Wall-sheared thermal convection: heat transfer enhancement and turbulence relaminarization

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 $Ra = 10^{8}$ and fixed Prandtl number $Pr = 5.3$ , while the wall-shear Reynolds number ( $Re_{w}$ ) is in the range $60 \leqslant Re_{w} \leqslant 6000$ . 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 that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that 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 find that for larger $Re_{w}$ , the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.

Enhanced heat transfer and reduced flow reversals in turbulent thermal convection with an obstructed centre

“…While when the centre of the cell is obstructed, FRS is replaced by ASRS or SRS (depending on the diameter of cylindrical obstruction). It has been shown that the SRS is more efficient in heat transfer than the FRS (Xu et al 2020(Xu et al , 2023; thus this change of flow topology from FRS to ASRS/SRS leads to a prominent Ra heat transfer enhancement in the low-Ra regime. For high Ra, i.e.…”

“…This can be achieved through methods such as by introducing rough surfaces (Du & Tong 1998;Wei et al 2014;Xie & Xia 2017), inserting plates (Bao et al 2015;Liu & Huisman 2020), tilting the convection cell (Zwirner & Shishkina 2018;Zwirner et al 2020) or introducing a vibration (Wang, Zhou & Sun 2020). Moreover, heat transfer can also be enhanced through other means, such as introducing phase changes (Lakkaraju et al 2013;Wang, Mathai & Sun 2019), changing the properties of the working fluids (Buongiorno 2009;Benzi & Ching 2018) or shearing the boundaries (Xu, Xu & Xi 2023).…”

Unifying constitutive law of vibroconvective turbulence in microgravity

“…The emergence of a unified constitutive law is a hallmark of gravity-induced convective turbulence (Ahlers, Grossmann & Lohse 2009;Sreenivasan 2019;Wang, Mathai & Sun 2019;Chen, Wang & Xi 2020;Jiang et al 2020;Wang et al 2021;Li et al 2022;Zhao et al 2022;Ecke & Shishkina 2023), e.g. Nu ∼ Ra β with β ≈ 0.3 in the classical regime (Ahlers et al 2009;Huang & Zhou 2013;Xi et al 2016;Zhang, Zhou & Sun 2017;Plumley & Julien 2019;Iyer et al 2020;Ahlers et al 2022;Xu, Xu & Xi 2023;Li, Chen & Xi 2024) and β = 1/2 in the ultimate regime for paradigmatic Rayleigh-Bénard (RB) convection (Grossmann & Lohse 2011;He et al 2012;Toppaladoddi, Succi & Wettlaufer 2017;Lepot, Aumaître & Gallet 2018;Wang, Zhou & Sun 2020;Zou & Yang 2021;Jiang et al 2022), where the Nusselt number Nu quantifies the heat transport efficiency and the Rayleigh number Ra quantifies the strength of buoyancy forcing. However, in microgravity, as the gravity effect is, however, almost absent, gravity-induced convection becomes too feeble to transport matter and heat.…”