recirculating flow and usually combines with the cavity shear-layer combustion, coalescing into a large reaction zone. Since the residence time in the cavity recirculation region is much larger than that in the cavity shear layer, the combustion stabilized in the combined cavity shear-layer/recirculation mode is more robust than that only stabilized in the cavity shear-layer mode. Furthermore, the combined cavity shear-layer/recirculation combustion may release more heat around the cavity and enhance the near-field combustion, leading to earlier ignition of the entire fuel jet. A schematic of velocity profiles in the jet wake and cavity shear layer displayed in Fig. 16 can also help to understand the stabilization characteristics. When the fuel jet is transversely injected into the crossflow, a jet wake is formed. There exists a recirculation in the near-field region of the jet wake, where the streamwise velocity around the centerplane is negative (uc < 0). With increasing downstream distance, the wake
spreads and decays soon with uNeuc decreasing toward zero. The flame base could be possibly stabilized only in a limited region where the flow velocity and flame speed satisfy uc sf. As a result, the flame base will be readily blown off when it is perturbed downstream by a large flow disturbance since uc increases soon with increasing downstream distance while sf basically keeps constant. Thus, the flame stabilized in the jet wake is not very robust. On the other hand, since the flow around the cavity shear layer is basically self-similar, lowspeed regions are always available for the flame base to be stabilized with increasing downstream distance. Once a disturbance destroys the velocity balance, the flame base can adjust its position to achieve a new balance readily. Hence, the cavity-stabilized combustion modes are much more robust compared to the jet-wake stabilized combustion mode.
4.
Conclusions
Combustion experiments were conducted to investigate the characteristics of cavity assisted hydrogen jet combustion in a supersonic flow with a total pressure of 1.6 MPa, a total temperature of 1486 K, and a Mach number of 2.52, simulating flight Mach 6 conditions. Optical diagnosis-based measurements, including OH-PLIF, OH spontaneous emission, highspeed framing of flame luminosity and schlieren were introduced to characterize the combustion flow. Pressure and temperature measurements were also carried out for some typical cases to supply more quantitative information. A supersonic combustor with a constant cross-sectional area was employed with several cavity configurations, fueling schemes and equivalence ratios. It was found that stable combustion could not be obtained without a cavity, indicating that pure jet-wake stabilized combustion cannot be achieved and the cavity acts as a flameholder. Three combustion modes were observed: cavity assisted jet-wake stabilized combustion, cavity shear-layer stabilized combustion, and combined cavity shear-layer/ recirculation stabilized combustion. The cavity assisted jetwake stabilized combustion was observed to be the most unstable mode, accompanied by intermittent blowoff under the present conditions, while the combined cavity shearlayer/recirculation stabilized combustion mode seemed to be the most robust one. It was also observed that the combustion tended to shift to more robust mode with increasing cavity length or equivalence ratio, or decreasing distance between the fuel injection and the cavity. The fluid within the cavity was usually fuellean mixture under the present conditions with upstream transverse injection. Thus, any means that promoted the fuel transport into the cavity shear layer and cavity recirculation region could support the transition of combustion to the more robust cavity stabilized modes.
Acknowledgments Fig. 16 e Schematic of velocity profiles in the jet wake (left) and cavity shear layer (right).
This work is supported by the National Natural Science Foundation of China under Grant Nos. 50906098 and 91016028,
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and Fok Ying Tung Education Foundation under Grant No. 131055. The authors would like to thank Dr. Zhouqin Fan for the help in conducting the experiments.
Nomenclature
A D L Ma P s t T Y
cavity aft angle, degree depth of cavity, mm length of cavity, mm Mach number pressure, Pa upstream injection distance, mm time, ms temperature, K mass fraction of species
Subscript 0 stagnation parameters i index of species jet fuel jet parameters
references
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