External convective jumping-droplet condensation on a flat plate

Patrick Birbarah, Nenad Miljkovic

Research output: Contribution to journalArticlepeer-review

Abstract

Water vapor condensation is vital to many natural and industrial processes such as building environmental control, power generation, and water desalination. Jumping-droplet condensation of water has recently been shown to have a 10X heat transfer enhancement compared to state-of-the-art filmwise condensation due to the removal of condensate at much smaller length scales (∼1 μm) than what is capable with gravitational shedding (∼1 mm). However, the efficient removal of jumping droplets can be limited by droplet return to the surface due to gravity, entrainment in bulk convective vapor flow, and entrainment in local condensing vapor flow. If used appropriately, convective condensation has the potential to entrain droplets, hence impeding their return to the surface. In this work, a comprehensive model of external convective jumping-droplet condensation on a superhydrophobic flat plate has been developed for constant heat flux boundary conditions. Boundary layer analysis was used to model the vapor flow over the external plate with condensation modeled as a vapor suction at the wall. The model was used to analyze the effects of jumping droplet size (1 < Rd < 100 μm), condensation heat flux (0 < q < 10 W/cm2), initial jumping location along the plate (10 cm < x0 < 5 m), free stream velocity (1 < Uf < 30 m/s), and plate inclination (0–360°), on droplet trajectory and overall heat transfer performance. Analysis of droplet trajectories revealed that the total distance traveled by jumping droplets along the plate ranged from millimeters to meters, while jumping heights were limited to less than a centimeter for the parameters considered. In addition, multiple-droplet coalescence, along with multi-jump droplet dynamics were analyzed, helping to explain a previously observed multi-hop process, and providing a potential pathway to generate larger effective trajectories along the plate. This work provides a comprehensive physical model of the external convective jumping-droplet condensation process, offers guidelines for the design of jumping-droplet systems to maximize heat transfer and minimize flooding, and develops a theoretical framework for the analysis of future convective jumping-droplet condensation processes important to more industrially relevant internal flow situations such as channels and tubes.

Original languageEnglish (US)
Pages (from-to)74-88
Number of pages15
JournalInternational Journal of Heat and Mass Transfer
Volume107
DOIs
StatePublished - Apr 1 2017

Keywords

  • Boundary layer
  • Condensation
  • Convective condensation
  • Flat plate
  • Forced convection
  • Jumping droplet
  • Suction
  • Superhydrophobic

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanical Engineering
  • Fluid Flow and Transfer Processes

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