A mechanistic model of critical heat flux for pool boiling based on supply failure mechanisms depending on the contact angle

Abstract

Experimental and theoretical analyses are conducted to develop a mechanistic model of critical heat flux (CHF) for application to pool boiling of fluids with contact angles between 0 degrees and 180 degrees. To accurately predict the CHF over the entire range of the contact angle, it is postulated that an unquenchable dry patch occurs when liquid fails to be supplied to the dry patch; A supply failure mechanism is classi-fied into two different mechanisms depending on the wettability of the boiling surface. The first is an evaporation mechanism which is dominant for non-wetting surfaces. This type occurs when the liquid supply rate in the liquid film due to the capillary force becomes lower than the evaporation rate. The second mechanism is a levitation mechanism which is dominant for wetting surfaces. This type occurs when the surface temperature under the dry patch exceeds the Leidenfrost point. To check the validity of the postulate, experiments involving the pool boiling of water and FC-72 are conducted on a titanium surface, which is a non-wetting surface for water but a wetting surface for FC-72. In the experiments, the temperature and heat flux distributions on the boiling surface are measured using infrared thermography for a quantitative characterization of boiling at the CHF. Experimental analyses confirm the postulate by showing that the CHF occurs due to the evaporation mechanism for the pool boiling of water, whereas the levitation mechanism for the pool boiling of FC-72. It is also observed that a large area of liquid film exists for the pool boiling of water, whereas this liquid film hardly exists for the pool boiling of FC-72. This observation indicates that the change in the dominant mechanism is due to the dependence of the liquid film area on the wettability of the boiling surface. After validating the postulate, a CHF correlation is developed that reflects the change of the supply failure mechanism depending on the contact angle. The developed correlation shows a good match to the extensive sets of CHF data previously obtained by various investigators to within 20%. (c) 2023 Elsevier Ltd. All rights reserved.