Time-Resolved Radiation and Convection Heat Transfer in Combusting Discontinuous Fuel Beds

Abstract

Time-resolved radiation and convection heat flux were measured in a series of experimental fires designed to explore heat transfer behavior during the combustion of discontinuous fuel beds. Fuel spacing and height were varied for both buoyancy- and wind-driven combustion. Peak radiation and convection heat fluxes as high as 130 kW/m² were recorded. Radiation flux had the effect of heating the fuel before flame arrival. Both positive (heating) and negative (cooling) convective heat transfer occurred before flame arrival. Surprisingly, the convection could also be positive or negative after flame arrival, indicating that even when engulfed in flames there were packets of cooler air moving across the sensor. In nearly all cases, short-duration convective heating pulses appear to precede the full onset of combustion, suggesting that convective heating may be critical as a pilot ignition source. Flame spread rate appears to be primarily governed by factors that affect the intensity of the convective transport. Rapid temporal fluctuations were observed in both radiation and convection, and spectral analysis revealed spectral content at frequencies as high as 50–70 Hz under buoyant flow conditions, and 150–200 Hz under the influence of wind.

Keywords:

• Discontinuous fuel
• Radiative and convective heat transfer

ACKNOWLEDGMENTS

This work was supported in part by funding provided by the U.S. Forest Service and the National Fire Plan. The assistance of Anita Hershman and Danette Paige in preparing fuel beds is appreciated.