Enhanced Heat Transport in Turbulent Convection over a Rough Surface

Abstract

A novel convection experiment is conducted in a cell with rough upper and lower surfaces. The heat transport across the rough cell is found to be increased by more than 76%. Flow visualization and nearwall temperature measurements reveal new dynamics for the emission of thermal plumes. The discovery of the enhanced heat transport has important applications in engineering and atmospheric convection.[ S0031-9007(98) PACS numbers: 47.27. Te, 05.60. + w, 44.25. + f, 47.27.Nz Turbulent flows over a rough surface are ubiquitous in nature. An example is convection in the atmosphere and oceans, where the underlying surfaces are almost always rough. The study of turbulence over a rough surface is of fundamental interest for understanding the structure and dynamics of turbulent boundary layers and is also relevant to many practical applications, ranging from effective heat transfer for a reentry vehicle to turbulent drag reduction of a commercial aircraft. Our current knowledge about the roughness effect on turbulent flows comes largely from experiments in wind tunnels and other open systems [1], where the disturbance flow produced by a rough wall is confined in the near-wall region and is quickly discharged to the downstream. Because of these reasons the surface roughness usually does not perturb the turbulent bulk region very much, and its effect can often be described by rescaling the relevant parameters with the surface roughness height k [2]. This situation is changed completely for flows in a closed cell, in which the disturbances produced by the boundaries are inevitably mixed into the turbulent bulk region.Turbulent Rayleigh-Bénard convection is an example of such a closed system, which has attracted much attention in recent years [3]. Intermittent bursts of thermal plumes from the thermal boundary layers and the coherent largescale circulation, which modifies the boundary layer via its shear, are found to coexist in the convection cell. These salient features are directly related to the heat transport across the cell, and have been adopted in several theoretical models [3][4][5] to explain the observed scaling laws in the heat flux and temperature statistics [4,6]. In this Letter, we describe a novel convection experiment in a closed cell with rough upper and lower surfaces. It is found that when k becomes much larger than the thermal boundary layer thickness d, the heat transport across the rough cell is increased by more than 76%. Flow visualization and near-wall temperature measurements reveal that the large-scale circulation interacts with the surface roughness and produces more thermal plumes from the tip of the rough elements. The striking effect of the surface roughness provides new insights into the nature of convective turbulence, and also has a myriad of applications in engineering, geography, and meteorology.The experiment is conducted in a cylindrical cell filled with water. Details about the apparatus have been described elsewhere [7], and here we mention only some key points. The rough upper and...

Boundary layer structure in a rough Rayleigh–Bénard cell filled with air

“…Though it does not always produce a change in γ , it still yields enhancement of plume emissions and a 20-76 % increase for Nu. This is larger than the increase due to the increase in heating area caused by the roughness (Shen, Tong & Xia 1996;Du & Tong 1998Qiu, Xia & Tong 2005). The result has been extended recently to the case where roughness is added on one plate only (Wei et al 2014).…”

Enhanced heat transport in partitioned thermal convection

“…Another important problem that has been carefully investigated is to find out ways to enhance heat transport through RBC, which is particularly useful in many industrial processes and is of fundamental interest. Methods have been proposed to achieve high heat flux in modified RBC systems, such as by creating roughness on the conducting plates (Du & Tong 1998), imposing pulsed heating power on the lower plate (Jin & Xia 2008), rotating the convection cell about a vertical axis (King et al 2009;Zhong et al 2009b), employing multiphase working fluids (Zhong, Funfschilling & Ahlers 2009a;Biferale et al 2012;Lakkaraju et al 2013) and performing lateral confinement of the turbulent flows . In this paper, we present, both experimentally and numerically, a novel and yet simple method that significantly enhances heat transfer.…”

Thermal transfer in Rayleigh–Bénard cell with smooth or rough boundaries

“…Tisserand et al (2011), Roche et al (2001a), Qiu et al (2005) and, in some configurations, Wei et al (2014) observe an increase of the scaling law exponent a: before the transition to Nusselt enhancement, a is close to 2/7 or 1/3 then it increases and reaches nearly 1/2. In several other configurations, the exponent a is unchanged but the prefactor C increases (Du & Tong, 1998;Wei et al, 2014).…”

“…Several geometries of structure are used such as square pyramids, Du & Tong (1998), Du & Tong (2000) for example, pyramidal grooves (Roche et al, 2001a), spheres (Ciliberto & Laroche, 1999) or square structures (Tisserand et al, 2011). Among these experiments, the one of Ciliberto and Laroche is particular because the roughness elements are glass spheres coated with copper varnish, so they can be considered as thermally insulating the plate.…”

“…Roughness can be added on only one of the horizontal plates (Ciliberto & Laroche, 1999;Tisserand et al, 2011;Wei et al, 2014), or on both plates (Du & Tong, 1998, 2000Qiu et al, 2005), or even on the entire cell (Roche et al, 2001a). Several geometries of structure are used such as square pyramids, Du & Tong (1998), Du & Tong (2000) for example, pyramidal grooves (Roche et al, 2001a), spheres (Ciliberto & Laroche, 1999) or square structures (Tisserand et al, 2011).…”