Experimental and Numerical Modelling of Gravity Currents Preceding Backdrafts

Abstract

This study investigates the turbulent mixing within gravity currents preceding backdrafts and validates the ability of the computational fluid dynamics (CFD) software Fire Dynamics Simulator version 4 (FDS) to simulate these flows. Backdrafts are rapid deflagrations, which occur after the introduction of oxygen into compartments containing unburned gaseous fuel. They may form large fireballs out of the compartment opening and present a significant hazard to the safety of fire-fighters. Gravity currents which precede backdrafts are responsible for the formation of flammable gas mixtures required for ignition. Scale saltwater modelling is used to generate Boussinesq, fully turbulent gravity currents for five different opening geometries, typical of fire compartments. Width-integrated concentration fields and two-dimensional velocity fields are generated using the non-intrusive light attenuation (LA) and particle tracking velocimetry (PTV) flow visualisation techniques respectively. Numerical simulations are carried out with FDS to replicate these flows. The experimental and numerical results are compared directly. Front velocities are shown to be governed directly by local buoyancy conditions, in the later stages of the flows, and therefore the initial conditions associated with the opening geometries only influence the front velocities indirectly. The internal concentration structure, internal velocity structure and location of potential flammable regions are found to be highly opening geometry dependent. In general, the results of the numerical simulations are quantitatively similar to those from experiment, which suggests that the numerical model realistically predicted the experimental flows. However, the numerical concentration fields appear slightly lumpier than those from the experiments, possibly due to unresolved turbulence on scales smaller than the numerical grid (0.01H, where H = compartment height).