Effect of freestream turbulence on the structure of boundary-layer flames

Abstract This study investigates the role of turbulence induced instabilities on three-dimensional boundary-layer flames. Boundary-layer diffusion flames under 0.99–2.43 m/s crossflow velocities were produced over a gaseous propane line burner. Incoming flows with turbulence intensities ranging from 0.5% to 16.8% were generated in a wind tunnel with fine wire meshes and perforated grids. It was observed that freestream turbulence initiated an earlier onset of visible coherent flame streaks, enlarged the initial streak spacing, and accelerated the growth of the streak spacing along the streamwise direction. Both spacing and fluctuation frequency of the flame streaks showed a nearly quadratic growth at high turbulence intensities. This instability promoted the transition of flames to a turbulent state, which ultimately modified the overall flame heating dynamics. The forward attachment length of the flame was found to be negatively correlated to the turbulence intensity; a dimensionless relationship was proposed to correlate the flame attachment length based on the Froude number, heat-release rate, and freestream turbulence intensity. Two heating modes, a momentum-dominated and a plume mode, were observed and found to be segregated by a critical Richardson number. The downstream heat flux was found to increase from 30 to 40  kW / m 2 in the momentum-dominated regime, when flow turbulence intensity changed from less than 1% to a level of 14.9–16.8%. Finally, it was observed that placing a bar upstream of the burner tripped the flow to the point where the downstream flame structure closely resembled flames under the highest turbulence intensity investigated, suggesting a simplistic configuration for future study. Ultimately, these results may improve our understanding of the structure and heating dynamics of boundary-layer flames as they transition to turbulence.