Moisture Content Gradient Research Articles

Two-dimensional wood drying stress simulation using control-volume mixed finite element methods (CVFEM)

The work was aimed at simulating two-dimensional wood drying stress using the control-volume finite element method (CVFEM). Stress/strain was modeled by moisture content gradients regarding shrinkage and mechanical sorption in a cross-section of wood. CVFEM was implemented with triangular finite elements and lineal interpolation of the independent variable which were programmed in Fortran 90 language. The model was validated by contrasting results with similar ones available in the specialised literature. The present model's results came from isothermal (20ºC) drying of quaking aspen (Populus tremuloides): two-dimensional distribution of stress/strain and water content, 40, 80, 130, 190 and 260 hour drying time and evolution of normal stress ( -2,5<σxx<1,2, MPa), from the interior to the exterior of wood.

Numerical simulation of coupled heat and mass transfer in wood dried at high temperature

The mutual effect between heat and mass transfer is investigated for wood dried at high temperature. A numerical model of coupled heat and mass transfer under the effect of the pressure gradient is presented. Based on the macroscopic viewpoint of continuum mechanics, the mathematical model with three independent variables (temperature, moisture content and gas pressure) is constructed. Mass transfer in the pores involves a diffusional flow driven by the gradient of moisture content, convectional flow of gaseous mixture governed by the gradient of gas pressure, the Soret effect and phase change of water. Energy gain or loss due to phase change of water is taken as the heat source. Numerical methods, the finite element method and the finite difference method are used to discretize the spatial and time dimension, respectively. A direct iteration method to solve the nonlinear problem without direct evaluation of the tangential matrix is introduced. The local convergence condition based on the contraction–mapping principle is discussed. The mathematical model is applied to a 3-D wood board dried at high temperature with the Neumann boundary conditions for both temperature and moisture content, and the Dirichlet boundary conditions for gas pressure.

Effect of radial origin on final moisture content and gradient, casehardening, cracking and deformations of dried Siberian larch (Larix sibirica) timber

Siberian larch (Larix sibirica Ledeb.) timber is a good raw material, particularly for the carpentry industry. However, in the radial direction varying wood properties, like density, may have an effect on the drying quality of sawn larch timber. Thus, final moisture content (MC), MC gradient, casehardening, cracking and deformations were studied according to radial origins, taking into account the orientation of annual rings. The effect of radial origin on MC properties was mainly caused by the differences in the basic density between different radial locations. Differing ring orientation between planks of different radial origin most probably affected the MC gradient. As casehardening and deformations had clear trends in radial direction, differences in them may be largely caused by MC differences. Thus including larch timber of similar density and ring orientation in the same drying lot would help achieve larch timber of uniform drying quality.

Simulation of High-Temperature Drying of Wood

An existing model of heat and mass transfer in wood at conventional temperatures was adapted to simulate the wood drying process at temperatures above the boiling point by adding a pressure equation. Two drying tests were conducted on white spruce (Picea glauca) sapwood and heartwood in a laboratory kiln at 105 and 115°C to validate the model. The predicted drying curves and the evolution of gas pressure in the board center were found to be in good agreement with the experimental data. Predicted and experimental results for the moisture content profiles were also found to be in fairly good agreement in the heartwood but the model tends to underestimate the moisture content gradients near the surface at high moisture contents.

Accelerated Drying of Plantation Grown Eucalyptus cloeziana and Eucalyptus pellita Sawn Timber

