Subcooled critical heat flux on laser-textured stainless-steel ribbon heaters in pool boiling of FC-72

Abstract

This study investigates critical heat flux (CHF) enhancement in pool boiling of FC-72 under saturated and subcooled conditions using laser-textured stainless-steel surfaces in combination with a hydrophobic coating. Multiple types of surfaces with capillary-length-spaced circular spots of different diameters are prepared with the spots being either untreated, hydrophobized or laser pretreated and hydrophobized. Pool boiling measurements are performed at atmospheric pressure on electrically heated ribbons. The CHF versus subcooling behavior is modeled using an established linear equation, while a modification into a second-degree polynomial is also tested to better account for nonlinear behavior of some surfaces. All developed surfaces are found to increase the CHF with enhancements ranging from 36 to 73% at saturation and 34–65% at 12 K of subcooling. Surfaces with mixed regions of low and high wettability generally exhibited the highest CHF enhancement at a 31% coverage with low wettability areas. Samples with laser-pretreated and hydrophobized spots exhibited the highest overall performance at saturation but a less steep trend of CHF versus liquid subcooling increase and CHF increase with increased superhydrophobic area ratio, indicating a different enhancement mechanism with hydrophobized microcavities presumably serving as preferential nucleation sites. Modeling of the CHF versus subcooling trend showed that (i) a second-degree polynomial equation is more appropriate for some surface types and (ii) the obtained constants differ significantly from surface to surface. Evaluation of a universal model based on all datapoints obtained within this study showed inadequate performance with error margins of ±13.5%. Consequently, the use of individual models for each type of enhanced surfaces with an overall accuracy of ±5.9% is recommended.