Heat Transfer Characteristics of CO2 in a Horizontal Tube under Subcritical and Supercritical Pressures

Supercritical CO2 heat transfer in vertical tubes has been widely studied, but limited work exists for that in horizontal tubes. In this work, experiments were conducted for CO2 in a horizontal tube with inner diameter of 8 mm under wide range of pressures (5.51-20.41 MPa), covering both subcritical and supercritical conditions. The ranges of heat flux and mass flux were 97.5-404.0 kW/m(2) and 510.2-1314.2 kg/m(2)s, respectively. The heat transfer for subcritical pressure far below critical pressure showed typical flow boiling characteristics, with a wall temperature peak along the tube corresponding to departure from nucleate boiling. For near-critical pressure, it showed characteristics similar to supercritical heat transfer, with continuous increase of wall temperature along the tube. The effects of different parameters including pressure, mass flux, and heat flux on the heat transfer behavior were analyzed and discussed. Moreover, using the pseudo-vapor quality concept of the supercritical three-regime-model introduced in a previous work, the heat transfer coefficient was found to behave very similarly between subcritical and supercritical conditions. The experimental results showed significant deviation from single-phase convection within the two-phase/two-phase-like region compared with the single-phase regions, and which validates the similarity between subcritical boiling and supercritical pseudo-boiling.