A flow regime map for refrigerant condensation in herringbone micro-fin tubes

Experimental and analytical work was performed and visual observations made to investigate the flow regimes during condensation of the refrigerants R-22, R-407C and R-134a in horizontal smooth, helical micro-fin, and herringbone micro-fin tubes at an average saturation temperature of 40°C, with mass fluxes ranging from 300 to 800 kg m^-2 s^-1. The flow regimes observed were mainly annular flow and intermittent flow. At low mass fluxes and low average vapour qualities, however, stratified wavy flow was observed. The experimental results show that the transition from annular to intermittent flow regimes occurred at average mass fraction values of 26%, 29% and 48% for the herringbone micro-fin, helical micro-fin, and smooth tubes, respectively. The Thome flow regime transition criterion for annular to intermittent flow, together with the Froude rate and Martinelli parameter, were used to characterize the prevailing flow regime. The combined analyses made it possible to adapt the condensation flow pattern map for helical micro-fin tubes in such a way that it can generate a valid and accurate flow regime map for condensation in herringbone micro-fin tubes. The new transition criterion effectively predicts the delay in the transition from annular to intermittent flow for all three refrigerants condensing in the herringbone micro-fin tube. This new flow regime map has never before been produced for enhanced tubes, and represents an important advance in the field of practical heat-exchanger design.