Study On The Simulation Of Heat And Mass Transfer For Microwave Drying Of Wood

Microwave drying is a fast and efficient drying technology. It has been widely used in various industries. However, the industrialization of wood drying is almost blank. Enriching and improving the theory of heat and mass transfer for microwave drying of wood has important theoretical meaning and practical value of reasonable controlling the microwave drying of wood and improving the quality of dried products. According to the transitive phenomena of moisture and heat in wood during microwave drying of wood and adopting of different heat and mass transfer mechanism in different stages of moisture content, this paper developed a numerical model of heat and mass transfer coupled equations for microwave drying of wood. This paper researched systematically the changeable and distributional laws of moisture content, temperature and vapor partial pressure during the process of microwave drying, analysed the influence factors of wood heat and mass transfer law during microwave drying, established the theoretical foundation of moisture content field and temperature field, and provided a scientific basis for appropriate application of the technology of wood microwave drying. In this paper, the main contents are as follows:This text analysed the process of transfer at different stage of moisture content in the microwave drying process. Free water and vapor is moved by the differential pressures, and bound water is moved by the role of chemical potential. Bound water is moved under the action of the chemical potential. Meanwhile, it also takes into the process of phase change.Based on volume averaging theory, assuming phase-change process in each phase in the local thermodynamics equilibrium, and vapor as an ideal gas, Considering the wood interior each phase's relations of mass and energy conservation, the paper set up a nonlinearity coupling mathematical model of heat and mass transfer in different moisture content stage in the microwave drying process. The basic parameters of the model are the moisture content and the temperature of wood, and the coefficients of the model are functions of basic parameters.A series of nonlinear equation group are obtained through diseretization of governing equations , boundary conditions and initial conditions with fully implicit finite difference technique. The iterative method is used in same time layer to change nonlinear equations into linearization step by step. The predicted values of state-variable of the next layer can be achieved by solving the linearity iterative equation groups, and the next layer of corrected values can be obtained by putting predicted values into discretization equation coefficients. Following the circulation system of predicted-adjustment, Finally, numerical results of heat and mass transfer in entire microwave drying process are realized.Taking Larix gmelinii as an example in the microwave drying, the paper analyzed the fitting relations of experimental datas and predicted values. The results showed, test results and simulation results fit better. The numerical computation results can predict parameter changes and heat and mass transfer process in microwave drying process.MATLAB is used to simulating the numerical results. The paper analysed the variation and distribution of moisture content, temperature and vapor pressure with time along the thickness direction in different stages of moisture content in the microwave drying process. Results indicated that the moisture content, temperature and steam pressure are more evenly distributed along wood thickness direction.This paper researched the impact of initial moisture content and thickness of the wood on the distribution of state-variable. And wood coupling efficiency of heat and mass was taken into account in the microwave drying process. Results indicated that the drying time, drying rate and the drying time ratio changed with the initial moisture content and thickness of wood. We can evaluate moisture content values (temperature values or steam pressure value) by fitting equation. dM/dT is not large and microwave drying rate is fast.