PREDICTING THE CROOK STABILITY OF LUMBER WITHIN THE HYGROSCOPIC RANGE

ABSTRACT

The crook instability of lumber at moisture contents below the fiber saturation point was studied using both experiments and mathematical modeling. The changes in overall length and in crook that were observed with environmental humidity cycling at ambient temperature suggest that crook instability at such conditions is determined largely by elastic phenomena. Consequently, a simplified mathematical model of crook instability based on only elastie paramelers was created and verified using property data and crook measurements from selected boards. Results indicate that the elastic model accurately describes crook instability in the hygroscopic moisture content range near ambient temperatures. Experimental observations also show that board-to-board differences in crook instability can be quite complex. A better fundamental understanding of wood shrinkage behavior would help to explain these differences.     

INFLUENCE OF DRYING CONDITIONS ON THE RHEOLOGICAL PROPERTIES OF PRUNES

Rheological properties of rehydrated prunes were obtained applying compression-relaxation tests by using a Texture Analyzer TAXT2i. A mathematical development was adopted to determine the stress and area, along the deformation. Experimental data of stress versus time was fitted by using three different rheological models: generalized Maxwell, Normand & Peleg and Maxwell. Results showed that generalized Maxwell model can be used to describe the viscoelastic behavior of the samples. The rheological parameters obtained indicated that prunes exhibited elastic behavior more pronounced at low moisture content and drying air temperature. At high moisture content and temperature the sample became a more viscous and less rigid.     

INFLUENCE OF DRYING CONDITIONS ON THE RHEOLOGICAL PROPERTIES OF PRUNES

ABSTRACT

Rheological properties of rehydrated prunes were obtained applying compression–relaxation tests by using a Texture Analyzer TAXT2i. A mathematical development was adopted to determine the stress and area, along the deformation. Experimental data of stress versus time was fitted by using three different rheological models: generalized Maxwell, Normand & Peleg and Maxwell. Results showed that generalized Maxwell model can be used to describe the viscoelastic behavior of the samples. The rheological parameters obtained indicated that prunes exhibited elastic behavior more pronounced at low moisture content and drying air temperature. At high moisture content and temperature the sample became a more viscous and less rigid.     

COMPRESSION ANALYSIS OF DEHYDRATED AGRICULTURAL PRODUCTS

ABSTRACT

The textural properties of apple, banana, carrot and potato were experimentally determined by uniaxial compressive tests of cylindrical specimens at a constant deformation rate of 5mm/min. Compression tests were performed, following air drying, at various moisture contents ranging from 0.2 to 6 kg/kg db. The tests were performed using a universal texture testing machine and simple mathematical equations were used to correlate the maximum experimental stress and the corresponding strain to the moisture content. It was shown that the maximum stress decreases as the moisture content decreases, until a critical moisture content of 1.8 kg water/kg dry solids. Further removal of water tends to increase the maximum stress. The maximum experimental strain was found to increase as water was removed. The stress-strain data of compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress (ow), the maximum strain (dim), the elastic parameter (E) and the viscoelastic exponent (p). The effect of the moisture content on the compressive behavior of dried materials was introduced through its effect on the model parameters. The shift in compression behavior at 1.8 kg water/kg solids leads to the conclusion that there is an important change of structure at this moisture content.     

Study of Desorption and Adsorption During Grain Aeration

One requirement in designing and operating and operating system is to minimize the moisture change in stored grain. Moisture transfer during the aeration process is generally believed to be caused by the partial pressure difference between water in grain and water in air (P₃-P₄). A mathematical model was developed for rough rice, which predicts the partial pressure difference under various grain and ambient conditions. Observations of moisture change over 10 aeration tests confirmed the good correlation between sorption phenomena and partial pressure differences. Studies indicated that air relative humidity, air temperature, grain temperature and grain moisture are significant factors influencing the partial pressure differences and, thus, affecting     

硅藻土基调湿材料中热湿耦合传递

建立了硅藻土基调湿材料内部热湿耦合迁移一维数学模型,模拟了不同孔隙率、不同环境温度和相对湿度下硅藻土基调湿材料中的热湿耦合迁移过程,结合硅藻土基调湿材料调湿性能实验测试结果以及硅藻土基调湿材料表面的显微图像分析,研究结果表明：随着孔隙率的减小,硅藻土基调湿材料的吸、放湿量均增大;环境温度对硅藻土基调湿材料热湿耦合迁移过程影响不显著,不同的环境温度（最大相差20℃）对硅藻土基调湿材料吸、放湿量的影响均在10%左右;硅藻土基调湿材料的孔径尺寸越小、小孔径孔隙数量越多,其调湿性能越好;硅藻土基调湿材料调湿性能的实测数据、模拟结果以及图像表征吻合良好,从而验证了理论模型的合理性。     

