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.     

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.     

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.     

2-D Hydro-Viscoelastic Model for Convective Drying of Highly Deformable Saturated Product

The aim of this work was to simulate in two-dimensions the spatio-temporal evolution of the moisture content, the temperature, and the mechanical stress within a highly deformable and water saturated product during convective drying. The material under study was an elongated potato sample with a square section placed in hot air flow. A comprehensive hydro-thermal model had been merged with a mechanical model, assuming a viscoelastic material, a plane deformation, and an isotropic linear hydric-shrinkage of the sample. This model was validated on the basis of the average water content and core temperature curves for drying trials under different operating conditions. The material viscoelastic properties were measured by means of stress relaxation tests at different water contents. The viscoelastic behavior was described by a generalized Maxwell model whose parameters were correlated to water content. The simulations of the spatio-temporal distributions of mechanical stress were performed and interpreted in terms of product potential damage. The sample shape was also predicted all aver the drying process with reasonable accuracy.     

High temperature internal friction measurements of 3YTZP zirconia polycrystals. High temperature background and creep

This work focuses on the high-temperature mechanic properties of a 3 mol% yttria zirconia polycrystals (3YTZP), fabricated by hot-pressureless sintering. Systematic measurements of mechanical loss as a function of temperature and frequency were performed. An analytical method, based on the generalized Maxwell rheological model, has been used to analyze the high temperature internal friction background (HTB). This method has been previously applied to intermetallic compounds but never to ceramics, except in a preliminary study performed on fine grain and nano-crystalline zirconia. The HTB increases exponentially and its analysis provides an apparent activation enthalpy which correlates well with that obtained from creep experiments. This fact shows on the one hand the plausibility of applying the generalized Maxwell model to ceramics, and on the other hand indicates the possibility of using mechanical spectroscopy as a complementary helpful technique to investigate the high temperature deformation mechanism of materials.     

粒度分布对超低水泥刚玉质浇注料流变性的影响

用浇注料流变仪研究了粒度分布对超低水泥刚玉质浇注料流变性的影响。结果表明 :粒度分布对超低水泥浇注料的流变性影响较大。随着粒度分布系数 (q)的减小 (从 0 .2 9、0 .2 6到 0 .2 3) ,浇注料的剪切应力、粘度和屈服应力增加 ,需水量上升 ,说明流变性变差。以 2 %的SiO2 微粉等量替代q =0 .2 3的浇注料中的Al2 O3微粉后 ,试样的流变性得到明显改善 ,更适合用于泵送施工 :浇注料的需水量减少 ,自流值提高。     

Flow properties and rheology of slag from coal gasification

We have studied the rheological characteristics of Texaco gasifier slag at high-temperature. Slag samples have been analyzed by X-ray fluorescence, X-ray diffraction, and scanning electron microscopy. The rheological behavior of the slag has been investigated experimentally using a high-temperature rheometer at temperatures between 1200 °C and 1340 °C. The effects of the shear rate and temperature on the rheological behavior of the slag have been explored. Moreover, the observed rheological behavior of the slag has been correlated with its solid-phase content, as calculated with the aid of the computer software package FactSage. The results show that the sensitivity of the slag viscosity to temperature decreases with increasing rotation speed. Above its liquidus temperature calculated by FactSage, the slag behaves as a Newtonian fluid; below its liquidus temperature, however, the rheological behavior of the slag becomes non-Newtonian owing to its increased solid-phase content. Meanwhile, Slag containing a number of crystalline particles shows dramatic shear-thinning and thixotropic behavior. Moreover, the shear-thinning behavior of the slag becomes ever more distinct as the temperature is decreased. The yield stress values of the slag and the number and particle size of the crystalline particles in the slag increase with decreasing temperature.     

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.     

Uniform shell patterning using rubber‐assisted hot embossing process. II. Process analysis

This two‐article sequence on rubber‐assisted embossing was aimed to understand the basic mechanisms affecting the pattern uniformity and replicability and to determine a process window for achieving uniform patterning and faithful replication. In Part I, the effects of major process and material parameters were identified and studied, and strategies for successful hot embossing with rubber as a pressure medium were proposed. In Part II, the rubber‐assisted embossing process was analyzed considering the unique rheological behavior of the materials involved to develop useful predictive capabilities for this new process. Specifically, a finite‐strain hyperelastic formulation was used for simulating the isothermal embossing stage, and a generalized Maxwell model was used to study the stress relaxation during the holding stage and to predict the elastic recovery after demolding. The rheological properties obtained in Part I were fitted to the constitutive models and implemented in the simulation procedures. The simulation results agreed with the major findings in the experimental work and provided a more quantitative insight into the process. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

WATER - CORN EQUILIBRIUM: TEMPERATURE DEPENDENCY OF THE G.A.B. MODEL PARAMETERS AND CALCULATION OF THE HEAT OF SORPTION

By using sorption data for com in the range of 4.4-60°C, the temperature (T) variations of the GAB. model parameters (M_m, C_B and k) were studied. As only the monolayer moisture (M_m) varied (decreased) with temperature, a generalized GAB equation of four total parameters was obtained. This, together with the Clapeyron equation allowed a method to calculate the heat of sorption (L_b) as a function of T and moisture content (M). For each temperature, Lb presented a maxinuin some 30% above the heat of vaporization of water (L_f), which occurred around the M_m, in congruence with the sorption theory. A similar behavior of L_b was found using the Othmer method. In contrast, the Gallaher equation predicted an ever increasing L_b up to 3 L_f at very low moistures. The GAB-Clapeyron method is then proposed to calculate L_b as a function of M and T.     

