A study of the turbulent structure of a two-dimensional diffusion flame formed behind a slender bluff-body

Abstract

An experimental investigation of confined turbulent reacting wake flows past a 2D square cylinder with planar fuel-jet injection into the vortex formation region is presented. Measurements of mean and turbulent velocities and temperatures and a sample of results on related statistics, obtained with laser velocimetry and thin digitally compensated thermocouples throughout the wake region for two Reynolds numbers and for a range of fuel-to-air velocity ratios (FAVR), are discussed. Counterpart isothermal flows with and without air-jet injection are also documented to facilitate a discussion of the effect of combustion on the turbulent wake aerodynamics. The study has shown that 2D slender bluff-body stabilized diffusion flames differ from their axisymmetric counterparts in that they produce longer recirculation zones and flame lengths. Their peak temperature and turbulence levels are located at the forward stagnation point, away from the burner face. In contrast to axisymmetric geometries, large-scale activity and periodic shedding are here drastically suppressed at medium and low FAVR operation. These intensify as the flame length shortens and global extinction conditions are approached, in a fashion similar to premixed bluff-body stabilized flames. On the basis of measured mean vorticity and turbulent intensity distributions within the recirculation region, a mechanism of vortex shedding suppression is suggested for the reacting wake. Vortex strengths, the double vortex structure, and the linear flame length dependence on FAVR are more similar for both geometries.