In this work the two-phase flow pattern transitions during forced convection evaporation in a horizontal, smooth tube of 6.0 mm inner diameter are investigated. The analysis is carried out comparing both image visualizations with a high speed frequency acquisition camera and local heat transfer coefficient measurements. In this way, two-phase flow patterns are recognized; in particular the slug, intermittent, annular and dry-out flow regimes. Tests are carried out with the refrigerant mixture R410A for saturation temperatures equal to 5.0 degrees C and 42.0 degrees C, mass fluxes ranging from 200 to 500 kg/m(2)s and wall heat fluxes equal to 5.0 and 20.0 kW/m(2). At low-medium vapour qualities slug and intermittent flow regimes are encountered. The higher the mass flux the lower the completion vapor quality of these flow regimes. Before the complete establishment of annular flow, a wide region of asymmetric annular flow was found (asymmetric annular flow regime is here defined as a superposition of annular and stratified-wavy flow regimes). The pure (symmetric) annular flow is never encountered for G=200 kg/m(2)s at T-sat=5 degrees C, and for G=200 kg/m(2)s and G=350 kg/m(2)s at T-sat=42.0 degrees C, but the asymmetric annular flow regime was found. The transition to dry-out flow regime was found in almost all the operating conditions and at vapor qualities close to unity for the lowest mass fluxes and heat fluxes; the dry-out flow regime occurs earlier increasing the mass flux, the heat fluxes or the saturation temperature (reduced pressure). A comparison was made with the Wojtan et al. (2005) flow pattern map, which is one of the most used and it was principally based on R410A experimental data. Good agreement in the prediction of slug flow regime was found for the lowest mass flux (G=200 kg/m(2)s) for both the saturation temperatures investigated. The transition from intermittent to the annular flow regime (asymmetric annular flow in the experiments) is established earlier than the transition predicted by the flow pattern map and at vapour qualities that decrease with the increase of mass flux. The annular to dry-out transition qualities are generally well predicted by the map.
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