Flow and Heat Transfer of a Stably Stratified Fluid Through an Enclosure

Abstract

The flow of a stably stratified fluid, or thermocline, through a cubical enclosure with: (a) strongly conducting; and (b) weakly conducting side-walls, was investigated experimentally. The flow Peclet number and enclosure aspect ratio were varied in the experiment. Flow visualization revealed buoyancy-affected motions in both side-wall configurations. During the charging period of an energy storage cycle, for the strongly conducting side-wall case, axial heat conduction along the side-wall induced an upward-directed buoyant force on the fluid adjacent to the wall in the lower (colder) part of the enclosure. This was in contrast to a downward-directed force on near-wall fluid in the weakly conducting side-wall case. However, the motions induced by the forces were relatively weak compared to the main flow, and in the weakly conducting wall case appeared to be confined to thin fluid layers adjacent to the enclosure side-walls. The cumulative influence of buoyant forces was most noticeable for long times, in flows with small Peclet number and conducting side-walls. Measurements of temperature and of the energy storage performance characteristics of the thermocline were made as a function of time. Predictions of these quantities based on the analytical solution of the transient, one-dimensional, energy conservation equation are in good agreement with the measurements.