Boiling heat transfer on finned tube bundle with lower tubes heated with constant heat flux

The boiling heat transfer on a finned tube bundle was experimentally studied in R11 at a pressure of 1 bar. The bundle is composed of 18 finned tubes in a 6 x 3 inline arrangement. The heat transfer coefficient of the bundle with a pitch-to-diameter ratio s/d = 1.6 or 1.15 and the time-averaged liquid velocity under the bundle were measured when the tubes of the lower rows were heated with high constant heat flux. The results indicate that the boiling heat transfer is strongly enhanced by the strong two-phase flow induced by the tubes of the lower rows, especially in the intermediate region, with the maximum enhancement of heat transfer relative to that with the lower rows not heated was as much as 150%. The greatest time-averaged liquid velocity under the bundle occurred with the increase of total heat flow. The mass flow rate through the bundle decreased with high heat flow. These results show us a way to enhance the boiling heat transfer in the intermediate region between natural convection and fully developed boiling and suggest that the bundle heat exchanger should be designed to work in the intermediate region.