Condensation on a horizontal surface with periodic asymmetrical structures – transient film growth. (May 2017)

• Record Type: Journal Article
• Authors: Natesh, Shashank; Truong, Eric; Narayanan, Vinod; Bhavnani, Sushil
• Is Part Of: International journal of heat and mass transfer. Volume 108:Part A(2017)
• Page Start: 1126
• Page End: 1139
• Publication Date: 2017-05
• Subjects: Filmwise condensation -- Asymmetric microstructures -- Passive directional motion -- Phase change -- Numerical modeling; Heat -- Transmission -- Periodicals; Mass transfer -- Periodicals; Chaleur -- Transmission -- Périodiques; Transfert de masse -- Périodiques; Electronic journals; 621.4022
• DOI: 10.1016/j.ijheatmasstransfer.2016.12.086
• Languages: English
• ISSNs: 0017-9310

Abstract

Highlights: Filmwise condensation on horizontal surface with asymmetric structures is studied. A transient film rise within an asymmetric structure (ratchet) is characterized. Film growth on an asymmetric ratchet is compared against a symmetric ratchet. Numerical results are validated by visualization experiments of condensate growth. Condensate rises preferentially towards the steeper slope of the ratchet. Condensate rise rate increases with an increase in ratchet surface asymmetry. Condensate growth is a function of capillary number. Condensate growth rate is independent of Bond number for ratchet pitches < 2.4 mm. Surface heat transfer coefficient increases with increase in surface asymmetry. Surface heat transfer coefficient increases fourfold from pitch of 2.4 mm to 80 µm. Abstract: Condensation of a highly wetting dielectric fluid on asymmetric millimeter-sized ratchets is characterized. The asymmetry is provided by ratchets or triangular grooves on the surface. The overall hypothesis to be tested is whether surface asymmetry would result in a directional self-propelled motion of the condensate film. As a part of this overall goal, the initial transient rise of condensate film in a ratchet is presented in this paper. The approach involves a numerical characterization of the condensation process of the fluid on an asymmetrically-structured horizontal surface. A numerical model for condensation, based on the kinetic theory of gases, is implemented to simulate the growth characteristics of the condensate film in the ratchet. The model is validated with experimental data and a parametric study to evaluate the effects of the ratchet pitch, angle of asymmetry and orientation of gravity on the condensate growth is conducted. The model and experimental results indicate a preferential rise of the condensate towards the steeper slope of the asymmetric ratchet. Parametric studies connote that the condensate growth rate is (a) a function of the capillary number (b) increases as the angle of the ratchet is increased, and (c) is independent of the direction of gravity.