Numerical study of the wall effect on the mass burning rate of small-scale methanol pool fires

Abstract

Dynamic predictions of the mass burning rate of pool fires under different burner conditions are essential to facilitate pool fire simulations without the need for artificially setting the inlet boundary conditions for the fuel surface. Such capability can remove the need for prescribed mass burning rates of pool fires in quantified assessment of the fire hazards. A fully coupled three-dimensional (3-D) model based on a multi-zone approach has been developed. In the gas-phase region, a compressible solver was employed. In the liquid-phase region, an incompressible solver with temperature-dependent thermophysical properties was utilized to directly solve fuel flow, accounting for the Marangoni effect, buoyancy effect, and incident radiation. In the solid-phase region, the 3-D heat transfer equation was resolved. The heat and mass transfer processes between different regions were simulated using conjugate heat transfer and an evaporation model based on "film theory". The proposed model has been validated through comparison with the 9 cm diameter methanol pool fire experiments. The predictions showed promising agreement with experimental measurements and empirical corrections, with the error in mass burn rate being within 3.1 %. Additionally, the predictions have captured a pair of vortices in sizes and directions closely resembling experimental observations. The sizes of the predicted vortices increased with the rising temperature at the base of the pool due to buoyancy and shear force. The analysis revealed that the wall effect not only leads to differences in the number of vortices and Marangoni velocity but also leads to a smaller mass burning rate in the burner with a high thermal conductivity than in the one with a poor thermal conductivity in the 9 cm diameter methanol pool fire. Neglecting the wall heat transfer would result in up to 18 % underprediction of the mass burning rate.