3D Modeling and Simulation of Mass Transfer in Vapor Transport through Porous Membranes

A 3D mathematical model is developed to predict the transport of water vapor through porous membranes. The model is based on solving the continuity, momentum as well as energy equations for water in the membrane contactor. The model's equations are numerically solved using the finite element method to obtain the concentration and temperature distributions of water in the membrane contactor. The model findings were in good agreement with experimental data. The proposed 3D model proved to be appropriate for predicting the performance of a membrane evaporator. Simulations were carried out in order to study the influence of different operating parameters and membrane structure on the membrane evaporation effectiveness. The results of simulation indicate that the gas velocity is a favorable parameter in the membrane evaporation process due to its tendency to keep the process far from the thermodynamic equilibrium.     

水平表面结霜过程的实验研究

对水平表面在低温高湿条件下结霜过程进行实验研究, 分析了冷面温度、湿空气温度和表面特性等因素对水珠冻结和霜层生长的影响。结果表明:冷面温度越低, 过冷水珠冻结时间越短、冻结直径越小;在湿空气温度与冷面温度温差相等的条件下, 湿空气温度越低, 过冷水珠冻结时间越短、冻结直径越小;疏水表面上过冷水珠的冻结时间比裸铝表面晚。冷面温度越低, 湿空气流速越快, 则霜层生长越快。     

Experimental identification of ice formation in small concrete pores

Freezing of water or salt solution in the concrete pores may be the cause for severe damage and significant reduction of service life. Although being one of the main research subjects during the past decades, a complete understanding of the deterioration mechanisms is still missing. This is probably because of a lack of understanding in the freezing of a liquid solution in the very small (some 10 nm) pores. In a series of experiments, the freezing process, its initiation, continuation and the resulting damage, was studied. Calorimetric, expansion and acoustic (ultrasonic pulse transit time, acoustic emission) methods were applied to monitor heat release, mechanical deformation and damage during a series of frost cycles. The results give an insight into supercooling, salt segregation, ice front penetration and thawing characteristics. Based on these data, a qualitative sequential damage model is proposed.     

表面特性对结霜和融霜排液的影响

对亲水表面、裸铝表面和疏水表面上结霜和融霜排液过程进行实验研究,分析了表面特性对冷凝水珠冻结、霜层生长和融霜排液的影响。结果表明：疏水表面上冷凝水珠呈规则球缺状、冻结较晚,而亲水表面和裸铝表面上冷凝水珠形状不规则;相比于亲水表面和裸铝表面上平整霜层,疏水表面上霜层不平整,有凹穴和凸起;疏水表面上霜层平均厚度增长较亲水表面和裸铝表面缓慢;在湿空气温度和冷面温度较低的情况下,表面特性对霜层生长的影响减弱;亲水表面具有较好的排液效果,其循环再结霜量最小。     

Modeling and Simulation of a Co-Current Rotary Dryer Under Steady Conditions

A model which joins the overall design algorithm of a rotary dryer with the drying kinetics equations derived from experimental data and with a finite segment algorithm is implemented in order to verify the dryer dimensions obtained from a basic sizing procedure. Total energy and mass balances and well-known correlations for the overall heat transfer coefficient are employed to develop it. Moreover, a one-dimensional finite segment model is solved to obtain the length profiles of temperature and water content for the air and solid phases. An experimental correlation for the mass transfer coefficient between solid and air phases is included in the finite segment model. The chosen heat transfer unit number for the basic sizing is verified with the outlet temperature and water content calculated by the finite segment scheme.     

Study on Shrinkage Deformation of Food in Microwave–Vacuum Drying

Drying shrinkage is an important problem in the food industry. Focusing on microwave–vacuum drying, we study the mechanism of deformation due to shrinkage of the food structure. A relationship between the strain and the water content is introduced for a finite element analysis. The temperature and water distributions are obtained by a finite difference method with the use of a variable permeability and diffusion coefficient depending on the water content. Comparisons with experimental data on radishes, carrots, and tofu indicate that the present model can express the deformation as well as the water content inside the materials.     

