Simplified Theortical Analysis For Smoldering Process

Theoretical analysis on smoldering is of great importance for fire safety, development of new technology to utilize biomass via smoldering. The objective of this study is to explain some observations during agro-stalk smoldering experiments and a simplified mathematical model for better investigation for the process was set up in future.Firstly, effect of fuel size on the possibility of flame combustion inside the smoldering bed was analyzed. Based on the relationship between fuel size and equivalent pore size in the fuel bed, the critical fuel size which flame will not exist was calculated according to the concept of quench length. The calculated results was analyzed and showed that: The flame will not appear inside the fuel bed when the dimension of fuel is smaller than 2～3 cm in ordered permutation. The packed density is smaller in disordered permutation , the critical dimension of the fuel is smaller. And it also sheds light on the phenomenon that mass loss rate of corn stalk, wheat straw smoldering is much higher than that of grass.Secondly, heat transfer in fuel bed for a typical smoldering case was investigated. Temperature inside the fuel bed was predicted according to the analytical solution of unsteady heat conduction in semi-infinite body. Compared with smoldering propagation velocity recorded in literature, heat transfer in fuel bed is faster than the smoldering propagation. This is part of the reason why temperature at different position inside the fuel bed increase simultaneously during smoldering experiments.In order to investigate the physical mechanism of smoldering, a new experimental method using char as fuel was proposed. A small experimental set-up was developed and pre-experiments have been performed. Results showed that smoldering propagated downward steadily and the propagation velocity is about 0.18 mm/min.Theoretical analysis of downward char smoldering was investigated based on some reasonable assumptions. According to mass and energy conservation, a one dimensional mathematical model was developed. Boundary condition and initial condition was determined. Solution of the model will be left for future work. Theoretical analysis provided a basis for the deep investigation of the process.