Large Eddy Simulation of medium-scale methanol pool fires - effects of pool boundary conditions

Abstract

This article reports Large Eddy Simulation (LES) of 30 cm diameter methanol pool fires in order to investigate the effects of the burner boundary conditions on the pool dynamics. The numerical model involves state-of-the-art subgrid scale (SGS) sub-models for mixing, combustion and turbulence-radiation interaction with all the model constants computed dynamically. The non-adiabatic steady laminar flamelet (SLF)/presumed filtered density function (FDF) combustion model is used whereas the radiation model combines the Rank Correlated Full Spectrum k-distribution (RCFSK) for spectral radiation with the finite volume method (FVM) as radiative transfer equation (RTE) solver. The baseline case considers a burner located one-pool diameter above the floor and a fuel lip height of 1 cm as specified in the experiments used for model validation. Model predictions for puffing frequency, mean and rms temperature and velocity, mean molar fractions of major species, radiative loss to the surrounding and radiative and total heat feedback to the fuel surface are in good agreement with the available experimental data. Two other burner boundary conditions are designed. The first disregards the fuel lip height while keeping the burner located one-pool diameter above the floor. The second modifies the first configuration by assuming that the burner rim is floor flush. Model results show that the burner boundary conditions affect significantly the flow structure and the pool fire dynamics by altering the flame base instability near the edge of the pan. A non-zero fuel lip height produces significantly wider and shorter flames which affect the heat feedback toward the fuel surface whereas altering the air entrainment at the pool basis enhances substantially the puffing frequency. This shows that the experimental burner boundary conditions have to be reproduced scrupulously for relevant model validations.