EFFECT OF VARIABLE STRAIN ON THE DYNAMICS OF DIFFUSION FLAME IGNITION

In many combustion applications, the strain rate determines the flame response. Experimental evidence and theoretical arguments indicate tha t the strain rate defines the conditions of ignition in many flow conf igurations. Ignition in time-variable strained flows is analysed in th is article. The response of a reactive element submitted to a time dep endent strain rate is considered with numerical and asymptotic methods . The results obtained for this generic problem are then used to predi ct the ignition of non-premixed shear flows formed by hot and cold rea ctants. The plane mixing layer and coaxial jet formed by streams of hy drogen and hot air are specifically considered. The spatial distributi on of the typical strain rate is first determined and a transformation into a local frame of reference is used to describe a spatially evolv ing strain rate as a time-dependent quantity. This provides the comple te strain rate history for an initial reactive interface. Asymptotic a nalysis gives the ignition time which is then transformed back into an ignition distance. A good agreement is found between theoretical resu lts and four test cases pertaining to supersonic combustion experiment s.