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1.
蔬菜真空冷却工艺的实验研究 总被引:1,自引:0,他引:1
康平 《制冷与空调(北京)》2012,(6):65-69
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。 相似文献
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CFD simulation of coupled heat and mass transfer through porous foods during vacuum cooling process 总被引:8,自引:0,他引:8
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. 相似文献
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The finite element method is used to analyse the two-dimensional transient heat transfer problem for cooling cooked meats with an ellipsoid shape under different environments offered by slow air, air blast and water immersion cooling units. The variations in physical properties of the cooked meat and in operating conditions during the cooling processes are included in the model. The models are solved by a user-friendly computer program developed by the authors. The numerical solutions are compared with the experimental results. The comparison shows that the maximum deviation between the predicted and experimental core temperatures for all the cooling processes is within 2.9°C. 相似文献
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在卷心菜真空冷却实验中,真空度是影响冷却时间和效果的主要因素。本文通过设定不同的自动补气阀工作压力范围来控制真空室的压力,进行卷心菜真空冷却实验,并分析了此过程中水分在低压下的气化过程。实验发现,自动补气阀工作压力范围分别为700 Pa-900 Pa、1000 Pa-1200 Pa和1300 Pa-1500 Pa时,当温度最低处(表叶)达到预定的0.7℃时,耗时分别为11 min、22 min和43 min,中心温度分别为3.86℃、6.68℃和9.11℃。结果表明:700 Pa~900 Pa的自动补气阀工作压力范围,对卷心菜进行真空冷却实验的效果较好;对于特定食品的真空冷却,应该可以找到各自恰当的自动补气阀工作压力范围,以提高冷却的效率。 相似文献
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Vacuum cooling is known as a rapid evaporative cooling technique for any porous product which has free water. The aim of this paper is to apply vacuum cooling technique to the cooling of the iceberg lettuce and show the pressure effect on the cooling time and temperature decrease. The results of vacuum cooling are also compared with conventional cooling (cooling in refrigerator) for different temperatures. Vacuum cooling of iceberg lettuce at 0.7 kPa is about 13 times faster than conventional cooling of iceberg lettuce at 6 °C. It has been also found that it is not possible to decrease the iceberg lettuce temperature below 10 °C if vacuum cooling method is used and vacuum pressure is set to 1.5 kPa. 相似文献
6.
Study of a novel silica gel–water adsorption chiller. Part I. Design and performance prediction 总被引:6,自引:4,他引:2
D.C. Wang Z.Z. Xia J.Y. Wu R.Z. Wang H. Zhai W.D. Dou 《International Journal of Refrigeration》2005,28(7):1073-1083
A novel silica gel–water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evaporation/condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance especially when the chiller is driven by a low temperature heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is more than 10 kW under a typical working condition with hot water temperature of 85 °C, the cooling water temperature of 31 °C and the chilled water inlet temperature of 15 °C. The COP exceeds 0.5 even under a heat source temperature of 65 °C. 相似文献
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为了找出大型低温环境室室内正压设计方法存在的问题,对低温环境室在不同室温下的竖向气压变化特点进行分析,提出室内竖向气压分布的计算方法,该算法的准确性得到了实测数据的验证,其平均相对误差为5.9%。分析结果表明:大型深冷低温环境室的竖向气压变化很大,且在降温过程中室内正压不断增大。对于高度为20 m、室温为-165℃的大型深冷低温环境室,其下部正压值可超过421 Pa,远高于低温环境室室内正压以往的设计值(5—10 Pa)和安全防护值(200 Pa),因此应大幅度提高其室内正压的设计值和安全防护值。为确保室内全部空间维持正压,低温环境室的室内正压控制点应设置在顶部。 相似文献
10.
M. P. SanSoucie J. R. Rogers V. Kumar J. Rodriguez X. Xiao D. M. Matson 《International Journal of Thermophysics》2016,37(7):76
The NASA Marshall Space Flight Center’s electrostatic levitation (ESL) laboratory has recently added an oxygen partial pressure controller. This system allows the oxygen partial pressure within the vacuum chamber to be measured and controlled in the range from approximately \(10^{-28}\,{\mathrm {to}}\,10^{-9}\) bar, while in a vacuum atmosphere. The oxygen control system installed in the ESL laboratory’s main chamber consists of an oxygen sensor, oxygen pump, and a control unit. The sensor is a potentiometric device that determines the difference in oxygen activity in two gas compartments (inside the chamber and the air outside of the chamber) separated by an electrolyte. The pump utilizes coulometric titration to either add or remove oxygen. The system is controlled by a desktop control unit, which can also be accessed via a computer. The controller performs temperature control for the sensor and pump, has a PID-based current loop and a control algorithm. Oxygen partial pressure has been shown to play a significant role in the surface tension of liquid metals. Oxide films or dissolved oxygen may lead to significant changes in surface tension. The effects on surface tension and viscosity by oxygen partial pressure in the surrounding environment and the melt dissolved oxygen content will be evaluated, and the results will be presented. The surface tension and viscosity will be measured at several different oxygen partial pressures while the sample is undercooled. Surface tension and viscosity will be measured using the oscillating droplet method. 相似文献
11.
