理论分析循环风速对木材常规干燥速率的影响

木材常规干燥过程中,除干燥室中介质的温度和湿度外,循环风速也是一个影响干燥速度的重要参数。本文根据传热传质理论,主要建立常规干燥传质数学模型、边界条件以及界面条件,分析了风速对木材表面蒸发速度的影响。结合木材内部水分传输速度与表面蒸发速度,理论分析了循环风速对干燥速率的影响。     

刨花板热压过程中的传热传质研究现状和展望

回顾了国内外在刨花板热压过程中传热传质方面进行的研究,特别是对近30年来基于传热传质机理建立起来的数学模型所取得的探索性进展作了系统的概述。     

白橡锯材热压干燥特性与工艺研究

以白橡锯材为研究对象,采用热压机对其进行热压干燥处理,系统研究了热压温度、热压压力和试件宽度等因素对白橡木材干燥特性、颜色、吸湿性和尺寸稳定性的影响规律,获得了优化的热压干燥工艺。研究结果表明:采用热压干燥法可以实现白橡木的快速高效干燥,随着热压温度的升高,木材干燥速度显著加快,颜色逐渐加深,热压压力和试件宽度对干燥速率和颜色的影响不明显;热压干燥可以显著减小白橡木材的平衡含水率(EMC)和湿胀率,提高其尺寸稳定性,且随着热压温度的升高,白橡木材EMC和湿胀率降低,热压压力和试件宽度对木材EMC和湿胀率无明显影响;与对照材相比,热压干燥白橡木材的EMC降低9.97%~33.67%,径向和弦向湿胀率分别降低8.54%~33.96%、11.26%~30.02%;白橡木材的优化热压干燥条件为:热压温度为140~150℃热压压力为0.1 MPa,板材宽度为自然宽。     

人工林柚木间伐材过热蒸汽干燥工艺研究

为获得高效节能的柚木干燥工艺,以人工林柚木间伐材为研究对象,系统研究了不同过热蒸汽温度（110、120、130、140℃）对柚木锯材干燥过程中的传热、传质速率及干燥质量的影响规律,获得柚木过热蒸汽干燥预热恒湿应力释放、慢速升温恒速干燥及减速升温减速干燥三个不同阶段的临界干燥温度,分别为110、130℃和120℃。研发了分段梯度控温的柚木过热蒸汽干燥工艺,并进行工厂中试研究。结果表明：使用过热蒸汽干燥能有效提高人工林柚木间伐材的干燥质量,缩短生产周期并降低生产成本。     

刨花对流干燥过程的传热传质研究动态

详细介绍并归纳多孔介质与木材领域内的传热传质理论研究概况,从中寻找出适合研究刨花对流干燥过程传热传质的方法.经过理论分析,提出可根据非平衡热力学理论和相平衡理论,建立一个描述刨花对流干燥时刨花外部对流传热传质和内部热、湿迁移过程的非平衡热力学模型.     

木材热压干燥及表面强化研究综述

介绍了热压干燥技术在木材加工利用上的重要意义以及木材热压干燥、表面强化技术在国内外的研究动态。与常规干燥方法相比，热压干燥技术不仅能快速干燥木材，还能显著提高和改善干燥木材的物理力学性质。此外还对汽蒸、热压干燥与表面强化联合处理以及汽蒸与否对木材表面强化的影响等方面的研究进行了展望。     

行业简讯

第七届国际林联木材干燥会议在日举行　　题为“新世纪中环保住宅的湿度控制和木材干燥技术”的第七届国际林联木材干燥会议于今年 7月在日本筑波召开 ,来自 2 2个国家的近 15 0位从事木材干燥研究的学者参会。会议共收到论文 78篇 ,会场宣读 4 9篇 ,内容涉及综述、干燥数学模型、干燥应力、干燥过程传热传质、基本水分关系、干燥过程自动控制、各种干燥新技术及应用等8个方面。我国有 8位代表参会 ,3位代表宣读了各自的论文。国际林联木材干燥组联络员 ,法国 Perre、 Patrick教授在大会做题为“木材干燥——从发展到创新中基础研究的作用…     

