fluid and thermal engineering

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Two-phase Flow and Heat Transfer(ME – 657) 

by

Dr. Dipankar N. BasuAssistant Professor

Department of Mechanical Engg.

IIT Guwahati, Guwahati – 781039

Lecture 1

Introduction to Multiphase Flow

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  Course Outline

Importance & applications of multiphase flow

Fundamentals of two-phase flow  Basic definitions & conservation equations

Comparison with single-phase systems

Identification of flow patterns

Mathematical modeling

Homogeneous flow, Separated flow & Drift flux models Simplified treatment of different flow regimes

Boiling heat transfer

  Classifications: Pool & Flow boiling

Boiling curve

Bubble nucleation & boiling crisis Condensation heat transfer: 

Drop-wise & Film-wise condensation

Simple modeling & analysis

  Measurement of two-phase parameters

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  Multiphase Flow Systems: Some Examples

Examples of gas-liquid flow

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  Multiphase Flow Systems: Some Examples

Gas-solid flow

Liquid-liquid flow

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  Multiphase Flow Systems: Some Examples

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  Multiphase Flow Systems: Some Examples

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 Unique Features of Two-phase Flow

Analysis of two-phase flow is distinctly different from its single-phase

counter-part because of numerous factors, some of which are

mentioned below.

  Presence of multiple (possibly), deformable and moving interface

Drastic change in fluid properties across the interface

  Possible compressible nature of one or all the phases (particularlywhen gaseous medium is involved)

Involvement of several length and time scales

  Strong dependence of the nature of flow stream on geometrical

orientation, method of initiating the flow and local condition

Different nature of interaction of phases with boundaries (walls)

  Very high heat transfer coefficient associated with boiling and

condensation processes

Susceptibility of systems towards instabilities and fluctuations

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 Appearance of Different Flow Regimes

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Two-phase Flow and Heat Transfer(ME – 657) 

by

Dr. Dipankar N. BasuAssistant Professor

Department of Mechanical Engg.

IIT Guwahati, Guwahati – 781039

Lecture 2

Two-phase Flow Regimes

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 Flow Regimes in Vertical Adiabatic Flow (Upward)

These are the most common types of flow regimes appearing with

increasing fraction of lighter phase in the mixture.

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 Flow Regimes in Vertical Adiabatic Flow (Downward)

Bubbly

flow

Slug flow Falling

film flow

Bubbly falling

film flow

Churn flow Dispersed

annular flow

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 Flow Regimes in Horizontal Adiabatic Flow

Gravity plays a dominating role in deciding the flow regime

for horizontal and inclined tubes.

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 Flow Regimes in Horizontal Adiabatic Flow

All possible flow regimes may

not appear under every two-phase flow situation. Nature of 

the flow regime depends

strongly on local flow

characteristics, including the

relative fraction of individual

phases, phasic velocities,

interfacial properties etc. It is

also possible to have different

transition regimes dominating

the flow domain.

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 Flow Regimes in Vertical Diabatic Flow

Here the location identified as x=0 signifies

the position of boiling boundary calculated

considering Thermodynamic equilibrium.However, due to the presence of non-

equilibrium subcooled boiling, small amount

of vapour generation can takes place prior

to this point. x=1 similarly identifies the

equilibrium location of the end of boiling.

Drop/Mist flow (appearsonly in heated channels)

Rate of heat addition and hence dx/dz plays a

crucial role in determining the presence or absence

of any particular flow pattern in diabatic channels.

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 Flow Regimes in Horizontal Diabatic Flow

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 Flow Pattern Map for Vertical Flow (Hewitt & Roberts, 1969)

This map was developed for

low-pressure air-water and

high-pressure steam-water

upward flow through smalldiameter (1-3 cm) tubes.

This is not the most accurate

presentation & serves mostly

as a rough guide.

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 Flow Pattern Map for Horizontal Flow (Baker, 1954)

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 Unified Flow Pattern Map (Taitel & Dukler, 1976)

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 Flow Pattern Map for Vertical Down-flow (Oshimowo & Charles, 1974)

This map was generated

for air & differentliquids in a channel of 

2.54 cm diameter at

172 kPa pressure.