BUOYANCY-AFFECTED FLOW AND HEAT-TRANSFER IN ASYMMETRICALLY HEATED ROTATING CAVITIES

Finite-volume predictions are presented for the convective heat transf er rates in a rotating cavity, formed by two corotating plane disks an d a peripheral shroud, and subjected to a radial outflow of cooling ai r. The heating of the disks is asymmetric, the air entering the cavity through a central hole in the cooler (upstream) disk. The predicted N usselt number distributions for each disk are compared with unpublishe d data from the University of Sussex for dimensionless mass-flow rates in the range 2800 less than or equal to C-w less than or equal to 14, 000 and rotational Reynolds numbers, Re-theta, up to 5.2 X 10(6). A si ngle-grid elliptic procedure was used with turbulent transport represe nted via a low-Reynolds-number k-epsilon model and the turbulence Pran dtl number concept. In comparing the predicted and measured convective . heat fluxes, it is important to consider the radiative heat exchange between the disks. This is estimated using a conventional view-factor approach based on black-body emission. Under conditions of asymmetric heating, rotationally induced buoyancy forces can exert significant e ffect an the flow structure, the induced motion tending to oppose that imposed by the radial outflow. Indeed, flow visualization studies hav e revealed that, as the rotational Reynolds number is increased (for a fixed value of C-w), the flow in the source region initially becomes oscillatory in nature, leading eventually to the onset of chaotic flow in which the usual Ekman layer structure does not persist in all angu lar planes. The extent to which the effects of such flow behavior can be captured by the steady, axisymmetric calculation approach used here is questionable, but it is found that the turbulence model (used prev iously for the prediction of heat transfer in symmetrically heated cav ities) still leads to good (+/-10 percent) predictive accuracy for the heated (downstream) disk. However, the predicted Nusselt numbers for the cooler (upstream) disk generally show little accord with experimen tal data, often signifying heat flow into the disk instead of vice ver sa. It is concluded that the modeling of the turbulent heat transport across the core region of the flow is erroneous, especially at high ro tational Reynolds numbers: This is attributed to overestimated turbule nce energy production in that region due to the action of the radial-c ircumferential component of shear stress (<(uw)over bar>). Adoption of an algebraic-stress model for this shear stress is partly successful in removing the discrepancies between prediction and experiment.