马尾松微波间歇干燥对干燥效率与速率的影响

分析马尾松木材微波干燥速率随时间的变化规律,比较微波连续辐射和间歇辐射对木材干燥速率和微波能利用效率的影响.结果表明:木材微波干燥过程可以分成加速段、恒速段和减速段3个阶段;在微波干燥过程中,木材含水率在纤维饱和点以上时,其平均干燥速率和水分蒸发效率比在纤维饱和点以下时的高;采用适当间歇辐射对木材输入微波能,微波能利用率较高.     

微波干燥与常规干燥中木材内含水率动态分布

以黑胡桃木材为对象,采用X射线扫描含水率分布测试方法,分析、比较了微波干燥与常规热风干燥过程中木材内部含水率动态分布规律.结果表明:在微波干燥和常规热风干燥过程中,木材内层含水率均高于表层,存在着整体性的内高外低的含水率梯度场;在接个微波干燥过程中,木材内部虽然出现了部分内层含水率低于外层的情况,存在着含水率分布的局部不均匀,但并未出现与常规热风干燥相反的含水率梯度;在微波干燥中,木材内部各层与表层含水率的差值随着干燥时间的延长逐渐降低.随着干燥过程的进行,常规热风干燥中木材芯层与表层含水率差呈现出先增加后减小的变化趋势.     

竹材热压干燥过程中的水分迁移特性研究

研究了竹材热压干燥过程中的水分迁移特性.结果表明:在整个干燥过程中,前期含水率降低较快,后期含水率降低较慢.竹材平均干燥速度与次表层竹材的干燥速度相近;在含水率较高的干燥初期,水分迁移的阻力在竹材表面,水分迁移主要靠毛细管张力作用;在含水率较低的干燥后期,水分迁移的阻力在竹材内部,水分迁移主要以扩散方式进行,干燥速度取决于木材内部水分移动的速度.竹材热压干燥过程中的水分移动,主要受温度梯度和含水率梯度的共同作用.     

干燥过程中木材温度的变化规律以及与含水率关系的研究

通过对高温和常规温度干燥过程中木材温度及含水率变化与分布的测定,研究木材温度变化的特点和木材温度与含水率的变化关系。分析自由水蒸发面移动、水分状态改变对温度变化的影响。在此基础上,探讨利用木材温度控制干燥过程的可能性和实施方法,     

微波和对流联合干燥的试验研究

试验以速生杨树和马尾松为干燥对象,研究了传统的对流干燥、微波干燥及微波和对流联合干燥三种不同供热方式的干燥动力学规律。结果表明:在整个干燥过程中,微波和对流联合干燥能使木材的内、外层含水率分布保持一致;与传统的对流干燥相比,联合干燥可以改善木材质量,提高干燥速度,降低干燥能耗。     

间歇和连续微波干燥对木材内蒸汽压力与温度变化的影响

对间歇和连续微波干燥过程中木材内部温度、蒸汽压力的变化以及二者相互关系进行探索.结果表明:木材在连续微波干燥过程中,温度的变化大致分为快速升温段、恒温段和后期升温段;微波辐射功率增加,升温速度加快,恒温段温度提高,时间缩短;内裂通常在高含水率木材高功率连续加热时出现;在木材温度上升到100℃之后,适当减少微波功率输入,或采用间歇输入微波能的方法可有效避免内裂的发生;炭化通常出现在木材干燥后期.适当控制木材中含水率,避免过低,减少微波能输入或采用间歇输入微波能的方法,可有效防止木材炭化.     

