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Wall sheared thermal convection: heat transfer enhancement and turbulence relaminarization | | Ao Xu
; Ben-Rui Xu
; Heng-Dong Xi
; | | Date: |
1 Jan 2023 | | Abstract: | We studied the flow organization and heat transfer properties in
two-dimensional and three-dimensional Rayleigh-Bénard cells that are imposed
with different types of wall shear. The external wall shear is added with the
motivation of manipulating flow mode to control heat transfer efficiency. We
imposed three types of wall shear that may facilitate the single-roll, the
horizontally stacked double-roll, and the vertically stacked double-roll flow
modes, respectively. Direct numerical simulations are performed for fixed
Rayleigh number ($Ra$) of $Ra = 10^{8}$ and fixed Prandtl number ($Pr$) of $Pr
= 5.3$, while the wall shear Reynolds number ($Re_{w}$) is in the range of $60
le Re_{w} le 6000$. Generally, we found enhanced heat transfer efficiency and
global flow strength with the increase of $Re_{w}$. However, even with the same
magnitude of global flow strength, the heat transfer efficiency varies
significantly when the cells are under different types of wall shear. An
interesting finding is that by increasing the wall shear strength, the thermal
turbulence is relaminarized, and more surprisingly, the heat transfer
efficiency in the laminar state is higher than that in the turbulent state. We
found the enhanced heat transfer efficiency at the laminar regime is due to the
formation of more stable and stronger convection channels. We propose the
origin of thermal turbulence laminarization is the reduced amount of thermal
plumes. Because plumes are mainly responsible for turbulent kinetic energy
production, when the detached plumes are swept away by the wall shear, the
reduced number of plumes leads to weaker turbulent kinetic energy production.
We also quantify the efficiency of facilitating heat transport via external
shearing, and found that for larger $Re_{w}$, the enhanced heat transfer
efficiency comes at a price of a larger expenditure of mechanical energy. | | Source: | arXiv, 2301.00353 | | Services: | Forum | Review | PDF | Favorites | |
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