Numerical Study of Buoyancy Effects on Triple Flames

The structure and propagation properties of triple flames subject to buoyancy effects are studied numerically using a high accuracy scheme. A wide range of gravity conditions, heat release, and mixing widths for a scalar mixing layer are computed for ’upright’ and ’inverted’ triple flames (i.e, gravity pointed in parallel and in opposite directions of flame propagation). These results are used to identify non-dimensional quantities, which parameterize the triple flame response. Computed results show that buoyancy effects act primarily to modify the overall size of the triple flame in response to gas acceleration through the two premixed branches. The impact of buoyancy on the structure of triple flame is found less pronounced than the general physical shape of the branches. The trailing diffusion branch is affected by buoyancy primarily due to the changes in the overall flame shape. The overall size of triple flame is determined by the two premixed branches. Buoyancy affects the locations of these premixed flames and consequently modifies the rates of diffusion of excess fuel and oxidizer from the premixed branches to the diffusion branch. A simple analytical model for the triple flame, which accounts for both buoyancy and heat release is developed. Comparisons of the proposed model for the triple flame speed with the numerical results for a wide range of gravity, heat release and mixing width conditions, yield good agreement.