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1.
Determination of diffusion coefficients for sub-alpine fir 总被引：5，自引：0，他引：5
Liping Cai 《Wood Science and Technology》2005,39(2):153-162
Estimates of the drying rate for sub-alpine fir lumber and the effect of process variables on final moisture contents are valuable information for kiln operators. The diffusion approach offers one generalized way to estimate the drying time required and the final moisture content distribution. The diffusion coefficients of normal wood and wet wood of sub-alpine fir lumber were determined by two types of approaches in this study: the traditional method and the inverse moisture diffusion algorithm. The results from both approaches indicate that the diffusion coefficient of normal wood is greater than that of wet wood for sub-alpine fir. 相似文献
2.
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
3.
Summary A system was developed for grouping tropical wood species for kiln drying based on similar estimated drying times. Six tropical species were subjected to drying and water soaking tests; variables were test temperature and specimen thickness. Soaking time, specimen thickness, and test temperature were used in an empirical model to predict drying times. The capability of the model to predict drying times was considered acceptable based on an adjusted coefficient of determination equal to 0.929 and an average error of 14 percent. The overall capability of the method proposed for grouping tropical hardwoods for drying is considered promising and worthy of further investigation. 相似文献
4.
采用非稳态法测定龙竹竹材热压干燥过程中的水分扩散系数,并探讨了温度对水分扩散系数的影响.结果表明:干燥温度越高,干燥各阶段水分扩散系数及平均水分扩散系数也越大;初始高含水率阶段,随含水率逐渐降低,水分扩散系数呈逐步增加趋势,在纤维饱和点附近时达最大值;随后,随含水率逐渐降低呈逐步减少趋势. 相似文献
5.
Summary The dependence of the diffusion coefficient of water in aspen (Populus sp.) on moisture content over the range of 0 to 18 percent moisture content at 43 °C was determined. The method requires a series of adsorption experiments and application of a numerical method for solving the diffusion equation with a moisture-content-dependent diffusion coefficient. The diffusion coefficient was found to increase exponentially with moisture content. The numerical method includes variable boundary conditions, as defined by the surface emission coefficient, so that the effect of surface equilibrium moisture content can be examined. The experimental moisture content-time curves can be predicted by the numerical method and showed an average deviation of 0.2 percent moisture content with the largest deviation being 0.7 percent moisture content. Practical implications and uses of the information derived from the numerical method are also discussed.This research was partially funded under the USDA Competitive Grant program 相似文献
6.
Yutaka Ishimaru Kazutoshi Arai Masato Mizutani Katsuhito Oshima Ikuho Iida 《Journal of Wood Science》2001,47(3):185-191
Some properties of wood (hinoki:Chamaecyparis obtusa) moisture-conditioned by an adsorption process from a dry state and by two desorption processes (from a water-saturated state and from a state with a moisture content slightly below the fiber saturation point) were investigated. The moisture contents of wood conditioned by the adsorption process and by the desorption process continued to approach to one another for the moisture-conditioning period of over 50 weeks. Accordingly, sorption hysteresis should be regarded as a transitional phenomenon that occurs during the process of approaching the true equilibrium, which requires a long time. The wood conditioned by the desorption process beginning from a water-saturated state showed slightly smaller dimensions than those conditioned by the adsorption process with the same moisture content; however, the wood conditioned by the desorption process from a moisture content below the fiber saturation point showed slightly larger dimensions than those conditioned by the adsorption process. The wood conditioned by the adsorption process from a dry state showed a higher modulus of elasticity and modulus of rupture than did the wood conditioned from a water-saturated state with the same moisture content. The mechanical properties of the wood also varied based on the states at which the desorption process was started. This is a notable characteristic of the relation between the drying condition and the mechanical properties of wood. 相似文献
7.
木材干燥导水系数和换水系数的研究 总被引：3，自引：0，他引：3
木材(板、方材)的导水系数和换水系数是反映木材干燥或存放过程中水分迁移的重要物性参数。然而,我国对木材导水系数和换水系数的研究和测定工作十分有限。木材干燥有关的理论计算中,常用原苏联的数据。由于这些数据本身可能存在的误差及用于我国树种的可靠程度难以估计,故使理论结果的实际运用受到限制。本文采用等厚试件系数分离法研究和测定了木材干燥过程动态导水系数和换水系数。 相似文献
8.
