Flame brush thickness of lean turbulent premixed Bunsen flame and the memory effect on its development

Abstract

Flame brush thickness is a significant parameter that demonstrates the evolution of turbulent premixed flame. In this paper, the flame height and flame brush thickness of lean turbulent premixed Bunsen CH4/air and C3H8/air flames were investigated under low turbulence intensities. The turbulence flow field is generated and controlled by changing the outlet velocity and perforated plates, and it is measured using a hot wire anemometry system. The turbulent flame fronts are detected by OH-PLIF technique. Results show that under fuel-lean conditions, the characteristic turbulent Bunsen flame height and centerline flame brush thickness decrease with increasing equivalence ratio, while they increase with the augment of outlet velocity. Turbulence intensity has marginal effect on these two parameters. The variations of flame brush thickness indicate that there are three regions during the whole development of turbulent Bunsen flames under low turbulence intensity. The first is the turbulence dominating region, where the variations of horizontal flame brush thickness follow the turbulence diffusion theory. The second is the non-local effect manifesting region, where the equivalence ratio, which controls the flame instability wavelength, influences the horizontal flame brush thickness. The third is the flame tip region where the flamelets intersect and make the radial wrinkles offset while the axial wrinkles superimpose. This unique tip region of turbulent Bunsen flame can be demonstrated by the linear relationship between its flame height and centerline flame brush thickness. Turbulent premixed Bunsen flames suffer different non-local effects during the flame development to downstream. To minimize the non-local effect on the statistics of turbulent flame front structure, a new method based on the development time period is proposed. Compared with statistical analysis based on the whole development time, this method can better demonstrate the flame-turbulence interaction.