Effect of Superhydrophobic Surfaces on Rod Bundle Flow Dynamics

Pressurized water reactors are designed to operate in a single-phase flow. However, during a flow loss or other off-design conditions liquid temperature

may exceed saturation temperature and, if a continuous film of gas forms, a boiling casualty may result. If superhydrophobic surfaces are introduced

among the fuel cell assembly, vapor bubbles show an affinity to these surfaces and gas may coalesce to escape faster resulting in a larger margin to reach

Critical Heat Flux. In the present study we consider air and liquid water mixture examining the overall flow dynamics in a case with no bulk liquid

flow, reminiscent of a case with coolant pump failure. As the flow with high gas volume fraction becomes optically opaque and is sensitive to intrusive

instrumentation, a custom build photon-counting dual energy threshold X-ray computed tomography system is employed for measurement of the time

average phase fraction within the bundle non-intrusively. Two dual-plane wire mesh sensors upstream and downstream of the rod bundle are employed

to obtain comparison phase fraction, velocity profiles and bubble size distributions. Additionally, traditional pressure-based gas holdup measurements

are employed to calculate time and volume averaged phase fraction. The data shows that a superhydrophobic surface being present in the rod bundle

results in a significantly lower gas volume fraction compared to those in a similar rod bundle without superhydrophobic internals.