Thickness swelling and density variation in aspen flakeboards 总被引：3，自引：0，他引：3
Summary This paper reports the relationship between thickness swelling and density in labmade aspen flakeboards. The boards were manufactured with several levels of initial mat moisture content and urea resin content. Weight increase and thickness swelling at 12 locations on each board were recorded under specified environmental conditions over a period of 71 days. Each board was then cut into 12 specimens, each containing one of the 12 locations at its center, and density was calculated. No definite relationship was found between thickness swelling and density among the 12 specimens of each board. Our results suggest that high initial moisture content and high resin content can decrease thickness swelling. Variations in mat pressure and core temperature during the press cycle and weight increase at 80 percent relative humidity are discussed.The 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, and it is therefore in the public domain and not subject to copyright 相似文献
9.
A drying experiment with 36 mm thick softwood boards having an average initial moisture content of approximately 1.2 (dry basis) was performed. Drying temperatures of 40, 60 and 80°C were used. Relative humidity and superficial air velocity were maintained at 40% and 3.0 m s?1, respectively. Internal moisture content was monitored along the process in the single direction of the internal flux of water. Loss in mass of the entire timber board was also determined. An effective coefficient of mass transfer was tuned to internal experimental profiles of moisture content by involving the Fick’s second law. An explicit finite difference method for the numerical solution of the mass balance represented by the Fick’s equation was combined with the simplex method of optimization to obtain a mass transport parameter in the magnitude of 1.5–3.5 × 10?9 m2 s?1. A positive and significant effect of temperature on the effective diffusion coefficient, which was well described by an Arrhenius type expression, was deduced from this investigation. Although a negative effect of the average moisture content on the internal resistance to mass transfer was also observed, it was much less evident; mainly above the wood fiber saturation point. A negligible influence of the local moisture content on the investigated transport parameter was noticed when either a linear or a nonlinear model correlating these variables was adopted. 相似文献
10.
以40 mm厚的辐射松(Pinus radiata)和人工林杉木(Cunninghamia lanceolat)锯材为对象,研究高温干燥过程中干燥温度和锯材含水率对萜烯类产物释放浓度的影响,并与辐射松常规干燥中的萜烯类释放浓度作比较。高温和常规干燥过程中采抽气样次数均为5次,抽气速度为1.0 L/min,采样时间为30 min,采用Tenax管收集干燥过程中萜烯类产物,用气相色谱仪分析其成分及浓度。结果表明:高温干燥辐射松萜烯类释放浓度高于常规干燥辐射松与高温干燥人工林杉木,但萜烯类挥发物的组分有所差别。常规干燥辐射松锯材萜烯类释放浓度对环境是安全的,然而在高温干燥过程中,尤其在干燥中后期,当萜烯类挥发物浓度大于1.5 mg/m3时,对辐射松和人工林杉木释放萜烯类挥发物应采取回收措施,以确保对环境的安全。 相似文献
11.
针对移行材和早材皱缩等主要气干缺陷,拟定出赤桉半干材的干燥工艺,窑干前进行回复皱缩和减小表面硬化的汽蒸处理,窑干至目标含水率时进行终了调湿处理.结果表明:汽蒸预处理可以有效地回复皱缩材和显著地减小表面硬化;5 d内将试材的平均含水率25.61%下降到11.72%;3 h的汽蒸处理和24 h的湿空气处理均可有效地减小含水率梯度和残余塑性变定,但后者效果更佳;板材干燥质量达到国家标准二级质量的要求. 相似文献
12.
13.
意杨薄板材干燥工艺研究 总被引：1，自引：0，他引：1
应用高温干燥与常温干燥有机结合的工艺技术(简称蒸汽综合干燥法),对意杨25mm厚板材进行处理,在预热阶段进行充分的汽蒸处理,而后进入常压过热蒸汽干燥工艺阶段,当含水率降到25%时,转入常温干燥工艺阶段,直到干燥阶段结束。这样既缩短了干燥周期又提高了干燥质量,同时也提高了干燥窑的生产效率。 相似文献
14.
