Light refraction effects in counterflow non-premixed flames

Abstract

This paper presents the influence of light refraction on the broadening of CH∗ and C2 ∗ species experimental profiles in laminar counterflow non-premixed flames. In fact, by comparing CH∗ and C2 ∗ experimental and numerical profiles in a counterflow configuration, a broadening of these species experimental profiles is observed, and these species are frequently employed experimentally to determine important macroscopic combustion properties. Therefore in this work, in order to give an explanation of these phenomena, light refraction due to a high temperature gradient was considered. The Gladstone–Dale relation was used to estimate the medium refractive index along the burner axis, taking into account the gas density and composition. Then, a simple procedure for light refraction estimation of rays reaching the solid angle of the camera in counterflow non-premixed flames was proposed. Finally, the influence of refraction of light on CH∗ and C2 ∗ species thicknesses appears to be significant depending on the operating conditions of counterflow non-premixed flames. However, taking into account the difference between the experimental and numerical profiles thicknesses, this effect is not fully responsible for the experimental broadening. Nonetheless, the same procedure described here can be used in order to study the light refraction for other experimental configurations, such as those of transcritical flames, where the gas density variation is much higher, and consequently the light refraction would be greater.