The objective of this study was to gain an understanding for drying sawn timber produced from fast-grown, well-managed Queensland hardwood plantations using accelerated drying methods. Due to limited resources, this was a preliminary study and further work will be required to optimize schedules for industrial implementation. Three conventional kiln trials, including two for 38-mm-thick, 19-year-old plantation Gympie messmate (Eucalyptus cloeziana F. Muell.) and one for 25-mm-thick, 15-year-old plantation red mahogany (Eucalyptus pellita F. Muell.), and two vacuum kiln drying trials, one each for 38- and 25-mm-thick Gympie messmate, were conducted. Measurements of final cross-sectional moisture content, moisture content gradient, residual drying stress, and internal and surface checking were used to quantify dried quality. Drying schedules were chosen based on either existing published schedules or, in the case of the vacuum drying trials, existing schedules for species with similar wood density and dying degrade properties, or manipulated schedules based on the results of trials conducted during this study. The findings indicate that both species can be dried using conventional drying techniques with acceptable grade quality in approximately 75 percent of the drying time that industry is currently achieving when drying native forest timber of the same species. The vacuum drying time was 60 percent less than conventional drying for 38-mm-thick, 19-year-old Gympie messmate, although drying quality needs improving. The findings have shown that through careful schedule manipulation and adjustment, the grade quality can be optimized to suit the desired expectation. Additional research is required to further optimize the schedules to ensure acceptable grade qualities can be reliably achieved across all drying criteria and exploit opportunities to reduce drying times further.

Effects of basic density, annual growth and drying schedule on selected quality factors of dried Siberian larch (Larix sibirica) timber

Siberian larch (Larix sibirica Ledeb.) timber is suitable for many uses in the mechanical wood industry. Drying it without any decrease in value, however, is difficult and slow. The purpose of this research was to compare the drying quality of Siberian larch timber dried with three different conventional schedules taking into account the density and annual growth of wood. Five drying tests were performed. The final moisture content (MC), MC gradient, cracks, deformations (bow, crook, twist and cup) and case hardening were measured from the dried timber. The basic density particularly affected the MC, as shown in the differences regarding dried wood, with the denser wood having higher final MC and MC gradient. It was also found that large annual growth increased some deformations. Most of the measured factors were best after drying at the highest temperatures used; however, a slightly different trend was observed for bow, twist and cup. MC factors and twisting were the most problematic properties in drying according to this study. Sorting Siberian larch timber, particularly according to density, would improve the MC properties of dried timber by ensuring sufficient drying time, as economically as possible, for each timber piece.

Effects of Turning Frequency, Windrow Size and Season on the Production Of Dairy Manure/Sawdust Composts

On-farm composting offers the potential to significantly reduce problems associated with manure management including odors, pathogens, ground water pollution, and utilization costs. Three controllable variables that directly affect the efficiency of on-farm composting are weather, windrow size and windrow turning frequency. In temperate climates, low ambient temperatures and winter precipitation may lower composting temperatures and increase compost moisture contents, making composting rates harder to maintain. Windrow size directly impacts the depth of penetration of oxygen and the amount of compost exposed to high temperatures, potentially affecting composting rate. Windrow turning reduces moisture and temperature gradients within the compost windrows, but the benefits of frequent turning on composting rate have not been well documented. The objectives of this study were to understand the effects of these variables on composting rate, weight loss, volatile solids loss, moisture content, oxygen and temperature gradients, bulk density and particle size of unseparated dairy manure composted with hardwood sawdust (DM+S). Two composting windrow sizes (surface area to volume ratios of 0.9-1.1m2/m3 and 1.7-2.1m2/m3) and two turning frequencies (every three days and every ten days) in two different seasons (winter and summer) were evaluated. Results showed that the seasonal differences were primarily associated with compost moisture content, which varied as a function of ambient air temperature and humidity. Windrow size affected oxygen and temperatures in windrows. Before turning, the temperature averages (different depths) were significantly higher (p < 0.05) and oxygen concentrations averages were lower in the larger windrows (p > 0.1) than those observed in the smaller windrows. Turning frequency was positively correlated (r > 0.7) with bulk density, moisture and wet weight loss but not with volatile solid loss, particle size or oxygen and temperatures. Overall, the final compost properties (day 120) were very similar regardless of turning frequency, season or windrow size. In conclusion, dairy manure composted in larger windrows turned 3 times per month resulted in composts not significantly different than those made in smaller windrows turned 10 times per month.

