An advance image processing technique is developed to quantitatively characterize the liquid-vapor interfacial waves, vapor layer thickness, minimum film boiling temperature (Tmin), quenching temperature (TQ), quenching time, and quench front velocity in the film boiling heat transfer regime. A facility was constructed with the purpose of performing high-temperature quenching of a simulated fuel rod in a near-saturated/subcooled water pool at atmospheric pressure. Inconel-600 tubular test sample with an outer diameter of 9.5 mm and a length of 25 cm was used. The test section has an embedded thermocouple that is connected to a data acquisition system for recording the temperature transients during quenching. An inverse heat conduction code was used to calculate the surface temperature and the corresponding heat flux. The latter was used to determine Tmin, which represents the minimum heat flux point on the boiling curve.

When a heated test section at a sufficiently high temperature plunges in a saturated or subcooled pool, a stable and continuous vapor layer is formed around it, preventing the liquid from being in a direct contact with the heated surface during film boiling. As the surface temperature of the rod gradually decreased, the vapor film starts to collapse at Tmin. Subsequently, the rod temperature dropped dramatically as the regime of heat transfer changed from transition boiling to nucleate boiling. Visualization of the boiling behavior was captured by a high-speed camera at a frame rate of 750 frames per second (fps) from which the vapor film thickness and the behavior of the liquid-vapor interface in the film boiling regime were analyzed frame by frame. The vapor-liquid interfacial waves as well as their temporal evolution are visualized for a range of wall superheat and various degrees of liquid subcooling. The thermocouple data and the taken videos are synchronized to couple the behavior of the vapor layer with the thermal behavior of the heated rod. Through the intensive image analyses, it was concluded that the vapor film thickness decreases contributing to a higher Tmin. Additionally, more oscillations of the vapor-liquid interface were found in the case of near-saturated pool. The quench front speed was observed to be constant for each subcooling.