수렴-발산 노즐을 이용한 냉동기의 성능특성에 관한 연구

Abstract

Existing cooling technologies are dominated by vapor compression technology, but there are efficiency limitations. The technology using waste heat or absorption technology has problems such as installation space and maintenance due to its large scale and complexity. Converging-diverging nozzle is manufactured to increase the cooling effect by using the phenomenon in which the temperature decreases as the fluid passes through the nozzle. The refrigeration system using converging-diverging nozzles has fewer parts compared to existing technologies, the safety of the device is improved, and energy efficiency can be increased by improving the efficiency of the heat exchanger and reducing the power consumption of the compressor. In this study, a converging-diverging nozzle is attached to the inlet of the evaporator to improve the cycle efficiency of the refrigeration system by simultaneously realizing evaporation and expansion of the refrigerant. The preliminary research for the manufacture of nozzles simulates a change in the length of a converging part, when the length of a converging part is 10 mm, the section that has a cooling effect on the diverging part was the longest. Based on the simulation results, a converging-diverging nozzle with a length of 10 mm, a nozzle diameter of 1 mm is fabricated. Before implementing the refrigeration system, the cooling effect according to the temperature, pressure and flow rate at the nozzle inlet is investigated using air as the working fluid. It is found that the temperature of the air that passes through the converging part rapidly decreases, that heat exchange takes place towards the rear end of the nozzle, the temperature of the air is gradually compensated. In addition, as the air pressure and flow rate in the nozzle increase, the cooling area and cooling effect within the diverging part is increased. When the air pressure and flow rate are 7 bar and 50 LPM, the cooling effect occurs up to about 15 mm from the inlet of the diverging part. It is found that the flow rate of air passing through the converging part has the most influence on the cooling effect and cooling area in the diverging part. Through previous research that prove the cooling effect of a single nozzle, five converging-diverging nozzles are manufactured, using R-134a refrigerant, the cooling effect and COP are identified. In all experiments except for the case when mass flow rate is 11.3 g/s, the temperature difference at inlet and outlet of the nozzles are increased as the refrigerant and nozzles are used less. When using one nozzle, the COP are increased until mass flow rate is 21.1 g/s and then decrease, showing a maximum performance of 4.34. Nozzles are attached to the inlet of the evaporator to obtain a COP. The COP is increased with the increase of air flow and internal temperature of the chamber. It is found that the COP is decreased as the cross-sectional area of the nozzles are increased and the amount of mass flow rate is greater than 13.5 g/s.