Investigation of flow patterns in two-phase carbon dioxide in horizontal and vertical pipes

Modern particle detectors crucially depend on efficient cooling systems. Two-phase carbon dioxide (CO2) is a suitable solution as a cooling agent. This publication presents the observations and results of investigations of horizontal and vertical flow of two-phase CO_2 at a temperature of T = -15 degrees C and a pressure of approximately 23 bar. Heat fluxes between 98.5 kW/m(2) and 200 kW/m(2) were applied to the CO2, covering the range expected to occur in the future ATLAS Pixel detector being built for the high-luminosity phase of the Large Hadron Collider. Flow speeds ranged from 11.8 m/s to 28.1 m/s. Dedicated sensors measured the temperature and the pressure before and after heating the CO2. Two-phase flow patterns occuring in the pipe after heating the CO2 were recorded with a high-speed camera. Stratified, wavy and slug flow are found to be the predominant patterns for horizontal flow, while upward vertical flow is mainly found to be slug or churn. Based on the recorded images the void fraction of the CO2 after heating is determined and compared for the different setups. The results are summarised in a flow-pattern map. A clear distinction between vertical and horizontal flow is found, with horizontal flow exhibiting a significantly higher void fraction than vertical flow. Based on the pressure measurements, the pressure drop after heating the CO2 is measured and the corresponding Euler number is computed. While the pressure drop increases with the heat flux for horizontal flow, the pressure drop reduces with the heat flux in the case of upward vertical flow.