Geometric and radiation effects on steady and unsteady strained laminar flames

Abstract

A detailed numerical investigation was conducted on the effects of the domain size and radiation on thedynamics of opposed jet, strained, laminar premixed, and diffusion flames. The simulation was performed by solving the conservation equations along the stagnation streamline of the counterflow and by using detailed description of the chemistry, molecular transport, and nonluminous thermal radiation at the optically thin limit. Results indicate that the hydrodynamic extinction strain rate increases with the nozzle separation distance because of the reduction of the values of the strain rate distribution within the main reaction zone. The predictions of extinction strain rates in the present study agree well with available experimental data on premixed flames, after using nozzle separation distances equal to the experimental ones. Furthermore, the inclusion of the Soret effect was found to increase the predicted strain rates for near stoichiometric flames. The effect of radiation was assessed for both steady and unsteady flames. The unsteadiness was introduced through sinusoidal variation of the nozzle exit velocities around some mean values. The results indicate that the effect of the radiation on the flame response and extinction becomes important only for near-limit premixed flames and weakly strained diffusion flames, which are characterized by large thicknesses. An asymmetry was also identified in the response of unsteady diffusion flames for which the trough value of the strain rate is near-zero, while the peak value causes substantial straining. The results were explained based on the competition between the mechanisms of reactant leakage and radiative loss as the strain rate is reduced.