A Demonstrative Study on the Two-phase vs. Single-phase Modeling of Buoyancy-driven Flows of Enclosed Nanofluids

ABSTRACT

A demonstrative numerical study on natural convection of water-based nanofluids in square enclosures with different boundary conditions imposed at the walls, and different orientations with respect to the gravity vector, is performed using both the single-phase and the two-phase approaches, with the main scope to evaluate in what measure the single-phase approach fails in describing the basic heat and fluid flow features, as well as in determining the thermal performance of nanofluids. The system of the mass, momentum and energy transfer governing equations is solved by way of a computational code based on the SIMPLE-C algorithm. Empirical correlations are used for the calculation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient. The following configurations are investigated: a tilted cavity differentially-heated at two opposite walls; a vertical cavity partially-heated at the bottom wall and cooled at both sides; and a vertical cavity differentially-heated at the vertical and horizontal walls. It is found that the non-uniform distribution of the suspended solid phase throughout the enclosure gives rise to a solutal buoyancy force, whose competition with the thermal buoyancy force results in a periodic flow detectable only if the two-phase approach is applied. Moreover, the impact of the dispersion of the nanoparticles into the base liquid, which turns out to be notably higher at higher average temperatures, is found to be systematically underestimated by the single-phase approach.

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Notes on contributors

Marta Cianfrini

Marta Cianfrini is a post-doctoral student at Roma Tre University of Rome, Italy. She received her M.S. degree in Biomedical Engineering in 2009 and her Ph.D. in Energetics in 2014 from Sapienza University of Rome, Italy. Her main research interests include heat transfer and convection of pure fluids and nanofluids. She has co-authored more than 10 papers in archival journals and conference proceedings.

Massimo Corcione

Massimo Corcione is a Full Professor of Thermal Sciences and HVAC Systems at Sapienza University of Rome, Italy. He received his M.S. degree in Mechanical Engineering in 1990 and his Ph.D. in Thermophysical Properties of Materials in 1995 from Sapienza University of Rome, Italy. His current research interests include convection of pure fluids and mixtures, heat transfer in nanofluids, and optimal design of HVAC systems. He is a member of ANS (American Nano Society), ASME (American Society of Mechanical Engineers) and UIT (Unione Italiana di Termofluidodinamica). He has authored or co-authored more than 130 refereed journal and conference publications, besides contributing to a number of books. He is a member of editorial boards of severasl journals.

Alessandro Quintino

Alessandro Quintino is a post-doctoral student at Sapienza University of Rome, Italy. He received his M.S. degree in Aeronautical Engineering in 2000 and his Ph.D. in Thermal Sciences in 2011 from Sapienza University of Rome, Italy. His main research interests include convection of pure fluids and mixtures, heat transfer in nanofluids, and optimal design of HVAC systems. He has authored or co-authored more than 30 papers in archival journals and conference proceedings.

Elisa Ricci

Elisa Ricci is a Ph.D. student at Sapienza University of Rome, Italy. She received her M.S. degree in Biomedical Engineering in 2015, discussing a thesis on the effects of Brownian and thermophoretic diffusion in natural convection of enclosed nanofluids. Her research field is convection heat transfer of pure fluids, mixtures and nanofluids.