Experimental investigation on flooding in a fluted tube with and without a twisted insert

Experimental flooding data were obtained for a smooth tube and a fluted tube with and without a twisted insert. Fluted tubes have been used to enhance both heat and mass transfer for many thermal systems including absorption heat pump systems. A plastic auger (twisted insert) was inserted in the fluted tube to increase heat and mass transfer. The effects of the twisted insert and angle of inclination on the flooding were examined. A water and ethyl alcohol solution (simulating an ammonia-water solution) flows downward in the fluted tube while air flows counter current in an upward direction. The onset of flooding and the flooding mechanism were analyzed at variable angles of inclination by visual observation. The experimental results showed that the effect of the twisted insert on the flooding curve decreased as the inclination angle varied from a horizontal to a vertical position. The effect of the enhanced surface was not affected by the inclination angle. The flooding velocity was the highest between the inclination angle of 20 and 50 depending on the liquid mass flow rates for the smooth tube, between 20 and 55 for the fluted tube without the insert, and between 40 and 60 for the superficial liquid velocities of 42.4 m/hr and lower for the fluted tube with an insert. The enhancement factor for the flooding velocity was obtained for the fluted tubes with/without inserts. Flooding correlations were developed as a function of inclination angle for the smooth tube, and the fluted tube with/without a twisted insert with an error band of ± 8.5%, ± 5.0% and ± 10.0%, respectively.