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. The parameter ranges examined span Reynolds number based on gas superficial velocity up to 5300 and produced bubble with bond numbers from 1 to 270, and Weber numbers based on estimated terminal velocity from 0.6 to 150. The Morton number was fixed at 2.63x10-11\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2.63\times 10<^>{-11}$$\end{document}. As the flow with high gas volume fraction becomes optically opaque and is sensitive to intrusive instrumentation, a custom-built photon-counting dual energy threshold X-ray computed tomography system is employed for measurement of the time average void fraction within the bundle non-intrusively. Two dual plane wire mesh sensors upstream and downstream of the rod bundle are employed to obtain comparison void fraction, velocity profiles and bubble size distributions. Additionally, traditional pressure-based gas holdup measurements are employed to calculate time- and volume-averaged void fraction. The data show that the presence of superhydrophobic surface being present in the rod bundle results in a significantly lower gas volume fraction and higher localized void fraction at the superhydrophobic coating, as compared to those in a similar rod bundle without superhydrophobic internals.