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Abstract not provided.

Abstract not provided.

Most attempts to quantify differential diffusion (DD) are based on the difference between different definitions of the mixture fraction. This paper presents a general method for evaluating differential diffusion in premixed or nonpremixed systems based on conservation equations for the elemental mass fractions. These measures form a basis for analysing differential diffusion. Casting these in terms of a mixture fraction gives particular insight into differential diffusion for nonpremixed systems, and provides a single DD measure. Furthermore, it allows direct evaluation of the validity of the traditional assumptions involved in writing a mixture fraction transport equation. Results are presented for one-dimensional opposed flow simulations of hydrogen and methane flames as well as direct numerical simulations (DNS) of CH4/H2-air and CO/H2-air flames. For a common definition of the mixture fraction, the DD measure can be approximated well by considering only the contribution of H2 and CH4 in methane-air flames. Differential diffusion is largely driven by production of H2 in the flame zone for hydrocarbon flames. Effects of strain rate and filter width on the relative importance of differential diffusion are examined. © 2005 Taylor & Francis Group Ltd.