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Simulation of bubbly flow between parallel plates. Turbulent structures are shown in yellow and cyan; bubbles are shown in light gray.Image Credit: Igor Bolotnov

Multiphase Flows

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Dr. Mohammad Jadidi(Ph.D. in Mechanical Engineering)

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Presented by: Mohammad Jadidi 2

For example, different-sized solid particles of the same material can be treated as different phases because each collection of particles with the same size will have a similar dynamical response to the flow field

In multiphase flow, a phase can be defined as an identifiable class of material that has a particular inertial response to and interaction with the flow and the potential field in which it is immersed.

Note:

What is Multiphase Flows?

Two or more distinct phases flowing simultaneously in mixture, and

Have some level of phase separation at a scale well above the molecular level

Multi-phase flows is simply any fluid flow system consisting of:

Multiphase Flows


Multiphase Flows

Multiphase flow regimes can be grouped into four categories:

1. Gas-Liquid or Liquid-Liquid Flows

2. Gas-Solid Flows

3. Liquid-Solid Flows

4. Three-Phase Flows

Classification of Multiphase Flows


Classification of Multiphase Flows: Gas-Liquid or Liquid-Liquid Flows

Bubbly flow: discrete gaseous or fluid bubbles in a continuous fluid. Droplet flow: discrete fluid droplets in a continuous gas. Slug flow: large bubbles in a continuous fluid. Stratified/free-surface flow: immiscible fluids separated by a clearly-defined

interface.

Bubbly flow Droplet flow Slug flow Stratified/free-surface flow

Multiphase Flows

Black color: liquid phase , White color: gas phase


Classification of Multiphase Flows: Gas-Liquid Flows

Gas–liquid flows can, in principle, assume several different configurations.

Classification of gas–liquid flows (adapted from Ishii & Hibiki, 2006).

Dispersed Flows Mixed or transitional flows Separated flows

Multiphase Flows


A generic two-phase vertical flow map

A very important factor is the diameter of the flow line, if the liquid and gas flow ratesare kept constant and the flow line size is decreased from 4” to 3”, both the superficial gasand liquid velocities will increase by a factor 16/9. Hence, in the two-phase flow map thispoint will move up and right along the diagonal to a new position. This could cause achange in flow regime, e.g. changing from bubbly flow to slug flow or changing fromslug flow to annular flow

Multiphase Flows

Multiphase Flow Regime Maps


NOTE: See appendix for more examples of flow regime maps



Flow Regimes in Vertical Pipe Configuration

Multiphase Flows


Multiphase Flows Classification of Multiphase Flows: Gas-Solid Flows

Particle-laden flow: This is flow of discrete particles in a continuous gas.

Pneumatic transport: This is a flow pattern that depends on factors such as solid loading, Reynolds numbers, and particle properties. Typical patterns are dune flow, slug flow, and homogeneous flow.

Fluidized bed: This consists of a vessel containing particles, into which a gas is introduced through a distributor. The gas rising through the bed suspends the particles. Depending on the gas flow rate, bubbles appear and rise through the bed, intensifying the mixing within the bed.

Particle Laden Flow through Industrial Cyclone

Gas–solid flow, identified as gas–solid or gas–droplet flows, is concerned with the motion of suspended solid or droplet in the gas phase

The following regimes are gas-solid flows


Multiphase Flows

Fluidization Regimes for Gas-Solid Suspension Flow

4 ton/hour fluidized bed incinerator for industrial wastes

Classification of Multiphase Flows: Gas-Solid Flows




Video: Low-Velocity, Dense-Phase Pneumatic Conveying System,by Mactenn, mactenn.com




VIDEO: semi dense phase pneumatic conveying of bulk material ((AZO Ltd.)

VIDEO: a simulation of a bulk material dilute phase pneumatic conveying (AZO Ltd.)



Pneumatic transport:

Dense Phase - Full Pipeline• Pressure is higher• Material velocity is lowest• Material nearly fully fills pipe line

Dense Phase - Slug Flowdense phase dune flowPressure is higherMaterial velocity remains quite low.Material forms moving dunes as it is conveyed.

Dense Phase - Bed FlowPressure is middling, lower than full dense phase. Material velocity is mixed. Bed material moves slow.Material above the moving bed is flowing above saltation velocity.

Lean Phase (Dilute Phase)Low pressure conveying Material moving at high velocity.All material is above saltation velocity, and is entrained in gas flow in pipe.Highest wearing conveying regime, also has high air consumption.


Multiphase Flows

Depending on the particle number density, Gas-Solid Flows can be characterized as either being dilute or dense

Classification of Multiphase Flows: Gas-Solid Flows

A dilute flow, is one in which the particle motion iscontrolled by the fluid forces (drag and lift)

A dense flow, on the other hand, is one in which the particle motion is controlled by collisions


Multiphase Flows Classification of Multiphase Flows: Liquid-Solid Flows

Liquid–solid flow is the transport of solid particles in liquid

The following regimes are liquid-solid flows

Slurry flow: This flow is the transport of particles in liquids.