细叶云南松天然林和人工林木材干燥特性

利用百度试验法对细叶云南松天然林和人工林木材的干燥特性进行研究,分别制定厚度为25 mm的细叶云南松天然林和人工林木材的干燥基准。结果表明,依据百度干燥试验中干燥缺陷及干燥速度分级标准,细叶云南松天然林和人工林试件均无内裂现象,干燥速度快,其特性等级均为1级;细叶云南松天然林试件的主要干燥缺陷是初期开裂和扭曲变形,其特性等级均为2级,截面变形的特性等级为1级,综合特性等级为2级;细叶云南松人工林试件的主要干燥缺陷是截面变形,其特性等级为3级,初期开裂和扭曲变形的特性等级均为1级,综合特性等级为3级。试验结果可为细叶云南松天然林和人工林木材实际生产过程中干燥工艺提供参考。     

热压工艺对翅荚木芯细木工板性能的影响

为探讨速生翅荚木人工林木材作为芯板制作细木工板的可行性及其适宜的热压工艺参数，以10年生翅荚木人工林木材为研究对象，对翅荚木芯细木工板制备的工艺技术进行了优化，分析了热压工艺因素对翅荚木芯细木工板性能的影响。结果表明：热压工艺因素对翅荚木芯细木工板含水率的影响程度从高到低依次为温度>时间>压力；对翅荚木芯细木工板密度、横向静曲强度、浸渍剥离长度的影响程度从高到低依次为温度>压力>时间。翅荚木芯细木工板适宜的热压工艺参数为：热压温度135℃，热压压力1.2 MPa，热压时间3 min。以翅荚木作芯层研制的细木工板均达到GB/T5849—2016《细木工板》要求，说明翅荚木适合作为细木工板的原料。     

木材干燥应力的研究方法与进展

木材干燥应力的研究对丰富和完善木材干燥理论、建立和完善干燥工艺、提高干燥质量和经济效益具有重要意义,许多研究者从不同角度对木材干燥应力应变进行了试验测定及数值计算.本文对其中的主要研究方法进行了较详细的归纳,并对今后的研究提出了看法和建议.     

微波处理中木材内温度分布的数学模拟

从热传导的物理规律出发,建立微波加热过程中木材内部热传导模型,并通过理论模拟揭示不同微波加热方式对预处理中木材内部温度分布的影响规律。结果表明:微波处理过程中,木材内部的温度分布规律及均匀性与微波加热方式直接相关;当采用单向微波辐射的方式进行加热时,沿着微波入射方向,木材温度逐渐降低,木材内部温差较大,且温度分布均匀性较差;当使用双向微波辐射的方式进行加热时,木材内能形成内高外低的温度梯度,且温度分布均匀性较好。     

计算流体力学数值仿真技术及其在木材干燥研究中的应用

计算流体力学（CFD）数值仿真技术是随着计算机科学发展起来的通过数值计算获得流场流动与传热规律的新技术,已逐渐成为木材科学领域中研究木材干燥技术的重要方法。文中对CFD仿真原理和方法进行阐述,从干燥室设计和优化、干燥室性能分析与干燥过程热质耦合传递等方面归纳总结国内外CFD技术在木材干燥研究中的应用现状,指出目前研究存在的不足并展望未来研究热点。     

Mass and heat transfer mechanism in wood during radio frequency/vacuum drying and numerical analysis

The mass and heat transfer mechanisms during radio frequency/vacuum(RF/V) drying of square-edged timber were analyzed and discussed in detail, and a new one-dimensional mathematical model to describe the transport phenomena of mass and heat during continuous RF/V drying was derived from conservation equations based on the mass and heat transfer theory of porous materials. The new model provided a relatively fast and efficient way to simulate vacuum drying behavior assisted by dielectric heating. Its advantages compared with the conventional models include:(1) 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;(2) The calculated data from different parts of the specimen can be displayed in the evolution curves, and the change law of the parameters can be better described. After analyzing the calculated results, most of the important phenomena observed during RF/V drying were adequately described by this model.     

