Visualization of the Thermal Flow Around a Thin Fin on the Hot Wall of a Differentially Heated Rectangular Cavity

The transient flow around a thin fin on the sidewall in a differentially heated cavity is visualized using a shadowgraph technique. The transition of the flow from sudden heating to a quasi-steady state may be classified into three stages: an early stage, a transitional stage and a quasi-steady stage. Such classifications are similar to those in the case with a smaller square fin on the sidewall (Xu et al., 2006). However, there are distinct differences of the flow features between the cases with a small square fin and a larger thin fin. In the early stage, a lower intrusion front forms below the fin which induces strong turbulent mixing downstream of the fin and is convected away. In the transitional stage, as the fluid in the cavity is stratified, the flow around the thin fin separates and oscillates. The oscillations break the double-layer structure of the downstream thermal boundary layer adjacent to the sidewall in the quasi-steady stage. It is found that the separation and oscillations of the flow around the thin fin are dependent on the geometry of the fin. The frequency of oscillations has a good linear correlation with the Rayleigh number, but is lower than that of unstable travelling waves in the thermal boundary layer for the case without a fin (approximately 1/2).