Mathematical Model for Freeze-Thaw Durability of Concrete

Although the equations governing the individual basic physical processes involved in freezing and thawing of concrete are known, a mathematical model for this complex phenomenon is unavailable. Its formulation is attempted in the present study. Desorption and absorption isotherms for concrete below 0°C are constructed on the basis of isotherms for concrete above 0°C, using pore size distribution functions. Water movement during freezing or thawing is described as a double diffusion process, involving both macroscopic diffusion through concrete and local diffusion of water into or out of air-entrained bubbles. Heat conduction is formulated taking into account the latent heat of freezing. Pore pressures are used in a two-phase material model, which makes it possible to predict the stress in the solid structure of concrete caused simultaneously by freezing and applied loads. This in principle reduces the freeze-thaw durability problem to the calculation of stresses and strains. However, development of the model to full application would require various new types of tests for calibration of the model, as well as development of a finite element code to solve the governing differential equations. Such a mathematical model could be used to assess the effect of cross-section size and shape, the effect of cooling rate, the delays due to diffusion of water and of heat, the effect of superimposed stresses due to applied loads, the role of pore size distribution, the role of permeability, and other factors which cannot be evaluated at present in a rational manner.     

Principles and Applications of Gas Separation in Nonfrosting Technology

Abstract

Fan coils in low temperature storage rooms or freezing chambers are usually covered with frost during long-term operation. The fan coils can be installed side-wall or overhead in the freezing chamber, and the coils have cast aluminum fins. The usual solutions for frosting on the surfaces of freezing coils is to heat or spray with water to get rid of the frost periodically. However, these solutions are not only energy-consuming, but they also tends to spoil the refrigerated objects in the chamber. The main idea of this research is to achieve nonfrosting by using the technology of absorption-dehumidification to separate the water vapor from the circulating air in the freezing chamber during operations. Therefore, an experimental determination of nonfrosting technology using absorption principles was employed in this study. In the absorption-dehumidification device, the water vapor was absorbed into the liquid desiccant solution, and the diluted solution was regenerated by using the heat sources combined with waste heat from the freezing machine and major heat from a modified water heater. The dried air from the absorption-dehumidification device was circulated in the freezing chamber. The chamber operated at a temperature of ?5 to ?10°C. Since most of the water vapor in the circulating air is removed by the absorption-dehumidification device, less water vapor can be condensed on the coils. Therefore, no frosty coils were found in this freezing chamber during long-term operation.     

波纹表面结霜的实验研究

结霜工况下,换热器翅片表面结霜会增大换热热阻,堵塞空气流道。在低温高湿条件下对波纹表面的结霜过程进行了实验研究,分析了冷面温度、湿空气流速及波纹结构等因素对波纹表面冷凝水珠冻结和霜层生长的影响。结果表明：冷面温度越低,冻结的冷凝水珠越小,冻结时间也越短;冷面温度越低或湿空气流速越快,霜层生长越快;在相同的实验条件下,波纹表面上霜层的高度比平直表面的低。     

Impact of sodium sulfate on soil frost heaving in an open system

The existence of solutes and their redistribution during freezing have a deep influence on the process of soil freezing. We performed unidirectional freezing experiments in an open system with red clay collected at the Beiluhe test site along the Qinghai–Tibet Railway. The groundwater supply of the soil was simulated in the laboratory experiment by attaching a liquid replenishment system to the bottom of the sample container. In order to see the influence of the salt on the evolution of the sample during unidirectional freezing, two types of experiments were performed. (i) The soil samples were supplied with a sodium sulfate solute of 5% concentration, and (ii) only distilled water without any salty component was added. Based on the freezing temperature measurements of salty soil, migration of sodium sulfate solution towards a lower temperature zone during freezing 0 °C isotherm in the soil moved gently towards deeper layers, but frost depth of the soil ascended slightly with time when the sample was constantly cooled. Compared to the distilled water replenishment, the amount of frost-heaving was smaller in the soil column with sodium sulfate solution replenishment. Based on the frost depth curve, the solubility curve of the Na₂SO₄–H₂O system and the amount of frost heaving and salt expansion in the soil column we have calculated the amount of frost heaving and salt expansion. In an early stage of the experiment deformation of the soil column was mainly caused by frost heaving, while in a later phase crystallization of the sodium sulfate played a lager role. This conclusion is confirmed by the results of the dry density measurement after the experiments.     

