Insights from numerical simulators for compartment fire spread

Many fire safety design problems demand a detailed quantification of the evolution of fire conditions in large and/or complex spaces. Fire development arises from the coupling between the combustion processes and the environmental conditions, including ventilation flows and heat transfer interactions with compartment boundaries. Comprehensive models based on Computational Fluid Dynamics (CFD) can in principle address these problems but are beset by challenges including the range of scales and uncertainties on the inputs needed to adequately represent the coupled solid and gas phase processes. For a subset of problems which involve fire spread over fuel bed structures composed of meshes of wood sticks, i.e. either isolated cribs or uniformly distributed fuel loads, significant experimental and theoretical analysis has been undertaken in recent years with the aim of describing and quantifying burning regimes. The classical theories of compartment fire dynamics provide approaches for Regime I (i.e. ventilation control) but there are much greater uncertainties in Regime II (i.e. fuel control); at the level of the fuel bed, the role of preheating is found to be critical in determining fire spread behaviours, with spread and burn-out determined by the energy balance at the fuel bed surface; this energy in turn is supplied from a range of sources including the local fire plume, the hot smoke layer and the compartment boundaries, each of which are evolving and thermally interacting with each other, while subject to the influence of the dynamic ventilation conditions; the fuel-bed controlled regime extends also to processes occurring internally within the crib structure, which are geometry dependent. With the advent of more advanced CFD approaches which adopt an exact geometric “stick-by-stick” representation of the fuel bed, in conjunction with fuel degradation described by a “complex pyrolysis model” through three parallel Arrhenius-like reactions for wood major constituents, all implemented in the Fire Dynamics Simulator code, progress is now being made on unravelling the drivers of fire spread, and hence the evolving compartment fire dynamics. The modelling strategy requires key parameters to be tuned in comparison to test results for isolated cribs and thereafter applied to prediction of spread in full-scale compartments. Post-processing analysis then reveals the relationship of burning rates to flux history evolution at the level of the surface of each stick, discriminating the vertical and horizontal surfaces, and hence the role of pre-heating from the fire plume and the hot layer, in conjunction with ventilation effects at compartment scale. These studies provide a level of detail which is impractical experimentally and are generating new insights into the essential fire spread processes, which are of great interest for fire safety design.