Numerical Prediction of Turbulent Diffusion Flames Formed by Cylindrical Tube Injector

Abstract

This work summarizes numerical results for a diffusion flame formed from a cylindrical tube fuel injector, issuing gaseous fuel jet vertically in a quiescent atmosphere. Both pure fuels as well as fuel mixtures are examined. The primary objective is to predict the flame base height as a function of the jet velocity. A finite volume scheme is used to discretize the time-averaged Navier-Stokes equations for the reacting flow, resulting from the turbulent fuel jet motion. The turbulent stresses, and heat and mass fluxes are computed from the Reynolds stress turbulence model. A chemical kinetics model involving a two-step chemical reaction mechanism is employed for the oxidation of methane. The reaction rate is determined from a procedure which computes at each point the minimum (process limiting) rate from an Arrhenius (kinetically controlled) expression and the eddy dissipation (turbulent mixing controlled) model. The Reynolds stress model (RSM), in conjunction with the two-step kinetics and the eddy dissipation model, produces flame base height and other flame characteristics that are in good agreement with experimental results. Numerical results are also in agreement with the hypothesis of Vanquickenborne and van Tiggelen concerning the stabilization mechanism of lifted diffusion flames. Furthermore, computed results also indicate that the flame base location can be approximately located by consideration of the turbulent mixing of the fuel jet in the non-reacting case. For propane, numerical results, obtained using one-step kinetics, show good agreement with the experimental data. Results pertaining to a methane-hydrogen mixture are obtained by using the RSM with three-step kinetics and the eddy dissipation model. The results for pure fuels and fuel mixtures indicate that the lift-off height for all the fuels considered in this study increases linearly with respect to the jet exit velocity. The study also analyzes the effect of swirling motion on the flame stabilization characteristics of the methane jet. The characteristics of methane flame are also determined by another combustion model which employs the probability density function (PDF) in conjunction with the flame sheet model. Results from this model differ in the near field from those predicted from the RSM-eddy dissipation model. However, in the far field the two combustion models yielded results that are in good agreement.