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1.
In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum
drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature
gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results
indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly
were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface
was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established,
and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly
correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased.
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Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100] 相似文献
2.
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
3.
分析和讨论了影响马尾松木材横向液体渗透性及其各向异性的机理.对马尾松木材的径向液体渗透性和弦向液体渗透性进行了测量.结果表明,马尾松木材的径向液体渗透性大于弦向液体渗透性,并且,马尾松木材的径向液体渗透性和弦向液体渗透性都是从边材向心材逐渐减小,并随着含水率的下降而升高.其原因是径向液体流动的主要通道是木射线和水平树脂道,弦向主要是管胞壁和交叉场上的纹孔. 相似文献
4.
ZHANG Biguang YI Songlin CHANG Jianmin LI XianjunMaterial Science Technology School of Beijing Forestry University Beijing P.R.China 《中国林业科技(英文版)》2004,3(4):65-71
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). 相似文献
5.
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
6.
Li Xian-jun Li Wen-jun Zhang Bi-guang 《中国林学(英文版)》2007,9(1):63-67
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... 相似文献
7.
8.
Dr. S. Avramidis Dr. S. G. Hatzikiriakos Dr. J. F. Siau 《Wood Science and Technology》1994,28(5):349-358
Summary A model that predicts heat and moisture transfer through wood in the hygroscopic range and which is based on the principles of irreversible thermodynamics, was evaluated with unsteady-state nonisothermal moisture desorption experimental data. The model predicted the phenomenon of thermal diffusion during the initial stages of desorption and results in a very good simulation of the desorption curve and the center's temperature change with time.Symbols Cp
specific heat of air (= 0.24 cal/g K @ 70 °C)
- CT
specific heat of wood, cal/g K
- D
transverse diffusion coefficient, cm2/s
- Eb
activation energy, cal/mol
- Eo
heat of vaporization, cal/mol
- EL
differential heat of sorption, cal/mol
- G
specific gravity of wood
- H
relative humidity, %
- hT
convective heat transfer coefficient, cal/cm2 s K
- hc
convective mass transfer coefficient based on the concentration of moisture in wood, cm/s
- hv
convective mass transfer coefficient based on the concentration of moisture in the air in equilibrium with the wood surface, cm/s
- KM
coefficient for diffusion due to moisture gradient, g/cm s %
- KT
transverse thermal conductivity coefficient, cal/cm K s
- M
moisture content, %
- Po
saturated vapor pressure, atm
- R
universal gas constant, cal/mol K (= 82.056 cm3 atm/mol K)
- t
time, s
- T
temperature, K
- x
distance, cm
Greek Letters
evaporation or condensation criterion
-
wood density, g/cm3
- W
water density (=1), g/cm3
- a
air density, g/cm3
Department of Wood Science and Forest Products Virginia Polytechnic Institute and State University Blackburg, Virginia 24061-0503 相似文献
9.
Summary There are few references in the wood science literature to nonisothermal moisture movement. Some experiments by Voight, Babbitt, and Choong indicate that thermal diffusion in wood may be very significant.Three equations are presented to represent nonisothermal moisture movement through wood in the transverse direction. The first, described in detail in a previous paper, is based upon two driving forces: Soret potential which results in thermal diffusion and chemical potential which results from a gradient of equilibrium relative humidity. All three equations include the same term for Soret potential and, in the second two equations, the thermal-diffusion term was derived by the application of activation theory. The isothermal term in these two equations utilizes moisture content gradient as the driving force and therefore a knowledge of the sorption isotherm is not necessary. The third equation contains an additional term for moisture-content activation to account for the increase in flux with an increased moisture content. All three equations give approximately the same result with the same input data at low moisture contents. The results diverge at high moisture contents and experimental data are required to determine which equation is most representative of the physical phenomenon of nonisothermal moisture movement. 相似文献
10.
