On the use of natural and forced convection correlations in predictive CFD simulations of liquid pool fires

Abstract

This paper presents a detailed analysis of predictive numerical simulations of liquid pool fires, i.e., a 1 m – diameter methanol pool, a 0.7 m × 0.8 m ethanol pool, and a 0.18 m – diameter heptane pool. The burning rate is predicted using the ‘film’ model, including empirical correlations for heat and mass transfer. Although the forced convection approach (used in Fire Dynamics Simulator, FDS 6.7.5) yields relatively good results in fuel evaporation rate, it is somewhat questionable from a fundamental standpoint for quiescent burning conditions. Therefore, the natural convection approach is implemented in FDS 6.7.5. The predicted burning rates are similar to the forced convection approach, but the fuel surface temperature is closer to the boiling point by the natural convection approach. Based on the extensive analysis for the methanol pool fire, a modified natural convection correlation is proposed. The latter is tested for the ethanol test with satisfactory results for the peak Heat Release Rate (HRR), burning time, and surface temperature (closer to the boiling point). The modified natural convection approach is further tested for the heptane pool fire. Improved predictions are obtained for the peak burning rate, the transient stage, and the surface temperature.