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Heat Transfer Research
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ISSN Druckformat: 1064-2285
ISSN Online: 2162-6561

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Heat Transfer Research

DOI: 10.1615/HeatTransRes.2018020194
pages 685-702

THREE-DIMENSIONAL NATURAL CONVECTION AND ENTROPY GENERATION IN TALL RECTANGULAR ENCLOSURES FILLED WITH STRATIFIED NANOFLUID/AIR FLUIDS

Mahmoud Salari
Department of Mechanical Engineering, Imam Hossein University, Tehran, Iran
Abbas Kasaeipoor
Department of Mechanical Engineering, Imam Hossein University, Tehran, Iran
Emad Hasani Malekshah
Faculty of Engineering, Department of Mechanical Engineering, University of Isfahan, Hezar Jerib Avenue, Isfahan 81746-73441, Iran; Department of Mechanical Engineering, Imam Hossein University, Tehran, Iran

ABSTRAKT

Fluid flow, heat transfer, and volumetric entropy generation due to the three-dimensional natural convection within a tall rectangular enclosure filled with two immiscible/stratified fluids have been studied comprehensively as a simplified thermal model for each cell of lead–acid batteries. The stratified fluids consist of an MWCNT–SiO2 (15%–85%)/EG nanofluid at the bottom and air in the top region of the enclosure. The Navier–Stokes equations are solved based on a three-dimensional form, and finite volume approach is utilized. The boundary condition for the interface involve heat and mass transfer and shear stress. The heated side walls have a constant heat flux, the bottom and top parts of the side walls have a symmetry condition showing the existence of similar fluid flow in the neighbor cell. The top and bottom walls are cooled by environment temperature. The three-dimensional flow structure and temperature field are obtained and analyzed at mid-depth in a two-dimensional form. Different operating parameters such as the aspect ratio (12 < AR < 120), Rayleigh number (103 < Ra < 106), and the solid volume fraction (φ = 0.005–0.02) are considered with fluid flow, heat transfer, and volumetric entropy generation. The results show that the dominant heat transfer mechanism is conduction at the tall enclosures with a high aspect ratio. Moreover, the interface between the nanofluid and air phases is acting like an insulation medium banning the heat energy to escape from the nanofluid region to the top cold wall. The Nusselt number enhances with increasing Rayleigh number and solid volume fraction. Higher volumetric entropy generation occurs at higher Rayleigh number and lower aspect ratio and solid volume fraction.


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