果蔬减压冷却过程中的传热与传质分析

果蔬减压冷却过程中的传热传质是优化运行管理、设计高效节能型真空冷却设备的基础,对减压冷却过程进行理论模拟研究能减少实验投入,缩短实验周期,具有重要的理论意义和现实意义.本文详细讨论并总结了真空冷却过程中球形果蔬的传热传质数学模型,这个模型对于减压冷藏实验的分析能够提供必要的数学模拟手段.     

真空冷却过程中水分蒸发沸腾的数值模拟（Ⅰ）：模型的建立

真空冷却过程是复杂的相变传热传质过程。在真空冷却期间,当真空室内的真空压力低于或者等于熟肉温度对应的饱和压力时,熟肉内部的水分将会沸腾蒸发以产生冷却效应。在能量和质量守恒的基础上,经过适当的简化,建立了真空冷却过程中水分迁移的数学模型。此模型能够预测真空冷却过程中熟肉内部的温度、压力和水分含量分布。     

ANSYS有限元软件在减压冷却保鲜研究中的应用

利用真空冷却过程中球形果蔬的传热传质数学模型,用ANSYS软件对温度场模型进行了模拟求解.理论模拟结果的温度场变化值与樱桃减压冷却实测的数值基本吻合,证明了对于水分含量较大的果蔬在实际减压冷却过程用ANSYS软件对温度场模型进行了模拟求解的可行性.     

低温捕集器内部流动及传热传质过程分析

分析了ＰＨ３与Ｎ２的二元混合气体在ＰＨ３低温捕集器中的二次相变流动，从理论上研究了其传热传质过程的机理。分析结果认为，此过程可分为：凝华、液膜生长及流动和残气凝固三个阶段。针对各阶段建立了相应的数学模型。     

真空冷却的理论模拟与实验研究

对球体、无限大平板、无限长圆柱3种最基本形状的食品进行理论模拟,利用有限差分的方法模拟真空冷却的非稳态过程,并分析冷却过程中的传热、传质。通过实验验证,其理论模拟同实验结果较吻合。     

果蔬真空预冷过程中热质传递的理论分析

从基本理论和基本定义出发建立了描述球形果蔬在真空冷却中传热、传质的数学模型。通过数值解算得到系统总压、产品温度(表面温度、中心温度、质量平均温度)、产品质量随时间的变化曲线。并用文献中可得实验资料对模拟的趋势加以验证,得到了较好的模拟效果,表明数学模型用来预测真空冷却中球形产品的特征参数如温度、压力和质量的变化过程是正确的,具有一定的理论意义。     

固体绝缘介质真空干燥的传热传质研究

在分析固体绝缘介质真空干燥传热传质机理的基础上 ,建立了数学模型 ,给出了解析求解及数值求解方法 ,为改进电工产品的真空处理工艺提供了理论依据。     

梨的真空贮藏过程传热传质分析及数值模拟

以梨为研究对象,理论分析在真空贮藏的条件下果蔬传热传质过程特性.建立相应的物理模型和数学模型,基于Gauss-Seidel迭代方法进行数值求解,得出真空贮藏过程中果蔬表面及内部的水分和温度的变化规律.模型的计算结果为果蔬的冷冻冷藏工程实践提供理论依据.     

固体绝缘介质真空干燥的传热传质研究

在分析固体绝缘介质真空干燥传热传质机理的基础上，建立了数学模型，给出了解析求解及数值救解方法，为改进电子产品的真空处理工艺提供了理论依据。     

纯金属真空蒸馏研究(Ⅰ):数学模型

建立了纯金属真空蒸馏过程数学模型。模型包括挥发过程、气相传质过程、冷凝过程和传热过程。应用迭代法对数学模型求解，可得出纯金属在不同实验温度、真空度下的蒸发速率。     

CFD simulation of coupled heat and mass transfer through porous foods during vacuum cooling process

A numerical simulation by using a computational fluid dynamics (CFD) code is carried out to predict heat and mass transfer during vacuum cooling of porous foods on the basis of mathematical models of unsteady heat and mass transfer. The simulations allow the simultaneous prediction of temperature distribution, weight loss and moisture content of the meats at low saturation pressure throughout the chilling process. The simulations are also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, thermal shrinkage, and anisotropy of the food. The model is verified by vacuum cooling of cooked meats with cylindrical shape within an experimental vacuum cooler. A data file for pressure history was created from the experimental pressure values, which were applied in the simulations as the boundary condition of the surface temperature.     

