3.1. Analysis of the Detonation Induced by a Single-Jet
Figure 3 shows the initiation process of single-jet detonation of supersonic gas. The jet diameter is set to 8 mm to ensure that the single-jet and symmetric-jet inject the same amount of energy per unit time. As shown in
Figure 3a, a hot jet injected at the local speed of sound enters the supersonic gas and blocks the incoming flow, thus inducing a bow shock wave in front of the jet. Under the continuous injection of the hot jet, the bow shock gradually develops outward and later collides with the upper wall surface.
Figure 3b shows the reflected shock wave. Despite the further increase in density and pressure of the gas compressed by the reflected shock, at this time the reflected shock strength still cannot achieve detonation, only the formation of shock wave-induced combustion. With the intensity of the reflected shock wave increasing, the regular reflection at the wall is transformed into a Mach reflection, and the Mach stem is developed at the original reflection point. In
Figure 3c, Mach reflection is formed near the upper wall at this time. This is a sign indicating the formation of local detonation. Since the intensity of the jet-induced bow shock is greatest at the jet nozzle, the primary triple point is propagating down, as shown in
Figure 3d. The flow field structure presented in the numerical simulation is generally consistent with the experimental observations of Cai et al. [
42]. This is particularly true for the bow shock, wall reflection shock, and transverse wave. However, it should be noted that the sliding reflection boundary conditions are used for both the upper and lower walls in the calculations, and that the complex interactions of the boundary layer at the wall with the shock (such as the separation zone) in the experiment could not be represented in the numerical simulation results.
Figure 4 demonstrates the contour of reaction progress variable of the supersonic combustible mixture during the movement to the lower wall of the primary triple point
T along with the bow shock, after the Mach stem is generated on the upper wall surface. As shown in
Figure 4a, the Mach stem is closely coupled to the post-wave combustion products, indicating that localized detonation combustion has formed behind the Mach stem. Because of the relatively weak intensity of the bow shock, an unburned jet is formed below the slip line of the triple point
T. The slip line is created by the sharp transverse motion of the transverse wave and the triple point. As the triple point gets closer to the lower wall surface, the bow shock intensity increases, resulting in a faster motion, which also leads to significantly rolled suction vortices on the shear layer of the slip line as shown in
Figure 4b. These vortex structures formed by the KHI promote turbulent mixing of combustion products with unburned material, further contributing to the consumption of unburned jets.
Figure 5 demonstrates the propagation of the detonation under the single-jet initiation method. The local detonation expands to the lower wall surface as the triple point moves downward, as shown in
Figure 5a. When the triple point collides with the lower wall surface, detonation combustion of the whole channel is achieved. In
Figure 5a,b, the pressures after the transverse wave are 240.5 and 287.2k Pa, respectively, indicating that after the collision with the wall, the transverse wave intensity increases. Moreover, the triple points are reversed and propagate up. After the transverse wave sweeps through the shear layer of the hot jet, the pressure wave is generated, and the detonation propagates forward as a single-headed mode. The shear layer formed under the continuous injection of hot jets is also equivalent to an aerodynamic ramp that continues to drive the forward propagation of the detonation wave.
3.2. Analysis of the Detonation Induced by Symmetric-Jet
As shown in
Figure 6a, the hot jets from the upper and lower walls are injected into the flow field to induce two bow shock waves. The structure of the bow shocks is symmetrical because the parameters of the two hot jets are set identically. Under the current conditions, the strength of the hot jets is not sufficient to achieve instantaneous detonation. After the gas mixture is compressed by the bow shock, the pressure rises, and thus a combustion zone is created behind the bow shock. The continuous injection of the hot jet further develops the bow shock and gradually extends to the opposite side. As shown in
Figure 6b, the two bow shocks interact with each other in the center of the channel to form a regular reflection, and a high-pressure region is formed at the interaction. With the enhancement of the reflected shock wave, the regular reflection is transformed into two double Mach reflections. As shown in
Figure 6c, the reflection point develops into a Mach stem. The structure of the shock front from top to bottom is bow shock-Mach stem-bow shock. At this time, Mach reflections on the wall caused by the reflected shock can also be observed. The propagation of the triple points raises the Mach stem height and develops further from the center of the channel to both sidewalls. As shown in
Figure 6d, two symmetrical triple points
T1 and
T2 are observed at the shock front, and the red arrows indicate the direction of motion of the triple points.
