Experimental Research On Heat Transfer Blockage In Small-scale PMMA Combustion

Heat transfer blockage is considered to be a fundamental phenomenon in fire research. As solid combustibles burn, fuel vaporization and combustion at the surface create a dense intervening layer of soot, pyrolysis gases and combustion products,which partially block exterior heat radiation to the fuel surface due to the absorption and scattering of radiative heat fluxes by the gas-soot mixture. This effect is enhanced by the increasing mass flux of the sample surface, as the pyrolysis process is accelerated with exterior heat radiation.Previous studies main focused on observation and heat transfer blockage in pyrolysis process, with little attention to heat transfer blockage effect in combustion process. Small-scale PMMA combustion and pyrolysis tests were conducted using cone calorimeter to analyze influences of thickness, location and exterior heat radiation on the experimental process. The heat transfer mechanism on the fuel surface was studied systematically for horizontal and vertical PMMA fire tests in laboratory.In this study, cone calorimeter was used to investigate the effect of sample thickness on the combustion process of small-scale vertical PMMA. When the sample thickness decreased, the period of steady combustion phase was shorten, however, the mass loss rate during steady combustion phase varied slightly. Pyrolysis tests of PMMA under an inert atmosphere were conducted to study the effects of exterior heat radiation and sample orientation. It was found that for both sample orientations, the mass loss rate increased linearly with increasing exterior heat radiation during steady state pyrolysis stage. The absorption ratio of exterior heat radiation for steady state pyrolysis stages were 4.9% and 0.2% for horizontal and vertical PMMA respectively,while the proportion did not change with the exterior heat radiation. When considering the attenuation of pyrolysis gases, the gasification heat of PMMA was obtained with a value of 1.51 kJ·g-1.For small-scale horizontal PMMA fires, the mass loss rate increased linearly with increasing exterior heat radiation. Flame height had a linear relationship with mass loss rate to the power 2/3. The flame shape was similar to a cone, the degree of whose deviation (?) did not change with exterior heat radiation. For horizontal PMMA fires,the radiation blockage fraction was the highest at the sample center, while it was the lowest at the sample edge. The mean radiation blockage fraction increased with exterior heat radiation and gradually reached an equilibrium value. The convective heating to the sample surface was weakened, which maybe induced by the reduced temperature gradient between the sample surface and the vapor layer. The simulation results of horizontal PMMA fires by FDS showed that flame radiation increased with increasing exterior heat radiation. For high exterior heat radiation, the increment of flame radiation had a linear relationship with exterior heat radiation.For small-scale vertical PMMA fires, mass loss rate also had a linear relationship with exterior heat radiation. It was suggested that heat transfer blockage fraction did not vary with exterior heat radiation. For vertical PMMA fires, the radiation blockage fraction was the lowest at the lower edge, which increased with height. The mean radiation blockage fraction increased with exterior heat radiation and gradually reached an equilibrium value. The convective heating to sample surface was enhanced with the increasing exterior heat radiation. The simulation results of horizontal PMMA fires by FDS showed that flame radiation also increased with the increasing exterior heat radiation, while it kept unchanged under high exterior heat radiation.