可压缩燃烧反应转捩混合层直接数值模拟

DIRECT NUMERICAL SIMULATION OF A COMPRESSIBLE TRANSITIONAL MIXING LAYER WITH COMBUSTION CHEMICAL REACTIONS

针对三维时间发展可压缩氢/氧非预混燃烧反应平面自由剪切混合层,采用5阶迎风/6阶对称紧致混合差分格式以及3阶显式Runge-Kutta时间推进方法,直接数值模拟了伴随燃烧产物生成和反应能量释放,流动受扰动激发失稳并转捩的演化过程.在转捩初期,获得了Λ涡、马蹄涡等典型的大尺度拟序结构,观察到了流动失稳后发生双马蹄涡三维对并的现象,大尺度结构呈较好的对称性.在流动演化后期,大尺度结构逐次破碎形成小尺度结构,混合层进入转捩末期,呈明显的不对称性.

A fifth-order upwind/sixth-order symmetric compact hybrid difference scheme coupled with a third-order explicit Runge-Kutta time-marching method is used as a direct numerical simulation algorithm to investigate a three-dimensional temporally-developing compressible plane free shear mixing layer with H2/O2 non-premixed combustion. The reacting mixing layer with product formation and energy release is perturbed by a pair of conjugate oblique waves, and hence experiences an instable transitional evolution. At the beginning of transition, some well-known large scale coherent structures, such as A vortex and horseshoe vortex, are found, and a three-dimensional paring phenomenon of two horseshoe vortices following flow instabilities is also revealed. Furthermore, these large scale structures are approximately symmetric. At the last stage of the development of this flow, large scale structures break down continuously, and small scale structures gradually get dominant. The reacting mixing layer finally behaves very similar to turbulence and shows clear asymmetry. On the analogy of the stability theory for incompressible boundary layer, the large scale structures obtained are due to a subharmonic secondary instability mechanism, which is well known as Herbert type secondary instability.