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Combustion characteristics of a dual-mode scramjet combustor with cavity flameholder
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Combustion Institute
Proceedings of the Combustion Institute 32 (2009) 2397–2404
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Combustion characteristics of a dual-mode scramjet combustor with cavity flameholder Daniel J. Micka *, James F. Driscoll Department of Aerospace Engineering, University of Michigan, 1320 Beal Avenue, Ann Arbor, MI 48109, USA
Abstract Combustion characteristics of a laboratory dual-mode ramjet/scramjet combustor were studied experimentally. The combustor consists of a sonic fuel jet injected into a supersonic crossflow upstream of a wall cavity pilot flame. These fundamental components are contained in many dual-mode combustor designs. Experiments were performed with an isolator entrance Mach number of 2.2. Air stagnation temperatures were varied from 1040 to 1490 K, which correspond to flight Mach numbers of 4.3–5.4. Both pure hydrogen and a mixture of hydrogen and ethylene fuels were used. High speed imaging of the flame luminosity was performed along with measurements of the isolator and combustor wall pressures. For ramjet mode operation, two distinct combustion stabilization locations were found for fuel injection a sufficient distance upstream of the cavity. At low T 0 , the combustion was anchored at the leading edge of the cavity by heat release in the cavity shear layer. At high T 0 , the combustion was stabilized a short distance downstream of the fuel injection jet in the jet-wake. For an intermediate range of T 0 , the reaction zone oscillated between the jet-wake and cavity stabilization locations. Wall pressure measurements showed that cavity stabilized combustion was the steadiest, followed by jet-wake stabilized, and the oscillatory case. For fuel injection close to the cavity, a hybrid stabilization mode was found in which the reaction zone locations for the two stabilization modes overlapped. For this hybrid stabilization, cavity fueling rate was an important factor in the steadiness of the flow field. Scramjet mode combustion was found to only exist in the cavity stabilized location for the conditions studied. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Supersonic combustion; Dual-mode combustor; Scramjet; Flame stabilization; Dynamics
1. Introduction Dual-mode scramjet engines have the potential to provide airbreathing propulsion at high flight Mach numbers (M flight ). These engines generally consist of a constant area isolator followed by a combustor with a diverging area. The aerothermodynamics of dual-mode engines are quite com-
*
Corresponding author. Fax: +1 734 763 0578. E-mail address: [email protected] (D.J. Micka).
plex and lead to significantly different flow conditions in the combustor for ramjet and scramjet mode operation [1–3]. These engines produce no thrust at rest and must be accelerated to a moderate M flight of 3–4 before ignition. At these moderate flight Mach numbers, the engine operates in the thermally choked, ramjet mode. A pre-combustion shock train in the isolator slows the flow to subsonic speeds before the fuel is burned. A thermal throat in the diverging section of the combustor allows the exhaust to be reaccelerated to supersonic speeds. As the engine
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.06.192
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accelerates to an M flight of 6–7, the pre-combustion shock train becomes weaker until the isolator exit Mach (M i;exit ) number is supersonic and scramjet mode operation is achieved. Eventually the precombustion shock train is swallowed completely. Combustion stabilization is a significant challenge in dual-mode combustors due to the high velocities involved. After ignition at moderate M flight , the incoming air temperature is insufficient to provide consistent auto-ignition and the combustion must be stabilized as a flame. In this flame regime, there must exist a location of favorable equivalence ratio, temperature, pressure, and velocity where the reaction base can stabilize and serve as a source of heat and radicals to the remaining fuel–air mixture [4]. Conversely, at very high M flight the incoming air temperature is sufficiently large that the auto-ignition delay time becomes negligible. In this auto-ignition regime, the combustion stabilization problem reduces to a mixing problem. At intermediate M flight , both auto-ignition and flame properties are expected to be important. In a practical dual-mode combustor, the combustion must be stabilized over a wide range of conditions including ram-to-scram mode transition. Through these different regimes, the combustion stabilization location and mechanism may change. Wall fuel jet injection with a cavity flameholder is a desirable configuration for a dual-mode scramjet combustor due to the low pressure losses and cooling requirements [4–6]. The cavity recirculation zone provides a long residence time for the fuel and air to mix and burn. The cavity flame provides a source of heat and radicals to ignite and stabilize combustion in the main flow. The main fuel injection may be normal to the air flow to achieve maximum penetration or it may be angled to recover some of the jet momentum. Previous studies have focused on combustors with such features [4,5,7–15], but the combustion stabilization mechanism and the role of the cavity is still not fully understood. The current study focuses on combustion stabilization for conditions where auto-ignition is not expected to be dominant. The laboratory combustor studied employs basic flow elements that have been proposed for practical dual-mode combustors such as sonic wall fuel injection and a wall cavity flameholder. Therefore it is expected to exhibit combustion stabilization and ignition properties which are applicable to this type of combustor in general. Ramjet mode combustion was studied for air stagnation temperatures (T 0 ) of 1040–1490 K. These values correspond to M flight of 4.3–5.4. One case of scramjet mode combustion also is reported. Gaseous hydrogen fuel was used for most tests due to its fast kinetics. This allowed flame stabilization mechanisms to be explored that would not be encountered until higher temperatures for hydrocarbon fuels. Results for a blend of 50% hydrogen
and 50% ethylene by volume were compared to results for pure hydrogen. 2. Experimental setup Experiments were performed in the supersonic combustion facility at the University of Michigan. A drawing of the test section is shown in Fig. 1. A two dimensional Mach 2.2 nozzle is followed by a constant area isolator with a cross section of 25.4 mm by 38.1 mm. This constant area section extends 402 mm up to the leading edge of a rectangular cavity which is 50.8 mm long, 12.7 mm deep, and spans the width of the test section. At the rear edge of the cavity begins a 349 mm long 4° diverging section which dumps into a 152 mm diameter exhaust. Room temperature gaseous fuel was injected sonically through a single port at either 44.5 or 14.0 mm upstream of the cavity leading edge on the combustor centerline. A 2.18 or 2.49 mm diameter injection port could be used at either location depending on the desired fuel flow rate. The ports were never used simultaneously. Pilot fuel was directly injected into the cavity through three spanwise 1.19 mm diameter ports in the cavity floor or rear wall. The floor ports were located 6.4 mm downstream of the cavity leading edge while the rear wall ports were located 3.8 mm above the cavity floor. Previous studies reported different flame structures and stability properties when the cavity was fueled at different locations [6,16,17]. A spark plug in the cavity floor was used to ignite the cavity pilot flame and was the only ignition aid used. The range of flow conditions explored is given in Table 1. An electric heater and hydrogen fueled vitiator were used to achieve the air stagnation temperatures (T 0 ) of 1040 1490 K. Make-up oxygen was added to maintain a 0.21 O2 mole fraction in the vitiator products. The run times were kept under 10 s in order to prevent thermal damage to the uncooled combustor. All the flow control and data acquisition equipment controlled by a Labview program. For each run, the air was first heated to 450 K by the electric heater. The vitiator oxygen flow was then started, followed 2 s later by the spark ignited vitiator fuel. Four seconds after vitiator ignition, the cavity fuel (if any) and cavity spark were turned on. One second later, the main fuel injection began. The main fuel flow was maintained for 2 s before all flow streams except the main air were terminated. A few runs were completed with up to 4 s of main fuel in order to acquire additional data. No differences were observed for the cases with longer run times. The air stagnation temperature was measured by a K-type thermocouple in the settling chamber just upstream of the nozzle. The temperature generally increased by approximately 50 K during the
D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404 constant area isolator 358mm
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combustor 44.5mm
nozzle
50.8mm
14.0mm M=2.2
P0,i = 590kPa T0 =1040 - 1490K
normal shock train (when ram-mode)
reaction zone for upstream fuel injection jet-wake stabilized mode H = 25.4mm
y x
reaction zone for upstream fuel injection cavity stabilized mode 12.7mm
downstream upstream main fuel main fuel injection location injection location
4o cavity rear wall fuel injectors
cavity floor fuel injectors
Fig. 1. Drawing of test section with flow features. Isolator and nozzle not to scale.
