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This work aimed to develop numerical models to predict the moisture content and deformation of potato slices during convective drying (40–80 °C, 0.5 m·s−1). Three-dimensional slices were considered in cylindrical, cubic, parallelepiped, and prism geometries. The first classic model coupled the linear constant drying rate period with the analytical solution of Fick's law in spherical coordinates, evaluating the mass diffusion coefficients (4.2–15.5·10−10 m2·s−1), critical drying time (1640–5085 s), and critical moisture content (1.8–2.4 kg·kg−1). The Finite Element Method (FEM) was a more robust model, that combined momentum and mass transfer to three-dimensional solid deformation of polyhedrons by ALE method, evaluating the mass diffusivity (1.4–6.5·10−10 m2·s−1). The FEM model could predict the shrinkage due to water molar flux removal on moving solid boundaries and explain the pseudo-constant drying rate detected in experimental data. The developed models accurately described the drying of food materials with a high shrinkage ratio. [ABSTRACT FROM AUTHOR]

The mass transfer (diffusion) properties of a colloidal capillary-porous material (pears cut into plates 5 mm thick for convective drying) as a function of the moisture content of the material at different temperatures of the drying agent (air) were studied experimentally. Since the pear sample shrinks significantly during drying, the change in sample thickness during the process was taken into account when calculating the mass conductivity coefficient. At the same time, the linear shrinkage of the specified material was studied. The dependences of the mass conductivity coefficient on the moisture content of the material k = f(u) were obtained from drying curves based on the zonal method at a constant temperature of the drying agent tdr. They represent dependences concave to the abscissa axis that increase with moisture content, which is typical of dried materials. The minimum values of the mass conductivity coefficient on the graphs k = f(u) vary from 0.3∙10–10 m2/s at 30°C to 2.2∙10–10 m2/s at 60°C. With a change in the moisture content of the material, the mass conductivity coefficient changes multiply. The shrinkage data for engineering calculations was described by equation. The possibility of describing the temperature dependence of the mass conductivity coefficient (moisture diffusion) by Arrhenius equation is shown. The values of the Arrhenius equation parameters are obtained. [ABSTRACT FROM AUTHOR]

A computational model is generated to analyse the influence of shrinkage on convective drying problems. The object considered here is cranberry which is assumed as 100% spherical. The drying air temperatures were considered from 313 to 348 K. There were four models used in this study, namely one-dimensional models without and with shrinkage, two-dimensional models without and with shrinkage. A finite difference scheme was used to discretize the heat and mass transport equations. Four separate computer codes were written in MATLAB to solve the discretized equations. The Arrhenius model is used to couple the heat and moisture transport equations and solve them simultaneously. The Gauss–Seidel iterative method was used to solve the equations. An empirical shrinkage model was used to calculate the shrinkage effect. The temperature and moisture distributions of cranberry were obtained with drying time. Moisture distributions without and with shrinkage results were calculated and compared to identify the effect of shrinkage. The cranberry lost a maximum of 35% of its size during drying. There was not much variation found in the results between 1- and 2D models. The maximum difference in centre moisture content of cranberry without shrinkage model was 56%, and the difference in mean moisture content was 49.2%. The numerical outcomes were validated with experimental data, and the model with shrinkage was a good fit with them than without shrinkage. Therefore, shrinkage needs to be considered in convective drying phenomena. [ABSTRACT FROM AUTHOR]

• Drying characteristics and mechanisms of hydrogel with a constant temperature stage are clarified and revealed. • A maximum duration (3 h) of constant temperature stage with an evaporation heat flux 268.7 W/m2 is observed in the hydrogel drying. • The maximum reduction in duration of stage II is 100 % when initial Biot number (Bi 0) increases from 0.39 to 0.47. • Under the same drying temperature, a critical Bi 0 (Bi 0,c) exists for the same hydrogel below the Bi c the drying rate increases with Bi 0 significantly. In this study, the convective drying characteristics of hydrogel with a constant temperature stage at different drying temperatures and initial Biot numbers (Bi 0) are experimentally presented. Additionally, a numerical model coupling the mass, energy conservation equations and solid momentum balance equation is developed and validated to reveal the drying mechanisms. The results show that there are three drying stages, i.e., the temperature rise (stage I), constant temperature (stage II), and second temperature rise (stage III). Numerical results demonstrate that the duration of stage II is related to the internal moisture transport resistance which increases rapidly and leads to the end of stage II. A maximum duration of 3 h for stage II with evaporation heat flux of 268.7 W/m2 is observed at drying temperature of 35 ℃. The three drying stages persist as the drying temperature increases, but the duration of stage II decreases rapidly. In particularly, under the same drying temperatures, there is a critical Bi 0 (Bi 0,c) below which the drying rate increases with Bi 0 significantly, but above it, the drying rate increases slightly for the same hydrogels. The duration and temperature of stage II decrease with increasing Bi 0. [ABSTRACT FROM AUTHOR]

