Laminar flow of fluids through short capillaries in conifer wood

Summary The viscous and slip flow terms of the Hanks-Weissberg and Petty-Puritch gas flow equations are compared. A Couette correction of 3/8 x radius must be added to the capillary length in the viscous term of the latter equation. The slip flow terms in the two equations are equivalent to within ±10%. A modified method is presented for calculating the radius and number of short capillaries from gas flow measurements. The applicability of this method was confirmed by measurements of gas flow through artificial membranes containing short capillaries. Earlier published values for the number and radius of pit membrane pores are recalculated using the modified method. The possible effects of very close spacing of pores in a membrane are discussed and it is suggested that the Couette correction is probably still applicable in this case.This work was supported in part by a Research Grant from the Science Research Council.     

Corrections for capillaries used in permeability measurements of wood

Summary Owing principally to kinetic energy losses, flow in which the flow rate is not linearly proportional to the applied pressure difference occurs to a considerable extent when the Reynolds number exeeds the length-to radius ratio of the capillary. When capillaries are used for flow or viscosity measurements corrections must be made for kinetic energy losses, end-effects and gas expansion. The results obtained for air flow through straight circular capillaries imply that non-linear flow may occur in wood and other heterogeneous porous media at Reynolds numbers in the region of 0.1 to 10.This work was carried out at the University of Aberdeen while J. F. Siau was on sabbatical leave and was partially supported by the NATO Senior Scientists Programme. The authors thank Professor J. D. Matthews for providing facilities for this work.     

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.     

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     

Water vapour diffusion through historically relevant glutin-based wood adhesives with sorption measurements and neutron radiography

In this work, the sorption and moisture diffusion behaviour of historically relevant glutin-based adhesives (i.e. bone glue, hide glue, fish glue) is characterized. The adhesive’s sorption isotherms were assessed on thin film samples revealing fundamental differences between the glutin-based adhesives and the synthetic reference adhesive (polyurethane). Furthermore, the water vapour diffusion parallel to the fibre was examined by means of neutron imaging on bonded two-layer samples of Norway spruce wood. In contrast to previous studies using neutron imaging, a new evaluation approach is presented, which allows for nonzero initial moisture conditions and takes into account and compensates for the geometry changes in the sample caused by swelling and shrinkage, thus allowing for a characterization of the diffusion behaviour within the glue line. The diffusion coefficients determined with neutron imaging were interpreted in terms of a theoretical model which takes into account the glue line microstructure. Although the diffusion coefficients were on average larger values for the glutin-based adhesives compared to the reference polyurethane adhesive, the significant variation observed in the sorption measurement is not reflected. This can partially be ascribed to excessive penetration of the adhesives into the wood substrate in fibre direction, which impedes a continuous adhesive layer. Furthermore, deformation and densification of the wood structure was assessed in the vicinity of the adhesive joint. This effect can be ascribed to the surface roughness, which results in very high local stresses leading to buckling and deformation of the tracheids. This situation is similar to that found for adhesive joints in or close to the fibre direction such as finger or butt joints.     

Turbulence in the wood system with small and short stresses

Small compressive stresses (500 N, 1000 N, 2000 N) of small duration were exerted on 2 × 2 × 2 cm³ wood specimen of Eucalyptus hybrid. Two relative humidity levels of 93% RH and 27% RH were maintained in dessicators. The per‐cent change in Equilibrium Moisture Content (EMC) was measured. It was found that the initial percent change of EMC was 2.62 times higher in low moisture content specimens when compared to the higher moisture content specimens. The EMC was found to decrease on applying compressive stress irrespective of the stress level. An explanation for this effect in terms of re-alignment of hydrogen bonds is presented in this paper. Received 8 December 1997     

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)     

