Experimental research on flow mode transition of falling film flow on metal foam wrapped tubes

Abstract

Flow mode transitions on metal foam wrapped tubes in shell and tube heat exchanger may affect the condensate characteristics obviously because the condensate inundation severely depends on the flow mode. In the present study, the flow mode transitions were experimentally investigated by observing the flow modes on metal foam wrapped tubes and determining the Reynolds numbers (Re) of flow mode transitions. The experimental conditions cover the metal foam pore density from 5 to 40 PPI (pore per inch), the tube spacing from 6 to 20 mm, working fluids of pure water and water/ethylene glycol mixture, and the flowing process of increasing flow rate and decreasing flow rate. The experiment results show that, as the PPI increases, the Reynolds numbers of droplet/droplet-column (D/DC) transition and droplet-column/column (DC/C) transition both increase, but the Reynolds numbers of column/column-sheet (C/CS) transition and column-sheet/sheet (CS/S) transition both decrease; as the Galileo number increases, the Reynolds numbers of flow mode transition increases. It is also found that, the Reynolds numbers of the D/DC and DC/C transition does not change obviously with the tube spacing, while the Reynolds numbers of the C/CS and CS/S transition increases sharply at first and then slightly with the increase of tube spacing. Correlations for flow mode transitions on metal foam wrapped tubes are developed, and the predicted results of the correlations agree with 94% of experimental data within a deviation of 20%.