In this study, the heat transfer characteristics of single-phase forced convection of R134a through single circular micro-channels with 1.7, 1.2, and 0.8 mm as inner diameters were investigated experimentally. The results were compared both to correlation

Flow vaporization heat transfer coefficient and pressure drop of carbon dioxide was measured in an extruded microchannel tube with 25 flowchannels of 0.8 mm ID and 0.5 m length. The test tube was heated by a water jacket, and the internal heat transfer coefficient was derived based on measured overall heat transfer and a regression-based expression for water-side heat transfer. Test principles are discussed and special emphasis is given to measurement uncertainties, including the propagation of uncertainty through the water-side regression. Studies of two-phase flow pattern were conducted in a separate test rig, using a 0.98 mm heated glass tube and a high-speed digital camera. Heat transfer and pressure drop measurements were conducted at varying vapour fraction for temperatures 0–25 °C, mass flux 190–570 kg m−2 s−1, and heat flux 5–20 kW m−2. Heat transfer results show significant influence of dryout, particularly at high mass flux and high temperature. Nucleate boiling dominates prior to dryout. Two-phase flow observations show increasing entrainment at higher mass flux, and a dominance of annular flow. Heat transfer data can be correlated reasonably well with a combination of models for nucleate boiling, convective evaporation, dryout incipience, and post-dryout heat transfer.