Drying-Induced Strain and Stress: A Review

This paper is to review the works on strains and suesses in materials during drying.The strains and suesses are caused when temperature and moisture gradients are generated in mterials whose volume changes with heating and moisture removal. In such materials. failure and irregular deformion may be generated which affect considerably the qudity of the products after drying. In the first part. modeling procedure is introduced for the analysis of the strain-stress behavior in elaslic. viscous and visccelastic materials combined with heat and moisture transfer. An overview of the works on swains and stresses and drying characteristics are presented for malerials such as porous media. clay. sol-gels. agricultural products and foods in the second part. There are some materials that show both elasticity of the solid phase and viscosity of the fluid phase ( water or solvena∥ or viscoelasticiry. The suesses are often correlated with a suction pressure of fluids in pores and the flow rate is based on Dacy's equation for the elastic and viscous tnedia and a kind of viscoelastic media. The general canstitulive equalions. for suains and svesses are often analyzed with the stain behavior given by a function of moisturr for some media ai well. The emohasis is on the inuoduclion of comprehensive criteria for undersunding the problems of strain and stress development in materials subjected to drying.     

INFLUENCE OF THE DRYING MEDIUM PARAMETERS ON DRYING INDUCED STRESSES

ABSTRACT

A thermomechanical model of drying of capillary-porous materials whose material constants depend on moisture content and temperature is presented in the paper. The finite element method is used for the solution of two-dimensional problem of convective drying of a prismatic bar. The moisture distributions, temperature distributions, drying induced strains and stresses for various drying medium parameters are determined. The effect of these parameters on moisture distribution and in particular on drying induced stresses is discussed.     

Rehydration of Dehydrated Foods

Dehydrated products readily take up moisture when immersed in a liquid medium, leading to significant changes in their thermophysical properties. The rehydration kinetics, the structural properties (apparent density, true density, specific volume and internal porosity), the viscoelastic behavior (compression tests), and the flavor losses were investigated during rehydration of various fruits and vegetables. The effect of temperature on the above properties was also investigated for some fruits and vegetables dehydrated by different drying methods (convective, vacuum, freeze, and osmotic drying). The results showed that the water temperature influences the rehydration kinetics and the equilibrium moisture content of the rehydrated. The structural properties of rehydrated foods appear to show a hysteresis compared to those measured during dehydration processes. The shrinkage that takes place during dehydration prevents rehydration and produces products with lower apparent density and higher porosity. Structural damages that occur during drying seem to affect also the viscoelastic behavior of the rehydrated foods and the hysteresis phenomenon is also observed at the textural properties, with a degree varying between the different drying methods. More specifically, freeze-dried materials present the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pretreatment seems to help freeze-dried materials to keep their elastic nature probably due to solids gain. Air- and vacuum-dried materials showed the smallest hysteresis tense, keeping their viscoelastic characteristics during rehydration close to those of dried materials. Flavor losses seem to have lower rates during rehydration comparing to those observed during drying. The maximum retention of flavor has been observed in the rehydrated products after freeze-drying, and flavor levels are close to those observed for fresh boiled foods.     

A Sharp Drying Front Model

Abstract

The drying of materials is often described by nonlinear diffusion equations. Up to now the only way to solve these equations is by numerical simulations. Recently an analytic solution has been proposed for the drying problem. Based on this solution a sharp drying front model is presented. Measured moisture profiles during drying and the drying curve of gypsum are compared with approximate models.     

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.     

A Moisture Transmembrane Transfer Model for Pore Network Simulation of Plant Materials Drying

The drying of plant materials with cellular tissue is often viewed as drying of porous media that is assumed to consist of cell cytoskeleton and intercellular space. Various approaches have been reported in the literature to describe heat and mass transfer during drying of such porous materials. However, the fact remains that the water in a cellular tissue is mostly intracellular and it should be driven out of the cells across cell membranes before transporting in cell gaps, as in a general porous media. In the present study, the transport process of moisture in a cellular tissue was analyzed. A mathematical model for moisture transport across the cell membrane was established, which was correlated to a self-developed, dual-scale pore network model (cell and pore network) for drying of plant materials. The relationship between mass volumetric flux and average intracellular moisture content was developed based on the microscopic images and the drying experiments.     

