Transient behaviors of mixed convection in a square enclosure with an inner impulsively rotating circular cylinder

This work numerically investigated the unsteady mixed convective heat transfer between a square enclosure and an inner concentric circular cylinder maintained at different temperatures. The cylinder undergoes a transition of motion whose physics is significant for engineering applications such as cooling of rotating shaft and lubricating of journal bearing. It keeps stationary at first and steady-state natural convective flow is achieved, then impulsively rotates at a constant angular velocity and the flow arrives at a new final steady-state. Although the mixed convective heat transfer for rotating cylinder configurations are analyzed in a number of works, the transient behaviors of the thermal and flow characteristics from the onset of cylinder rotation to the final steady-state are not investigated, which restrains the thorough understandings on the relevant flow physics and mechanisms. The present work performs the first numerical investigation on the targeted physical problem. The objective is to explore the transient characteristics of the mixed convective heat transfer, and to identify the effects of cylinder rotation and rotation speed (tangential velocity at cylinder surface, V0) on the flow and heat transfer behaviors. The results are presented and analyzed by the temporal variations and spatial distributions of the thermal field, mean heat transfer rate, local heat transfer rate and velocity/temperature in certain regions where the flow is significantly affected. Our numerical results for the first time identify two flow regimes depending on the tangential velocity: mixed buoyancy-shear stress dominated flow for V0 ≤ 1.0 where the effect of natural convection is pronounced and the flow is driven by both buoyancy and shear stress from the cylinder surface, and shear stress dominated flow for V0 ≥ 2.0 where the flow is almost entirely driven by rotating cylinder and heat transfer is realized by conduction. The transient behaviors of the thermal and flow quantities are characterized and categorized based on the two regimes.