Study on a critical heat flux model for pool boiling based on two different mechanisms: rewetting limit and capillary limit

Abstract

In the current study, mechanisms behind the Departure from Nucleate Boiling (DNB) process are experimentally investigated, and a theoretical model for predicting Critical Heat Flux (CHF) is developed based on the investigation. Generally, when a heating element is immersed in stationary fluid, nucleate pool boiling occurs, accompanied by numerous bubbles on the surface of the heating element. However, as the surface heat flux reaches CHF, the DNB process occurs, and the surface temperature abruptly increases to about 1000℃. Therefore, to avoid the failure of heating elements, it is required to identify the cause of the DNB process and accurately predict CHF based on it. In this study, temperature and heat flux distributions on the boiling surface are measured using a high-speed IR camera to figure out the cause of the DNB process. Experiments are performed about FC-72 and water, and as a result, it is observed that the DNB process caused by two different mechanisms depending on the working fluid. Firstly, for the case of FC-72, the DNB process occurs when pre-existed dry patches merge into a vapor film as the temperature of the dry patches reaches the Leidenfrost temperature. On the other hand, for the case of water, the DNB process occurs when a local dry spot appears at the liquid film region under bubbles and expands to the entire surface. Based on the two different phenomena observed in the current study, two CHF mechanisms are suggested: Rewetting limit and Capillary limit. A theoretical model for predicting CHF is developed based on the two suggested mechanisms and validated using experimental data. Also, parametric analyses are conducted to investigate the effect of the surface conditions, such as the contact angle, the heater size, and the surface thermal conductivity, on CHF.