关于真空冷却实验中的一些问题

近年来,不少有关真空冷却的文章讨论了用小型真空冷却机组进行的实验研究。其中有些论文在实验设计上可能存在问题,包括实验的对象、样品的数量、真空度、样品的失水量等。因此实验所得到的数据和结论,能否用于指导实际的真空冷却,是需要斟酌的。这里从真空冷却的机理出发,介绍了真空冷却的主要应用场合,讨论了真空腔的压力,食品的降温效率和失水量等问题,并给出了水、菠菜、牛肉、面包的最大理论温降和最小理论失水率。     

真空冷却过程的机理分析

真空冷却过程是复杂的相变传热传质过程,该过程涉及到扩散、传热、传质和相变等机理.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的数学模型.通过对结果分析发现,数值模拟的结果与实验数据吻合很好,误差在5%以内.数学模型可以准确预测真空冷却过程中的温度变化和质量损失,将有助于进一步理解和研究真空冷却机理.     

球性食品真空冷却非稳态过程的数值分析

通过引入食品材料透气性参数和食品内部到表面的最短距离,建立了食品真空冷却过程的传热模型。在此基础上,采用用户自定义函数编程,首次基于FLUENT模拟了球形食品真空冷却过程。以卷心菜真空冷却为例进行了实验验证,结果显示模拟值和实验值吻合性较好,并对模拟结果进行了分析。研究结果表明:食品材料的透气性和形状对真空冷却过程中食品内部温度场的分布有显著的影响,透气性越好、食品内部到表面的最短距离越小,温度分布越均匀。FLUENT模拟为研究食品真空冷却提供了新途径。     

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。     

蔬菜真空冷却工艺的实验研究

真空冷却是一种能提高果蔬贮藏品质并可延长货期的快速预冷方法。本文以卷心菜、大葱、豆角和韭菜为研究对象,利用BSVC5真空预冷机,针对加水、包装、降压和复压等工序进行实验研究。结果表明,真空冷却前对蔬菜加水和包装能够明显加快冷却速度和降低失重率,且加水量应低于6%;采用分阶段降压能使蔬菜降温更均匀,并根据蔬菜自身性质终压应控制在200～750 Pa范围内,采取引入冷空气复压的方法可以避免蔬菜温度回升。     

果蔬真空气化冷却技术的研究

本文着重介绍了真空气化冷却原理，真空气化冷却装置及其操作技术，影响真空冷却装置及其操作技术，影响真空冷却效果的主要因素等问题。     

快捷食品真空冷却的对比实验研究

真空冷却是一种快速有效的冷却方法。通过真空冷却实验研究,讨论了蔬菜与其半成品真空冷却的异同,并对其影响因素进行了分析。结果发现:西葫芦表皮质硬,水分不易蒸发,不适合真空冷却;韭菜叶部较根部比表面积大、冷却速率大;快捷食品汤料含量比较多,水分易于蒸发,冷却速率比较快,为了防止产品温度低于0℃,应控制最佳冷却时间;韭菜538 g、炒西葫芦628 g、韭菜鸡蛋547 g的最佳冷却时间分别为19 min、8 min、7 min。     

真空冷却过程中水分迁移的数学模型

从真空冷却的基本理论和基本定义出发,分析了真空冷却过程中产品内部的水分迁移过程,建立了真空条件下水分迁移的数学模型.通过数值模拟得到了产品内部、产品表面水分含量随时间的变化曲线及产品的质量损失随时间的变化曲线.利用文献中的试验数据对模拟结果进行了验证,模拟结果与文献中的试验结果基本一致.这表明数学模型可以用来预测真空冷却过程中产品的水分含量和质量损失,具有一定的理论意义.     

真空冷却过程中气体温度变化特性的研究

为了研究真空冷却过程中真空室内气体温度的变化特性，以水为实验对象，在真空室内布置了压力及多个温度测点，进行多组真空冷却实验，从而测得在抽真空和复压过程中真空室内的压力和各点的气体温度值。研究表明，当真空室内压力发生变化时，真空室内气体温度的变化具有某些特征，并且气体温度还受容器壁温、水温等的影响，本文针对这些特征进行了相关实验及理论分析，并得到了相似的变化规律。     

Modelling vacuum cooling process of cooked meat—part 1: analysis of vacuum cooling system

A mathematical model is developed to analyse the performance of a vacuum cooler. The model is based on the mass conservation of air and vapour in the vacuum chamber. In the chamber, the vapour evaporated from foods under the vacuum and the vapour removed by the vapour-condenser and vacuum pump contribute to the variation in the vapour partial pressure, and the ingress air and the air released by the pump cause the change of air partial pressure. Experiments were carried out on vacuum cooling of water to validate the model. The predicted vacuum pressure and temperature histories are compared with the measured values. The maximum deviation between the predicted and measured vacuum pressure is within 110 Pa (for the chamber pressure between 12,000 and 2200 Pa), while the maximum deviation between the predicted and measured temperature of water is less than 2 °C. The model can, therefore, be used to predict the transient vacuum pressure profiles for analysing the vacuum cooling process of foods such as cooked meat.     

