Abstract

Contemporary architecture and new structural materials challenge the classical premise of fire safety: compartmentalisation. The historical classification between Regime I and Regime II is no longer sufficient to analyse the new fire behaviours we are encountering. This article presents an analysis of the impact of geometry on the development of flows in a compartment fire, particularly in the back-wall effect. The study contrasts the case of an almost cubic compartment with one that doubles its length and with one that doubles its height, using a CFD model. The cubic model is validated using data from an experiment, where temperature fields were measured, and from which the flow fields are subsequently derived. Through computational models, the impact of the back-wall effect on the magnitude and direction of velocities is observed. For the cases evaluated, this effect is shown to increase the local vertical velocity component by approximately 35 % at selected measurement locations in the cubic compartment, while also altering the flow direction; this comparison is based on probes at equivalent locations and does not represent a spatially averaged increase in overall velocity magnitude. The analysis was conducted using FDS simulations, validating the model with experimental data. Streamline plots reveal that in cubic compartments, the buoyancy of the flame does not directly influence the change in flow direction; instead, it is primarily due to collision with the rear wall. Conversely, in the elongated case, the fluid dynamics are significantly affected by the buoyancy of the fire plume, which deflects it towards the ceiling.