樟子松木材干燥过程传热传质多尺度单元表征模型

了解木材干燥过程中的水分迁移和热量传递规律有助于提高木材的干燥质量,改善干燥工艺,节约能源。以樟子松(Pinus sylvestris)为材料,建立能够较准确模拟木材干燥过程中含水率和温度分布变化的多尺度单元表征模型,模型由宏观尺度上3个耦合方程――2个水分扩散方程和1个热量平衡方程,以及微观尺度上的单个细胞水分迁移的平衡方程组成。解析模型的过程为:分析初始条件和边界条件、有限元网格的生成、方程离散化、查找相应物性参数、MATLAB软件编程求解。最后通过试验分析验证了建立的多尺度模型的准确性:在80℃进行切片称质量法和容积密度法试验值和模拟值的比较,2种方法的试验值与模拟值之间均没有显著性差异(P>0.05),表明2种方法都可以反映出模拟值的准确性; 切片称质量法P<0.5,容积密度法P>0.5,表明容积密度法与模型结果更加吻合。用容积密度法进行40,60和80℃下试材平均含水率的变化试验,比较试验结果和模拟值,两者吻合较好,证明多尺度单元表征模型可以反映出干燥的传热传质行为。
Understanding the theory of heat and mass transfer in wood drying is conducive to wood drying process improvement and energy saving. Pinus sylvestr was used as experimental material and a multiscale mathematical model was construct, which can accurately simulate the temperature and moisture content changes during the wood drying process. The model was made up of three coupled equations: two water diffusion equations and one energy balance equation in macroscopic level, and water transfer balance in single cell equation in microscopic level. The process of parsing the model was as follows: analyzing of the initial and boundary conditions, forming of the finite element mesh, the discretization of the equation, searching the corresponding parameters, and solving the programme by MATLAB software. Finally, the accuracy of the established multi-scale model was verified by the experimental analysis. The results showed that:(1)Comparing the experimental value and analog value by slice weighing method and density volumetric method at 80℃, the P value of slice weighing method was lower than 0.5, while the P value of density volumetric method was higher than 0.5, proving that the density volumetric method was more coincident with model method than slice weighing method was;(2)The average moisture content of sawn timber change experiment was performed by the density volumetric method at 40℃, 60℃ and 80℃, experiment and model results were congruence. The multi-scale model could reflect the heat and mass transfer behavior at the wood drying process. Establishing the heat and mass transfer mathematical model from different scales can accurately reflect the moisture content changes in the wood and spatial distribution