CFD Predictions of Fire Spread over Wood Cribs in Large Open-plan Compartments: New Insights

Abstract

A CFD-based numerical simulator, coupled with a multi-component pyrolysis model established for an isolated wood crib fire test, successfully predicts fire spread behaviours and the evolution in compartment boundary conditions in a large-scale test with an extended, uniform fuel bed. Five distinct fire spread modes are identified, showing a gradual transition from 2D to 1D planar fire spread patterns. Our new model is no longer constrained to follow prescribed burning rates and an analysis of the drivers of fire spread at the level of the individual wood sticks shows linear correlations of burning rates with the incident heat fluxes at the fire’s leading edge, and similar within the crib during the main travelling fire phase, but higher rates when preheating effects dominate later in the fire. There is also relative insensitivity to exposures on the surface of the crib at the trailing edge, and it is believed that burning rates here are effectively isolated from preheating effects by the development of rigid char layer with high thermal resistance, thereby decoupling them from the compartment boundary conditions. This is a feature long recognised in experimental studies, but which is very challenging to corroborate through direct experimental measurements.