Integral Flow Parameters and Material Characteristics Analysis in Through Air Drying: Part I

The extension of the Darcy law (the Forchheimer flow equation) relating second-order nonlinear pressure drop with flow velocity is studied during fast transient through air drying of sheets of porous biobased materials such as paper. A range of the paper materials with open structure consistent with tissue and towel products (basis weights 25 and 50 g/m²) made using different production processes are analyzed for the factor-specific influences with regard to changes in the fluid resistance from the removal of moisture from the material interstices. A characteristic dimension suitable for the drying process is applied from viscous and inertial momentum transport analysis.     

Selective drying of hygroscopic materials wetted with binary mixtures

ABSTRACT

Drying experiments were carried out in a laboratory apparatus with differently hygroscopic materials wetted with binary mixtures. Composition curves were determined under variation of the drying conditions, the sample thickness, and the drying method. The results obtained are presented and compared with those of non-hygroscopic materials.

In particular, the interactions between moisture and solid when using strongly hygroscopic products influence the composition curve from the very beginning of the drying process, contrary to weakly hygroscopic materials, where the selectivity is controlled by the sorption equilibrium only at the end of the drying process.

A method to calculate the selectivity in knowledge of the binary adsorption equilibrium is given. For this purpose, it is necessary to split the total moisture into a free and a bounded share.     

A Simulation Study on Convection and Microwave Drying of Different Food Products

Abstract

It is now well recognized that matching the external drying condition with the drying kinetics of a material can lead to substantial savings of energy and in the case of heat-sensitive products, even to higher quality product. In this work, the effect of convection and microwave heat input and other product parameters on the batch drying characteristics of model materials, potato and carrot slabs, whose thermo-physical data are readily available in the literature, was modeled using a one dimensional liquid diffusion model. The influence of various thermo-physical properties of the product in drying of heat-sensitive materials was quantitatively assessed. Heat of wetting, temperature and moisture dependent effective diffusivity and thermal conductivity are considered in this model. The effect of moisture diffusivity on drying using convection and a microwave field is simulated in view of the interest in predicting the drying performance by simplified method. Conditions under which the drying rate is controlled by the external drying conditions and the internal thermo-physical properties of the product are computed and discussed.     

R & D Needs - Drying of Ceramics

ABSTRACT

New research and development needs for the drying of ceramics are prooosed. The researches on the ceramic drying are briefly reviewed and the role of the drying in the ceramic production process iS summarized. The drying must deal with molded materials and the problem is of significant difference from another drying with particulate materials. It means that the drying must be performed with keeping the molded feature and the insufficient completion of drying and the careless operation influence directly on the product quality The importnance of the R & D on the heat and moisture transfer. shrinking mechanism. the deformation behavior and the strain-stress formation in the molded clay issuggested. The subjects are also pointed out for the further improvement of the ceramic drying process and the precise design of molding with high quality.     

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.     

Rheological Behavior of Douglas-fir As Related to the Process of Drying

ABSTRACT

Large inelastic strain occurs inside a piece of lumber during drying. The strain consists of several components such as elastic, plastic, creep, shrinkage and mechano-sorptive effect. The drying behavior of the whole board is determined by the behavior of the individual components and their interactions. Whereas limited investigations have been made on those strains under moderate conditions, there is a lack of comprehensive research aimed to examine the behavior at elevated temperatures and to incorporate the various strain components into a process model. This research provides experimental data for various strain components of small wood samples and an analytical tool for evaluating the drying behavior of full-size boards.

Small test specimens of Douglas-fir were loaded tangentially in both tension and compression under constant and varying moisture conditions at different temperatures. Experiments were conducted using a small testing machine contained within a pressure vessel. The strain fields for loaded and unloaded test samples were measured using a high resolution video camera. The required moisture change at controlled temperatures was achieved by controlling the total pressure in absence of air with saturated steam. Moisture content was monitored by a quartz spring sorption balance.

The total deformation due to loading and moisture change was decomposed into instantaneous, creep, shrinkage and mechano-sorptive components. Constitutive equations for each component were developed as a function of stress, temperature, moisture, time and moisture change. These equations were incorporated into a process model to simulate the development of stress and strain in large pieces of lumber during drying.

A slicing method was used to measure the distribution of moisture and strain through the thickness of full-size boards at different stages of drying. The process model was used to predict drying stress and strain based on the measured moisture distribution and material properties. The effect of drying conditions and types of wood on the development of drying stress was demonstrated. The predicted drying strains under different drying conditions were compared with the corresponding measurements.     

