Settling of heated particles in homogeneous turbulence

Abstract

We study the case of inertial particles heated by thermal radiation while settling by gravity through a turbulent transparent gas. We consider dilute and optically thin regimes in which each particle receives the same heat flux. Numerical simulations of forced homogeneous turbulence are performed taking into account the two-way coupling of both momentum and temperature between the dispersed and continuous phases. Particles much smaller than the smallest flow scales are considered and the point-particle approximation is adopted. The particle Stokes number (based on the Kolmogorov time scale) is of order unity, while the nominal settling velocity is up to an order of magnitude larger than the Kolmogorov velocity, marking a critical difference with previous two-way coupled simulations. It is found that non-heated particles enhance turbulence when their settling velocity is sufficiently high compared to the Kolmogorov velocity. Energy spectra show that the non-heated particle settling impacts both the very small and very large flow scales, while the intermediate scales are weakly affected. When heated, particles shed plumes of buoyant gas, further modifying the turbulence structure. At the considered radiation intensities, clustering is strong but the classic mechanism of preferential concentration is modified, while preferential sweeping is eliminated or even reversed. Particle heating also causes a significant reduction of the mean settling velocity, which is caused by rising buoyant plumes in the vicinity of particle clusters. The turbulent kinetic energy is affected non-monotonically as the radiation intensity is increased due to the competing effects of the downward gravitational force and the upward buoyancy force. The thermal radiation influences all scales of the turbulence. The effects of settling and buoyancy on the turbulence anisotropy are also discussed.

Turbulent thermal convection driven by heated inertial particles

“…cluster and create hot plumes of upward moving fluid, a phenomenon also observed in settling of hot particles through forced turbulence (Frankel et al 2016). In figure 23 the particle temperature versus the Froude number is also plotted.…”

“…The small peak observed in the particle temperature around Fr ≈ 1 can be attributed to the fact that at this regime the mean vertical velocity of the particles is almost zero, leading to higher residence time of the particles in the plumes. In a recent study of DNS of hydrodynamically forced turbulence laden with heated inertial particles (Frankel et al 2016), it was shown that the buoyant plume shed by the clusters breaks down the preferential sweeping mechanism by which particle settle faster as they favour the downward side of turbulent eddies (Wang & Maxey 1993).…”

“…They also considered the sedimentation of a cluster of hot particles, and showed a decrease of settling velocity, as well as a faster breakdown of the cluster due to thermal plumes. Recently Frankel et al (2016) explored the two-way coupling between forced homogeneous turbulence and heated inertial particles, demonstrating large effects of buoyant plumes shed by the heated clusters on both settling velocity and turbulent kinetic energy. The work of Mercier et al (2014) also considers the interplay between thermal convection and heat exchange from a solid particle.…”

Effects of orbital shaking on settling velocity of mineral particles in aqueous solutions using split‐plot designs

“…In a turbulent flow, the sedimentation phenomenon does not depend only on characteristics of particles and fluid, but also on the Kolmogorov length and time scales which are related to the eddies (Fung ; Dejoan ; Frankel et al . ; Fornari et al . ).…”

“…The real situation for industrial thickeners and settling tanks involves complex interactions between particles, flocculation and turbulence that substantially alter from one case to another. In a turbulent flow, the sedimentation phenomenon does not depend only on characteristics of particles and fluid, but also on the Kolmogorov length and time scales which are related to the eddies (Fung 1998;Dejoan 2011;Frankel et al 2014;Fornari et al 2015a,b). Basically, particles smaller than the Kolmogorov length behave differently than those larger than this scale.…”