Effects of radiation models on steady and flickering laminar non-premixed flames

Abstract

Four laminar non-premixed methane/air flames with increasing Reynolds numbers are investigated using a newly developed conjugate combustion-radiation laminar flame solver. The baseline model includes a forward Monte Carlo ray tracing solver coupled with a line-by-line spectral model, a 16-species skeletal chemical mechanism and no soot model. A backward Monte Carlo solver is developed for the nongray inhomogeneous combustion mixtures and verified using the forward Monte Carlo solver. A 25-species skeletal chemical mechanism is adopted for parametric studies. Effects of soot radiation on unsteady flames are investigated using a two-equation soot model. Good agreement with experiments in radiative flux and radiant fraction is observed for all flames using the baseline model. The radiative heat flux predicted by the backward Monte Carlo solver agrees well with that obtained from the forward Monte Carlo solver, with well-controlled standard deviations. Contours of radiation-related scalars indicate that the laminar flames are within the optically-thin limit. Results obtained from conjugate combustion-radiation simulations with various radiation solvers and spectral models show minor differences for the two smaller flames. For the two unsteady flames, radiative re-absorption has a more significant impact on the overall flame structure, especially when soot is considered. Parametric studies using the longest flickering flame suggest that the choice of chemical mechanism, the consideration of radiation, and the inclusion of soot radiation, affect the prediction of the instantaneous flame height. The prediction of flame puffing frequency is insensitive to the variations in chemistry, soot and radiation models. The radiative heat flux prediction is sensitive to the thermal boundary condition of the fuel nozzle, the inclusion of soot and its radiation with re-absorption. It is recommended that soot should be considered for future studies when assessing radiation models using this set of target flames.