Effects of fuel mass flux and burner boundary condition on turbulent transport properties and flame shape of free turbulent buoyant diffusion flames

This paper presents an experimental and analytical investigation of the turbulent transport and flame geometric characteristics of free turbulent buoyant diffusion flames under different fuel mass fluxes and burner boundary conditions (i.e. with/without a flush floor). The stereo particle image velocimetry technique was utilised to measure the three-dimensional instantaneous velocity fields of the free methane buoyant flames with a burner diameter ( d ) of 0.30 m and dimensionless heat release rates ( $\dot{Q}^{*}$ ) of 0.50–0.90. The results showed that, compared with the configuration without a floor, the time-averaged axial velocity fluctuations squared and the time-averaged radial velocity fluctuations squared decreased, and the peak values of the time-averaged radial velocity, the time-averaged radial velocity fluctuations squared and the time-averaged axial and radial fluctuation product shifted towards the burner centreline in the configuration with a flush floor. Based on the dimensional analysis and the gradient transport assumption, the mean turbulent viscosity within the mean flame height ( $\nu _{t}^{=}$ ) was scaled. Compared with the configuration without a floor of under equal $\dot{Q}^{*}$ , the turbulent viscosity decreased in the configuration with a flush floor, resulting in an increase in mean flame height and a reduction in mean flame width. Based on the concepts of turbulent mixing and equal axial convection and radial diffusion times, semi-physical models were derived for the mean flame height and the mean flame width, respectively. The two correlations agreed well with the experimental data of this work for the two burner configurations with and without a flush floor.