Characterisation of the Thermoflow due to the Dry Nitrogen Flushing Scheme in the ATLAS Inner Tracker using Computational Fluid Dynamics

TL;DR

This study uses computational fluid dynamics to analyze the dry nitrogen flushing scheme in the ATLAS Inner Tracker, ensuring it remains dry and within humidity specifications during the High Luminosity upgrade.

Contribution

It provides a detailed CFD model to evaluate and optimize the nitrogen flushing scheme for the ATLAS Inner Tracker upgrade.

Findings

CFD model offers quantitative insights into humidity control.

Design modifications improve dryness and prevent condensation risks.

Operational and failure scenarios are thoroughly characterized.

Abstract

The planned High Luminosity upgrade to the Large Hadron Collider at CERN aims to increase the instantaneous luminosity peak to about 7.5 x 10^{34} cm^{-2}s^{-1}. The ATLAS detector will be extensively re-designed to meet the challenges of this upgrade. This paper focuses on the use of computational fluid dynamics to characterise the thermoflow in order to model the dry nitrogen flushing scheme in the Common Environmental Monitoring and Interlock System for the ATLAS Inner Tracker as part of the upgrade process. The Technical Design Report considers the possibility for the bi-phase CO2 coolant temperature to drop to as low as -55 degrees C in the case of a fault. The specification for the highest Relative Humidity within the ITk volume is therefore equivalent to a dew point temperature at or below -60 degrees C in order to prevent condensation which could damage the detector electronics. The design accommodates for humidity monitoring to detect the onset of such events and dry nitrogen flushing to remove moisture. Therefore, it is important to thoroughly understand all consequences of atmospheric air ingress due to air-leaks and/or air-ingress from the outlets due to the over-pressure. The computational fluid dynamics model presented in this study was used to provide quantitative and qualitative insight into the various operational and failure conditions, informing engineering design changes to optimise the flushing scheme and ensure that the ITk remains dry and within the design specification of the acceptable dew point range.