Numerical Simulation Study On Heat And Mass Transfer During Drying Process Of Chinese Fir Plantation Wood

Drying quality and energy consumption was influenced by heat and mass transfer ratesduring the sawn drying. The transfer law of moisture and heat during wood drying can bedescribed quantitatively by heat and mass model, which provided theoretical basis foroptimizing drying process and energy conservation. Therefore, the reaserch on heat and masstransfer model building and analysis had important theoretical significance and practical value.A mathematical model about earlywood and latewood density radial distribution wasestablished by taking Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) plantation woodas an example. Based on this model, considering the effect of non-uniform earlywood andlatewood density on wood drying, two one-dimensional mathematical model were establishedfor separately describing heat and mass transfer in quarter-sawn lumber and flat-sawn lumberdrying process. Moisture content (MC.) distribution in small flawless and average MCvariation in sawn lumber during drying was separately detected by X-ray method and weighingmethod, which was used for testing the accuracy and feasibility of the mass and heat transfermodels. In the end, based on the models about heat and mass transfer in quarter-sawed lumber,the effect of drying medium, physical parameters, lumber thickness on drying rate wasqualitatively analyzed. The results showed that:1. Tree growth temperature cycling was quantitively described by sine wave function, andthe function fitting coefficient of determination was0.907. Cyclical changes in rainfall duringtree growth was quantitively described by piecewise linear function. Tree growth rate modelgot from them can accurately analyze temperature and rainfall effects on tree growth rate. Themodel about earlywood and latewood density based on tree growth rate model can accuratelydescribe wood density ridial distribution, which provided theoretical basis for the wood densityon the macro and micro intrinsic link, as well as mathematical foundation for quantitivelyanalyzing effects of earlywood and latewood non-uniform density on wood drying. 2. Heat and mass transfer during wood drying process was separately expressed byFourier law and Fick’s law of diffusion. Based on the density distribution function ofquarter-sawed lumber and flat-sawed lumber drying, a one-dimensional mathematical modeldescribing heat and mass transfer “multi field-phase transitions-diffusion” was established.Using finite difference method and FORTRAN language programming, with boundaryconditions, numerical solutions of this model was obtained.3. Using slicing method to test the applicability of X-ray scanning method in layer MCdetection. The result showed that there was no significant differece between them (P>0.05),and we can get accurate MC distribution in wood by X-ray scanning method. With this method,layer MC variation of quarter-sawed lumber and flat-sawed lumber under dry bulb temperature60℃(wet bulb temperature40℃) and dry bulb temperature90℃(wet bulb temperature75℃)were detected. Then, it compared with the result got from heat and mass transfer model. Itindicated that the model can reflect the MC distribution during wood drying process. Thevariation of average MC got from drying test that quarter-sawed lumber and flat-sawed lumberunder different dry bulb temperature and wet bulb temperature was good agreement with theresult obtained from heat and mass transfer model. Therefore, it showed that the heat and masstransfer model can reflect heat and water transfer in wood drying.4. Through the heat and mass transfer model, we can can draw some condusions: in woodconstant drying stage, drying rate depended on the surface evaporation rate, and dryingmedium was main factors affecting the rate of surface evaporation. The higher the dryingmedium temperature, the smaller the relative humidity, the greater the flow rate, and the fasterthe drying rate of sawn drying. When the flow rate of the air surpassed1.5m/s，it had nosignificant effects on wood drying rate. In the slowly drying stage, drying rate depended onvolume evaporation rate and interface evaporation rate, which were determined by woodphysical properties. The equivalent mass diffusion rate and thermal conductivity had positivecorrelation with wood drying rate; while timber thickness had negative correlation with drying rate. The effect of thermal conductivity was smaller than equivalent mass diffusion rate.