I. Mixing in doorway flows. II. Entrainment in fire plumes

Abstract

The motion of hot combustion products through a burning structure plays a dominant role in fixing the spread of the fire and the spread of toxic gases. The first part of this report is concerned with the effect of door geometry on the rate of turbulent mixing between the hot and cold layers near a doorway. The second part of the report deals with the entrainment rates in the near field of a buoyant diffusion methane flame. In the study of the motion of the hot combustion products near a doorway, the gas in the burning room is assumed to be divided into two homogeneous layers--the ceiling layer which contains the hot combustion products and the floor layer which contains the denser fresh air. Temperature and carbon dioxide measurements are taken from a half-scale room which uses a pump and furnace to simulate the entrainment and heating of the fresh air by the fire plume. The mass transfer rates are calculated from these measurements and are found to be a function of a Richardson number that uses the interface height measured from the floor as the characteristic length and the average inflow velocity of fresh air as the characteristic velocity. This form for Ri(0) is derived from an analysis based on Taylor's entrainment hypothesis. For a given fire size, i.e., for a given mass flow of fresh air into the room, the effect of reducing the door area is to reduce the value of Ri(0), and hence increase the mixing rate in the room. The door height is found to be a much stronger influence on the mixing rate and interface height than the door width. The entrainment rates of fresh air in the near field of a buoyant diffusion methane flame whose flames extend well above the interface between the hot gas layer and the fresh air layer are measured for several interface heights. In the experiments, the ceiling layer-fire plume interaction is simulated by placing a large steel hood over an axisymmetric burner. The hood may be raised or lowered to change the interface height. The entrainment measurements are obtained from a chemical analysis of a dried sample of combustion products taken from the ceiling layer. The measured species concentrations are compared with the theoretical equilibrium composition of the ceiling layer gas. For a given interface height, the ceiling layer gas temperature is found to be a linear function of the fuel-air ratio up to the stoichiometric fuel-air ratio. Increasing the fuel-air ratio above the stoichiometric value did not change the gas temperature significantly. This maximum temperature is observed to decrease with interface height. The air entrainment rates are found to be a weak function of the fuel-air ratio at very low elevations of the interface.