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1.
马尾松木材在高温干燥中的水分扩散性 总被引：9，自引：0，他引：9
对马尾松木材在高温干燥过程中的水分非稳态扩散进行了研究 ,结果表明当含水率高于纤维饱和点时 ,水分扩散系数随含水率的降低而增加 ;当含水率低于纤维饱和点时 ,水分扩散系数随含水率的下降而减少。马尾松木材的径向扩散系数大于弦向扩散系数。随着温度的升高和相对湿度的降低 ,木材的横向水分扩散系数增大 相似文献
2.
短周期工业材干燥过程中木材内部水分迁移特点的研究 总被引：8，自引：0，他引：8
研究了短周期共业材新伐材和气干材在干燥过程中木材厚度方向含水率分布和水分迁移特点,考察了干燥过程中介质温度变化及其对表层产中心层干燥速度的影响。研究表明,原木初含水率径向、从向存在变异、导致板材初含水率分布无规律。在非稳态干燥中,逐渐呈外低内高近似抛物线分布,直至平衡处理后,内外层含水率才接近一致。平均干燥速度随介质温度的提高而增加,不同干燥阶段介质温度的影响也不同,在相同的平衡含量水率条件下,提 相似文献
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樟子松木材高温干燥过程中水分的非稳态扩散和干燥能耗的研究 总被引：1，自引：0，他引：1
为更加高效广泛地利用樟子松材,对不同初含水率(IMC)的木材进行高温干燥试验。探究相同厚度下不同初含水率木材高温干燥能耗。并依据非稳态扩散理论,在一定的实验条件下探讨不同初含水率对水分径向扩散系数的影响。结果表明:初含水率越高,扩散系数越大,在纤维饱和点附近达到最大值。当含水率在纤维饱和点以下时,水分扩散系数随着初含水率的增加而增大;而当木材含水率在纤维饱和点以上时,其扩散系数基本保持恒定。樟子松材高温干燥过程中,干燥时间越长,能耗越大;不同初含水率试件干燥结束时的能耗不同,初含水率越高,高温干燥过程用时越长,能耗越大。木材干燥过程中依据含水率的差异进行分级干燥可以达到节能的目的。 相似文献
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锯材在纤维饱和点以下干燥时,水分的迁移过程主要是以非稳态扩散形式进行的,非稳态扩散指木材内部的水流通量和含水率梯度随时间和空间而变化。为了给木材干燥提供依据,采用非稳态法在一定的试验条件下测定了木材的水分扩散系数,探讨了不同材种、干燥介质温度、初始含水率、纹理方向对扩散系数的影响。结果表明:扩散系数与树种有一定的关系,通过比较,樟子松的扩散系数为7.06×10~(-6)cm~2/s,杨树的扩散系数为9.61×10~(-6)cm~2/s,杨树的扩散系数大于樟子松的扩散系数;在纤维饱和点以下,初始含水率30%时樟子松的木材水分扩散系数为7.54×10~(-6)cm~2/s,25%时为7.06×10~(-6)cm~2/s,20%时为6.61×10-6cm~2/s,15%时为6.37×10~(-6)cm~2/s,初始含水率越高,水分扩散系数越大;在相同条件下,干燥介质的温度为90℃、80℃、70℃、60℃、50℃时,对应的樟子松木材水分扩散系数分别为8.04×10~(-6)cm~2/s、7.32×10-6cm~2/s、6.63×10~(-6)cm~2/s、5.94×10~(-6)cm~2/s、4.91×10~(-6)cm~2/s,扩散系数随着温度的升高而增大;在不同的纹理方向,木材水分扩散系数不同,樟子松纵向、径向、弦向的扩散系数分别为7.06×10~(-6)cm~2/s、3.63×10-6cm~2/s、2.14×10~(-6)cm~2/s,由大到小依次为纵向、径向、弦向。 相似文献
6.