木材干燥预热过程的水分流和热流分析

本文将非平衡态热力学的水分流和热流线性耦合方程应用于多孔介质 (木材 )的干燥预热过程 ,从机理上分析了其水分流和热流的特性 ,同时运用在预热样品边界上积分的方法求解耦合流方程 ,针对木材干燥预热过程进行了具体计算 ,所得的有关结论与以往的实验结论相吻合。这一结果表明线性流耦合方程是研究多孔介质干燥过程中水分流和热流的特性及其相互作用规律的有效方法 ,既可揭示耦合过程中流的本质又可对其进行定量描述。因Soret效应预热过程中存在着进入木材内的水分热扩散流 ,预热结束后木材含水率将有所增加 ,而水分和热量的输运主要集中在预热开始的前 2h内。本文关于木材干燥预热的一些结果 ,可用于木材干燥预热的生产实际中     

木材干燥过程中声发射信号分析北大核心

对木材在干燥过程中产生的声发射信号进行采集和特征分析,结果表明:木材干燥过程中主要采集到两种不同类型的声发射信号:一种信号来自木材内部自由水的蒸发,与木质部导管内水分空穴化过程的声发射信号一致;另一种信号与木材干燥过快时自身形变开裂相关。作为一种木材干燥质量的控制方法,测量和分析声发射信号,可为木材干燥温度和湿度的控制提供依据,实现防止木材干裂、提高木材干燥质量的目的。     

木材干燥过程中声发射信号分析

对木材在干燥过程中产生的声发射信号进行采集和特征分析,结果表明:木材干燥过程中主要采集到两种不同类型的声发射信号:一种信号来自木材内部自由水的蒸发,与木质部导管内水分空穴化过程的声发射信号一致;另一种信号与木材干燥过快时自身形变开裂相关.作为一种木材干燥质量的控制方法,测量和分析声发射信号,可为木材干燥温度和湿度的控制提供依据,实现防止木材干裂、提高木材干燥质量的目的.     

微波干燥木材的应用

对木材进行干燥处理,使其含水率达到一定的使用要求是保证木制品质量的重要手段,因此在使用前都要进行干燥。当前,一般是采用热空气加热木材的传统干燥方法,这种方法,不管其热源方式如何,都是用热空气从外部加热木材,使其表面水分蒸发,形成表面和内部含水率梯度,促使水分由内向外扩散达到干燥木材的目的。若使表面蒸发速度与内部扩散速度相平衡,木材加热均匀、温湿度适当,木材就能快速干燥。因此,在干燥木材时,为抑制初期表面水分扩散,应采用高温高湿的方法增加导热系数,但因木材本身导热性能差,这种方法很难实现。１微波干…     

Study on Wood Drying Properties under Vacuum-floating Pressure

Using Masson pine as specimens, when drying mediums are overheated steam and hot-air respectively, preheating characteristics of wood under vacuum-floating pressure drying are discussed by measurement of internal temperature field and variation of moisture content during period of preheating. The relationship between drying medium conditions and drying rate of wood reveals that, firstly, compared with process of air-drying, time needed to be preheated under vacuum-floating decreased dramatically, meanwhile, the magnitude of condensation on the surface is relatively larger. Secondly, drying rate of wood under vacuum-floating pressure increases coupled with temperature of medium, reduction of absolute pressure, and raise of fluctuation rate of pressure. The sequence of factors contribution to drying rate is listed as following: temperature of medium (t) > fluctuation rate of pressure (f)> absolute pressure (p).     

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。     

A microwave applicator for on line wood drying: Temperature and moisture distribution in wood

Summary An especially designed open microwave applicator was analysed using wood as the material to be heated and dried. The idea was to develop an on line microwave construction consisting of several small open applicators, each fed by a small standard magnetron (for example 1.4 kW main power). The process was analysed by measuring the wood temperature during heating using an IR-camera and detecting the moisture distribution during drying by CT-scanning. Pine and birch wood samples were used in the experiments, mainly 40 mm in thickness. The experiments show that the power distribution differs between dry wood and moist wood. The analysis of the temperature fields captured by the IR-camera during the first minutes allows a rather accurate determination of the MW power. Consequently, the drying proceeds unevenly in the wood specimens, especially in the longitudinal direction. The dimensions of the applicator and its relation to the wood dimension are very important. However, the wood was not destroyed, the temperature and moisture gradients did not affect the wood in terms of checks or deformations. The drying rate in different positions of the specimen varied between 0.30 and 0.80 percentage moisture content/min. The uneven energy, meaning temperature and field distribution, is to be compensated in the future by a moving wood load and by alternating the position of each applicator in a larger scale microwave pilot plant. Received 25 February 1997     

