热泵干燥条件下刺参有效扩散系数的研究

有效扩散系数D_(eff)是干燥传质研究的重要基础数据之一,现有D_(eff)研究大多侧重于考虑干燥温度t而忽视干基含水率X、收缩变形的影响。本文建立了刺参干燥有限元模型和D_(eff)计算模型,以X数据的模拟值与实验值之差最小为优化目标,计算得到D_(eff),对比分析了收缩变形、t和X对刺参D_(eff)的影响。研究表明:收缩变形、t及X均对D_(eff)影响较大,同时考虑上述3个影响因素得到的D_(eff)比仅考虑t及X影响时小3.9%～14.6%,而考虑收缩变形和t影响得到的D_(eff)比仅考虑t影响时小8.7%～14.5%,且刺参D_(eff)均随t和X的升高而增加。考虑收缩变形、t和X等对D_(eff)的影响后,刺参干燥模型模拟得到的X与实验值吻合最好,相关系数高于0.994。模拟得到的刺参内部D_(eff)按由内向外逐渐降低的规律分布,干燥从1 h进行至15 h,内外D_(eff)差值由6.4×10~(-10) m~2/s降至0.9×10~(-10) m~2/s,中心处D_(eff)则由14.6×10~(-10) m~2/s降至8.9×10~(-10) m~2/s。     

Experimental Analysis and Simultaneous Heat and Moisture Transfer with Coupled CFD Model for Convective Drying of Moist Object

Convective drying of rectangular-shaped moist object has been analyzed both experimentally and numerically. Transient mass of the potato sample is measured experimentally. Moisture content, diffusivity, and density of the object are calculated at different drying air temperatures from 40°C to 70°C with an air velocity of 2 m/s. A three-dimensional (3D) finite volume method (FVM) based numerical model is developed to predict the temperature and moisture distribution. A computational fluid dynamics (CFD) code is used for predicting heat and mass transfer coefficients required in the boundary conditions of the heat and mass transfer model. The experimental and numerical data are compared and good agreement is observed.     

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.     

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.     

An improved procedure for obtaining and maintaining well characterized partial water saturation states on concrete samples to be used for mass transport tests

A conditioning procedure is proposed allowing to install into the concrete specimens any selected value of water saturation degree with homogeneous moisture distribution. This is achieved within the least time and the minimum alteration of the concrete specimens. The protocol has the following steps: obtaining basic drying data at 50 °C (water absorption capacity and drying curves); unidirectional drying of the specimens at 50 °C until reaching the target saturation degree values; redistribution phase in closed containers at 50 °C (with measurement of the quasi-equilibrium relative humidities); storage into controlled environment chambers until and during mass transport tests, if necessary. A water transport model is used to derive transport parameters of the tested materials from the drying data, i.e., relative permeabilities and apparent water diffusion coefficients. The model also allows calculating moisture profiles during isothermal drying and redistribution phases, thus allowing optimization of the redistribution times for obtaining homogeneous moisture distributions.     

低温真空干燥对胡萝卜品质的影响

以胡萝卜为对象,研究了低温真空干燥对胡萝卜的营养成分(如还原糖、维生素C等)及复水特性影响,并将低温真空干燥与真空冷冻干燥及热风干燥进行了对比。实验表明:经过热风干燥和真空冷冻干燥后胡萝卜的维生素C损失率分别是低温真空干燥的2.47倍和2.14倍。低温真空干燥可以较好的保持胡萝卜的组织结构完整性,并且减少营养物质的流失。     

Moisture distribution in drying ordinary and high performance concrete cured in a simulated hot dry climate

Adequate moisture is very important during early age of portland cement concrete. The Single Point Magnetic Resonance Imaging technique was used to study the effects of various lengths of moist curing, and the use of curing compound, on the amount and distribution of evaporable water during drying of ordinary and high performance concrete. The specimens subjected to six different curing regimes, were cast in triplicate for a total of 72. After moist curing at 38°C, the specimens were subjected to uniaxial drying in an environmental chamber at 38°C and 40% relative humidity that simulated hot dry climate conditions. As the specimens were drying, Magnetic Resonance Imaging was used to study the evaporable water distribution, non-destructively and with millimetric resolution. The Magnetic Resonance Imaging profiles indicated a reduced moisture loss with increasing length of moist curing. Extended moist curing was especially beneficial for the two self-compacting concrete mixtures, particularly for the cover concrete. In all mixtures the use of curing compound was only marginally better than one day moist curing, but was significantly better than air curing, particularly for the cover concrete. The moisture diffusivity was evaluated from the transient moisture distribution profiles using the Boltzmann transformation method. The results indicated a strong dependence of the moisture diffusivity on the moisture content when above 80% saturation, whereas below this value it remains almost constant. The moisture diffusivity is significantly reduced with increased moist curing period.     

