Augmentation of convective heat transfer by a torsionally-oscillating endwall disk

Abstract

An investigation is made of fluid flow and heat transfer characteristics in a vertically-mounted circular cylinder. Motions are generated by the top endwall disk, which oscillates about the central axis with rotation rate Ω = ελ cos(λt) . The temperature of the top disk is higher than that of the bottom disk, producing a stable stratification of Brunt-Vaisala frequency N. Numerical solutions are acquired to the time-dependent Navier-Stokes equations. Comprehensive velocity and temperature data are obtained, which illustrate salient features of quasi-steady periodic flows. As the stratification increases, the steady meridional streaming is confined to a narrow region close to the top disk. Resonance is identified at particular values of (N/λ), when the system is excited at correct natural frequencies. An elementary inviscid analysis indicates the modes of inertial-gravity oscillations, and the present numerical data are in close agreement with the inviscid results. The amplitudes of fluctuating parts of meridional flow and of Nusselt number display distinctive peaks under resonance conditions. Details of evolutions of fluctuating velocities and temperatures are scrutinized to offer physical explanations for resonance.