Effect of orientation on the burning and flame characteristics of PMMA slabs under different pressure environments

To investigate the combined effects of orientation and environmental pressure on flame characteristics, steady burning experiments of inclined polymethyl methacrylate (PMMA)slabs were conducted at reduced environmental pressures ranging from 0.5 to 1 atm. According to the visual observations of flame appearance and fitting results of burning rate at different inclination angles, diffusion flames can be classified into three regions: pool-fire region, transition region and wall-fire region. Experimental result shows that flame pulsation frequency depends only on sample width in pool-fire region, however, mainly on fuel inclination angle in wall-fire region. A theoretical analysis is performed to predict the flame pulsation frequency as a function of modified gravity and the outlet diameter of gas-phase flame at different fuel inclination angles. In addition, the dimensionless flame pulsation frequency St is found to correlate well with the modified Froude number Fr^∗, St=0.461/Fr^∗0.53. Flame length shows a tendency to peak at 0.8 atm except 2.5 cm wide samples. Correlations between flame length and heat release rate are found with a power-law exponent of 1.41 for pressures smaller than 0.8 atm and 1.04 for larger pressures. Both experimental result and the developed correlations of flame length suggest that the roles of molecular diffusion and air entrainment played in diffusion burning are closely related to the environmental pressure and sample dimension.