AN ANALYSIS OF HEAT AND MASS TRANSFERPHENOMENON IN WOOD DURING DRYING

INTRODUCTIONTheconventionaltwingmethodisgener-all3usedduringwoocldryinginChinaandothercountriesatpresent.Fluidvelocity,tem-peratUreandrelativehumidityofdryingmediaarecontrolledmainlybymoisbecontentofwood'dunngwtingprocess.Soitisveryim-portanttounderstandthephenomenonofheatandmasstransferinwoodduringdIyingproc-essformakingreasonablythewtingtechnoI-ogy-controlwtingprocess,heightenwtingquality'.decreasewtingenergyconsumptionanddiminishwtingtime.Thephenomenonofheatandmasstrans-ferinwooddurin…     

Some considerations in heterogeneous nonisothermal transport models for wood: a numerical study

This study compares a number of coupled heat and mass transfer models and presents numerical comparisons of phenomenological coefficients between the four models (Stanish, Perre, Pang, and Avramidis) that are most frequently used in the literature to describe wood-drying processes. The USDA sorption isotherm, the Hailwood-Horrobin model, was adopted to calculate the relations between moisture content in wood and water vapor pressure at any temperature. Due to different assumptions about the driving forces of heat and mass transfer, coefficients in each model represent different values for moisture content and temperature and are closely related to each other. In the case of isothermal mass transfer, the moisture diffusion coefficient in the transverse directions from the Stanish and Pang models increased with decreasing moisture content. This contradicts the Avramidis and Perre models and numerous experimental results. Thermal diffusion effects on the drying process may not be predominant because the nonisothermal state is relatively short. Therefore, the Perre model, which does not consider the thermal diffusion effect, has been used successfully in the drying simulation. However, it may be erroneous in certain cases when the nonisothermal state prevails over the system, such as building physics. The Pang model cannot explain the phenomena of thermal diffusion and moisture thermodiffusion. It might be reasonable to modify the thermal diffusion of the Avramidis model, which is lower than that of the Stanish model. The apparent heat diffusivity was higher than the true heat diffusivity.     

Experimental determination of the convective heat and mass transfer coefficients for wood drying

The knowledge of the convective heat and mass transfer coefficients is required for the characterization of the boundary conditions of the heat and mass transfer equations of a wood drying model based on water potential. A new experimental method for the determination of the convective mass transfer coefficient is presented. This method is based on the measurement of the moisture content, and indirectly the water potential, at the surface of a wood specimen at different drying times. Drying experiments were performed on red pine (Pinus resinosa Ait.) sapwood from nearly saturated to dry conditions at 56 °C, 52% relative humidity and air velocities of 1.0, 2.5 and 5.0 m s⁻¹. The results show that the convective mass transfer coefficient is constant until the wood surface moisture content reaches about 80% and then decreases more or less gradually as the moisture content decreases further. The convective mass transfer coefficient increases with air velocity. A regression analysis shows that there is no significant improvement in considering the water potential gradient near the wood surface when the difference in water potential between the surface and the surrounding air (ψ_s − ψ_∞) is used to determine the convective mass flux at the surface. Also, ψ_s − ψ_∞ is more appropriate than the water vapour pressure difference (pv_s − pv_∞) as the responsible driving force of the moisture flux leaving the wood surface. The convective heat transfer coefficient was determined during the same experiments. A plateau is observed at high values of moisture content corresponding to the constant drying rate period. Received 27 February 1998     

木材干燥过程的热质迁移及其耦合关系

应用不可逆过程热力学研究木材干燥过程热质迁移及其耦合效应。根据不可逆过程的熵的产生率推导出含有耦合效应的传热传质关系式 ,提出湿度梯度不仅引起水分物质流 ,也引起热流 ,以及驱动水分物质流的力应是扩散势梯度。     

Introducing an overall mass-transfer coefficient for prediction of drying curves at low-temperature drying rates

The aim of this study is to justify that the drying rate of wood can be represented by a phenomenological model defining a driving force expressed as the difference between the average wood moisture content and the equilibrium wood moisture content. The results show that the mathematical relation proposed is valid when introducing an overall mass transfer coefficient, K, at low temperature wood-drying rates.     

木材干燥过程含水率和温度变化的数学模型研究

木材干燥过程存在着热质互换和能量的转移,这为从热力学的角度研究木材干燥过程提供了前提条件,因此提出利用热力学原理分析木材干燥过程含水率和温度变化规律的数学模型.首先建立描述含水率和温度变化的高阶非线性偏微分方程组,然后采用有限元分析原理将方程组离散化,再经过一系列数学运算得到方程的收敛解,具体方法为牛顿迭代法,最后通过对木材含水率和温度变化情况仿真结果的分析,验证该方法的可行性及数学模型的准确性.