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1.
This study was carried out to investigate the effects of steam explosion treatment on drying rates, moisture distribution, shrinking, and checking during radio-frequency/vacuum drying of Korean larch pillar combined with a longitudinal kerf. All the pillars except the nonkerf/control with high initial moisture content (MC) could be dried within 89 h from the green condition to about 15% MC. In pillars with high initial moistures or in the early stage of drying, the drying rates were sharply accelerated by steam explosion treatments. The final moisture gradients along the transverse direction were gentler for the steamexploded pillars and for pillars with a longitudinal kerf, respectively, than for the unexploded pillars and for pillars without a longitudinal kerf. The moisture gradients along the longitudinal direction on all the layers were gentler for the pillars with a longitudinal kerf than for those without the kerf. Formation of checking was significantly controlled in the pillars with a longitudinal kerf. All the steam-exploded pillars except the kerf/1-cycle, however, were more severely damaged by checks than the unexploded pillars. A prong test revealed an extremely low level of residual stress for all the dried pillars. 相似文献
2.
《Wood material science & engineering》2007,2(2):55-65
During drying, timber changes its shape, mainly as a result of specific properties such as shrinkage anisotropy, radial differences in longitudinal shrinkage and spiral grain. The distortion, causing severe downgrading, can be reduced by restraining the timber and by using special drying schemes. The research described here is related to a project on the improvement of shape stability of Norway spruce. In the present part, different larch species from three stands were dried at high temperatures (80, 120 and 170°C). The effects of restraint during presteaming, drying and steaming on short-term twist reduction were investigated. The permanency of drying distortions was investigated in subsequent moisture cycling. Results showed a clear dependency of twist on the distance to pith. Restrained specimens sawn close to pith experienced reduced twist. This twist reduction was permanent during subsequent moisture variations. Drying temperature did not significantly influence twist and twist amplitude in moisture cycling. 相似文献
3.
《Wood material science & engineering》2013,8(2):55-65
Abstract During drying, timber changes its shape, mainly as a result of specific properties such as shrinkage anisotropy, radial differences in longitudinal shrinkage and spiral grain. The distortion, causing severe downgrading, can be reduced by restraining the timber and by using special drying schemes. The research described here is related to a project on the improvement of shape stability of Norway spruce. In the present part, different larch species from three stands were dried at high temperatures (80, 120 and 170°C). The effects of restraint during presteaming, drying and steaming on short-term twist reduction were investigated. The permanency of drying distortions was investigated in subsequent moisture cycling. Results showed a clear dependency of twist on the distance to pith. Restrained specimens sawn close to pith experienced reduced twist. This twist reduction was permanent during subsequent moisture variations. Drying temperature did not significantly influence twist and twist amplitude in moisture cycling. 相似文献
4.
Tomy Listyanto Keisuke Ando Hidefumi Yamauchi Nobuaki Hattori 《Journal of Wood Science》2013,59(4):282-289
To investigate the effect of CO2 laser incising under five drying methods on drying characteristics of Sugi lumber, the squares (120 mm × 120 mm) of Sugi lumber with length of 650 mm were used. A half of samples were incised by CO2 laser with incising density of 2,500 holes/m2. Five types of drying methods were used: microwave drying, steam injection drying, and three combinations of microwave heating and steam injection drying. Steam injection drying was conducted by injecting superheated steam of 120 °C through a perforated plate heated to 140 °C of an injection press. Microwave was irradiated with the power of 3 kW at frequency of 2.45 GHz. The results indicated that incising helps heat through a specimen and thus the whole temperature raised rapidly, which was up to threefolds compared to that of no-incised one. Incised specimens dried by a combination of microwave heating for 1 h and steam injection showed the highest drying rate, which was up to 5.3 %/h. Incising and microwave heating contributed positively to dry lumber under more uniform distribution of moisture content and to reduce surface and internal checks. Incised specimen dried by microwave showed the most uniform distribution of moisture content without surface and internal checks. 相似文献
5.
Hermann Pleschberger Christian Hansmann Ulrich Müller Alfred Teischinger 《Wood Science and Technology》2013,47(6):1323-1334
Specific fracture energy measurements were applied to identify changes caused by wood-drying processes of solid wood. Specimen design and geometry as well as parameters and specifications for a fracture energy test were determined experimentally. The specific test set-up was applied on plantation teakwood sample sets of standard as well as alternating convection kiln dryings and one oven drying. The results show that alternating changes of the drying temperature along with the equilibrium moisture content (EMC) in a kiln schedule have a small but significant decreasing effect on the specific fracture energy in the radial/longitudinal as well as the tangential/longitudinal testing direction. Furthermore, oven drying at constant high drying temperature along with low EMC did not result in a significant change of specific fracture energy compared with standard drying, but caused greater scattering values in both transverse crack propagation systems. 相似文献
6.
