木材干燥产生缺陷的预防措施及补救方法

缺陷对木材的加工及使用影响很大 ,本文对木材干燥产生的缺陷及产生的原因进行了探讨 ,提出干燥引起缺陷的预防措施。     

CLASSIFICATION AND COMPARISON OF WOOD DRYING MODELS

The Various models established by many scholar in the near years areclassified and compared between them.     

非接触式无损检测木材干燥应力的方法

木材干燥应力是干燥过程中产生干燥缺陷的主要因素。文章根据数字散斑相关方法(DSCM)检测应力的原理设计出了一种非接触式无损检测木材干燥过程中应力的方法,用木材干燥表面测点位移的变化速率即应变速率来表示木材干燥过程中的应力状态,初步提出了建立木材干燥应力评价体系的设想。研究结果表明:木材表面测点位移变化量与时间存在着对数关系,与含水率变化量则呈线性关系。     

橡胶木的防腐及干燥

橡胶木作为商品材具有双重经济价值，在木材利用方面因其易受真菌和昆虫的侵蚀，妨碍了对其材质的使用。现从木材干燥的角度论述：如何最大限度地减少真菌的产生和昆虫的侵蚀，合理地利用橡胶木。     

Diffusion of bound water in wood

Summary Choong's (1963) data for isothermal sorption of water vapor by wood are used to compute pressures, chemical potentials, and entropies of water in the wood specimens of his nonisothermal mass equilibrium experiments. Entropies of both the bound water and water vapor were reasonably constant. A balance existed between thermal diffusion and mass diffusion, as indicated by gradients in temperature and chemical potential. This balance also is suggested by opposing gradients in spreading pressure and vapor pressure. Equal chemical potentials showed that the vapor and bound water were in equilibrium. The model proposed by Siau (1980) for nonisothermal diffusion is consistent with these results. Expressions are given for the two unknown parameters in this model: moisture conductivity and heat of transfer. The constant entropy of water vapor is used to show that the heat of transfer exceeds the activation energy for bound water diffusion by about 25 percent.The author wishes to thank Dr. Christen Skaar for his helpful comments during preparation of this paper for publication     

Diffusion of bound water in wood

Summary A model for isothermal bound water diffusion in wood is derived from Babbitt's (1950) analysis of diffusion in adsorbing solids. Calculations of the energy required for water molecules to become dissociated from their sites is identified as one component of the activation energy for diffusion. Consideration of the resistance to diffusion leads to a second component of activation energy associated with overcoming the attraction of water molecules for themselves. Also, an approximate expression for the resistance coefficient is developed. These results are combined into a transport model for bound water. The model shows that equations for bound water movement based on fluid mechanics (Babbitt 1950) and thermodynamics (Katchalsky, Curran 1965) are identical when the driving force for diffusion is defined as the moisture flux per unit transport coefficient. Activation energies and diffusion coefficients derived from the model compare favorably with literature values.The author wishes to thank Dr. Christen Skaar for his interest and advice during preparation of this paper for publication     

熏烟木材干燥窑的技术改造

熏烟木材干燥窑通过技术改造,干燥后的板材含水率均匀,应力小,周期短,干燥质量完全达到GB6491-1999《锯材干燥质量》标准。     

Diffusion of bound water in wood

Summary Fick's law states that during steady-state diffusion, moisture flux is proportional to the gradient in moisture concentration, making concentration gradient the driving force for diffusion. Although Fick's law permits convenient comparisons between predicted and measured moisture contents, fundamental and experimental difficulties with it have been reported. In this paper, the dependence of spreading pressure on vapor pressure and temperature is developed. This information is used to show that gradients in spreading pressure and chemical potential are not proportional. Spreading pressure gradient is considered the correct driving force because the associated transport law can be derived from first principles.The author wishes to thank Dr. Christen Skaar for his interest and advice during preparation of this paper for publication     