微波预热MDF板坯的实验研究 总被引：2，自引：0，他引：2
利用微波对MDF板坯进行预热处理的研究结果表明,微波预热处理不仅能够在很短时间内提升板坯的温度,还能使板坯内的水分重新分布。通过处理,板坯表层的水分得到了提高,芯层水分被降低。这一结果有利于板坯在热压时热量从表层向芯层传递,从而缩短热压时间,提高生产效率。 相似文献
11.
《Wood material science & engineering》2013,8(2):95-110
Abstract Wood moisture content of coated panels of Scots pine sapwood was recorded during 24 months natural weathering in Vienna by logging electric resistance and temperature near the surface. Eight coating systems with various film thicknesses were used, including three solvent borne alkyd stains, three water borne acrylic stains and two water borne acrylic paints. At five sites in Europe wood moisture content of panels coated with three solvent borne alkyd stains, a brown acrylic stain and a white opaque acrylic paint was recorded weekly by changes in panel mass. Fluctuations in wood moisture content were influenced by the film thickness, water permeability and colour of the coating systems used. Degradation phenomena led to decreasing moisture protection of less durable coating systems over time of exposure. The exposure site clearly affected wood moisture content in the panels. 相似文献
12.
汽车车厢底板用竹木复合板的研制 总被引：6，自引：1，他引：6
采用马尾松作芯板、竹片作表板、酚醛树脂作胶粘剂，制造汽车车厢底板用竹木复合板。在确定的涂胶量、热压温度、热压时间条件下，研究了不同的热压压力与竹木复合板性能的关系，得出了适宜的热压工艺条件。研究结果表明，竹木复合板的物理力学性能达到林业行业标准ＬＹ１０５５—９１规定的指标值，为充分、合理、经济、有效地利用竹木资源提供了依据 相似文献
13.
Bruno Esteves António Velez Marques Idalina Domingos Helena Pereira 《Wood Science and Technology》2007,41(3):193-207
Heat treatment of Pinus pinaster and Eucalyptus globulus woods, two important species in Portugal, was performed in the absence of air by steaming, inside an autoclave, for 2–12 h
at 190–210°C. Mass losses increased with treatment time and temperature reaching 7.3% for pine and 14.5% for eucalypt wood.
The wood behaviour with moisture was improved. The equilibrium moisture content decreased by 46% for pine and 61% for eucalypt,
the dimensional stability increased (maximum anti-shrinking efficiency in the radial direction of 57 and 90% for pine and
eucalypt, respectively) and the surface wettability was lowered. In relation to mechanical properties, the modulus of elasticity
was little affected (maximum decrease of 5% for pine and 15% for eucalypt) but the bending strength was reduced (by 40% at
8% mass loss for pine and 50% at 9% mass loss for eucalypt wood). The variation of properties was related to treatment intensity
and mass loss but significant improvements could already be obtained for a 3–4% mass loss without impairing the mechanical
resistance. The response of eucalypt was higher than that of pinewood. Heat treatment of eucalypt wood shows an interesting
potential to improve the wood quality for solid timber products. 相似文献
14.
In this article, a one-dimensional and a two-dimensional approach to the evaluation of local diffusion coefficients for Norway
spruce sapwood from measured moisture content (MC) values are presented. A studied wood sample was dried from the initial
green condition to about 15% mean MC, but here only the diffusive part of the drying process between approximately 25% and
15% mean MC was treated. Measured local MC values were based on nondestructive X-ray computed tomography data. Finite element
calculations were performed with two alternative diffusion coefficients to test the appropriateness of the diffusion coefficients
that were evaluated from the measured MC values. The evaluated diffusion coefficients show interesting dependence on MC and
distance from the evaporation surface. The advantage of using the methods presented is that the diffusion coefficient is calculated
on a local level without having to define a function for the diffusion coefficient’s dependency on other parameters. 相似文献
15.
木材真空-浮压干燥特性的试验研究 总被引：1，自引：0，他引：1
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) 相似文献
16.
