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Solid Fuel Flame Spread and Mass Burning in Turbulent Flow.


pdf icon Solid Fuel Flame Spread and Mass Burning in Turbulent Flow. (3354 K)
Zhou, L.

NIST GCR 92-602; 232 p. March 1992.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Available from:

National Technical Information Service (NTIS), Technology Administration, U.S. Department of Commerce, Springfield, VA 22161.
Telephone: 1-800-553-6847 or 703-605-6000;
Fax: 703-605-6900; Rush Service (Telephone Orders Only) 800-553-6847;
Website: http://www.ntis.gov
Order number: PB92-181049

Keywords:

solid fuels; flame spread; turbulent flow; ceilings; floors; paper; plastics; polymethyl methacrylate; regression rate

Abstract:

An experimental study has been carried out to investigate the controlling mechanisms of solid fuel flame spread and mass burning in turbulent flows. The effects of flow velocity, turbulence intensity and buoyancy on concurrent and opposed flame spread rate and surface regression rate have been examined in both flow and ceiling configurations. It is found that for opposed flows, the flame spread rate of thermally thick PMMA sheet increases initially with the flow velocity, reaches a peak value and then decreases as the flow velocity increases further. The flow turbulence effect is to increase the flame spread rate initially and then decreases it at higher turbulence intensity. The flame spread rate of thermally thin paper sheet in an opposed flow decreases monotonically with the flow velocity and turbulence intensity. The flow turbulence also has a significant effecton the flame extinction conditions, resulting in a smaller extinction velocity for larger flow turbulence intensity. For concurrent flow flame spread, it is found that the flow turbulence decreases the flame spread rate for both floor and ceiling geometries, mainly as a result of the flame length shortening at high turbulence intensity. I is also found that flow velocity intensifies the spread of the flame. The experimental data of flame spread rate, flame length and surface heat flux agree well with the formula obtained from a simplified thermal model, indicating that the heat transfer from flame to solid surface is the dominant controlling mechanism in the turbulent concurrent flame spread and, that the gas phase chemical reaction is of secondary importance. For solid fuel mass burning, it is found that the solid fuel surface regression rate decreases with the downstream distance and the flow velocity in both floor and ceiling configurations. The flow turbulence increases the surface regression by enhancing the mixing and bringing the flame closer to the solid surface. Empirical correlations betwen the non-dimensional surface regression rate and the non-dimensional flow parameter are obtained, which indicates the possibility of incorporating the flow turbulence intensity explicitly in a non-dimensional analysis or a numerical simulation of the problem. The results from floor and ceiling geometries are compared to determine the effect of buoyancy on flame spread and mass burning. It is shown that for ceiling configuration, buoyancy enhances the heat transfer from the flame to the solid surface by pushing the flame closer to the wall. However, it also causes the gas phase chemical reactions to proceed less completely through insufficient gas mixing and surface quenching.