Table 1 Test conditions Parameter
Test conditions
P 0;i T0 Fuel type
59015 kPa 1040–1490 K H2 , or 50–50 H2 /C2 H4 by vol. 0.18–0.42 0–0.12 1.75, 0.55 Rear wall, Floor, Both
/ m_ cav fuel =m_ total fuel ðx=H Þmain fuel Cavity fuel injection
2 s the main fuel was on during each run. The average temperature measured during this time is reported as the nominal value of T 0 . The initial air stagnation pressure (P 0;i ) was also measured in the settling chamber. The wall static pressure (P w ) was recorded at 40 Hz at eight locations in the combustor and isolator. The combustion region was imaged through fused silica windows that were 305 mm long. A Vision Research Phantom 9.0 camera was used to record high speed movies of the flame luminosity. Images of 768 by 240 pixels were acquired at 4000 Hz for 2 s after the main fuel flow was initiated. The field of view was approximately 150 mm in length and spanned the height of the test section and cavity. For a few cases a wall pressure signal was recorded at high frequency and synchronized with the high speed camera. For this purpose a transducer was mounted on the sidewall in the isolator 140 mm upstream of the cavity leading edge. The pressure was recorded at 50 kHz for 2 s and a 1 ms moving average was computed from the signal.
engine. Results for hydrogen fuel with / ¼ 0:21 0:27 and m_ cav fuel =m_ total fuel ¼ 0:0 0:12 at T 0 ¼ 1040 1410 K are presented. Higher values of / led to the shock train moving into the nozzle at high temperatures. The isolator exit Mach numbers were computed to vary between 0.68 and 0.82 from values of P w =P 0;i at x=H ¼ 3:25 using the 1-D method given by Curran et al [2]. Results for the blended fuel are presented for / ¼ 0:42 at T 0 ¼ 1250 1490 K. This higher equivalence ratio was necessary for the blended fuel in order to thermally choke the flow and cause ramjet mode operation. For these cases M i;exit was computed to be between 0.66 and 0.85. The higher temperature cases led to lower isolator exit Mach numbers due to the changing location of the heat release as discussed in Section 3.1.1. 3.1.1. Hydrogen fuel – combustion stabilization modes Two distinct combustion stabilization modes were found for upstream main fuel injection. The average combustion luminosity for these two modes can be seen in Fig. 2. In the cavity stabilized mode, the reaction zone was anchored at the leading edge of the cavity and spread into the main flow at an approximately constant angle. In the jet-wake stabilized mode, the reaction zone was stabilized well upstream of the cavity in the
(a) cavity stabilized combustion main fuel (b) jet-wake stabilized combustion
3. Results and discussion 3.1. Ramjet operation – upstream fuel injection Ramjet mode operation (subsonic M i;exit ) was studied with upstream main fuel injection (ðx=H Þmain fuel ¼ 1:75) for both pure hydrogen fuel and the hydrogen–ethylene blend. For all cases presented, the shock train was contained in the isolator as is desirable in an operational
main fuel
Fig. 2. Average combustion luminosity images for upstream injection of hydrogen fuel. T 0 ¼ 1250 K, / ¼ 0:23, no cavity fueling. Yellow line is isoluminosity contour. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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wake of the fuel injection jet and had a curved leading edge. Cases were found of steady cavity stabilized combustion, steady jet-wake stabilized combustion, and oscillation between the two modes. The bimodal nature of the combustion is illustrated by Fig. 3. For this figure, 22,500 total images were analyzed for 5 runs with hydrogen fuel at / ¼ 0:21, m_ cav fuel =m_ total fuel ¼ 0:02 (rear wall injection), and T 0 ¼ 1130 1400 K. An isoluminosity contour characteristic of the reaction zone leading edge was defined. For each image, the average axial location of this contour was calculated for y=H ¼ 0:1 0:4 above the cavity. From Fig. 3 it can be seen that there are two distinct regions where the combustion can be stabilized. The upstream peak represents the jet-wake stabilized location and the downstream peak represents the cavity stabilized location. The fraction of time the combustion spent in each stabilization mode during a run was determined from the high speed movies. The definition of reaction zone leading edge in Fig. 4 was used and a critical axial location which separated the
number of images
jet-wake stabilized mode
cavity stabilized mode
600 400 200 0
—1.5
—1 —0.5 0 0.5 1 1.5 reaction zone leading edge (x/H)
2
2.5
Fig. 3. Histogram of reaction zone leading edge location for five cases with 22,500 images total. Upstream injection of hydrogen with T 0 ¼ 1130 1400 K, / ¼ 0:21, m_ cav fuel =m_ total fuel ¼ 0:02.
jet-wake stabilized mode
f = fraction of time combustion is in
1.0
0.8
Cavity stabilized regime
Intermediate, oscillatory regime
Jet-wake stabilized regime
0.6
0.4
φ = 0.21, mcav fuel / mtotal fuel = 0.02 φ = 0.21, mcav fuel / mtotal fuel = 0.12
0.2
0 1000
φ = 0.26, mcav fuel / mtotal fuel = 0.0
1100
1200 1300 T0 (K)
1400
1500
Fig. 4. Combustion stabilization mode vs. T 0 for hydrogen fuel. Upstream main fuel injection, cavity rear wall fueling.
two modes of ðx=H Þcrit ¼ 0:6 was set. Each image where the reaction zone leading edge was upstream of this value was considered to be in the jet-wake stabilized mode. The fraction of time in each mode was not very sensitive to the selection of ðx=H Þcrit due to the small percent of time spent in transition. The results for runs with three combinations of / and m_ cav fuel =m_ total fuel (all rear wall cavity fueling) for T 0 ¼ 1040 1410 K are plotted in Fig. 4. It can be seen that T 0 is the dominant variable in determining the combustion stabilization mode. At high T 0 ( J 1350 K), the combustion was virtually always stabilized in the jet-wake mode. For low T 0 ( K 1150 K), the combustion was virtually never stabilized in the jetwake mode, i.e. it was always stabilized in the cavity mode. There was a range of intermediate T 0 for which the combustion oscillated between the two stabilization modes. The fraction of time in the jet-wake stabilized mode (f) may be approximated by Eq. 1 for all conditions. Eq. 1 is plotted as a dashed line in Fig. 4. 1 1 T 0 1250 K ð1Þ f ¼ þ jT 0 1250 Kjerf 2 2 75 K For cavity stabilized combustion, the reaction zone spread into the flow at a relatively constant angle between 26 and 30°. The larger angles were associated with higher T 0 . Very similar reaction zones were found in previous studies of angled ethylene injection upstream of a wall cavity by Mathur et al. [5] and Lin et al. [15]. In all these cases, the constant spreading angle indicates that the reaction is likely occurring as a premixed flame. The flame is anchored in a low speed region in the cavity shear layer and spreads into the flow at an angle which matches the local flame speed to the normal flow velocity. The fact that the spreading angle increases with temperature is consistent with this explanation. This constant spreading angle would not be expected if auto-ignition played a prominent role in the main flow reaction because the flow is shock free (subsonic) in this region. For jet-wake stabilized combustion, the leading edge of the reaction zone (as defined earlier from the luminosity) was located on average 20– 30 mm downstream of the fuel injection location for T 0 ¼ 1410 1230 K. The higher temperatures were associated with the shorter distance. In a previous study with ethylene fuel, Mathur et al. [5] found that the combustion sometimes moved upstream of the cavity, but only along the sidewall in the separated boundary layer. In the current study, images from the top of the test section, such as Fig. 5, showed that the jet-wake stabilized combustion was indeed occurring in the wake of the fuel jet and not along the side walls. Studies with very high temperature air [18] or fuel [7] have found combustion in the fuel jet-wake, but this has been attributed to auto-ignition. As be will
D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404
Fig. 5. Top view, jet-wake stabilized, average combustion luminosity. Upstream injection of hydrogen fuel with T 0 ¼ 1370 K, / ¼ 0:27, m_ cav fuel =m_ total fuel ¼ 0:07.
be discussed in the Section 3.1.2, the dynamic behavior of the jet-wake stabilized combustion indicates that it is likely a flame and not autoignition. Figure 7 shows the average pressure in the test section for the two flame stabilization modes and the oscillatory case. The jet-wake stabilized case has a shock train which extends further upstream and a slightly higher peak pressure than the cavity stabilized case. This is caused by the heat release occurring further upstream for the case of jetwake stabilized combustion. 3.1.2. Hydrogen fuel – dynamics For intermediate temperatures (1150 K 6 T 0 6 1350 K), the reaction zone was observed to oscillate between the jet-wake and cavity stabilized positions. This is a fundamentally different type of instability than the thermoacoustic and fluid dynamic instabilities that have been T0 = 1130K, cavity stabilized T0 = 1270K, oscillating modes T0 = 1400K, jet-wake stabilized
0.075
0.6 0.5 0.4
0.045
0.3
0.03
0.2
0.015
0.1
σPw/P0,i
0.06
0 —16
—12
—8
—4
0
P w /P 0,i
0.09
0 4
x / H (from cavity leading egde)
reaction zone leading edge (x/H)
Fig. 6. Wall pressure standard deviation (solid symbols) and time average (open symbols). Upstream hydrogen fuel injection with different stabilization modes. / ¼ 0:21, m_ cav fuel =m_ total fuel ¼ 0:02. −2
fuel inj. jet-wake stabilized cavity stabilized
0 2
P w /P 0,i
0.55
0.45
0.35 500
1000
1500
2000
2500
time from start of main fuel (ms)
Fig. 7. Synchronized reaction zone leading edge location and wall pressure data. Upstream hydrogen fuel / ¼ 0:27, injection with T 0 ¼ 1220 K, m_ cav fuel =m_ total fuel ¼ 0:05.
