Gradient mixed wettability surfaces for enhancing heat transfer in dropwise flow condensation. (November 2021)

• Record Type: Journal Article
• Authors: Abbasiasl, Taher; Chehrghani, Mirvahid Mohammadpour; Sadaghiani, Abdolali Khalili; Koşar, Ali
• Is Part Of: International journal of heat and mass transfer. Volume 179(2021)
• Publication Date: 2021-11
• Subjects: Flow condensation -- Functional surface -- Wettability contrast -- Nanostructured surface -- Droplet dynamics -- Heat transfer enhancement -- Minichannel; Heat -- Transmission -- Periodicals; Mass transfer -- Periodicals; Chaleur -- Transmission -- Périodiques; Transfert de masse -- Périodiques; Electronic journals; 621.4022
• DOI: 10.1016/j.ijheatmasstransfer.2021.121664
• Languages: English
• ISSNs: 0017-9310

Abstract

Highlights: Dropwise flow condensation on gradient mixed wettability surfaces was investigated. A parametric study was conducted to present the optimum conditions at different steam mass fluxes. The heat transfer coefficient could be increased by 37% compared to a plain hydrophobic surface. A higher ratio of the hydrophobic area provided better conditions for droplet nucleation and growth. Larger superhydrophobic regions in the downstream section were advantageous for droplet removal and sweeping. Abstract: Steam condensation plays a pivotal role in a broad range of industrial applications. Dropwise condensation, which occurs on hydrophobic and superhydrophobic surfaces, has been proven to have a larger heat transfer coefficient than filmwise condensation because of enhanced droplet detachment. However, hydrophilic surfaces possess a better droplet nucleation rate compared to hydrophobic and superhydrophobic surfaces. These two characteristics (high droplet nucleation rate and better droplet removal) require counteracting properties. Many studies increased the nucleation rate by creating nanostructured surfaces. Wettability contrast mechanism is another approach, which could achieve both high nucleation rate and droplet removal, and have attracted much attention in enhancing a better heat transfer performance. In this study, gradient mixed wettability surfaces were developed on copper surfaces to improve heat transfer during steam flow condensation inside a minichannel. Circular hydrophobic islands were fabricated on a nanostructured superhydrophobic background surface. Various configurations of different island and pitch sizes are considered. Real-time images captured by a high-speed camera are used to examine the droplet dynamics and the influence of wettability on droplet size distribution. Thus, a parametric study is conducted to present the optimum conditions at different steam mass fluxes. According to the heat transfer analysis results, the heat transfer coefficient could be enhanced by 37% compared to a plain hydrophobic surface. A higher ratio of hydrophobic area provided better conditions for droplet nucleation and growth, while larger superhydrophobic regions in the downstream section was advantageous for droplet removal and sweeping. As a result, the role of mixed wettability in achieving the best performance was revealed for dropwise flow condensation.