Adiabatic two-phase pressure drop of carbon dioxide in different channel orientations

Adiabatic pressure drop measurements of two-phase Carbon Dioxide (CO$_2$) have been carried out in horizontal, vertical upward and downward direction with a dedicated test facility at the European Organization for Nuclear Research (CERN). A database of more than 1100 measurements, consisting of 512 data points in horizontal and 295 data points for each vertical up- and downflow respectively, has been compiled within this study. The experiments cover saturation temperatures from − 25 °C⩽$T_{\textrm{sat}}$⩽ +5 °C, mass velocities from 100 kg m$^{-2}$ s$^{-1}$⩽$G$⩽ 450 kg m$^{-2}$ s$^{-1}$ and are carried out in test sections with an inner diameter of 8 mm. The analysis of the pressure drop signals reveals that pressure fluctuations are increasing around the slug flow regime and the oscillation effects get magnified in vertical direction. The measurements in horizontal direction are compared to 18 frictional pressure drop models. For the comparison of the vertical data, 21 void fraction correlations accounting for the static head have been combined with the frictional models. The data sets of the horizontal and vertical upflow measurements are well predicted by several pressure drop models with an acceptable statistical significance. However, the prediction models perform less accurately for vertical downflow and it has been found that the static head is not well described by the void fraction correlations. In particular at low mass velocities, the effects of phase separation due to dominant buoyancy forces are not well incorporated by the void fraction models considered. In addition to the comparison of the entire data sets, subdivided analyses accounting for the flow patterns are provided within the present work.