Study of CO2 evaporation in micro-channels for next-generation silicon detectors : experimental and numerical approaches

Because of its good thermo-physical properties and low global warming power (GWP), for certain scientific applications, CO2 is considered a suitable refrigerant for cooling electronic devices. The next generation of trace detectors at the LHC (CERN), scheduled for installation in 2027, will be cooled to temperatures between +10°C and -40°C, by evaporating liquid CO2 circulating in titanium mini-channels attached to 4cm² silicon PIXEL sensors. For the next generation, to be installed on the Future Circular Collider (FCC) by 2040, a new option explored by CERN is being studied by the LEGI-LAPP team. This innovative solution involves circulating CO2 through microchannels integrated into the silicon substrate directly behind the sensors, minimizing thermal interfaces. To achieve this, it is necessary to evaluate the thermal performance of this solution in order to validate this technological choice. As part of this thesis, several silicon/pyrex multichannel samples were produced in a cleanroom environment. These samples consist of an expansion zone (capillaries) connected to an evaporation zone (parallel microchannels). For saturation temperatures between -30°C and -40°C, mass velocities ranging from 250 kg/(m²s) to 1400 kg/(m²s), and vapor contents between 4% and 18%, the heat transfer coefficient between the silicon wall and the CO2 was evaluated for heat fluxes ranging from 12 kW/m² to 161 kW/m². The results show that the heat transfer coefficient is strongly degraded by increasing mass velocity, but remains relatively independent of heat flux density. Numerical 2D flow simulations using the Volume Of Fluid approach confirm this trend.