Thermal diffusion in Masson pine wood

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.     

Thermal diffusion of bound water in wood

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.     

Nonisothermal diffusion model based on irreversible thermodynamics

Summary A comparison is made between a nonisothermal diffusion model derived from irreversible thermodynamics and three other equations that have been applied to several series of nonisothermal experiments.     

Determination of diffusion coefficients for sub-alpine fir

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.     

Recreational Multifunctionality and its implications for agroforestry diffusion

Recreational Multifunctionality refers to those farms providing at least one recreational service to members of the farm household or the public. Based on the types of recreational services offered, two types of landowners have been identified: Productivists and Ruralists. They differ on their extent of Recreational Multifunctionality, farm household attributes and farm representation. Both types of landowners also have different behavior and understanding of agroforestry. This study builds on the Recreational Multifunctionality construct to identify message content, channels and agencies that can better facilitate the diffusion of agroforestry. In 2006, 353 randomly selected landowners from Missouri were interviewed and clustered in two groups: Productivists (38.0%) and Ruralists (62.0%). These groups were examined to identify differences regarding their perceptions of being a good farmer (i.e., message content); their preferred source of information (i.e., communication channel); and their preferred agencies for learning purposes (i.e., communication agencies). Study results show differences between Productivists and Ruralists in all three aspects of diffusion examined (i.e., message content, channels and agencies), suggesting that all landowners should not be approached in the same way when promoting agroforestry. Messages conveyed to Productivists should emphasize the economic benefits of agroforestry, while messages to Ruralists should emphasize the conservation benefits of agroforestry. Touring farms with active tree management is a good way to disseminate agroforestry practices among Productivists while state and federal extension agents appear to be more suitable to approach Ruralists. Both groups can effectively be reached using printed materials, especially through official conservation magazines.     

Individual-based simulation of the diffusion process of Eupatorium adenophorum

In order to investigate the diffusion patterns of Eupatorium adenophorum, this study uses a computer to simulate various plant diffusion processes under different environments. The study develops binary cellular automata as a framework reflecting the growth and semination of plants by the change of cellular state and transition function. This model allocates 16 integers, namely 1 to 16, to indicate a life history of plant starting from seed. In each iteration (a year), Monte-Carlo simulation is applied to decide whether an individual survives into the next year, the probability of which is estimated from previous literature. If a plant survives youth, mature or senile period, its propagation will also be considered and short-tailed dispersal kernel was employed. Applying this basic model, this study also simulates the diffusion process of E. adenophorum under four typical ecological environments. Examining the age structure of the four populations, we found that, although with significantly different numbers of plants left, most individuals were in the infancy period. Besides, their dispersal velocity showed different periodic law. These observations provided reasonable suggestions on what control strategies to be taken respectively.     

Nonisothermal moisture diffusion experiments analyzed by four alternative equations

Summary Five steady-state nonisothermal diffusion experiments were performed with one surface maintained at approximately 70°C and the other at 35°C, with the latter at a relative humidity of 65%. Relative humidities on the warm side varied from 11% to 65% resulting in equilibrium moisture contents from 2.1% to 8.9%. A reversal of flux direction was observed as the relative humidity of the warm side was decreased below that on the cool side indicating a strong influence of the temperature gradient. This reversal was predicted by two nonisothermal equations: one based on a gradient of activated moisture content and the other on a gradient of chemical potential. The flux reversal was not accounted for by the isothermal forms of Fick's law based on gradients of moisture content and partial water-vapor pressure.The author is grateful to Dr. R. V. Jelinek of S. U. N. Y. College of Environmental Science and Forestry for his programming of the computer to solve the differential equations for calculation of the fluxes and moisture-content profiles     

A further note on the activation energy of diffusion

Summary The activation energy of diffusion can be expressed in terms of thermodynamic quantities such as entropy and enthalpy of sorption. A substantial simplification to such a published expression is presented.     

