Influence of micro-structured superhydrophobic surfaces on nucleation and natural convection in a heated pool. (February 2019)

• Record Type: Journal Article
• Authors: Cowley, Adam; Maynes, Daniel; Crockett, Julie; Iverson, Brian D.
• Is Part Of: International journal of heat and mass transfer. Volume 129(2019)
• Page Start: 1095
• Page End: 1109
• Publication Date: 2019-02
• Subjects: Superhydrophobic -- Natural convection -- Heat transfer -- Mass transfer -- Nucleation; Heat -- Transmission -- Periodicals; Mass transfer -- Periodicals; Chaleur -- Transmission -- Périodiques; Transfert de masse -- Périodiques; Electronic journals; 621.4022
• DOI: 10.1016/j.ijheatmasstransfer.2018.10.030
• Languages: English
• ISSNs: 0017-9310

Abstract

Highlights: Superhydrophobic surfaces act as nucleation sites for mass transport. Nucleating bubble behavior depends on the underlying micro-structure. Large bubbles form and impede heat transfer. Abstract: This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobic surfaces. All surfaces tested were submerged in a 20 mm deep pool of water and heated from below to maintain a constant surface temperature, while the side walls of the pool were insulated, and the top was covered. Three thermocouples positioned in the pool obtain the average pool temperature. A heat flux sensor is placed directly beneath the surface to measure the heat flux supplied to the pool. Free convection heat transfer coefficients are obtained for the sub-boiling temperature range of 40–90 °C. Six surface types are studied: smooth hydrophilic, smooth hydrophobic, superhydrophobic with rib/cavity structures, superhydrophobic with rib/cavity structures and additional sparsely spaced ribs to close off the cavities, circular posts, and circular holes. It is found that structured superhydrophobic surfaces provide cavities for nucleation to occur. More dissolved air effervesces from the water as the surface temperature increases due to an increased level of supersaturation and convection. The nucleation leads to large air bubble formations that reduce the overall convection coefficient when compared to the smooth surfaces. For the rib/cavity structured surfaces, the bubbles form in an anisotropic manner and are aligned with the surface structure. More bubbles are observed on the superhydrophobic surfaces where the cavities are bounded. Since water's ability to dissolve air is dependent on temperature, heat and mass transfer cannot be treated independently on any of the superhydrophobic surfaces studied here.