气候敏感的马尾松生物量相容性方程系统研建 总被引：1，自引：0，他引：1
【目的】构建气候敏感的马尾松生物量相容性方程系统,分析气候因子对马尾松各分项生物量的影响,为森林碳汇监测和森林可持续经营提供技术支撑。【方法】基于150株马尾松单木生物量数据,采用非线性联立方程组法构建气候敏感的马尾松生物量相容性方程系统,各分项生物量(干材、干皮、树枝、树叶和地上总生物量)选用以直径和树高为自变量的二元生物量方程作为基础模型,利用一阶交叉验证法对所构建的生物量相容性方程系统进行评价。【结果】与传统未考虑气候因子的各分项生物量模型相比,气候敏感的马尾松生物量相容性方程系统预测精度明显提高,且该生物量相容性方程系统可定量描述不同气候带亚区生物量的差异程度,保证干材、干皮、树枝和树叶与地上总生物量相容。【结论】气候敏感的马尾松生物量相容性方程系统能有效分析气候因子对各分项生物量的影响,可应用于其他树种的生物量预估。 相似文献
17.
Summary The transport of heat and moisture during the convective drying of southern pine was examined experimentally. Moisture distribution within the wood samples was measured using gamma attenuation. The accuracy of these measurements was found to be moisture content dependent with estimated uncertainties ranging from 29.5% at 10% moisture content to 6.8% at 120% moisture content. Local moisture content measurements reveal large inflections at high moisture contents. Peaks in moisture content are closely correlated with low fwood density and vice versa. Above the fiber saturation point the measured drying rate is not a well defined function of the moisture content. This is perhaps due to the biological variability of wood as well as grain orientation, although no definite conclusions could be drawn from the data with regard to the effect of grain orientation. The drying rate does not appear to be highly temperature dependent at high moisture contents. Below the fiber saturation point where diffusion is expected to be the dominant transport mechanism, the drying rate is less affected by biological variability and grain orientation, however, the effects of temperature are evident.The authors wish to acknowledge the support of this work by the National Science Foundation under Grant Number CME-7906367. We must also express our appreciation to staff members at Weyerhaeuser Technology Center who supplied us with wood samples. In addition to the authors, Mr. T. R. Brown, S. H. Moen, and D. Chow played a critical role in gathering much of the data 相似文献
18.
文章以马尾松、尾叶桉、马占相思和南洋楹4种木材为研究对象,采用KJQ、TRQ、WAQ 3种涂料与防腐剂复配处理木材,研究复配涂料对木材耐腐性能的影响。结果表明KJQ复配涂料处理对提高马尾松的耐腐性能较其它2种涂料好。经KJQ复配涂料处理后,马尾松、尾叶桉、马占相思和南洋楹木材的耐腐性能由原来的稍耐腐等级提高到强耐腐等级,其中KJQ涂料与DDAC、IPBC和SGB 3种防腐剂活性成分复配后的处理材耐腐性能较好。 相似文献
19.
20.
Isoko Takahashi Yasuo Takasu Takanori Sugimoto Youji Kikata Yasutoshi Sasaki 《Wood Science and Technology》2010,44(4):607-619
In this study the thermoplastic flow behavior of steamed wood flour was investigated. First it was demonstrated that steamed
Japanese beech flour flowed out of the nozzle under compression at high temperature in a thermal flow test with a capillary
rheometer. The effects of the steaming temperature, steaming time, compressive pressure, and moisture content of wood flour
on the thermal flow temperature were examined. It was shown that the higher the steaming temperature and compressive pressure,
the lower the thermal flow temperature. Also, the thermal flow temperature of the sample steamed at 200°C for 20 min became
lowest and increasingly higher over time. Furthermore, the thermal flow temperature became linearly low with increasing moisture
content of the sample under 15%, whereas it became essentially constant over 15%. It is clarified that compressive pressure
and moisture content as well as the steaming conditions profoundly affect the thermoplastic flow behavior of steamed wood
flour. 相似文献