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Annals of the Assembly for International Heat Transfer Conference 13

ISBN 1-56700-225-0 / CD 1-56700-226-9

Volumes per year:

various

Year 2006

DOI: 10.1615/IHTC13.p5

REFLECTIVITY BASED STUDY OF PENTANE EVAPORATION FROM A MENISCUS ON A 1μm THICK NANOPOROUS COATING

M. Ojha
The Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, USA

J. L. Plawsky
The Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, USA

Peter C. Wayner, Jr.
The Isermann Department of Chemical and Biological Engineering, Rensselaer, Polytechnic Institute, Troy, NY 12180-3590, USA

ABSTRACT

The interaction of a liquid with a nanoporous medium is a challenging problem that is critical to advanced heat transfer processes. In the presence of a heat flux, fluid transport in porous media is a coupled problem encompassing liquid evaporation from the pores, capillary flow inside the porous matrix as well as fluid transport at the surface. The objective of this work was to develop a reflectivity based technique to study the evaporation of a meniscus on a preformed nanoporous coating. Nanoporous silica (NPS) was used as the coating material and pentane as the fluid. A microscope was used to monitor the reflectivity of the drying sample at a high magnification. NPS has an interconnected pore structure with a pore size distribution range of 1−10 nm. Pentane completely wets the NPS coating (thickness, < 1 μm) and also fully saturates the pores. Evaporation of the pentane changes the reflectivity of the porous coating due to a change in the optical path length (n×d) through the coating. An effective medium based optical model was developed to determine the refractive index of the liquid filled porous coating (n_NL). The fraction of pentane in the pores was calculated from the effective refractive index and the drying characteristics of this system were studied as evaporation progressed. The behavior of an evaporating pentane sidewall meniscus on an NPS coating was also studied. Higher heat fluxes led to meniscus oscillations and streaming flows within the nanoporous coating. These flows were easily visualized due to changes in the coating's refractive index and reflectivity as the pores were alternately filled and drained.