Experimental estimation of thermal expansion and vorticity distribution in a buoyant diffusion flame

Abstract

The flow induced by buoyant diffusion flames results from thermal expansion caused by heat release and vorticity generated because of density gradients in the flame. The thermal expansion source induces a potential velocity field, and the vorticity induces a solenoidal velocity field. Baum and McCaffrey's technique for the calculation of fire-induced flow field requires specifications of thermal expansion and vorticity source terms. We estimate the thermal expansion using the laminar flamelet method in conjunction with measurements of major gas species concentrations. Mixture fractions are calculated based on the major species concentration data, and gradients of mixture fraction are obtained from the curve fits to the radial profiles of mixture fractions. The temperature, density, and mass diffusivity of the gases are determined using laminar flamelet state relationships from OPPDIF simulations of a natural gas/air diffusion flame. These quantities are needed for estimating the source term for thermal expansion. The mean velocity field is measured using particle imaging velocimetry. The mean vorticity is obtained by differentiating this field using a finite difference. Near the burner surface, the vorticity components based on the axial and radial velocity gradients are approximately equal. A few centimeters from the burner surface, the component involving the axial velocity becomes dominant. We have computed the fire-induced flow field using the method of Baum and McCaffrey in conjunction with the present source term measurements. The results of these computations agree reasonably well with experimental data.