Computation of Flame Base Height of a Turbulent Diffusion Flame Resulting from a Methane Jet.

Abstract

This work summarizes numerical results for a diffusion flame formed from a cylindrical tube fuel injector, issuing a gaseous methane jet vertically in a quiescent atmosphere. The primary objective is to predict the flame base height and other flame characteristics as a function of the fuel 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 a Reynolds stress turbulence model. A chemical kinetics model involving two-step chemistry 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 Van Quickenbourn and van Tiggelen concerning the stabilization mechanism of lifted diffusion flames. The present results show the existence of the condition of tangency, as postulated by Van Quickenbourn and Van Tiggelen, between the jet axial velocity and the flame velocity profiles at the flame base. Furthermore, the burnout rate calculations indicate a high degree of premixing of air and fuel upstream of flame base for moderate to high fuel jet velocity cases.