Determination of moisture content and moisture content profiles in wood during drying by low-field nuclear magnetic resonance

In this article, the low-field nuclear magnetic resonance was used to measure the moisture content (MC) and MC profiles in poplar wood during real-time drying. The T₂ distribution curve at each drying stage measured using the Carr–Purcell–Meiboom–Gill pulse sequence provided detailed information in the dynamic change of free water as well as bound water of the whole wood sample. In addition, a new developed SE-SPI pulse sequence was first used to evaluate the spatially resolved T₂ distribution of the successive nondestructive sliced layer of wood. Combined with the area integration method, the moisture content in each layer was calculated, and the change of MC profiles within wood at the MC above and below fiber saturation point was well reflected.     

Modeling and On-line Measurement of Drying Stress of Pinus massoniana Board

A drying stress model was established by considering that the total shrinkage of wood is the sum of free shrinkage, instantaneous strain, viscoelastic strain, and mechanosorptive strain. From the stress model, the stress can be calculated once the actual wood shrinkage and moisture content gradient are known. Based on this theory, on-line measurement of the drying stress has been realized by measuring the moisture content (MC) gradient between the surface and the core layers, and the actual shrinkage of the board for Pinus massoniana.

A sensor for measuring wood shrinkage was developed based on electric resistance and strain relationship in a selected element material within the sensor. A resistance type of MC sensor was used for the MC gradient measurement. These sensors are reliable and can meet the requirement of the measurements in practical drying. The technique reported in this article for detecting drying stress from the on-line measurements of board shrinkage and MC gradient can be applied to develop optimized drying schedule in commercial drying.     

Online measurement of moisture content,moisture distribution,and state of water in corn kernels during microwave vacuum drying using novel smart NMR/MRI detection system

Moisture content is unevenly distributed and hard to measure when agricultural products are dried using microwave drying. A low-field nuclear magnetic resonance/imaging (NMR/MRI) and microwave vacuum drying (MVD) combination equipment was developed. The residual moisture content, distribution, and state of water (free, immobilized, and bound) in fresh corn kernels during MVD were quickly measured in real time. NMR results indicated that the amplitude of free and immobilized water decreased very rapidly at the early stage of MVD, while the amplitude of bound water experienced a similar rapid decrease at the last stage. MRI results indicated that the moisture content was always distributed unevenly during MVD, especially at the early stage. The moisture distribution tended to become uniform when drying progressed and the bound water became dominant. The residual moisture content of corn kernels and integral (total) amplitude of NMR were found to fit well with a linear model (R²?>?0.991, P?相似文献   

Modeling and On-line Measurement of Drying Stress of Pinus massoniana Board

A drying stress model was established by considering that the total shrinkage of wood is the sum of free shrinkage, instantaneous strain, viscoelastic strain, and mechanosorptive strain. From the stress model, the stress can be calculated once the actual wood shrinkage and moisture content gradient are known. Based on this theory, on-line measurement of the drying stress has been realized by measuring the moisture content (MC) gradient between the surface and the core layers, and the actual shrinkage of the board for Pinus massoniana.

A sensor for measuring wood shrinkage was developed based on electric resistance and strain relationship in a selected element material within the sensor. A resistance type of MC sensor was used for the MC gradient measurement. These sensors are reliable and can meet the requirement of the measurements in practical drying. The technique reported in this article for detecting drying stress from the on-line measurements of board shrinkage and MC gradient can be applied to develop optimized drying schedule in commercial drying.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

Modeling the Heat and Mass Transfer in Microwave Drying of White Oak

A 2D comprehensive heat and mass transfer model was developed to simulate the free liquid, vapor, and bound water movement in microwave drying of white oak specimens. The experimental and model results showed that, for white oak, moisture movement was easily impeded and high gradient of internal vapor pressure occurred. The internal vapor pressure was affected by sample dimension (length and thickness). At the same input power density, the internal pressure generated in the core increased with the sample length and thickness. However, as compared with sample length, sample thickness has less effect on the pressure gradient because of the high ratio of permeability between longitudinal and transverse directions.     

