Direct Numerical Simulation of a Compressible Turbulent Mixing Layer with Combustion Chemical Reactions

Abstract

Scientists paid a long-term attention on the non-premixed compressible combustion mixing layer which can interpret the mechanism of inner flow of Scramjet. Except mass diffusion and compressibility, energy release makes the mechanism of the transition intricate. Proverbially, heat release impacts instability of the flow, changes gradient of density or pressure, enhances the viscosity. But how do these work? 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 evolution from transition to full turbulence. At the beginning of transition, some well-known large scale coherent structures, such as Λ vortex and horseshoe vortex, are found, as well as the results of ideal gas simulations. It means that the powerful inflexion non-viscosity instability dominates both the real gas flow and ideal gas flow. By studying the ideal gas, mechanism of the scramjet can be made known. Then dual-vortex paring phenomenon following flow instabilities is also revealed. In common sense, transition of the shear flow come through from large scale structures to turbulence directly; Sandham advanced a view that dual-vortex paring phenomenon and quadric instability would occur at the top of the Λ vortex in compressible mixing flow. The simulations show that guess of Sandham is correct, and vortex pairing is an essential step in the transition of shear flows. In the later stages of the development of this flow, large scale structures break down continuously, and small scale structures gradually get dominant. The reacting mixing layer finally becomes fully developed turbulence and shows clear asymmetry. At the dense area of vorticity, rate of the reaction is enhanced. Simulations of spatial evolution of compressible mixing layer with combustion are carried out to validate the results of time-development simulations. The validations are in accord with the former results.