Abstract:
In this work, the momentum and heat transfer characteristics of a time-dependent flow of
Bingham plastic fluid over a heated isothermal sphere have been investigated numerically
over wide ranges of conditions: Reynolds number, 5 ≤ Re ≤ 120, Bingham number, 0.1 ≤
Bn ≤ 100, Prandtl number, 0.7 ≤ Pr ≤ 100, frequency, π/4 ≤ ω∗ ≤ π and amplitude, 0 ≤
A ≤ 0.8. The influence of the fluid yield stress and inertia due to the imposed flow pulsations on the flow and thermal fields have been examined in detail. Detailed structure
of the flow and temperature fields are analysed in terms of the instantaneous streamlines,
isothermal contours, yielded/unyielded zones, surface pressure profiles, time-average drag
coefficient and surface- and time-average Nusselt number. The influence of the frequency
and amplitude of pulsations on the size of yielded (fluid-like) and unyielded (solid-like)
zones is considered in order to understand convective heat transport. The size of yielded
zones is seen to be in phase with the imposed pulsating velocity. However, the yield stress
effects suppress the influence of flow pulsations. The temporal evolution of the drag coefficient and Nusselt number lag the imposed pulsating flow by different degrees thereby
indicating the inherently different evolution of the momentum and thermal boundary layers. Broadly, the pulsating flow conditions may lead to up to 20% augmentation in heat
transfer provided there is a moderate degree of advection and/or when the fluid yield
stress effects are weak, i.e., small Bingham numbers. Thus, the maximum benefits of pulsating flow accrue in Newtonian fluids only. Finally, the present values of the time-average
Nusselt number have been consolidated in the form of a predictive expression