Effect of Outer Surface Sealing Treatment on the Reduction of Surface Check Occurrence During the Drying of Center-Bored Round Timber

A significant amount of time and energy is required to dry green timber with a large cross-section. Due to long-lasting internal moisture gradients, internal stress is high during the drying of large cross-sectional timber, and the potential for check occurrence is significant. Although many researchers have aimed to develop a method for drying large pieces of wood without the occurrence of drying defects, a procedure for rapidly drying wood without cracks has not yet been developed. In the present study, an outer surface sealing method and center-boring process (i.e., drilling a hole along the central longitudinal axis) was developed to dry timber with a large cross-section without the occurrence of checks. The proposed center-boring procedure reduces the movement of heat and moisture inside the wood and expedites the drying process by sustaining a small MC gradient. Moreover, the outer surface sealing treatment changes the drying stress direction and controls check occurrence. By kiln drying center-bored (80-mm diameter) and outer-surface-sealed round timber pitch pine (Pinus rigida) with an initial MC of 30% and an outer diameter of 140 mm, a final MC of 6% was obtained within 40 hours without the occurrence of any drying defects.     

Thermal Residual Stresses in Adhesively Bonded In-plane Functionally Graded Clamped Plates Subjected to an Edge Heat Flux

In this study we have carried out the thermal residual stress analyses of adhesively bonded functionally graded clamped plates for different edge heat fluxes. The material properties of the functionally graded plates were assumed to vary with a power law along an in-plane direction not through the plate thickness direction. The transient heat conduction and Navier equations describing the two-dimensional thermo-elastic problem were discretized using the finite-difference method, and the set of linear equations was solved using the pseudo singular value method. The plate material properties near the interfaces played an important role in the interfacial adhesive stresses. The compositional gradient affected considerably both in-plane temperature distributions and heat transfer periods. The type of in-plane heat flux had only a minor effect on the temperature profiles but affected both the temperature levels and heat transfer period. Both plates undergo considerable compressive normal strains and stresses, but shear strains were more effective. Peak equivalent strains were observed for a constant heat flux and plates with a metal-rich composition. The compositional gradient and direction played important role in the profiles and levels of normal, shear and equivalent stresses as well as strains. The equivalent stress and strains concentrated along the free edges of the adhesive layer. The adhesive layer experienced a considerable distortional deformation rather than volumetric deformation. The equivalent stress exhibited small changes through the adhesive thickness and along the overlap length. The equivalent stress remained uniform in a large region of the overlap length and increased to a peak level around the free edges of the first plate–adhesive interface, whereas it increased to a peak level in a large region of the overlap length from a minimum level around the free edges of the second plate–adhesive interface. The strains and equivalent strains were higher for a metal-rich material composition. The direction of the material composition of the plates affected both stress and strain levels; thus, the CM–CM and CM–MC plates exhibited lower strain and stress levels than those in the MC–CM and MC–MC plates. However, only the adhesively bonded CM–MC plate configuration could achieve the lowest deformations and stresses in both plates and adhesive layer.     

Comparing two intermittent drying schedules for timber drying quality

Intermittent drying techniques for drying timber may provide various benefits by improving timber quality and addressing energy efficiency through saving in energy consumption. The purpose of this study was to compare two intermittent drying schedules applied in the treatment of Eucalyptus delegatensis boards, through assessing surface and internal check development, moisture content (MC) profiles during drying, and timber distortions at the end of drying. The study used identical conditions during the heating phase at 45°C/60% relative humidity (RH), except for RH during the nonheating phase (80 and 90%). The results, discussed in this paper, analyzed the timber quality during and at the end of drying. The different RH during the nonheating phase did not generate a significant difference in MC at the case boards between the two drying schedules. The assessed quality of timber at the end of drying was based on AS/NZS 4787:2001. MC gradient, drying stress residual, internal checking and collapse were graded as class “A” (class A is the highest grade and D is the lowest). Bow, cupping, and spring were under the permissible levels based on grading standard AS 2082–2007. Measured data were validated using Drytek® simulation software showing MC movement in case boards.     

Moisture conversion and migration in single-wheat kernel during isothermal drying process by LF-NMR

Conversion among different moisture-binding types of single Hebei wheat kernels during isothermal drying processing at 60?°C was studied by low field nuclear magnetic resonance (LF-NMR), and moisture migration was studied by MRI. Water inside wheat kernels exists in four types: chemically combined water (T₂₁), strong bound water (T₂₂), loosely bound water (T₂₃), and free water (T₂₄). A new method to obtain the contents of different moisture-binding types is forwarded according to the transverse relaxation time distribution curve of dried wheat kernels. Moisture conversion inside wheat kernels during drying occurs mainly through the following mechanisms: high-temperature drives T₂₄ to diffuse to lower moisture areas and transform into other moisture-binding types, and moisture-gradient drives T₂₂ to transform into T₂₄. The drying process can be divided into two stages. Moisture migrates from endosperm to epidermis during drying. As the drying rate of the wheat kernel significantly decreases, the drying conditions and parameters need to be adjusted to improve the drying rate while ensuring the quality of wheat kernels.     