Jean-Marie Chapp Nicolas Martin Guy Terwagne Jan Lintymer Joseph Gavoille Jamal Takadoum 《Thin solid films》2003,440(1-2):66-73
Thin films of titanium oxynitride were successfully prepared by dc reactive magnetron sputtering using a titanium metallic target, argon, nitrogen and water vapour as reactive gases. The nitrogen partial pressure was kept constant during every deposition whereas that of the water vapour was systematically changed from 0 to 0.1 Pa. The evolution of the deposition rate with an increasing amount of water vapour injected into the process was correlated with the target poisoning phenomenon estimated from the target potential. Structure and morphology of the films were analysed by X-ray diffraction and scanning electron microscopy. Films were poorly crystallised or amorphous with a typical columnar microstructure. Nitrogen, oxygen and titanium concentrations were determined by Rutherford backscattering spectroscopy and nuclear reaction analysis, and the amount of hydrogen in the films was also quantified. Optical transmittance in the visible region and electrical conductivity measured against temperature were gradually modified from metallic to semiconducting behaviour with an increasing supply of the water vapour partial pressure. Moreover, an interesting maximum of the electrical conductivity was observed in this transition, for a small amount of water vapour. 相似文献
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Cooling rate and heat transfer characteristics of cooked meats using four different cooling systems of vacuum cooling, air blast cooling, water immersion cooling and slow air cooling were investigated. The experimental results show that only the vacuum cooling can achieve the requirement of cooling the cooked meats from about 74 to 10°C within 2.5 h. The vacuum cooling shows different heat transfer characteristics during the cooling process, as compared with other cooling methods. Vacuum cooling rate is controlled by the evaporation rate of water from the cooked meats, while the cooling rates of the other three cooling methods are governed by the thermal conductivity of the cooked meats. Therefore, it is impossible for air blast, water immersion and slow air cooling to obtain high cooling rates since these three methods are different only in the convective heat transfer from the surface of the cooked meat to the cooling medium. 相似文献
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真空冷却过程中气体温度变化特性的研究 总被引:1,自引:2,他引:1
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。 相似文献
15.
Kazuhiro Fukuyo Taichi Tanaami Haruko Ashida 《International Journal of Refrigeration》2003,26(2):249-255
The authors developed a new air-supply system for improving the thermal uniformity and the cooling rate inside a fresh food cabinet of a household refrigerator. For these purposes, we added a blower and jet slots to a conventional cooled air supply system. The jet slots circulate the air inside the cabinet at a higher velocity to optimize airflow velocity and its distribution. The jet stirs the air inside the cabinet and improves thermal uniformity, which resulted in half the temperature deviation in the cabinet as that of the conventional systems. The jet also improves the heat transfer on the surface of foods. We achieved a four times higher cooling rate with the new system than that with the conventional ones. In order to cut down the development period, we applied computational fluid dynamics to study air distribution inside the cabinet with the new system. We also derived the model of the cooling process by the jet using theoretical and empirical equations and applied it to decide the jet velocity for the rapid cooling. 相似文献
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The vapour–liquid phase equilibrium was measured using an apparatus based on the recirculation method for the 2,3,3,3-tetrafluoroprop-1-ene + propane binary system. A total of 55 data points were obtained over the temperature range from 253.150 to 293.150 K. The experimental standard uncertainties were less than 5 mK for the temperature, 800 Pa for the pressure, and 0.005 for the mole fraction. The experimental data were correlated by the PR-VDW model and PR-HV-NRTL model. Both models can present the experimental data well and the PR-HV-NRTL model achieves a higher degree of precision. The relative deviations of pressure are within ±0.50% and the maximum absolute deviation of vapour mole fraction is 0.006 for the PR-HV-NRTL model. A positive azeotropic behaviour can be observed for the binary system. 相似文献
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A. Abrutis M. Lukosius Z. Saltyte R. Galvelis P.K. Baumann M. Schumacher J. Lindner 《Thin solid films》2008,516(15):4758-4764
Metal-organic chemical vapour deposition (MOCVD) of various phases in PrOx system has been studied in relation with deposition temperature (450–750 °C) and oxygen partial pressure (0.027–100 Pa or 0.2–750 mTorr). Depositions were carried out by pulsed liquid injection MOCVD using Pr(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) precursor dissolved in toluene or monoglyme. By varying deposition temperature and oxygen partial pressure amorphous films or various crystalline PrOx phases (Pr2O3, Pr7O12, Pr6O11) and their mixtures can be grown. The pure crystalline Pr2O3 phase grows only in a narrow range of partial oxygen pressure and temperature, while high oxygen pressure (40–100 Pa) always leads to the most stable Pr6O11 phase. The influence of annealing under vacuum at 750 °C on film phase composition was also studied. Near 90% step coverage conformity was achieved for PrOx films on structured silicon substrates with aspect ratio 1:10. In air degradation of Pr2O3 films with transformation to Pr(OH)3 was observed in contrast to Pr6O11 films. 相似文献
19.
The main factor to affect lower limit of static expansion vacuum standard is outgassing of the inner surface. A new method
has been demonstrated to extend the lower limit by using a non-evaporable getter pump (NEGP) to eliminate the outgassing influence.
Because NEGP has no pumping speed for inert gases, it can maintain the ultra-high vacuum background without changing gas quantity
in the calibration chamber when an inert gas is used as the test gas, which makes the standard pressure accurately calculable
by Boyle’s law. By this method, the lower limit of static expansion vacuum standard has been extended to 10−7 Pa. 相似文献
20.
The aim of this paper is to present the development of a very specific ultra-high vacuum system for the space application PHARAO. In order to reach the specified pressure (2.6×10−8 Pa) during 3 years in a self-contained system, specific solutions have been developed. A calculation of partial pressures of different chemical species (typically hydrogen, rare gases and cesium) in the different areas of the vacuum tube has been computed. Experiments have been also performed in order to verify the possibility of using different kind of materials (outgassing rates of bulk and porous titanium, silicon carbide reinforced aluminum, graphite, etc.) and pumping systems such as getters and a specially developed ion pump. 相似文献