The authors have discussed the physical regularities of internal mass transfer in drying colloidal capillary-porous materials, which is characterized by the fact that in addition to the phenomenon of mass conductivity (diffusion of moisture) in a fixed coordinate system, the transfer of moisture by the matrix of the material due to its shrinkage is observed. As applied to this case, for a body in the shape of an unbounded plate dried symmetrically from both surfaces, a mathematical model has been written that describes mass conductivity and convective transfer of moisture in the body. The convective transfer of moisture depends on the normal coordinate to the plate's surface: it is maximum at the surface of the plate and is equal to zero in its central plane. The authors have formulated the problem describing the process of symmetric drying of an unbounded plate on the basis of the convective-diffusion equation with a uniform initial distribution of the concentration and a boundary condition of mass transfer of the third kind. It has been noted that the formulated problem can be solved by numerical methods. For analysis of the degree of influence of the convective component on internal mass transfer, an analytical solution to the problem at constant parameters of the process has been obtained. According to the obtained solution, calculations of the change in the volume mean relative concentration as a function of the Fom number have been performed for the internal problem (Fom = 100) and the mixed diffusion problem (Fom = 10). It has been shown that the maximum effect of the influence of the convective component is observed for the intradiffusion problem. [ABSTRACT FROM AUTHOR]

Convective drying is an ancient technique to dry moist materials. Many experimental and numerical studies on convective drying enriched the knowledge of researchers and industrialists for further research and developments. In this study, a thorough review is made on convective drying of food materials. A brief history of convective drying, its importance, methods, applications, and statistical data are clearly reviewed. The impact of important drying parameters such as shrinkage and influence of pretreatment during convective drying have been critically appraised. Instruments used, experimental setup developed, and properties measured and estimated have been listed and analyzed. A novel technology of taking snapshots during drying using the thermal camera is discussed. Different mathematical models such as analytical and numerical methods and its present challenges are extensively reviewed and discussed. An advanced level solution for convective drying at present is explained. Gaps in numerical methods to solve convective drying problems have been discussed and effective summaries are drawn. [ABSTRACT FROM AUTHOR]

Highlights • The apple is dried utilizing convective drying method. • The experiments are studied via RSM and utilizing desirability function. • The drying time, energy consumption, and shrinkage are optimized. • The optimal conditions are designated by maximum desirability function (D = 0.781). Abstract In this work, response surface methodology (RSM) is applied to state an optimized system for convective drying of apple slices using desirability function. The interaction of the independent parameters including air temperature (T = 70–90 °C), air velocity (V = 4–5 m/s), and apple slice geometry (G = circle, square, and triangle) with the dependent variables consist of drying time, energy consumption, and shrinkage is determined. Minimum drying time, energy consumption and shrinkage is regarded as the optimizing drying conditions of apple slices. Experimental results are adapted by a second-order polynomial model where analysis of variance is utilized to define model compatibility and optimal drying conditions. The optimal conditions of combined optimized responses were 90 °C inlet temperature, 5 m/s inlet velocity, and square geometry which designated by maximum desirability function (D = 0.781). The air temperature had a notable influence on total responses, but the effect of air velocity and apple slice geometry were important in shrinkage. Generally, the results marked that a greater desirability value could be obtained in the higher air temperature and higher air velocity with the square geometry. [ABSTRACT FROM AUTHOR]

A self-designed hot-air convective dryer coupled with contacting ultrasound system was used for the dehydration of garlic slices. Contacting ultrasound significantly accelerated the drying process. Compared with non-ultrasound treated samples, the drying time of garlic samples sonicated at 1513.5 W/m 2 was shortened by 35.0% at 50 °C, 48.5% at 60 °C and 50.0% at 70 °C. According to water transport simulation using a diffusion model considering sample shrinkage, both water effective diffusivity and external mass transfer coefficient were enhanced when garlic samples contacted ultrasound directly during drying. In the meantime, internal water diffusion was more affected by contacting ultrasound. Furthermore, contacting ultrasound had a positive effect on the preservation of organosulfur compounds in garlic slices during drying. The browning problem of garlic products during hot-air drying was also alleviated if contacting ultrasound treatment was performed properly. Besides, no over-heating problem was observed during contacting ultrasound enhanced drying within the studied experimental range. [ABSTRACT FROM AUTHOR]