木材真空-浮压干燥特性的试验研究

以马尾松为试验材料 ,首先分别以过热蒸汽和空气作为干燥介质 ,在预热阶段通过对木材内部温度场和含水率变化的测定 ,探讨了木材真空 -浮压干燥预热阶段的特性 ;然后 ,通过对干燥介质条件与干燥速率之间影响关系的研究 ,总结出木材真空 -浮压干燥的特性。结果表明 :真空 -浮压干燥预热阶段进行得非常迅速 ,同时其表面水分的凝结量也较空气干燥大 ;木材的浮压干燥速率随着干燥介质温度的增加、绝对压力的减小和浮动频率的加大而增加。上述因素中对干燥速率影响程度从大到小的排列顺序为 :介质温度 (T) >压力浮动频率 (Hz) >绝对压力 (P)     

Moisture content gradient in a softwood board during drying: simulation from a 2-D model and measurement

Summary A 2-D mathematical model was developed to simulate moisture movement and heat transfer in width and thickness directions within a softwood board during drying. The model is based on wood physiological features and behaviour observed during drying. In sapwood, liquid water movement is assumed to be a consequence of capillary action between liquid and gas phases inside the cell lumens. However, liquid flow does not occur in wood close to the exposed surfaces because at timber surfaces the wood cells are damaged during the sawing process and consequently the liquid column is broken. In heartwood, liquid flow is negligible since the pits are normally aspirated during the formation of heartwood in the growing tree. Water vapour moves under a partial vapour pressure gradient while bound water diffuses within the wood material due to differences in chemical potential. The model was solved numerically to predict moisture-content profiles. Experiments were undertaken to measure the moisturecontent gradient. Samples were removed from a tunnel dryer at intervals throughout drying, frozen overnight and then cut into slices for moisture-content determination. The experimental results were used to verify the model.This work is supported by the New Zealand Foundation of Research, Science and Technology under contract CO4415     

Research on the effect of microwave pretreatment on moisture diffusion coefficient of wood

Based on the pathway of moisture diffusion in wood below FSP, an improved moisture diffusion model has been proposed. The theoretical solution of the mathematic model shows that the diffusion coefficient for microwave pretreated wood was increased no more than 3% compared with that of wood without being treated even under extreme condition. Experiments indicate that the pit membrane structure can be effectively destroyed during microwave pretreatment, but the moisture diffusion coefficient can not be significantly improved. In practice, the effect of microwave pretreatment on moisture diffusion coefficient can be ignored. The drying rate of wood during the sequent conventional drying can not be significantly improved by microwave pretreatment.     

两种干燥环境下思茅松干燥前后抽提物变化的初步研究

采用传统的化学分析和高效液相色谱分析方法,研究思茅松木材干燥前后抽提物成分的变化情况.结果表明:思茅松木材经高温干燥,不仅会发生水分扩散等物理变化,还会发生裂解等化学变化.认为这是由于可挥发性化合物在干燥过程中随水分的蒸发和迁移而从木材中挥发出来,此外,干燥温度升高使木材提取物中部分成分由不可裂解物质变成了可裂解物质,并且从未材中挥发出来.     

Microwave vacuum drying characteristics of Pinus massoniana wood

Microwave-vacuum (MV) drying characteristics of plantation Masson pine (Pinus massoniana) were studied experimen-tally for various levels of microwave radiation time, initial moisture content (MC), vacuum level and wood thickness. The results show that the process of MV drying for wood can be significantly divided into a short accelerating rate drying period, a long constant rate drying period and falling rate drying period, and the second drying period can extend to levels of mean MC below the fiber satu-r...