Drying of Rubber Wood Sawdust Using Tray Dryer

The present study attempts to understand drying characteristics of rubber wood sawdust in a tray dryer as it is the simplest and oldest of the dryers known commercially. An increase in temperature, flow rate of the heating medium, and initial moisture content was found to increase the drying rate. However, an increase in the particle diameter and bed height was found to reduce the drying rate. The increase in drying rate with temperature and moisture content was attributed to increase in the diffusion coefficient, while the increase due to the flow rate is attributed to reduction in the external mass transfer resistance during early stages of drying while the drying rate was high. An increase in bed height as well as particle size increases the diffusion path length for moisture, which contributes to the reduction in drying rate. The experimental data were modeled using Fick's diffusion equation, and the effective diffusivity coefficient was evaluated by minimizing the error between the experimental data and the prediction using the model equation. The effective diffusion coefficient was found to increase with increase in temperature, initial moisture content, and the flow rate of the heating medium, while it was found to decrease with increase in particle size. The diffusion coefficient was not found to vary with the bed height/solid loading. The effective diffusion coefficient was found to vary within 9.1 × 10^-9to 22 × 10^-9 m²/min. The standard deviation of error between the experimental data and prediction using the model, using the estimated effective diffusivity coefficient, was found to be less than 0.07 for the entire set of data, indicating the appropriateness of the model in predicting drying kinetics.     

The Prediction of the Emissivity and Thermal Conductivity of Powder Beds

The present study attempts to understand drying characteristics of rubber wood sawdust in a tray dryer as it is the simplest and oldest of the dryers known commercially. An increase in temperature, flow rate of the heating medium, and initial moisture content was found to increase the drying rate. However, an increase in the particle diameter and bed height was found to reduce the drying rate. The increase in drying rate with temperature and moisture content was attributed to increase in the diffusion coefficient, while the increase due to the flow rate is attributed to reduction in the external mass transfer resistance during early stages of drying while the drying rate was high. An increase in bed height as well as particle size increases the diffusion path length for moisture, which contributes to the reduction in drying rate. The experimental data were modeled using Fick's diffusion equation, and the effective diffusivity coefficient was evaluated by minimizing the error between the experimental data and the prediction using the model equation. The effective diffusion coefficient was found to increase with increase in temperature, initial moisture content, and the flow rate of the heating medium, while it was found to decrease with increase in particle size. The diffusion coefficient was not found to vary with the bed height/solid loading. The effective diffusion coefficient was found to vary within 9.1 × 10^?9to 22 × 10^?9 m²/min. The standard deviation of error between the experimental data and prediction using the model, using the estimated effective diffusivity coefficient, was found to be less than 0.07 for the entire set of data, indicating the appropriateness of the model in predicting drying kinetics.     

The effect of wall absorption on dispersion in annular flows

Summary The effect of the irreversible boundary reaction on the dispersion of tracers in annular flows is studied. The solution of the mathematical model, based on the generalized dispersion model, brings out the dispersive transport following the injection of a tracer in terms of the three effective transport coefficients, viz. the exchange, the convection and the dispersion coefficients. It is found that the exchange and the convection coefficients are enhanced while the dispersion coefficient is reduced with the increase in the value of the absorption parameter.The mathematical model is applied to a specific problem in clinical medicine: dispersion of solutes in blood flow in a catheterized artery. The model predicts that the insertion of a catheter in general and any further increase in its size will enhance the process of the dyes or other solutes getting out of the blood vessels, and this is found to be independent of the absorption parameter representing the absorption properties of the blood vessel wall.     

Comparison between models with constant and variable diffusivity to describe water absorption by composite materials

An important property of composite material is related with its water absorption characteristic. Several articles present studies about this property and, in general, in these articles an analytical solution of the diffusion equation is used to describe the process. In this case, the effective water diffusivity is considered with a constant value. However, the water penetration modifies the internal structure of the material and this can modify the effective mass diffusivity of the product. In this article, a one‐dimensional numerical solution of the diffusion equation in Cartesian coordinates is presented, enabling to consider a constant value or a variable expression for the effective water diffusivity. A comparison between the models with constant and variable diffusivity was performed and the results indicate that the model with variable diffusivity as a decreasing function of the local moisture content describes better the process than the model with constant diffusivity.     