卷心菜真空冷却过程的CFD传热传质模拟

有效的真空预冷模拟能减少实验投入,缩短试验周期,具有重要的理论意义和现实意义。综合考虑了非稳态传热和质量守恒,建立了球形果蔬真空冷却的温度和质量数学模型,并以卷心菜为例进行了模拟和实验。实验结果表明,模拟数据与实验数据较吻合,温度误差在4%以内,质量损失误差在6.5%以内,说明该数学模型可用来模拟真空冷却过程,对实际应用具有一定的指导意义。     

Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers

A coupled heat and mass transfer model is developed to extract the transfer coefficients for falling-films from the measurements on a tubular absorber. The mass transfer coefficients obtained from the coupled model and the log-mean-difference approach agree within about 10%. For the heat transfer coefficient, the values given by the two models can differ quite significantly. The cooling water temperature distribution predicted by the coupled model agrees well with measurements. The transfer coefficients obtained from experimental measurements using the various methods reported in the literature show wide variations.     

液体真空制冷的理论分析及实验验证

本文建立了真空制冷的数学模型，对绝热和有热交换情况下的真空制冷过程进行了理论分析，推导了两种情况下液体温度随时间的变化函数；讨论了影响液体温度变化的各种参数，其中系统压强ps、液体初始温度Tf0、汽液交界面面积A和外界热源Q是影响液体温度变化的主要因素。在理论计算基础上，进行了实验验证，并对二者进行了分析比较。实验数据和理论计算结果非常吻合，理论计算能够很好地预测水温的变化趋势。     

Steady state CFD modeling of airflow, heat transfer and moisture loss in a commercial potato cold store

Storage loss beyond permissible limit is one of the most important problems in Indian potato cold stores, which has been hindering further growth of this industry. The losses in the stored potatoes have a direct relation to the intricate coupled transport phenomena of heat, mass and momentum transfer therein. Therefore, airflow, heat transfer and moisture loss was investigated in a potato cold store of commercial scale under steady state condition using the computational fluid dynamics technique. The developed CFD model was a two-dimensional simplification of the cold store. Heat and mass transfer at the cooling coils were not modeled, instead temperature and relative humidity in the air space were specified based on measured values. The model was validated in a commercial scale cold store and was found to be capable of predicting the air velocity as well as product temperature with an average accuracy of 19.5% and 0.5 °C, respectively and also the simulated average total moisture loss was found to be only 0.61% water (w.b.) higher than the experimental one for a storage period of 6 months. The main deficiencies of the airflow pattern which resulted in wide variations in temperature and moisture loss within the stored commodity can be investigated. The model located the probable zones of hot and cold spots, excessive product dehydration and moisture condensation within the storage facility, which might lead to qualitative and quantitative deterioration in stored product. This modeling tool could very well be applied to incorporate necessary design improvements with a view to improve the airflow distribution and heat transfer in order to limit the storage losses within the permissible limit.     

水滴在真空室内结晶过程的模拟

模拟真空室内水滴结晶过程是模拟真空雾化法制备冰浆系统的第一步。模拟了液滴在真空室内从较高的温度降温到零度,之后到一定的过冷度,然后发生相变为液-固两相,到最后为全部固相的整个过程。在该过程中热质传递迅速发生,模拟结果表明蒸发或者升华控制了整个过程。以扩散模型和以对流模型所得到的结果都与实验结果在一定程度上吻合,这是因为通过对流或者导热所传递的热量比相变潜热要小几个数量级。在整个结晶过程中液滴温度的变化表明:液固相变要占据较长的时间,是一个更为重要的过程。考虑重力、浮力和粘滞力的影响给出了液滴在结晶过程中的运动方程。从该模型计算出的液滴结晶过程中下降距离比仅考虑重力作用下给出的要准确。研究结果为更好的设计真空闪蒸室提供了理论依据和参考。     

A computational fluid dynamic model of the heat and moisture transfer during beef chilling

The unsteady heat and mass transfer process during beef carcass chilling was modelled for a three-dimensional beef carcass geometry. A three-step method was used to simulate the simultaneous heat and mass transfer process in order to reduce the computational time. In the first step, a steady state simulation of the flow field was conducted. In the second step, the local heat and mass transfer coefficients were calculated. Finally, the third step consists of the simultaneous heat and mass transfer process simulation on the meat carcass only. A separate 1-D grid was used to calculate the moisture diffusion in the meat. The simulation of a 20-h chilling run takes 5 days on a 2.5 GHz Pentium 4 computer. The model allows calculating and predicting the heat load, temperatures, weight loss and water activity. Local variations in the heat and mass transfer coefficients, temperature and water activity were found around the beef carcass. The CFD model gives temperature predictions that agree with experimental data better than any previous model. The weight loss tends to be over-predicted probably due to neglecting the resistance caused by the fat cover.     

Selection of convective heat transfer coefficient from comparison of experimental data with results of mathematical modeling

We propose a simple method for determining the convective heat transfer coefficient from a vacuum arc remolten ingot into a helium atmosphere upon cooling after removing power from the furnace. To this end, the consumable electrode end temperature was measured experimentally upon furnace depressurization. The heat transfer coefficient was determined with the aid of a mathematical model of the process under consideration by the trial-and-error method.