Numerical study on airflow performance and mechanical characteristics of centrifugal fan

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

Numerical simulation of pollutant diffusion in public toilets

Compared with residential restrooms, public toilets usually have higher utilization rates. How to ensure the air quality in the toilets through reasonable ventilation is of great significance to human health. In this study, an office building with a public toilet is selected as the research object, and the Airpak 3.0 software is adopted to simulate the airflow velocity distribution in the toilet with different air change rates and exhaust vent heights. Variations of the air velocity distribution, ammonia concentration, ventilation efficiency, and other parameters, are compared and analyzed. The results show that increasing the air change rate could reduce the concentration of pollutants in the toilet, but it has negative effects on ventilation efficiency. After comprehensive analysis, the desirable air change rate is chosen with the value of 15h-1. The exhaust outlet is set near the source of pollution to facilitate the discharge of pollutants. This work may provide a theoretical basis for amelioration of the toilet ventilation environment

Investigation on the pressurized discharge performance from a liquid oxygen tank under different injected gas temperatures

Accurate prediction on the pressurized discharge performance is significant to the safety operation of cryogenic propellant system. In the present study, a two-dimensional numerical model is established to simulate the fuel outflow with high-temperature gas injection. Both the environmental heat invasion and interfacial phase change are detailedly considered. The volume of fluid method is used to predict the distribution of the liquid-vapor interface, and the low Re k-ε turbulent model is adopted to simulate the pressurized discharge of liquid oxygen. The liquid hydrogen discharge tests, under the gas hydrogen injection, are selected to validate the developed numerical model, and the fluid temperature of the symmetry axis of the liquid hydrogen tank is selected as the comparison parameter. It shows that the present numerical model has good prediction accuracy with calculation deviations being less than 20%. Based on the developed numerical model, the effect of the injected gas temperature on the pressurized discharge of liquid oxygen is investigated and analyzed. Some valuable conclusions are achieved. The present work could strengthen the researchers’ understanding on the thermodynamic behavior during pressurized discharge and might supply some technique supports for the design and optimization of cryogenic propellant systems

Operation performance of an ultralow-temperature cascade refrigeration freezer with environmentally friendly refrigerants R290-R170

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

Experimental test on the performance of a −80 °C cascade refrigeration unit using refrigerants R290-R170 for COVID-19 vaccines storage

With the urgent demand for ultralow-temperature refrigerators worldwide, the operation reliability and stability of the refrigeration system becomes greatly crucial. In this study, a -80 oC ultralow-temperature cascade refrigeration system (CRS) is developed. From aspects of global warming potential (GWP) and ozone depletion potential (ODP), the environmentally friendly refrigerants R290 and R170 are utilized in the high-temperature cycle and low-temperature cycle of CRS. The experimental measurement is conducted in a Type-laboratory with a dry bulb temperature of 25.0 °C and a wet bulb temperature of 20.2 °C. The pull-down and stable operation performance of the CRS freezer are experimentally investigated. Both the inlet and outlet temperature and pressure of two compressors are monitored, and the operation characteristic of the CRS is analyzed. With some temperature test points arranged in the freezer, the temperature drop and temperature fluctuation variation of air are assessed. The operation power consumption of the CRS is also measured during the whole process. It shows that the −80 °C temperature can be generated and realized by the developed ultralow-temperature freezer

Experimental study on the pull-down performance of a −80°C cascade refrigeration freezer

To cope with global climate change, chlorofluorocarbons and hydrofluorocarbons refrigerants have been or will be abandoned by different countries worldwide. Natural refrigerants seem to be promising substitutes in the near future. In this study, eco-friendly refrigerants R290 and R170 are adopted as high-temperature and low-temperature fluid in a cascade refrigeration freezer to generate −80°C. The experimental measurements are carried out to investigate the pull-down performance of the freezer. The inlet and outlet pressure of two compressors are monitored and tested. Temperatures of different state monitors in high-temperature cycle and low-temperature cycle are measured to analysis the operation performance of the cascade refrigeration system. Different temperature test points are set to research the temperature drop and temperature distribution in the freezer. Some new findings are obtained. Due to the lack of ultralow temperature experimental data on cascade refrigeration system, this study can enrich and deepen the understanding on operation mechanism of ultralow temperature cascade refrigeration freezer