RDX干燥过程影响因素的综合分析

对黑索今（RDX）干燥过程的影响因素进行了综合分析。通过单因素实验和正交实验,对影响RDX干燥过程的主要因素及其影响显著性进行了研究,得到干燥RDX的最佳干燥条件。干基含水量、真空度、温度对RDX干燥过程有重要影响,影响的显著性顺序为：温度的影响显著性强于真空度,更强于干基含水量。建议RDX的干燥条件是：温度80℃,真空度0．05MPa,干基含水量10％。     

The effect of drying preconditioning on the South African durability index tests

This study investigated the effect of different drying regimes in the preconditioning stage on Durability Index (DI) test results. The moisture condition of specimens needs to be stable and uniform for the tests to be accurate and reliable. Three drying regimes were used: (a) standard oven drying method of 50 °C for 7 days, (b) oven drying at 50 °C to constant mass, and (c) drying using a solvent replacement method with isopropanol. Concrete mixes were designed using three w/c ratios (0.40, 0.50 and 0.65) and four binders. The isopropanol and oven drying to constant mass methods were found to remove different amounts of moisture compared with the standard drying method. Most (about 80%) of the moisture was removed within 7 days when oven dried. Statistical analysis suggested that, for certain mixes, the drying method had an effect on the DI results, with the chloride conductivity test being the most sensitive. For quality control purposes, it is impractical to wait until specimens are completely dry, which in some cases took up to 17 days, before performing the DI tests. The practical solution is for specimens to be tested at not less than 7 days and not more than 8 days of drying.     

Moisture absorption behavior of sugar palm fiber reinforced epoxy composites

In engineering practice, moisture absorption test is generally used for quality control purposes and to measure the degradation of the quality for the composite materials. The objectives of this study are to investigate the value of Fickian diffusivity constant, moisture equilibrium content and correction factor for the natural fiber composites. Tests were carried out on composite plates, which was a combination of sugar palm fiber and epoxy resins and two different fiber compositions have been chosen which were 10% and 20% by weight. Pure epoxy plates have been used for the control measures. The specimens were oven dried for 60 h in an air-circulating oven operated with 108 °C before being immersed in the constant temperature water bath, which the distilled water was set at 40 °C for the moisture absorption behavior test for 33 days. From this study, plates with 20% fiber loading possessed the highest amount of moisture prior to the moisture absorption behavior test, which is 0.93%. In moisture absorption behavior test, the corrected value of Fickian diffusivity constant for the 20% fiber loading is 3.76 × 10⁻⁷ mm²/s, which is the highest among other composites. It is shown that, for composite plates that contain higher fiber composition, the moisture absorption rate is even higher.     

Salt diffusion in brick structures Part II The effect of temperature, concentration and salt

This is our second article concerning salt diffusion measurements in porous brick materials. In the first article a method for measuring the rate of diffusion of salt in ceramic material was presented and applied to the measurement of diffusivity of NaCl in different brick materials. In this work the same method was applied to other water-soluble salts commonly found in building materials. The binary diffusion coefficients were measured for KCl, NaNO₃, CaCl₂, Na₂SO₄, MgCl₂ and Na₂CO₃. The result for the diffusivity of 0.05 molar salts in water in new Finnish red brick varied between (0.271–0.544)*10^–5 cm²/s at a temperature of 25 ± 0.05°C. The effect of temperature and concentration on the diffusivity of NaCl in new red brick was also measured. The rate of change of diffusivity as a function of temperature, concentration and the type of salt was very similar to that in free solutions. However, the values of the diffusion coefficients were lower in the porous medium than in solution. A mutual relation between the binary diffusion coefficient and the viscosity of the salt-water solution was observed in the porous brick.     

A comparison of the finite element and control volume numerical solution techniques applied to timber drying problems below the boiling point

Drying is a process which involves heat and mass transfer both inside the porous material, where a phase change in moisture occurs from the liquid to the gaseous state, and in the external boundary layer of the convected hot dry air, which heats the porous medium. The equations which govern this process consist of three tightly coupled, highly non-linear partial differential equations for the unknown system variables of moisture content, temperature and pressure. Due to the inherently complex boundary conditions and intricate physical geometries in any practical drying problem, an analytical solution is not possible. In order to obtain a transient drying solution it is necessary to resort to a numerical technique. The numerical solution techniques which were employed in this research were the finite element method and the control volume method. The transient numerical results were compared and contrasted for two timber drying problems, first, at a dry bulb temperature of 50°C, and secondly, at 80°C, both cases being below the boiling point of water.     

