An experimental investigation of dropwise and filmwise condensation of low pressure steam in tube banks
Research to date has highlighted a number of conditions where dropwise condensation may offer heat transfer enhancements over filmwise condensation. Previous studies have shown at pressures above or around atmospheric, dropwise condensation offers significant benefit over filmwise. However, some of this research suggests that as the system pressure is reduced below atmospheric, the benefits of dropwise condensation diminish rapidly, to the extent that, at pressures around 50mbar the benefits of dropwise over filmwise are minimal. This thesis details a series of experiments which were conducted to investigate the heat transfer and pressure drop distributions in tube bundles during both dropwise and filmwise condensation of steam. The primary objective of the work was to determine the design implications associated with switching the mode of condensation of a electricity generating steam turbine condenser from the current filmwise mode, to dropwise. Experimental data were obtained from a new purpose build apparatus containing seventy-five, 150mm long titanium tubes, arranged in an in line configuration of five columns and fifteen rows. Dropwise and filmwise data were recorded from each row at test cell inlet pressures down to 50mbar using both pure steam and steam air mixtures. Filmwise heat transfer data indicated that, under most conditions, heat transfer coefficients were generally in agreement with those obtained by previous workers. Heat transfer data obtained during dropwise condensation suggested that the benefits of dropwise condensation are not significantly diminished at low pressure, and that, unlike filmwise condensation, inundation has little or no effect in a fifteen row bundle. The data also indicated that the pressure drop characteristics and effects of air are, within experimental error, identical during both modes of condensation and in line with models and theories proposed by previous workers.