Stress-strain state of wood at kiln drying

Summary A model has been suggested which allows the calculation of stresses arising in kiln drying and humidification of wood, as well as the total residual strain, i.e. set strain _s, consisting of purely residual strain _r and the so-called frozen strain _f. Frozen strains arise under the operating influence of loading when the stiffness is increased because of a decrease in moisture content (or temperature) of the wood. The process of formation of set strains _s has been shown to depend on the history of loading, variations of the moisture content of the wood, as well as cooling of the section before the measurement of _s. The possibility of using set strain as a parameter of the state of stress of wood in kiln drying has been noted.Presented at the VIII. International Symposium on Fundamental Research of Wood. Warsaw, Poland, October 1990     

现代热泵木材干燥技术的发展

论述了现有热泵木材干燥技术的发展，现代热泵木材干燥技术吸收了常规木材干燥技术的某些优点，具有木材干燥周期越来越短、被干燥木材质量和节能效果明显等特点。     

马尾松板材的脱脂处理

以福建产马尾松为研究对象，探讨了利用碱性药剂对马导松板进行脱脂处理的工艺。通过正交试验分析了温度、浓度，时间以及水与木材体积比对脱脂效果的影响与原因。     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

Investigations on the Heating and drying of wood with infrared radiation

Summary By means of a newly developed measuring device the temperature distribution on the surface and inside of dry samples of beech and spruce was determined at one-sided short and long wave infrared radiation in dependency on time and intensity of radiation and on the thickness of the wood. Furthermore, the surface temperatures of moist samples were measured as a function of moisture content. The results which are important for the practice of infrared radiation were plotted in diagrams from which e. g. the increase of the temperature at the radiated surface in dependence on thickness and species of the wood and on the duration and intensity of short and long wave infrared radiation can be read. The drying tests, made on wood samples of different thickness by means of one- and two-sided short and long wave infrared radiation, gave valuable information on the practical application of infrared wood drying.

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Zusammenfassung In ausgedehnten Versuchsreihen wurden mit einer neuentwickelten Meßanordnung die Temperaturverhältnisse an der Oberfläche und im Inneren von trockenen Buchen- und Fichten-brettproben bei einseitiger Bestrahlung mit Hell- und Dunkelstrahlern in Abhängigkeit von der Bestrahlungsstärke, Bestrahlungsdauer und Holzdicke aufgenommen. Weiterhin wurden die Temperaturen an der Oberfläche feuchter Brettproben in Abhängigkeit vom Feuchtigkeitsgehalt bestimmt. Die Ergebnisse wurden in Diagrammen dargestellt, aus denen z. B. die für die praktische Anwendung der Infrarotstrahlung besonders wichtige Temperaturzunahme an der bestrahlten Oberfläche in Abhängigkeit von der Holzdicke, Holzart, Bestrahlungsstärke und Bestrahlungsdauer bei Hell- und Dunkelstrahlung abgelesen werden kann. Die mit ein-und zweiseitiger Bestrahlung an unterschiedlich dicken Brettproben mit Hell- und Dunkelstrahlern durchgeführten Trocknungsversuche lieferten wertvolle Hinweise für die praktischen Möglichkeiten der Holztrocknung mit Infrarotstrahlern und für die Wahl der Trocknungs-bedingungen.

     

Numerical and experimental analysis of a wood drying process

 Experimental investigation and computational analysis were performed to evaluate the influence of the ambient air parameters during the drying process on the temperature, moisture and resulting deformations and stresses in wood samples. The numerical procedure uses the Finite Volume Method to discretise the equations governing heat, mass and momentum balance and takes into account the anisotropic nature of wood. The comparison of the numerical and experimental results shows very good agreements, implying that the proposed numerical algorithm can be used as a useful tool in designing wood drying schedules. Received 31 March 1999     

