The experimental fuel equivalence ratio (ER) was designed according to the similarity principle of inflow/jet momentum ratio, as shown in Formula (1). The subscript “jet” represents the jet state, and “flow” represents the inflow state. The specific basis was to ensure that
was constant for different injectors. According to the above principle, the test conditions in this paper are shown in
Table 2.
3.1. Flame Stabilization Modes and Characteristics
Due to the pressure fluctuation by the ethylene valve, the stable period of 18.0–19.0s was selected as the research period in this paper, as the pressure was smooth enough. The pressure of 18.0–19.0 s was nondimensionalized by the isolator entrance static pressure
= 70 kPa (Ma = 2.4) or 41 kPa (Ma = 3.0). The Mach number along the combustor was obtained through one-dimensional analysis [
15]. The calculated mean and standard deviation of pseudo-color images of CH* chemiluminescence were 18.5–19.0 s, and the shadowgraph was 18.5 s. According to the pressure characteristics along the path, the working conditions can be divided into pure cavity flame stabilization mode, shear layer flame stabilization mode (weak and strong), and jet-wake flame stabilization mode. The following is a brief analysis of the flow and flame characteristics in these conditions.
Figure 5 shows the dimensionless pressure/Mach number along the
x axis and the mean pseudo-color image of CH* chemiluminescence at Case 9. The pressure is relatively stable in this test condition, and the pressure rise is relatively small. The dimensionless pressure
is slightly higher than 1, and the Mach number near the jet and cavity is slightly higher than 1 and greater than 1, respectively. Because the combustion flame has a long length, the pressure rise caused by combustion was offset by expansion, and the pressure maintains the pressure state of the inlet, which is equivalent to an isobaric combustion state. Corresponding to the CH* pseudo-color image, we found that the flame intensity is weak, and the flame is mainly stabilized in the shear layer, located in the downstream of the cavity. This feature is defined as weak shear layer flame stabilization mode in this paper.
Figure 6 showed the shadowgraph, where the depth of injection is relatively low, and the oblique shock wave in the combustor is clear and not greatly affected. It indicates that the flame is in the weak combustion stage of supersonic combustion.
Figure 7 shows the dimensionless pressure/Mach number along the
x axis and the mean pseudo-color image of CH* chemiluminescence at Case 11 (1.7D
0I0.144). The pressure is relatively stable, and the dimensionless pressure
is near 1 in front of the cavity. The highest pressure range is 2–3 times that of the referent pressure; the pressure rises significantly but has not reached the pressure of the normal shock wave. The pressure rise is concentrated in the cavity and extended to the rear edge of the cavity by about 200 mm, where the Mach number is less than 1. Compared with
Figure 5, we found that the flame intensity at
x = 528 mm is larger than Case 9 (1.7D
0I0.099), but the flame area is still stabilized in the shear layer. This feature is defined as strong shear layer flame stabilization mode in this paper.
Figure 8 shows the shadowgraph of Case 11 (1.7D
0I0.144), where the equivalent ratio is increased, and the ethylene injection depth is higher than Case 9 (1.7D
0I0.099). The intensity of the oblique shock wave in the combustor increases with a strong disturbance.
Figure 9 shows the dimensionless pressure/Mach number along the
x axis and the mean pseudo-color image of CH* chemiluminescence at Case 5 (1.2D
0II0.182). The pressure rise does not exceed 1, and it is basically in the cavity area. In this case, in the expansion section after the cavity, the flame is reignited after the flame is extinguished, which may be caused by the separation and re-attachment of the boundary layer. The reason for this result could be that the flame formed by the main jet is extinguished, and the fuel is entrained by the jet CVP to the cavity and burns. It is possible that the small jet diameter and a weaker jet CVP do not allow for the formation of a shear flame in order to form a stable flame with the main jet. The Mach numbers along the streamline are all greater than 1. The flame is mainly stable in the cavity whose strength is very weak, and this feature is defined as the pure cavity flame stabilization mode in this paper.
Figure 10 is the shadowgraph at Case 5 (1.2D
0II0.182). Due to the high fluid velocity, the shear layer is compressed in the cavity without expanding outward, which is why the flame in the cavity fails to meet the main jet.
Figure 11 shows the dimensionless averaged pressure/Mach number along the
x axis and the mean pseudo-color image of CH* chemiluminescence at Case 13 (1.7D0I0.169). The starting point of the pressure rise is in the isolator, and the magnitude of the pressure rise is very high. The Mach number near the jet and cavity is less than 1, which is typical subsonic combustion. Corresponding to the CH* pseudo-color image, the flame intensity near the jet is very high, and the flame is behind the jet-wake [
9,
10]. This state is defined as jet-wake flame stabilization mode in this paper.
