CO₂ SASS: A Modular Test Rig for the Scientific Assessment of Heat Transfer of Carbon Dioxide in the Supercritical State

Supercritical carbon dioxide (sCO2) is characterized by low viscosity and a peak in specific heat capacity near the pseudo--critical point, making it a promising coolant for microelectronics cooling. However, most existing sCO2 test rigs are designed for large-scale thermodynamic cycle studies and lack the capability for controlled, localized heat transfer measurements in small channels. This work presents CO2-SASS (Scientific Assessment of heat transfer in the Supercritical State), a modular, high-pressure test rig designed to measure local heat transfer coefficients and pressure drops in stainless-steel tubes with diameters in the order of 1-3 mm. The system provides independent control of pressure, mass flow and heating, with direct local wall and fluid temperature as well as precise absolute and differential pressure measurements. Particular emphasis is placed on high-accuracy temperature acquisition, including individual thermocouple calibration and cold-junction bias correction. A detailed uncertainty analysis highlights the dominant role of temperature measurement accuracy, especially for small wall-fluid temperature differences near the pseudo-critical point.