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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

Visualization of droplet condensation in membrane distillation desalination with surface modification: hydrophilicity, hydrophobicity, and wicking spacers

David Martin Warsinger
Rohsenow Kendall Heat Transfer Laboratory, Department of Mechanical Engineering Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139-4307, USA

Jaichander Swaminathan
Rohsenow Kendall Heat Transfer Laboratory, Department of Mechanical Engineering Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139-4307, USA

Lucien L. Morales
Rohsenow Kendall Heat Transfer Laboratory, Department of Mechanical Engineering Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139-4307, USA

Margaret Bertoni
Rohsenow Kendall Heat Transfer Laboratory, Department of Mechanical Engineering Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139-4307, USA

John H. Lienhard V
Rohsenow Kendall Heat and Mass Transfer Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA

Abstract

Condensation performance is a key target for improving the energy efficiency of thermal desalination technologies such as air gap membrane distillation (AGMD). This study includes the first visualization of condensation in AGMD, through the use of a high conductivity, transparent sapphire condenser surface. The study examines how flow patterns are affected by several novel modifications, including varied surface hydrophobicity, module tilt angle, and gap spacer design. The experimental results were analyzed with numerical modeling. While the orientation of the mesh spacer, which holds the air gap apart, was found to have no substantial effect on the permeate production rate, the surface's hydrophobicity or hydrophilicity did result in different rates. The hydrophobic surface exhibited fewer droplets bridging the gap, more spherical droplets, and better droplet shedding. For gap sizes less than ~3 mm, the hydrophilic surface frequently had regions of water pinned around the surface itself and the plastic spacer. While the flow patterns observed were more complex than the film condensation typically used to model the process, the simplified numerical modelling yielded good agreement with the data when an adjustment factor was used to account for the gap size.

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