Analysis of the detonation initiation process of single- and symmetric-jet can be seen. In the single-jet initiation method, the bow shock is reflected on the upper wall and generates a Mach stem, resulting in the emergence of a local detonation and gradually initiating the whole flow. By contrast, in the symmetric-jet initiation method, the appearance of the Mach stem is due to the interaction of the upper and lower bow shock, and the emergence of the position is on the central axis of the channel. To analyze the combustion after the shock surface under the symmetric-jet conditions,
Figure 7 shows the emergence of the Mach stem and its development toward the walls on both sides. As shown in
Figure 7a, the Mach stem between the triple points
T1 and
T2 is tightly coupled to the combustion surface, indicating that a Mach detonation has formed. However, the weaker strength of the bow shock on both sides of the triple point
T1 and
T2 makes the formation of two unburned jets between the Mach stem and the bow shocks. In addition, the bow shock in the middle of the channel has the lowest intensity and the weakest effect on the compression of the mixture. Therefore, even after the appearance of the Mach stem, there is still a large amount of incompletely reacted gas behind the interaction of the upper and lower bow shocks. Moreover, multiple mushroom vortex structures formed by the RMI are also observed in this region. These vortex structures are developed from the rear of the slip lines. Comparing
Figure 7a,b, it is found that these vortex structures generated by RMI enhance the turbulent mixing of combustion products and reactants to promote turbulent combustion in the middle of the flow. In addition, the small-scale KHI vortices along the slip line also greatly enhance the mixing efficiency of reactants and combustibles and promotes the consumption of unburned jets. This process gradually makes the combustion reaction after the detonation complete, while the release of a large amount of chemical energy can also effectively promote the development and propagation of the detonation wave.
To further explore the large-scale RMI vortices behind the slip line,
Figure 8 illustrates the generation and consumption mechanism of this mushroom vortex structure. The inconsistency in the directions of the density and pressure gradient produces a large baroclinic torque (∇
×∇p), causing the RMI at the interface of the unburned jet [
43]. The left side of
Figure 8 shows the contours of the baroclinic torque. Comparing
Figure 8a,b, after the Mach stem in the middle of the channel appears, the gas mixture reacts quickly through its adiabatic compression, which also causes the combustible gas and products of high-speed reaction to be squeezed, forming a backward jet and a forward jet. The contours of the baroclinic torque show that there is a strong baroclinic torque at the backward jet. This also leads to a faster growth rate of the backward jet, as shown in the contour of the product on the right. The “cap” structure in the RMI vortex is defined as
Ci, and
i is used to classify the number of observed vortices. The stronger baroclinic torque leads to the continuous creation of new vortices. The increasing number and size of the RMI vortices enlarge the contact surface between unburned and burned materials, intensifying the chemical reaction. Comparing
Figure 8c,d, it can be seen that the baroclinic torque near the forward and backward jets disappears with the propagation of the triple point. The right-hand product cloud also further shows that during this period, new mushroom vortices are no longer generated, while turbulent mixing effects lead to the gradual depletion of the previous RMI vortices. The gas mixture in the middle of the flow field gradually reacts completely.
Figure 9 shows the propagation of the triple points after the local detonation is generated using the symmetric-jet initiation method. It also indicates the detonation initiation process of the whole flow and the propagation of the detonation wave in the channel. The black arrow identifies the direction of propagation of the triple point. As shown in
Figure 9a, after the local detonation is formed, the triple points
T1 and
T2 propagate up and down the wall, respectively. At the same time, the Mach stem height is raised, and the local detonation is expanded. When the triple points
T1 and
T2 collide with the wall and then move in the opposite direction, the initiation of the entire flow field is achieved, as shown in
Figure 9b. In
Figure 9c, the two triple point collision leads to an increase in local pressure and temperature, and a rapid increase in the rate of the chemical reaction. The triple points again reverse their motion and approach the walls on both sides, as shown in
Figure 9d. In the symmetric-jet initiation method, two triple points with opposite propagation are formed on the detonation front, and the detonation shows a double-headed self-sustaining propagation. In addition, under the continuous injection of hot jets, the shear layer formed by hot jets at the upper and lower walls constitutes a compression channel, which raises the pressure behind the detonation wave and promotes the forward propagation of the detonation wave.
From the above analysis, it is clear that there are differences in the detonation initiation process under the two different initiation methods, which also lead to significant differences in the location of the local detonation and the characteristics of the detonation front.
Figure 10a shows the trajectory of the detonation front based on the two jet initiation methods. The trajectory can be classified into two stages. In the first stage, the hot jet is injected into the flow field and induced to form a bow shock. At this time, the trajectory captures the position of the bow shock, and the line segment is relatively smooth. The second stage is the detonation front after the flow field is initiated. When the hot jet is continuously injected, the trajectories of wave surface under the two initiation methods are straight lines, and the slopes are almost the same. This indicates that regardless of whether the single- or symmetric-jet is used at this stage, under the same intensity of the hot jet, the propagation velocity of the formed detonation wave is almost equal. Based on the slope of the curve, the average forward velocity
of the detonation wave is approximately 266 m/s, and the corresponding overdrive degree
is 1.363 (
. When the double-jet is shutdown at 200μs, the detonation wave surface exhibits regular periodic oscillations, whose period is approximately 40 μs. At this time, the average relative propagation velocity of the detonation wave is
= 57.9 m/s, the forward velocity is decreased by approximately 70%, and the corresponding overdrive degree
is decreased to 1.073. It can be assumed that the overdrive detonation wave gradually decays into periodic-oscillation CJ detonation.
