Effects of saturation temperature on the boiling properties of carbon dioxide in small diameter pipes at low vapour quality: Pressure drop

The thermal management of most of the silicon pixel detectors at the heart of the Large Hadron Collider experiments at CERN relies on boiling carbon dioxide inside compact heat exchangers of small hydraulic diameter at saturation temperatures between +15 ∘C and −30 ∘C. Due to the current lack of suitable predictive models, a long-term study has been launched to create a consistent and reliable experimental database studying the peculiarities of boiling carbon dioxide in mini- and micro-channels. This study presents results on the total pressure drop of boiling carbon dioxide at low vapour quality (0 < x < 0.4) in 200 mm-long stainless steel tubes with an inner diameter of 2.15 mm, 1 mm and 0.5 mm. By means of a dedicated test setup equipped with high precision sensors, a wide range of saturation temperatures (+15 ∘C to −25 ∘C) and mass fluxes (100 to 1800 kg/m2s) have been explored and diabatic tests were carried out for heat fluxes from 5 to 35 kW/m2. The data presented focus on the influence of the saturation temperature on the two-phase pressure drop. It is suggested that the combination of shifting physical properties of carbon dioxide and different confinement conditions causes a change in the phenomenological behaviour of the flow and that a transition between macro- and micro-scale most likely occurs within the range of test parameters. It is further shown that a shift in the applicability of existing prediction methods is caused by those effects and no correlation is able to predict the experimental data and trends in the whole temperature range. A selective approach to the use of existing correlations is also proposed.