In the present study, the operation performance of an ultralow-temperature cascade refrigeration freezer is experimentally researched. The natural refrigerants R290-R170 are adopted as high-temperature and low-temperature fluids. The experimental test is conducted in a type laboratory with a dry bulb temperature of 32.0 °C and a wet bulb temperature of 26.5 °C. Different state monitors are set to display the system operation performance, and several temperature monitors are arranged to study the pull-down performance and temperature variations in the freezer. Based on the established experimental rig, three freezing temperatures, including − 40 °C, − 80 °C, and − 86 °C, are measured and compared. The results show that it takes about 240 min for the freezer to be pulled down to − 80 °C. During the pull-down period, different monitors all experience rapid temperature drop, and the power consumption reduces from 1461.4 W to 997.5 W. Once the target temperature is achieved, the freezer comes into periodic start–stop operation. With the set temperature ranging from – 40 °C to – 86 °C, the inlet temperature of two compressors gradually decreases, while the discharge temperature has an increase trend. The cooling effect of the pre-cooled condenser reduces with the freezing temperature, while the long connection pipe has opposite variation profile. Moreover, it is observed that for different freezing temperatures, most of the space in the freezer can be cooled down to the target temperature. It is confirmed that the present ultralow-temperature freezer can be used for the storage and transportation of COVID-19 vaccines. However, it is also found that the cascade refrigeration system is not suitable for high freezing temperature, due to high power consumption and extensive start–stop switch of refrigeration system.
As small fans are widely used to dissipate the heat of the electronic components, a series of special requirements are put forward on the airflow performance and stress characteristics. In the present study, the computational fluid dynamics (CFD) method is adopted to study the airflow characteristics of a specific type of fan, including the fluid pressure distribution, flow velocity field and fluid streamline distribution. The stress characteristics of the fan blades are systematically analyzed based on the fluid-solid coupling and thermal-solid coupling methods. The results show that with the rotation speed of 1400 rpm, the airflow velocity in the air duct is unevenly distributed, and some eddy disturbances form and occur in the flow field. To improve the operating efficiency of the fan, appropriate optimization schemes should be adopted to reduce the intensity and range of the eddy influence. When the inlet temperature is 20 °C, the stress on the impeller is mainly caused by centrifugal force and thermal load. As the inlet temperature increases, the effect of the thermal load becomes increasing. While for the centrifugal force, its influence on the impeller gradually disappears and completely disappears when the temperature reaches 50 °C.
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