Wildland fire research has been extensive and on going since before 1950. The motivation behind this research is to prevent loss of property and lives. In spite of this research, the heat transfer of fuel ignition and flame spread is not well understood. This dissertation seeks to fill gaps in this understanding through modeling and also by experimentation. The effect of water vapor on the transmission of thermal radiation from the flame to the fuel was investigated. The Spectral Line Weighted-sum-of-gray-gases approach was adopted for treating the spectral nature of the radiation. The study reveals that water vapor has only a moderate effect even at 100 percent humidity. Experiments were conducted wherein wood shavings and Ponderosa pine needles in quiescent air were subjected to an imposed radiant heat flux. The internal temperature of these particles was measured and compared to steady-state model predictions. Excellent agreement was observed between the model predictions and the experimental data. Exercise of the model led to the conclusion that ignition of the fuel element by radiation heating alone is unlikely. Time-resolved radiation and convection heat flux were measured in a series of experimental laboratory fires designed to explore heat transfer behavior during combustion of discontinuous fuel beds. Convection heat flux was shown to fluctuate between positive and negative values during flame engulfment, indicating the presence of alternating packets of hot combustion gas and cool ambient air within the flame. Rapid temporal fluctuations were observed in both radiation and convection. Spectral analysis revealed content at frequencies as high as 150 to 200 Hz. Time-resolved radiation and convection heat flux histories were also collected on fourteen controlled burns and wildfires. The data reveal significant temporal fluctuations in both radiation and convection heat flux. Spectral analysis using a Fast Fourier Trans-form (FFT) revealed content as high as 100 Hz using data sets that were sampled at 500 Hz. The role of the higher frequency convective content in fuel thermal response was explored using a one-dimensional transient conduction model with a convective boundary condition. It was shown that high-frequency (i.e., short-duration) convective pulses can lead to fine fuel ignition.
College and Department
Ira A. Fulton College of Engineering and Technology; Mechanical Engineering
BYU ScholarsArchive Citation
Frankman, David J., "Radiation and Convection Heat Transfer in Wildland Fire Environments" (2009). All Theses and Dissertations. 1813.
heat transfer, radiation, convection, forest fire, SLW, exchange factor, view factor, radiation ignition, spectral content, heat flux