Effect of Surface Oxidation on the Onset of Nucleate Boiling in a Materials Test Reactor Coolant Channel

Abstract

The onset of nucleate boiling (ONB) serves as the thermalhydraulic operating limit for many research and test reactors. However, boiling incipience under forced convection has not been wellcharacterized in narrow channel geometries or for oxidized surface conditions. This study presents experimental data for the ONB in vertical upflow of deionized (DI) water in a simulated materials test reactor (MTR) coolant channel. The channel gap thickness and aspect ratio were 1.96آ mm and 29∶1, respectively. Boiling surface conditions were carefully controlled and characterized, with both heavily oxidized and native oxide surfaces tested. Measurements were performed for mass fluxes ranging from 750 to 3000  kg/m2 s and for subcoolings ranging from 10 to 45آ°C. ONB was identified using a combination of highspeed visual observation, surface temperature measurements, and channel pressure drop measurements. Surface temperature measurements were found to be most reliable in identifying the ONB. For the nominal (native oxide) surface, results indicate that the correlation of Bergles and Rohsenow, when paired with the appropriate singlephase heat transfer correlation, adequately predicts the ONB heat flux. Incipience on the oxidized surface occurred at a higher heat flux and superheat than on the plain surface.