The effect of turbulent mixing and differential diffusion on the autoignition delay of a H_2/N_2 jet flame in vitiated coflow using Linear Eddy Model

摘要

The effect of turbulent mixing and differential diffusion on the autoignition delay of a H_2/N_2 jet flame in vitiated coflow is investigated using the Linear Eddy Model. The main objectives of the current work are to investigate numerically the effect of turbulent mixing and differential diffusion on ignition delay under atmospheric pressure and elevated pressure conditions. A broad range of turbulent Reynolds numbers is investigated. It is found that differential diffusion simulations always ignite faster than the equal diffusivity computations. The difference between the two transport models diminishes with increasing the turbulence intensity. The autoignition of the jet flame demonstrates a high sensitivity to the coflow temperature especially under elevated pressure conditions. The effect of turbulent mixing on the ignition delay is classified in two regimes depending on the autoignition delay and the turbulent time scale. Within one regime, mixing is the rate-limiting process and the ignition delay is decreasing with increasing turbulence. The other regime is distinguished by the initial conditions and the early history of scalar dissipation rate at the ignition onset location. Turbulence intensity has only a small effect. These two regimes can be differentiated for all pressures considered.