Modelling of Conditional Scalar Dissipation Rate in Turbulent Premixed Combustion

Abstract

In turbulent premixed flames, for the mixing at a molecular level of reactants and products on the flame surface, it is crucial to sustain the combustion. This mixing phenomenon is featured by the scalar dissipation rate, which may be broadly defined as the rate of micro-mixing at small scales. This term, which appears in many turbulent combustion methods, includes the Conditional Moment Closure (CMC) and the Probability Density Function (PDF), requires an accurate model. In this study, a mathematical closure for the conditional mean scalar dissipation rate, <Nc|ζ>, in Reynolds, Averaged Navier–Stokes (RANS) context is proposed and tested against two different Direct Numerical Simulation (DNS) databases having different thermochemical and turbulence conditions. These databases consist of lean turbulent premixed V-flames of the CH4-air mixture and stoichiometric turbulent premixed flames of H2-air. The mathematical model has successfully predicted the peak and the typical profile of <Nc|ζ> with the sample space ζ and its prediction was consistent with an earlier study.

Modelling of a Bluff-Body Stabilised Premixed Flames Close to Blow-Off

“…It signifies the dissipation rate of progress variable variance, which is predominantly affected by the coupling of turbulence, diffusion, and chemical reaction. Hence, this work aimed to implement an alternative model [21] for this term in the RANS-CMC framework and to use the method to compute lean premixed bluff-body stabilised turbulent flame. The PDF of the progress variable was modelled using a presumed shape with a beta function.…”

“…This assumption is invalid for some flames with high Damkhøler numbers. Unlike the algebraic model, the proposed model is based on a mathematical approach [21]; therefore, it can be applied to a broad spectrum of combustion regimes.…”

“…On the other hand, the contribution of CMC for premixed combustion would be predicting species with slow time scales, including the effects of turbulence on the chemical structure with complex chemical kinetics [19,20]. The proposed model for the conditional scalar dissipation rate term in the CMC transport equation is based on a mathematical approach and has no preassumptions [21].…”