The influence of osmotic pressure on poulticing treatments for cultural heritage objects

The crystallization of salts is widely recognized as one of the most significant causes of irreversible damage to many cultural objects such as wall paintings, stone sculptures, historic buildings. The removal of salts from these objects is however difficult and often poultices are used. In these methods a wet poultice is applied to the surface of the substrate to be treated and is kept in place for some period of time before being removed. Many studies up to now on poulticing have focused on the salt and moisture transport solely in terms of advection and diffusion. The objective of this study is to demonstrate the potential contribution of osmotic pressure to salt extraction during poulticing treatments. To this end we have conducted a series of experiments where we have measured the moisture and salt transport during poulticing for some well defined materials. Here we have used nuclear magnetic resonance to measure non-destructively the moisture and ion transport during these experiments. This study shows that osmotic pressure can exert a significant influence on salt extraction by poulticing methods during drying. Importantly, as salt is transported from the substrate and into the poultice, this results in a build-up of osmotic pressure within the poultice decreasing the effective pore-size of the poultice. Therefore the build-up of osmotic pressure enhances the salt extraction and thus increases the efficiency of the poulticing treatment.     

响应面法分析干燥条件对TNT静态干燥的影响

通过单因素实验和响应面分析法（RSM）,对影响TNT（梯恩梯）干燥过程的主要因素及其影响显著性进行了研究,得到干燥TNT的最佳干燥条件。结果表明：含水量（干基,rm）、真空度（p）、温度（T）对TNT干燥过程有重要影响,它们都是影响RDX干燥过程的显著性因素,影响显著性顺序为：真空度影响最大,其次是温度,含水量影响最小,建议,TNT的适宜干燥条件是：真空度0．03MPa,温度70℃,含水量8％。     

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.     

Modeling of rotary vane compressor applying artificial neural network

The thermal modeling of rotary vane compressor (RVC) was performed in this paper by applying Artificial Neural Network (ANN) method. In the first step, appropriate tests were designed and experimental data were collected during steady state operating condition of RVC in the experimental setup. Then parameters including refrigerant suction temperature and pressure, compressor rotating speed as well as refrigerant discharge pressure were adjusted.With these input values, the operating output parameters such as refrigerant mass flow rate and refrigerant discharge temperature were measured. In the second step, the experimental results were used to train ANN model for predicting RVC operating parameters such as refrigerant mass flow rate and compressor power consumption. These predicted operating parameters by ANN model agreed well with the experimental values with correlation coefficient in the range of 0.962-0.998, mean relative errors in the range of 2.79-7.36% as well as root mean square error (RMSE) 10.59 kg h⁻¹ and 12 K for refrigerant mass flow rate and refrigerant discharge temperature, respectively. Results showed closer predictions with experimental results for ANN model in comparison with nolinear regression model.     

椰肉的保色抑菌研究

以椰肉为研究对象,通过对新鲜椰肉经盐、糖和醋不同浓度溶液浸泡处理,然后用不同包装材料对处理过的椰肉进行密闭包装,在一定时间内定期检测其色差、失重、硬度及感官评定,研究时间对产品品质的影响,探究不同浓度的盐、糖和醋溶液及包装材料对椰肉的保色抑菌效果。结果表明:在质量分数为0.3%的盐溶液、1.1%的糖溶液、0.1%醋溶液,通过PVDC/PE复合薄膜的包装下得到的果肉产品品质是较好的。     

Determination of Moisture Diffusivity as a Function of Both Moisture and Temperature

The effect of moisture and temperature on liquid water transport in porous media was studied. Specimens of autoclaved aerated concrete were subjected to one-sided water penetration in isothermal conditions at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C. After specified time intervals, moisture profiles were determined gravimetrically. The moisture diffusivity was calculated for a particular temperature as a function of moisture content, using an inverse analysis. The results demonstrate the dependence of the moisture diffusivity on the moisture content and the temperature of the samples. The moisture diffusivity for high moisture content can be as much as one order of magnitude greater than for the lowest moisture content studied. The moisture diffusivity was found to increase by as much as a factor of two when the temperature is increased from 20 °C to 80 °C.