Simulation of freezing step in vial lyophilization using finite element method

Determination of heat transfer and temperature profile during freezing step is fundamental to predict the final structure of a lyophilized product. The aim of this study was to develop and optimize a dynamic model based on finite element method for simulation of freezing step in order to study final product morphology in both vertical and radial directions. Different factors have been taken into account: chamber pressure, shelf temperature, vial shape, initial solution temperature, nucleation temperature and phase changes. The dynamic axisymmetric model proposed could simulate temperature of each point in the vial and position of liquid-solid interface, without necessity of fitting parameters or questionable assumptions. In addition, the model was extended to predict the average crystal size in each element and the influence of different factors was examined.     

MATHEMATICAL SIMULATION OF TEMPERATURE AND MOISTURE FIELDS WITHIN A GRAIN KERNEL DURING DRYING

Non-linear partial differential equations are presented for two dimensional heat and mass transfer within a single grain kernel during drying. In this model, the moisture evaporation inside the kernel is considered. The moisture is assumed to diffuse to the outer boundary of the kernel in liquid form and evaporate on the surface of the kernel. The influence of temperature and moisture content on grain properties is also considered in the simulation. The Non-linear partial differential equations are solved using the finite element method and simulation data is verified on a thin layer dryer for wheat kernels. The comparison shows that the simulated results have a high accuracy with average relative error of about 5%. The results of the finite element analysis can be used for grain quality evaluation, drying simulation studies and stress analysis of grain kernel.     

水在冷却和冻结过程温度场的实验与理论研究

在自行开发的“多通道温度数据智能采集系统”的基础上，进行了圆柱形容器内水的冷却和冻结实验，并基于ANSYS的有限元分析法对其冷却和冻结过程进行了计算机模拟。结果表明：自行研发的“多通道温度数据智能采集系统”人机界面友好、使用方便、运行可靠、成本低廉、用途广泛，极具推广性；计算机模拟与实验结果相吻合。本文提出的基于ANSYS热分析的有限元分析方法在冷却和冻结过程的温度场研究中具有诱人的应用前景；冻结过程越靠近筒壁相变越早发生，相变持续时间就越短；相变前越靠近筒壁，降温速率越大，降温曲线的线性分布越明显，相变后则与此相反。     

Failure of Capillary Theory of Frost Damage as Applied to Ceramics

The behavior of solid bars of porous ceramics saturated with water was investigated during the freezing and melting of the water. The length change, differential temperature, and temperature were monitored continuously during the experiments. The behavior of the saturated bars during the freezing of water was analyzed in detail. It was shown that quasi-equilibrium conditions, as assumed by the capillary theory of frost damage, did not satisfactorily describe the results of these experiments because the plasticity of the ice phase is too low to permit pressure equilibration.     

MATHEMATICAL SIMULATION OF TEMPERATURE AND MOISTURE FIELDS WITHIN A GRAIN KERNEL DURING DRYING

ABSTRACT

Non-linear partial differential equations are presented for two dimensional heat and mass transfer within a single grain kernel during drying. In this model, the moisture evaporation inside the kernel is considered. The moisture is assumed to diffuse to the outer boundary of the kernel in liquid form and evaporate on the surface of the kernel. The influence of temperature and moisture content on grain properties is also considered in the simulation. The Non-linear partial differential equations are solved using the finite element method and simulation data is verified on a thin layer dryer for wheat kernels. The comparison shows that the simulated results have a high accuracy with average relative error of about 5%. The results of the finite element analysis can be used for grain quality evaluation, drying simulation studies and stress analysis of grain kernel.     

