Development of a numerical model for liquid pool evaporation

Abstract

The paper presents a detailed modelling approach of liquid pool evaporation that is suitable for pool fires and which relies on a combination of mass and heat transfer-based models. The mass transfer approach is based on the so called ‘film theory’ where mass diffusion is driving the molecular transport process across the film thickness. This driving mechanism is dominant during the early stages of the fire. However, as the liquid surface approaches the boiling point, heat transfer takes over and the evaporation rate becomes controlled by the heat fluxes at the liquid surface. In order to illustrate the potential of the proposed ‘comprehensive approach’, an in-house code has been developed and validated against a standardized experimental test for the vaporization rate of water in the ASTM E2058 fire propagation apparatus under an external heat flux of 50 kW/m2. The results show that, when the film thickness is calculated assuming a natural convection regime instead of a turbulent forced convection regime, the prediction of the early stage of the vaporization process is significantly improved in comparison to previously published results. However, the water evaporation case appeared to be solely driven by mass transfer (i.e., water vapor concentration gradients). Therefore, additional illustrative numerical tests with several liquid fuels and higher heat fluxes have been carried out. These tests point out the importance of combining the mass transfer-based approach with the heat transfer-based approach when the liquid surface reaches the boiling point.