When solid wood dries from a green condition to a moisture content used for further processing, moisture-induced fracture and stresses can occur. The drying stresses arise because of internal deformation constraints that are strongly affected by the cross-sectional moisture gradient differential shrinkage and the inhomogeneity of the material. To obtain a better understanding of how stresses develop during climatic variations, the field histories of stresses (and strains) in cross sections in their entirety need to be studied. The present paper reports on experiments and numerical simulations concerned with analysing the development of strains and stresses during the drying of 15-mm-thick discs of Norway spruce timber log. The samples were dried at 23 °C and relative humidity of 64 % from a green condition to equilibrium moisture content. The moisture gradient in the longitudinal direction was minimised by use of thin discs simplifying the moisture history of the samples studied. The strain field history was measured throughout the drying process by use of a digital image correlation system. Numerical simulations of the samples agreed rather well with the experimental strain results obtained. The stress results also indicated where in the cross section and when fractures could be expected to occur during drying. More optimal drying schemes showed markedly reduced stress generation. 相似文献
7.
8.
木材微波干燥内部压力对水分移动的影响 总被引:1,自引:0,他引:1
在微波干燥过程中,能量是以电磁波的形式直接渗透到木材的内部,并通过微波电磁场与水分子及木材中极化分子(羟基)的相互作用而迅速产生大量的热,导致木材内水分移动机理与常规干燥很大的不同。实验结果表明:微波干燥过程中,存在内高外低的压力场,内中蒸汽压力是水分移动的驱动力。 相似文献
9.
A microwave applicator for on line wood drying: Temperature and moisture distribution in wood 总被引:7,自引:0,他引:7
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 相似文献
10.
Steam conditioning of softwood boards after kiln drying is of critical importance for relief of residual drying stresses
and to improve distribution of final moisture content. The conditioning practice in New Zealand includes two steps: immediately
after high temperature (HT) drying the load is cooled until the core wood temperature is 75 to 90°C, and then the stack is
steam conditioned for a period of 1 to 4 hours depending on the lumber thickness and moisture content after drying. In this
work, experimental and theoretical studies were performed to better understand the conditioning process and to investigate
factors which influence its effectiveness. In the experiment, 50 mm thick Pinus radiata sapwood boards were first dried at 120/70°C for 11, 12, 13, 16 and 18 hours, respectively, to varying moisture contents,
and then cooled and steam conditioned for 1 hour. To assess the effectiveness of conditioning, moisture pick-up, moisture
gradient, and transverse residual drying stress (indicated by cup and strain) were measured. It was found that drying wood
to a low moisture content (below 6%) increased the conditioning effectiveness. A separate matched stack was conditioned for
4 hours after 13 hours drying which showed better results than 1 hour conditioning. A mathematical model for wood drying was
extended to include both the cooling and conditioning phases. The model was numerically solved to examine the wood temperature
and moisture content changes during the whole process of drying, cooling and final steam conditioning. Increase in wood temperature,
moisture pickup and moisture gradient during steam conditioning were predicted and validated by the experimental data. This
information is currently being used at the New Zealand Forest Research Institute in simulation of stress development and relief
for drying of Pinus radiata lumber.
Received 6 July 1998 相似文献
11.
This work examines the effect of moisture content on the accuracy of nondestructively and simply estimating weight, density, and Young’s modulus by a vibration test without measuring specimen weight. The resonance frequencies with and without concentrated mass were measured by longitudinal vibration and bending vibration tests. The wet specimens whose initial moisture contents were 93–134% were dried at 20 °C and 65% relative humidity, and their weight, density, and Young’s modulus were estimated. The accuracy of the estimation was affected by the increase in the resonance frequency, caused by the drying process, during the vibration tests. The resonance frequency without the concentrated mass should, therefore, be measured after obtaining the resonance frequency with the concentrated mass. The accuracy of the estimation in the bending vibration test was higher than that in the longitudinal vibration test. This tendency can be explained by the error in the measurement of resonance frequencies with and without the concentrated mass. 相似文献
12.