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previously studied in similar combustors [13,14]. Figure 7 shows a typical time history of the reaction zone leading edge (as defined in Section 3.1.1) for an oscillatory case. It can be seen that the reaction zone is generally clearly stabilized in either the cavity or jet-wake location, with movement between the two locations happening very quickly. The oscillation between the modes does not occur at a set frequency and is generally in the range of 5–20 Hz. A FFT of the signal did not reveal any dominant frequencies in the range expected for instabilities caused by thermoacoustics or periodic shed vortices (hundreds to thousands of Hz). Figure 7 also shows a high frequency wall pressure signal at x=H ¼ 5:5 that is synchronized with the high speed imaging. From this graph it can be seen that the largest pressure changes are associated with the movement of the reaction zone. When the combustion is in the jet-wake stabilized mode, the pressure in the isolator is higher than when it is in the cavity stabilized mode. The reaction zone movement precedes the pressure change by approximately 8 ms. Thus the large pressure fluctuations are caused by the reaction zone movement and not vice versa. The tendency of the reaction zone to oscillate between two distinct, relatively stable locations indicates that it is likely a flame, and not autoignition, in both the cavity and jet-wake stabilized modes. If the combustion was primarily due to auto-ignition in the jet-wake stabilized mode, we would not expect to see the reaction zone leading edge located only in the small stable region illustrated in Fig. 8. Instead, any fluctuations which pushed the reaction zone downstream of its average location would give the fuel/air mixture more time, and thus make it more likely, to auto-ignite. The observed flame stabilization behavior may be explained in the following way. Cavity fueling does not play a role in the stabilization location because sufficient main fuel is entrained into the cavity for the cavity reaction to stabilize the main flame at all conditions studied. For cases when a jet-wake location is available for stabilization, the flame is often not capable of flashing forward through the intermediate region (between the cavity and jet-wake stabilization locations) which has a higher average velocity and/or a less favorable local equivalence ratio than the jet-wake stabilization location. At low T 0 , the flame remains in the cavity stabilized location until a fluctuation in the jet-wake occurs which is large enough to allow the reaction zone for jet-wake stabilized combustion
Mi,exit
main fuel injection
intermediate region stable region for jet-wake flame base
reaction zone for cavity stabilized combustion
stable region for cavity flame base
Fig. 8. Flame base stabilization locations.
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3.1.3. Blended ethylene–hydrogen fuel For the 50–50 blend of ethylene and hydrogen fuel, both cavity and jet-wake stabilized combustion was observed as shown in Fig. 9. The reaction zone shapes are similar to that found for hydrogen, but the jet-wake stabilized reaction zone is located farther downstream than in the hydrogen fuel case. The reaction zone leading edge varied from approximately 35 to 50 mm downstream of
(a) cavity stabilized combustion main fuel
cav fuel (b) jet-wake stabilized combustion cav fuel
main fuel
Fig. 9. Average combustion luminosity images for upstream injection of blended ethylene–hydrogen fuel. T 0 ¼ 1370 K, / ¼ 0:42, m_ cav fuel =m_ total fuel ¼ 0:05.
the fuel injection location on average for T 0 ¼ 1490 1330 K. This is likely due to the much higher equivalence ratio used for the blended fuel compared with hydrogen. The higher / creates a longer mixing distance between the injection location and the presumably near stoichiometric flame base. The cavity stabilized flame is similar to the hydrogen flame but with a lower spreading angle of 18–22°. The effect of T 0 on the combustion stabilization mode is shown in Fig. 10. As expected, the blended fuel exhibits the same behavior seen for hydrogen fuel, but the transition from cavity to jet-wake combustion occurs at a higher temperature. The error function approximation shown for the blended fuel is simply shifted to the right by 120 K compared with Eq. 1. 3.2. Ramjet operation – downstream fuel injection The fuel injection location at ðx=H Þmain fuel ¼ 0:55 was used to cause the two stabilization locations (jet-wake and cavity shear layer) to overlap. The goal was to minimize the reaction zone oscillations that occurred when the two stabilization locations were separated in space. Ramjet mode combustion from this downstream fuel 1.0 Hydrogen fuel, φ = 0.21-0.26
jet-wake stabilized mode
flame to flash forward to a relatively stable location in the jet-wake. The flame will then remain in the jet-wake stabilized location until another fluctuation makes the position unstable and it flashes back to the cavity stabilized location. As T 0 increases, the flame speed increases. Thus the magnitude of the fluctuations required for the flame to flash forward to the jet-wake location become smaller, and the magnitude of the fluctuations which cause it to flash back become larger. The flame then spends more and more time in the jet-wake stabilized location as T 0 increases. At a high enough T 0 , there are no fluctuations present in the flow which are sufficient to cause the jetwake location to become unstable. The flow fluctuations which cause the flame to flash forward to the jet-wake position or back to the cavity position may be related to fluid dynamic, acoustic, or facility dependent fluctuations (or freestream fluctuations in a flight vehicle). The lack of a dominant frequency and relatively low frequencies at which this oscillation occurs, however, suggest that any periodic shed vortices or acoustic modes do not couple with the reaction zone location oscillations. There is a large difference in the steadiness of the pressure field for the different stabilization modes. Figure 6 shows the standard deviation of the non-dimensional wall pressure (rP w =P 0;i ) from the 40 Hz measurement at eight locations in the isolator and combustor. Pressure fluctuations are a marker of the steadiness of the combustion in a thermally choked flow because any change in heat release rate or distribution will cause a change in the pre-combustion shock train length and pressure rise. The cavity stabilized mode is the steadiest mode because the base of the flame is located in a low speed region in the upstream part of the cavity shear layer. This part of the shear layer is relatively steady because it is fixed by the geometry of the cavity. In the jet-wake stabilized mode, the base of the flame must be located in a low speed region of the jet-wake with a proper local equivalence ratio. The wake behind the fuel jet is expected to be much less steady than the recirculation in the fixed geometry cavity and thus the combustion is less steady. The location of the heat release moves significantly when oscillating between the two stabilization modes, so the pressure is least steady in this case.
f = fraction of time combustion is in
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0.8
Blended fuel, φ = 0.42
0.6
0.4
0.2
0 1000
1100
1200 1300 T0 (K)
1400
1500
Fig. 10. Combustion stabilization mode vs. T 0 for hydrogen and blended fuel with upstream main fuel injection.
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injection location was studied for hydrogen fuel with T 0 ¼1250 K, / ¼ 0.18 and 0.27, and m_ cav fuel =m_ total fuel ¼ 0.0 and 0.10 through floor and rear wall injectors. This temperature is equivalent to the most unsteady oscillatory case observed for upstream main fuel injection. For all conditions the pre-combustion shock train was contained in the isolator. Wall pressures and average luminosity images for the / ¼ 0:27 cases are shown in Figs. 11 and 12. It can be seen that cavity fueling from either location makes the flow significantly more steady. For no cavity fueling, the combustion appears to be in the jet-wake stabilized mode. The shape and location of the reaction zone with respect to the fuel injector is similar to that seen for the upstream injection, jet-wake stabilized case shown in Fig. 2b. Additionally the magnitude of the pressure fluctuations are similar to the jet-wake stabilized case shown in Fig. 6. With cavity fueling, shown in Fig. 12b and c, the reaction zone extends into the upstream part 0.6
0.06
0.5
0.04
0.4
0.03
0.3
0.02
0.2
0.01
0.1
0 −16
−12
−8
−4
0
P w /P 0,i
σ Pw/P0,i
0.05
no cavity fuel rear wall cavity fuel floor cavity fuel
0 4
x / H (from cavity leading egde)
Fig. 11. Wall pressure standard deviation (solid symbols) and average (open symbols) for downstream injection of hydrogen. T 0 ¼ 1250 30 K, / ¼ 0:27, m_ cav fuel =m_ total fuel ¼ 0:0 or 0.10.
