Abstract
A test facility to investigate flow pattern transitions of vertical two-phase flow of CO2 has been built within the scope of the high-luminosity detector upgrades at the European Organization for Nuclear Research (CERN). Adiabatic flow pattern observations for both vertical up- and downflow are recorded with high-speed imaging in tubes of 8 mm inner diameter, with saturation temperatures in the range of −25°C to +5°C and mass velocities ranging from 100kgm−2s−1 to 450kgm−2s−1. A database of 431 flow pattern observations in upward and 123 in downward direction has been compiled. The recorded data have been analysed with machine learning techniques and a previously trained Frame- and Flow-Regime-Classifier is used for the flow regime classification. The observed two-phase flow pattern transitions did not match the transition lines of existing flow pattern maps. As a consequence, new transition lines for the bubbly-to-slug, slug-to-churn and churn-to-annular transitions have been developed for both vertical upflow and downflow respectively and condensed into new flow pattern maps. It is concluded, that the flow regime transitions are strongly depended on vapour quality, mass velocity, the flow direction and the fluid properties. Compared to horizontal flow, a dryout region is not observed and the liquid film of the annular flow regime dries out symmetrically at vapour qualities close to x=1.
Highlights
The global warming potential (GWP) of refrigerants and their impact on climate change have been gaining considerable attention within the last decades
The power dissipation of the ATLAS Inner Tracker (ITk) and the CMS Inner Detector will be 0.3 MW and 0.5 MW respectively, while, in order to cope with the high radiation level present at the Large Hadron Collider (LHC), both detectors will be operated at temperatures down to −40◦C [5]
A database of 431 flow pattern observations in upward and 123 in downward direction is set up and the records are analysed with machine learning techniques, that automatically identify the flow pattern transitions
Summary
The global warming potential (GWP) of refrigerants and their impact on climate change have been gaining considerable attention within the last decades. Carbon dioxide (CO2) has already shown good performance in several cooling applications in High Energy Physics experiments [1,2] and is, due to its beneficial thermo-physical properties, a promising candidate for replacing synthetic working fluids [3,4]. Experiments in High Energy Physics are positioned far from their ancillary service systems and are typically connected to the cooling plants by long transfer lines – in the order of 100 m – with both horizontal and vertical segments. This layout, combined with the need of stable and precisely controlled operational temperatures, is well suited for TwoPhase Accumulator Controlled Loops (2PACL). The concept has already proven to be highly reliable and to yield good performance in several HEP experiments [6,7], and even in space applications like the AMS experiment on the International Space Station (ISS) [8]
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