A numerical study of blockage and inclination effects on natural convection in a uniformly heated air flow channel

Abstract

The present study is concerned with the flow behaviour and thermal performance of an air flow channel subject to uniform heating with an adiabatic circular cylinder symmetrically positioned in the channel. A two-dimensional numerical study is conducted, covering a range of blockage ratios ($\beta$, the ratio between the cylinder diameter to the channel width), inclination angles ($\phi$, relative to horizontal plane), cylinder positions ($h$, the distance from the inlet), and Rayleigh numbers ($Ra$, up to 6.0 × 10¹¹). It is observed that vortices shed from the cylinder interact with the thermal boundary layers (TBLs) adjacent to channel walls, which disturbs downstream TBLs and enhances mixing in the channel. For $Ra$ = 6.0 × 10¹¹, the averaged lateral wall temperature of a vertical channel ($\phi$ = 90°) drops by 31 % at $\beta$ = 0.50, and the mass flow rate through the channel increases by 40 % at $\beta$ = 0.25 compared to an unblocked vertical channel (i.e., without the cylinder). In a channel inclined at $\phi$ = 30°, up to 29 % reduction of the averaged wall temperature is achieved compared to an unblocked inclined channel at $Ra$ = 6.0 × 10¹¹. However, the inclination of the channel from the vertical position generally deteriorates its thermal performance. Moreover, at $\beta$ = 0.75 and $Ra$ = 6.0 × 10¹¹, the flow skews towards one lateral wall at $\phi$ = 30° and 90°, resulting in an irregular wake, but the flow structures are more symmetric at $\phi$ = 60°. The results reported here provide a passive strategy to design obstacles in convective flow channels for optimising thermal performance.