The effects of inclination angle on flooding in a helically fluted tube with a twisted insert

This paper develops an experimental flooding correlation as a function of the inclination angle for 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 flooding were examined. Water-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 flooding for both vertical and nearly horizontal tubes were developed. The experimental data from this paper were compared with those for smooth tubes from the literature. The results show that flooding was initiated from the top for inclination angles less than 60° while it started from the bottom of the tube for the angles larger than 60°. The fluctuation in pressure drop was very strong when flooding started from the bottom while it was not as strong when it started from the top. The effect of the position of the insert was not significant for the nearly horizontal tube while it had some effect for the vertical tube. For the fluted tube with an insert, the flooding vapor velocity was the highest for the inclination angle between 40° and 60° for superficial liquid velocities less than 1.178 × 10^-2 m/s. When the superficial velocity of liquid flow is higher than 1.178 × 10^-2 m/s, the superficial flooding vapor velocity increases with an increase of the inclination angle.