Figure 10 is the shadowgraph of Case 13 (1.7D
0I0.169). The depth of injection increases significantly. Due to the strong back pressure generated by combustion heat release, the shock wave train is pushed to the upstream of the jet, as shown the solid blue line in
Figure 12.
The jet wake flame stabilization mode is unstable in a certain range. If the shock wave system formed before the main flow decelerates to subsonic speed, but the total temperature and total pressure of flow are not enough to maintain the jet flame stabilization mode, an oscillation mode will occur; with the flame switching from jet wake stabilization to shear layer stabilization, the shock wave also oscillates at same time. The amplitude of flame oscillation varies in different jet scales and equivalence ratios.
Figure 13 shows the standard deviation pseudo-color image of CH* chemiluminescence at Case 11 (1.7D
0I0.144) and Case 13 (1.7D0I0.169).
Figure 14 and
Figure 15 show that pressure along the
x axis at Case 11 and Case 13 during 18.0–19.0 s. From the trend of color change over time, the pressure fluctuation of Case 11 is small, and the fluctuation area is concentrated in the cavity and the expansion part rear of the cavity. The pressure fluctuation at Case 13 is large, and the fluctuation area is concentrated in the jet and cavity.
3.2. Factors Affecting Flame Stabilization Modes
Many factors will influence the flame stabilization mode, such as air stagnation temperatures [
8], equivalence ratio [
9], injection distance [
16], and fueling schemes [
17]. In this paper, ER, jet scale (diameter), and distance between adjacent jet orifices are studied in detail.
Figure 16 shows the influence of the ER on the flame stabilization mode. Analyzing the pressure diagram of the combustor for
ϕ1.7 × 3 with different ERs in Ma2.4, we found that with the increase of ER, the pressure gradually increases in the combustor, and the pressure rising point moves upstream. The corresponding flame stabilization mode is from blowoff to weak shear layer flame stabilization, then to strong shear layer flame stabilization, and finally to jet-wake flame stabilization. For the same injector, along with the increase of the ER, the jet/inflow momentum ratio and the penetration depth of fuel increases, which forms positive feedback with back pressure caused by heat release, and further improves mixing and combustion of fuel/inflow. The change trend of flame stabilization modes is from weak shear layer to strong shear layer, then to jet-wake stabilization mode with the increase of ER. The change process of flame stabilization between shear layer and jet-wake is abrupt, while the change of internal pressure between shear layer and jet-wake is stable, without abrupt change.
Through the experimental study of injectors with different jet scales (D/H, jet diameter/isolator height), we found that the increase of jet scale will be beneficial to combustion at a large ER, while the increase of jet scale is not conducive to combustion at a small ER.
According to the experiments in this paper, the boundary between large and small ER is whether a strong shear layer can be formed.
Figure 17 shows the influence of jet scale with a large ER: 1.5D
0I0.141 and 1.2D
0 I0.168 form a jet-wake stabilization flame, and 1.2D
0I0.150 forms a strong shear layer stabilization flame. Comparing the ER of the three test conditions, it can be found that the ER of 1.5D
0I0.141 is less than the ER of 1.2D
0I0.168, but the pressure rise of 1.5D
0I0.141 is larger, and even causes the change of the flame stabilization mode [
14]. A larger recirculation zone in the upstream of the jet caused by a larger jet scale is more conducive to flame stabilization, resulting in the enhancement of combustion heat release. However, for a small ER, the conclusion is the opposite, as shown in
Figure 18. The pressure rise in 1.5D
0I0.074 is actually smaller than 1.2D
0I0.051 for reasons that require further analysis.
Additionally, the distance between adjacent jet orifices also affects the flame stabilization mode, as shown in
Figure 19. We found that 1.7D
0I0.144 forms a strong shear layer stabilization flame and 1.5D
0I0.141 forms a jet-wake stabilization flame. This indicates that the increase of jet orifice spacing is not conducive to heat release, and even offsets the combustion gain brought by the increase of the jet scale. Compared with the working 1.7D
0I0.057 and 1.2D
0I0.051, when the ER is small, the increase of jet orifice spacing will lead to blowoff.
Figure 20 summarizes flame stabilization modes of different jet scales at different ERs, in which “0” represents blowoff, “1” represents weak shear layer flame stabilization, “2” represents strong shear layer flame stabilization, and “3” represents jet-wake flame stabilization. For the jet scale of 1.2D
0, weak shear layer flame stabilization is transformed to strong shear layer flame stabilization when the ER is 0.051–0.075, and a strong shear layer flame stabilization is transformed to jet-wake flame stabilization when the ER is 0.15–0.168. For the jet scale of 1.5D
0, the flame stabilization transition point from weak shear layer to strong shear layer is delayed with the ER of 0.074–0.095, and the flame stabilization transition point from strong shear layer to jet-wake is advanced with the ER of 0.117–0.139. For the jet scale of 1.7D
0, when the ER is 0.057, the flame in the combustor is blowoff. In the ER of 0.115–0.144, the flame stabilization transition point from weak shear layer to strong shear layer is later than 1.5D
0. In the ER of 0.144–0.168, the flame stabilization transition point from strong shear layer to jet-wake is also later than 1.5D
0. The above facts show that both jet diameter and jet orifice spacing have an impact on the flame stabilization mode.
3.3. Physical Mechanism of Different Flame Stabilization Modes
Based on the above research results and characteristics of flame stabilization modes, this paper proposes physical mechanisms under different flame stabilization modes, as shown in
Figure 21. The fuel distribution of the transverse jet and the cavity will form different modes according to the intensity of the jet flow condition.
