Combustion and Flame, Vol.225, 237-254, 2021
Large eddy simulation of the unstable flame structure and gas-to-liquid thermal feedback in a medium-scale methanol pool fire
The objective of the present study is to perform fine-grained Large Eddy Simulations (LES) of a canonical 30-cm-diameter methanol pool fire configuration in order to evaluate the ability of current fire models to predict the flame structure and the rate of heat transfer to the liquid fuel surface. The simulations are performed using an LES solver called FireFOAM, a prescribed fuel evaporation rate, and a "wall-resolved" approach; the simulations also include a description of the fuel pan lip. Great attention is paid to the design of the computational grid and to the control of spatial resolution. Less attention is paid to the LES model formulation: except for the treatment of radiation for which we consider both a simplified model using a prescribed global radiant fraction and a more advanced model based on the Weighted-Sum-ofGray-Gases approach, the simulations use baseline FireFOAM treatments to describe subgrid-scale turbulence and combustion. Results suggest that grid-converged solutions are achieved at a spatial resolution of 2 mm in the near-pool-surface region and 5 mm in the bulk of the flame region. Consistent with experimental observations, the simulated flames feature a strong instability characterized by the cyclic formation of thin boundary layer flames at the liquid pool surface and vortex rings at the burner rim that grow into large puffs and determine the structure of the entire flame. Predictions of the mean and root-mean-square temperature and velocity are found to be in good agreement with measurements taken from the literature. Equally encouraging results are obtained for the mean radiative and convective heat fluxes near the pool surface; quantitatively, simulations are found to overestimate the intensity of the thermal feedback by 25%. These results suggest that provided that the computational grid is fine enough, current fire models can be used to predict the gas-to-fuel thermal feedback. (C) 2020TheCombustionInstitute. PublishedbyElsevierInc. Allrightsreserved.
Keywords:Pool fire;Buoyancy-driven turbulent diffusion flame;Puffing instability;Gas radiation;Thermal feedback;Large Eddy simulation