A UNIFORM STRAIN MODEL OF ELEMENTAL FLAMES IN TURBULENT COMBUSTION SIMULATIONS

The paper describes a multistep chemistry, combustion-zone submodel fo r turbulent combustion simulation which can be used in the regime of f lamelet combustion. The flowfield in the submodel is simplified by ass uming a uniform strain rate across the flame structure as opposed to t he variable strain observed in the corresponding boundary-layer soluti on. This assumption allows the decoupling of the momentum equation fro m the energy and species concentration equations, and, together with a series of coordinate transformations, the reduction of the system of governing equations to a set of reaction-diffusion equations which can be accurately and easily integrated. Study shows that the accuracy of the model can be greatly improved if an effective uniform strain rate instead of the imposed strain rate at the far field is used. We devel op an expression for the effective uniform strain rate as a function o f the flow-imposed strain and the adiabatic flame temperature. Results obtained over a wide range of strain rates show that the results of t he uniform strain-rate model agree well with those obtained using the boundary-layer approximation in terms of the steady-state flame struct ure and evolution of the burning velocity in response to a stepwise ch ange in strain rate for both premixed and diffusion flames. It is also shown that the uniform strain-rate model with reduced chemical kineti cs is able to produce the extinction S curve, steady-state profiles, a nd a transient response which are close to that of the exact solution. The importance of the dynamics of flame-flow interaction is shown in the paper by comparing the actual response of a flame to sudden variat ions in strain rate with the quasisteady response implied in the ''fla melet library'' approach. It is shown that the latter could lead to er rors in the prediction of burning velocity. (C) 1997 by The Combustion Institute.