Experimental study on heat and mass transfer of falling liquid films in converging-diverging tubes with water

To improve the heat and mass transfer performances of falling films and develop a new type of falling film evaporator, the heat and mass transfer characteristics of falling film evaporation and sensible heating in four different sizes of converging-diverging (CD) tubes were experimentally explored in this study. By analyzing the relationship between the heat transfer coefficient and the falling film flow rate, it was found that the CD tube is suitable for falling film evaporation and sensible heating with a large Reynolds number. For different sizes of tubes, the key factor affecting the heat transfer performance in falling film evaporation and sensible heating is the rib height. At the same rib height and rib pitch, the longer the converging segment of the CD tube, the better the heat transfer performance. According to the comparative analysis on heat and mass transfer performances of falling film evaporation and sensible heating inside the four CD tubes, the CD tube 3# is the best. The evaporation heat transfer coefficient (when the Reynolds number of the liquid films is 2356), evaporation mass transfer rate (when the perimeter flow rate of the liquid films is 0.173 kg/(m.s)), and sensible heating heat transfer coefficient (when the Reynolds number of the liquid films is 1635) of CD tube 3# are 62%, 38% and 63% higher than that of the smooth tube, respectively. For the same CD tube, the evaporation heat transfer coefficient is greater than the sensible heating heat transfer coefficient, but both increases as the flow rate increases. The heat transfer correlations of falling film evaporation and sensible heating in the experimental range were obtained. It will provide a useful reference for future engineering design and industrial applications. (C) 2018 Elsevier Ltd. All rights reserved.