Vertical U-pipe flow characteristics in absorption heat pump: Experimental study under vacuum conditions

摘要

The U-pipe is a significant pressure separation and flow device in the lithium bromide absorption heat pump. Contrary to conventional understanding, in this study, flashing evaporation is observed in U-pipe flow in a vacuum condition absorption heat pump by replacing the metal U-pipe with a transparent one. An experimental unit is developed to further investigate the two-phase flow characteristic inside a vertical U-pipe under vacuum. The flashing occurs on the upward side of the U-pipe owing to decrease in pressure. The flow phenomenon is described and the flashing evaporation mechanism is discussed in this paper. Small bubbles from the downward side of the free surface vapor entrainment travel toward the upward side and serve as the nuclei for flashing. The temperature and pressure profile is measured and discussed in this work. A large pressure drop is observed at the outlet of the U-pipe, and a method is suggested to calculate this pressure. This study changes the present understanding of U-pipe flow patterns in absorption heat pump under vacuum. The experimental results will be helpful in improving existing U-pipe pressure separation device designs for absorption heat pumps.