Summary: This paper presents a thermo‐hydro‐mechanical framework to model the drying behavior of Boom clay. First, the experimental campaign conducted Noémie Prime is briefly presented because it is used to validate the model. The data acquisition and processing is emphasized because of the use of X‐ray microtomography to be able to more accurately compare experimental and numerical strain fields. The different submodels are introduced. Numerical simulations are performed to illustrate the capability of the proposed model to reproduce the observed behavior. Finally, a comprehensive sensitivity study on several key model parameters associated with the water retention curve, and the permeability of the medium, is performed to get a better understanding of the physics behind the coupled model. [ABSTRACT FROM AUTHOR]

The finite element method was employed to model the simultaneous heat and mass transfer in cylindrical samples of banana during convective drying. The Arbitrary Lagrangian-Eulerian (ALE) approach was implemented to incorporate the axial and radial shrinkage effects in the FEM model. The results showed that the FEM model was satisfactorily able to predict the moisture content (R² > 0.97) and core temperature (R² > 0.98). Therefore, the proposed model can be used as a promising tool to optimise the drying process of banana from quality and efficiency points of view. Also, the model can be easily extended for drying of similar products with cylindrical shape. [ABSTRACT FROM AUTHOR]

The shrinkage of biological materials (agri-food), is a common phenomenon developed during a drying operation that directly affects the product quality. In this work a dimensionless mathematical model including the reduction at the interface of a high moisture content food (tuber) in 1D geometry was developed. The mass and heat conservation equations coupled to the Leibniz-Reynolds Transport Theorem (LRTT) at the interface of a high moisture content food were solved. The system of nonlinear unsteady state equations was solved by using a multi-frontal massively parallel sparse direct Solver. In order to validate this model, tuberous crop slices were dried at three temperatures: 40, 50 and 60 °C, relative humidity of 25% and a constant airflow of 1.5 m/s. Drying kinetics and the evolution of temperature within the product were logged. The model is able to simulate the moisture loss and predict the highest thickness change. The simulations show a good agreement with the experimental drying kinetics, as well as for the evolution of temperature. Simulations show 1-D shrinkage is not linear and the model improves its prediction when the drying temperature is higher (intensive mass flux). [ABSTRACT FROM AUTHOR]

The finite element method was employed to model the simultaneous heat and mass transfer in cylindrical samples of banana during convective drying. The Arbitrary Lagrangian–Eulerian (ALE) approach was implemented to incorporate the axial and radial shrinkage effects in the FEM model. The results showed that the FEM model was satisfactorily able to predict the moisture content (R2> 0.97) and core temperature (R2> 0.98). Therefore, the proposed model can be used as a promising tool to optimise the drying process of banana from quality and efficiency points of view. Also, the model can be easily extended for drying of similar products with cylindrical shape. [ABSTRACT FROM AUTHOR]

Boom clay is studied as a potential host rock for underground nuclear waste storage. This experimental work analyzes its response under the convective drying conditions that may be applied in case of contact with the ambient atmosphere (during disposal drilling, for instance) or with gallery ventilation (because of damage in gallery lining). The drying kinetics is first established. In a second phase, the cracking and shrinkage onset and development are focused on, thanks to an X-ray microtomography device. The results show a phase of ideal shrinkage and a phase of shrinkage with desaturation, which are closely related to the kinetics. Cracking and shrinkage progressively develop from the drying surface, up to values representing about 3 and 12% of the cross section area, respectively. [ABSTRACT FROM AUTHOR]

This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of the original sludge. With increasing amounts of sawdust, the initial and final bed volumes, initial and final total exchange surfaces, and initial void fraction increase linearly, but the bed volume shrinkage and final void fraction decrease linearly. [ABSTRACT FROM PUBLISHER]

