查看更多>>摘要：According to the phase change property of carbon dioxide and the rapid cooling ability of liquid nitrogen, an experimental system of quick-frozen strawberry by combined jet of carbon dioxide and liquid nitrogen was designed. Through a series of experiments, the pressure changes and jet outlet temperature at different nitrogen jet speeds at different temperatures were studied. Then, the effects of nitrogen injection speed at different low temperatures and different inlet positions in the quick-frozen room on the quick-frozen strawberry freezing rate was investigated. The final results show that when the jet speed of low-temperature nitrogen increases, the pressure loss in the flow process also increases gradually. At the same time, the speed of mixing will increase, and the disturbance will increase, and the outlet jet temperature will also decrease. When the nitrogen inlet speed was 0.2 m/s, the temperature between the strawberry was uniform. Different inlet and outlet positions had no influence on the freezing time of strawberry. Quick-freezing room (c) improves the heat transfer efficiency and makes the temperature of strawberries more uniform. The results obtained lay a foundation for further research and development of an efficient quick-frozen product device.
查看更多>>摘要：Subcooling systems are positioned in recent years as one of the best solutions to improve the efficiency of transcritical CO2 cycles. Specifically, the integrated mechanical subcooling cycle allows the improvement of these systems only using CO2 as a refrigerant. This integrated cycle can be designed with three different architectures: extracting the CO2 from the gas-cooler outlet, from the subcooler outlet or from the liquid tank. In this work, the three configurations are experimentally analysed and the main differences between them are studied. An experimental plant has been tested at three heat rejection levels (25.0, 30.4 and 35.1 °C) and a fixed temperature of the secondary fluid at the evaporator inlet of 3.8 °C. The results show that from an energy efficiency point of view, all the configurations have practically the same COP, with certain variations in the cooling capacity and the greatest differences in the cycles are found in the subcooler.
查看更多>>摘要：Considering the problems of high energy consumption, complex system structure and low drying temperature in current industrial drying system of metal parts, a closed loop heat pump drying system with auxiliary condenser was designed and established. The effects of auxiliary condenser operation parameters on the circulating air temperature, coefficient of performance (COP), moisture extraction rate (MER) and specific moisture extraction rate (SMER) were experimentally investigated, where the compressor rotating speed, circulating air volume, cooling water inlet temperature and flow rate respectively were 3900 r?min?1, 539.8 m3?h?1, 5～30 °C and 30.6～50.1 kg?h?1. The results showed that the drying temperature gradually increased with increasing cooling water inlet temperature and decreasing cooling water flow rate. When the cooling water inlet temperature and flow rate respectively were 20 °C and 34.3 kg?h?1, the drying temperature was 70.02 °C, meeting the high temperature requirement of industrial drying. Besides, the cooling water flow rate had little influence on the system COP, while the system COP increased at first and then decreased with increasing cooling water inlet temperature, reaching its maximum value 5.64 when the cooling water inlet temperature was 20 °C. Moreover, when the cooling water inlet temperatures respectively were 20 °C and 5 °C with constant cooling water flow rate 34.3 kg?h?1, the system MER and SMER could reach their maximum values of 3.8 kg?h?1 and 1.68 kg?kW?1?h?1, showing excellent dehumidification capacity. These results can provide guidance for the popularization and application of heat pump drying technology in the industrial drying.
查看更多>>摘要：The vacuum sublimation - rehydration thawing (VSRT) as a new vacuum thawing method consists of a vacuum sublimation stage and a rehydration heating stage. The heat and mass transfer are involved in the two stages mentioned above, and the heating plate (heat source) temperature is a key factor affecting the heat and mass transfer. In this work, the influences of heating plate temperature on the thawing efficiency (thawing time, thawing rate), thawing effect (thawing loss, texture parameters, color and luster) and system specific energy consumption during the process of VSRT were experimentally investigated to further optimize the process conditions. The results showed that there was an optimal value of the heating plate temperature within a certain range, so that the thawing efficiency and effect could reach the optimal state simultaneously. When the heating plate temperature was 25 °C, the thawing time was relatively short (67 min), the thawing rate was relatively fast (5.29 cm h?1), and the thawing loss was the lowest (0.4%). Meanwhile, the texture parameters, color and luster of thawed pork were not significantly different from that of the fresh pork, and the system specific energy consumption was lower (0.035 kWh kg?1) than the microwave thawing (0.04 kWh kg?1). Compared with the common method of natural air thawing (NAT), the thawing time was shortened by 40%, the thawing rate was increased to 2.4 times, the thawing loss could be reduced by 86%, and the better texture parameters, color and luster of thawed pork could be maintained under the method of VSRT.
查看更多>>摘要：Supercooling is of particular interest in the food industry since it has a great potential to improve shelf-life without freezing damage during storage. Food is a complex matrix of multi-components and its supercooling behavior may depend upon the physical properties of each component. In this study, the effect of a pulsed electric field (PEF) and an oscillating magnetic field (OMF) on supercooling of beef at different fat levels were explored. Lean-rich beef was supercooled with PEF only, while the simultaneous application of PEF and OMF was required for fat-rich beef. The finite element method (FEM) simulation demonstrated that fat caused the non-uniform current flow and temperature gradient due to the low electrical conductivity, and subsequently negative impacts on the supercooling extension of fat-rich beef meat. The OMF combined with PEF ensured maintaining a supercooled state of fat-rich beef at ?4 °C by synergistically inhibiting ice nucleation. The beef steaks preserved in the supercooled state showed significantly lower drip loss, compared to the conventionally stored controls, and there were no significant differences in color between supercooled and fresh steaks during the 7 day-preservation.
