Research: Multiphase Flows
Compressible Multiphase Flow  ♦  Cluster-Induced Turbulence  ♦  Electrically-Charged Particles  ♦  Reactive Gas-Solid Flows

Dispersed multiphase flows (solid particles suspended in a carrier phase, liquid droplets in gas, or gaseous bubbles in liquid) are commonly found in many engineering applications, and are often turbulent. These flows are ubiquitous in a range of energy conversion processes, and typically control device efficiencies and overall operating conditions. However, the complexities that arise from the chaotic nature of the background turbulent flow, coupled with a random assembly of interacting particles, makes understanding and predicting such flows very difficult. A main focus of our research involves developing robust and scalable numerical tools to investigate the multiphysics/multiscale phenomena in various flow conditions. Below are several examples of multiphase flow applications with both engineering and science objectives.


Compressible Multiphase Flow

Background
Compressible multiphase flows exhibit a wide range of interesting phenomena that play important roles in engineering and science. Our group focuses on developing methods capable of accurately capturing the interactions between a collection of particles (e.g., solid particles or liquid droplets) and shock waves in compressible viscous flows. Here we apply these methods to understand the role of droplet-turbulence interactions on jet noise, in addition to fundamental instabilities that form as shock waves pass through a curtain of particles.

Compressible multiphase flow calculations. (A) 25 M particles in a Mach 2.5 shear layer, (B) particle-laden shock tube, and (C) Sod shock tube demonstrating shock-capturing capability.


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Cluster-Induced Turbulence

Background
Cluster-induced turbulence (CIT) occurs in fluid-particle flows when (i) the mass loading, defined by the ratio of the specific masses of the particle and fluid phases, is order one or larger and (ii) the difference between the mean phase velocities is non-zero. In statistically stationary flows, fluctuations in particle concentration can generate and sustain fluid-phase turbulence even in the absence of mean shear, referred to as fully-developed CIT. Because the density ratio of the two phases is very large in gas-particle flows, CIT is ubiquitous in practical engineering and environmental flows when body forces or inlet conditions generate a mean velocity difference, such as the gravity-driven flow shown in the figure to the right. Surprisingly, despite its importance, very little is known about the fundamental properties of CIT.

Simulation results from fully-developed CIT show that momentum coupling between the two phases leads to significant differences from the behavior observed in very dilute systems with one-way coupling. In particular, entrainment of the fluid phase by clusters results in an increased mean particle velocity that generates a drag production term for fluid-phase turbulent kinetic energy that is highly anisotropic. Using a novel filtering technique, the local instantaneous granular temperature can be separated from the phase-average particle-phase turbulent kinetic energy. Observations from the simulations indicate that high values of the granular temperature correspond to regions in front of falling clusters where the particle-phase velocity is strongly compressed in the vertical direction, referred to as compressive heating.

Eulerian-Lagrangian simulation of gravity-driven fully-developed CIT. Time evolution of particle positions (left) and fluid velocity (right).

Relevant papers
  • Effect of domain size on fluid-particle statistics in homogeneous gravity-driven cluster-induced turbulence (2016) Journal of Fluids Engineering DOI
  • On fluid-particle dynamics in fully-developed cluster-induced turbulence (2015) Journal of Fluid Mechanics DOI
  • Mass loading effects on turbulence modulation by particle clustering in dilute and moderately dilute channel flows (2014) Journal of Fluids Engineering DOI
  • Numerical study of collisional particle dynamics in cluster-induced turbulence (2014) Journal of Fluid Mechanics DOI

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    Electrically-Charged Particles

    Background
    Non-trivial interactions between electrically-charged particles and turbulence can have important effects in many engineering and environmental flows, including atmospheric clouds, colloids, electrosprays, and protoplanetary nebula (dusty plasma). Due to the long-range nature of electrostatic forces, properly accounting for Coulomb interactions in systems with many particles must be handled carefully for accurate predictions that avoid N^2 computations. The images to the right show particles settling under gravity in a two-dimensional periodic flow. Momentum coupling with the background fluid leads to significant clustering of the particle phase. In the presence of an electric field, Coulomb repulsion homogenizes the system.

    In spray combustion systems, clustering of droplets delays the evaporation rate and significantly reduces operating efficiencies. Charged injection atomizers that generate electrically charged sprays can potentially offer several benefits to conventional fuel injection systems. In addition to enhanced mixing and reduction in droplet agglomeration via Coulomb repulsion, control over the droplet size distribution and flame shape downstream of the injection nozzle is possible. Moreover, as droplets evaporate, they retain their charge until reaching a critical radius, in which the charge is transferred to the gas phase via a corona discharge. Meanwhile, very little is known about this process.

    In addition to possible applications in combustion, advancing the predictive capabilities of electrically-charged droplets in turbulence is crucial for understanding the onset of rain formation in clouds. Throughout the atmosphere, ion pairs are produced by cosmic rays that attach to aerosol particles and droplets. Even in fair-weather clouds, attachment of air ions to cloud droplets significantly reduces the ion concentration. Ionization of atmospheric air together with the potential difference that exists between the upper atmosphere and the earth’s surface generates a non-negligible vertical electric field. Interactions between this electric field and charged droplets will thus directly affect coalescence and collisions that dictate the onset of rainfall.

    Cluster mitigation by an imposed electric field on settling charged particles. Uncharged (top), charged (bottom).


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    Reactive Gas-Solid Flows

    Background
    Reactive gas-solid flows play a major role in a wide variety of engineering devices. Heterogeneous reactions occurring in circulating fluidized bed reactors have been used in a range of technical processes, including fluid catalytic cracking (FCC), gasification and combustion of coal, and more recently thermochemical conversion of biomass. Currently, catalytic-like processes remain the primary conversion technique within the petrochemical industry. A common feature of all reactive gas-solid systems is the complex processes occurring at the surface of each particle, including heat and mass transfer between the two phases, adsorption on and desorption from the solid surface, and the actual chemical reaction between the adsorbed gas and the solid. Meanwhile, the macroscopic flow dynamics (e.g., turbulence in the fluid phase, clustering and bubbling in the particle phase) can have a large impact on these microscale processes. Simultaneously accounting for both the chemical reactions occurring at the surface of each particle and macroscopic particle dynamics in turbulent flows poses significant challenges in developing predictive models.

    A volume-filtered formulation that describes chemically-reacting flows in the presence of solid catalytic particles was recently derived and implemented in an Eulerian-Lagrangian framework. The work was published here. The image to the right shows how the non-homogeneity in particle concentration due to clusters affects the conversion of chemical reactants in vertical pipe flow. The reactant is quickly depleted in clusters falling at the pipe walls, while unconverted gas persists in the central region of the flow. Clusters were found to delay the conversion process by up to 85% compared to a homogeneous distribution of particles under identical conditions.

    Heterogeneous reaction occurring in a vertical riser flow. Clusters falling at the walls are seen to hinder mixing between the phases, degrading the overall conversion efficiency.

    Relevant papers
  • Numerical investigation and modeling of reacting gas-solid flows in the presence of clusters (2015) Chemical Engineering Science DOI