Abstract:

Objective: The shrinkage model of wood control volume is established, the wood profile density measured by X-ray method is used to calculate the moisture content distribution, which provides a new method for the calculation of wood moisture content distribution. Method: The wood before oven drying is divided into control volume along the thickness direction, the oven-dry shrinkage parameter is introduced, the thickness of the wood control volume after oven drying is calculated according to the wood shrinkage principle, the density of the control volume is calculated by the wood profile density, and the calculation model for computing the moisture content of the wood control volume before oven drying is constructed. In the calculation model, the moisture content of the wood control volume before oven drying and the thickness and density of the wood control volume after oven drying are highly coupled. Therefore, the results of the above variables are computed by developing an iterative algorithm and compiling a calculation program in Matlab software. The algorithm takes the actual thickness of the wood after oven drying as the constraint condition to continuously correct the oven-dry shrinkage parameter of the wood, it can effectively avoid the calculation error caused by the deviation between the introduced value of oven-dry shrinkage and the real value. At the same time, this study carried out Populus tomentosa in the convective drying test of wood, the calculation method of this study is used to calculate the moisture content distribution of wood in different drying stages, and compared with the actual average moisture content of wood (weighing method) and the moisture content distribution calculated by method of assuming uniform shrinkage in the thickness direction of wood. Result: The average moisture content of wood measured by the calculation method in this study is very close to that measured by the weighing method, and the correlation coefficient square (R²) is more than 0.999 2. Compared with uniform shrinkage method the distribution of wood moisture content calculated by the two calculation methods is consistent above FSP (fiber saturation point); Below FSP, the moisture content calculated by the calculation method in this study is relatively low in the wood surface layer, relatively high in the wood core layer, and large moisture content gradient in the wood core and surface layer. When the wood is dried for 45 h, 69 h and 134 h respectively, the moisture content differences of core and surface wood calculated by the calculation method of this study are 22.61%, 15.65% and 5.58% respectively. The moisture content differences of core and surface wood calculated by existing uniform shrinkage method are 18.06%, 11.51% and 4.27% respectively. The absolute deviations of the two calculation methods are 4.55%, 4.14% and 1.34% respectively. Compared to the uniform drying method, the accuracy of the moisture content distribution of wood measured by the calculation method in this study was improved by 3.80%, 6.00% and 4.04% at 45 h, 69 h and 134 h respectively. Conclusion: The control volume shrinkage model constructed in this study fully considers the non-uniform shrinkage caused by moisture content distribution, which can make up for the deficiency of assuming uniform shrinkage of wood in existing uniform shrinkage method so as to improve the calculation accuracy of wood moisture content distribution and provide an effective technical means for the dynamic detection of wood moisture content distribution.

Key words: shrinkage model, X-ray scanning method, moisture content distribution, control volume, iterative calculation