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14-multi

# 14-multi - Lecture 14 Multiphase Flows Applied...

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1 Lecture 14 - Multiphase Flows Applied Computational Fluid Dynamics Instructor: André Bakker http://www.bakker.org © André Bakker (2002-2006) © Fluent Inc. (2002)

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2 Definitions Multiphase flow is simultaneous flow of: Materials with different states or phases (i.e. gas, liquid or solid). Materials with different chemical properties but in the same state or phase (i.e. liquid-liquid systems such as oil droplets in water). The primary and secondary phases: One of the phases is continuous (primary) while the other(s) (secondary) are dispersed within the continuous phase. A diameter has to be assigned for each secondary phase to calculate its interaction (drag) with the primary phase. A secondary phase with a particle size distribution is modeled by assigning a separate phase for each particle diameter.
3 Dilute versus dense phase: Refers to the volume fraction of secondary phase(s). Volume fraction of a phase = Laminar versus turbulent: Each phase can be laminar or turbulent. Fluid flow (primary phase) may be turbulent with respect to the secondary phase but may be laminar with respect to the vessel. Volume of the phase in a cell/domain Volume of the cell/domain Definitions (2)

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4 Why model multiphase flow? Multiphase flow is important in many industrial processes: Riser reactors. Bubble column reactors. Fluidized bed reactors. Scrubbers, dryers, etc. Typical objectives of a modeling analysis: Maximize the contact between the different phases, typically different chemical compounds. Flow dynamics. Rushton CD-6 BT-6
5 bubbly flow droplet flow particle-laden flow slug flow annular flow free-surface flow Multiphase flow regimes Bubbly flow: discrete gaseous bubbles in a continuous liquid. Droplet flow: discrete fluid droplets in a continuous gas. Particle-laden flow: discrete solid particles in a continuous fluid. Slug flow: large bubbles in a continuous liquid. Annular flow: continuous liquid along walls, gas in core. Stratified and free-surface flow: immiscible fluids separated by a clearly-defined interface.

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6 Flow regimes: vertical gas-liquid flow ) ( ) / ( ) / ( : 2 3 m A s m Q s m v Velocity l Superficia sg = ) ( ) / ( 60 ) ( 3 3 m V s m Q VVM Q = Evaporator
7 Multiphase flow regimes User must know a priori the characteristics of the flow. Flow regime, e.g. bubbly flow, slug flow, annular flow, etc. Only model one flow regime at a time. Predicting the transition from one regime to another possible only if the flow regimes can be predicted by the same model. This is not always the case. Laminar or turbulent. Dilute or dense. Secondary phase diameter for drag considerations.

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8 Empirical correlations. Lagrangian. Track individual point particles.
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14-multi - Lecture 14 Multiphase Flows Applied...

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