Suitability of auger drill sampling for the measurement of moisture content gradient in kiln-dried South African grown Eucalyptus grandis poles

Moisture content (MC) gradient at the time of preservation plays a pivotal role in determining the success of the preservation process. The South African pole drying industry relies on auger drill sampling to determine whether the MC of dried poles is acceptable for preservative treatment. The objectives of this study were to determine: (i) the reliability and (ii) potential sources of errors of the auger drill MC sampling method on kiln dried E. grandis poles. Destructive oven-dry MC determination on sample blocks of 12.5 mm thickness and 25 mm width as the benchmark method and auger drill samples in increments of 25 mm were used to determine the MC gradient at the theoretical ground line (TGL) of poles. Results showed that the MC of 25 mm deep increment auger drill samples did not differ from those measured using the block method. However, there were differences between MC values measured using the auger drill and block methods when samples of 50 mm or 75 mm in length were considered. Differences in MC values measured after drying also occurred in parts of the poles where moisture was still high, i.e., above fibre saturation point. It was concluded that SANS 5983 (South African Bureau of Standards. Standard method for preparation of test specimens for moisture content determination in timber and timber products, 2008), which specifies taking single radial auger drill samples of depth 70±5 mm at mid length, is a reliable method for measuring MC in a kiln dried E. grandis pole. It is recommended that MC is determined in auger increments of 25 mm at the most critical zone of a pole, such as the TGL rather than mid length.

Experimental Study on Moisture Migration in Unsaturated Loess under Effect of Temperature

In this paper, moisture migration in loess considering temperature effect is studied by tests on unsaturated loess samples with different densities and initial moisture contents. Test results reveal that obvious changes in moisture content distribution in a loess sample can be observed after temperature difference is exerted on the two ends of the sample. Moisture content at the cold end increases and that at the hot end decreases. Under the effect of temperature difference, moisture content difference at the two ends of a soil sample is related to the initial moisture content, soil density, and magnitude of the temperature difference. Generally speaking, larger temperature differences and smaller soil densities result in more obvious moisture migration and larger moisture content differences at the two ends of the soil sample. When the initial moisture content is large, the moisture content difference caused by a temperature difference is small; when the initial moisture content is small, the moisture content difference caused by a temperature difference is also small; when the initial moisture content is moderate, the moisture content difference caused by a temperature difference is large. After the analysis of test results, taking the soil density and moisture content into account, a formula is obtained to determine the moisture content gradient resulting from the temperature gradient. Reliability of the formula is verified by comparing the measured and calculated data. Because of the reverse migration of liquid water and water vapor at the end of the experiment, it is difficult to determine the thermal potential and matrix potential. Based on the experimental data, this paper probes into the water potential equation that can be used for stability analysis. The equation considers the comprehensive impact of soil density, temperature gradient, moisture content, and moisture content gradient on water potential. It only applies to analyze stable distributions of temperature and does not apply to unstable temperature distributions.

CORRELATION BETWEEN DRYING DEFECTS, THEIR PARAMETERS AND MOISTURE GRADIENT IN KILN-DRIED, SOUTH AFRICAN GROWN EUCALYPTUS GRANDIS POLES

Non-destructive testing for drying defects in Eucalyptus grandis utility poles would be highly advantageous. These defects can negatively affect creosote preservative treatment and in-service performance. The objective of this study was to assess correlations between drying defects such as surface checking, honeycombing, collapse, their parameters and moisture content (MC) gradient in poles, to possibly find a simple and quick but reliable method to assess internal and external drying defects. Defects and moisture content gradients were measured in 39 kiln-dried E. grandis poles. After measuring surface check length, width and depth using a measuring tape, a ruler and a depth gauge, destructive sampling at the theoretical ground line (TGL) was done to measure the MC gradient between the shell and core of poles. Digital image analysis of cross-sections of discs cut at TGL was used to measure honeycomb check width, length and area, as well as counting individual closed surface checks. Collapse was assessed using qualitative methods. Results showed that honeycombing and collapse were positively, and surface checking and MC gradient were negatively correlated. Surface check width, length and depth were also correlated. Honeycomb count, check width, length and area were strongly correlated. It was concluded that measuring any of these surface check and/or honeycomb parameters may give meaningful deductions about the extent of surface checking and honeycombing respectively.