Comparing the abscissas of points
A and
B as shown in
Figure 10a, it is discovered that using the single-jet requires a longer time to complete initiation. However, in the symmetric-jet initiation mode, the two triple points propagate toward the wall at the same time, which can shorten the time of initiation. This also leads to a more rapid increase in the height of the local detonation wave (Mach stem) under a double-jet injection, as shown in
Figure 10b. Furthermore, by comparing the coordinates (the ordinate values of points
A and
B) at the moments when the initiation is achieved under the two methods, it is clear that the initiation achieved within a certain distance by the symmetric-jet is approximately 4.6 mm shorter than that for single-jet. This is useful for practical applications.
Through a detailed analysis of the above two initiation methods, it is found that the change in the flow area after the detonation wave caused by changing the initiation method of the jet has little effect on the propagation speed of the detonation. However, regardless of the mode of initiation, the intensity of the jet-induced bow shock is critical to initiation. For the symmetric-jet initiation mechanism, the low intensity of bow shock may lead to insufficient energy generated by collision reflection of shock to form Mach reflection, thus affecting the detonation initiation process. Therefore, it is necessary to further induce the effect of the intensity of the bow shock on the symmetric-jet initiation.
3.3. Effects of Diameter of Hot Jet Dj on Detonation Initiation
In the previous section, it was shown that the intensity of the jet-induced bow shock depends mainly on the diameter of the hot jet
Dj, pipe height H
p, and the momentum flux ratio
, expressed as follows:
This section explores the effect of jet diameter Dj on the double jet detonation initiation process.
Based on a basic calculation example (Dj = 4.0 mm), four other jet diameters are considered (while other parameters are maintained), leading to five in total as follows: Dj1 = 2.5 mm, Dj2 = 3.0 mm, Dj3 = 3.5 mm, Dj4 = 4.0 mm, and Dj5 = 4.5 mm.
The penetration height of jet formula is expressed as follows [
44,
45]:
When the jet diameter increases from 2.5 mm to 4.5 mm, the momentum flux ratio
remains unchanged, whereas the jet penetration increases from 4.5028 to 6.8710. This means the strength of the bow shock increases accordingly.
Table 4 lists the penetration height H
p and the momentum flux ratio
of the dual-jet for different diameters.
Figure 11 shows the ignition time
t1 and the time
t2 of the initiation completed for five different jet diameters, with a time step of 2.5 μs.
Table 5 lists the location of the ignition point (Mach stem) and the times
t1 and
t2 during the process of detonation initiation under five numerical simulation conditions. It can be seen in the table that the stronger jet penetration under the larger diameter of jet results in stronger and steeper induced bow shocks. As a result, the intersection point of the two bow shocks is more forward, which also leads to a more forward location for the initiation point to appear. In addition, as the jet diameter increases, the detonation initiation becomes more rapid.
Observing the right hand part of
Figure 11, in the case of the jet diameter of 3–4.5 mm, the secondary reflected shock wave formed at the two wall sides by the reflected shock behind the Mach stem is observed. However, at a jet diameter of 2.5 mm, this reflected shock is not observed. To analyze the cause of this phenomenon,
Figure 12 shows the shock wave interaction process before initiation at a jet diameter of 2.5 mm. As shown in
Figure 12a, the interaction between the bow shocks produces reflected shock waves, and as the intensity of the reflected shocks increases, the Mach reflections are formed at the wall. Thereafter, the Mach reflections move toward the intersection of the bow shocks. As shown in
Figure 12b, the two Mach reflections interact with each other to form multiple shocks and expansion waves between the upper and lower slip lines. When the triple points of the two Mach reflections collide with each other, the large amount of energy generated drives the formation of the Mach stem in the middle of the flow field to achieve initiation.
As shown in
Figure 13, after the initiation is completed, the detonation waves under different cases are overdriven forward at a stable speed. As shown in
Table 6, when the jet diameter increased, the relative propagation velocity
, absolute propagation velocity
V, and overdrive degree
of the detonation wave also increased. This is because as the jet diameter increased, the jet penetration expanded and the height of the shear layer formed by the hot jets rose, which further compressed the gas mixture after the detonation wave, thereby accelerating the propagation of the detonation wave.
Figure 14 shows the critical moment of initiation for single-jet and symmetric-jet for different jet diameters. As can be seen from the figure, in the interval of 3.0–4.5 mm jet diameter, comparing the time
t1 between symmetric- and single-jet, it can be seen that the local detonation appears earlier and the velocity of initiation is more rapid with the symmetric-jet. Comparing time
t2 when the jet diameter of 3.0–4.5 mm, the symmetrical-jet can be seen to shorten the time of the first collision between the triple point and the wall, and speed up the completion of the initiation. When the jet diameter is 2.5 mm, the bow shock induced by two jets is not strong enough. The interaction of bow shocks cannot directly form a local detonation, but this can be initiated by the collision of two Mach reflections behind the intersection.