Role of k-w closure coefficients for free jet turbulence modelling

The present work is focused on the prediction of the fluid dynamics behaviour for natural gas burners characterized by low NOX emissions by using a Computational Fluid Dynamics (CFD) analysis. One of the major critics addressed to CFD simulations for such complex physics is linked to the accuracy of turbulence models not able to properly predict the behaviour of the jet entering the chamber. For this reason some well known turbulence models are examined in this work in order to validate the CFD approach for both high and low Reynolds jet. The authors want to show that a case by case understanding of the flow regime and the physics may help in extending the range of validity of RANS turbulence models based on the eddy viscosity concept and, in particular, of the k-two-equations turbulence models given by Wilcox in 1988 and 1998. The experimental data used as reference, come from two set of tests related to different isothermal flow behaviour: high Reynolds number (Re= 68000) and low Reynolds number (Re= 5400). Based on the evidence that at low Reynolds number the hypothesis of homogeneous and isotropic turbulence eddies for the dissipative scale is no longer valid, a modification of k-turbulence model’s closure coefficients has been proposed for the low Reynolds case, accordingly to the hypothesis of free-shear mixing (hereafter referred to as “Briggs’s approach”). This leads to a better agreement with the experimental data.