International Journal of Heat and Mass Transfer 154 (2020) 119640 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/hmt Enhancing filmwise and dropwise condensation using a hybrid wettability contrast mechanism: Circular patterns Karim Egab a,c , Mohammed Alwazzan a , Benli Peng a , Saad K. Oudah a , Zongqi Guo b , Xianming Dai b , Jamil Khan a , Chen Li a,∗ a Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States b Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 75080, United States c Department of Electromechanical Systems Engineering, Southern Technical University, Iraq a r t i c l e i n f o Article history: Received 11 September 2019 Revised 7 February 2020 Accepted 10 March 2020 Keywords: Condensation Heat transfer Wettability contrast Hybrid Circular patterns Droplet Dynamics Copper Tubes a b s t r a c t Condensation surfaces using a wettability contrast mechanism is one of the effective methods to enhance heat transfer rate. The pattern design plays a critical role in realizing various contrast degrees of wet- tability. In this study, water vapor condensation of two series of hybrid circular-patterned designs were developed and experimentally characterized under the atmospheric pressure with the presence of non- condensable gasses (air). The outer surfaces of copper tubes in the horizontal orientation were used as the condensation surface with the designed patterns. The condensation heat transfer rates were com- pared to that of the complete dropwise condensation (DWC) and complete filmwise condensation (FWC), respectively. In the first series, the pattern design consists of hydrophobic (β ) circular patterns formed on a less-hydrophobic (γ ) background. The diameter of the patterns was varied to find an optimum con- figuration, which was observed with the pattern diameter and pattern distance at 1.5 mm and 0.5 mm, respectively. Significantly enhanced condensation rate was obtained compared to FWC. In the second se- ries, the design consists of γ -patterns contrasting with a β -background, which is an inverse design of the first series in terms of wetting conditions. A parametric study was conducted to examine the in- fluence of the pattern dimensions on the condensation. In the second series pattern designs, significant enhancements compared with both FWC and DWC were achieved. The optimal pattern was found to be at a diameter of 1.5 mm and gap of 1.5 mm, leading to a 79% higher heat transfer rate compared to that of the complete DWC. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Condensation is an essential phase change process found in countless numbers of applications. Increasing condensation rates through efficient use of modified condensing surfaces can reduce global energy consumption and create a greener environment. Thus, realizing efficient, durable, and cost-effective condensers be- come a central goal of condensation heat transfer. Today, numer- ous investigations have being developed to advance condensation. Despite the various approaches, the aim is always toward increas- ing the condensation mass and heat transfer rates by increasing droplets mobility and droplets shedding rates, which require ulti- ∗ Corresponding author. E-mail addresses: karimelectron@gmail.com (K. Egab), mohd.alwazzan@gmail.com (M. Alwazzan), pengbenli@gmail.com (B. Peng), soudah@email.sc.edu (S.K. Oudah), zxg161030@utdallas.edu (Z. Guo), Dai@utdallas.edu (X. Dai), khan@cec.sc.edu (J. Khan), LI01@cec.sc.edu (C. Li). mately modifying the condensation surfaces. Reducing the surface free energy is one of the most adapted approaches due to its effec- tiveness in promoting dropwise condensation (DWC), which can be up to 10 times higher than that of FWC in terms of heat transfer rates [1–9]. Nevertheless, the final condensation mode is governed by many factors such as surface free energy [10,11], surface features or roughness [12–17], testing conditions [18,19], and working fluid properties [20–22]. These factors eventually would alter the force interactions and the relationship between the liquid surface ten- sion and the free surface energy, which would eventually deter- mine the condensation mode (i.e., DWC, FWC, or mixed). Yet, the most common and challenging condensation conditions found in many practical applications was adapted herein, which is conden- sation of saturated water vapor under the atmospheric pressure with the presence of non-condensable gasses (air). Wettability contrast mechanism is another effective approach that leads to significant improvements of condensation rates, https://doi.org/10.1016/j.ijheatmasstransfer.2020.119640 0017-9310/© 2020 Elsevier Ltd. All rights reserved.