(a) no cavity fuel
main fuel
main fuel
main fuel
(b) rear wall cavity fuel cav fuel
(c) floor cavity fuel cav fuel
Fig. 12. Average combustion luminosity images for downstream main fuel injection of hydrogen. T 0 ¼ 1250 30 K, / ¼ 0:27, m_ cav fuel =m_ total fuel ¼ 0:0 or 0.10. Image (b) is blacked out in rear corner of cavity due to contaminant buildup on the window which was glowing brightly.
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of the cavity. As noted before, this is a relatively steady area of low speed flow. Thus the steady combustion in this area provides heat and radicals to the main flow reaction, which appears to be primarily jet-wake stabilized. This hybrid stabilization is steadier than pure jet-wake stabilization, but not quite as steady as pure cavity stabilization. It can be seen that the cavity shear layer reaction is significantly stronger for cavity floor fueling, which is likely why this configuration produced a slightly more steady flow field. Pure cavity stabilized combustion (with a constant spreading angle) was not found for downstream main fuel injection. It is expected that cavity stabilized flames are burning in a premixed fashion. For downstream fuel injection there is not enough distance between the injection location and the cavity leading edge for this premixing to occur. The same basic trends in combustion stabilization and steadiness were seen for / ¼ 0:18. For the lower equivalence ratio though, the difference in pressure fluctuations between cases with and without cavity fueling was less. For this lower equivalence ratio the main fuel jet does not penetrate into the flow as far and so more main fuel may be entrained into the cavity. Therefore the flame base is located farther upstream in the cavity shear layer which plays a role in stabilizing the flame. 3.3. Scramjet operation Scramjet mode operation of the combustor was achieved by lowering the equivalence ratio below that necessary for thermal choking. Ramto-scram mode fluctuations were observed to occur for / < 0:20 with hydrogen fuel and for / < 0:40 with the fuel blend. Steady scramjet mode combustion was not observed because flame blowout occurred if the equivalence ratio was lowered sufficiently. To obtain fully steady scramjet mode combustion in this combustor would require raising T 0 , lowering / (while still igniting), moving the heat release further downstream, or some combination of the these. Figure 13 shows the average pressure in the combustor during ramjet and scramjet mode operation for one case where a low frequency ram-to-scram oscillation of about 5 Hz occurred. M i;exit was computed to be 0.97 and 1.69 from wall pressure at x=H ¼ 3:25 for the ramjet and scramjet mode combustion, respectively. The actual Mach number just upstream of the combustion for both cases was a bit lower due to the fact that Mach number is decreasing in the downstream direction and due to the assumptions of the 1-D method. It can be seen that the pressure in parts of the isolator changes significantly between ramjet and scramjet mode combustion. This is due to the fact that there must be a step change in the isolator exit Mach number to go from thermally choked subsonic to
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no fuel scramjet mode combustion ramjet mode combustion
0.3 0.2 0.1 0 −16
−12
−8
−4
0
4
Fig. 13. Average wall pressure for ramjet mode combustion, scramjet mode combustion, and the no fueling case. Downstream injection of hydrogen fuel at T 0 ¼ 1410 K, / ¼ 0:18, m_ cav fuel =m_ total fuel ¼ 0:09.
supersonic heat addition. This can lead to an instability with large pressure fluctuations when undergoing ram-to-scram transition. The reaction zone was found to appear in only the cavity stabilized location for the scramjet mode conditions studied. This is likely due to the fact that the higher velocity and lower static temperature associated with a supersonic M i;exit (for a fixed T 0 and isolator entrance Mach number) made the jet-wake location unsuitable for flame stabilization. 4. Conclusions A laboratory dual-mode scramjet combustor was studied for a range of air stagnation temperatures (1040–1490 K), two fuel injection locations, and two fuel types. The reaction zone locations, average pressures, and pressure fluctuations are reported. The stagnation temperatures studied correspond to a range of flight Mach numbers (4.3–5.4) where a wall cavity pilot flame is needed to assist in ignition and flame stabilization. Two distinct combustion stabilization modes were found for ramjet operation with fuel injection sufficiently upstream of the cavity: jet-wake stabilized (at large T 0 ) and cavity stabilized (at low T 0 ). Cavity stabilized combustion is anchored at the leading edge of the cavity shear layer and spreads into the main flow at an approximately constant angle. Jet-wake stabilized combustion is located in the wake of the fuel injection jet a short distance downstream. The fuel appears to be burning as a flame rather than auto-ignition in both stabilization modes. There was an intermediate range of T 0 for which the combustion oscillated between the two stabilization modes causing large pressure fluctuations in the isolator. The variation of T 0 in this experiment is analogous to acceleration through a range of Mach numbers in an operational dual-mode engine. Thus an engine of similar
geometry to the one tested is likely to encounter this instability. The impact of this instability may be minimized by placing the main fuel injection with respect to the cavity such that the two flame stabilization locations overlap. For this hybrid stabilization mode there is no large movement of the heat release zone and so the pressure fluctuations are minimized. Cavity fueling was required to realize the full benefits of this hybrid stabilization for the conditions of this study. The proper location of the main fuel jet to achieve this hybrid stabilization mode likely depends on /, fuel type, and the detailed engine geometry. For scramjet mode operation, only the cavity stabilized combustion mode was found. A strong instability was encountered at the ram-to-scram transition due to the step change in isolator exit Mach number required at this shift.
References [1] W.H. Heiser, D.T. Pratt, Hypersonic Airbreathing Propulsion, AIAA Education Series, AIAA, 1994. [2] E.T. Curran, W.H. Heiser, D.T. Pratt, Annu. Rev. Fluid Mech. 28 (1996) 323–360. [3] F.S. Billig, J. Propul. Power 9 (4) (1993) 499–514. [4] A. Ben-Yakar, R.K. Hanson, J. Propul. Power 17 (4) (2001) 869–877. [5] T. Mathur, M. Gruber, K. Jackson, et al., J. Propul. Power 17 (6) (2001) 1305–1312. [6] M.R. Gruber, J.M. Donbar, C.D. Carter, K.-Y. Hsu, J. Propul. Power 20 (5) (2004) 769–778. [7] K.H. Yu, K.J. Wilson, K.C. Schadow, J. Propul. Power 17 (6) (2001) 1287–1295. [8] J.M. Donbar, M.R. Gruber, T.A. Jackson, C.D. Carter, T. Mathur, Proc. Combust. Inst. 28 (2001) 679–687. [9] G. Yu, J. Li, J. Zhao, L. Yue, X. Chang, C.-J. Sung, Proc. Combust. Inst. 30 (2005) 2859–2866. [10] T. Mitani, M. Izumikawa, Proc. Combust. Inst. 28 (2001) 689–695. [11] R.A. Baurle, D.R. Eklund, J. Propul. Power 18 (5) (2002) 990–1002. [12] A.J. Neely, I. Stotz, S. O’Byrne, R.R. Boyce, N.R. Mudford, A.F.P. Houwing, 13th International Space Planes and Hypersonics Systems and Technologies, AIAA 2005-3358, 2005. [13] J.-Y. Choi, F. Ma, V. Yang, Proc. Combust. Inst. 30 (2005) 2851–2858. [14] K.-C. Lin, K. Jackson, R. Behdadnia, et al., Joint Propulsion Conference and Exhibit, AIAA 20075382, 2007. [15] K.-C. Lin, C.-J. Tam, I. Boxx, C. Carter, K. Jackson, M. Lindsey, Joint Propulsion Conference and Exhibit, AIAA 2007-5381, 2007. [16] C.C. Rasmussen, S.K. Dhanuka, J.F. Driscoll, Proc. Combust. Inst. 31 (2007) 2505–2512. [17] J. Driscoll, C.C. Rasmussen, J. Propul. Power 21 (6) (2005) 1035–1044. [18] B.K. McMillin, J.M. Seitzman, R.K. Hanson, AIAA J. 32 (10) (1994) 1945–1952.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 32 (2009) 2397–2404
www.elsevier.com/locate/proci
Combustion characteristics of a dual-mode scramjet combustor with cavity flameholder Daniel J. Micka *, James F. Driscoll Department of Aerospace Engineering, University of Michigan, 1320 Beal Avenue, Ann Arbor, MI 48109, USA
Abstract Combustion characteristics of a laboratory dual-mode ramjet/scramjet combustor were studied experimentally. The combustor consists of a sonic fuel jet injected into a supersonic crossflow upstream of a wall cavity pilot flame. These fundamental components are contained in many dual-mode combustor designs. Experiments were performed with an isolator entrance Mach number of 2.2. Air stagnation temperatures were varied from 1040 to 1490 K, which correspond to flight Mach numbers of 4.3–5.4. Both pure hydrogen and a mixture of hydrogen and ethylene fuels were used. High speed imaging of the flame luminosity was performed along with measurements of the isolator and combustor wall pressures. For ramjet mode operation, two distinct combustion stabilization locations were found for fuel injection a sufficient distance upstream of the cavity. At low T 0 , the combustion was anchored at the leading edge of the cavity by heat release in the cavity shear layer. At high T 0 , the combustion was stabilized a short distance downstream of the fuel injection jet in the jet-wake. For an intermediate range of T 0 , the reaction zone oscillated between the jet-wake and cavity stabilization locations. Wall pressure measurements showed that cavity stabilized combustion was the steadiest, followed by jet-wake stabilized, and the oscillatory case. For fuel injection close to the cavity, a hybrid stabilization mode was found in which the reaction zone locations for the two stabilization modes overlapped. For this hybrid stabilization, cavity fueling rate was an important factor in the steadiness of the flow field. Scramjet mode combustion was found to only exist in the cavity stabilized location for the conditions studied. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Supersonic combustion; Dual-mode combustor; Scramjet; Flame stabilization; Dynamics
1. Introduction Dual-mode scramjet engines have the potential to provide airbreathing propulsion at high flight Mach numbers (M flight ). These engines generally consist of a constant area isolator followed by a combustor with a diverging area. The aerothermodynamics of dual-mode engines are quite com-
*
Corresponding author. Fax: +1 734 763 0578. E-mail address: [email protected] (D.J. Micka).
