Numerical study of buoyancy effects on the structure and propagation of triple flames

Abstract

The structure and propagation properties of diffusion neutral 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 downward-propagating (in the same direction as the gravity vector) and upward-propagating (in the opposite direction to the gravity vector) triple flames. These results are used to identify non-dimensional quantities, which parametrize the triple flame responses. Results show that buoyancy acts primarily to modify the overall span of the premixed branches in response to gas acceleration across the triple flame. The impact of buoyancy on the structure of triple flame is less pronounced than its impact on the topology of the branches. The trailing diffusion branch is affected by buoyancy primarily as a result of the changes in the overall flame size, which 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 speed, which accounts for both buoyancy and heat release is developed. Comparisons of the proposed model with the numerical results for a wide range of gravity, heat release and mixing width conditions, yield very good agreement. The analysis shows that under neutral diffusion, downward propagation reduces the triple flame speed, while upward propagation enhances it. For the former condition, a critical Froude number may be evaluated, which corresponds to a vanishing triple flame speed.