MEASUREMENTS AND MODELING OF A BLUFF BODY STABILIZED FLAME

An axisymmetric bluff body stabilized nonpremixed turbulent flame of 27.5% CO/32.3% H2/40.2% N2-in-air was investigated. The recirculation zone stabilized the flame and provided greater strain rates than possible in jet or even piloted-jet flames. Major species, density, and temperature were measured using a laser Raman scattering system, which was modified to operate in a chemiluminescent environment. The computational model was based on partial equilibrium in the radical pool, an assumed shape pdf over the two thermochemical variables required, and the k-epsilon-turbulence model for closure of the density-weighted averaged Navier Stokes equations. The equations were solved in the elliptic form appropriate to recirculating flow. Enough grid was added to reduce the transverse cell Reynolds numbers to below two, ensuring second-order accurate and stable discretization of convection operators and so eliminating artificial diffusion. Mean properties such as density were obtained at each node by convolution with the joint pdf over the two thermochemical scalars. The k-epsilon-turbulence model gave too rapid an initial decay. Agreement was encouraging on mixture fraction mean and variance, temperature, and species concentration fields. The bluff body provides an intensely turbulent flowfield for interactions with combustion chemistry, and is within the scope of numerical analysis. To improve the turbulence model, and to have a formalism that permits three or more scalars as required for hydrocarbon fuels, pdf transport methods should be merged with conventional solvers for the mean hydrodynamics.