SUMMARY

Flow mechanisms affect transport processes during condensation. Most studies on two-phase flow regimes are qualitative in nature, typically providing only information to guide the identification of the respective regimes and the transitions between them. These studies have, however, not yielded quantitative information to assist the development of pressure drop and heat transfer models. Such qualitative studies have also yielded results with considerable variability between investigators. A comprehensive investigation of flow mechanisms, void fraction, pressure drop and heat transfer during condensation of R404A in microchannels is proposed here. In contrast to all prior investigations, high speed video recordings and image analysis will be used to directly measure void fraction, slug frequencies, vapor bubble velocity, vapor bubble dimensions and liquid film thicknesses in tube diameters ranging from 0.5 to 3 mm. Experiments will be conducted at reduced pressures and mass fluxes ranging from 0.38 to 0.77 and 200 to 800 kg m-2 s-1, respectively, to document their influences on the condensation process at local vapor qualities ranging from 0 to 1. This information will be used to develop models for the void fraction in condensing flows. A complementing set of heat transfer and pressure drop measurements will also be conducted on the same geometries at similar conditions, and the void fraction models will be used in conjunction with these measurements to yield improved heat transfer and pressure drop models. It is expected that this comprehensive set of experiments and analyses will yield a self consistent and accurate treatment of high-pressure condensation in small hydraulic diameter geometries.