Effect of kiln drying schedule on the quality of South African grown Eucalyptus grandis poles

In specific cases, kiln drying prior to preservative treatment is the prescribed method of utility pole seasoning in South Africa. The slow drying nature of Eucalyptus grandis poles makes rapid kiln drying potentially detrimental due to development of defects such as excessive surface checking, honeycomb and collapse. To control these defects while maintaining the fastest drying rate possible, the effect of drying schedule on the drying quality of E. grandis poles was investigated. Full length pole drying was simulated in thirty 2.1 m long butt sections of poles of which the top ends were end sealed. Three industrially used schedules that differed in dry and wet bulb temperature and duration of drying (Tdb, Twb and time) were chosen: 80°C/59°C/8 days, 80°C/69°C/10 days and 70°C/59°C/12 days. Surface check length of each pole was measured after drying. Cross-sections cut at the theoretical ground line (TGL), 1.5 m from the butt end, were used to measure the moisture content (MC) gradient between the shell and core of poles. Digital image analysis of cross-sections of discs cut at TGL was used to measure the area of honeycomb and disc cross-section. Collapse was assessed qualitatively.

Antibiotic resistance profiles of soil bacterial communities over a land degradation gradient

This study profiled soils over a land degradation gradient to obtain formulae as integrative measures for describing the gradient as a result of deforestation in Thailand. We applied antibiotic resistance most-probable-number profiling to the soil bacterial communities, and then described the gradient. Soil samples were collected on the gradient represented by dry evergreen forest (the original vegetation), dry deciduous forest (moderately disturbed) and bare ground (the most degraded) in February (dry season), March (shortly after temporal precipitation) and June (rainy season) 2001. In the period of this study, the degradation was consistently shown as soil conditions like sandy texture, high bulk density, lower pH, high exchangeable acidity, poor mineral and organic nutrients and dryness. Soil fertility index and soil evaluation factor, as the integrative measures of the intensity of land degradation, were described by scores on the first or the second principal component derived from the soil bacterial community profiles for each sampling time (R>0.457, p 0.501, p<0.001), suggesting great dry to moist seasonal effects. Further, the changes had significant relationships with gradients of soil moisture content, acidity and/or soil nitrogen content. The data sets on the soil bacterial community profiles had more complicated data structures than the physicochemical data sets, suggesting effects of the physicochemical changes on the soil bacterial community. The differences between the bacterial and the physicochemical aspects suggest that it is advantageous to observe multiple aspects of soil quality when describing a soil-related gradient of interest.

A distributed parameter model for particles in the spray drying process

A multicomponent particle drying model has been developed to describe the compositions and temperatures of spray-dried particles using distributed drying kinetics. This means that component migration is expected to occur within the particles, thus temperature and moisture content gradients through the particles are present and might not be uniform. Particles are divided into a number of shells and may be spray dried from a solution of several solutes dissolved in water, such as sugars and proteins. The model simulates the drying kinetics within a concurrent spray dryer and also accounts for the effects of the solubilities of the solids, as well as surface activity of the components. The model has been successfully compared with an analytical solution for evaporation alone, to test the modelling of the external heat and mass-transfer processes. The model has also been compared with an analytical solution for internal diffusion in a sphere, which has tested the modelling of the internal heat and mass-transfer processes.

Moisture content gradient and ventilation in stored wheat affect movement and distribution of Oryzaephilus surinamensis and have implications for pest monitoring

