Research On Detonation Initiation And Propagation Mechanisms In Supersonic Premixed Flows

The characters of detonation initiation and propagation in supersonic flows are very different from those in quiescent gas. The detonation wave may present two types configuration which are oblique detonation wave(ODW) and normal detonation wave(NDW) with various coming flow conditions and stabilization mechanisms. Through experimental observation and numerical simulation, the mechanisms of detonation initiation by oblique shock wave and hot jet have been investigated. The transformation of ODW to NDW and the characters of NDW propagation in supersonic flows have also been studied.The experimental system for studying detonation in supersonic flow is designed in this paper. It has various functions via replacing the system components. NS equations have been used to study the influence of boundary layer on detonation propagation in which viscosity is considered. In other cases, Euler equations have been adopted.Detonation initiation in supersonic flow has been investigated. Characters of detonation initiation induced by oblique shock wave with different coming flow conditions and wedge angles have been studied. The results show that whether a subsonic zone forms behind precursor shock wave is the main factor that determines a saltation or smoothness oblique detonation wave formation. The detonation initiation will be easier with higher equivalence ratio and larger wedge angle. There are two factors impact the detonation initiation if the coming flow Mach number is changed. That leads the relations between the detonation wave and Mach number are not confirmed. The detonation wave maybe propagates upstream if the temperature of coming flow decreases. That is due to the impacts of the walls.If the wedge with turning angle is adopted and the ODW is overdriven initially, there are three possible results occurring: If the turning point is far away from the wedge front, it can transform into a CJ(Chapman-Jouguet) ODW gently. If the turning point is near the wedge front, the ODW will decouple. But if the oblique shock wave induced by wedge after turn point can ignite the premixed coming flow all the same, an ODW corresponding the wedge angle after turn point will form again. For the CJ ODW, if the turning point is behind the CJ plain, the ODW will not be influenced. But if the turn point is in front of the CJ plain, the ODW angle will decrease and the ODW will decouple.Based on the detonation initiation induced by oblique shock wave, another means of detonation initiation which is hot jets initiation has also been studied. There are three types of hot jets which are low speed, high speed and detonation jets adopted in the experiments. The low speed hot jets can induce a DDT process, which will initiate the premixed flow. The detonation jets can initiate the premixed flows via the effects of hemisphere shock reflection and the shock-coupled flame surface together when the test section size is large. For the high speed hot jets, the speed of coming flow is very important to the detonation initiation mechanism. If the coming flow speed is low, hemisphere shock reflection and the shock-coupled flame surface are needed to initiate detonation wave. However, if the coming flow speed is high, hot jets may induce a DDT process to initiate a detonation wave.In investigation of detonation initiation induced by oblique shock wave, a phenomenon that the detonation wave propagates upstream has been observed. The mechanism of this phenomenon has also been studied. A mach reflection of detonation wave occurs on the wall is necessary for the detonation wave propagation upstream. The mach stem is an overdriven normal detonation wave in a convergent channel. Its speed is higher than theoretical CJ value. The conditions in which mach reflection occurs have also been shown.The boundary layer influences the detonation intensively as detonation propagating upstream. The boundary layer may separate near the wall. An overexpansion detonation initiation method is adopted. The detonation wave propagates upstream. The experimental results show that the detonation speed is higher than CJ speed when the separation zone is not very large. According to the numerical simulation results, the flow area is changed behind detonation surface. It is the main factor that increases the detonation speed. The convergence-divergence flow channel behind detonation wave may cause the detonation speed higher than CJ speed under some conditions. In experiment detonation wave decoupling and re-initiation have also been observed. This is because the separation zone is too large to sustain the detonation wave. Subsequently the shock waves behind the detonation wave re-initiate it again.