Experimental and Theoretical Investigation of the Dilution, Pressure and Flow-Field Effects on the Extinction Condition of Methane-Air-Nitrogen Diffusion Flames

摘要

Laminar opposed flow diffusion flames, established in the forward stagnationregion of a porous cylinder or between two opposed jets from circular tubes, have been used extensively to study diffusion-flame structure and extinction, in order to quantify the effect of flame stretch on the interaction of transport and chemical processes. The results provide valuable physical insight and quantitive data for application of laminar flamelet concepts in modeling turbulent diffusion flames. At fixed nozzle separation distance, increasing the opposed-jet exit velocities increases the axial velocity gradient (strain rate) and the fuel and oxidizer concentration gradients in the mixing layer, thereby decreasing the local diffusion time in the vicinity of the flame. The second Damkohler number, defined as the ratio of the diffusion time to chemical reaction time, also decreases, subjecting the flame increasingly to nonequilibrium effects and eventually resulting in stretch-induced extinction. The critical strain rates beyond which the flame cannot be stabilized have been studied experimentally and theoretically.