查看更多>>摘要：This paper proposes a vacuum ice production device by solid adsorption. An analysis model related to exergy loss and exergy efficiency is established in each component of the vacuum flash ice making system. By comparing the exergy analysis results of adsorption vacuum ice production device and condensing vacuum ice production, this paper has drown an effective method to reduce exergy loss. It has been found that the exergy efficiency of the adsorption vacuum ice production device is 9.64% higher than that of the condensing vacuum ice production device. The exergy efficiency of water vapor adsorption in the adsorption chamber is the highest, while the exergy efficiency of water vapor adsorption in the condensation chamber is the lowest. In order to reduce the energy demand of the device, the heat released by the condenser should be fully utilized to reduce the exergy loss of the condenser. At the same time, the heat and mass transfer of the flash evaporation process should be enhanced when designing the flash chamber, such as optimizing the ice making working fluid to improve the exergy efficiency of the flash chamber.
查看更多>>摘要：Propane (R290) has excellent thermophysical properties and development potential. The dynamic characteristics of R290 rotary compressors and room air conditioners have been studied mainly on the start-up and defrost processes. There is little research on the shutdown process. Unlike hydrofluorocarbons (HFCs), the density of liquid propane is lower than that of lubricant. It is difficult for propane to dissolve into the oil sump and downward after the refrigerant separation after shutdown. The shutdown state affects the distribution of refrigerant in the oil sump and the mixture viscosity of the restart-up, which is related to the compressor reliability. In this paper, an R290 room air conditioner system was built and the variations of system pressures, temperatures, and oil sump parameters of an R290 high-pressure rotary compressor with three different lubricants were measured after shutdown in two conditions. As the experiments indicated, driven by the refrigerant supersaturation (the difference between the actual refrigerant solubility and saturation solubility) resulting from the rapid pressure drop after shutdown, a large amount of refrigerant separated from the oil sump, and the refrigerant solubility dropped fast. As the compressor temperature decreased, the bottom viscosity reached a peak. For the low-temperature heating condition, the peak was about 120.4～160.1mPa?s. However, when the degree of supersaturation decreased to a certain value, the refrigerant no longer separated out. There is always a solubility gradient of refrigerant in the oil sump of R290 high-pressure compressor after shutdown. Its size was related to the pressure drop and the miscibility of lubricants and refrigerants.
查看更多>>摘要：Aimed at the optimal chiller loading problem in parallel chillers system, an improved parallel particle swarm optimization (IPPSO) algorithm is proposed. This algorithm uses random and chaotic sequence mechanisms to initialize particles respectively, so that the two populations have different characteristics at the beginning of generation. Meanwhile, a new immigration operator is proposed to break the internal balance of the population, enhance the diversity of the population and promote the population evolve to a higher level. Besides, according to the characteristics of the two populations, different improvement strategies for inertia weight are adopted to accelerate the convergence speed of the algorithm further. Finally, the performance of the proposed IPPSO algorithm is tested with two well-known parallel chillers system cases, and its experimental results are compared with other algorithms. The experimental results show that compared with other algorithms, the IPPSO algorithm can find the better optimal solution and present an obvious energy-saving effect. The convergence ability, computational complexity and robustness are also verified after the detailed comparisons.
查看更多>>摘要：The flow boiling heat transfer of environmentally friendly refrigerant HFO-1234yf in a horizontal circular microchannel is experimentally investigated. The inner diameter and heating length of the test channel are 0.5 mm and 300 mm, respectively. Experiments are conducted at relatively high mass flux ranging from 750 to 1500 kg m?2 s?1, heat flux from 25 to 85 kW m?2 and inlet pressure between 0.604 and 0.634 MPa. The heat flux is stepwise imposed to the test channel until dryout occurs. The flow boiling heat transfer characteristics are analyzed in detail. In the pre-dryout region, the heat transfer coefficient (HTC) is found to be mainly determined by the heat flux, while the effect of mass flux is weak. Two different HTC behaviors are observed at low and high mass fluxes, which can be attributed to the prominent effect of thin liquid film evaporation on heat transfer and the occurrence of partial dryout. Moreover, the oscillation of mass fluxes and wall temperatures are observed at high mass flux conditions, resulting in heat transfer deterioration. In the post-dryout region, the critical heat flux (CHF) is found to increase with mass flux, while the critical vapor quality would decrease. Finally, the experimental data of HTC and CHF are compared with correlations from the literature and good agreements are presented.
查看更多>>摘要：In refrigeration and air-source heat pump systems, water vapor in moist air circulating across the heat exchanger fins condenses into liquid nuclei when the surface temperature is below the dew point. The supercooled liquid droplets grow and coalesce until a liquid-to-solid phase transition occurs, preceding the growth of crystals on the ice beads and the subsequent development of a frost layer within and on top of the crystals. This paper describes a comprehensive, three-stage model for frost nucleation, in which a novel crystal growth model stage was integrated with droplet growth and frost layer growth models. The overall model was experimentally validated for both hydrophobic and hydrophilic surface types and various environmental conditions focused on surface temperatures between 0 and ?10 °C, relative humidities between 70 and 90%, and air temperatures between 0 and 20 °C. Model results indicated that the surface contact angle affected the ice beads' shape and size. The air's water vapor content affected the frost layer's thickness and density. If the humidity increased, the mass transfer potential increased, prompting the crystal growth–frost growth transition to occur quickly and at high density values. The cold surface temperature also affected the stage transitions. At ?9 °C surface temperature, crystal growth lasted approximately 2 min while it increased to 21 min at ?5 °C. Frost thickness was initially higher at ?9 °C due to a shorter droplet growth stage, but the thickness increase slowed during frost layer growth, resulting in a temporarily higher thickness at ?5 °C.
NSTL
Elsevier