Heat transfer characteristics of cooked meats using different cooling methodsCaractéristiques du transfert de chaleur de viandes cuites réfrigérées par des méthodes différentes

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.     

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.     

Air cycle combined heating and cooling for the food industry

In the food industry there is often a need to both cook and cool food. Heating and cooling processes are rarely directly linked due to heat from refrigeration processes being insufficient to cook food. Therefore cooling and cooking is usually provided by separate pieces of equipment. This paper presents an air cycle system where the hot air was used for heating and the cold air used for fast freezing.The apparatus used a bootstrap unit developed for aircraft air conditioning which was unable to run at the low temperature required for food freezing, so a parallel compressor was added to enable the system to operate at the low temperature required for food freezing. This approach allowed temperatures as low as −140 °C and as high as 234 °C. The system cooked and froze beef burgers, and at the same time heated water up to 98 °C.     

The potential of solar energy use in desiccant cooling cycles

The use of heat produced by solar thermal collectors is an interesting option for thermal driven air conditioning processes. A thermal driven cooling technique which fits well to non-tracking solar collectors is the desiccant cooling technique. Recently several projects have been carried out which focus on the connection of desiccant cooling systems with solar thermal energy for regeneration of the sorbents. This communication deals with three main topics: (1) experiences achieved in a realized system which is coupled to a solar collector are discussed, (2) a new concept is presented, in which a solar air collector is integrated into the desiccant cooling cycle as the only heat source and (3) a comparative study is presented which compares system performance for different system configurations and different climatic situations.     

Prediction of air coil performance under partially wet and totally wet cooling conditions using equivalent dry-bulb temperature method

This paper mainly deals with the cooling and dehumidifying performance of air coils used in refrigeration and air-conditioning systems. A new method, equivalent dry-bulb temperature (EDT) method, is proposed for calculating the heat and mass transfer and for predicting the cooling modes (totally wet, partially wet, and totally dry) of moist air over the coil surface. A numeric model is further developed and validated with the experimental data of plain fin air coils. The deviation in both the cooling capacity and the vapor condensate estimated by the model is within the range of ±10%, and the prediction for the cooling modes of moist air over the coil surfaces is fairly exact.     

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.     

Experimental investigation of a free-piston Vuilleumier refrigerator

This paper presents experimental data for a free-piston Vuilleumier machine, which was designed as a refrigerator for CFC-free cooling and air conditioning. The basic features of the Vuilleumier cycle and the free-piston operation, which is known from free-piston Stirling engines, are discussed briefly. Additionally, the design of the prototype is described. From the variety of the experimental data, the influence of the mean pressure and the three cycle temperatures on the refrigerating capacity and the COP are selected for presentation and detailed discussion in this paper. The results are used to deduce some general operating characteristics of free-piston Vuilleumier refrigerator, which reveal its potential for CFC-free cooling and air conditioning.     

Description and experimental results of a semi-indirect ceramic evaporative cooler

Evaporative cooling is used in industrial and air conditioning processes to reduce temperature in different fluids. Direct evaporation systems can lead to environmental problems such as Legionnaire's disease, and indirect systems reduce system efficiency.This work presents the manufacture, test bed set up and trials carried out on a ceramic evaporative cooling system which acts as a semi-indirect cooler. Depending on air characteristics, it may act as a sensible or enthalpic exchanger. The water cooled in a cooling tower, using the return air coming from the conditioned room (22 °C and 50% comfort conditions) goes through the ceramic pipes, exchanging sensible and latent heat with a current of outdoor air.The use of this recovery system is mainly in climates with a high temperature and humidity such as tropical environments where the system yields a decrease in supply air humidity, using the cooling power of return air.The tests presented show the system behaviour for various supply air conditions.     

Dimensionless correlations of frost properties on a cold plate

This study proposes dimensionless correlations for predicting the properties of frost formed on a cold plate. Frosting experiments are carried out to obtain the correlations with various environmental parameters including the air temperature, air velocity, absolute humidity, and cooling plate temperature. The thickness, density, surface temperature, effective thermal conductivity, average heat and mass transfer coefficients of the frost layer are correlated as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature by using a dimensional analysis. The correlations proposed in this study agree well with the experimental data within a maximum error of 10%, and can be used to predict the average frost properties in the following ranges: the air temperature of 5–15 °C, air velocity of 1.0–2.5 m s⁻¹, absolute humidity of 0.00322–0.00847 kg kg_a⁻¹, and cooling plate temperature of −35–−15 °C.     

