Colloquia

Samenvatting

In this thesis, the cooling behavior of carbon dioxide in its supercritical state flowing through a tube subjected to moderate and extreme heat fluxes is numerically investigated. The extreme heat flux scenario is particularly relevant for nuclear fusion reactors, which experience heat loads on the order of 10 MW/m2 during operation and up to 20 MW/m2 during shut-down transients. Like many thermal systems, fusion reactors are currently cooled using water as a coolant. However, given the superior thermophysical properties of supercritical CO2 (sCO2) under certain conditions compared to water, and its chemical inertness, this thesis proposes sCO2 as a potential alternative coolant. The study first examines the feasibility of simulating the so-called pseudo-boiling phenomenon, where sCO2 transitions from a liquid-like to a gas-like state under moderate heat fluxes at supercritical pressure. This is demonstrated by qualitatively comparing simulation results with experimental and numerical data available in the literature. Subsequently, the flow of sCO2 is simulated in a geometry specific to fusion reactors, namely a tube with an externally mounted tungsten monoblock that is exposed to extreme heat loads of 10 MW/m2 and 20 MW/m2. The numerical results indicate that, for similar pumping power as water, sCO2 exhibits superior thermal performance compared to water. Specifically, the maximum temperature of the plasma facing surface of the tungsten block is significantly lower when sCO2 is used as the coolant. This improved performance is attributed to the high heat transfer coefficients associated with pseudo-boiling transitions of sCO2 in the flow channel beneath the heated monoblock, which is a mechanism absent in the case of single-phase water cooling. By analyzing the predicted temperature, density, and velocity profiles, further physical insights are gained into the buoyancy effects that lead to the accumulation of gas-like CO2 near the top inner wall of the tube, which causes heat transfer deterioration for the shut-down transient of 20 MW/m2. Recommendations are made to mitigate this effect, particularly by increasing the inlet velocity and introducing modifications to the inner surface of the tubing.