Numerical simulation of low-concentration CO2 adsorption on fixed bed using finite element analysis

Accurately predicting distributions of concentration and temperature field in fixed-bed column is essential for designing adsorption processes.In this study,a two-dimensional (2D),axisymmetric,nonisothermal,dynamic adsorption model was established by coupling equations of mass,momentum and energy balance,and solved by finite element analysis.The simulation breakthrough curves fit well with the low-concentration CO2 adsorp-tion experimental data,indicating the reliability of the established model.The distributions of concentration and temperature field in the column for CO2 adsorption and separation from CO2/N2 were obtained.The sensitivity analysis of the adsorption conditions shows that the operation parameters such as feed flow rate,feed concentra-tion,pellet size,and column height-to-diameter ratio produce a significant effect on the dynamic adsorption performance.The multi-physics coupled 2D axisymmetric model could provide a theoretical foundation and guidance for designing CO2 fixed-bed adsorption and separation processes,which could be extended to other mixed gases as well.     

Numerical and physical modeling of the permeability of paper to CMC and coating liquids

The penetration of CMC solutions and coating liquids into paper sheets was investigated experimentally and modeled using a Lucas-Washburn equation (Lepoutre, 1978) adequately modified to account for the dynamic contact angle and the coating fluid rheology. A numerical simulation of the fluid penetration based on the finite element resolution of the Navier-Stokes equations was carried out as well. The numerical predictions and the predictions of the Lucas-Washburn equation were compared with the experimental data showing the limitations of the models.     

Ice formation in hardened cement paste,Part I — room temperature cured pastes with variable moisture contents

Ice formation in room temperature cured mature hardened Portland cement paste with different moisture contents have been measured. The measurements were carried out continuoslly in the temperature range from +20° C to −60° C.For moisture contents higher than that corresponding to approximately three equivalent BET-monolayers, the initial freezing temperature increases with increasing moisture contents. Moisture contents below approximately three monolayers is not freezable. For specimens containing freezable water, there is an increase in the amount of non-frozen water content with increasing moisture contents.Further, there is a slight increase in the amount of non-frozen water with increasing water/cement ratios for specimens in equilibrium with the same relative water vapor pressure.It is postulated that frost problems only can be expected in specimens containing more water than corresponding to a relative water vapor pressure of about 0.9.     

水泥基材料的结晶压机理研究

本文通过仪器测定了水泥基材料冻结时产生的冻胀力,试验结果发现水泥基材料冻结时也会产生冻胀力,且净浆试件冻胀力稍微大于砂浆试件,同时发现毛细孔充满液苯的砂浆试件冻结时也会产生冻胀力.结合冻胀力试验结果以及水和苯结晶过程的分析,推断冻结过程中水泥基材料内部产生冻胀力,而冻胀力产生的机理主要是结晶压.     

Moving carbonation fronts in concrete: A moving-sharp-interface approach

We present a new modeling strategy for predicting the penetration of carbonation reaction fronts in concrete. The approach relies on the assumption that carbonation reaction concentrates macroscopically on an a priori unknown narrow strip (called reaction front) moving into concrete gradually changing its mechanical and chemical properties. We propose a moving-interface model to forecast the maximum penetration depth of gaseous CO₂ in the porous concrete matrix. The main questions driving this research are: How fast does the carbonation front move? and How long does it take until the front reaches the reinforcement?. As model output, we determine simultaneously the position of the carbonation front and the profiles of the active concentrations. The model equations are solved using a specially tailored finite element scheme and are validated relying on experimental data from the Ph.D. thesis by D. Bunte Zum Karbonatisierungsbedingten Verlust der Dauerhaftigkeit von Außenbauteilen aus Stahlbeton, Ph.D. thesis, TU Braunschweig (1994). Our approach should be viewed as an alternative to the standard carbonation models.