Temperature and mixing field measurements in stratified lean premixed turbulent flames

Abstract

Turbulent combustion under lean stratified conditions appears in many practical applications. However, relatively few experimental or theoretical studies have investigated stratified flames in detail. We present qualitative mixing and quantitative temperature field measurements of jet-like flames in a canonical burner geometry with the focus on comparisons of stratified and premixed flames. Planar laser-induced fluorescence of OH and acetone are used to mark the flame front and the mixing field, respectively. Laser Rayleigh scattering is used to probe the detailed structure of the temperature field. The burner consists of three staged concentric tubes, and flame stabilization is facilitated by a pilot flame burning inside the central tube. This arrangement minimizes the effects of the pilot flame on the shear and stratification layer in-between the two annular slots. Operating conditions are selected in accordance with previous flow field studies in this burner. Shear and stratification between the annular slots are varied independently from each other. Comparisons between stratified and premixed flames show that differences emerge primarily at large scales and are dominated by shear layers. The effect of stratification appears in instantaneous temperature profiles but is washed out in radial profiles of mean and rms temperatures. In combination with earlier flow field measurements, these data serve as well for validation of numerical simulations.