Fruits and vegetables are porous in nature and undergo pronounced shrinkage during convective drying process. Therefore, shrinkage and porosity should be taken into consideration while predicting heat and mass transfer. This work was conducted to study shrinkage and porosity changes along with simultaneous heat and mass transport during the process. Potato slices were subjected to drying for 7 h at 62 °C. It was observed that shrinkage varies linearly with respect to moisture content and reduction in radial dimension of potato slices was around 35%. Porosity undergoes rapid increase after attaining certain moisture content in final stages of drying. The work was extended to study the influence of shrinkage and porosity on heat and mass transfer. Simulated results were validated with experimental values. This model can be employed to predict temperature, moisture, density profiles and to study shrinkage and porosity of various fruits and vegetables. [ABSTRACT FROM AUTHOR]

The effect of drying temperature (40-65 °C) and air rate (2-6 m s−1) on the formation of Maillard reaction indicators and vitamins content of carrots dehydrated by convection has been investigated. The range of assayed processing conditions, based on a previous central composite face design (CCD), led to moderate changes in the studied parameters, even under the most severe conditions. In addition, the drying kinetic of the process was studied taking into account the experimental quantitation of shrinkage, which allowed the determination of a first drying period with a constant rate of water evaporation per unit of exchange surface. The slope of the first drying period, the moisture loss during the first hour of drying and the level of quality parameters (Maillard reaction indicators and vitamins) were correlated with processing conditions with high accuracy. For the prototype here used, the optima temperature and air rate to maximise the desirability function (0.77) were 46 °C and 4.9 m s−1. [ABSTRACT FROM AUTHOR]

The influence of seed coat on the behaviour of heterogeneous structure of cocoa bean (frosteros sp.) during convective drying with tempering period was investigated in an isothermal condition. It was found that the removal rate of moisture and air diffusion between the cocoa kernel and seed coat is not the same especially at the temperature of 55°C and initial drying time for all temperatures used. When the drying temperature increased to 70°C and 81 °C, the cocoa kernel and the seed coat shrinks simultaneously, indicating that the rate of moisture and air removal from the kernel through the seed coat are the same. Change in sphericity of the bean was more pronounced at the temperature of 55 °C for the seeds with seed coat while for all other drying conditions it remains fairly constant after one-hour drying. [ABSTRACT FROM AUTHOR]

Abstract: In many productions, Arthrospira sp. biomass is dried in cylinders or layers form. Drying involves shrinkage that changes organoleptic qualities and aspects. To understand the change, similar production forms and sizes were convective dried at 45°C with 2–6mm initial diameter cylinders and 1 and 4mm thin layers. Total layer size- and cylinder length shrinkage ratio is only 95%. Thicknesses direction shrinkage is the most significant with 28–65%. Cylinders present anisotropicity and shrink more in the perpendicular direction to the support than in the parallel one. All curves present several stages. The first ideal shrinkage stage of layers goes until the half of initial moisture content that involves lower final porosity by layers than by cylinders. End of ideal shrinkage moisture content decreases when cylinder initial diameter decreases. Cylinders initial porosity is around 15–20% and increases only during the 2nd stage up to final porosities between 68% and 80%. [Copyright &y& Elsevier]

Abstract: The paper analyses the results of the research on drying a variety of giant pumpkin (Justynka – 957). The research involved drying kinetics of pumpkin as well as changes in the volume of 10mm pumpkin cubes, dried at the temperature of 80°C, using free (natural) convection and in the forced convection in the tunnel dryer, with the drying factor speed of 1.2ms−1 and in the fluid bed dryer. Drying kinetics of pumpkin pieces dried in the tunnel dryer, fluid bed dryer and during two-stage initial drying in the tunnel, and in the fluid bed dryer were examined. Measurements results were used to verify the theoretical models of drying kinetics of the first period taking volume shrinkage into consideration, and the second period of the examined processes. [Copyright &y& Elsevier]

The purpose of this study was to determine the influence of the internal structure of dried apples on such physical properties as density, porosity, or shrinkage. Apple slices were dried using convection, microwave-convection, and infrared-convection methods. The apple slices dried by means of the microwave and infrared methods showed between 11 to 12% lower shrinkage, 30-34% lower volume, 18-23% lower density, and porosity of 25-28% higher than the slices dried exclusively by the convection method. Considerable differences in the density and porosity of the apple slices dried using the above methods were confirmed by analyzing the tissue images made with the help of scanning electron microscopy. Convective drying caused significant changes in the actual size and distribution of the apple cells. The cells dried using both the microwave and infrared methods had a larger cross-sectional area compared with the convective-dried material. [ABSTRACT FROM AUTHOR]