plex and lead to significantly different flow conditions in the combustor for ramjet and scramjet mode operation [1–3]. These engines produce no thrust at rest and must be accelerated to a moderate M flight of 3–4 before ignition. At these moderate flight Mach numbers, the engine operates in the thermally choked, ramjet mode. A pre-combustion shock train in the isolator slows the flow to subsonic speeds before the fuel is burned. A thermal throat in the diverging section of the combustor allows the exhaust to be reaccelerated to supersonic speeds. As the engine
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.06.192
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accelerates to an M flight of 6–7, the pre-combustion shock train becomes weaker until the isolator exit Mach (M i;exit ) number is supersonic and scramjet mode operation is achieved. Eventually the precombustion shock train is swallowed completely. Combustion stabilization is a significant challenge in dual-mode combustors due to the high velocities involved. After ignition at moderate M flight , the incoming air temperature is insufficient to provide consistent auto-ignition and the combustion must be stabilized as a flame. In this flame regime, there must exist a location of favorable equivalence ratio, temperature, pressure, and velocity where the reaction base can stabilize and serve as a source of heat and radicals to the remaining fuel–air mixture [4]. Conversely, at very high M flight the incoming air temperature is sufficiently large that the auto-ignition delay time becomes negligible. In this auto-ignition regime, the combustion stabilization problem reduces to a mixing problem. At intermediate M flight , both auto-ignition and flame properties are expected to be important. In a practical dual-mode combustor, the combustion must be stabilized over a wide range of conditions including ram-to-scram mode transition. Through these different regimes, the combustion stabilization location and mechanism may change. Wall fuel jet injection with a cavity flameholder is a desirable configuration for a dual-mode scramjet combustor due to the low pressure losses and cooling requirements [4–6]. The cavity recirculation zone provides a long residence time for the fuel and air to mix and burn. The cavity flame provides a source of heat and radicals to ignite and stabilize combustion in the main flow. The main fuel injection may be normal to the air flow to achieve maximum penetration or it may be angled to recover some of the jet momentum. Previous studies have focused on combustors with such features [4,5,7–15], but the combustion stabilization mechanism and the role of the cavity is still not fully understood. The current study focuses on combustion stabilization for conditions where auto-ignition is not expected to be dominant. The laboratory combustor studied employs basic flow elements that have been proposed for practical dual-mode combustors such as sonic wall fuel injection and a wall cavity flameholder. Therefore it is expected to exhibit combustion stabilization and ignition properties which are applicable to this type of combustor in general. Ramjet mode combustion was studied for air stagnation temperatures (T 0 ) of 1040–1490 K. These values correspond to M flight of 4.3–5.4. One case of scramjet mode combustion also is reported. Gaseous hydrogen fuel was used for most tests due to its fast kinetics. This allowed flame stabilization mechanisms to be explored that would not be encountered until higher temperatures for hydrocarbon fuels. Results for a blend of 50% hydrogen
and 50% ethylene by volume were compared to results for pure hydrogen. 2. Experimental setup Experiments were performed in the supersonic combustion facility at the University of Michigan. A drawing of the test section is shown in Fig. 1. A two dimensional Mach 2.2 nozzle is followed by a constant area isolator with a cross section of 25.4 mm by 38.1 mm. This constant area section extends 402 mm up to the leading edge of a rectangular cavity which is 50.8 mm long, 12.7 mm deep, and spans the width of the test section. At the rear edge of the cavity begins a 349 mm long 4° diverging section which dumps into a 152 mm diameter exhaust. Room temperature gaseous fuel was injected sonically through a single port at either 44.5 or 14.0 mm upstream of the cavity leading edge on the combustor centerline. A 2.18 or 2.49 mm diameter injection port could be used at either location depending on the desired fuel flow rate. The ports were never used simultaneously. Pilot fuel was directly injected into the cavity through three spanwise 1.19 mm diameter ports in the cavity floor or rear wall. The floor ports were located 6.4 mm downstream of the cavity leading edge while the rear wall ports were located 3.8 mm above the cavity floor. Previous studies reported different flame structures and stability properties when the cavity was fueled at different locations [6,16,17]. A spark plug in the cavity floor was used to ignite the cavity pilot flame and was the only ignition aid used. The range of flow conditions explored is given in Table 1. An electric heater and hydrogen fueled vitiator were used to achieve the air stagnation temperatures (T 0 ) of 1040 1490 K. Make-up oxygen was added to maintain a 0.21 O2 mole fraction in the vitiator products. The run times were kept under 10 s in order to prevent thermal damage to the uncooled combustor. All the flow control and data acquisition equipment controlled by a Labview program. For each run, the air was first heated to 450 K by the electric heater. The vitiator oxygen flow was then started, followed 2 s later by the spark ignited vitiator fuel. Four seconds after vitiator ignition, the cavity fuel (if any) and cavity spark were turned on. One second later, the main fuel injection began. The main fuel flow was maintained for 2 s before all flow streams except the main air were terminated. A few runs were completed with up to 4 s of main fuel in order to acquire additional data. No differences were observed for the cases with longer run times. The air stagnation temperature was measured by a K-type thermocouple in the settling chamber just upstream of the nozzle. The temperature generally increased by approximately 50 K during the
D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404 constant area isolator 358mm
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combustor 44.5mm
nozzle
50.8mm
14.0mm M=2.2
P0,i = 590kPa T0 =1040 - 1490K
normal shock train (when ram-mode)
reaction zone for upstream fuel injection jet-wake stabilized mode H = 25.4mm
y x
reaction zone for upstream fuel injection cavity stabilized mode 12.7mm
downstream upstream main fuel main fuel injection location injection location
4o cavity rear wall fuel injectors
cavity floor fuel injectors
Fig. 1. Drawing of test section with flow features. Isolator and nozzle not to scale.