An investigation on drying of millet in fluidized beds

Experiments were conducted to estimate the drying kinetics of millet in a batch fluidized bed. The drying rate was found to increase significantly with increase in temperature and with flow rate of the heating medium, while decrease with increase in solids holdup, in line with the qualitative understanding of the kinetics of drying in batch fluidized beds. Among the various popular semi empirical models tested with the experimental data, the simplest of all the models, page model was found to match the experimental data with the root mean square error (RMSE) of less than 1.4%.The effective diffusivity coefficient was evaluated using Fick’s diffusion equation assuming surface moisture in equilibrium with the surrounding atmosphere, by minimizing the error between the experimental data and the model prediction. The evaluated effective diffusivity coefficient was found to vary from 0.53 × 10^?11 to 2.8 × 10^?11 with RMSE less than 3.4%. The effective diffusivity of millet is found to be approximately three times lower than that of ragi (food grain) and corn.     

Effect of flexure induced transverse crack and self-healing on chloride diffusivity of reinforced mortar

Cracks in reinforced concrete are unavoidable. Durability is of increasing concern in the concrete industry, and it is significantly affected by the presence of cracks. The corrosion of reinforcing steel due to chloride ions in deicing salts or sea-water is a major cause of premature deterioration of reinforced concrete structures. Although, it is generally recognized that cracks accelerate the ingress of chlorides in concrete, a lack of consensus on this subject does not yet allow reliable quantification of their effects. The present work studies the relationship between crack widths and chloride diffusivity. Flexural load was introduced to generate cracks of width ranging between 29 and 390 μm. As crack width was increased, the effective diffusion coefficient was also increased, thus reducing the initiation period of corrosion process. For cracks with widths less than 135 μm, the effect of crack widths on the effective diffusion coefficient of mortar was found to be marginal, whereas for crack widths higher than 135 μm the effective diffusion coefficient increased rapidly. Therefore, the effect of crack width on chloride penetration was more pronounced when the crack width is higher than 135 μm. Results also indicate that the relation between the effective diffusion coefficient and crack width was found to be power function. In addition, a significant amount of self-healing was observed within the cracks with width below 50 μm subjected to NaCl solution exposure. The present research may provide insight into developing design criteria for a durable concrete and in predicting service life of a concrete structures.     

Exact analysis of unsteady convective diffusion in Casson fluid flow in an annulus – Application to catheterized artery

Summary The dispersion of a solute in the flow of a Casson fluid in an annulus is studied. The generalized dispersion model is employed to study the dispersion process. The effective diffusion coefficient, which describes the whole dispersion process in terms of a simple diffusion process, is obtained as a function of time, in addition to its dependence on the yield stress of the fluid and on the annular gap between the two cylinders. It is observed that the dispersion coefficient changes very rapidly for small values of time and becomes essentially constant as time takes large values. In non–Newtonian fluids the steady state is reached at earlier instants of time when compared to the Newtonian case and the time taken to reach the steady state is seen to depend on the values of the yield stress. It is observed that a decrease in the annular gap inhibits the dispersion process for all times both in Newtonian as well as in non–Newtonian fluids. When the yield stress is 0.05, depending upon the size of the annular gap (0.9–0.7) the reduction factor in the dispersion coefficient varies in the range 0.58–0.08. The application of this study for understanding the dispersion of an indicator in a catheterized artery is discussed.     

Influence of variable permeability on the dispersion of a chemically reacting solute in porous media

In this paper in the first part, the influence of variable permeability on the two MHD basic flows in porous media is studied numerically. It is seen that the variable permeability due to random packing causes the hydromagnetic channelling in both the flows. Further, it is also observed that the flow in both the cases is retarded for any increase in the magnetic parameter.

In the second part, the influence of variable permeability and the effects of homogeneous and heterogeneous reactions on the dispersion of a solute in the above two basic flows are investigated. In the case of diffusion with a first order homogeneous reaction, in both the flows, the variation of permeability increases the effective dispersion coefficient while it decreases with increase in the magnetic parameter. In the case of diffusion combined with homogeneous and heterogeneous chemical reactions, the variation in permeability again increases the effective dispersion coefficient while with increase in magnetic parameter causes a decrease in the effective dispersion coefficient. Several earlier results follow as particular cases of the present study.     

