Conjugate heat transfer in the unbounded flow of a viscoelastic fluid past a sphere

29 Apr 2020  ·  F. Pimenta, M. A. Alves ·

This work addresses the conjugate heat transfer of a simplified PTT fluid flowing past an unbounded sphere in the Stokes regime ($Re = 0.01$). The problem is numerically solved with the finite-volume method assuming axial symmetry, absence of natural convection and constant physical properties. The sphere generates heat at a constant and uniform rate, and the analysis is conducted for a range of Deborah ($0 \leq De \leq 100$), Prandtl ($10^0 \leq Pr \leq 10^5$), Brinkman ($0 \leq Br \leq 100$) and conductivity ratios ($0.1 \leq \kappa \leq 10$), in the presence or absence of thermal contact resistance at the solid-fluid interface. The drag coefficient shows a monotonic decrease with $De$, whereas the stresses on the sphere surface and in the wake first increase and then decrease with $De$. A negative wake was observed for the two solvent viscosity ratios tested ($\beta$ = 0.1 and 0.5), being more intense for the more elastic fluid. In the absence of viscous dissipation, the average Nusselt number starts to decrease with $De$ after an initial increase. Heat transfer enhancement relative to an equivalent Newtonian fluid was observed for the whole range of conditions tested. The temperature of the sphere decreases and becomes more homogeneous when its thermal conductivity increases in relation to the conductivity of the fluid, although small changes are observed in the Nusselt number. The thermal contact resistance at the interface increases the average temperature of the sphere, without affecting significantly the shape of the temperature profiles inside the sphere. When viscous dissipation is considered, significant changes are observed in the heat transfer process as $Br$ increases. Overall, a simplified PTT fluid can enhance heat transfer compared to a Newtonian fluid, but increasing $De$ does not necessarily improve heat exchange.

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