Mathematical models for the drying of rigid porous materials

This paper presents a resume of literature on theories and mathematical models for drying of rigid porous materials. Key work on drying by soil physicists has been neglected in the engineering drying literature. We have included these works here to bring this literature to the attention of engineers. A new and general model for moisture and energy transport in rigid porous media during drying is presented. It is demonstrated that under certain simplifying assumptions, the general model reduces to less general models which have previously been proposed. Experimental and simulation results are given for the drying of Valentine sand. Under the drying conditions studied, the drying rate during the falling rate period is controlled by the capillary flow of water to an evaporation zone in the porous media. The models simulated here are of varying complexity and rigor. The capabilities and limitations of these models are discussed.     

THERMOMECHANICS OF THE DRYING PROCESS OF FLUID-SATURATED POROUS MEDIA

ABSTRACT

Aspects of the thermomechanics of fluid-saturated porous solids relating to the drying process of materials are discussed in the paper. From the balance equations and the principles of thermodynamics of irreversible processes, the differential equations describing the drying body deformations and induced stresses, the temperature, and the moisture content distributions are developed. The porous body is assumed to be elastically deformed and the pore space is filled with an immiscible liquid-gas mixture-the moisture. The phase transitions of liquid into gas are taken into account     

A Moisture Transfer Model for Isothermal Drying of Plant Cellular Materials Based on the Pore Network Approach

Plant materials with cellular structure, like fruits and vegetables, are often viewed as porous media in terms of model building of the drying process, on the basis of a hypothesis that all of the moisture of a plant tissue is trapped in a continuous and connected pore network system. However, most of the moisture in the plant tissue is contained naturally in enclosed cells. In the course of drying, the trapped moisture has to cross the cell membranes and then migrates in the extracellular space. Based on this concept, a pore network model for isothermal drying of plant materials was developed in which two stages of moisture movement—transmembrane transfer and extracellular transfer in the pore network—were considered. Finally, the isothermal convective air-drying processes of a potato slice were simulated. The calculated results were validated by the experiments conducted under the simulation conditions.     

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.     

HEAT,MOMENTUM, AND MASS TRANSFER MEASUREMENTS IN INDIRECT AGITATED SLUDGE DRYER

ABSTRACT

Whereas indirect agitated drying has been extensively studied for granular materials, little is known in the case of pasty products. We describe an experimental set up specially designed for the investigation of drying kinetics, of heat transfer coefficient evolution, and of the mechanical torque necessary for stirring. This device was applied to municipal sewage sludge. Preliminary experiments were performed to investigate the influence of ageing of sludge on the drying kinetics. It appears that ageing does have no effect except for the first two days. The influences of the wall temperature, the stirrer speed, the dryer load and the location of the stirrer against the heated wall were studied. Three different rheological behaviors were observed during sludge drying. In particular, the sludge goes through a “glue” phase, and high levels of mechanical strain are recorded when the compact mass begins to break up. A critical stirrer speed is found within the range 40–60 rpm. To shorten the drying time, it seems better to adjust the wall temperature in accordance with the moisture content of the sludge.     

Sticky Region in Drying—Definition and Identification

Abstract

A device has been developed, to characterize the stickiness of materials prone to shearing that is based on torque measurement during intermittent stirring. Experiments were carried out, using materials of various origin, and over a range of moisture contents that span the pasty to free-flowing powder consistencies. To reflect the complex nature of stickiness, often noted by researchers during the drying of slurries and pastes, the concept is proposed, of a stickiness region, adjacent to the stickiness curve. Referring to the two-dimensional visualization of a drying process with evolving moisture content and material temperature, possible trajectories are suggested, to deal with the stickiness problem.     

DRYING OF BIOLOGICAL PRODUCTS WITH SIGNIFICANT VOLUME VARIATION. EXPERIMENTAL AND MODELING RESULTS FOR POTATO DRYING.

ABSTRACT

This work is devoted to the experimental investigation and theoretical modeling of biological products with high initial moisture content which implies significant volume contraction. Firstly, experimental equipment which was designed to continuously determine variations in the global moisture content and in the temperature of the sample is introduced. Furthermore, the equipment is controlled by a PC which records experimental information in real time. In particular, measurement of the radial and temporal distributions of moisture content and temperature during the drying process of the homogeneous porous media potato are given. The mean moisture content and temperature curves are also given.