降低真空冷却失水的方法研究

提出相对失水率的概念,作为比较真空冷却过程中实际失水高于理论失水程度的判断依据.以水作为研究对象,采用不同的方法进行降低无效失水的实验,结果是水面上不加遮挡物时,其相对失水率高达2.0以上;加了遮挡物,失水明显降低;采用双层遮挡物,使相对失水率降至1.2左右,得到理想的结果;同时容器内水位的高低对失水也有影响,较低的水位其失水也较少.     

减压降温过程对样品温度均匀性影响分析

针对减压过程产生的热分层现象对样品温度均匀性的影响进行了数值模拟,得到了样品在不同时刻的温度分布,而且通过分析获得样品温度均匀的结论,并通过实验验证了计算结论的准确性.     

Factor affecting the cooling rate of lettuce in vacuum cooling installations

Commercial-scale experiments carried out during the last five years have shown that the cooling time of a head of lettuce, wrapped in PVC film and packed in a carton, can vary from 25 to 65 min, depending on the design of the vacuum installation. Experiments were, therefore, carried out with a commercial four pallet vacuum cooler, provided in accordance with the requirements of the Agricultural Engineering Institute, with three vacuum pumps and two compressors. The influence of pumping speed, cooling unit capacity and initial temperature on the cooling time and on the final temperature of lettuce leaves and butts was investigated. The results were compared with the cooling rate obtained with other vacuum cooling installations. Design data are proposed.     

Analysis of vacuum bag resin transfer molding process

An analytical model is developed to analyze the resin flow through a deformable fiber preform during vacuum bag resin transfer molding (VBRTM) process. The force balance between the resin and the fiber preform is used to account for the swelling of fiber preform inside a flexible vacuum bag. Mold filling through multiple resin inlets is analyzed under different vacuum conditions. The formation of dry spots is demonstrated in the presence of residual air. Molding of a three-dimensional ship hull with lateral and longitudinal stiffeners is simulated to demonstrate the applicability of the model.     

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.     

真空泵系统的级间冷却方案

探讨了真空泵系统中冷却装置的方案,定性讨论了真空系统级间冷却方式的选择,给出了理论选型原则,通过实例分析,指出在真空系统工程设计中,需要给出系统工程化解决方案,更能保证真空系统高效安全运行.     

低温绝热容器中皮拉尼真空计取代热偶真空计的可行性分析

在低温绝热容器夹层真空度的测量中,皮拉尼真空计和热偶真空计较适用,目前热偶真空计应用较多。从测量范围、测量精度、测量温度、互换性、远程监控、检漏能力和性价比等方面,对皮拉尼真空计和热偶真空计的性能进行了比较,得出皮拉尼真空计更适用于低温绝热容器夹层真空度测量的结论。最后列举了目前国外几种典型的商用皮拉尼真空计和热偶真空计的技术参数。     

Analysis of helium II thermal links for instrument cooling in low gravity

The Low Temperature Microgravity Physics Facility (LTMPF) is a reusable, cryogenic facility that will accommodate a series of low temperature experiments to be conducted at the International Space Station. The facility will use a He II cryostat to cool the instruments. Some configurations of the science instruments in the cryostat will require an enhanced thermal link between the He II bath and parts of the instruments. Such an enhanced link can be made with plumbing filled with He II. This paper reports the results of analysis that was performed using the BATC proprietary helium flow software called SUPERFLO, on four different concepts for this link. The four concepts analyzed were: a simple tube with the heated end closed, a closed end tube with a porous plug at its entrance, a closed end tube filled with capillary tubes, and a porous plug driven flow loop. It was found that the concepts that used a porous plug were more robust since they were much less prone to boiling. This is due to the low gravity which causes all of the liquid in helium tank and plumbing to be very close to saturated conditions unless a porous plug is used to create a thermomechanical pressure. The effects of varying system parameters such as a acceleration, heat flux, pore size and tube size were also investigated and the results are reported.