Research On Optimization And Modeling Of Hybrid Wood Drying With High-Frequency And Convective Heating

Wood drying is an important premise to ensure the quality of wood products. Reasonable drying treatment is helpful to enhance the dimensional stability of wood, extend the life of wood products, improve wood processing performance and utilization value, and reduce the transportation cost of lumber. At present, overall performance of domestic convection drying equipment could only meet the basic requirements of wood drying. In order to control the price and cost, low thermal efficiency, lack of circulation rate of drying medium, inaccurate control of medium humidity, moisture content and other parameters are common problems of convective equipment.Different drying methods process special application scope and properties. Complying with development trend of drying technology, hybrid drying, which combined different drying methods effectively, has become one of the main research directions in the field of wood drying. This dissertation originated from Forestry Science Support Project "on-line Detection of Numerical Control Techniques in Wood Drying Process"(2006BAD18B0801) and948Project of State Forestry Administration "Technology introduction of Hybrid Drying with High-frequency and Convective Heating"(2006-4-105). Regarding the hybrid drying process as the research subject, aimed at improving drying rate, drying quality, and reducing energy consumption, this dissertation studied matching strategy of double heating resources, internal flow field of drying kiln, influence of operation period, timing of high-frequency generator, and sticker thickness on drying process. On the basis of these studies, a one-dimension mathematical model was established to described internal heat and mass transfer mechanism of wood during hybrid drying process, which provides a theoretical foundation for the application of this dual heating resources wood drying method. The main work of this study was summarized as following:In order to investigate the operation timing of High-frequency(HF) resource in process of hybrid drying. HF generator worked respectively during early period, middle period, and late period of convective drying. Experimental results indicated that on constant conditions of power and frequency, there are significantly negative correlations between moisture content(MC) and HF penetrability. Temperature gradient was more easily generated when HF generator worked in late period of drying. And MC gradient distributed in the same direction during whole drying process. Combined with surface check data, drying stress generated during heating was smaller.Aiming at problems that slow and uneven distribution of medium velocity in circulating drying kiln. Computational Fluid Dynamics(CFD) was adopted to carry out numerical simulation of axial fan and drying workshop. Total pressure, dynamic pressure, and static pressure in different area of fan flow field were discussed in this chapter. It was found that total pressure increased monotonously with the increasing of installation angle. The range of static pressure zone enlarged gradually to blade tip. So was the range of wake losses. Simulation results of dry workshop indicated that baffle exerted an obvious influence on upper part of piles. Control boards improved inner flow field of piles. With the increasing width of vertical vent, inlet velocity of drying medium increased at first then decreased, and inlet velocity variation of every layer tended to be stable.120mm thickness wood was used to carried out hybrid drying experiments in different experiment conditions. According to drying curve, distribution of MC in thickness direction, energy consumption, surface checks, and temperature curve, there was negative correlation between sticker thickness and cooling down time of piles. It was found that piles with15mm thickness stickers possessed higher drying rate, lower energy consumption, and less drying stress. HF heating for1.5min and continued to5min, deviation between every layer of wood and environment temperature tented to be constant. There was litter difference of heating rate among different MC stages.Based on energy conservation law and mass conservation law, mathematical model was established to describe mass and heat transfer in the thickness direction of wood. According to Whitaker volume averaging theory, the model and boundary conditions were transferred to difference equations by means of finite difference method(FDM) and finite volume method(FVM). The results of experiments demonstrate that the mathematic model was able to simulate drying process of wood under hybrid drying with high accuracy.