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

Abstract

The dynamic behavior of conveyor-belt dryers involving externally controlled heat and mass transfer phenomena has been studied via digital simulation. The investigation concerned an industrial dryer used for the moisture removal from wet raisins. The dryer consisted of three drying chambers and a cooling section, all involving the same conveyor belt. For each chamber, perfect temperature control was assumed for the drying air temperature, while its humidity was left uncontrolled. The effect of material temperature and moisture content at the entrance of the dryer and the drying air temperature on material temperature and moisture content at the exit of the dryer and the corresponding drying air humidity, have been explored by step forcing the disturbance and manipulated variables in the non-linear dryer model simulator. Results showed that material moisture content at the exit of the dryer is greatly affected by material moisture content at the entrance as well as by the drying air temperature. Reliable transfer functions for each process module were obtained by fitting several transfer function models on the simulated data using a least-squares approach. It was found that when input material moisture content could be instantly measured, the system responded slowly enough so that excellent control could be achieved for material moisture content at the exit of each chamber. In this case a Pi-feedback cascade temperature controller was used. When a 15 sec delay measuring sensor was introduced, poor performance was observed. A simplified lead-lag feedforward controller, added to the system, in conjunction with the primary Pi-feedback cascade controller, resulted in good control performance of the delay sensor system.     

Effect of Carbon Nanotubes on the Rheological and Electrical Percolation of PP/EPDM Blend

The rheological and electrical percolation of single-walled carbon nanotubes on a thermoplastic-elastomer based on polypropylene/ethylene-propylene-diene was investigated. Polypropylene-grafted maleic anhydride was used to improve the nanotube dispersion. The shear stress and viscosity decreased with increasing temperature from 200 to 220°C. The flow activation energy for the nanocomposites increased with increasing nanotube content. The morphology and degree of dispersion of the nanotubes in the thermoplastic-elastomer matrix were investigated using SEM. The obtained rheological and electrical properties of the nanocomposites indicate that they were affected by the nanotube-nanotube network structure, which was related to the morphological behavior of nanotubes uniform dispersion.     

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

The dynamic behavior of conveyor-belt dryers involving externally controlled heat and mass transfer phenomena has been studied via digital simulation. The investigation concerned an industrial dryer used for the moisture removal from wet raisins. The dryer consisted of three drying chambers and a cooling section, all involving the same conveyor belt. For each chamber, perfect temperature control was assumed for the drying air temperature, while its humidity was left uncontrolled. The effect of material temperature and moisture content at the entrance of the dryer and the drying air temperature on material temperature and moisture content at the exit of the dryer and the corresponding drying air humidity, have been explored by step forcing the disturbance and manipulated variables in the non-linear dryer model simulator. Results showed that material moisture content at the exit of the dryer is greatly affected by material moisture content at the entrance as well as by the drying air temperature. Reliable transfer functions for each process module were obtained by fitting several transfer function models on the simulated data using a least-squares approach. It was found that when input material moisture content could be instantly measured, the system responded slowly enough so that excellent control could be achieved for material moisture content at the exit of each chamber. In this case a Pi-feedback cascade temperature controller was used. When a 15 sec delay measuring sensor was introduced, poor performance was observed. A simplified lead-lag feedforward controller, added to the system, in conjunction with the primary Pi-feedback cascade controller, resulted in good control performance of the delay sensor system.     

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

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.     

粘弹性流体非等温本构方程

本文将Giesekus提出的单模式平均构型分子模型推广到非等温情形,然后利用人们熟知的粘温关系式来计及温度和热历史对应力张量的影响,得到一个描述聚合物浓溶液和熔体非等温流变性质的本构方程,不必在分子模型中引入人为的假定来考虑非等温效应.对于非等温应力发展的计算表明,这一本构方程能避免以前研究者提出的本构方程的缺点,合理地描述聚台物流体的非等温流变性质.     

高分子稀溶液中溶剂对分子特性与流变性能的影响

本文研究了不同溶剂对离子性和非离子性高分子稀溶液的分子构型和流变性能的影响.在部分水解的聚丙烯酰胺水溶液中加入无机盐,测量并计算了分子扩张因数,特征粘度与盐浓度、盐离子价数、溶液pH值的关系.同时使用不同的有机溶剂测量聚丁烯溶液的Maxwell模型松弛时间和分子扩张因数.研究表明,高分子稀溶液可藉调整溶剂性质的方法以达到期望的流变特性.     

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.