Laminar Smoke Point Based Subgrid Soot Radiation Modeling Applied to LES of Buoyant Turbulent Diffusion Flames

Abstract

Abstract. Large eddy simulations (LES) of gaseous buoyant turbulent flames have been conducted with the application of a flamelet based soot-radiation model. The subgrid model applies a turbulent eddy description of soot formation, oxidation and radiation and is based on the laminar smoke point concept. Two parameters, a local turbulent strain rate and prior enthalpy loss/gain fraction influence the soot formation and radiation. Radiation heat transfer is simulated by solving the finite volume discretized form of the radiative transfer equation (RTE) with the subgrid soot-radiation model implemented. The radiant heating of surfaces in close proximity of the flames is computed and predicted heat fluxes and surface temperatures are compared against experimental data. Fire growth in a rack storage arrangement is simulated with the application of a pyrolysis model. Computed heat release rate (HRR) is compared against experimental data. IntroductionThermal radiation, primarily due to soot, is responsible for fire spread in industrial scale rack storage scenarios. Flue spaces in rack storage setups are typically in the range of 0.15-0.6 m. Radiation in these spaces tends to be in the optically thin regime for moderately sooting pyrolysates from solid fuel surfaces. Accurate spatial and temporal prediction of radiant emission from the turbulent flame sheet therefore becomes the primary factor in predicting fire growth. This article presents results from CFD studies of (1) radiant heat transfer from a 60 kW propane fire to inert parallel panels [1], and (2) fire growth in an industrial-scale rack storage setup [2].Studies by Markstein [3] have established that radiation from turbulent buoyant diffusion flames correlates with the fuel's laminar smoke point height. Soot formation, oxidation and radiation all scale with the laminar smoke point. Using this concept, a model for soot radiation was developed by Lautenberger et al [4]. CFD simulations with this model showed good agreement of computed results with laminar diffusion flame experimental data. In turbulent flames the Kolmogorov micro-time scale governs the processes of soot formation, oxidation and radiation. This Kolmogorov mixing occurring in small reaction volumes must be described.A subgrid soot-radiation model based on the laminar smoke point model [5] is applied to compute radiant emission from buoyant turbulent diffusion flames. The subgrid soot-radiation model includes the effect of turbulence on the soot generation and oxidation rates. The model