Large eddy simulation of turbulent vertical wall fires supplied with gaseous fuel through porous burners

Abstract

This paper presents a large eddy simulation (LES) study of vertical turbulent wall fires, which aims at bringing fundamental insight into the near-wall flame structure and heat transfer characteristics. The LES simulations are wall-resolved, i.e., the simulations are performed with sufficient grid resolution to capture the wall gradients and do not require a wall heat transfer model. The wall fires considered herein correspond to a simplified configuration in which gaseous fuel is supplied from an array of vertical porous burners, featuring a series of meter-scale wall flames with different fuel mass flow rates and different fuel types. Four fuels (methane, ethane, ethylene and propylene) are studied with prescribed fuel flow rates. Simulations are performed using an LES solver called FireFOAM. The Wall-Adapting Local Eddy-viscosity (WALE) model is used for turbulence modeling. The Eddy Dissipation Concept (EDC) model is used for combustion modeling. The thermal radiation model uses the discrete ordinate method with the simplifying assumption of an optically-thin medium characterized by a fixed radiant fraction. In the fuel blowing region, grid convergence is achieved using a 3 mm computational grid; while in the downstream flame preheating region, a 1.5 mm near-wall grid spacing is required to fully resolve the wall convective heat transfer. The change in grid requirement is due to the presence or absence of fuel blowing which affects the thickness of the wall viscous sub-layer. The simulations are in good qualitative and quantitative agreement with experimental data in the turbulent flame region; in particular: the radiative heat flux increases with elevation and with the fuel mass loss rate, while the convective heat flux remains approximately constant with elevation and decreases with the fuel mass loss rate. The present wall-resolved simulations are considered a first step on the route to developing accurate wall models needed for engineering simulations of wall fires.