In the present work, a diffusion-based model was adopted to represent the convective drying behavior of cylindrical banana samples, taking into consideration the shrinkage along drying. The developed model simulated a significant number of situations resulting from the variations of some operating conditions. The temperatures tested were 50, 60, 67 and 70°C, the air velocities were 3, 4 and 4.5 m/s and the relative humidity range of the drying air was from 3.5 to 11.5%. The calculated drying curves were compared to the experimental ones in order to determine apparent moisture diffusivity. An empirical equation was suggested, describing the apparent moisture diffusivity within the banana versus product temperature and local moisture content. A good agreement was found between experimental and calculated drying kinetics. [ABSTRACT FROM AUTHOR]

Abstract: Non-isotropic shrinkage of potato slabs was studied within the frame of the systematic approach to food engineering systems (SAFES) methodology. Digital images of potato slabs were taken and lateral projected areas (A lat) of material during drying were recorded. Two characteristics peaks in graphs A lat vs. time were detected. Also, cellular surface area of potato cells was recorded and related to moisture content by means of an empirical equation with in conjunction with SAFES methodology and A lat values at the two afore mentioned peaks, allowed to evaluate intra- and extracellular moisture contents. Fractal dimension (D f) of solid–air interfaces during drying were also calculated. Graphs of D f vs. time also showed two peaks which appeared at approximately the same drying times than those for A lat which may indicate that inner structures composed of solid–air matrixes are projected towards the surface in the form of fractal solid–air interfaces. [Copyright &y& Elsevier]

Abstract: Kinetics of osmotic dehydration (OD) and effects of sucrose impregnation on thermal air-drying of pumpkin slices were investigated. A simplified model based on the solution of Fick’s Law was used to estimate effective diffusion coefficients during OD and air-drying. In order to take into account shrinkage, average and variable thicknesses were considered. Pumpkin slices were dehydrated in sucrose solutions (40%, 50% and 60%, w/w, 27°C). The effective water diffusion coefficients were higher than the sucrose, and low diffusivity dependence with solution concentration was observed. Samples non-treated and pre-treated in 60% osmotic solutions during one hour were dried in a hot-air-dryer at 50 and 70°C (2m/s) until equilibrium was achieved. Pre-treatment enhanced mass transfer during air-drying. Great volume reduction was observed in pre and non-treated dried samples. Using variable thickness in the model diminished the relative deviations between predicted and experimental OD and drying data. [Copyright &y& Elsevier]

Modeling of drying of capillary-porous materials is a mathematically complex problem. It takes into consideration simultaneous heat and mass transfer inside the material with physicochemical properties changing during the drying process. Modeling of the process mentioned above consists of describing the heat and mass transfer balances by means of differential equations. Moisture diffusion coefficient as a function of moisture content and temperature of the material is a crucial parameter that controls the process. An additional problem occurs when moving boundary of the shrinking material is taken into account. In the present work, the identification of diffusion coefficient as a function of moisture content and temperature on the basis of two different models is shown. The two models include the Pakowski model (defined in the stationary coordinates) and the Kechaou model (defined in moving coordinates). Experimental data necessary to verify the models were obtained on the basis of series of tests for different boundary conditions performed on an apple tissue. During the drying process, samples of apple undergo significant volumetric shrinkage. In this article, the comparison of the two models describing the convective drying process of shrinking material is presented together with the comparison of the identified moisture diffusion coefficient. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to report potato moisture diffusivity data determined from experimental convective drying kinetics. Two different methods have been applied to evaluate this coefficient. The first one was based on the analytical solution of a fickian diffusive model which enabled diffusivity determination by simple linear regression over experimental data. The regression was done using the real sample thickness in order to account for shrinkage. The second one was based on a numerical solution of solid mass balance equation and moisture transfer diffusion/convection equation, these equations being coupled by the solid phase velocity due to shrinkage. At each time step of the simulation, the diffusivity was adjusted to minimize the difference between calculated and measured sample mean moisture content. Due to several simplifying hypothesis the first method could only provide approximate results, while the second could be considered as reference. Predictive correlations for moisture diffusivity as function of temperature and moisture content are presented. [Copyright &y& Elsevier]

Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation. [ABSTRACT FROM AUTHOR]

