The Effects of Inlet Turbulence Intensity and Computational Domain on a Nonpremixed Bluff-Body Flame

Abstract

A bluff body burner was investigated using computational fluid dynamics (CFD) to assess the effects of inlet turbulence intensity and compare the combustion characteristics with and without the bluff-body modeled in the computational domain. The effects of the CFD modeling techniques were assessed for inlet turbulence intensity, using a two-dimensional (2D) versus three-dimensional (3D) computational domain, and whether to include the bluff body in the domain. The simulations were compared with experimental data from the Turbulent Nonpremixed Flames workshop. The results showed that the turbulence intensity specified as a boundary condition at the fuel-jet inlet had a substantial impact on the axial decay of mixture fraction and temperature, which was overlooked by previous researchers when the bluff body was not modeled. The numerical results of the 2D axisymmetric and 3D domains without the bluff body showed that the 3D domain provided the best predictions when the turbulence intensity was defined using a published correlation versus experimental estimates since the k–ε turbulence model underestimated dissipation. It was shown that a 2D axisymmetric domain can be used to obtain predictions with acceptable numerical errors without the inclusion of the bluff body, and that a uniform inlet velocity can be specified, provided that the inlet turbulence intensity is defined using the correlation by Durst et al. (“Methods to Set Up and Investigate Low Reynolds Number, Fully Developed Turbulent Plane Channel Flows,” ASME J. Fluids Eng., 120(3), pp. 496–503.). Finally, further analysis of flow and flame characteristics demonstrated that when the bluff-body was included for the 2D axisymmetric domain, predictions improved and the flow was insensitive to inlet turbulence intensities because the bluff-body provided an entrance region for the flow to develop before mixing, thus reducing inlet effects. Thus, if experimental inlet data are not available, the addition of the bluff-body in the computational domain provides a more accurate jet velocity profile entering the reacting domain and eliminates errors caused by the inlet boundary condition.