Radio Frequency Vacuum Drying Of Square-edged Timber With Pith: Mathematical Model And Numerical Analysis

Drying is one of the most important components of the lumber manufacturing process. The development and implement of the drying technology with efficient and high quality are significant for improving the performance and the quality of the wood, prolonging the service life of wood products, saving the energy, protecting the environment and high efficient using of the wood. Radio-frequency/vacuum (RF/V) drying is an advanced combined drying technology with high product quality and short drying times, but it is difficult to detect the major parameters such as temperature and the mositure content (MC). Though there is the appropriate drying process, it is difficult to implement, thus the technoloyg is hard to popularize in wood drying process. In this paper, combining with the characteristics of high frequency vacuum drying, the mass and heat transfer mechanism along fiber direction of the wood with the pith was in-depth analyzed, and a one-dimensional mathematical model to describe the transport phenomena of mass and heat during continuous RF/V drying of was developed from mass and energy conservation equations based on the mass and heat transfer theory of porous materials.The variation and distribution laws of the parameters (MC, temperature, vapor pressure and etc.) were systematic studied by solving the governing equations with numerical analysis method. Meanwhile, it can lay the foundation for the automatic control of wood drying and provide essential information for improving drying quality. The main content of the research and the results are as follows:The mass and heat transfer mechanism of the wood along fiber direction was stated in detail:For the square-edged timber which the length was shorter than 200cm and cross section is larger than 12cm, the free water and vapor were transferred in the form of bulk flow along the fiber direction under the total pressure gradient, and most of the moisture was evaporated at first, and then transferred in the form of vapor to the surrounding; The energy was transferred in the form of heat conduction. The Fick’s and Darcy’s were unified by theoretical derivation, and the form of bulk flow was expressed by the form of diffusion equationIn order to facilitate the model’s construction and solution, the structure of the wood and the physical drying process were simplified and assumed, considering the conservation relations of momentum, mass and energy between each phase of the wood, a one-dimensional mathematical model to describe the transport phenomena of mass and heat during continuous RF/V drying of was developed. In this model, time and space were independent variable, and MC, temperature, density of vapor, vapor pressure, the evaporation rate were dependent variable.According to Whitaker’s average volume theory, the governing equations and corresponding boundary conditions of the one-dimensional mathematical model were discretized in the range of calculation using the finite volume method, and an explicit scheme method was selected for time and space integrations, the softwere Matlab which has the advantages of fast calculation speed and high precision was used for programming and calculations. Compared with conventional models, it has the following advantages:Each independent variable has a separate control equation and is solved independently by converting the partial differential equation into a difference equation with the finite volume method; The calculated data from different parts of the wood specimen is displayed in the evolution curves because the specimen is divided into several units.The effects of the permeability, length, initial MC, RF power on the change and distribution laws of the MC and temperature were analyzed by changing the parameters of the model. The results indicate that the parameters mentioned above had an effect on the drying time, drying rate and temperature, and the effects of RF power and permeability were higher. Under the same drying condition, if the RF power was larger, the drying time is shorter, the drying rate is higher, the rising rate of temperature is higher; if the permeability is higher, the drying time is shorter, the drying rate is higher, the rising rate of temperature is lower.The square-edged timber Sugi specimen was dried in a laboratory RF/V dryer and the total average MC and local temperature were monitored as a function of time. By analyzing the calculated and experimental results, it can be concluded that most of the important phenomena observed during RF/V drying can be adequately described by this model.