| In this paper,wood combustion charring behavior was systematically researched with six kinds of commonly used timber.The impact factors influencing on the charring rate were deeply analyzed including the density,gas permeability,annual ring width,grain direction and heat flux.Charring rate models were established for single timber and mixed timber and their precise and applications were examined.Mechanical strength,chemical composition and microscopic structure of wood charred transition layer were studied.The main conclusions were summarized as follows:(1)Wood density,gas permeability and heat flux significantly affected the wood charring rate,while the impact with factors of ring width and wood grain direction on the wood charring rate was not significant.The wood charring rate increased with the increase of gas permeability and heat flux and decreased with the increase of the density.The degree of influence on the gas permeability of the wood was stronger than wood density and heat flux.(2)For single softwood or hardwood,the significant negative correlation was shown between wood density and wood charring rate.While the correlation between wood density and wood charring rate was not found for mixed timber.The significant positive correlation between gas permeability and wood charring rate for mixed timber only is found in the longitudinal section.(3)Charring rate model was established based on conservation of energy and thermodynamic equation for single timber and mixed timber,which was exponential form with two parameters including thermal diffusivity and burning time.For single timber charring rate model,the relative error of the charring rate theoretical and actual values was less than 20%.For mixed timber charring rate model,the minimum relative error of the charring rate theoretical values and furnace measured values was 10.96% when the heat flux was 50kW/m~2.The cone calorimeter could be equivalent to the furnace in the study of wood charring behavior when the heat flux considered.(4)The optimal charring rate model was acquired at 50kW/m~2 heat flux for mixed timber and its accuracy was greater with the extension of the burning time.The optimal charring rate model was compared with other charring rate models.The timber results showed that.Its applicability was greater than EC5 model,Australia model,White model and Ulg model,while unsuitable for species with densities less than 290 kg/m.The optimal charring rate model was less than Ulg model in precision.(5)There was still some structural strength of nearly wood charring limit area,which can reach more than 50% compared with the uncharred layer after charred.ATR-FTIR spectra showed that part of hemicellulose and cellulose had been pyrolysed nearly charring limit.In the micro structure,cytoskeleton structure of wood charring limit was intact,while some cell wall and cell lumen contracted and break.There was a certain degree of cracking and breaking for fibrous and ray cell. |
