Experimental And Theoretical Investigation On Heat And Mass Transfer In Wood Prior To Fire

Experimental and theoretical investigation on the processes of heat and mass transfer in wood prior to fire is conducted in this paper. In the experiment, different aspects of wood including thermophysical properties, porous staicturc and pyrolysis character, which affect the heat and mass transfer, arc analyzed systematically. An integrated heat conductive model comprised of heat, desiccation and pyrolysis is established, and the rules of heat and mass transfer in earlier phases of fire have also been researched theoretically.In first, a new short time formula of one dimension heat conduction differential equation, based on the theory of one dimension transient method, is presented for thermophysical property measurement of wood. The result of testing on red pine and larch wood arc satisfied by new method.In succession, the relationship between heat conductivity of wood and moisture, density, porosity, temperature and grain is studied experimentally in the range from 25 to 100℃. The result shows that, the heat conductivity is in direct proportion to density. but in inverse proportion to porosity. The heat conductivity of air-dried wood increased probably linear with the temperature rising. And that heat conductivity along the grain of wood is higher than that cross the grain. The former is 1.8 or 2 times of the latter.The testing results on thermophysical property of wet wood, which moisture is below the FSP (Fiber Saturated Point), illustrate that, when the moisture is less than 10%, the specific heat of wood is dependent on temperature linearly, whereas exponentially when the moisture is more than 10%.The outcome of the experiment shows the specific heat of dry wood is independent on species. In temperature range from 25 to 200 ℃ , The heat conductivity along the grain of wood is approximately 2.4 or 3 times of that cross the grain. And this result is relative to composing orientation of the molecules interlink inside the cellular wall of wood. In addition, although the heat conductivity in radialgrain is different from which in tangential grain, the deviations become smaller as the temperature rising.The microscopical observation on porous wood structure confirms that heat transfer characteristic is dependent on the distribution and magnitude of porosity inside wood. Under the same moisture in wood, the capability of heat conduction reduces with higher distribution and smaller size of the porosity.The thcrmo-gravity experiments in different atmosphere are performed. The results clearly demonstrate that, the volatile gas released from wood during the process of pyrolysis under the air is facile to be oxygenated. In contrast to the inert gases, the oxidation effects arc more intensive. However, the char conversion rate under the nitrogen is higher than that under the air. At a certain atmosphere, the characteristic temperature and mass loss proportion during wood pyrolysis is not quiet different from species. The result also shows that the increasing of temperature rising rate will promote the reaction of pyrolysis of wood.Finally, a three dimension transient heat conduction model and moisture migratory model for heat and mass transfer in wood exposed to a radioactive resource with high temperature is founded. After that, the finite clement method is introduced to resolve the transient temperature profiles and moisture distributions in wood. A small proving experimental device has also been built. Correspondingly, the numerical simulation results have been validated. And the computational results show that the estimation on ignition temperature and time of wood is approach to the experimental values, the relative errors arc within 7%. In a point of view for whole temperature distribution variety, the temperature variational trend exhibited by computational result is consistent with that of experimental result ideally, particularly in the process before wood ignition.