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. 相似文献
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圆竹材的合理干燥是决定竹材利用质量优劣和寿命的重要环节,圆竹材的微波真空干燥,是一项探索性研究课题。笔者采用微波真空干燥方法,在一定的干燥基准条件下,依据含水率变化曲线,将干燥过程分为加速、恒速和减速干燥三个阶段。研究结果表明,干燥初期随着温度逐渐上升,水分蒸发量逐渐增加,中期恒速干燥阶段,大量液态水转化为水蒸气排出,持续时间长,后期减速干燥阶段含水率较低,主要是结合水的蒸发,含水率变化很小,三个阶段所占干燥时间分别约为20、90 min和40 min;沿壁厚方向干缩率不明显,沿着竹竿高度方向由下向上,干缩率逐渐增大。 相似文献
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百度试验条件下木材含水率场的数学分析及其实用意义 总被引：2，自引：0，他引：2
为将控制工程理论中的系统辩识方法应用于木材干燥技术，本文建立起在进行百度试验时沿厚度方向木材含水率场随时间变化的数学模型并求出其数学解。这一数学解表述了在百度试验条件下木材初含水率、干燥时间、即时含水率和相应的扩散系数之间的动态函数关系。据此，可以通过试验的方法得到在此条件下不同树种木材水分在不同含水率条件下的扩散系数，从而为制定木材干燥基准给出必要的数据。本文试图为用控制工程理论研究木材干燥技术提供一条新的思路。 相似文献
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采用常规热压法对刨花板板坯进行热压,探讨热压时中心层温度变化规律与板坯含水率、板厚、板材密度及热压温度等的关系.结果表明:在快速升温段,升温速度随板厚的增加而明显减小,随热压温度的提高而加快;在慢速升温段,升温速度随板厚的增大而显著加快,随热压温度的升高而明显加速,升温速度受目标密度和板坯含水率影响很小;板坯内水分蒸发所需时间随板厚、板坯含水率、热压温度、板材密度的增长而增加;板坯内水分蒸发温度随板材密度的增加而升高,随板厚的减少而升高,热压温度和板坯含水率对其几乎没有影响;加入胶粘剂会使快速升温段的升温速度有所加快,而使恒温段的水分蒸发温度有所降低. 相似文献
11.
Li Xian-jun Zhang Bi-guang Li Wen-jun 《中国林学(英文版)》2005,7(2):47-51
In order to analyze the effects of the temperature gradient on moisture movement during the highly intensive microwave-vacuum drying, thermal diffusion of Masson pine wood was studied. Internal distribution of temperature and moisture in Masson pine samples sealed by epoxy resin and aluminum foil was measured, the magnitude of thermal diffusion was calculated and the influencing factors of thermal diffusion were discussed. Results showed that with the transfer of moisture toward the low temperature in wood, opposite temperature and moisture gradient occurred. The initial moisture content （MC）, temperature and time are important factors affecting this process; the thermal diffusion is in proportion to wood temperature, its initial moisture and time. The temperature and distance from hot surface is strongly linearly correlated, and the relationship between MCs at different locations and distance from the hot end surface changes from logarithmically form to exponentially form with the increase in experimental time. 相似文献
12.
W. T. Simpson 《Wood Science and Technology》1993,27(6):409-420
Summary Diffusion analysis can be used to estimate the time required to dry lumber. However, more accurate calculations require additional information on the relationship between the diffusion coefficient as a function of moisture content and on the effects of temperature, equilibrium moisture content, board thickness, and air velocity on drying time and moisture gradients. The primary objectives of this study were (1) to determine the diffusion coefficient of northern red oak (Quercus rubra) as a function of moisture content and (2) to compare experimentally determined sorption times and moisture content gradients with those calculated by the diffusion model. The diffusion coefficient was found to increase approximately exponentially with moisture content over a range of 6 to 30 percent at 43°C. This relationship was similar in both adsorption and desorption tests. Experimental adsorption and desorption times and desorption moisture content gradients were compared with finite difference solutions to the diffusion equation. Practical uses of solutions to the diffusion equation are illustrated for kiln drying wood that has first been predried to below the fiber saturation point. Drying time is also related to variable kiln conditions and board thickness.This research was partially funded under the U. S. Department of Agriculture Competitive Grant programThe Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time 相似文献
13.
