Impact of multi-component diffusion in turbulent combustion using direct numerical simulations

Abstract

This paper presents the results of DNS of a partially premixed turbulent syngas/air flame at atmospheric pressure. The objective was to assess the importance and possible effects of molecular transport on flame behavior and structure. To this purpose DNS were performed at with two proprietary DNS codes and with three different molecular diffusion transport models: fully multi-component, mixture averaged, and imposing the Lewis number of all species to be unity.Results indicate that1.At the Reynolds numbers of the simulations (Returb = 600, Re = 8000) choice of molecular diffusion models affects significantly the temperature and concentration fields;2.Assuming Le = 1 for all species predicts temperatures up to 250 K higher than the physically realistic multi-component model;3.Faster molecular transport of lighter species changes the local concentration field and affects reaction pathways and chemical kinetics.A possible explanation for these observations is provided in terms of species diffusion velocity that is a strong function of gradients: thus, at sufficiently large Reynolds numbers, gradients and their effects tend to be large. The preliminary conclusion from these simulations seems to indicate molecular diffusion as the third important mechanism active in flames besides convective transport and kinetics. If confirmed by further DNS and measurements, molecular transport in high intensity turbulent flames will have to be realistically modeled to accurately predict emissions (gaseous and particulates) and other combustor performance metrics.