Study of Desorption and Adsorption During Grain Aeration

ABSTRACT

One requirement in designing and operating and operating system is to minimize the moisture change in stored grain. Moisture transfer during the aeration process is generally believed to be caused by the partial pressure difference between water in grain and water in air (P₃?P₄). A mathematical model was developed for rough rice, which predicts the partial pressure difference under various grain and ambient conditions. Observations of moisture change over 10 aeration tests confirmed the good correlation between sorption phenomena and partial pressure differences. Studies indicated that air relative humidity, air temperature, grain temperature and grain moisture are significant factors influencing the partial pressure differences and, thus, affecting     

MATHEMATICAL MODEL DEVELOPMENT AND SIMULATION OF HEAT PUMP FRUIT DRYER

ABSTRACT

A mathematical model of a heat pump fruit dryer was developed to study the performance of heat pump dryers. Using the moisture content of papaya glace' drying, the refrigerant temperature at the evaporator and condenser and the performance, was verified. It was found that the simulated results using closed loop heat pump dryer were close to the experimental results. The criteria for evaluating the performance were specific moisture extraction rate and drying rate. The results showed that ambient conditions affected significantly on the performance of the open loop dryer and the partially closed loop dryer. Also, the fraction of evaporator bypass air affected markedly on the performance of all heat pump dryers. In addition, it was found that specific air flow rate and drying air temperature affected significantly the performance of all heat pump dryers.     

Numerical modeling assessment of mechanical effect in bovine leather drying process

Leather manufacturing involves a crucial energy-intensive drying stage in the finishing process to remove its residual moisture and generates important heat gradients. The numerical model presented in this study has been developed to describe the drying process of porous medium: bovine leather that undergoes deformation due to shrinkage. The mathematical formulation of fundamental heat, mass and momentum transfers’ phenomena during drying summarizes a two-dimensional model considering elastic behavior of bovine leather. The evolution of moisture content, temperature, and mechanical stresses during drying was discussed. The model was validated with experimental results. Numerical simulations show good agreement with experimental results. The study shows that the elastic model keeps the stress sign at the final stage of drying. The deformations induce tensional stresses near the surface equilibrated by compressive stresses within the product. They reached their maximum for normal stresses equal to 5.97 and 3.52?MPa at around 2145 and 868?s, respectively, for normal stresses along x and y directions and then decrease.     

VISCOELASTIC BEHAVIOR OF DEHYDRATED CARROT AND POTATO

Viscoelastic properties for carrot and potato were experimentally determined by stress relaxation tests using uniaxial compressive tests of cylindrical specimens at various deformation rates (5,10 and 20 cm/min). Stress relaxation tests were performed following conventional drying at 70°C and 15% humidity for various moisture contents ranging from 10 to 80%. The tests were performed using a Zwick testing machine and the stress relaxation data were modeled using a two-term Maxwell model. It was shown that the relaxation behavior of carrot and potato was not affected by deformation rate, but it was sensitive to moisture content. The remaining force and relaxation time of the elastic component were found to be depended on moisture content. Drying tends to decrease the remaining force and the elastic relaxation time of carrot and potato until a certain moisture content (1.7 and 1.9 kg/kg db). Further uptake of water tends to increase the remaining force and the elastic relaxation time for both materials. The viscous component of Maxwell model does not seem to be affected by moisture content.     

VISCOELASTIC BEHAVIOR OF DEHYDRATED CARROT AND POTATO

Abstract

Viscoelastic properties for carrot and potato were experimentally determined by stress relaxation tests using uniaxial compressive tests of cylindrical specimens at various deformation rates (5,10 and 20 cm/min). Stress relaxation tests were performed following conventional drying at 70°C and 15% humidity for various moisture contents ranging from 10 to 80%. The tests were performed using a Zwick testing machine and the stress relaxation data were modeled using a two-term Maxwell model. It was shown that the relaxation behavior of carrot and potato was not affected by deformation rate, but it was sensitive to moisture content. The remaining force and relaxation time of the elastic component were found to be depended on moisture content. Drying tends to decrease the remaining force and the elastic relaxation time of carrot and potato until a certain moisture content (1.7 and 1.9 kg/kg db). Further uptake of water tends to increase the remaining force and the elastic relaxation time for both materials. The viscous component of Maxwell model does not seem to be affected by moisture content.     

