Effects of Soret diffusion on turbulent non-premixed H2 jet flames

Abstract

Recent studies have shown that Soret diffusion (SD), driven by temperature gradients, could play an important role in both laminar and turbulent premixed H2 flames. However, comparatively little effort has been made to investigate SD effects on turbulent non-premixed H2 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 H2 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 H2 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 ZBilger) 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−ZBilger and (ii) increasing the degree of misalignment between the gradients of Z and ZBilger. 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 H2, 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 H2 flames and into modeling SD effects with flamelet-based combustion models.