Observation of departure from nucleate boiling under flow using optical visualization and IR thermometry

Abstract

Departure from nucleate boiling (DNB) is a limiting phenomenon in the safety of nuclear reactors. Several mechanistic models have been suggested for predicting the critical heat flux at the DNB. However, existing experiments could not fully reveal the underlying phenomena of mechanistic models. In this study, we closely examined the phenomenon of rewetting failure of a dry patch during DNB to provide a deeper understanding of the mechanism of DNB occurrence and to assist in subsequent mechanistic modeling. Specifically, we utilized the total reflection technique and IR thermometry to obtain the synchronized side-view, total reflection view, temperature, and heat flux profile images at the DNB. We established a water flow loop under atmospheric pressure with a vertical rectangular test section comprising an ITO heater coated on a sapphire substrate. The ITO heater was 10 mm wide and 120 mm long. The long length of the ITO heater enabled the development of elongated bubbles upstream of the DNB location. A series of tests were conducted under various flow rates. Slug flow was observed in all tests conducted. An alternate passage of elongated bubbles and water slugs led to expansion and shrinkage of the dry patches. At the DNB, the dry patch size continued to increase, but the developing pattern was dependent on the flow rate. The peripheral temperature of the dry patches at the DNB was found to be ∼150°C.