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ISBN : **978-1-56700-517-2 (Flash drive)**

5-6th Thermal and Fluids Engineering Conference (TFEC)

This study numerically investigates the laminar natural convection from differentially heated equal diameter cylinders, horizontally submerged in a viscoplastic fluid filled in a square enclosure having its top surface cooled whereas the remaining walls are insulated. The flow and heat transfer are assumed to be occurred in the steady, two-dimensional regime. The present results extend over the range of Rayleigh number, 10^{2} ≤ *Ra* ≤ 10^{6}, Prandtl number, 10 ≤ *Pr* ≤ 100, Bingham number, 0 ≤ *Bn* ≤ 100 for different vertical positions of the cylinders with respect to the center of the enclosure. In all, seven cases ranging from -0.25 ≤ *δ* ≤ 0.25, *δ* is defined as the distance from the center of the cylinder to the horizontal mid-plane of the enclosure (scaled using the side of the enclosure). The resulting flow and temperature fields are analysed in terms of streamlines, yielded/unyielded zones, isotherms and the average Nussetl number. For given values of *Ra, Pr, δ*, there exists a limiting Bingham number (*Bn*_{max}) above which heat transfer occurs solely by conduction and the value of *Bn*_{max} is strongly influenced by the values of *δ*, and the other dimensionless parameters. The average Nusselt number increases with Rayleigh number while it bears an inverse relation with Bingham and Prandtl numbers. On the other hand, the positioning of the two cylinders above or below the horizontal mid-plane adversely impacts heat transfer with reference to the central position. Finally, it is demonstrated that the onset of conduction-dominated regime occurs at a fixed value of (*Bn/Gr*^{-1/2})_{c} regardless of the values of *Ra* and *Pr*.

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