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

A numerical method for accurate simulation of the time and spatial characteristics of the inner and outer vortex structures in transitional H-2/N-2 jet diffusion names is presented. The direct numerical simulation, incorporating buoyancy, a simple one-step chemistry model, and transport coefficients that depend on temperature and species concentration, is described in detail. The species and energy equations are simplified 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 implicit, third-order-accurate, upwind numerical scheme having very low numerical diffusion is used to simulate the inner small-scale structures and the outer large-scale structures simultaneously. Although the outer structures develop without introducing perturbations, the inner structures are manifested from artificially introduced computer generated 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 distance without losing their identity or spreading radially. These vortices undergo an unusual axial-growth and merging process that is shown to result from their interactions with the outer vortices. The complex spectral characteristics of the name are interpreted in terms of the dynamics of this interaction process. The inner vortices appear to have very little impact on the flame since the flame surface is located well outside the jet shear layer.