- Author
- McBryde, J. D.
- Title
- Experimental and Numerical Modelling of Gravity Currents Preceding Backdrafts.
- Coporate
- University of Canterbury, Christchurch, New Zealand
- Report
- Fire Engineering Research Report 08/1, January 2008, 264 p.
- Keywords
- backdraft | gravity current | turbulent mixing | computational fluid dynamics | deflagration | oxygen | compartments | fuels | fireballs | fire fighters | gas mixtures | flammable gases | ignition | salt water | equations | validation | velocity fields | flow visualization | literature reviews | light attenuation | specifications | sensitivity analysis | mass flux | velocity | experiments
- Identifiers
- scale salt water modeling; gravity currents preceding backdrafts; particle tracking velocimetry; Fire Dynamics Simulator (FDS); co-ordinate system; non-dimensional variables; bulk front characteristics; flammable regions; internal concentration structure; internal velocity structure; effect of opening geometries; relative concentration field time sequences; magnitude of difference in relative concentration; predicted flammable region time sequences
- 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).