International Journal of Refrigeration-revue Internationale Du Froid最新文献

A R744 (CO₂) refrigeration system has been designed to cool down the Large Hadron Collider (LHC) silicon detectors ATLAS and CMS, located at CERN, Switzerland. The silicon detectors are subjected to high radiation levels. The system is composed of a pri- mary CO₂ trans-critical booster vapor compression loop operated with piston compressors, and an oil-free liquid pumped loop on the evaporation side, to preserve the detectors. To ensure the system's reliability, the cooling facility is designed to operate under a parallel operation mode of several modular 70 kW plant units providing evaporation temperature as low as -53 °C. This layout, is also useful in case of components failure and maintenance. A numerical model is developed using a dynamic simulation software Dymola that is based on the open source Modelica modelling language. The simulation results are proven on a first demonstration plant (System A) experimentally to explore the systems control logic and to validate the reliability of the system before it is built on the detectors side. In this paper the models development is explained and the results of the experimental validation of the numerical model are shown.

Domestic refrigerator/freezers account for approximately 6 % of all energy consumption around the globe and mainly rely on vapor compression cycles to operate. Researchers have investigated alternative cycle architectures such as dual-loop cycles and parallel circuit cycles to improve their efficiencies. Despite the demonstrated energy saving potential of these advanced cycles, additional implementation costs are often not justifiable. However, to meet forthcoming stricter energy standards while ensuring flexible multi-temperature operation of domestic refrigerator/freezers, advanced cycle architectures are needed. In this paper, a state-of-the-art bypass circuit cycle triple-evaporator domestic refrigerator freezer with R-600a and a reciprocating compressor has been used as the baseline cycle investigate an alternative cycle configuration. Specifically, this work presents a two-stage vapor-injected cycle with a multi-evaporator system to enable energy savings and cost-effectiveness. The cycle establishes two separate evaporation temperatures to better match the cabinet temperature of fresh food and freezer compartment. The reduced difference between cabinet temperature and its evaporation temperature decreases the irreversibilities in the heat exchanger and improves overall system efficiency. Moreover, the addition of the economization line from the medium temperature evaporator reduces the compressor work. To capture the complex transient behavior of both the baseline system and proposed cycle architecture with their control strategies, a dynamic model has been developed and validated with experimental data. The validated dynamic model with the baseline cycle was modified to consider the two-stage vapor-injected cycle and its control logic, and simulation results yielded up to 13 % energy consumption reduction with respect to the baseline system.

Considerable efforts have been made to minimize the adverse effects of freezing on the quality of frozen products. In this study, ultrasound-assisted osmodehydrofreezing (UOD) was used to accelerate the freezing process. The effects of osmotic solution concentration, temperature, and application of ultrasound on freezing characteristics and quality attributes of frozen carrot were investigated. The changes in water loss (WL) and solid gain (SG) were predicted using kinetic models, with effective diffusivities determined according to Fick's second law of diffusion for a finite cylinder. The results from the kinetic modeling demonstrated the good fitness of the new proposed exponential model. Effective diffusivities for WL and SG ranged from 4.49 × 10⁻⁸ m² s⁻¹ to 8.42 × 10⁻⁸ m² s⁻¹ and 4.30 × 10⁻⁸ m² s⁻¹ to 5.12 × 10⁻⁸ m² s⁻¹, respectively, with the highest diffusivities observed in ultrasound-assisted dehydrated samples at 45 °C in a 60 % sucrose solution. Dehydration pretreatment could shorten the freezing time significantly. Pretreated samples (after thawing) showed higher firmness and lower phenolic contents, as compared to the control samples (freezing without osmodehydration). In addition, the UOD process minimized drip loss and electrolyte leakage. The high duration of ultrasound exposure resulted in structural damage, which was confirmed by scanning electron microscopy. In general, it was revealed that the UOD process prior to freezing was more useful in preventing the quality loss of the samples, as compared to osmodeydration without sonication. These findings, thus indicate that applying UOD is a promising approach for freezing food materials.

Ammonia leakage in food cold storage have adverse impact on the quality of food, which may be related to ammonia concentration and exposure time, but there is a lack of data support. In this paper, the quality of boxed non-dairy cream exposed to ammonia environment (0 (CK), 100, 500, and 1000 mL m⁻³) at −18 °C for 6 day were evaluated. Results showed that no ammonia residue was detected in the boxed non-dairy cream exposed for 6 days. There were no significant differences (P > 0.05) in the color value, odor, pH value, and sensory score of the non-dairy cream under the four ammonia groups during the same exposure time. And peroxide values was far below the standards limit, and benzo (α) pyrene content of all the cream groups were not detected The results indicated that all the boxed non-dairy cream in this experiment had no quality deterioration and food safety risk.

During the period of COVID-19, the health problems caused by the contamination of imported cold chain food by pathogenic microorganisms attracted widespread attention, which brought new risks and challenges to cold chain products. The traditional liquid disinfection method has obviously been unable to meet the existing disinfection and sterilization needs, and establishing a complete set of cold chain product disinfection system has become the focus of research. This paper takes the process of importing cold chain food from Tianjin's primary cold storage as an example, and proposes the closed-loop cold storage disinfection system of "Human-Cargo-Apparatus-Waste". The establishment of the disinfection system determined the working characteristics of different areas of the primary cold storage and the risk level of virus transmission through on-site analysis and simulated aerosol diffusion experiments. According to the characteristics of different working areas in the cold storage, targeted sealing measures and disinfection techniques are adopted. It can effectively ensure the complete disinfection of cold chain products and avoid the negative impact of disinfection and sterilization methods. The expected research results provide certain technical support for the development and application of safe virus elimination throughout the entire process of imported cold chain.

