A NUMERICAL INVESTIGATION OF A LIFTED H_2/N_2 TURBULENT JET FLAME IN A VITIATED COFLOW

摘要

Numerical calculations of a lifted H_2/N_2 turbulent jet flame in a vitiated coflow of hot gases are presented. The calculations are performed using Magnussen's Eddy Dissipation Concept (EDC) for turbulent combustion, and are an extension to previously reported EDC modeling results presented by Cabra et al. (2002). Four different turbulence models are employed to investigate in more detail the turbulence modeling effect on the EDC combustion model with detailed chemistry. A series of simulations are presented that indicate the extent to which turbulence models influence the predicted lift-off height with the EDC combustion model. Several flow conditions were tested. For all calculations, EDC predicts more lift-off by using the standard k-ε model than by using Reynolds-stress-equation (RSE) models, whereas a modified k-ε model predicts less lift-off than the RSE models. The reason for the lower predicted lift-off with the modified k-ε model is because a modified turbulence Prandtl or Schmidt number in the scalar equations in the modified k-ε model allows an earlier mixing of the hot coflow with the fuel jet. All models overpredict the lift-off height for the standard flow conditions. Recent experiments and numerical calculations by others have shown that the vitiated coflow flame is extremely sensitive to variations in the coflow temperature. The present calculations show that this sensitivity is captured by the EDC combustion model, however to a smaller degree than that previously reported. Calculations with variations in coflow temperature and jet flow velocity indicate that for each flow condition, the various turbulence models predict the same percentage increase or decrease in lift-off height. These EDC calculations show that the turbulence model effect on the EDC predicted lift-off height is important and that a better flame structure is predicted with the RSE model by Jones and Musonge than with the other turbulence models.