End correction for gaseous diffusion through short capillaries in wood

Summary It is shown that to calculate the rate of gaseous diffusion through an isolated capillary of radius a an end correction of a/2 must be added to the physical length of the capillary. This correction is applicable to diffusion through pit apertures in conifer wood. Mutual interference between diffusion through closely spaced capillaries reduces the end correction. Two approaches to the calculation of this effect are discussed. The pores in many bordered pit membranes are so closely spaced that the end correction becomes negligible. It is shown theoretically that when the end correction is taken into account the tracheid lumen/cell wall pathway provides the path of least resistance for transverse moisture diffusion through conifer wood.     

On the activation energy of diffusion of water in wood

Summary The diffusion equation for water in wood is expanded in terms of temperature and moisture gradient on the assumption that the driving force for the diffusion of water in wood is the partial pressure of water vapour. An analytic expression is then developed for the activation energy of diffusion in terms of enthalpy and entropy changes associated with the sorption process. The expression is compared with another published curve and some similarity was observed.Symbols C water concentration, kg/m³ - D diffusion coefficient for water vapour in wood with vapour pressure as the driving potential, kg/ms Pa - D_c diffusion coefficient for water vapour in wood with water concentration as the driving potential, m²/s - D_c a constant value of D_c, m²/s - E activation energy of diffusion, J/kg - F flow density, kg/m² s - f h/l - h specific enthalpy, J/kg - L l/R T - l latent heat of vapourization of free water, J/kg - l_s latent heat of vapourization of sorbed water, J/kg - p partial pressure of water vapour, Pa - p_s pressure of water vapour at saturation, Pa - R specifc gas constant for water, J/kg K - r relative humidity - s specific entropy, J/kg K - w dry basis moisture content - x length coordinate, m - a constant temperature equal to 6,800 K - -/ln r - _w density of wood (dry mass/moisture volume) at a given moisture content, kg/m³ - s/R - L style as 2 lines above - free water relative to sorbed water The author is grateful to the Editorial Board in relation to the use of (4)     

Dissolved carbon dioxide gas diffusion in green Nothofagus fusca heartwood

Summary Non-steady state desorption of dissolved carbon dioxide gas from green Nothofagus fusca heartwoood boards saturated with carbon dioxide gas at a pressure of 1.5 MPa was used to measure transverse dissolved carbon dioxide gas diffusivities at 1, 20 and 30 °C. Mean transverse diffusion coefficients were 0.6×10^-11m²s^-1 at 1 °C, 4.2×10^-11m²s^-1 at 20 °C, and 8.4×10^-11m²s^-1 at 30 °C. These are 0.6, 2.5 and 3.8% of the dissolved carbon dioxide gas diffusivities in water at 1, 20 and 30 °C respectively. The activation energy of dissolved carbon dioxide gas diffusion in green N. fusca heartwood is 59. 4 kJ mol^-1. This is three times the activation energy of dissolved carbon dioxide gas diffusion in water. This suggests the presence of a reaction mechanism between the dissolved carbon dioxide molecules and the cell wall constituents.The Forest Research Institute, Ministry of Forestry, New Zealand, supported this work by way of a postgraduate studentship.     

Local water vapor diffusion coefficient when drying Norway spruce sapwood

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.     

钛与不锈钢扩散连接工艺的研究

本文阐述了钛与不锈钢连接的现状以及必须采用中间过渡夹层的原因，并对工业纯钛TA2和不锈钢1Cr18Ni9Ti的真空扩散连接进行了试验研究。定性分析了温度，压力和时间对接头性能的影响。指出了工业纯钛TA2和不锈钢1Cr18Ni9Ti扩散连接的最佳工艺参数。     

Solute diffusion into cell walls in solution-impregnated wood under conditioning process III: effect of relative humidity schedule on solute diffusion into shrinking cell walls