Portable forced-air tunnel evaluation for cooling products inside cold storage rooms

Freezing process efficiency is affected by the required conditions to keep the air flow and temperature at the product surface. The objective of this work was to obtain results on comparative studies with air exhaustion and blowing using an experimental portable forced-air freezing tunnel. The device was designed to improve cooling rates inside storage room without the need for a cooling/freezing tunnel. A heterogeneity factor was proposed for air circulation evaluation and compared with convective heat transfer coefficient (h_ef) values. Lower modules of heterogeneity factor values represent smaller temperature differences among samples. Comparing two different air flow processes, heterogeneity factor values were similar for regions where the cooling air could flow without obstructions. However, larger differences were observed for regions with hampered air circulation. Results indicated that the air distribution, as well as the heat transfer, occurs more uniformly around the products in the exhausting process than in the blowing system.     

冷却方式对MMT/LDPE介电性能的影响

采用熔融插层法制备了蒙脱土/低密度聚乙烯(MMT/LDPE)纳米复合材料, 探讨空气自然冷却、空气快速冷却、水冷却和油冷却四种制备工艺对复合材料介电性能的影响。利用XRD、FTIR、AFM、PLM、DSC和TSC等分别对复合材料和纳米MMT粒子的微观形态、复合材料的电导、击穿、介电频谱和空间电荷特性进行表征。结果表明, 经表面修饰的纳米MMT粒子在基体聚合物中已经剥离并均匀分散; 不同冷却方式对复合材料的结晶度有一定的影响, 其中油冷却试样结晶速率最高, 结晶尺寸最小; 纳米MMT的加入使复合材料内部陷阱密度和深度均有所增加, 且试样的介电性能有不同程度的改善。油冷却试样抑制空间电荷的作用比较明显, 在20和40 kV/mm的场强下, 试样中正电荷的峰值与空气自然冷却试样相比分别下降了63.57%和51.39%; 且油冷却试样的电导率最小, 击穿场强值最大; 在1~10⁵ Hz的测试频率范围内, 与空气自然冷却试样相比, 其他三种试样的介电常数和介质损耗角正切值都有不同程度的降低。     

A method for evaluating the heat and mass transfer characteristics in a reversibly used water cooling tower (RUWCT) for heat recovery

In sub-tropical regions, a standard water cooling tower may be reversibly used, as part of a desuperheater heat recovery system for service hot water heating, to extract free heat from ambient air in colder seasons when building cooling load is reduced. Chilled water is pumped into a reversibly used water cooling tower (RUWCT) where it is heated by warmer ambient moist air. This paper presents a method by which the heat and mass transfer characteristics in a counter-flow RUWCT can be evaluated. The method is developed by introducing to the Merkel's equation for standard water cooling towers the revisions that account for the differences in heat and mass transfer characteristics between a water cooling tower and a RUWCT. Field experimental results from a RUWCT installed in a sub-tropical region in China indicated that the method developed could be used to evaluate the thermal performance of a RUWCT with an acceptable accuracy.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

Seasonal performance of a central air conditioner in Dhahran, Saudi Arabia. A computer simulation study

Seasonal performance of four standard, residential-sized, central air conditioners in a typical hot and humid climate of Gulf environment was measured. A computer model was developed to stimulate the residential building cooling loads and the resulting seasonal energy consumption. For an average 100 m² residence located in Dhahran, equipped with a 17.6 kW air conditioner, simulations show a consumption of ≈22200 kWh of electricity for space cooling over 2600 operation hours, this being within 13% of the measured value. The seasonal performance of three lower capacity (10.5, 12.3 and 14.0 kW) air conditioners for the same duty was also predicted. With the 14.0 kW capacity unit the electricity consumption is reduced by ≈ kWh and can satisfy the load for 96.6% of the time. The smallest (10.5kW) of the four systems can supply the house cooling load for only ≈78% of the time with a seasonal energy consumption of 17 350 kWh.     

