Investigation of receding capillary flow on engineered surfaces and its effect on critical heat flux enhancement

Abstract

Enhancing critical heat flux (CHF) is beneficial for gaining greater departure from nucleate boiling ratio (DNBR), which indicates improved thermal safety margin of nuclear fuel claddings. In developing an accident tolerant fuel (ATF) cladding, the key concept is to achieve resistance capability against high-temperature oxidation. To realize this, conventional metal surfaces are engineered to be more suitable for mitigating the steam-metal chemical reaction as well as gaining thermal safety by enhancing the CHF. Depositing accident tolerant materials on the fuel cladding surface enables surface structures to be controlled in micro/nano scale, whose morphological change promotes a capillary wicking phenomenon inducing the enhanced CHF. Based on recent achievements for the development of ATF cladding surface in the aspect of CHF enhancement, largely three technical questions have remained unanswered to the presence; how much CHF enhancement is feasible within the roughness limit (typically < 0.3 μm in Ra) of the nuclear fuel cladding surface; how to characterize the capillary wicking phenomenon on the surface; and finally, how to predict separate effects of the micro/nano scale surface roughness and wettability on the CHF. Based on experimental (measurements of surface characteristics and CHF on 17 test surfaces) and modeling (correlating the surface parameters to CHF) efforts, this thesis answers those questions as follows. First of all, satisfying the roughness limit of nuclear fuel cladding surface while excluding porous structures, the CHF of the Cr-coated surface was enhanced by 80%. This significant enhancement was achievable by fabricating the hierarchical surface, for which Cr was coated on ground surfaces. Without the grinding process, Cr coating on a mirror surface slightly enhanced the CHF by 10%, which falls in a measurement error range considering thermal-hydraulic condition in nuclear core. To achieve meaningful CHF enhancement on the ATF cladding surface while satisfying essential features of the surface (e.g., not-porous and not-highly rough (< 0.3 μm in Ra)), the surface should include hierarchical structures even after surface coating. Second, this thesis introduced a new creative wicking experiment to characterize the capillary wicking phenomenon of receding capillary flow, which mimicked in-situ hydrodynamic behavior of a triple contact line during nucleation. The key idea was to consider an expansion pressure source, which plays a role of a vapor recoil pressure in triggering the CHF. This first-of-a-kind observation of the receding capillary flow facilitated the analysis of how the triple contact line receded from expanding dry area as the surface parameters change separately. Lastly, based on the understanding on the mechanism of the receding capillary flow, the wicking heat flux model was developed as a function of the arithmetic roughness height, nanoscale surface area ratio, and apparent contact angle, and thereby allowing the model to be easy-to-use. Without any adjusting constants, the present model mechanistically predicted the 60 CHF data included in 7 experimental groups (5 from literatures) within the error of ±20% showing the superior accuracy than the existing predictions based on wettability and roughness. In conclusion, the experimental results as well as developed CHF model are expected to contribute on elucidating the role of the surface parameters on the CHF within a practical roughness range of nuclear fuel cladding surface. In addition, this work will be useful for optimizing micro/nano scale design of surface structure for improved thermal safety of the ATF cladding and even for generic thermal applications demanding high heat flux boiling.