Nonpremixed combustion in an accelerating transonic flow undergoing transition

Mixing layers composed of fuel and air streams passing through straight and curved channels are studied by performing two-dimensional numerical simulations. Both non-reacting and reacting mixing layers are studied. For the mixing layer without imposed streamwise acceleration, the flow remains at low subsonic speed throughout the channel. For the mixing layer with imposed streamwise acceleration, the flow accelerates from low subsonic speed at the inlet to low supersonic speed at the exit. In this research, we focus on the development of the mixing layer from laminar flow to the transition regime. The full Navier-Stokes equations coupled with multiple-species equations and the energy equations with chemical reactions are solved using a finite difference numerical scheme. All the reacting cases are more unstable than the corresponding non-reacting cases based on the turbulent kinetic energies. The reacting mixing layer exhibits large local streamwise velocity peaks which become more pronounced when imposed favorable streamwise pressure gradient is present. Positive and negative vorticities are generated in the reacting mixing layer by the baroclinic effect associated with the large density gradient across the combustion zone. For the curved mixing layers, the centrifugal and R-T instabilities can coexist. The centrifugal effect associated with the velocity profile and the R-T mechanism can individually impose a stabilizing effect on one side of the peak but a destabilizing effect on the other side. The instability mechanism becomes even more complicated in the situation where multiple streamwise velocity and density peaks exist across the transverse direction. For the flow parameters and the channel dimensions considered in this research, we have found that the mixing layer with faster and lighter air stream on the outside of the curve is more unstable than the mixing layer with heavier and slower fuel stream on the outside of the curve. The imposed streamwise acceleration tends to stabilize the mixing layer and delay the roll-up of the mixing layer.