Increasing wood production through old age in tall trees

How long forest trees can sustain wood production with increasing age remains an open question, primarily because whole-crown structure and growth cannot be readily measured from the ground or on felled trees. We climbed and directly measured crown structures and growth rates of 43 un-suppressed individuals (site trees) of the two tallest species – Eucalyptus regnans and Sequoia sempervirens – representing a wide range of tree sizes and ages. In both species, ground-level measurements of annual growth, including height, ring width, and basal area increment, exhibited the oft-reported trend of decreasing growth (or no change in growth) with age, yet wood production of the entire main trunk and whole crown both increased with size and age up to and including the largest and oldest trees we measured. The balance between structural metrics of whole-crown respiratory demands (cambium area, inner bark volume, sapwood volume, and heartwood deposition area) and photosynthetic capacity (leaf area and green bark area) was statistically independent of size but not age. After accounting for the effect of size, trees with lower potential respiratory demands grew more than trees with higher potential respiratory demands per unit photosynthetic area. The strongest determinant of tree energy balance was the ratio of aboveground cambium area to leaf area. Among the site trees we examined, over 85% of the variation in annual wood production was explained by variation in size, and the proportion of total aboveground wood production in appendages (branches, limbs, and reiterated trunks) increased linearly with size. With increasing age in both species, the proportion of annual wood production converted to heartwood increased in main trunks and appendages. The oldest tree we measured produced more heartwood in its main trunk over 651 years (351 m³) than contained in any tree we measured <1500 years old. The two tallest tree species achieve similar stature despite divergent growth dynamics and ecologies. At one extreme, E. regnans attains great size quickly but dies relatively young because trees are susceptible to fire and fungi. At the other extreme, S. sempervirens attains great size more slowly but has a long lifespan because trees resist fire and prioritize investment in decay-resistant heartwood. Increasing wood production as trees age is a mechanism underlying the maintenance of biomass accumulation during forest development and the carbon-sink capacity of old-growth forests.     

Stumpage value in the short wood system for the conversion into high forest of a oak coppice

This study aims to analyze the factors affecting the costs of different logging systems, in particular considering three different bunching-extraction methods. Moreover the stumpage value of each plot was calculated, and a comparison analysis of the results is presented. The work was carried out in Central Italy, in an aged Quercus cerris L. coppice in hill zones, with a 45% average slope. Short wood system harvesting was applied. Felling and processing were performed by chainsaw, while extraction with three different methods: plot No. 1 with mules, plot No. 2 with a tractor with winch, plot No. 3 with polyethylene chute line. Transport of firewood from the forest track to the landing was made by a tractor loaded as a mule. In the plot No. 1 the costs per unit mass (30.76 ? 33% -1 ) were higher despite having the lower cost per hour (39.99 ? -1 ). The reason was the lower productivity (1.3 t 33% ·h -1 ). The plot No. 2 had the highest cost per hour (66.79 ? -1 ). However, productivity was the highest (2.7 t 33% ·h -1 ). This aspect was notable from the financial point of view (24.74 ? 33% -1 ). Plot No. 3 was the less expensive (23.92 ? 33% -1 ). Comparing the three methods, a reduction of the costs through more appropriate extraction systems may increase the stumpage value from 8.3 to 9.65 ? 33% -1 .     

An investigation of the external and internal resistance to moisture diffusion in wood

Summary An attempt was made to resolve the resistance to moisture diffusion in wood into its components, namely, the external and internal resistances by using Newmann's solution of Fick's second law. The effect of specimen thickness, moisture content, and temperature on the coefficients were also investigated.This research was supported by the National Science Foundation     

Application of a thermodynamic model to experiments on nonisothermal diffusion of moisture in wood

Summary Five series of nonisothermal diffusion experiments originally analyzed by two theoretical equations were reanalyzed using a model derived from irreversible thermodynamics.     

The effect of openings on combined bound water and water vapor diffusion in wood

This study was undertaken to estimate the effect of openings between cell walls on combined bound water and water vapor diffusion in wood. Using a newly developed model, the radial and tangential moisture diffusion coefficients can be predicted depending on the opening area. The new model explicitly involves a term for water vapor diffusion through the openings, as well as a term for the combined diffusion of bound water and water vapor. A classical model developed by Stamm and Choong had higher longitudinal moisture diffusion coefficients than that in the parallel model at higher moisture content, which is inconsistent with the Wiener bound rule. The new model suggested in this article is useful for analyzing the experimental results and understanding the variability of the diffusion coefficients.     

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.     

Implementation of a relaxation equilibrium term in the convective boundary condition for a better representation of the transient bound water diffusion in wood

In this work, a relaxation term was added to the convective boundary condition to increase the accuracy of the transient bound water diffusion modeling in wood. The implemented term accounts for a relaxation time constant in the equilibrium moisture content. The inverse finite element analysis approach was used to determine the values of all coefficients of the modified diffusion model. This procedure was performed for beech wood (Fagus sylvatica L.) in the radial and longitudinal directions. The experimental data obtained by Perré et al. (2007) for transient diffusion configurations were used here. The accurate control of moist air parameters and the improved procedure for mass measurements of a sample during sorption experiments were used. The influence of the modification of the boundary condition on accuracy of diffusion modeling was analyzed.