Investigation Of The Unsteady Characteristics Of The Cavity Shear Layer In Supersonic Combustion

The unsteady characteristics of two-dimension cavities shear layer in supersonic combustion were numerically simulated by the method of hybrid RANS/LES. The turbulence model for RANS is two-equation SST and the sub-grid scale model for LES is one-equation Yoshizawa. The influences of the length-depth ratio, the back ramp angle, the transverse fuel injection upstream of the cavity and the heat release of combustion on the shape and oscillation characteristic of the shear layer were especially studied. Then the ignition and combustion characteristics of the cavities were studied experimentally by means of spontaneous radiation imaging and high speed photography.In single supersonic flow, both of the length-depth ratio and back ramp angle have obvious influences on the shear layer. The first one mainly influences the evolvement and shape of the shear layer, and the latter mainly influences the intensity of cavity-shear layer self-oscillation. The shear layer of the long cavity develops more sufficiently, so it becomes severely unstable and surges intensively. The cavity with vertical back ramp oscillates more intensively because of the strong interaction of the compress wave with its shear layer, which is caused by the low effectiveness of the ejection of the impingement energy.In the case of the transverse fuel injection upstream of the cavity, the cavity shear layer becomes thicker and rises lightly, the cavity-shear layer self-oscillation becomes weaker, the oscillation energy shifts to higher frequency, and the influence of the back ramp angle becomes weaker.After the join of the combustion, the cavity shear layer keeps becoming thicker and rising obviously, the impingement shock wave on the back ramp disappears, the cavity-shear layer self-oscillation becomes much weaker, the oscillation energy shifts to lower frequency rapidly. The randomicity of pressure oscillation greatly increases, which may indicate the appearance of chaos. The mixing and combustion performances of the long cavity are obviously better than the short ones.It is showed in the experiments that for the supersonic combustion chamber with transverse fuel injection upstream of the cavity, the ignition and flame-holding characteristics have great correlation with development of the cavity shear layer and the injection mixing layer as well as the interaction of them. So the parameters of the injection such as position, pressure and flux must be matched with the development of the cavity shear layer to gain the best performances.It is better to inject fuel and air into the cavity directly in a proper way and organize combustion there, which could greatly reduce the influence of the uncertainty of the flow field and make the ignition and flame-holding function of the cavity act more effectively.