真空冷却过程中水分蒸发沸腾的数值模拟（Ⅱ）：模型的验证

利用圆柱型的熟肉为实验材料,通过实验验证了真空冷却过程中水分迁移的数学模型。结果发现：产品在70min内从70℃被冷却到5℃。当真空冷却结束时,产品中心和表面的实验温度分别是7．01。c和3．98℃,对应的产品的中心和表面的模拟温度分别是6．97℃和3．78℃。真空冷却实验过程中,产品的平均水分含量从72．60％降到61．03％。模拟过程中,产品的平均水分含量从71．00％降低到62．37％。模拟结果显示真空冷却的质量损失为8．63％,然而实验过程中真空冷却的质量损失为11．57％,其偏差在3％左右。模拟结果与实验结果基本一致,建立的数学模型能够正确地预测真空冷却过程中温度、压力和水分含量的变化过程。     

A simple model for falling film absorption on vertical tubes in the presence of non-absorbables

Most water–lithium bromide (LiBr) absorption chillers have a purge system to remove non-absorbable gases that cause a reduction in cooling capacity. Generally, the non-absorbables are originated in corrosion/passivation processes inside the machine, but leaks can also be a source of concern. However, since leaks must be corrected immediately to avoid machine deterioration, this study is mostly aimed at the non-absorbables evolved during operation. This paper analyses the effect of inlet non-absorbable air concentration, outlet purge velocity, absorber pressure and cooling water temperature on the falling film absorption process inside a vertical tube absorber, based on a simple transport coefficient model. This model consists of three ordinary differential equations solved with as method for initial-value problem, and a set of auxiliary equations. The study shows that the effect of non-absorbables can be significant, and furthermore provides a quantitative framework to aid in purge design. The nominal working conditions in this study were a solution Reynolds number of 100, an absorber pressure of 1.3 kPa, a cooling water temperature of 35 °C and an inlet solution concentration of 62% LiBr by weight. The results indicate that a minimum vapour velocity is required to sweep the non-absorbables along the absorber towards the purge, thus preventing reduced absorption fluxes. At a cooling water temperature of 35 °C, an inlet air concentration of 20% (by mole) resulted in a 61% reduction in mass absorption flux.     

Force estimation of mechanical removing processes of mushy structure in an aqueous solution

In a system of producing liquid ice with mechanical removing processes of mushy structure in an aqueous solution, a solid layer as thermal resistance does not stack up on a cooling surface. It prevents refrigerator's COP dropping and also prevents ice production rate dropping. However, some extra energy is needed for mechanical removing processes of the mushy structure from cooling surfaces. In this paper, the required force for removing the mushy structure is expressed as a function of experimental conditions based on bending-beam theory. The mushy structure is regarded as an elastic body. The estimated values of modulus of elasticity and values of adhesion force are reasonable compared with the observed results. Parameters that affect on mode difference, maximum force and steady force are shown.     

Potential energy benefits of integrated refrigeration system with microturbine and absorption chiller

This paper presents and analyzes the performance potential of a refrigeration system that is integrated with a microturbine and an absorption chiller (RMA). The waste heat from the microturbine operates the absorption chiller, which provides additional cooling. This additional cooling capacity can be utilized either to subcool the liquid exiting the condenser of the refrigeration system or to precool the air entering the condenser in the refrigeration system. Moreover, any surplus cooling capacity not utilized in the subcooler can be utilized to precool the microturbine intake air. The additional assistance to the refrigeration system enhances the efficiency of the refrigeration cycle, which in turn reduces the required microturbine size. The smaller size of the microturbine enhances the part load efficiency, especially in lower ambient temperatures. With increased microturbine efficiency, RMA with subcooler, RMA with subcooler and microturbine intake air precooler, and RMA with condenser air precooler can reduce the annual energy consumption by 12, 19, and 3%, respectively, as compared to a refrigeration system operating without any waste heat utilization from the microturbine. Therefore, RMA with subcooler and microturbine intake air precooler has the best potential of energy savings. The payback period of RMA with subcooler and microturbine intake air precooler is estimated in 3 years, which facilitates it as an economically feasible solution among the options investigated.     

Evaluation of an underground railway carriage operating with a sustainable groundwater cooling system

In London and Merseyside in the UK, which are served by deep underground railway networks, rising water tables are proving an increasing problem, leading to a serious deterioration of track, traction supply and signalling systems. Also in these transport systems there is an increasing demand for an energy efficient and environmentally sustainable comfort cooling system. This paper outlines a novel cooling system that aims to reduce the water table by using the groundwater for ‘free’ cooling the railway network. This paper investigates the potential for cooling the underground network and trains in this way. It uses a purposely-developed mathematical model to show that additional cooling to the existing rolling stock may be provided, by cooling the tunnels within which they operate. It has been shown theoretically, that by cooling the air within the tunnels by 9K, the temperature in the typical carriage operating under peak load conditions will reduce by approximately 6K to a much more acceptable level. Finally the paper identifies areas for further work, that are required in order to realise the preferred way of achieving a sustainable cooling scheme.