This study was carried out primarily to ascertain whether the movement of Oryzaephilus surinamensis and Sitophilus granarius from low to high humidity zones occurs in bins of wheat and whether aeration of the grain (10 m 3/h/t) affects this movement. The second aim was to ascertain the best placement of insect detection traps under the different conditions. Insects were introduced into the lower half of the grain in the bins and their movement was monitored using traps placed at various depths in the grain. Sitophilus granarius did not move through the grain into the top layer regardless of the moisture content, temperature or aeration status of the grain. More O. surinamensis were caught in unventilated bins than in ventilated bins. More insects were caught in the ventilated bins containing layers of both dry and wet grain than in the bins containing only dry grain. The spatio-temporal distribution of O. surinamensis varied significantly. The depth at which insects were trapped varied between treatments: in ventilated dry grain, most insects were trapped at the surface; in ventilated wet and dry grain, most insects were trapped at 10 cm and 0.75 m; in unventilated wet and dry grain, the vast majority of the insects were trapped at 0.75 m. Very few insects were trapped at 1.75 m regardless of the treatment. The proportions of the initial population of O. surinamensis which were recaptured in the top layer of grain varied between treatments. Most were recaptured in the unventilated bins containing wet and dry grain followed by ventilated bins containing wet and dry grain. The smallest proportion of the population was recaptured in the ventilated bins containing only dry grain. Immediate practical implications for pest monitoring based on physical control measures in use are discussed.

Moisture distribution in a rice kernel during tempering drying

The intra-kernel moisture content profiles are important in understanding the effect of tempering process on the moisture content gradient and rice fissuring. Intermittent drying of long-grain (var. L201) and short-grain (var. Akitakomachi) rough rice was performed and the moisture content distributions within a rice kernel were estimated by a simplified sphere drying model. The results showed that the model was capable of accurately simulating drying and tempering processes of grain. Moisture content gradients were produced during drying periods, which remarkably decreased at the early stage of tempering processes and then decreased gradually. Tempering at 50 °C for 120 min removed about 80% of the moisture content gradients created in a rice kernel during drying. Moreover, moisture contents of the internal layers of the kernels always decreased in the whole intermittent drying while that of the outer layers of the kernels decreased during the drying periods and increased during the tempering periods.

Possible interactions between environmental factors in determining species optima

AbstractQuestions: 1. Indicator values, such as those of Ellenberg, for different environmental factors are seen as independent. We tested for the presence of interactions between environmental factors (soil moisture and reaction) to see if this assumption is simplistic. 2. How close are Ellenberg indicator values (IV s) related to the observed optima of species response curves in an area peripheral to those where they have been previously employed and 3. Can the inclusion of bryophytes add to the utility of IVs?Location: South Uist, Outer Hebrides, Scotland, UK.Methods: Two grids (ca. 2000 m × 2000 m) were sampled at 50‐m intervals across the transition from machair to upland communities covering an orthogonal gradient of both soil pH (reaction) and soil moisture content. Percentage cover data for vascular plants, bryophytes and lichens were recorded, along with pH and moisture content of the underlying sand/soil/ peat. Reaction optima, derived from species response curves calculated using HOF models, were compared between wet and dry sites, and moisture optima between acidic and basic samples. Optima for the whole data set were compared to Ellenberg IVs to assess their performance in this area, with and without the inclusion of bryophytes.Results: A number of species showed substantially different pH optima at high and low soil moisture contents (18% of those tested) and different soil moisture optima at high and low pH (49%). For a number of species the IVs were poor predictors of their actual distribution across the sampled area. Bryophytes were poor at explaining local variation in the environmental factors and also their inclusion with vascular plants negatively affected the strength of relationships.Conclusions: A substantial number of species showed an interaction between soil moisture and reaction in determining their optima on the two respective gradients. It should be borne in mind that IVs such as Ellenberg's may not be independent of one another.

Roadside revegetation by native plants: I. Roadside microhabitats, floristic zonation and species traits