This study has focused on solute diffusing into cell walls in solution-impregnated wood under conditioning, process of evaporating solvent. The purpose of this paper was to clarify the RH- (relative humidity-) schedule that promotes the solute diffusion into shrinking cell walls during conditioning. The wood samples impregnated with a 20 mass% aqueous solution of polyethylene glycol (PEG1540) was conditioned with a temperature of 40?°C to the equilibrium point at the RH where the samples swelled maximally. The samples were subsequently conditioned at 40?°C under the schedules including four ways of RH-decrease steps where the cell walls shrunk. The amount of solute (PEGs) diffused into cell walls during the conditioning logarithmically increased with increasing the number of the RH-decrease steps. This was well explained by the theoretical model that describes the solute diffusion into shrinking cell walls. It is clarified from the model that the RH, or moisture content of the sample, should be decreased as gradually as possible to increase the total amount of diffused solute into shrinking cell walls, and that the amount of diffused solute is smaller for the lower moisture content. The model also suggests that effect of change in RH schedule on change in total amount of diffused solute does not depend on solute diffusivity in the sample under drying in a vacuum over phosphorous pentoxide, and that impregnated wood should be conditioned under natural convection rather than forced convection for promoting the diffusion into shrinking cell walls.     

Infectious ideas: Modelling the diffusion of ideas across social networks

Will the practice of collecting wild honey wearing no clothes become a widespread practice in Zimbabwe? Or will beekeeping take over as the main way that people acquire honey? Both practices impact on forest resources; how can the foresters influence the uptake of these ideas? This paper describes an exploratory modelling study investigating how social network patterns affect the way ideas spread around communities. It concludes that increasing the density of social networks increases the spread of successful ideas whilst speeding the loss of ideas with no competitive advantage. Some different kinds of competitive advantage are explored in the context of forest management and rural extension.     

Effect of collapse on fitted diffusion coefficients for Victorian ash eucalypts

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.     

防止天牛扩散及暴发的防护林体系模式探讨

通过调查,选择出抗青杨天牛及青杨虎天牛危害的的树种及易受2种天牛危害的树种。通过放飞试验,确定了2种天牛的扩散特性,从而提出防止2种天牛扩散及暴发的防护林体系结构模式。     

An irreversible thermodynamics model for unsteady-state nonisothermal moisture diffusion in wood

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 C_p specific heat of air (= 0.24 cal/g K @ 70 °C) - C_T specific heat of wood, cal/g K - D transverse diffusion coefficient, cm²/s - E_b activation energy, cal/mol - E_o heat of vaporization, cal/mol - E_L differential heat of sorption, cal/mol - G specific gravity of wood - H relative humidity, % - h_T convective heat transfer coefficient, cal/cm² s K - h_c convective mass transfer coefficient based on the concentration of moisture in wood, cm/s - h_v convective mass transfer coefficient based on the concentration of moisture in the air in equilibrium with the wood surface, cm/s - K_M coefficient for diffusion due to moisture gradient, g/cm s % - K_T transverse thermal conductivity coefficient, cal/cm K s - M moisture content, % - P_o 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     

DETERMINATION OF STATIC AND DYNAMIC DIFFUSION COEFFICIENTS OF MOISTURE IN PARTICLEBOARDS

The static and dynamic diffusion coefficients are important coefficients todescribe the moisture transfer processes in particleboard.In this paper,the formula ofculculating the static and dynamic diffusion coefficients were deduced.At first,the staticdiffusion coefficients of four kinds of particleboards were determined by using diffusion cupmethod.The results demonstrated that the static diffusion coefficients parallel to panelsurface were 10-20 times as large as that of perpendicular to panel surface for test boards.To determine both dynamic diffusion coefficients and surface emission coefficients ofmoisture in particleboards in one experimental period,specimens in four different thickness-es of each kind of particleboard were used in the experiment.Then the method ofregression was used and the dynamic diffusion coefficients and surface emission coefficien-ts were determined based on the slope and intercept of the regressive line.