Direct numerical simulations of strained laminar and turbulent nonpremixed flames: Computational and physical aspects.

Abstract

For direct numerical simulations of compressible reacting flows, a generalized formulation of the characteristic boundary conditions is proposed. The improved approach resolves several issues of spurious solution behavior encountered in compressible flow simulations. This is accomplished by accounting for all the relevant terms in the determination of the characteristic wave amplitudes and by accommodating a relaxation treatment for the transverse terms with a coefficient determined by the low Mach number asymptotic expansion. The improved boundary conditions are applied to a comprehensive set of test problems and are demonstrated to perform consistently superior to existing approaches. Dynamics of edge flames encountered upon local quenching of nonpremixed flames is studied considering detailed chemistry of hydrogen-air combustion. The density-weighted displacement speed and scalar dissipation rate are found to be appropriate parameters in characterizing the edge flame speed. It is also found that the edge flame speed depends strongly on the transient and history effects of the flow field in addition to the local scalar dissipation rate. Negative edge speed is observed during the early phase of the interaction due to the transverse enthalpy loss induced by large strain. The effects of fuel Lewis number on the edge speed are found to be consistent with the previous theoretical predictions. The dynamics of soot formation in ethylene-air nonpremixed counterflow flames is studies using a semi-empirical soot model and a radiation model based on the discrete ordinate method. Transient characteristics of soot behavior are studied in both laminar and turbulent counterflow configurations. The detailed analysis reveals that the soot number density depends predominantly on flame temperature, while the soot volume fraction is more sensitive to the surface growth mechanism such that it depends on the combined effects of the local conditions of flow, temperature and fuel concentration. The results suggest that accurate prediction of soot volume fraction in turbulent combustion requires consideration of transient and history effects on the evolution of each soot particle.