Effects of Soret diffusion on turbulent non-premixed H_2 jet flames

摘要

Recent studies have shown that Soret diffusion (SD), driven by temperature gradients, could play an important role in both laminar and turbulent premixed H-2 flames. However, comparatively little effort has been made to investigate SD effects on turbulent non-premixed H-2 flames, in spite of the relevance of these flames in safety and their potential application in clean power and propulsion systems. To this end, the impact of SD on turbulent non-premixed H-2 combustion is investigated numerically in this work by comparing two three-dimensional direct numerical simulations of temporally evolving turbulent jet flames. In one simulation, a mixture-averaged diffusion (MD) model is used to approximate multicomponent transport, while in the other the MD model is supplemented with a Soret term to consider SD effects. The emphasis is placed on examining and interpreting the impact of SD on flame structure, differential diffusion, and flame-tangential diffusion. It is found that H and OH mass fractions are significantly affected by SD, while SD has a negligible impact on temperature, heat release rate and H-2 mass fraction. This is due to the fact that larger SD flux of H radical is strongly coupled with the main chemical reactions. However, for H2 its larger SD flux is located in the fuel-rich zone and decoupled from the main reactions. The difference between a conserved scalar (mixture fraction Z) and a non-conserved scalar (Bilger mixture fraction Z(Bilger)) is employed as a diagnostic parameter to characterize differential diffusion, and the results show that the effects of SD on differential diffusion are reflected in two aspects: (i) increasing the absolute value of Z - Z(Bilger) and (ii) increasing the degree of misalignment between the gradients of Z and Z(Bilger). Furthermore, the analysis of the contribution of SD to flame-tangential diffusion occurring in mixture fraction isosurfaces indicates that for H radical SD can augment the relative contribution of flame-tangential diffusion, especially in the region of high scalar dissipation rate. On the other hand, for H-2, SD has a negligible impact on both flame-normal and flame-tangential diffusion. The present study contributes to providing insights into how SD affects turbulent non-premixed H-2 flames and into modeling SD effects with flamelet-based combustion models. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.