Convective heat transfer along ratchet surfaces in vertical natural convection

Abstract

We report on a combined experimental and numerical study of convective heat transfer along 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 markedly different orientations of the large-scale circulation roll (LSCR) and different heat transport efficiencies. Statistical analysis shows that the heat transport efficiency depends on the strength of the LSCR. When a large-scale wind flows along the ratchets in the direction of their smaller slopes, the convection roll is stronger and the heat transport is larger than the case in which the large-scale wind is directed towards the steeper slope side of the ratchets. Further analysis of the time-averaged temperature profiles indicates that the stronger LSCR in the former case triggers the formation of a secondary vortex inside the roughness cavity, which promotes fluid mixing and results in a higher heat transport efficiency. Remarkably, this result differs from classical Rayleigh–Bénard convection (RBC) with asymmetric ratchets (Jiang et al., Phys. Rev. Lett., vol. 120, 2018, 044501), wherein the heat transfer is stronger when the large-scale wind faces the steeper side of the ratchets. We reveal that the reason for the reversed trend for VC as compared to RBC is that the flow is less turbulent in VC at the same $Ra$ . Thus, in VC the heat transport is driven primarily by the coherent LSCR, while in RBC the ejected thermal plumes aided by gravity are the essential carrier of heat. The present work provides opportunities for control of heat transport in engineering and geophysical flows.

Numerical simulation of a mixed vertical ventilation system

“…where Prandtel number (Pr = ) is assumed to be Pr = 1 for dry air. Therefore one partial elliptical differential Equation (7)…”

“…Experimental studies along with numerical estimations of ventilation room have been performed by [10], [21] and [23]. The cases of 3D cavity with heat source have been performed by [4,7]. Finally, more information about CFD numerical discussion can be found in [12,19,24].…”

Mean velocity and temperature profiles in turbulent vertical convection

“…However, it is recognised widely that the mean velocity profile in wall shear flows cannot be applied to describe vertical convection (VC), due to the effect of buoyancy (Ke et al 2020). The VC is the buoyancy-driven flow along a vertical heated wall, and it usually includes three configurations (Hölling & Herwig 2005;Howland et al 2022): (a) a single heated vertical plate immersed in an ambient fluid (Tsuji & Nagano 1988a,b;Wells & Worster 2008;Ke et al 2020), abbreviated as 'plate VC' hereafter; (b) a closed cavity with two opposite sidewalls heated at different temperatures (Shishkina 2016;Jiang et al 2019;Wang et al 2021;Zwirner et al 2022), abbreviated as 'cavity VC' hereafter; (c) a vertical infinite channel with periodic boundary conditions in the wall-parallel directions (Pallares et al 2010;Ng et al 2013Ng et al , 2015Howland et al 2022;Ching 2023), abbreviated as 'channel VC' hereafter. This study focuses on the last configuration of VC, as shown in figure 1(a).…”

Natural convection and pattern interaction in a two-dimensional vertical slot

“…Further work has examined the modifications created by ratchet surfaces (Jiang et al. 2019). All these studies typically involved bounding surfaces kept at prescribed uniform temperatures.…”