Unified descriptor for enhanced critical heat flux during pool boiling of hemi-wicking surfaces

• Published in: International journal of heat and mass transfer 2022-02, Vol.183, p.122189, Article 122189
• Main Authors: Song, Youngsup, Zhang, Lenan, Díaz-Marín, Carlos D., Cruz, Samuel S., Wang, Evelyn N.
• Format: Article
• Language: English
• Subjects: Boiling; Capillary wicking; Critical heat flux; Heat flux; Heat transfer; Phase change heat transfer; Roughness; Thin films; Wettability
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2021.122189

•Experimentally demonstrated that pool boiling CHF enhancement on hemi-wicking surfaces cannot be explained by roughness or wickability alone.•Experimental results of systematically designed micropillar surfaces showed that CHF depends on both roughness and wickability.•Performed a scaling analysis to derive a relationship for CHF with a unified descriptor associated with thin film density and volumetric wicking rate.•Thin film density and volumetric wicking rate enhance CHF by accelerating bubble departure frequency and delaying the dry-out at bubble base.•CHF values from our experiments and literature data showed a positive linear correlation with the unified descriptor. Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.