NUMERICAL INVESTIGATIONS OF TRANSITIONAL H-2 N-2 JET DIFFUSION FLAMES/

A numerical method for accurate simulation of the time and spatial cha racteristics of the inner and outer vortex structures in transitional H-2/N-2 jet diffusion names is presented. The direct numerical simulat ion, incorporating buoyancy, a simple one-step chemistry model, and tr ansport coefficients that depend on temperature and species concentrat ion, is described in detail. The species and energy equations are simp lified by introducing two conserved scalars beta(1) and beta(2) and by assumming that the Lewis number of the flow is equal to unity. An imp licit, third-order-accurate, upwind numerical scheme having very low n umerical diffusion is used to simulate the inner small-scale structure s and the outer large-scale structures simultaneously. Although the ou ter structures develop without introducing perturbations, the inner st ructures are manifested from artificially introduced computer generate d random noise. The buoyancy-driven outer instabilities and the shear- driven inner ones are found to roll up into vortices at frequencies of similar to 14 and 350 Hz, respectively. Unlike the structures in cold jets, the shear driven vortices in flames propagate over a long dista nce without losing their identity or spreading radially. These vortice s undergo an unusual axial-growth and merging process that is shown to result from their interactions with the outer vortices. The complex s pectral characteristics of the name are interpreted in terms of the dy namics of this interaction process. The inner vortices appear to have very little impact on the flame since the flame surface is located wel l outside the jet shear layer.