Particle transport and deposition in wall-sheared thermal turbulence

TL;DR

This study investigates how particles of different sizes behave and deposit in turbulent thermal convection with wall shear, revealing size-dependent transport patterns and developing a new model for particle settling.

Contribution

The paper provides new insights into particle transport in wall-sheared thermal turbulence and introduces a novel model for medium Stokes number particle settling.

Findings

Small particles circulate within large-scale rolls or near walls depending on flow dominance.

Medium particles show inhomogeneous distribution and preferential concentration.

The new model accurately predicts residence times for various particle sizes and flow conditions.

Abstract

We studied the transport and deposition behaviour of point particles in Rayleigh-B\'enard convection cells subjected to Couette-type wall shear. Direct numerical simulations (DNSs) are performed for Rayleigh number ($Ra$) in the range $10^7 \leq Ra \leq 10^9$ with a fixed Prandtl number $Pr = 0.71$, while the wall-shear Reynolds number ($Re_w$) is in the range $0 \leq Re_w \leq 12000$. With the increase of $Re_w$, the large-scale rolls expanded horizontally, evolving into zonal flow in two-dimensional simulations or streamwise-oriented rolls in three-dimensional simulations. We observed that, for particles with a small Stokes number ($St$), they either circulated within the large-scale rolls when buoyancy dominated or drifted near the walls when shear dominated. For medium $St$ particles, pronounced spatial inhomogeneity and preferential concentration were observed regardless of the prevailing flow state. For large $St$ particles, the turbulent flow structure had a minor influence on the particles' motion; although clustering still occurred, wall shear had a negligible influence compared with that for medium $St$ particles. We then presented the settling curves to quantify the particle deposition ratio on the walls. Our DNS results aligned well with previous theoretical predictions, which state that small $St$ particles settle with an exponential deposition ratio and large $St$ particles settle with a linear deposition ratio. For medium $St$ particles, where complex particle-turbulence interaction emerges, we developed a new model describing the settling process with an initial linear stage followed by a nonlinear stage. Unknown parameters in our model can be determined either by fitting the settling curves or using empirical relations. Compared with DNS results, our model also accurately predicts the average residence time across a wide range of $St$ for various $Re_w$.