【作者】 李志伟；

【导师】 曹军；

【作者基本信息】 东北林业大学 ， 机械设计及理论， 2012， 博士

【摘要】 木材干燥是确保木质产品质量的重要前提。对木材进行合理的干燥,能够增强木材的尺寸稳定性,延长制品的使用寿命；改善木材的加工性能,提高利用价值,降低木材的运输费用。目前国产木材对流干燥设备,总体性能指标基本可以满足生产要求,但由于价格、成本等因素的制约,设备普遍存在热效率偏低,干燥介质循环速度不足,介质湿度、含水率等参数检控不准确的问题。顺应国际干燥技术的发展趋势,对把不同干燥方法的适用范围和特点进行有效整合的联合干燥方法的研究,已经成为木材干燥领域主要的研究方向之一。论文结合林业科技支撑计划专题“干燥在线检测数控调控技术”(2006BAD18B0801)以及国家林业局948项目“高频-对流联合加热木材干燥技术引进”(2006-4-105),把高频-对流木材干燥过程作为研究对象,以加快干燥速度、提高干燥质量、节省干燥能耗为优化目标,讨论了高频-对流双热源的匹配、干燥室内部流场、高频发生器的运行时间、隔条厚度等因素。在此基础上,分析了木材内部的传热、传质机理,建立了高频-对流干燥过程的数学模型,为这种双热源木材干燥方法的应用提供了理论依据。本文的主要工作内容包括：通过分别在对流干燥的前期、中期、后期进行高频加热,对联合干燥过程中高频热源的运行时机进行试验研究。结果表明：在高频发生器的功率和频率恒定条件下,材内水分越多,高频电磁波的穿透深度越浅,在干燥后期进行高频加热更容易形成温度梯度。在木材含水率低于20%时加入高频,整个干燥过程中存在与对流干燥相同的含水率梯度分布。结合表面开裂数据的统计结果,发现在干燥后期加入高频对于干燥应力的控制比较理想。针对对流干燥室中介质流速较低且分布不均的问题,使用计算流体动力学(CFD)方法,对轴流风机和干燥间内部流场进行数值模拟。分析了风机流场不同区域,总压、动压、静压的变化趋势；通过建立Reliable k-E两方程湍流模型,探讨了随着安装角度改变对风机出口截面总压和静压的影响,发现随着安装角度增大,叶片要部的低静压区向叶片顶部扩大,总压压升增加,叶片尾迹损失的范围逐渐扩大。通过建立RNG k-ε两方程湍流模型,模拟了带有导流装置的干燥间内部流场分布情况。结果表明：导流板对材堆上部入口气流的速度和分布均匀性有明显影响；底部控制板改善了材堆内部介质的流动；随着垂直气道宽度的增加,材堆入口气流的流量、流速先增大后减小,各层材堆入口速度变化趋于稳定。不同试验条件下,进行联合干燥,比较分析了干燥曲线、分层含水量分布、残余应力、干燥能耗、开裂情况,温度变化曲线。结果表明：高频-对流联合加热的效率随着含水率的降低而增加。随着隔条厚度的减小,材堆降温所需时间有所延长。采用15mm厚度隔条的材堆拥有较高的平均干燥速度、较低的十燥能耗和较小的残余应力。高频加热持续1.5mmin以上至5min,木材各层温度与环境温度差随加热时间变化的比值基本固定,不同含水率阶段升温速度差别很小。基于质量守恒、能量守恒定律,建立了高频-对流联合加热过程中的沿木材厚度方向的热质迁移模型。根据Whitaker体积平均理论,通过有限差分法(FDM)和有限体积法(FVM)把模型和边界条件的控制方程转化离散方程。试验验证表明,模型能够有效反应试材含水率和温度的变化趋势。试验值和模拟值之间的相关系数不低于0.9,对于材内温度的拟合精度低于含水率的拟合精度。更多还原

【Abstract】 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.更多还原