Influence of convective heat transfer modelling in CFD simulations of upward flame spread

Abstract

Focusing on convective heat transfer modelling upward flame spread scenarios, different methods for calculating the convective heat fluxes are explored. These approaches involve: 1) calculating convective heat transfer based on the stagnant film theory; 2) using a simplified approach based on the stagnant film theory; 3) using Nusselt correlations for natural convection; and 4) modelling convection based on the effective (i.e., laminar plus sgs) thermal diffusivity. Large eddy simulations (LES) of a series of Single Burning Item (SBI) experiments with Medium Density Fiberboard (MDF) and plywood walls are performed. Even though discrepancies are observed, the approach which considered the sgs effects performs overall better and is able to predict both the initial flame growth period and the decay phase with reasonable accuracy (i.e., deviations in the peak HRR of approximately 5% and 34% for the MDF and plywood tests cases, respectively). The predicted total heat fluxes are, in most cases, in fairly good agreement with the experiments (i.e., deviations of up to 15% and 20% for the MDF and plywood tests cases, respectively). The predictions with the other methods are far less satisfactory. Overall, it is demonstrated that accurate convective heat transfer modelling can be important in flame spread scenarios.