Effect of finite dimension on downward flame spread over PMMA slabs: Experimental and theoretical study

This work investigates experimentally and theoretically the heat and mass transfer process of downward flame spread over finite width polymethyl methacrylate (PMMA) slabs in quiescent air. A series of experiments with different sample dimensions, 3, 4, 5 and 6 mm thick and 1.5-12 cm wide, are conducted and a solid phase flame spread model is developed to predict the behaviors during the process. Being different from the 2-D condition, an inverse 'V' shape leading edge, accelerated mass loss rate and spread rate exist due to the lateral combustion of samples and enhanced oxygen diffusion. The angle of leading edge increases with increasing width for 3 mm slabs, whereas no significant change occurs for thicker samples. The analytical model indicates that the mass loss rate and spread rate are functions of thermal parameters, leading edge angle and geometry of material. For fixed thickness, a linear relationship exists between the mass loss rate and width, while the spread rate is inversely proportional to the width of sample. The model also supplies two methods to estimate the heat flux ahead of leading edge and on the pyrolysis surface: by measured mass loss rate or flame spread rate. The validity of the proposed model is verified by the good agreement between the experimental and predicted results.