Differences between constant and intermittent drying in surf clam: Dynamics of water mobility and distribution study

Low-field nuclear magnetic resonance (LF-NMR) has been increasingly popular as analytical tools for evaluating the dynamics of water mobility and distribution. In this study, dynamics of moisture mobility and constitution of surf clam during constant drying process and intermittent drying process were evaluated by LF-NMR, while the differences of physical and chemical indexes were measured. Intermittent drying improved the product quality of clam, such as moisture content, shear force, color indices, sugar content, peroxide value, thiobarbituric acid value, and the bulk water ratio, which were closely related with moisture distribution and microstructure. The moisture constitution of constant drying process and intermittent drying process were distinctly different. Tempering process reduced drying time and resulted in lower moisture content in dried surf clam. In the meanwhile, the boundary between A₂₁ and A₂₂ was acquired by LF-NMR, revealed that bound water and immobilized water transformed from each other. During tempering process, the myofibril stretched out, verifying that moisture approached a relatively homogeneous. In addition, R² value reached 0.9897 and 0.9926 for calibration and validation, respectively, displaying good linear correlations between the T₂₁ parameters and moisture content. This study interpreted the dynamics of water mobility and distribution on the proton level to explain the reason that tempering processes to improve physicochemical indexes of surf clam.     

MODELLING OF STRESS DEVELOPMENT DURING DRYING AND RELIEF DURING STEAMING IN PINUS RADIATA LUMBER

ABSTRACT

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available.     

Water status and distribution in shiitake mushroom and the effects of drying on water dynamics assessed by LF-NMR and MRI

Abstract

Water status and distribution in shiitake mushroom and the effects of drying temperature on water migration were investigated by low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) technique. Three water components assigned to bound, immobilized water and free water were observed in fresh shiitake mushroom by LF-NMR, and higher moisture content was found in the lamella and outer layer of stalk by MRI. During drying, the transverse relaxation time and peak area of immobilized and free water in pileus significantly decreased. Higher drying temperature could promote the changes in water mobility and population. MRI results revealed the water evaporated from external surface of shiitake mushroom, then the water in inner region migrated to the surface during the drying. The drying properties, color and shrinkage of shiitake mushroom were also measured. The strong correlation between moisture content and total peak area of water with a coefficient of 0.9792 revealed the potential of LF-NMR as a rapid and nondestructive method to monitor drying degree of shiitake mushroom.     

Investigation on Moisture Transfer Mechanism in Porous Media During Rapid Drying Process

Abstract

In this work, moisture transfer mechanism in wet porous media during rapid drying process is investigated experimentally and analytically. By use of scanning electron microscopic device, the rapid drying processes for potato, carrot, and radish species were observed and recorded. The microscopic drying experiments show that during high intense and rapid drying process, the mechanism of moisture migration in materials is mainly considered as a displacement flow driven by pressure gradient along a capillary passage. A simplified displacement flow model during rapid drying process is proposed and the time needed for moisture transfer in porous media is calculated. To examine this drying mechanism, one-dimensional displacement flow test device is built up and a set of experiments under different pressure gradients and temperatures are conducted. Glass beads of 0.8 mm in diameter are used as the porous material. The experimental results show that when pressure gradient is getting greater at constant temperature, the moisture removal time is getting smaller. On the other hand, under the same pressure gradient, when liquid temperature increases, the time for moisture transfer from the internal to the external surface decreases. The calculated moisture removal times are well agreed with the experimental data.     

ANALYSIS AND OPTIMIZATION OF THE CONDITIONING PHASE IN TIMBER DRYING

The conditioning phase of the conventional timber drying process has been investigated. Due to wood shrinkage, tensile stress will develop in the surface layer of the board during drying. This stress combined with the change in MC causes mechano-sorptive creep. If the process is abruptly terminated when the target MC is reached, then there will be a considerable internal MC gradient and the creep deformation will cause distortions in subsequent machining. A conditioning phase is often introduced at the end, in order to reduce these problems. The conditioning phase has been analyzed with a wood drying simulation model. It is shown that traditional conditioning cannot completely reverse the deformations, even at very long conditioning times. An optimizing procedure has thus been used to create a new conditioning “schedule” which gives the best possible result within a given time. Features of such optimized schedules are presented. As the surface layer is elongated during drying, there is a risk that considerable stress develops as the moisture profile levels out. In some cases internal checking will be the result. The optimization procedure is thus modified to avoid such situations and the impact on the conditioning schedule is discussed.     