Table 1 Test conditions Parameter
Test conditions
P 0;i T0 Fuel type
59015 kPa 1040–1490 K H2 , or 50–50 H2 /C2 H4 by vol. 0.18–0.42 0–0.12 1.75, 0.55 Rear wall, Floor, Both
/ m_ cav fuel =m_ total fuel ðx=H Þmain fuel Cavity fuel injection
2 s the main fuel was on during each run. The average temperature measured during this time is reported as the nominal value of T 0 . The initial air stagnation pressure (P 0;i ) was also measured in the settling chamber. The wall static pressure (P w ) was recorded at 40 Hz at eight locations in the combustor and isolator. The combustion region was imaged through fused silica windows that were 305 mm long. A Vision Research Phantom 9.0 camera was used to record high speed movies of the flame luminosity. Images of 768 by 240 pixels were acquired at 4000 Hz for 2 s after the main fuel flow was initiated. The field of view was approximately 150 mm in length and spanned the height of the test section and cavity. For a few cases a wall pressure signal was recorded at high frequency and synchronized with the high speed camera. For this purpose a transducer was mounted on the sidewall in the isolator 140 mm upstream of the cavity leading edge. The pressure was recorded at 50 kHz for 2 s and a 1 ms moving average was computed from the signal.
engine. Results for hydrogen fuel with / ¼ 0:21 0:27 and m_ cav fuel =m_ total fuel ¼ 0:0 0:12 at T 0 ¼ 1040 1410 K are presented. Higher values of / led to the shock train moving into the nozzle at high temperatures. The isolator exit Mach numbers were computed to vary between 0.68 and 0.82 from values of P w =P 0;i at x=H ¼ 3:25 using the 1-D method given by Curran et al [2]. Results for the blended fuel are presented for / ¼ 0:42 at T 0 ¼ 1250 1490 K. This higher equivalence ratio was necessary for the blended fuel in order to thermally choke the flow and cause ramjet mode operation. For these cases M i;exit was computed to be between 0.66 and 0.85. The higher temperature cases led to lower isolator exit Mach numbers due to the changing location of the heat release as discussed in Section 3.1.1. 3.1.1. Hydrogen fuel – combustion stabilization modes Two distinct combustion stabilization modes were found for upstream main fuel injection. The average combustion luminosity for these two modes can be seen in Fig. 2. In the cavity stabilized mode, the reaction zone was anchored at the leading edge of the cavity and spread into the main flow at an approximately constant angle. In the jet-wake stabilized mode, the reaction zone was stabilized well upstream of the cavity in the
(a) cavity stabilized combustion main fuel (b) jet-wake stabilized combustion
3. Results and discussion 3.1. Ramjet operation – upstream fuel injection Ramjet mode operation (subsonic M i;exit ) was studied with upstream main fuel injection (ðx=H Þmain fuel ¼ 1:75) for both pure hydrogen fuel and the hydrogen–ethylene blend. For all cases presented, the shock train was contained in the isolator as is desirable in an operational
main fuel
Fig. 2. Average combustion luminosity images for upstream injection of hydrogen fuel. T 0 ¼ 1250 K, / ¼ 0:23, no cavity fueling. Yellow line is isoluminosity contour. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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wake of the fuel injection jet and had a curved leading edge. Cases were found of steady cavity stabilized combustion, steady jet-wake stabilized combustion, and oscillation between the two modes. The bimodal nature of the combustion is illustrated by Fig. 3. For this figure, 22,500 total images were analyzed for 5 runs with hydrogen fuel at / ¼ 0:21, m_ cav fuel =m_ total fuel ¼ 0:02 (rear wall injection), and T 0 ¼ 1130 1400 K. An isoluminosity contour characteristic of the reaction zone leading edge was defined. For each image, the average axial location of this contour was calculated for y=H ¼ 0:1 0:4 above the cavity. From Fig. 3 it can be seen that there are two distinct regions where the combustion can be stabilized. The upstream peak represents the jet-wake stabilized location and the downstream peak represents the cavity stabilized location. The fraction of time the combustion spent in each stabilization mode during a run was determined from the high speed movies. The definition of reaction zone leading edge in Fig. 4 was used and a critical axial location which separated the
number of images
jet-wake stabilized mode
cavity stabilized mode
600 400 200 0
—1.5
—1 —0.5 0 0.5 1 1.5 reaction zone leading edge (x/H)
2
2.5
Fig. 3. Histogram of reaction zone leading edge location for five cases with 22,500 images total. Upstream injection of hydrogen with T 0 ¼ 1130 1400 K, / ¼ 0:21, m_ cav fuel =m_ total fuel ¼ 0:02.
jet-wake stabilized mode
f = fraction of time combustion is in
1.0
0.8
Cavity stabilized regime
Intermediate, oscillatory regime
Jet-wake stabilized regime
0.6
0.4
φ = 0.21, mcav fuel / mtotal fuel = 0.02 φ = 0.21, mcav fuel / mtotal fuel = 0.12
0.2
0 1000
φ = 0.26, mcav fuel / mtotal fuel = 0.0
1100
1200 1300 T0 (K)
1400
1500
Fig. 4. Combustion stabilization mode vs. T 0 for hydrogen fuel. Upstream main fuel injection, cavity rear wall fueling.
two modes of ðx=H Þcrit ¼ 0:6 was set. Each image where the reaction zone leading edge was upstream of this value was considered to be in the jet-wake stabilized mode. The fraction of time in each mode was not very sensitive to the selection of ðx=H Þcrit due to the small percent of time spent in transition. The results for runs with three combinations of / and m_ cav fuel =m_ total fuel (all rear wall cavity fueling) for T 0 ¼ 1040 1410 K are plotted in Fig. 4. It can be seen that T 0 is the dominant variable in determining the combustion stabilization mode. At high T 0 ( J 1350 K), the combustion was virtually always stabilized in the jet-wake mode. For low T 0 ( K 1150 K), the combustion was virtually never stabilized in the jetwake mode, i.e. it was always stabilized in the cavity mode. There was a range of intermediate T 0 for which the combustion oscillated between the two stabilization modes. The fraction of time in the jet-wake stabilized mode (f) may be approximated by Eq. 1 for all conditions. Eq. 1 is plotted as a dashed line in Fig. 4. 1 1 T 0 1250 K ð1Þ f ¼ þ jT 0 1250 Kjerf 2 2 75 K For cavity stabilized combustion, the reaction zone spread into the flow at a relatively constant angle between 26 and 30°. The larger angles were associated with higher T 0 . Very similar reaction zones were found in previous studies of angled ethylene injection upstream of a wall cavity by Mathur et al. [5] and Lin et al. [15]. In all these cases, the constant spreading angle indicates that the reaction is likely occurring as a premixed flame. The flame is anchored in a low speed region in the cavity shear layer and spreads into the flow at an angle which matches the local flame speed to the normal flow velocity. The fact that the spreading angle increases with temperature is consistent with this explanation. This constant spreading angle would not be expected if auto-ignition played a prominent role in the main flow reaction because the flow is shock free (subsonic) in this region. For jet-wake stabilized combustion, the leading edge of the reaction zone (as defined earlier from the luminosity) was located on average 20– 30 mm downstream of the fuel injection location for T 0 ¼ 1410 1230 K. The higher temperatures were associated with the shorter distance. In a previous study with ethylene fuel, Mathur et al. [5] found that the combustion sometimes moved upstream of the cavity, but only along the sidewall in the separated boundary layer. In the current study, images from the top of the test section, such as Fig. 5, showed that the jet-wake stabilized combustion was indeed occurring in the wake of the fuel jet and not along the side walls. Studies with very high temperature air [18] or fuel [7] have found combustion in the fuel jet-wake, but this has been attributed to auto-ignition. As be will
D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404
Fig. 5. Top view, jet-wake stabilized, average combustion luminosity. Upstream injection of hydrogen fuel with T 0 ¼ 1370 K, / ¼ 0:27, m_ cav fuel =m_ total fuel ¼ 0:07.
be discussed in the Section 3.1.2, the dynamic behavior of the jet-wake stabilized combustion indicates that it is likely a flame and not autoignition. Figure 7 shows the average pressure in the test section for the two flame stabilization modes and the oscillatory case. The jet-wake stabilized case has a shock train which extends further upstream and a slightly higher peak pressure than the cavity stabilized case. This is caused by the heat release occurring further upstream for the case of jetwake stabilized combustion. 3.1.2. Hydrogen fuel – dynamics For intermediate temperatures (1150 K 6 T 0 6 1350 K), the reaction zone was observed to oscillate between the jet-wake and cavity stabilized positions. This is a fundamentally different type of instability than the thermoacoustic and fluid dynamic instabilities that have been T0 = 1130K, cavity stabilized T0 = 1270K, oscillating modes T0 = 1400K, jet-wake stabilized
0.075
0.6 0.5 0.4
0.045
0.3
0.03
0.2
0.015
0.1
σPw/P0,i
0.06
0 —16
—12
—8
—4
0
P w /P 0,i
0.09
0 4
x / H (from cavity leading egde)
reaction zone leading edge (x/H)
Fig. 6. Wall pressure standard deviation (solid symbols) and time average (open symbols). Upstream hydrogen fuel injection with different stabilization modes. / ¼ 0:21, m_ cav fuel =m_ total fuel ¼ 0:02. −2
fuel inj. jet-wake stabilized cavity stabilized
0 2
P w /P 0,i
0.55
0.45
0.35 500
1000
1500
2000
2500
time from start of main fuel (ms)
Fig. 7. Synchronized reaction zone leading edge location and wall pressure data. Upstream hydrogen fuel / ¼ 0:27, injection with T 0 ¼ 1220 K, m_ cav fuel =m_ total fuel ¼ 0:05.