Hydrotransport: This describes densely-distributed solid particles in a continuous liquid.

Sedimentation: This describes a tall column initially containing a uniform dispersed mixture of particles. At the bottom, the particles will slow down and form a sludge layer. At the top, a clear interface will appear, and in the middle a constant settling zone will exist.

Temporal evolution particle distribution during gravity-dominated sedimentation.

Image Credit: (Center for Turbulence Research , Annual Research Briefs 2003)

The Stokes number gives a measure of temporal correlation between particle velocity and the

fluid velocity and is defined as

𝑺𝒕 = 𝝉𝒑 / 𝝉𝑭 If St <<1, the particle response time is much

less than the characteristic time associated with the flow field. In this case the particles will have ample time to respond to changes in flow velocity and, the particle and fluid velocities will be nearly equal

If St>>1, then the particle will have essentially no time to respond to the fluid velocity changes and the particle velocity will be little affected by fluid velocity change


Multiphase Flows

Slurry flow: This flow is the transport of particles in liquids. In slurry flows, the Stokes number is normally less than 1.

VIDEO: Detailed large-eddy simulation of a turbulent slurry flow in a horizontal pipe with the formation of a static bed at the bottom. Performed using NGA by Jesse Capecelatro and Olivier Desjardins.

Classification of Multiphase Flows: Liquid-Solid Flows


Multiphase Flows

Hydrotransport: This describes densely-distributed solid particles in a continuous liquid.

Classification of Multiphase Flows: Liquid-Solid Flows


Multiphase Flows

Three-phase flows are combinations of the other flow regimes listed in the previous sections.

Classification of Multiphase Flows: Three-phase flows

A trickle-bed reactor (TBR) is a chemical reactor that uses the downward

movement of a liquid and the downward (co-current) or upward (counter-current)

movement of gas over a packed bed of (catalyst) particles. It is considered to be

the simplest reactor type for performing catalytic reactions where a gas and

liquid (normally both reagents) are present in the reactor and accordingly it is

extensively used in processing plants. Typical examples are liquid-

phase hydrogenation, hydrodesulfurization, and hydrodenitrogenation inrefineries.

https://en.wikipedia.org/wiki/Chemical_reactor

https://en.wikipedia.org/wiki/Catalyst

https://en.wikipedia.org/wiki/Hydrogenation

https://en.wikipedia.org/wiki/Hydrodesulfurization

https://en.wikipedia.org/wiki/Hydrodenitrogenation


Appendix

Multiphase Flows


Multiphase Flows Examples of flow regime maps (Gas-Liquid Flows)

Flow regime map for the horizontal flow of an air/water mixture in a 5.1cm diameter pipe. Hatched regions are observed regime boundaries,

lines are theoretical predictions. Adapted from Weisman (1983).

Sketches of flow regimes for flow of air/water mixtures in ahorizontal, 5.1cm diameter pipe. Adapted from Weisman (1983).


Multiphase Flows Examples of flow regime maps (Vertical pipe flow)

Sketches of flow regimes for two-phase flow in a vertical pipe. Adapted from Weisman (1983).

A flow regime map for the flow of an air/water mixture in a vertical, 2.5cm diameter pipe showing the

experimentally observed transition regions hatched; adapted from Weisman (1983).

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Multiphase Flows

The evolution of the steam/water flow in a vertical boiler tube.

Presented by: Mohammad Jadidi

It should be added that flow regime information such as that presented in previous slides appears to be valid both for flows that are not evolving with axial distance along the pipe and for flows, such as those in boiler tubes, in which the volume fraction is increasing with axial position.


Multiphase Flows Examples of flow regime maps (Slurry flow regimes)

When the particles are small so that their settling velocity is much less than the turbulent mixing velocities in the fluid and when the volume fraction of solids is low or moderate, the flow will be well-mixed. This is termed the homogeneous flow regime

Flow regimes for slurry flow in a horizontal pipeline.

NOTE: Typically homogeneous flow regime only occurs in practical slurry pipelines when all the particle sizes are of the order of tens of microns or less.


Multiphase Flows Examples of flow regime maps (Slurry flow regimes)

Flow regimes for slurry flow in a horizontal pipeline.

When particle sizes are larger than tens of microns, vertical gradients will occur in the concentration and the regime is termed heterogeneous

When a packed bed develops, the flow regime is known as a saltation flow. In a saltation flow, solid material may be transported in two ways, either because the bed moves or because material in suspension above the bed is carried along by the suspendingfluid.

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Multiphase Flows

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Dr. Mohammad Jadidi(Ph.D. in Mechanical Engineering)

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