There are three regions in the cavity flame stabilization structure: the flow region inside the cavity, the shear layer region, and the jet wake region, respectively. The three are related in that the main function of the flow zone inside the cavity is to exchange material and energy with the shear layer by the recirculation zone of the combustion products. The shear layer zone is the main region of the turbulent flame. The expansion angle of the shear layer is a function of turbulent flame velocity and incoming flow velocity. The jet wake area is based on the transition from the non-premixed flame to the premixed flame. If the flame can be stabilized in the non-premixed area, the jet wake area can self-sustain the flame, and the flame expands to the jet wake area. If not, it will develop into a strong shear layer flame, and part of the flame overlaps with the flame in the shear layer area to form an extended turbulent flame. The jet wake flame is ignited by the shear layer. If the flame is not successfully connected, a weak shear layer flame is formed, and part of the wake fuel enters the shear layer. The flame mode in the cavity formed independently because the turbulence intensity in the shear layer region is higher than the critical condition for flame formation, so flameout occurs.
The jet has an independent mixing zone, and the material exchange with the cavity is carried out through CVP. The influence of the jet size has a great influence on the amount of CVP. The large-scale jet has less generation with the shear layer due to the high penetration depth. The exchange mass is higher, which is also the reason for the higher range of the large-scale blow-out equivalence ratio. However, small-scale jets are more likely to form shear flames because their entire CVP is closer to the shear layer. Therefore, the key to forming a stable flame is the mass exchange between the jet and the shear layer. However, in the process of forming a stable flame, whether it can switch to the jet stable mode is determined by the Da number, where
SL is the laminar flame speed,
l is the integral length scale,
u’ is the fluctuating component of the velocity, and
lf is the flame thickness. The turbulent dissipation rate is
ε,
k is the turbulent kinetic energy,
r is the spatial coordinate, and
C is a universal constant called the Kolmogorov constant [
18].
In the process of combustion flame stabilization, the jet diameter is a more important scale, which affects the evolution process of flame stabilization. It can be seen from Equations (2) and (3) that it mainly affects the integral scale, that is, the scale that affects the vortex structure. The proportional scale of jet diameter and height affects the distribution of flame zones.
The increase of the equivalence ratio means the increase of the jet velocity, which mainly affects three factors. The first is the turbulence intensity of jet mixing. Due to the increase of the convective Mach number, the integral scale of the jet increases, and the Da number increases, and the flame tends to be easily stabilized. The second factor is the spatial distribution of the fuel, which affects the spatial relationship between the jet wake region and the cavity shear layer. The third factor is the change in the total fuel equivalence ratio. The heat release affects the velocity, density, and static temperature of the combustor inlet. These factors ultimately affect the ignition and flame distribution.
For increasing equivalence ratio, the combustion reaction will be intensified, and the heat release area will be enlarged. The non-premixed fuel will also be ignited, but the heat release at this time is not enough to form a precombustion shock wave system in the isolator. Therefore, this state is still a shear layer stabilization flame, but its intensity is significantly higher than that of the weak shear layer stabilization flame, and the heat release area is mainly concentrated in the rear of cavity, so it is defined as a strong shear layer stabilization flame.
When equivalence ratio is further increased, there will be two states according to the jet diameter. One is that the shear layer flame is easy to blow out abruptly [
15]. At this time, the fuel at the bottom of the jet wake is brought into the shear layer region of the cavity. The flame stability of the shear layer needs to satisfy the triple flame theory. Another state is that the combustion heat release is enhanced, and a precombustion shock wave is produced by higher back pressure. The large-scale vortex structure exists upstream of the jet, and the penetration depth of fuel injection is increased. A low-speed zone is formed on the side of the jet to stabilize the flame, and the shear layer and cavity provide high-temperature gas for the heat release zone to maintain the reaction.
The effect of the increase of the inlet Mach number is similar to that of the decrease of the equivalence ratio. With the increase of the inlet Mach number, the combustion will change from a strong shear layer stabilization flame to a weak shear layer stabilization flame, or from a weak shear layer stabilization flame to a cavity stabilization flame to blowoff.