Magnetic fields are a new food-processing technology, with application in freezing, refrigeration, thawing, and preservation. In this work, the effects of static magnetic field (SMF) on freeze-thaw behavior, water distribution, retrogradation, and quality of fried rice are investigated. Results from the freezing curve show freezing time of fried rice during 2 mT treatment is the shortest, followed by 5 mT. Over a number of freeze-thaw cycles (FTC), the retrogradation enthalpy, relative crystallinity, and short-range ordered degree increased, while the application of the SMF can alleviate these processes. Changes in texture properties of samples treated by SMF are the significantly delayed in comparison with 0 mT, especially the adhesiveness index. The color and flavor of fried rice also show a delay downward trend. Therefore SMF can improve the quality of fried rice. The static magnetic field freezing method is appropriate for the actual production of frozen fried rice.

Improving the performance of a refrigerated container is crucial in effectively preserving thermal-sensitive cargo. The combined effect of vertical airflow resistance and airflow short-circuiting in air gaps play a vital role in cargo cooling. This study numerically investigates the effects of cargo vertical airflow resistance and air gaps between cargo pallets using two commonly used apple packaging boxes, each stacked in six different cargo pallet arrangements. A heat transfer enhanced refrigerated container design and a numerical model developed from the present authors’ previous studies are used. The numerical model uses the porous medium model to incorporate the refrigeration unit and cargo within the refrigerated container. Additionally, the numerical domain includes the T-bar floor in the refrigerated container. Initially, the airflow characteristics based on vertical airflow resistance and air gaps are discussed. Then, the corresponding effects on cargo cooling are addressed. Results show that the air gaps between the cargo pallets have positive and negative effects on cargo cooling. In cargos with lower vertical airflow resistance, an increase in air gaps leads to reduced cargo cooling and elevated temperature non-uniformity. Conversely, cargos with higher vertical airflow resistance experience increased cooling rates and reduced temperature non-uniformity with the increase in air gaps. However, avoiding airflow short-circuiting through the air gaps between the cargo pallets and improving the vertical airflow inside the cargo pallets can provide better cargo cooling, which outweighs the positive cooling effect of the air gap.

The freezing of aquatic products without causing significant changes in quality is recently attracting much attention. In this study, the effects of air freezing (AF, -20 °C), blast freezing (BF, -35 °C), liquid nitrogen freezing (LNF), and immersion freezing (IF, -35 °C) by 20 % ethanol, 20 % propylene glycol, 10 % glycerol, and 10 % sodium chloride aqueous solutions, respectively, on muscle qualities and myofibrillar protein properties of the red drum were investigated. The results indicated that the freezing rate of IF was 2.66 cm/h, which was 6.33 times higher than that in AF groups. Compared with AF and BF, the ice crystals were smaller and more uniformly shaped, with properties of lower water migration and less texture softening after cryogenic liquid freezing of IF and LNF group aquatic products. Immersion freezing showed the same superiority in maintaining the freshness of fish fillets and retaining the protein structure as liquid nitrogen storage. Overall, IF has similar effects to LNF in maintaining muscle qualities and myofibrillar protein properties. This study describes a new freezing method that uses 20 % ethanol, 20 % propylene glycol, 10 % glycerol, and 10 % sodium chloride aqueous solutions as immersion freezing medium. The multi-compound freezing medium has the potential to be used for the freezing of aquatic products.

The effect of immersion freezing on water distribution and calcium ion migration in pears was studied. The samples were frozen at -20 °C and -40 °C. Results showed that the -40 °C combined immersion freezing (-40IF-T) was better in inhibiting sample browning and sensory quality, compared with other groups. Water activity and freezable water content were closer to that of the unfrozen sample. Compared with -20IF, the freezing time of -40IF-T was reduced by 74.90 %. Both the drip loss and calcium ion content showed that -40IF-T was better than other groups. Low-field nuclear magnetic resonance results showed that -40IF-T reduced water migration during freezing, indicating a more uniform distribution of water in -40IF-T tissues. The results of relative conductivity showed that -40IF-T could effectively reduce the damage of cell membrane caused by freezing. In conclusion, low temperature and cryoprotective fluid can be used as effective methods to maintain sample quality.

Further efficiency improvements of screw compressors rely on computer modeling and mathematical analysis at the design stage. As speed and capacity increase, the uniformity assumption used in the conventional chamber model for twin-screw compressors is no longer suitable. At the same time, CFD simulations are complicated and time-consuming for design optimization. Therefore, in this paper, a chamber model considering pressure distribution is proposed, which is fast enough and more applicable. Each working unit of a screw compressor is considered a one-dimensional compressible transient flow with two moving pistons, which allow the passage of fluid. Numerical methods of the model are verified by a shock wave test. A twin-screw compressor was also simulated and the results of the chamber model were compared with the CFD results. Pressure fluctuations like those in previous experimental studies can be captured in the simulated indicator diagram. General trends with different speeds, suction ports, and wrap angles could also be recognized in the simulation results. The compression process and the influence of structural parameters on the compressor's functioning are revealed intuitively. By combining existing chamber models, the applicability of the new model can be further extended, and it is expected to be a helpful tool for compressor design.