Experimental Investigation of the Transient Pool Boiling Heat Transfer on the Quenching of Vertical Rodlet in Water

Abstract

Quenching is an important phenomenon when the emergency core cooling system is put into operation during loss of coolant accident (LOCA) in a nuclear reactor. In this study, an experimental apparatus is designed and constructed with the purpose of conducting transient pool boiling experiments with quenching method for zirconium (Zr-4) cylindrical test samples. Three thermocouples are inserted in the test sample to investigate the effect of axial distance on the minimum film boiling temperature. The Zr-4 rodlet is heated up to a temperature well above the minimum film boiling temperature (up to 600 °C), and then plunged vertically in a quiescent pool of subcooled water. A data acquisition system is used to record the temperature history of the embedded thermocouples. Data reduction is performed by an inverse heat conduction code to calculate the surface temperature and corresponding surface heat flux. A visualization study is conducted to record the quench behavior of the test sample by using a high-speed camera. It is found that the minimum film boiling temperature decreases with the axial distance, while temperature at critical heat flux (CHF) is relatively insensitive to the axial distance. The film boiling heat transfer coefficient decreases with surface temperature, and seems to be independent of axial distance. The quench front is observed to originate from the bottom and move upward. It is found that the quench front velocity remains nearly constant in the lower region of the test sample, and significantly increases in the upper region.