Nonlinear instabilities leading to rapid mixing and combustion in confined supersonic double-shear-layer flow

Abstract

Direct numerical simulations conducted in the present study show that a slow fuel gas stream issued between supersonic high-temperature air streams confined in a constant-area channel can mix with air quickly to cause explosive combustion along the following processes: (1) linear flaw instability excitation, (2) eddy formation without shocks, fuel flow acceleration to supersonic speed and enhanced mixing with air, associated with fuel layer meandering, (3) explosive combustion, and (4) thermally choked burnt gas flow. The underlying physics of the supersonic instabilities involved are revealed by interpreting the simulation results in an attempt to find an effective mixing enhancement technique.

The basic flow configuration consists of a confined, plane, double shear/mixing layer flow with forcing fluctuations at the inlet. The difference in velocities between inlet air and fuel streams is supersonic. The reflection condition imposed at the walls serves to disturb acoustically the double shear layer flow in such a way that the walls reflect Mach waves radiated from the inlet disturbance. The most unstable wave excited downstream is skew-symmetric with respect to the centerline, thus leading to the meandering of fuel layer accompanied by Karman-vortex-like eddies. A series of instability excitations couples with the fuel layer meandering in a confined flow region, enhances the exchange of momentum and species between the fuel and air streams, thus accelerating the mixture to a supersonic speed within a short distance prior to explosive combustion. The behavior of the flame front resembles that of lifted turbulent-jet flames. Flame flashback, stationary flame front and flame blowout take place, depending on the inlet condition. Their criteria are provided in terms of the Chapman-Jouguet detonation wave speed.