Convective heat transfer over ratchet surfaces in vertical natural convection

We report on a combined experimental and numerical study on convective heat transfer over ratchet surfaces in vertical natural convection (VC). Due to the asymmetry of the convection system caused by the asymmetric ratchet-like wall roughness, two distinct states exist with different orientations of the large-scale circulation roll and different heat transport capacities. The statistical analysis shows that the heat transport efficiency depends on the strength of the large-scale recirculation. We observe that, when the large-scale wind sweeps the ratchets along the smaller slopes, the convective roll is stronger and the heat transport larger than the case in which the mean flow has the opposite direction. The result on the heat transfer differs from standard Rayleigh-B\'enard convection (RBC) where the heat transfer is stronger in the second configuration. We show that the reason for the opposite behavior of VC as compared to RBC is that the flow is more turbulent in RBC than that in VC at the same Ra. This work helps to understand the mechanisms of heat transport in turbulent thermal convection with asymmetric roughness and sheds new light on heat transport enhancement and flow control in engineering.