Some considerations in heterogeneous nonisothermal transport models for wood: a numerical study 总被引：1，自引：1，他引：0
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. 相似文献
14.
短周期工业材干缩率和干燥应变规律的研究 总被引：1，自引：0，他引：1
研究了短周期工业材在干燥过程中木材表层和内部各层干缩率及弹性和塑性应变的变化规律。结果表明,基准硬度水平对应变值的大小和最大变就值的到达时间有显著影响。干燥前期,木材表层伸张弹性应变最大值随着温度和平衡含水率的增加而减少,木材表层塑性应变则随平衡含水率的增加而增加。正确调节基准初期阶段的平衡含水率水平,能有效地控制干燥前期应变最大值,从而达到控制干燥质量的目的。 相似文献
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The effects of high temperature steam pretreatment on the change in wood moisture content (MC) and characteristics of vacuum drying were investigated in this study. Poplar and manchurian walnut woods were pretreated with high temperature steam at 100°C and 140°C, prior to vacuum drying. A comparison of the characteristics of vacuum drying between steam pretreated wood and untreated wood was carried out. The results show that during steam pretreatment, the MC of wood decreased within a few hours. The reduction of MC varied with the temperature; the higher the temperature, the faster the MC dropped. During the vacuum drying stage, the rates of drying of pretreated samples were higher than those of untreated samples when MC was below the fiber saturation point. Furthermore, the total drying time of samples treated at a steam temperature of 140°C was lower than that of untreated samples. Therefore, a vacuum procedure after steam pretreatment can effectively shorten the drying time when drying wood. 相似文献
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南方阔叶树材干燥初期应变特点的研究 总被引：6，自引：2，他引：6
在不同温度和平衡含水率条件下研究南方阔叶树材干燥初期应变特点。试验结果表明，干燥初期表层呈现伸张弹性应变和塑性应变，内层为压缩应变。伸张弹性应变的最大值是产生表裂的主要原因，塑性应变而引起的表层固定变形是产生内裂的原因。 相似文献
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Analysis of moisture diffusivity of larch timber during convective drying condition by using Crank＇s method and Dincer＇s method 总被引：4，自引：0，他引：4
Two analytical procedures （Crank＇s method and Dincer＇s method） for porous solid materials were reevaluated and used to determine moisture diffusion coefficients and moisture transfer coefficients for larch lumber subjected to drying. A diffusion-like equation was used to describe drying process data. The lumber was idealized in the modeling as infinite plates. The moisture transport process inside the board was assumed to be one-dimensional. The macroscopic drying kinetics curves of larch timber at particular conditions were determined experimentally. Based on these data, calculation for both the moisture diffusion coefficients and moisture transfer coefficients by the Dincer＇s analytical procedure were made. The dynamic moisture diffusion coefficients by the traditional Crank＇s method were calculated. In general, diffusion coefficients calculated by the Dincer＇s method were all higher than those by Crank＇s method. These results could be due to the differences between two analytical methods and also different characteristics between solid moisture diffusion process and heat transfer process. Therefore the analysis and solution procedures of moisture diffusion differential equations need to be adapted in the future. With drying temperature＇s increasing moisture diffusion coefficient （D） and moisture transfer coefficient （k） increases accordingly. Also the relationships between diffusion coefficients and temperature as well as material moisture contents were analyzed by using Arrhenius equation and bound water transport theory. 相似文献
18.
Wet cores, recently observed in highly collapsed boards towards the end of pre-drying, suggest that a previously unobserved
relationship between collapse and the rate of internal moisture diffusion may exist. In this study, a drying model has been
used to fit the diffusion coefficient and other model parameters to the drying data from the earlier study. The model parameters
were fitted to six sub-samples from 20 sample boards (a total of 120 sub-samples). The model was only fitted to the drying
data from green to 50% average moisture content. An inter-relationship between basic density, collapse and the fitted diffusion
coefficient was established and discussed. It is suggested that the effect of collapse on diffusion coefficient could be the
result of two different mechanisms. 相似文献