Recognizing the severity of road effects and need for developing a natural and self-sustained roadside vegetation cover, this study aimed to provide an ecological basis for selecting desirable native plants based on their autecological attributes by floristic analysis of naturally colonized plants in roadside microhabitats. We hypothesized that (i) vegetation zonation along roadsides is a function of the different microtopography and substrate types (microhabitats) created by road construction and (ii) plant colonization in these microhabitats is dependant upon the presence of suitable regeneration traits adapted to the specific microhabitats. We identified four distinct microhabitats namely shoulder, side slope, ditch and back slope from the edge of the road to the edge of the forest. We conducted vegetation and soil analyses in these microhabitats along 34 random transects running perpendicular to the road to the edge of forest in a 14 km section of the Trans Canada Highway (TCH) in Terra Nova National Park (TNNP), Newfoundland, Canada. The multi-response permutation procedure (MRPP) confirmed that the plant communities of the four roadside microhabitats were significantly different from each other. Canonical correspondence analysis (CCA) showed that composition of roadside plant communities was related to gradients of soil moisture content, bulk density, organic matter depth and pH. Several indicator plants, determined by indicator species analysis (ISA), were abundant in their respective microhabitats, indicating their affinity to particular sets of environmental conditions. The root–shoot analysis of selected dominant plants across the roadside habitats revealed that plants of a particular microhabitat had similar above and below ground spread and biomass allocation patterns. We found that floristic zonation along roadsides is a function of roadside microtopography, substrate type and environmental gradients created by the road building process. The similarities in above and below ground spread and biomass allocation patterns of dominant plants of respective microhabitats could be used as a basis of interpreting the role of autecological attributes of the species that enable them to establish in specific microhabitats. Several native plants, such as Empetrum nigrum, Juniperus communis, Vaccinium angustifolium, Trifolium repens, and Anaphalis margaritaceae are naturally abundant in side slopes and possess autecological attributes such as low stature, widespread above- and below-ground parts, and drought tolerance. Presence of these desirable properties and their perennial habit make them excellent candidates for roadside revegetation.

Effects of combined heat and mass transfer on heating load in building drying period

In initial used building with EPS walls, the wall still contains a considerable amount of water that evaporates over time. In order to analyze the effect of moisture levels on a drying building energy performance, the heat and moisture coupling transfer of building wall with multilayer composition in the cold serious area Harbin, China, was simulated. The freezing of liquid moisture in exterior envelope was considered and the moisture content gradient was used as mass transfer driving forces. The effect of some impact factors, as initial moisture content, vapor retarder, interior wallpaper and exterior glazed brick on the wall energy consumption were also analyzed. It is concluded that the effect of drying are most significant during the first year of building initial use. The drying rate of the new building was significantly high in the first year, especially in the first few months. These changes during initial drying often account for a significant fraction of the overall load and have notable effect on the building energy performance.

A laboratory equipment for the study of moisture processes in thermal insulation materials when placed in a temperature field

The aim of this paper was to describe design and use of a laboratory set-up that makes a very well controlled moisture load to detect material properties of thermal insulation materials. The outer shell of the set-up is a plastic box with thermal insulation on its outer side. This box is placed on a balance for recording of weight loss of the entire system. The sample to be tested is placed on top of this box. The inside of the box is heated electrically and the air circulation is driven by a fan. There is a water reservoir in the box, which is also placed on a balance in order to record the amount of moisture added to the air in the box. To obtain the highest moisture contents it was found necessary to use a wick in the reservoir that increases transport of moisture to the air in the box. The whole set-up was placed in a climate chamber in order to get well controlled temperature and moisture content for the outside of the box. In this way, well controlled gradients of temperature and moisture content in air were achieved around the specimen. The set-up was found to function well and was in this study useful for measurement of the samples moisture resistance factor.

Numerical modelling of moisture movement and related stress field in lime wood subjected to changing climate conditions

Numerical modelling was used to follow the evolution of the moisture content gradient and the stress field resulting from the restrained differential dimensional response across a wooden cylinder, simulating sculptures, in response to variations in temperature (T) and relative humidity (RH). Material properties of lime wood (Tilia sp.) were used in the modelling as this wood species was historically widely used. The allowable RH variations, below which mechanical damage will not occur, were derived as functions of the amplitude, time period and starting RH level of the variation. Lime wood can endure step RH variations of up to 15% in the moderate RH region, but the allowable domain narrows when RH levels shift from the middle range. The allowable amplitude of the variations increases when time allowed for the change increases. The stress field does not vanish even for slow, quasi-static changes in RH due to structural internal restraint resulting from the anisotropy in the moisture-related dimensional change.