Effect of Variable Strain on the Dynamics of Diffusion Flame Ignition

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Abstract
In many combustion applications, the strain rate determines the flame response. Experimental evidence and theoretical arguments indicate that the strain rate defines the conditions of ignition in many flow configurations. Ignition in time-variable strained flows is analysed in this article. The response of a reactive element submitted to a time dependent strain rate is considered with numerical and asymptotic methods. The results obtained for this generic problem are then used to predict the ignition of non-premixed shear flows formed by hot and cold reactants. The plane mixing layer and coaxial jet formed by streams of hydrogen and hot air are specifically considered. The spatial distribution of the typical strain rate is first determined and a transformation into a local frame of reference is used to describe a spatially evolving strain rate as a time-dependent quantity. This provides the complete strain rate history for an initial reactive interface. Asymptotic analysis gives the ignition time which is then transformed back into an ignition distance. A good agreement is found between theoretical results and four test cases pertaining to supersonic combustion experiments.