An image processing technique for the study of vapor film dynamics during quenching of a downward facing hemisphere

Understanding two-phase heat transfer mechanisms on downward facing hemispherical vessels is crucial during external reactor vessel cooling (ERVC) under severe accident conditions. Film boiling could be the predominant heat transfer regime in the initial stages of quenching under these circumstances. In this work, the process of downward facing film boiling on the outer surface of a hemispherical vessel is studied using high-speed photography. This technique does not require any invasive sensors, which may alter the natural liquid-vapor interface in film boiling. Using a high-speed camera and several image processing techniques, more accurate measurements of film thickness have been made at different degrees of subcooling (0°C, 3°C, 5°C, and 10°C) and several angular locations (0°, 14°, 28°, and 42°) on a hemispherical vessel. With increased liquid subcooling and decreasing angular locations, the vapor film thickness has been found to decrease. Average film thicknesses at 0°C subcooling (resp. 5°C), one second after immersion were found to be approximately 2 mm (resp. 0.5 mm). High-speed videos (with frame rates as high as 650 fps) have shown significant oscillations at the liquid-vapor interface during film boiling. Additionally, oscillations in the film thickness and its wave characteristics have been analyzed at the prescribed angular locations and degrees of subcooling. From the visual data, insights can be made regarding the heat transfer behavior of film boiling; an attempt has been made to qualitatively relate the characteristics of oscillation waves to the vessel temperature.