NONPREMIXED TURBULENT REACTING FLOW NEAR EXTINCTION

Direct numerical simulations (DNS) are used to study fundamental processes in non-premixed turbulent reacting flows. A broad range of Damkohler number, Da, and reaction zone thicknesses are studied, encompassing: stable, near-equilibrium reaction; local extinction; and global extinction. A simple thermochemical model for one-step reversible reaction is employed, in which the state of the fluid is described by the mixture fraction xi(x, t) and the perturbation from equilibrium y(x, t). A pseudo-spectral method, with grid sizes up to 128(3), is used to solve the Navier-Stokes equations and the conservation equations for xi and y. These equations are augmented with artificial forcing, so that the resulting velocity and mixture fraction fields are statistically stationary, homogeneous and isotropic. At sufficiently high Damkohler number, the perturbation field is also statistically stationary, corresponding to stable reaction. But at lower Da, y increases without bound, corresponding to global extinction. The critical Damkohler number Da(crit) at which global extinction occurs is determined from the simulations, and is found to be significantly different from that predicted by simple models (flamelet, conditional moment closure, etc.). A simple statistical model is used to show that the discrepancy can plausibly be explained by statistical variability. The simulation results are used to assess the accuracy of simple models, especially for Damkohler numbers at which there is globally stable reaction, but significant local extinction.