A probability-based method for calculating effective diffusion coefficients of composite media

A probability-based method is proposed for calculating the concentration of chloride at specified points of deterministic and random heterogeneous microstructures. The method, referred to as local method, is used to calculate effective diffusion coefficients of composite media. The effective diffusion coefficient of a heterogeneous material specimen is defined as the diffusion coefficient of a fictitious homogeneous specimen with the same global properties as those of the original specimen. The local method is based on properties of diffusion processes and Itô’s formula; it is simple to code, always stable, accurate and suitable for parallel computing. We use the method to calculate effective diffusion coefficients of a material specimen with a deterministic diffusivity field and a random heterogeneous mortar specimen calibrated to an experiment reported in Ref. Caré (2003) [11].     

荷载作用下饱和水泥浆体中氯离子扩散性能研究

混凝土类水泥浆复合材料中各种尺度的孔隙,如凝胶孔、毛细孔、掺入的气体气泡以及微裂纹等影响着氯离子的扩散性能。孔隙结构参数(如孔隙率)在外荷载作用下会产生变化,进而影响了水泥浆体中氯离子扩散性能。外荷载作用对氯离子扩散行为的影响,可以等效为外荷载所引起的孔隙率的改变对氯离子扩散性能的影响。从微观角度出发,将饱和水泥浆体看作由水泥浆体基质(其孔隙率为零)和孔隙水夹杂相所组成的两相复合材料介质。基于弹性力学理论推导并获得了饱和水泥浆体达到其强度前(即未产生新裂纹前)当前孔隙率与材料初始孔隙率及体应变之间的定量关系,得到了水泥浆体中氯离子扩散系数与这些参数的定量关系。基于Fick第二定律分析了外荷载(体应变)和孔隙率变化对氯离子扩散性能的影响。研究表明:氯离子在饱和砂浆中的扩散系数随孔隙率增大而显著增大;氯离子在砂浆中的扩散系数随压缩体应变的增大而减小,随拉应变增大而增大。     

Probing Membrane Viscosity and Interleaflet Friction of Supported Lipid Bilayers by Tracking Electrostatically Adsorbed,Nano‐Sized Vesicles

Particle tracking is used to measure the diffusional motion of nanosized (≈100 nm), lipid vesicles that are electrostatically adsorbed onto a solid supported lipid bilayer. It is found that the motion of membrane‐adhering vesicles is Brownian and depends inversely on the vesicle size, but is insensitive to the vesicle surface charge. The measured diffusivity agrees well with the Evans–Sackmann model for the diffusion of inclusions in supported, fluidic membranes. The agreement implies that the vesicle motion is coupled to that of a nanoscopic lipid cluster in the upper leaflet, which slides over the lower leaflet. The diffusivity of membrane‐adhering vesicles is therefore predominantly governed by the interleaflet friction coefficient, while the diffusivity of single lipids is mainly governed by the membrane viscosity. Combined with fluorescence recovery after photobleaching analysis, the interleaflet friction coefficient and the membrane viscosity are determined by applying the Evans–Sackmann model to the measured diffusivity of membrane adhering vesicles and that of supported membrane lipids. This approach provides an alternative to existing methods for measuring the interleaflet friction coefficient and the membrane viscosity.     

Simulation of time-discontinuous chemically-aided intergranular fracture

In this work, based on experimental observations, a model for the pressure dependent diffusion and accumulation of hydrogen ahead of a propagating intergranular crack front is developed. In the model, the pressure dependency of the diffusion is incorporated into the activation energy of an Arrhenius form of the material's diffusivity tensor along high-diffusivity grain boundaries. Inherent to the model is that large amounts of hydrogen can be absorbed into the crack's process zone, due to the high triaxial stress in that region, which produces a larger effective diffusivity locally. Further ahead, the effective diffusivity quickly decreases. The combined effect forms a barrier to further rapid penetration, which leads to the mentioned accumulation of the hydrogen in the process zone. Theoretical aspects of the model are discussed and numerical simulations of the transient distributions of hydrogen and subsequent crack front propagation are performed to illustrate the main characteristics of the model.     

Dispersion of a solute in pulsatile non-Newtonian fluid flow through a tube

The unsteady dispersion of a solute by an imposed pulsatile pressure gradient in a tube is studied by modeling the flowing fluid as a Casson fluid. The generalized dispersion model is applied to study the dispersion process, and according to this process, the entire dispersion process is expressed in terms of two coefficients, the convection and the dispersion coefficients. This model mainly brings out the effects of yield stress and flow pulsatility on the dispersion process. It is observed that the dispersion phenomenon in the pulsatile flow inherently differs from the steady flow, which is due to a change in the plug flow radius during a cycle of oscillation. Also, it was found that the dispersion coefficient fluctuates due to the oscillatory nature of the velocity. It is seen that the dispersion coefficient changes cyclically, and the amplitude and magnitude of the dispersion coefficient increases initially with time and reaches a non-transient state after a certain critical time. It is also seen that this critical time varies with Womersley frequency parameter and Schmidt number and is independent of yield stress and fluctuating pressure component. It is observed that the yield stress and Womersley frequency parameter inhibit the dispersion of a solute. It is also observed that the dispersion coefficient decreased approximately 4 times as the Womersley frequency parameter increases from 0.5 to 1. The study can be used in the understanding of the dispersion process in the cardiovascular system and blood oxygenators.     

