Multiphase flows

Particle-laden flow

Turbulent flow with particles or droplets can be found everywhere in nature and industry. Flows involving particles can be classified into different categories: one-way coupling and two-way coupling. In the case of one-way coupling, the particles are dispersed by the flow, but they have negligible effect on the flow. For two-way coupling, momentum exchange exists between the dispersed phase and the flow, because the particle size or mass/volume fraction is large. In this case, turbulence levels of the base flow can be decreased or increased by the particles (Figure 1).

Figure 1. Difference between one-way coupling and two-way coupling.

Preferential concentration is a phenomenon that occurs in the one-way coupling regime, where particles are inhomogeneously distributed in the carrier medium and form clusters in specific regions (Figure 2). The degree of preferential concentration varies with the particle Stokes number, and has been shown to be maximum when the Stokes number is close to unity. Preferential concentration is caused by centrifugal forces, which cause the particles to be slung out of eddies and collect in low-vorticity regions. It can also be explained by the sweep-stick mechanism, which causes particles to stick to low-acceleration points. This phenomenon has been investigated by dispersing inertial particles in our homogeneous isotropic turbulence (HIT) chamber. A laser-based measurement system visualizes the dispersed particles. The degree of preferential concentration can be quantified by the box counting method or Voronoi analysis.

Figure 2. Preferential concentration of particles in homogeneous isotropic turbulence (HIT) [1].

We are additionally investigating the two-way coupling phenomenon. Turbulence itself has very complex properties, and when particles are added to it, turbulence becomes even more difficult to analyze. Therefore, we are utilizing ideal homogeneous isotropic turbulence (HIT), which can be generated from eight woofer speakers as shown in Figure 3 (a). To characterize HIT, quantitative non-intrusive measurements are made with laser-based diagnostics such as particle image velocimetry (PIV), as shown in Figure 3 (b).

Figure 3. (a) HIT chamber with eight speakers [2]. (b) Instantaneous velocity and vorticity field.

When particles are dispersed into HIT, they can globally change the properties of the turbulence. Figure 4 (a) shows the isotropy ratio according to the particle mass loading. Heavy particles fall due to gravity and cause wakes behind them, which increase perturbations in the direction of gravity. Likewise, as shown in Figure 4 (b), the turbulent kinetic energy varies according to the change in the mass loading and particle Stokes number. When a large number of heavy particles are dispersed in the flow, the turbulent kinetic energy gradually decreases.

Since various properties of particles and turbulence have a complex effect on two-way coupling, no clear relationship or interpretation has been made. So as, to understand this fundamental phenomenon, we are conducting various experiments on the interaction between particles and turbulent flow.

Figure 4. Change in (a) isotropy ratio and (b) turbulent kinetic energy, according to particle mass loading [3].

[1] Han, K., Lee, H., & Hwang, W. (2020). Pseudo real-time continuous measurements of particle preferential concentration in homogeneous isotropic turbulence. Experimental Thermal and Fluid Science, 112, 109968.
[2] Lee, H., Han, K., Park, H., Jung, H., & Hwang, W. (2018). Generation and characterization of homogeneous isotropic turbulence. Journal of the Korean Society of Visualization 16(1), 21.
[3] Hwang, W., & Eaton, J. K. (2006). Homogeneous and isotropic turbulence modulation by small heavy (St ~ 50) particles. Journal of Fluid Mechanics, 564, 361.

Aerosol and droplet dispersion

In order to understand how infectious diseases such as COVID-19 are spread by respiratory droplets (Figure 5), the dynamics of droplet dispersion in the atmosphere needs to be understood. Droplet dispersion varies depending on the initial conditions, ambient environmental conditions, droplet chemical composition, and coupling between heat and mass transfer processes. For accurate quantitative analysis, it is necessary to construct a system in which these complex factors can be separately considered. We are currently focusing on the effects of environmental conditions, such as humidity, temperature, etc. An environmental chamber (Figure 6) is being utilized, which can control some of these parameters.

Figure 5. Droplet dispersion (image from Chris Rogers, Getty Images).

Figure 6. Environmental chamber.

Particle-droplet interaction

Collisions between liquid droplets and particles are easily found in nature and industry, such as atmospheric particulate matter (PM) adhering to rain drops, and wet scrubbers for particulate removal in gas streams. In previous studies, droplet collisions with stationary particles have been examined. We are taking this one step further and investigating the interaction between freely moving particles and droplets. We are examining the various adhesion/collision mechanisms of the particles and droplets, such as that shown in Figure 7. The goal of this research is to gain a deeper understanding of particle-droplet interaction in turbulent flow, and to improve the particle removal performance of applications such as wet scrubbers.

Figure 7. Mechanisms that affect the collection process of aerosol particles by water droplets [4].

[4] Ardon-Dryer, K., Huang, Y., & Cziczo, D. (2015). Laboratory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets on a single-droplet basis. Atmospheric Chemistry and Physics, 15(16), 9159.