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previously studied in similar combustors [13,14]. Figure 7 shows a typical time history of the reaction zone leading edge (as defined in Section 3.1.1) for an oscillatory case. It can be seen that the reaction zone is generally clearly stabilized in either the cavity or jet-wake location, with movement between the two locations happening very quickly. The oscillation between the modes does not occur at a set frequency and is generally in the range of 5–20 Hz. A FFT of the signal did not reveal any dominant frequencies in the range expected for instabilities caused by thermoacoustics or periodic shed vortices (hundreds to thousands of Hz). Figure 7 also shows a high frequency wall pressure signal at x=H ¼ 5:5 that is synchronized with the high speed imaging. From this graph it can be seen that the largest pressure changes are associated with the movement of the reaction zone. When the combustion is in the jet-wake stabilized mode, the pressure in the isolator is higher than when it is in the cavity stabilized mode. The reaction zone movement precedes the pressure change by approximately 8 ms. Thus the large pressure fluctuations are caused by the reaction zone movement and not vice versa. The tendency of the reaction zone to oscillate between two distinct, relatively stable locations indicates that it is likely a flame, and not autoignition, in both the cavity and jet-wake stabilized modes. If the combustion was primarily due to auto-ignition in the jet-wake stabilized mode, we would not expect to see the reaction zone leading edge located only in the small stable region illustrated in Fig. 8. Instead, any fluctuations which pushed the reaction zone downstream of its average location would give the fuel/air mixture more time, and thus make it more likely, to auto-ignite. The observed flame stabilization behavior may be explained in the following way. Cavity fueling does not play a role in the stabilization location because sufficient main fuel is entrained into the cavity for the cavity reaction to stabilize the main flame at all conditions studied. For cases when a jet-wake location is available for stabilization, the flame is often not capable of flashing forward through the intermediate region (between the cavity and jet-wake stabilization locations) which has a higher average velocity and/or a less favorable local equivalence ratio than the jet-wake stabilization location. At low T 0 , the flame remains in the cavity stabilized location until a fluctuation in the jet-wake occurs which is large enough to allow the reaction zone for jet-wake stabilized combustion
Mi,exit
main fuel injection
intermediate region stable region for jet-wake flame base
reaction zone for cavity stabilized combustion
stable region for cavity flame base
Fig. 8. Flame base stabilization locations.
D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404
3.1.3. Blended ethylene–hydrogen fuel For the 50–50 blend of ethylene and hydrogen fuel, both cavity and jet-wake stabilized combustion was observed as shown in Fig. 9. The reaction zone shapes are similar to that found for hydrogen, but the jet-wake stabilized reaction zone is located farther downstream than in the hydrogen fuel case. The reaction zone leading edge varied from approximately 35 to 50 mm downstream of
(a) cavity stabilized combustion main fuel
cav fuel (b) jet-wake stabilized combustion cav fuel
main fuel
Fig. 9. Average combustion luminosity images for upstream injection of blended ethylene–hydrogen fuel. T 0 ¼ 1370 K, / ¼ 0:42, m_ cav fuel =m_ total fuel ¼ 0:05.
the fuel injection location on average for T 0 ¼ 1490 1330 K. This is likely due to the much higher equivalence ratio used for the blended fuel compared with hydrogen. The higher / creates a longer mixing distance between the injection location and the presumably near stoichiometric flame base. The cavity stabilized flame is similar to the hydrogen flame but with a lower spreading angle of 18–22°. The effect of T 0 on the combustion stabilization mode is shown in Fig. 10. As expected, the blended fuel exhibits the same behavior seen for hydrogen fuel, but the transition from cavity to jet-wake combustion occurs at a higher temperature. The error function approximation shown for the blended fuel is simply shifted to the right by 120 K compared with Eq. 1. 3.2. Ramjet operation – downstream fuel injection The fuel injection location at ðx=H Þmain fuel ¼ 0:55 was used to cause the two stabilization locations (jet-wake and cavity shear layer) to overlap. The goal was to minimize the reaction zone oscillations that occurred when the two stabilization locations were separated in space. Ramjet mode combustion from this downstream fuel 1.0 Hydrogen fuel, φ = 0.21-0.26
jet-wake stabilized mode
flame to flash forward to a relatively stable location in the jet-wake. The flame will then remain in the jet-wake stabilized location until another fluctuation makes the position unstable and it flashes back to the cavity stabilized location. As T 0 increases, the flame speed increases. Thus the magnitude of the fluctuations required for the flame to flash forward to the jet-wake location become smaller, and the magnitude of the fluctuations which cause it to flash back become larger. The flame then spends more and more time in the jet-wake stabilized location as T 0 increases. At a high enough T 0 , there are no fluctuations present in the flow which are sufficient to cause the jetwake location to become unstable. The flow fluctuations which cause the flame to flash forward to the jet-wake position or back to the cavity position may be related to fluid dynamic, acoustic, or facility dependent fluctuations (or freestream fluctuations in a flight vehicle). The lack of a dominant frequency and relatively low frequencies at which this oscillation occurs, however, suggest that any periodic shed vortices or acoustic modes do not couple with the reaction zone location oscillations. There is a large difference in the steadiness of the pressure field for the different stabilization modes. Figure 6 shows the standard deviation of the non-dimensional wall pressure (rP w =P 0;i ) from the 40 Hz measurement at eight locations in the isolator and combustor. Pressure fluctuations are a marker of the steadiness of the combustion in a thermally choked flow because any change in heat release rate or distribution will cause a change in the pre-combustion shock train length and pressure rise. The cavity stabilized mode is the steadiest mode because the base of the flame is located in a low speed region in the upstream part of the cavity shear layer. This part of the shear layer is relatively steady because it is fixed by the geometry of the cavity. In the jet-wake stabilized mode, the base of the flame must be located in a low speed region of the jet-wake with a proper local equivalence ratio. The wake behind the fuel jet is expected to be much less steady than the recirculation in the fixed geometry cavity and thus the combustion is less steady. The location of the heat release moves significantly when oscillating between the two stabilization modes, so the pressure is least steady in this case.
f = fraction of time combustion is in
2402
0.8
Blended fuel, φ = 0.42
0.6
0.4
0.2
0 1000
1100
1200 1300 T0 (K)
1400
1500
Fig. 10. Combustion stabilization mode vs. T 0 for hydrogen and blended fuel with upstream main fuel injection.
D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404
injection location was studied for hydrogen fuel with T 0 ¼1250 K, / ¼ 0.18 and 0.27, and m_ cav fuel =m_ total fuel ¼ 0.0 and 0.10 through floor and rear wall injectors. This temperature is equivalent to the most unsteady oscillatory case observed for upstream main fuel injection. For all conditions the pre-combustion shock train was contained in the isolator. Wall pressures and average luminosity images for the / ¼ 0:27 cases are shown in Figs. 11 and 12. It can be seen that cavity fueling from either location makes the flow significantly more steady. For no cavity fueling, the combustion appears to be in the jet-wake stabilized mode. The shape and location of the reaction zone with respect to the fuel injector is similar to that seen for the upstream injection, jet-wake stabilized case shown in Fig. 2b. Additionally the magnitude of the pressure fluctuations are similar to the jet-wake stabilized case shown in Fig. 6. With cavity fueling, shown in Fig. 12b and c, the reaction zone extends into the upstream part 0.6
0.06
0.5
0.04
0.4
0.03
0.3
0.02
0.2
0.01
0.1
0 −16
−12
−8
−4
0
P w /P 0,i
σ Pw/P0,i
0.05
no cavity fuel rear wall cavity fuel floor cavity fuel
0 4
x / H (from cavity leading egde)
Fig. 11. Wall pressure standard deviation (solid symbols) and average (open symbols) for downstream injection of hydrogen. T 0 ¼ 1250 30 K, / ¼ 0:27, m_ cav fuel =m_ total fuel ¼ 0:0 or 0.10.
