高频-对流联合加热干燥对木材温度梯度及干燥质量的影响

Effect of Radio-frequency Convection Drying on Wood Temperature Gradients and Drying Quality

对断面120 mm×120 mm的落叶松含髓心方材进行高频-对流联合加热干燥,通过控制双热源匹配获得木材横断面上的不同温度梯度,检测不同温度梯度干燥过程中木材的干燥速率、开裂、含水率分布、应变等变化,进而解析温度梯度对干燥速率、应力及质量等的影响。研究结果表明:(1)通过调节高频-对流双热源匹配(高频加热和停歇的时间),可控制木材干燥过程中的温度梯度及其变化范围。(2)试材正向(内高外低)温度梯度适当增大,其干燥速度加快,终含水率分布均匀。(3)正向温度梯度对减小木材芯表层含水率差的作用,体现在干燥过程的中后期,且作用效果随温度梯度增大而增强。(4)干燥前期,表层产生较大拉应变,中间及芯层则产生压应变,且拉、压应变皆随温度梯度增大而增大。干燥中期,中间层由前期的压应变转为拉应变,其随温度梯度的增大而减小;干燥中后期,表层及中间层产生压应变,芯层产生拉应变,其均随温度梯度的增大而增大;干燥后期,温度梯度对试材各层应变的影响不大。

The box-heart larch with a cross-section of 120 mm×120 mm was dried under radio frequency combined with convective drying,and the different temperature gradients in the cross-section was obtained by controlling the match of two heat sources. Some main parameters,including drying rate,checks,distribution of moisture content,and drying strains,were investigated with different temperature gradients. The effect of temperature gradients on drying rate,drying stress and drying quality were analyzed. The variation of temperature gradients could be handled by changing the match of radio frequency and convection heating. The increase of the positive temperature gradient accelerated the drying rate and made the distribution of final moisture content evenly. The influence of positive temperature gradient on the decrease of the moisture content gradient was reflected in the mid and later drying stage,and the effect was in line with the temperature gradients. In the early stage of drying,the tensile strain was presented in the surface layer,but the compressive strain was in the middle and core layers,and all of the drying strains were increased with the increased temperature gradients. In the middle stage of drying,the compressive strain was replaced by tensile strain in the middle layer,and the strain was decreased with the increased temperature gradients. In the mid and later drying stage,the compressive strain was presented in the surface and middle layers,but the tensile strain was in the core layer. In the latter stage of drying,there was little effect of temperature gradients on the drying strains for each layer.