Thermal residual stresses in adhesively bonded in-plane functionally graded clamped circular hollow plates

This study investigates the effects of in-plane compositional gradient exponent and direction on the thermal residual stress and deformations in adhesively bonded functionally graded clamped circular plates. The material composition was assumed to vary with a power law along an in-plane direction not through the plate thickness direction. The transient heat conduction and Navier equations in polar coordinates describing the two-dimensional thermo-elastic problem were discretized using finite-difference method, and the set of linear equations were solved using the pseudo-singular-value method. The material composition direction is designed as Ceramic-Metal (CM)–CM, CM–Metal-Ceramic (MC), MC–CM, and MC–MC for the inner and outer plates. The temperature decreased radially along the plates, but exhibited a sharp decrease across the adhesive layer. The compositional gradient exponent and direction affected evidently temperature levels and heat transfer period. The compressive radial and shear strains are more effective on the deformation in the adhesive layer and the plate regions near the plate–adhesive interfaces. The adhesive layer is subjected to considerable shear deformations. The equivalent strain and stresses are very low in a large region of the plates but exhibit sharp peaks on the plate regions near the plate–adhesive interfaces, and decrease towards the adhesive interfaces. These stress and strain peaks in the plates and adhesive layer are affected by the compositional gradient and direction. For an outer edge flux, the largest equivalent strain and stresses are observed in the CM–MC joint but the lowest levels occur in the MC–CM or secondly CM–CM joint. In addition, an inner edge flux results in the lowest and highest peak strains and stresses in the MC–CM and CM–MC joints, respectively. The MC–MC and CM–CM joints result in lower temperature, stress and strain levels around the adhesive layer and along the adhesive interfaces for outer and inner edge fluxes, respectively.     

A study on drying models and internal stresses of the rice kernel during infrared drying

Due to the limited penetration of infrared, it is very difficult to develop an infrared drying model of rice kernels. In this study, two kinds of simplified drying models, which assumed the penetration depth is infinity and zero, were developed to investigate the effects of penetration on drying characteristics of thin layer infrared drying. The results show each model can predict temperature and moisture contents (MC) accurately. The maximum temperature difference of rice kernels in both models was always less than 1.5°C, so it is reasonable to exclude the influence of thermal stresses due to nonuniform temperature. This study also developed the internal stresses model with the mechanical properties from literatures. Mechanical properties were with the changes of temperature and MC. These models were solved with COMSOL Multiphysics and there are two stress concentration areas. One is near the surface of the endosperm, another close to the center. Comparison between the von Mises stress distribution and the moisture gradient was made after the simulation. The maximum MC in the endosperm appeared at its surface, which reached 190 1/m at 110?s, and the maximum stress appeared at the same place, which, a little later, reached 7?MPa at 160?s. Moisture gradient at the center was zero due to the existence of symmetry, while there was a significant stress, which reached 3.2?MPa during drying.     

ANALYSIS AND OPTIMIZATION OF THE CONDITIONING PHASE IN TIMBER DRYING

The conditioning phase of the conventional timber drying process has been investigated. Due to wood shrinkage, tensile stress will develop in the surface layer of the board during drying. This stress combined with the change in MC causes mechano-sorptive creep. If the process is abruptly terminated when the target MC is reached, then there will be a considerable internal MC gradient and the creep deformation will cause distortions in subsequent machining. A conditioning phase is often introduced at the end, in order to reduce these problems. The conditioning phase has been analyzed with a wood drying simulation model. It is shown that traditional conditioning cannot completely reverse the deformations, even at very long conditioning times. An optimizing procedure has thus been used to create a new conditioning “schedule” which gives the best possible result within a given time. Features of such optimized schedules are presented. As the surface layer is elongated during drying, there is a risk that considerable stress develops as the moisture profile levels out. In some cases internal checking will be the result. The optimization procedure is thus modified to avoid such situations and the impact on the conditioning schedule is discussed.     

HEAT AND MASS TRANSFER DURING SAPWOOD DRYING ABOVE THE FIBRE SATURATION POINT

ABSTRACT

Pine sapwood was dried in an air convection kiln at temperatures between 60-80 °C. Temperature and weight measurements were used to calculate the position of the evaporation front beneath the surface. It was assumed that the drying during a first regime is controlled by the heat transfer to the evaporation front until irreducible saturation occurs. Comparisons were made with CT-scanned density pictures of the dry shell formation during initial stages of drying of boards.

The results indicate a receding evaporation front behaviour for sapwood above approximately 40-50% MC when the moisture flux is heat transfer controlled. After that we finally reach a period where bound water diffusion is assumed to control the drying rate.

The heat transfer from the circulating air to the evaporation front controls the migration flux. In many industrial kilns the heating coils therefore have too small heat transfer rates for batches of thin boards and boards with high sapwood content.