Backdraft Phenomena

Book excerpt: The purpose of this project was to develop a fundamental physical understanding of backdraft phenomena. The research was divided into three phases: exploratory simulations, gravity current modeling, and quantitative backdraft experiments. The primary goal of the first phase was to safely simulate a backdraft in the laboratory. A half-residential-scale compartment was built to conduct exploratory experiments. The initial experiments concluded with a scenario describing the fundamental physics of backdrafts. The importance of the gravity current which enters the compartment after opening was identified. In the second phase, the gravity current speed and the extent of its mixed region was investigated in a series of scaled salt water experiments. The scaled compartment (0.3m x 0.1 5m x 0.1 5m) was fitted with a variety of end openings: full, slot, door, and window. Video and photo data indicate that the mixing layer which rides on the gravity current in the full opening case, expands to occupy nearly the entire current in the partial opening cases. The Froude number and nondimensional head height are independent of ~ and are in good agreement with numerical simulations and special limits from the literature. In the final phase, 28 backdraft experiments were conducted in a 1.2 m by 1.2 m by 2.4 m compartment. A methane burner was ignited inside a closed compartment and allowed to burn as long as oxygen was available. After the flame extinguished due to oxygen starvation, the burner was left on to allow the unburned fuel fraction to increase. Upon opening the hatch a gravity current enters the compartment and travels across the floor to the ignition source. After ignition a deflagration rips through the compartment and out the opening cul ninating in a large fireball. Histories recorded included: fuel flow rates, upper layer temperatures, lower layer temperatures, opening velocities, compartment pressures, upper layer species concentrations for O2, CO2, CO, and HC. Results indicate that unburned fuel mass fractions >15% are necessary for a backdraft to occur and that the backdraft severity strongly depends on the delay time and species concentrations.