Transient pool boiling heat transfer of oxidized and roughened Zircaloy-4 surfaces during water quenching

The sensitivity of surface oxidation and surface roughness to transient pool boiling heat transfer was investigated by performing water quenching experiments of Zircaloy-4 rodlets under increased pressure or subcooled water. The results demonstrated that quenching behavior was notably affected by the surface oxidation and the surface roughness regardless of environmental conditions (saturated water at 0.5 MPa or ΔTsub = 15 K). The minimum film boiling temperature increased with the thickness of surface oxide (3.6 ± 0.2 to 55.6 ± 2.2 µm). Rough surfaces (Ra = 11.4 ± 2.5 µm) showed a large surface heat flux with vigorous vapor generation even in the early stages of quenching. To explain the augmented quenching heat transfer by the surface modifications, a hypothesis that incorporates instantaneous heat transfer during liquid-solid contacts in the stable film boiling regime was proposed. The theoretical model with assumptions elucidated the bubble dynamics of the modified surfaces qualitatively and predicted minimum film boiling points depending on the degree of surface oxidation, which were in good agreement with experimental results. Both surface properties and parameters affecting liquid-solid contact are dominant factors in determining transient pool boiling heat transfer during water quenching.