Flow Boiling of Carbon Dioxide at Different Channel Orientations for the Thermal Management of Future Detector Technologies at the High-Luminosity Large Hadron Collider at CERN

The global warming potential (GWP) of working fluids in thermodynamic cycles and their environmental impact have been gaining considerable attention within the recent years. The global objectives on climate protection are becoming increasingly ambitious, whereas the general demand for refrigeration appliances with enhanced cooling capacities is constantly growing all over the world. One promising approach to reconcile these contradictory expectations is the replacement of synthetic working fluids with natural refrigerants.The European Organization for Nuclear Research (CERN) supports the development of green technologies within the upgrades of their future accelerator facilities and Carbon Dioxide (CO2) has been identified as an eco-friendly candidate with excellent heat transfer characteristics that has the potential for replacing detrimental refrigerants in future scientific and industrial cooling applications. Inspired by the positive experiences with several mid-scale refrigeration systems, the ATLAS and CMS experiments decided to use CO2 as working fluid for the thermal management of their future Particle Tracker detectors requiring cooling capacities in the order of 0.5 MW.Pressure drop and heat transfer properties are key factors for designing thermodynamic cycles properly. Motivated by the lack of experimental data of flow boiling CO2 in vertical directions, a comprehensive research program has been launched to investigate the two-phase flow behaviour of CO2, in particular in vertical up- and downward direction. As a first step, a test facility to study the two-phase flow characteristics has been built. The setup allows measurements of the significant two-phase flow parameters in horizontal, vertical up- and downward direction in macro-scale pipes with inner diameters of 8 mm. The test conditions of the experiments cover saturation temperatures from -25 °C to +5 °C and the mass velocity ranges from 100 kg m^-2 s^-1 to 450 kg m^-2 s^-1.The performance of pressure drop models has been analyzed with data sets of 512 measurements in horizontal and 295 data points for each vertical up- and downflow direction respectively. 18 frictional pressure drop models have been compared to the horizontal data set and 21 void fraction correlations have been combined with the frictional pressure drop models accounting for the static head, resulting in 378 factorial combinations to anticipate the pressure losses in vertical directions. The best models have been identified for the entire data sets for all flow directions. Moreover, a subdivided analysis, split up according to the flow regimes, has been carried out to highlight the most accurate prediction methods with regard to the flow regimes.The heat transfer properties of flow boiling CO2 have been investigated in vertical upward direction with diabatic measurements of 5 kW m^-2 and 11 kW m^-2 respectively. The dryout phenomena is observed to be influenced by the heat flux, the mass velocity and the saturation temperature. To avoid operation at reduced heat transfer coefficients, a correlation to predict the dryout inception is suggested. The comparison of the data set to established prediction methods reveals, that the heat transfer coefficients of vertical upflow are generally underpredicted. As a consequence, an enhancement factor for vertical upflow of two-phase CO2 is suggested for two prediction models.