Two-phase flow heat transfer of CO2 vaporization in smooth horizontal minichannels

Abstract Experiments were performed on the convective boiling heat transfer in horizontal minichannels with CO 2 . The test section is made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm and with lengths of 2000 and 3000 mm, respectively, and it is uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 20–40 kW m −2 , a mass flux range of 200–600 kg m −2  s −1 , saturation temperatures of 10, 0, −5, and −10 °C and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The reduction of heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux, mass flux and saturation temperature, and with a smaller inner tube diameter. The experimental heat transfer coefficient of CO 2 is about three times higher than that of R-134a. Laminar flow appears in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for CO 2 was developed with 8.41% mean deviation.