Release of potassium sorbate from active films of sodium alginate crosslinked with calcium chloride

The release of potassium sorbate from alginate films was evaluated considering different active agent concentrations in the film and three levels of alginate crosslinking. The mechanism involved in the diffusional process was investigated using the Power Law Model. The results indicated that potassium sorbate diffusion in alginate films has characteristics of Fickian and non‐Fickian behaviour. Effective diffusion coefficients obtained using the solution in series derived from Fick's Second Law are close to values obtained by the short‐time solution, indicating that the influence of swelling on effective diffusivity, although perceptible, is small, allowing the use of an average effective diffusivity. The values of effective diffusivity found indicate that alginate films have a potential use as systems for release of active substances. Copyright © 2009 John Wiley & Sons, Ltd.     

Modeling of chloride diffusion in hetero-structured concretes by finite element method

With a two-dimensional structure model, the finite element method was used to simulate the chloride diffusion behavior in a heterogeneous concrete composed of two phases with distinct diffusivities, compared with its derived homogeneous medium that was given a single effective diffusion coefficient by the mean field theory. In contrast to the smooth chloride concentration profiles for the homogeneous medium, a three-dimensional concentration distribution network has been found in the hetero-structured concrete, characterized by a complicated band-like profile along the diffusion depth. Moreover, it features an undulating and narrowing width as the diffusion time and diffusion depth increase. The modeling results also manifest that the chloride diffusion in hetero-structured concretes appears to lag behind that for the homogeneous ones, showing an increasingly notable discrepancy between the two chloride concentration profiles as the diffusion proceeds. Nonetheless, such a concentration difference may be remarkably reduced when the chloride binding and time-reducing effect of diffusivity are supposed to happen within the cement paste phase of the hetero-structured concrete and its derived homogeneous medium.     

Unsteady free and forced convective diffusion

The paper is concerned with the dispersion of a solute in combined free and forced convective flow through a channel in the presence of uniform axial temperature variation along the walls. Using a generalised dispersion model, we have evaluated the longitudinal diffusion coefficients which are valid for all time after the injection of the slug in the flow. When both Grashof number and time are small, the dispersion coefficient increases very rapidly with Grashof number whereas it increases very slowly with time but after certain lapse of time it increases and levels off quickly to the asymptotic value for large time. Further for large Grashof number, the dispersion coefficient is practically independent of time and the above results are valid for both heating and cooling of the plate.     

考虑纤维/基体界面相的复合材料湿扩散模型

通过类比复合材料湿扩散与热传导的控制方程以及边界条件,以Halpin & Tsai模型为基础,发展了一个考虑了纤维/基体界面相的三相复合材料湿扩散模型,并研究了纤维界面随机损伤对湿扩散的影响。建立了纤维周期排布、随机排布、界面相损伤随机分布3种细观有限元模型。用上述模型分析了单向复合材料横向有效湿扩散系数（TEMDUC）随纤维和界面相体积分数、湿扩散性能以及界面相损伤率变化的规律,理论预测与有限元计算结果一致。研究发现：界面相或纤维相的扩散系数存在一个临界值,当扩散系数小于该临界值时,TEMDUC随纤维体积分数的增大而减小;反之,TEMDUC随纤维体积分数的增大而增大,此临界值的大小与纤维体积含量无关。研究还发现纤维界面损伤率相同的条件下,其分布的随机性对复合材料的有效湿扩散系数影响不大。     

Effect of trapping on the solubility and diffusivity of hydrogen in palladium (α-phase)

The solubility and diffusivity data of hydrogen in palladium with different concentrations of defects are discussed within the framework of the two-level model with local equilibrium (Oriani's model). Analytical expression of the solubility isotherm and of the variation of the apparent diffusion coefficient depending on the total bulk concentration are derived. A good agreement is found between the experimental results on the solubility and diffusivity of hydrogen and the calculated values using the theoretical expressions.