Matched sample boards from 20 quarter-sawn boards of Victorian Ash (Eucalyptus regnans F. Muell and E. delegatensis R.T. Baker) were dried using three different levels of ramped pre-drying schedules to investigate the effects of moisture
gradients on collapse recovery and internal checking. Prior to reconditioning, most wet cores were found in highly collapsed
boards with low density. Reducing the gradients in these boards is crucial for recovering collapse and closing internal checks.
If time allows the boards to be equilibrated prior to steam reconditioning, a target mean moisture content of ≤20% with a
moisture gradient of close to 5% (core to surface moisture content) is likely to recover slightly more collapse than targeting
a mean moisture content close to ≥15%. However, if time or kiln restraints limit equilibration it is likely to be better to
target a percentage moisture content of closer to 15% in order to ensure that the core to surface moisture gradients are below
8–10%. The slight reduction in collapse recovery with this second approach is less important than the possibility that collapse
and internal checks in the centre of boards with wet cores will not be closed. Care needs to be used with this latter approach
not to over-dry some boards, since moisture contents below 15% will progressively reduce collapse recovery. For boards within
these moisture content guidelines, the application of heat, rather than moisture pick-up, appears to be the most important
component of the steaming reconditioning process. Hence, steaming only needs to be undertaken for long enough to heat the
core of the board close to the target temperature of 100°C. A simple method for estimating this heat-up time for different
thicknesses and species was demonstrated based on a key dimensionless group for heat transfer, the heat-transfer Fourier number. 相似文献
15.
16.
Measuring moisture content profiles in a board during drying: a polychromatic X-ray system interfaced with a vacuum/pressure laboratory kiln 总被引：1，自引：0，他引：1
A standard X-ray source and an energy-sensitive detector were placed on both sides of a wood dryer which operates over a wide
range of drying conditions. The spectrometric X-ray intensities transmitted through the wood sample were related to the mass
concentration of wood and water using a rigorous calculation needed in the case of a polychromatic X-ray beam. The source
and the detector are carried by a motion controller in order to scan the sample at different drying times and determine the
moisture content (MC) profile along the board thickness. Treatment on spectrometric data and the evolution of the MC profiles
in a Norway spruce board (Picea abies) are presented in this work. 相似文献
17.
Changes in the modulus of elasticity (MOE), modulus of rupture (MOR), and stress relaxation in the radial direction of wood (hinoki:Chamaecyparis obtusa) moisture-conditioned by the adsorption process from a dry state and by the desorption process from a moisture content slightly below the fiber saturation point were investigated. The MOE and MOR of wood conditioned by the adsorption process showed significant increases during the later stages of conditioning when the moisture content scarcely changed. However, with the desorption process they did not increase as much during later stages of conditioning, though they increased during early stages of conditioning when the moisture content greatly decreased. The stress relaxation of wood decreased with an increase in the conditioning period with both the adsorption and desorption processes. These results suggest that wood in an unstable state, caused by the existing state of moisture differed from that in a true equilibrium state shows lower elasticity and strength and higher fluidity than wood in a true equilibrium state. Furthermore, the present study demonstrates that the unstable states of wood induced during the course of drying, desorption, and possibly adsorption of moisture are slowly modified as wood approaches a true equilibrium state. 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
19.
Björn A. Källander 《Wood Science and Technology》2002,36(6):477-486
The European Drying Group (EDG) proposal on a wood drying quality standard defines demands on final moisture content variation of the dried wood. The final moisture content variation will depend on material parameters as well as the production process and the wood will always show a “natural” moisture content variation after drying. Thus the drying process has to be defined well enough to allow for the natural moisture content variation in order to fulfil the demands of the drying standards. As the average equilibrium moisture content of the wood in a vacuum drying kiln with pure steam atmosphere is determined by the pressure and the temperature, the demands on the climate control system to fulfil the demands of the drying standard can be calculated with regard to the natural moisture content variation of the wood. In the first part of this contribution the demands on climate control in vacuum dryers are calculated based on the EDG-standard and the natural moisture content variation. In the second part of the contribution the demands on climate control are compared with climate and moisture content measurements from industrial production in vacuum kilns. Critical factors in kiln design and climate control system design necessary to maintain a controlled drying climate are listed. 相似文献
20.
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