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Fire Propagation in Concurrent Flows. Final Progress Report. September 1, 1992-August 31, 1993.


pdf icon Fire Propagation in Concurrent Flows. Final Progress Report. September 1, 1992-August 31, 1993. (2022 K)
Fernandez-Pello, A. C.

NIST GCR 94-644; 66 p. June 1994.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Available from:

National Technical Information Service
Order number: PB94-193844

Keywords:

fire spread; buoyant flow; fire research; flame size; polymethyl methacrylate; turbulent burning; turbulent flow; turbulent heat transfer; oxygen concentration

Abstract:

A research program has being conducted to study the mechanisms controlling the spread of flames in an oxidizing gas flow moving in the direction of flame propagation. During this reporting period research has been conducted to study the effect of the oxidizer flow characteristics on the concurrent flame spread over thick PMMA sheets. The parameters varied in the experiments are the oxidizer flow velocity, turbulence intensity and oxygen concentration, and the geometrical orientation (floor and ceiling). Their effect on the flame spread process is studied by measuring the rate of flame spread, flame length, surface heat flux, products of combustion and soot. The results of the experiments show that the combined effect of flow velocity, turbulence intensity, and oxygen concentration has a complex influence on the flame spread process. At low flow velocity, the flame spread rate increases monotonically with turbulence intensity. At high flow velocity, however, the flame spread rate increases with flow turbulence at low turbulence intensities, but it decreases at high turbulence intensity values. The effect is more pronounced at high oxygen concentration. These trends appear to be due to a strong influence of the turbulence intensity on the flame temperature and length, and on the heat flux from the flame to the solid fuel. Turbulence enhances mixing, which increases the flame temperature and then the heat flux. The effect of turbulence on the flame length comes from two opposing factors. In one hand the enhanced mixing results in a stronger reaction with faster reactant consumption, which tends to produce a shorter but hotter flame. On the other hand, the higher flame temperature results in an increased mass burning rate, which tends to increase the flame length. It appears that at low flow turbulence, the latter effect dominates and thus there is an increase in the flame length. As the turbulence level continues to rise, the reactant consumption dominates, which leads to a decrease in the flame length. For the present experiments, the transition between the two regimes shifts from u'/U=5% at U=2.0 m/s, to u'/U=15% at U=1.0 m/s, and no transition point is observed at U=0.5 m/s within our experimental conditions. The flame spread rate is the outcome of the combined effect of the flame length and the heat flux. Under all flow velocities and turbulence intensities, the flame spread rate increases with the oxygen concentration. For low oxygen concentrations, a linear dependence is observed between the flame spread rate and the oxygen concentration. For high oxygen concentrations, the dependence of the flame spread rate on the oxygen concentration follows a second power law. By comparing the floor and ceiling results, it is found that buoyancy has two opposite effects, one is enhancing the heat transfer to the surface by reducing the flame stand-off distance and the other reducing the chemical reaction completeness by intensifying the flame quenching at the wall. The overall buoyancy effect on the flame spread and mass burning processes depends on the flow condition.