Numerical Simulations of Low Temperature Ignition Chemistry with Flow, Temperature, and Species Fluctuations in High Pressure Counterflow Flames

摘要

A numerical model for the simulation of the low temperature ignition with the flow, temperature, and species turbulent fluctuations in a high pressure counterflow diffusion flame is developed. In order to model the turbulent fluctuations, both time and space dependent perturbations are introduced. There are two types of perturbations studied in this research. The first one is to perturb the molar fraction of H_2O_2 to mimic the effect of exhausted gas recirculation and the second one is to perturb the strain rate (G) to model the velocity and temperature fluctuations in the turbulent combustion. In the simulation the perturbation size is fixed at 0.1 cm but the perturbation frequency varies from zero to several thousands hertz. The effects of these perturbations on both low and high temperature ignitions were investigated for uniform and nonuniform fuel and oxidizer boundary temperature cases. The results showed that (1) the H_2O_2 perturbation for the uniform temperature case causes a non-monotonic decrease of the high temperature ignition delay time with a critical perturbation frequency at which the high temperature ignition delay time is the minimum; this nonmonotonic decrease is due to the acceleration of the low temperature kinetics and the deceleration of the transition from the low to high temperature ignition by the perturbation according to the reaction pathway analyses. (2) The strain rate perturbation for the uniform temperature case causes a non-monotonic increase of the high temperature ignition delay time with a critical perturbation frequency near which the high temperature ignition is dramatically delayed as a result of the susceptibility of the transition from the low to high temperature ignition on the strain rate. (3) The strain rate perturbation for the nonuniform temperature case can introduce the temperature fluctuation and therefore both the low and high temperature ignitions can be either accelerated or decelerated due to the strong dependence of the chemical kinetics on the temperature. The above results are first presented and can shed a light on the understanding of the turbulence/chemistry interaction in the turbulent combustion.