ROLE OF INNER AND OUTER STRUCTURES IN TRANSITIONAL JET DIFFUSION FLAME

A jet diffusion flame was studied using a time-dependent, axisymmetric, third-order-accurate computational-fluid-dynamics-based model. In a previous investigation, this vertical, unconfined propane jet flame at transitional fuel-jet velocity was studied experimentally and found to have a double-vortex structure with large-scale buoyance-driven vortices outside the flame surface and smaller Kelvin-Helmholtz-type vortices in the shear layer of the fuel jet inside the flame surface. In the calculations the outer-vortex structures were developed as part of the solution, while a weak shear-layer perturbation was required to generate the inner structures. The computed shapes and the time and spatial evolution of these complex structures closely match those observed in the real flame. The model also correctly predicted the change in the direction of rotation of the inner vortices, which was observed 15 nozzle diameters downstream of the fuel-jet exit in the experiments. The computed results are used to explain this phenomenon. The detailed shapes of the time-averaged and root-mean-square temperature profiles at different heights in the flame are well predicted by the model and are shown to be due to the dynamic motion of the outer-vortex structures. Computational experiments have been performed to examine the long-standing question regarding why the vortices inside a transitional jet flame have a very long coherence length as compared with that of cold jets. Buoyancy was found to be a major factor. The buoyant acceleration of the hot gases of the flame entrains fluid that would normally be entrained by the inner vortices of the jet shear layer and causes their rapid dissipation. The buoyant acceleration also reduces the velocity gradient in the jet shear layer which, in turn, weakens the process that drives the growth of the vortices.