Dynamic viscoelastic behavior of wood under drying conditions

In this study heartwood from a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation was treated using a high-temperature drying (HTD) method at 115°C, a low-temperature drying (LTD) method at 65°C, and freeze vacuum drying (FVD), respectively. The dynamic viscoelastic properties of dried wood specimens were investigated. The measurements were carried out at a temperature range of −120 to 250°C at four different frequencies (1, 2, 5, and 10 Hz) using dynamic mechanical analysis (DMA). We have drawn the following conclusions: 1) the storage modulus E′ and loss modulus E″ are the highest for HTD wood and the lowest for FVD wood; 2) three relaxation processes were detected in HTD and LTD wood, attributed to the micro-Brownian motion of cell wall polymers in the non-crystalline region, the oscillations of the torso of cell wall polymers, and the motions of the methyl groups of cell wall polymers in the non-crystalline region in a decreasing order of temperatures at which they occurred; and 3) in FVD wood, four relaxation processes were observed. A newly added relaxation is attributed to the micro-Brownian motions of lignin molecules. This study suggests that both the HTD and the LTD methods restrict the micro-Brownian motion of lignin molecules somewhat by the cross-linking of chains due to their heating history. __________ Translated from Journal of Beijing Forestry University, 2008, 30(3): 96–100 [译自: 北京林业大学学报]     

Causes of color changes in wood during drying

The forest industry operates in a dynamic and global market where change and competition are the rule rather than the     

Modeling of cell wall drying stresses in wood

All applications of wood involve drying the material from the green state. The cell wall may be viewed as a laminate consisting of different layers. The layers have different orientations and therefore different moisture expansion characteristics. As a result, stresses will develop in the layers due to drying. Micromechanical models for fibre composite materials were used in combination with a laminate analogy in order to calculate these drying stresses in the cell wall layers S1, S2 and S3. Resulting stresses were very high. In reality viscoelastic effects will significantly reduce stresses at high moisture content. However, at lower moisture content irreversible cell wall damage is likely to form as a result of the stresses computed by the model. Received 20 October 1998     

木材微波干燥缺陷产生原因及其预防措施分析

从木材介电特性、微波穿透深度、微波设备、干燥工艺等方面分析了木材微波干燥缺陷产生的主要原因,并从优化干燥器设计,改善微波场分布均匀性,控制微波辐射功率和辐射时间,改变微波辐射方式等方面探讨了预防干燥缺陷产生的主要措施,以期为木材微波干燥技术的研究和应用提供参考。     

Osmotic stress in wood — part II: on the computation of drying stresses in wood

Summary Plastic stress arising in wood during drying is calculated according to the theoretical model developed earlier. The mechanism of stress reversal and the type of resudual stress corresponding to different values of material constants are shown. The results are in qualitative agreement with experimental evidence.List of symbols A coefficient of swelling below the fibre saturation point - C concentration of moisture in wood; weight of moisture per weight of dry wood - C ₀ uniform concentration of moisture in wood at the beginning of drying - C ₁ equilibrium concentration of moisture at the boundary during drying - C =C-C ₁ - non-dimensional concentration - D diffusivity - D ₀ first term in the expansion of diffusivity as function of concentration: D=D ₀(1+D ₁ C+...) - D ₁ secondterm in the expansion (see D ₀) - E Young's modulus - e _ij deviator of tensor of strain: - e _ij ^P deviator of plastic strain: - e _ij ^E deviator of elastic strain - F fibre saturation point (concentration at which the function (c) changes slope) - F =F-C ₁ - g(x,t) function which assumes the value 1 in the elastic zone and 0 in the plastic zone - k von Mises' yield stress - L half width of the sample - M total moisture content - P plastic power - S _ij deviator of stress - S _kk =S ₁₁+S ₂₂+S ₃₃ - S _ij ^E =2 e _ij - T _ij tensor of stress - T _kk =T ₁₁+T ₂₂+T ₃₃ - T non-zero component of stress in a beam or plate - non-dimensional stress - actual stress rate in an elastic zone, fictitious stress rate in a plastic zone - t time - t increment of time - x y z spatial coordinates - X increment of spatial coordinate - Y - Y ₀, Y ₁ terms in the expansion of Y(C): Y(C)=Y ₀(1+Y ₁ C+...) - non-dimensional Y - , (c) coefficient of osmotic expansion (dependent on concentration) - _ij tensor of strain - _kk =₁₁+₂₂+₃₃ - =_yy=_zz non-zero component of strain in the case of a plate or beam - modified strain - elastic constants of an isotropic body - non-dimensional spatial coordinate - Poisson's ratio - non-dimensional time