Research On The Temperature Distribution And Charring Rate In The Pyrolysis Process Of Wood Exposed To Variable Heat Fluxes Linearly Increasing With Time

When subjected to external heating, solid combustible start to decompose, giving a mixture of volatile species and, sometimes, char as products. The mixture of the volatiles emitting from the solid surface and the ambient oxygen will be ignited when the criteria of ignition are achieved. Charring material will product large amount of solid carbonaceous residual, mainly char, after combustion. Wood is to be considered as the most representative of charring materials and is one of the most popular combustibles in a fire. So the capability to predict the combustion characteristics of wood is quite important to fire safety engineering.Thermal radiation is the main method of heat transfer in a compartment fire. The previous experimental and theoretical studies were almost conducted with the external heat flux imposed on the specimen is fixed and is not varied with time. While, in a common compartment fire, the external heat flux imposed on the combustibles will vary continuously with the growth of fire. So it is necessary to examine the combustion characteristics of wood exposed to variable heat fluxes.Based on the former work, this paper experimentally and theoretically examines the pyrolysis, charring and ignition process of wood exposed to variable heat fluxes, as following:The radiation platform for studying the fire behavior of combustibles is utilized to research the pyrolysis and ignition of different wood species under heat fluxes linearly increasing with time. The influences of increasing rate of heat flux and wood density on the ignition time, surface temperature and internal temperature distribution are examined. It is found that both of them will affect the ignition time. The ignition time will decrease with a larger increasing rate of heat flux and a smaller wood density. This is determined by the external and internal heat transfer respectively. The critical increasing rate of heat flux for wood ignition under variable heat flux, with the value of 0.02 kW·m^-2·s^-1, can be estimated from the power function of ignition time and increasing rate of heat flux. The rate of temperature rise will increase with a larger increasing rate of heat flux and a smaller wood density. The surface temperature at ignition will decrease with a larger increasing rate of heat flux, and in the region of 340...