A two-dimensional numerical investigation of the oscillatory flow behaviour in rectangular fire compartments with a single horizontal ceiling vent

This paper presents a comparison of fire fields model predictions with experiment for the case of a fire within a compartment which is vented (buoyancy-driven) to the outside by a single horizontal ceiling tent. Unlike previous work, the mathematical model does not employ a mixing ratio to represent vent temperatures but allows the model to predict vent temperatures but allows the model to predict vent temperatures a priori. The experiment suggest that the flow through the vent produces oscillatory behaviour in vent temperatures with puffs of smoke emerging from the fire field model. While the numerical predictions are in good qualitative agreement with observations, they overpredict the amplitudes of the temperature oscillations within the vent and also the compartment temperatures. The discrepancies are thought to be due to three-dimensional effects not accounted for in this model as well as using standard 'practices' normally used by the community with regards to discretization and turbulence models. Furthermore, it is important to note that the use of the k-epsilon turbulence model in a transient mode, as is used here, may have a significant effect on the results. The numerical results also suggest that a linear relationship exists between the frequency of vent temperature oscillation (n) and the heat release rate (Q(0)) of the type n proportional-to Q(0)(0.29), similar to that observed for compartments with two horizontal vents. This relationship is predicted to occur only for heat release rates below a critical value. Furthermore, the vent discharge coefficient is found to vary in an oscillatory fashion with a mean value of 0.58. Below the critical heat release rate the mean discharge coefficient is found to be insensitive to fire size.