This study was conducted to evaluate the quality and structural changes in potatoes during microwave and convective drying. A modified microwave oven, operated in either the microwave or convective drying mode, was used to dry the samples. The quality attributes of the dehydrated potato samples were investigated on the basis of the ascorbic acid retention (vitamin C) and rehydratibility, and the structure in terms of the shrinkage behaviour. Ascorbic acid is an important indicator of quality and its selection was due to its heat labile nature. Ascorbic acid deterioration demonstrated first-order kinetic behaviour, and was further found to depend on air temperature, microwave power and moisture content. Reduced vitamin C destruction was found in the microwave dried samples. The volumetric shrinkage of the samples exhibited a linear relation with moisture content. With convective processing, the samples exhibited uniform shrinkage throughout, however, with microwave drying two shrinkage periods were observed. Microwave dried samples had higher rehydration potential. [Copyright &y& Elsevier]

Drying of heat-sensitive materials of high moisture content requires exact knowledge of kinetics and reliable control of a drying process. A physical-mathematical model was compiled for the description of heat and mass transfer, and was coupled with the elastic behavior of the material. To check the validity of the mathematical model, drying experiments were carried out. Potato was chosen as a testing material. Experimental conditions were as follows: drying temperatures of 38, 42, and 47°C, relative humidity of 14, 20, and 23%, and air velocities of 1.5, 3, and 4.5 m/s. Representative drying and temperature curves as well as moisture and temperature profiles are presented and discussed. Comparison between predicted and experimental results provides satisfactory agreement. Stress field developed during drying and obtained using the rheological behavior of the material was also presented and commented. [ABSTRACT FROM AUTHOR]

Shrinkage of foodstuffs is a common physical phenomenon observed during different dehydration processes. These changes affect the quality of the dehydrated product and should be taken into consideration when predicting moisture and temperature profiles in the dried material. The aim of this work is to give a physical description of the shrinkage mechanism and present a classification of the different models proposed to describe this behaviour in food materials undergoing dehydration. The models were classified in two main groups: empirical and fundamental models. Empirical models are obtained by means of regression analysis of shrinkage data. Fundamental models are based on a physical interpretation of the structure of food materials and try to predict dimensional changes due to volume variation of the different phases in the food system along the drying process. Several models referred to in this work were compared with experimental data on air drying of apple, carrot, potato and squid flesh. Average relative deviations between experimental and predicted values of shrinkage found were in most cases less than 10%. For some materials, models that neglect porosity change tend to show larger deviations. [Copyright &y& Elsevier]

X-ray microtomography is proposed as a new tool to investigate the evolution of size, shape and texture of soft materials during a drying operation. This study is focused on the drying of mechanically dewatered sludges from a secondary wastewater treatment. The shrinkage phenomenon is shown to play a crucial role in the control of the drying process. The shrinkage curves are determined by analysing the shape and size of cross sectional microtomographic images of sludge extrudates at different levels of drying. The observation of drying and shrinkage curves allows us to determine 3 critical water content values, which define different drying zones where extragranular, intragranular or mixed limitations prevail. When drying is externally controlled, the decrease of the drying rate observed during experiments can be related to the reduction of the external area of the sample, i.e., to shrinkage. When drying is internally controlled, resistances inside the solid govern the process. Between these two extreme situations, the drying rate reduction is the result of both the external area decrease and the development of internal resistances limiting drying. A multizone model is proposed to describe quantitatively these observations. The analysis of the internal texture of the sludge extrudates reveals crack formation at the end of the drying process. The onset of crack formation is clearly related to the appearance of internal transfer limitations, i.e., humidity and temperature gradients inside the material. [ABSTRACT FROM AUTHOR]

The convective drying of plate made of clay is considered in the paper. The model of drying with material coefficients dependent on the moisture content and the temperature is presented. The material is assumed to be brittle-elastic (linear elasticity with moisture dependent material constants). The strength of the material estimated by bending test, is moisture dependent. Numerical simulation of drying processes is done. Through the comparison of the stresses arising during drying with the strength of the material the possibility of dried body damage is shown. The range of constant drying conditions in which damage is expected is specified. [ABSTRACT FROM AUTHOR]

The subject of this paper is to study the shrinkage of strawberries as a function of the moisture ratio during microwave-convective drying. Strawberries were pre-treated and osmotically dehydrated and dried at power levels 0.1 and 0.2W/g, based on initial mass. The main results showed that the shrinkage has a linear relation with moisture ratio; the equivalent diameter of the strawberry has a reciprocal logarithmic function with moisture ratio; the change in volume was bigger for the fruits osmotically dehydrated than for the not osmotically dehydrated ones, power level exerts influence on shrinkage and equivalent diameter for pre-treated and osmotically dehydrated samples. [ABSTRACT FROM AUTHOR]