Numerical and experimental investigation of heat and mass transfer in rotating systems.

摘要

Heat and mass transfer processes in rotating cells are investigated numerically and experimentally. The experiments are also performed with gas evolution.; Numerical simulations are performed to study the velocity and temperature fields in detail. Based on the numerical results and scaling analysis, the basic flow structure and heat and mass transfer rate are discussed. There exist velocity boundary layers, called the Ekman layers, due to the Coriolis force. In addition, we have thermal boundary layers in the heat transfer experiment and solutal boundary layers in the mass transfer experiment. The flow can be classified into two different regimes depending on the ratio of the Ekman layer thickness to the thermal or solutal boundary layer thickness.; The Ekman suction driven convection regime occurs when the boundary layer ratio is less than unity, which is the case in the present heat transfer experiment. In this regime, the flow is very much suppressed by the Coriolis force. The thermal boundary layer thickness and the heat transfer rate are controlled by the Ekman suction flow. The computed Nusselt numbers agree well with the present experimental data. The experiment shows that the flow becomes oscillatory under certain conditions, probably due to the Coriolis force.; When the boundary layer ratio is greater than unity, we have the centrifugal buoyancy driven convection regime. This happens in the present mass transfer experiment with an electrochemical system. The basic characteristics of the flow are similar to those for buoyancy driven convection in rectangular enclosures, and the mass transfer rate is not affected by the Coriolis force. However, the Coriolis force is still important away from the solutal boundary layer. It is shown that unsteady secondary cells appear because of the Coriolis force.; The effect of gas evolution is investigated experimentally in the heat and mass transfer experiments. It is found that the heat transfer rate is not much affected by the gas evolution but the mass transfer is substantially increased by the bubbles.