(a) no cavity fuel
main fuel
main fuel
main fuel
(b) rear wall cavity fuel cav fuel
(c) floor cavity fuel cav fuel
Fig. 12. Average combustion luminosity images for downstream main fuel injection of hydrogen. T 0 ¼ 1250 30 K, / ¼ 0:27, m_ cav fuel =m_ total fuel ¼ 0:0 or 0.10. Image (b) is blacked out in rear corner of cavity due to contaminant buildup on the window which was glowing brightly.
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of the cavity. As noted before, this is a relatively steady area of low speed flow. Thus the steady combustion in this area provides heat and radicals to the main flow reaction, which appears to be primarily jet-wake stabilized. This hybrid stabilization is steadier than pure jet-wake stabilization, but not quite as steady as pure cavity stabilization. It can be seen that the cavity shear layer reaction is significantly stronger for cavity floor fueling, which is likely why this configuration produced a slightly more steady flow field. Pure cavity stabilized combustion (with a constant spreading angle) was not found for downstream main fuel injection. It is expected that cavity stabilized flames are burning in a premixed fashion. For downstream fuel injection there is not enough distance between the injection location and the cavity leading edge for this premixing to occur. The same basic trends in combustion stabilization and steadiness were seen for / ¼ 0:18. For the lower equivalence ratio though, the difference in pressure fluctuations between cases with and without cavity fueling was less. For this lower equivalence ratio the main fuel jet does not penetrate into the flow as far and so more main fuel may be entrained into the cavity. Therefore the flame base is located farther upstream in the cavity shear layer which plays a role in stabilizing the flame. 3.3. Scramjet operation Scramjet mode operation of the combustor was achieved by lowering the equivalence ratio below that necessary for thermal choking. Ramto-scram mode fluctuations were observed to occur for / < 0:20 with hydrogen fuel and for / < 0:40 with the fuel blend. Steady scramjet mode combustion was not observed because flame blowout occurred if the equivalence ratio was lowered sufficiently. To obtain fully steady scramjet mode combustion in this combustor would require raising T 0 , lowering / (while still igniting), moving the heat release further downstream, or some combination of the these. Figure 13 shows the average pressure in the combustor during ramjet and scramjet mode operation for one case where a low frequency ram-to-scram oscillation of about 5 Hz occurred. M i;exit was computed to be 0.97 and 1.69 from wall pressure at x=H ¼ 3:25 for the ramjet and scramjet mode combustion, respectively. The actual Mach number just upstream of the combustion for both cases was a bit lower due to the fact that Mach number is decreasing in the downstream direction and due to the assumptions of the 1-D method. It can be seen that the pressure in parts of the isolator changes significantly between ramjet and scramjet mode combustion. This is due to the fact that there must be a step change in the isolator exit Mach number to go from thermally choked subsonic to
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D.J. Micka, J.F. Driscoll / Proceedings of the Combustion Institute 32 (2009) 2397–2404 0.5 0.4
no fuel scramjet mode combustion ramjet mode combustion
0.3 0.2 0.1 0 −16
−12
−8
−4
0
4
Fig. 13. Average wall pressure for ramjet mode combustion, scramjet mode combustion, and the no fueling case. Downstream injection of hydrogen fuel at T 0 ¼ 1410 K, / ¼ 0:18, m_ cav fuel =m_ total fuel ¼ 0:09.
supersonic heat addition. This can lead to an instability with large pressure fluctuations when undergoing ram-to-scram transition. The reaction zone was found to appear in only the cavity stabilized location for the scramjet mode conditions studied. This is likely due to the fact that the higher velocity and lower static temperature associated with a supersonic M i;exit (for a fixed T 0 and isolator entrance Mach number) made the jet-wake location unsuitable for flame stabilization. 4. Conclusions A laboratory dual-mode scramjet combustor was studied for a range of air stagnation temperatures (1040–1490 K), two fuel injection locations, and two fuel types. The reaction zone locations, average pressures, and pressure fluctuations are reported. The stagnation temperatures studied correspond to a range of flight Mach numbers (4.3–5.4) where a wall cavity pilot flame is needed to assist in ignition and flame stabilization. Two distinct combustion stabilization modes were found for ramjet operation with fuel injection sufficiently upstream of the cavity: jet-wake stabilized (at large T 0 ) and cavity stabilized (at low T 0 ). Cavity stabilized combustion is anchored at the leading edge of the cavity shear layer and spreads into the main flow at an approximately constant angle. Jet-wake stabilized combustion is located in the wake of the fuel injection jet a short distance downstream. The fuel appears to be burning as a flame rather than auto-ignition in both stabilization modes. There was an intermediate range of T 0 for which the combustion oscillated between the two stabilization modes causing large pressure fluctuations in the isolator. The variation of T 0 in this experiment is analogous to acceleration through a range of Mach numbers in an operational dual-mode engine. Thus an engine of similar
geometry to the one tested is likely to encounter this instability. The impact of this instability may be minimized by placing the main fuel injection with respect to the cavity such that the two flame stabilization locations overlap. For this hybrid stabilization mode there is no large movement of the heat release zone and so the pressure fluctuations are minimized. Cavity fueling was required to realize the full benefits of this hybrid stabilization for the conditions of this study. The proper location of the main fuel jet to achieve this hybrid stabilization mode likely depends on /, fuel type, and the detailed engine geometry. For scramjet mode operation, only the cavity stabilized combustion mode was found. A strong instability was encountered at the ram-to-scram transition due to the step change in isolator exit Mach number required at this shift.
References [1] W.H. Heiser, D.T. Pratt, Hypersonic Airbreathing Propulsion, AIAA Education Series, AIAA, 1994. [2] E.T. Curran, W.H. Heiser, D.T. Pratt, Annu. Rev. Fluid Mech. 28 (1996) 323–360. [3] F.S. Billig, J. Propul. Power 9 (4) (1993) 499–514. [4] A. Ben-Yakar, R.K. Hanson, J. Propul. Power 17 (4) (2001) 869–877. [5] T. Mathur, M. Gruber, K. Jackson, et al., J. Propul. Power 17 (6) (2001) 1305–1312. [6] M.R. Gruber, J.M. Donbar, C.D. Carter, K.-Y. Hsu, J. Propul. Power 20 (5) (2004) 769–778. [7] K.H. Yu, K.J. Wilson, K.C. Schadow, J. Propul. Power 17 (6) (2001) 1287–1295. [8] J.M. Donbar, M.R. Gruber, T.A. Jackson, C.D. Carter, T. Mathur, Proc. Combust. Inst. 28 (2001) 679–687. [9] G. Yu, J. Li, J. Zhao, L. Yue, X. Chang, C.-J. Sung, Proc. Combust. Inst. 30 (2005) 2859–2866. [10] T. Mitani, M. Izumikawa, Proc. Combust. Inst. 28 (2001) 689–695. [11] R.A. Baurle, D.R. Eklund, J. Propul. Power 18 (5) (2002) 990–1002. [12] A.J. Neely, I. Stotz, S. O’Byrne, R.R. Boyce, N.R. Mudford, A.F.P. Houwing, 13th International Space Planes and Hypersonics Systems and Technologies, AIAA 2005-3358, 2005. [13] J.-Y. Choi, F. Ma, V. Yang, Proc. Combust. Inst. 30 (2005) 2851–2858. [14] K.-C. Lin, K. Jackson, R. Behdadnia, et al., Joint Propulsion Conference and Exhibit, AIAA 20075382, 2007. [15] K.-C. Lin, C.-J. Tam, I. Boxx, C. Carter, K. Jackson, M. Lindsey, Joint Propulsion Conference and Exhibit, AIAA 2007-5381, 2007. [16] C.C. Rasmussen, S.K. Dhanuka, J.F. Driscoll, Proc. Combust. Inst. 31 (2007) 2505–2512. [17] J. Driscoll, C.C. Rasmussen, J. Propul. Power 21 (6) (2005) 1035–1044. [18] B.K. McMillin, J.M. Seitzman, R.K. Hanson, AIAA J. 32 (10) (1994) 1945–1952.