EFFECT OF OSMOTIC DEHYDRATION ON VISCOELASTIC PROPERTIES OF APPLE AND BANANA

The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (σ_max) the maximum strain (ε_max), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.     

EFFECT OF OSMOTIC DEHYDRATION ON VISCOELASTIC PROPERTIES OF APPLE AND BANANA

ABSTRACT

The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (σ_max) the maximum strain (?_max), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.     

MODELING AND SIMULATION OF THE HOT-PRESSING PROCESS IN THE PRODUCTION OF MEDIUM DENSITY FIBERBOARD (MDF)

A mathematical model is presented that describes heat and moisture transfer during the hot-pressing of MDF. The model includes conduction of heat between the press platens and the central layer of the mattress, phase change of water from the adsorbed to the vapor state and the convective heat and mass transfer through the edges of the mattress. This approach results in a three-dimensional unsteady-state problem in which the properties of the wood fibers, water and the wood-water system are dependent on temperature, mattress moisture content and steam pressure. For solving this problem, a finite difference scheme is used for the discretization of-spatial variables and the resulting set of ordinary differential equations is solved using an appropriated ODE solver.

This theoretical model is used to predict the evolution of temperature, moisture content, steam pressure and relative humidity with time at a given position in the mattress under typical operating conditions. Several plots are shown displaying the profiles of these four dependent variables along the three axes of the panel. The effects of temperature, moisture content and vapor pressure gradients on the physical properties of the system are also analyzed. These simulation results are useful to identify the controlling factors of hot-pressing operation in order to improve the quantitative understanding of the complex mechanisms involved in panel formation.     

型煤干燥过程的研究

对型煤的干燥过程进行了分析, 通过实验对型煤水分和干燥时间的关系进行了探讨, 并建立了干燥数学模型, 以期对实际生产提供帮助。     

Effective Moisture Diffusivity Estimation from Drying Data. A Comparison Between Various Methods of Analysis

Moisture diffusivity is the most crucial property in drying calculations. Literature data are scarce due to the variation of both experimental measurement techniques and methods of analysis. The effect of using different methods of analysis on the same experimental drying data is examined in this work. Detailed and simplified mathematical models, incorporating moisture diffusivity as model parameter, are applied. It is proved, that significant differences in the calculated values of moisture diffusivity result when different models are used, and probably these differences explain the variation in literature data. Thus, the adoption of a standardised methodology will be of great importance in moisture diffusivity evaluation.

The above findings resulted from the application of four alternative models on the drying data of three common food materials, potato, carrot and apple. A typical pilot plant scale dryer with controlled drying air conditions was used for the experiments. The moisture content dependence of the diffusion coefficient was proved significant at the last drying stage, while the temperature dependence followed the well known Arrhenius relation. The effects of considering external mass transfer and volume shrinkage during drying, were also investigated.     

Compression mechanical properties of wood at temperatures simulating fire conditions

Compression mechanical properties of wood at temperatures simulating fire conditions have been obtained for use in structural response models to aid the design of light-timber framed walls as fire barriers in accordance with recent performance-based fire codes. The properties that have been measured are elastic modulus and strength. The properties were obtained from samples that had a number of similarities to studs in walls. The effects of knots, however, have been excluded from this study. The elastic modulus greatly reduces as temperatures approach the vaporization point of moisture. The properties were significantly lower compared with properties given in the literature. Input of the measured properties into a structural fire model for walls led to good comparisons with results of experiments on walls with low load ratios (that is, the ratio of the applied load to the collapse load capacity at ambient conditions). Such walls are typically used in building construction. However, the elastic moduli presented in this paper and in other literature do not lead to good model predictions for slender walls with high load ratios. Further research into creep is required. Copyright © 2001 John Wiley & Sons, Ltd.     

MODELING VACUUM-CONTACT DRYING OF WOOD: THE WATER POTENTIAL APPROACH

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.     

MODELING VACUUM-CONTACT DRYING OF WOOD: THE WATER POTENTIAL APPROACH

ABSTRACT

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.