Effect of the gap size in the start-up free convective flow around a square prism near a wall

Flow visualizations and thermal measurements have been employed to analyse the transient free convective flow in the vicinity of a square prism (D × 2D × 2D) near a vertical heated wall for a modified Rayleigh number Ra* < 4.5 × 1010. The effect of changing the gap, G, between the prism and the wall from G = 0 (prism touching the wall) to G → ∞ (no prism at all) was investigated. It is shown that the reduction of the gap height G/D ⩽ 0.1 results in more complex flow patterns due to the increase of the adverse pressure gradient leading to the separation of the viscous layer downstream of the prism. Consequently, as well enhancement (G/D = 0) as degradation (G/D = 0.1) in the heat transfer performance occur in the prism wake. These phenomena are more pronounced in the early start-up of the flow.