Effects of particle settling on Rayleigh-Bénard convection

Abstract

The effect of particles falling under gravity in a weakly turbulent Rayleigh-Bénard gas flow is studied numerically. The particle Stokes number is varied between 0.01 and 1 and their temperature is held fixed at the temperature of the cold plate, of the hot plate, or the mean between these values. Mechanical, thermal and combined mechanical and thermal couplings between the particles and the fluid are studied separately. It is shown that the mechanical coupling plays a greater and greater role in the increase of the Nusselt number with increasing particle size. A rather unexpected result is an unusual kind of "reverse one-way coupling", in the sense that the fluid is found to be strongly influenced by the particles, while the particles themselves appear to be little affected by the fluid, in spite of the relative smallness of the Stokes numbers. It is shown that this result derives from the very strong constraint on the fluid behavior imposed by the vanishing of the mean fluid vertical velocity over the cross sections of the cell demanded by continuity.

Analysis of centrifugal homogenization and its applications for emulsification & mechanical cell lysis

“…To demonstrate the capability of the homogenizer, we prepared emulsions consisting of small-sized droplets by centrifuging a mixture of oil, water, and surfactant (as described in Section 2.2) through the device. Though the results presented here are for an emulsification application, the trends observed are also relevant to other homogenizer applications such as cell lysis (as is demonstrated later in this paper), mixing of high-viscosity fluids, and breakup of aggregates [60,61].…”

Wind Reversal in Bubbly Natural Convection

“…In the colder region, it happens in exactly the opposite way. The thermal convection undergoes significant changes with phase transitions [1,2] and when polymers [3], bubbles [4,5], and particles [6] disperse in the continuous phase. The most relevant effect is the heat transfer enhancement of the multi-phase convection respect to the case of single phase convection.…”

“…In such a system, the dispersed phase interacts with the various temporal and spatial scales of the flow topologies according to the coupling of local thermal and mechanical interact between each particle and the surrounding fluid. The physics of particles and bubbles dispersion in convective flows is a key feature in understanding the turbulence dampening and the enhancement of the thermal convection [4][5][6].…”

Statistical properties of thermally expandable particles in soft-turbulence Rayleigh-Bénard convection

“…Particles were found to settle at the walls, depleting the RBC bulk flow of particles and forming a porous layer at the plates that eventually would cause a decrease of the heat transfer. In numerical studies particles are often prevented from getting stuck at the plates by neglecting gravity [3,12,35], by pointing gravity in the direction parallel to the walls [17,20] or by removing particles from the flow as soon as they reach one of the plates [15,23,22]. Here, the larger thermal expansion coefficient of particles alone ensures that particles eventually move away from the plates again.…”

“…The dynamics of thermally inertial particles (without thermal expansion) in RBC has already been studied numerically in the limit of bubbles (light particles) [14,15,23] and in the limit of particles which are heavier than the fluid [22]. In these studies a two-way coupling approach is used, i.e.…”