Abstract
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.
Highlights
The benefits of minimizing the size of a heat exchanger by using small hydraulic diameter tubes or channels have been confirmed multiple times in literature for various geometries and refrigerants [1,2,3,4]
The total pressure drop of CO2 boiling flows has been studied at low vapour quality in 200 mm-long, small-diameter stainless steel tubes with inner diameter of 2.15 mm, 1 mm and 0.5 mm
A new test setup has been used to explore with high precision measurements a wide range of saturation temperatures (+15◦C to −25◦C) and mass fluxes (100 to 1800 kg/m2s) and diabatic tests were carried out for heat fluxes from 5 to 35 kW/m2
Summary
The benefits of minimizing the size of a heat exchanger by using small hydraulic diameter tubes or channels have been confirmed multiple times in literature for various geometries and refrigerants [1,2,3,4]. This approach provides a higher surface area to volume ratio compared to macro-channels and results in higher heat transfer rates and lower equipment size [4]. The application of compact heat exchangers is especially appealing for very confined spaces such as those typical of modern silicon detectors in high energy physics (HEP) experiments, carried out at CERN (European Organization for Nuclear Research). One of the peculiarities of this application is the need for continuous operation in the Nomenclature B
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