| Superheated steam drying can be used for the whole process drying of easy drying wood and the second half drying of hard drying wood,which can significantly improve the drying efficiency of wood,reduce the drying energy consumption and reduce the exhaust gas emission.At the same time,the physical and mechanical properties of dried wood can be optimized.This article is based on thinning plantation in teak path level material as the research object,probe into its full moisture content domain fast drying method,and proved its moisture and heat transfer mechanism in the process of drying,deconstruct the hydrothermal coupling law of quantitative effects of temperature and moisture of superheated steam drying,obtain superheated steam drying wood water heat transfer mathematical model,providing theoretical basis for optimization of superheated steam drying process.Based on the above research contents,this paper draws the following conclusions:(1)The differences between medium size teak and mature teak were systematically compared.In macroscopic structure,juvenile teak was less artistic and aesthetic than mature teak,with light color,wide ring and less oil.In physical mechanics,the oven dry density decreased by 25.3%,the volume dry shrinkage increased by 22.8%,the flexural strength decreased by 19.5%,the flexural strength decreased by 20.5%,and the flexural modulus decreased by 15.2%.In the microscopic structure,the fiber length,fiber width,fiber aspect ratio,fiber double wall thickness and fiber wall cavity of medium-size thinning teak decreased by 6.0%,4.9%,6.0%,3.6%and 10.3%,respectively.In chemical composition,cellulose of medium diameter thinning wood teak decreased by 8.1%compared with mature wood,lignin increased by 4.1%,hemicellulose increased by 4.6%,phenol,1%Nao H and cold water increased by 11.5%,5.4%and 10.1%,respectively.In terms of drying characteristics,medium diameter thinning wood teak belongs to the tree species that is difficult to dry.(2)Using X profile density meter acquired the superheated steam drying of longitudinal,radial and string in three directions of moisture migration in time and space distribution,the results show that the conventional drying in slow drying stages when moisture content is 30%,and three different direction of the specimen into the superheated steam point of deceleration drying moisture content were 15%,20%and25%,high drying efficiency than conventional drying 50-70%.The moisture content of30mm thick quarter-sawed board and plain-sawn board presented a hump distribution.The maximum difference of moisture content of plain-sawn board was up to 72.01%,and the maximum difference of d quarter-sawed board was up to 70.61%,which had a great influence on drying.The equivalent diffusion coefficient was used to characterize free water seepage and suction water diffusion.Based on the genetic algorithm and experimental data,the equivalent diffusion coefficient of teak in three directions was calculated in reverse,where the longitudinal equivalent diffusion coefficient was D7)=9.68×10-9(24-2.25×10-9(23+3.46×10-11(22-1.00×10-8(2+9.81×10-9,radial direction equivalent diffusion coefficient was D7)=9.79×10-10(24-6.49×10-11(23+1.84×10-10(22-9.97×10-10(2+4.73×10-10,chordwise direction equivalent diffusion coefficient was D7)=1.00×10-9(24-2.63×10-13(23+2.36×10-10(22-1.00×10-9(2+5.24×10-10.(3)The temperature rise curve of teak with different initial moisture content under the first boundary condition was studied by using the intelligent digital press and the online temperature collection system.The temperature inside the wood presents a u-shaped deep groove distribution.At 120?,the temperature difference inside the wood can reach 70?at the beginning of heating.The huge temperature difference will lead to large thermal stress in the wood.The rate of temperature rise in the early stage should not be too large when the superheated steam is used for drying.The temperature rise should be slow to reduce the temperature gradient on the wood surface and inside,so as to make the wood heat permeate evenly,so as to avoid unnecessary drying defects.The effect of water content and temperature on teak thermal conductivity was characterized by power function,and the wood equivalent thermal conductivity was deduced with genetic algorithm.The results showed that the wood equivalent thermal conductivity gradually decreased with the increase of water content and increased with the increase of temperature.Its functional form was=(-2.591×10-8W2+2.648×10-8W+3.249×10-9)(0.546W2-0.834W+3.760)?(4)Starting from the mathematical point of simplified control equation of the moisture migration and combined with the third chapter and fourth chapter of the derivation of equivalent diffusion coefficient and the equivalent coefficient of thermal conductivity,heat water to build a strong wood superheated steam drying mathematical model with moving boundary,and based on the finite difference mathematical thought and Compaq Visual Fortran writed numerical simulation program,and use the research of the superheated steam equipment teak superheated steam drying experiment was carried out,the results showed that superheated steam drying teak,suggest early preheat to 85?above,and then adoptde the 110-140?to drying,when the average moisture content of timber reach 19%lower superheat temperature,damp heat coupling effect release of teak drying stress,later promoted to speed up the drying speed,temperature of superheated steam model prediction of the temperature and moisture content curve fitting with the experimental data very well.Build interface by the rate of evaporation and the rate of evaporation volume model can be characterized quantitatively mobile evaporation interface,prediction of any position in the process of wood drying temperature,moisture content,interface of evaporation rate,the rate of evaporation,water vapor volume density and the dynamic change regularity of relative humidity,quantitative characterization of critical moisture content,the maximum stress in the process of wood superheated steam in order to optimize the superheated steam drying technology to provide theoretical basis for different tree species. |
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