Heat Transfer Blockage In Small Scale Combustion

Focusing on fire hazards, this research addresses heat transfer processes andattenuation mechanism on heat feedback to the surface of a burning object from flamesas well as adjacent external radiators.As solid combustibles burn, fuel vaporization and combustion at the surface createa dense intervening layer of soot and gases that partially block heat feedback fromreaching the burning surface due to absorption and scattering of radiative heat fluxes bythe gas-soot mixture located immediately above the surface. This effect is furtherenhanced by mass transfer of pyrolysis gases leaving a burning surface and is foundconsiderable. The phenomenon is known as "flame heat transfer blockage".Blockage clearly affects burning rates and heat release rates of fires. In order toaccurately model fire growth and propagation, a quantitative understanding andprediction of flame heat transfers including the effects of blockage are generally needed.Accurate estimation of flame heat transfers is extremely important in determiningignition and fire spread hazards. Currently there is quite limited knowledge about theflame heat transfer blockage effect. This lack of knowledge has become a “bottleneck”for predicting the growth rate of hazardous-scale fires.In this research, experimental studies were performed taking advantage of theunique capability of the Fire Propagation Apparatus (FPA) of being able to vary theambient oxygen concentrations. The blockage phenomenon was clearly observed andquantitatively measured in experiments of small9.5cm diameter PMMA and POM poolfires subjected to external radiation burning in ambient atmospheres of various oxygenconcentrations. The measurements were further explained by a one-dimensionaldiffusion flame model developed based on the fundamental heat transfer andcombustion theories. This theoretical model has been verified for its accuracy andreasonableness with experimental data and the literature. It provides a greaterunderstanding of the blockage phenomenon for helping to better design experiments oranalyze the results of experiments as an important tool. It has been experimentally andtheoretically demonstrated that the overall heat transfer blockage factor can be up to0.3~0.4for PMMA and POM flames, and the blockage factors are nearly independent ofthe external radiation, but increase as the ambient oxygen concentration increases. Major accomplishments of this research were:(1) A general linear function ofburning mass loss rate was obtained both experimentally and theoretically, whichreveals the experimental concept and evidence of the flame heat transfer blockage andlays the foundation for experimental measurements of the blockage phenomenon;(2)An approximate band model was developed for fuel vapors and inserted into RADCALto compute radiances from inhomogeneous radiant gases;(3) The classical mass transferB Number was extended by including a radiation feedback factor. A universal masstransfer number BRwas analytically derived for semi-transparent flames involvingexternal radiation;(4) A simple dimensionless criteria characterizing flame transparencyto heat feedbacks was established so that a quantitative description of the flame heattransfer blockage was achieved. In addition, main causes for attenuation of heattransfers back to the fuel surface were revealed, which include, with increasing externalradiation and fuel burning rate, significant convective blockage; larger outward toinward radiation, decreasing radiative fraction of combustion heat release and blockedinward radiation by cold fuel gas near the surface with a large absorption coefficient.Results of this research provide strong support for improving flammability testingtechniques and methods of combustible materials.