Experimental correlation for flooding vapor velocity in a fluted tube with a twisted insert [heat pumps]

This paper develops a flooding correlation from experiments in a fluted tube with a twisted insert for application to absorption heat pump systems. Fluted tubes have been used to enhance both heat and mass transfer during absorption and desorption processes. In the present tests, a plastic twisted tape was inserted to increase the absorption rate in counter current absorption. The effects of the twisted insert and the angle of inclination on the flooding were examined. Water and ethyl alcohol solution flows downward in the fluted tube while air flows counter current in an upward direction. The flooding mechanism in a fluted tube with a twisted insert was analyzed by visual observation and experimental correlations for the flooding for both vertical and nearly horizontal positions were developed. The experimental data from this paper were compared with those for smooth tubes from the literature. The results show that the flooding initiated from the bottom for the vertical tube and from the top of the tube for the nearly horizontal tube. The fluctuation of the pressure drop for the vertical tube was much stronger than that for the horizontal tube. The effect of the position of the insert was not significant for the nearly horizontal tube while it was somewhat significant for the vertical tube. The superficial flooding vapor velocity for the nearly horizontal position is lower than that for the vertical position for a given liquid mass flow rate.