Near-critical-point Krypton ejector cooling cycle for ultralow temperature detector cooling at CERN

The increasing concern on the use of synthetic fluids in heat pumps and refrigeration applications have been increasing considerably over the last twenty years. Decarbonization of the industrial and commercial sector via the use of natural working fluids is by far one of the most effective measures to tackling global warming. However, this policy was also extended to international organizations such as CERN (European Organization for Nuclear Energy) which is highly energy intensive. In 2008, detector cooling systems at CERN were based on single-phase pumped loop and oilfree vapor compression cycle using perfluorocarbons (C6F14, C3F8). With the ongoing advancement of silicon detector technologies, which demanded stringent mass minimization within the detector volume, several alternative solutions were explored. CO₂ was identified as a radiation-hard coolant able to maximize the heat removal capability while minimizing the size of hardware inside the detectors, feature extremely appreciated for low-mass detectors. The developed CO₂ system, named 2PACL (two-phase accumulator pumped loop) has proven to be highly reliable and controllable. However, its efficiency, expressed as the ability of the coolant to evacuate the heat generated by the sensor and adjacent electronics via an isothermal process, deteriorates when approaching the low temperatures operation around -45°C. This reduction in efficiency is primarily attributed to the decreased evaporating pressure at such temperatures. In 2018, the primary system responsible for transferring the heat from the 2PACL unit to the external environment, was transitioned from HFC-based solution (R404A) to CO₂. Since then, CERN supports the development and implementation of green technologies as the only system alternative to be used within the upgrades of the different subdetectors. The future upgrade of the detectors under the HL-LHC program, which is necessary to increase the amount of data and the foreseen collision rates, will require highly-effective and low mass detector cooling down to temperatures around -80/-90 °C due to the higher radiation levels. Superior performance given by high working pressure fluids is essential for the thermal management of future detectors. To investigate new efficient coolants for the future domain of operation, a sub-project was launched. In this respect, as first part of this thesis, a preliminary study targeting natural working fluids points out krypton as a promising cooling agent. Silicon detector trackers are expensive and fragile components demanding very harsh requirement to the cooling system, mainly: high system’s stability to provide an accurate spatial and temporal thermal stability along the entire detector structure, remote control due to the harmful radioactivity level near the detectors, low mass contribution of the cooling system with respect the overall mass installed around the detector. All these requirements, together with the lack of any theoretical-experimental data on the use of krypton as refrigerant, pose a number of challenges. To address those challenges, an innovative ejector-driven cooling system able to cope with the demanding requirements of the detectors was designed, although the limited project budget constrained both the development and the use of suitable components. The comprehensive research program included the development of a test facility to study the dynamic of the cycle starting from room temperature down to -80°C, with particular emphasis on the system controllability and gradual cooldown of the detector. The setup allows for testing the gas, supercritical and two-phase state of krypton while providing measurements of the significant parameters necessary for tuning the numerical models used for sizing of the ejectors, expansions devices and heat exchangers. All the different transient modes, comprising the system startup, cooldown, supercritical and two-phase operation have both been modelled and tested experimentally. Design guidelines were suggested accordingly to the future operating range, mainly in terms of minimum operating temperature.