On adaptively reduced chemistry in large eddy simulations

Abstract

We demonstrate the use of adaptive chemistry in large scale turbulent combustion simulations, in this case a large eddy simulation (LES) of turbulent non-premixed ethylene flames. An additional transport equation for the mixture fraction is solved, enabling the chemistry to be treated using the methodology of conditional moment closure (CMC). In this framework the chemical kinetics can be reduced on-the-fly on a CMC sub-grid. The reduction method in hand is based on a time scale analysis, where species in an equilibrated state are constrained by the leading species treated in detail in the simulation. The equilibrated species are determined adaptively dependent on the local mixture fraction composition, which is divided into four carefully selected CMC sub-grid domains from fuel lean to fuel rich. The selection of constrained species is based on a pre-calculated level of importance (LOI) matrix, where the mathematical formulation of the resulting reduced chemistry is the Rate-Controlled Constrained Equilibrium (RCCE). Since concentrations are calculated also for the equilibrated species throughout the simulation, transition between domains can be treated directly. The total number of constrained (leading) species is kept constant, but the species which are treated in detail differ between the domains, and hence also between regions in the flame. Combining these procedures we find that the number of constrained species can be as low as 15 (out of a total 75 species) without losing important structures and details of the flame, including profiles of important radicals.