Effects of Soret Diffusion on Premixed Counterflow Flames

Abstract

Soret diffusion is a secondary mass diffusion and it affects laminar flames with very light or heavy species and large temperature gradients. To get a general understanding of Soret effects on stretched flames, we conduct theoretical analysis on a premixed counterflow flame with Soret diffusion. A deliberately idealized premixed counterflow flame model is analyzed asymptotically within the framework of large activation energy, potential flow, and thermal-diffusive model. A correlation among flame stretch rate, flame position, and flame temperature is derived and is used to assess the effects of Soret diffusion on counterflow flame structure, Markstein length, and extinction stretch rate. Results show that Soret diffusion quantitatively affects the premixed counterflow flame and that the influence of Soret diffusion strongly depends on Lewis number and stretch rate. For light fuels, the premixed counterflow flame becomes stronger after including Soret diffusion. The opposite trend occurs for heavy fuels. The influence of Soret diffusion is found to increase with the stretch rate. A linear change between normalized Markstein length and Soret diffusion coefficient is observed, indicating that flame becomes more sensitive to stretch rate after including Soret diffusion. Furthermore, Soret diffusion is shown to greatly increase the extinction stretch rate of light fuels. These results indicate that for highly-stretched premixed flames containing very light species, the impact of Soret diffusion cannot be neglected.