Investigation of the influence of the Reynolds number on extinction and reignition

摘要

In this study comparisons are made of the results of a series of simulations for different values of the Reynolds number, but with fixed Schmidt and Prandtl numbers (Sc = Pr = 0.7) and identical chemistry and approximately identical large-scale dynamics and initial scalar geometry. The goal is the investigation of the influence of the change in the diffusive scales on local extinction and reignition in a diffusion flame. Extinction occurs when the fluctuations of the scalar dissipation rate at the stoichiometric value of the mixture fraction (χ_(st)) become larger than a threshold value (χ_q). The fluctuations of the scalar dissipation rate reach higher values with increasing Reynolds number, but the probability that χ_(st) > χ_q does not increase with Reynolds number at the two highest values of the Reynolds number studied. The latter effect is related to the lognormal nature of the PDF of the scalar dissipation rate. As the fluctuations of χ_(st) become increasingly larger than χ_q, extinction takes place faster and results in lower temperatures. Reignition, however, appears to take place approximately similarly in the different cases. The Reynolds number dependence of the average temperature of the stoichiometric surface is mostly due to the increase in extinguished area fraction of the stoichiometric surface. Although the amount of extinction increases as the Reynolds number is increased, the rate of increase appears to slow down with increasing Reynolds number.