Hiroyuki Yamamoto Julien Ruelle Yoshiharu Arakawa Masato Yoshida Bruno Clair Joseph Gril 《Wood Science and Technology》2010,44(1):149-163
The mechanism responsible for unusual hygro-mechanical properties of tension wood containing the gelatinous layer (G-layer)
was investigated. Tension and normal wood specimens were sampled from the leaning stems of a 75- and a 40-year-old Kunugi
oak (Quercus
acutissima) tree, and the moisture dependencies of the longitudinal Young’s modulus and longitudinal dimensions were measured. The results,
which were analyzed in relation to the anatomical properties of the specimens, revealed that the ratio of increase in the
longitudinal Young’s modulus with drying was higher in the G-layer than in the lignified layer (L-layer); the longitudinal
drying shrinkage displayed a similar pattern. It was found that the lattice distance of the [200] plane in the cellulose crystallite
increased with drying, moreover, the half-width of the [200] diffraction peak increased with drying, which was remarkable
in the tension wood. Those results suggest that in the green state, the polysaccharide matrix in the G-layer behaves like
a water-swollen gel; however, it is transformed into a condensed and hard-packed structure by strong surface tension during
moisture desorption, which is a form of xero-gelation. However, in the L-layer, condensation and subsequent xero-gelation
of the polysaccharide matrix was prevented by the hydrophobic lignin that mechanically reinforces the matrix. 相似文献
13.
《Wood material science & engineering》2013,8(4):190-195
Abstract A Lumber Quality Model developed to predict lumber grade after drying was applied and validated in industry. The model was calibrated with a 198-piece lumber package that was dried at a local sawmill in British Columbia, Canada, and 30 sawmill grade reports were recorded from 2008 to 2010. The calibrated model was then used to predict the target moisture content that would result in the maximum lumber value. It was predicted that lumber value should increase by approximately 17 CAD (Canadian dollars) m?3 if the target moisture content were reduced from the historic 17% to an optimum of approximate 13%. The sawmill implemented the recommendations and the predictions were validated with another seven industrial drying runs. 相似文献
14.
Liquid water absorption in dried Norway spruce timber measured with CT scanning and viewed as a percolation process 总被引:2,自引:0,他引:2
Liquid flow in dried wood is complicated to study, since wood is a nonhomogeneous, hygroscopic-porous, anisotropic material. However, liquid flow is important to understand, since it has an influence on the durability of wood and on such processes like impregnation, drying, surface treatment, etc. In this study, simulations of liquid water absorption in wood as a fibre network, percolation, were compared with experimental water absorption in the longitudinal direction in spruce timber. With CT scanning, water distribution during liquid flow can be shown visually and measured by image processing. Liquid water absorption in end grain of spruce was measured with CT scanning after 1, 3, 7 and 14?days of liquid water absorption and shown as moisture content (MC) profiles in heartwood and sapwood. It was found that the amount of water absorbed could be expressed as a linear function of the square root of time. The slopes of the lines differed between sapwood and heartwood and also varied depending on the growth condition of the trees. The simulations according to the percolation method show generally good agreement with the measured results for sapwood. 相似文献
15.
Summary Dielectric measurements at 3 GHz were made for three softwoods, European pine, spruce and hemlock. The longitudinal, radial and tangential grain directions of the wood were considered as well as moisture contents ranging from 6% to 35%. The positive effect of the moisture content on the loss factor illustrates the selectivity of microwave drying techniques, while the observations also show that the longitudinal dielectric properties are substantially higher than the transverse ones. The specific effect of the wood species on the dielectric behaviour has to be ascribed mainly to those intrinsic characteristics of the species which influence the sorptive capacity of the wood. 相似文献
16.
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
17.
对40 mm厚杉木锯材制定两个高温干燥工艺并进行试验研究,检测和分析干燥周期、干燥速率、锯材干燥质量等指标。干燥工艺I采用高温湿空气进行干燥;干燥工艺II在干燥前期高含水率阶段采用过热蒸汽条件,干燥后期低含水率阶段采用高温湿空气进行干燥。结果显示:两个干燥工艺在各阶段的干燥速率差异明显,干燥前期工艺II的干燥速率为1.30%/h,较工艺I低约37.2%;但干燥后期工艺II的干燥速率为1.89%/h,较工艺I高约70.27%。干燥工艺II可以有效避免锯材内裂的发生,干燥质量满足GB/T 6491—2012《锯材干燥质量》二级材的指标要求,干燥效率提高。 相似文献
18.
19.
This study investigated the effect of pretreatment with high temperature and low humidity (HT-LH) on characteristics of radio-frequency/vacuum
(RF/V) drying of boxed heart timber of Japanese cedar (150 × 150 mm in cross section, 3600 mm long). From green to approximately
15% moisture content, the drying times including HT-LH treatment were 170 h for the nonkerfed control, 190 h for the kerfed
control, and 150 h for both the kerfed and nonkerfed HT-LH specimens. Surface checks were effectively prevented by the HT-LH
treatment while the kerfed HT-LH specimens were free from surface checks. In order to prevent the formation of internal checking
during RF/V drying, it is suggested that HT-LH treatment should finish around the fiber saturation point. 相似文献