UC Berkeley

This thesis addresses the influence of superhydrophobic material on bubble dynamics within a bubble column and simulated nuclear fuel cell channels through the use of X-ray Computed Tomography and iterative reconstruction algorithms. Pressurized water reactors operate in a single-phase flow. Localized nucleation sites arise to promote convectional heat transfer to the bulk liquid. During an event of flow loss or any condition that can result in bulk liquidtemperatures exceeding saturation temperatures, bubbles will form more frequently, creating a boiling casualty called Departure from Nucleate Boiling. A Departure from Nucleate Boiling condition results from the bulk liquid temperature reaching and exceeding Critical Heat Flux. If a superhydrophobic-coated material is introduced into a fuel cell assembly, the vapor bubbles will show an affinity to the air layer in these coatings and therefore escape faster from the flow regime, resulting in a larger margin to reach Critical Heat Flux. This evaluation is made based on our findings using only an air and liquid mixture with similar characterization to vapor bubbles that would be found in a saturated system. Additionally, bubble column reactors rely on bubbly flow to promote mixing to meet chemical requirements for their applications. Superhydrophobic internals may modify the bubble regime to promote larger mixing with minimal changes in superficial gas velocity. The bubble dynamics that arise from dispersed bubble flow to slug flow can be measured by its void fraction. A superhydrophobic surface will enable bubbles to escape faster to the surface and therefore result in a void fraction value that remains lower as compared to void fraction conditions without superhydrophobic coated internals. With higher multiphase flow rates, these techniques become complex and difficult to measure. Advanced measurement techniques are required to measure void fraction in these partially opaque systems. Our findings suggests that lower void fractions are achievable with a modification of surface wall characteristics to promote superhydrophobicity. Additionally, superhydrophobic coatings do promote a transition to churn-turbulent flow without any modification to superficial gas velocity or bubble column dimensions. We show how geometric constraints, varied by adjusting the rod configuration, also play a role in localized gas holdup but not as substantial as replacing a single fuel rod with a superhydrophobic-coated rod. Lastly, advanced measurement techniques, such as X-ray computed tomography, have proven effective at providing a non-intrusive and spatial and temporal resolution of void fraction measurements in dynamic multiphase flow systems.