The effect of flameholder geometry on combustion efficiency in ducted burners

Abstract

The turbulent flowfield in a backmixed combustor is modeled analytically by simultaneously solving the governing partial differential conservation equations. An algebraic and a two-equation eddy viscosity model are employed in the numerical simulation to account for the turbulence transport processes. Predicted distributions of isothermal flow properties are systematically compared to experimentally obtained data to appraise the eddy viscosity models. In addition, predicted distributions of reacting flow properties are presented to illustrate the applicability of current numerical methods to the prediction of continuous combustion flows.The turbulent momentum and mass transport properties are evaluated in isothermal flow for a range of mixing conditions. Both eddy viscosity models qualitatively describe the system hydrodynamics, but the detailed flow structure is inadequately represented. The mass transport predictions from the algebraic viscosity model agree favorably with experiment. The inferior performance of the two-equation viscosity model is remedied by refining the boundary condition specification for the turbulence energy dissipation rate. It is shown that the turbulence energy dissipation rate adjacent to critical solid walls strongly influences the overall mixing characteristics of the two-equation model.The isothermal flow results indicate that the algebraic eddy viscosity model provides cost-effective predictions of the general fluid flow patterns and mass transport trends in confined flows exhibiting strong recirculation. The two-equation eddy viscosity model provides better resolution of the small-scale turbulence processes but requires careful testing to ensure realistic predictions.The hot flow calculations accentuate the inadequate transport characteristics identified in the isothermal flow analysis and verify that considerable testing of the numerical model is required before proceeding to the complicating conditions of combustion.