Heat flux enhancement by regular surface protrusion in partitioned thermal convection

Abstract

We investigate the influence of the regular roughness of heated and cooled plates and adiabatic partition boards on the mean heat transport in a square Rayleigh–Bénard (RB) convection enclosure by two-dimensional direct numerical simulations. The roughness is in the form of isothermal protrusions with a rectangular base and triangular tip. The protrusion height varies from 10% to 25% of enclosure height. With increased protrusion height, the large-scale circulation cannot wash out the cavity between two consecutive protrusions. Thus, the overall heat transport of the enclosure impedes. We have inserted the partition boards between two successive protrusions with a gap between the conduction plate and the partition board to wash out the cavity. The partition board height varies from 20% to 99.8% of enclosure height. We have performed the simulations for the range of Rayleigh number 106–108 and at a fixed Prandtl number of 1. The tip of the triangular protrusion acts as an active plume-emitting spot. We observe a single large-scale elliptical roll with counter-rotating corner rolls for small partition board height. With an increase in partition board height, an elliptical large-scale roll breaks down into the number of large-scale rolls horizontally placed one beside the other. Finally, we observe multiple rolls stacked vertically when the partition boards almost touch the conduction walls. Heat flux enhancement strongly depends on large-scale flow structures. We found a maximum heat flux enhancement in protrusion with partitioned RB case approximately up to 4.7 times the classical square RB for an optimal gap between conduction plate and partition board. The maximum heat transport enhancement is due to the strong horizontal flow through the gap between the conduction plate and partition board, which locally reduces the thermal boundary layer's thickness. The interaction between the horizontal jets and the thermal boundary layers enhances heat transport.