LES/TPDF investigation of the role of reaction and diffusion timescales in the stabilization of a jet-in-hot-coflow CH4/H2 flame

Abstract

Moderate or Intense Low oxygen Dilution (MILD) combustion is a promising technology to meet the ever-stringent emission regulation while maintain high thermal efficiency. In this study, large eddy simulation (LES) in conjunction with transported probability density function (TPDF) method has been carried out for the first time to investigate the impact of reaction and diffusion timescales on the stabilization process of the jet-in-hot-coflow (JHC) CH4/H2 flame emulating MILD conditions. First it is demonstrated that the LES/TPDF simulations yield improved predictions of the species and temperature fields due to its capability in capturing finite-rate chemistry and resolving molecular transport at the filter scale. Then the impact of reaction and diffusion timescales on the stabilization process are investigated. It is found that the attenuation of chemical kinetics results in larger stabilization heights and unstable flame bases. More importantly the variation of stabilization height is found to be linearly proportional to that of auto-ignition delay time, illustrating the crucial importance of chemical kinetics during flame stabilization. The results show that the flame is initiated from the lean mixture away from the shear layer, which implies the importance of molecular transport during flame stabilization. Particle-level budget analysis further shows that the resolved molecular diffusion is important for flame base dynamics by contributing more than half of the overall conditional diffusion rate. Finally, a scaling rule for the characteristic flame stabilization time is proposed based on the auto-ignition delay time and characteristic time of diffusion, and it works reasonably well for all the cases considered. These findings shed light on the key physico-chemical mechanisms of the stabilization process for JHC flames under the MILD combustion mode. Moreover, the assessment on subgrid mixing and resolved molecular diffusion reveals that the simulation exhibits low sensitivity to the mixing model and mixing timescale while being highly sensitive to the resolved molecular diffusion, highlighting the key modelling aspects related to LES/TPDF simulation of this flame.