Effects of lewis number on extinction behavior of premixed flames in a stagnation flow

Abstract

The combined effects of the velocity gradient and the Lewis number of deficient reactantsin the premixed gas on the behavior and extinction of a premixed flame in a stagnation flow were studied experimentally. The flat twin flames established in the stagnation region of the two dimensional opposed flow of the same mixture were used in order to make an adiabatic and noncatalytic stagnation surface. The flame extinction behavior observed is purely attributed to the flow-transport properties-chemical reaction interaction. The mixture employed was hydrogen, methane, propane, or butane with air. The flame temperature and the distance between the two flames were measured for the wide variations of the fuel concentration and the velocity gradient. As the stagnation velocity gradient was increased by increasing the opposed flow velocity, the flat two flames approached each other, and finally the extinction occurred abruptly. By observing the flame behavior, the extinction process of the flame was classified into two patterns corresponding to the ranges of the Lewis number Le of the deficient reactant in the premixed gas (fuel in the lean mixture and oxygen in the rich mixture). Its ranges are Le 1. Under the condition of Le 1 (lean hydrogen or methane-air and rich propane or butane-air), as the velocity gradient is increased, the flame temperature increases and reaches a maximum point at a certain value of the velocity gradient, beyond which the flame temperature decreases. The flame is extinguished close to the stagnation surface or in contact with the stagnation surface. The results of this experiment are in good agreement with the theoretical predictions.