Experimental research on the flow field and flame geometry of free buoyant diffusion flames with low dimensionless heat release rates

Author：Yuan, Y. K.， Lei, J.， Liu, N. A.

Journal：Combustion and Flame

DOI:  10.1016/j.combustflame.2024.113659

Keywords: Free buoyant diffusion flame, Instantaneous velocity field, Turbulent viscosity, Flame height, Flame width, fire, entrainment, dynamics, height, Thermodynamics, Energy & Fuels, Engineering

Abstract: 

The flame geometry of turbulent diffusion flames is dominated by the turbulent mixing between the gaseous fuel and the entrained air. The significant variations of the power law for the mean flame height regarding the dimensionless heat release rate (Q(center dot)& lowast;) in different Q(center dot)& lowast; ranges implied that the turbulent mixing characteristics would change greatly among these Q(center dot)& lowast; regimes. This paper presents a combined experimental and analytical study to reveal the turbulent transport properties and flame shape of free turbulent buoyant diffusion flames of relatively low Q(center dot)& lowast; prevalent in large-scale fires. The instantaneous velocity fields of free buoyant methane diffusion flames (burner diameter: d = 0.30 m, Q(center dot)& lowast;= 0.22-0.40) were measured by the stereo particle image velocimetry (SPIV) technique. The results indicate that the radial profiles of the time-averaged axial velocity, axial normal stress, radial normal stress, shear stress, and turbulent kinetic energy exhibited self-similarity at different heights under all the heat release rates. The turbulent flow in the buoyant flame was found to be anisotropic. Based on the gradient transport approximation, the mean turbulent viscosity within the mean flame height (nu=t ) was scaled by g1/2d3/2, where g is the gravitational acceleration. The semi-physical correlations for the mean flame height and width were derived based on the concepts of turbulent mixing and equal axial convection and radial diffusion times, respectively. The two correlations agreed reasonably with the experimental data of this work.