Two phase flow – flow regimesHalldor Palsson

University of Iceland

Two phase flow – flow regimes – p. 1

Classification into regimes

Flow regimes determine the macroscopic behavior oftwo phase flow.

Different regimes are classified by visual observation.

It can be difficult to specify with certainty, which regimea particular flow belongs to.

Many different classifications exist in the literature.Scientists don’t agree on a unique set of flow regimes.

Classification can be useful, since different flow regimesaffect parameters in different manner, such as pressuredrop.

Two phase flow – flow regimes – p. 2

Geometrical difference

The flow regimes can be divided into three main classes:

1. Regimes for horizontal flow in pipes, where the heavierphase (water) tends to be located close to the bottom,because of gravity. In most cases the gas phasepushes the liquid phase along the flow direction.

2. Regimes for vertical flow in pipes. The liquid phasetends to be on the pipe walls, forming a stable or anunstable film. Flow velocity can be different and flowregimes form differently for upward and downwardflows.

3. Regimes for sloped pipes, which are not as well known.Here the slope angle is important as well as thedirection of the flow (upwards or downwards).

Two phase flow – flow regimes – p. 3

Horizontal – bubble flow

Small bubbles are present in the flow, and aredispersed everywhere in the cross section.

Bubbles gather in the top of the pipe for low velocities.

Bubbles become foam like at high velocities.

Two phase flow – flow regimes – p. 4

Horizontal – plug flow

Bubbles join and form larger gas plugs.

The plugs flow in the upper part of the pipe because ofgravity effects.

Two phase flow – flow regimes – p. 5

Horizontal – stratified flow

The phases are completely separated with gas in theupper part and liquid in the lower part of the pipe.

Surface is relatively smooth and variations are small.

This type of flow can only occur at low velocity.

Two phase flow – flow regimes – p. 6

Horizontal – wavy flow

Appears at higher velocities, compared to the stratifiedflow.

Waves form on the phase boundaries, resulting in morefriction between the two phases.

Waves travel in the direction of flow, especially if gasvelocity is considerably higher that liquid velocity.

Two phase flow – flow regimes – p. 7

Horizontal – slug flow

Waves in the flow reach the top of the pipe, closing thegas path in the top.

Different momentum of the phases (because of velocitydifference) results in sudden pressure changes whenthe path closes.

Shocks and vibrations are experienced in the flow,which should be avoided if possible.

Two phase flow – flow regimes – p. 8

Horizontal – annular flow

The liquid forms a coat all around the pipe walls.

Gas flows in the middle, possibly with liquid dropletstraveling along it.

Coat tends to be thicker at the bottom than at the top,because of gravity effects.

Two phase flow – flow regimes – p. 9

Horizontal – spray flow

Liquid droplets are dispersed in the flow of gas in thepipe.

The amount of liquid is much less in this case.

Liquid velocity is very similar to the gas velocity, orS ≈ 1.

Two phase flow – flow regimes – p. 10

Vertical – bubble flow

Gas bubbles are present in a liquiddominated flow.

Bubbles are mostly evenlydistributed, but tend to gather nearthe pipe center with increasedspeed.

Velocity is similar for both phases,S ≈ 1.

Two phase flow – flow regimes – p. 11

Vertical – slug flow

Gas bubbles gather and form bulletshaped voids in the pipe center.

The flow is unstable, resulting inpressure shocks and vibrations inthe pipe.

Two phase flow – flow regimes – p. 12

Vertical – churn flow

Gas bubbles gather and form agas path in the center of the pipe.

The flow is foam like and veryunstable, resulting in high pressurevariations.

The gas velocity is high, pushingthe liquid upwards.

Two phase flow – flow regimes – p. 13

Vertical – annular flow

The liquid forms an uniform layeron the pipe walls.

The layer can be unstable, with awavy surface, but is much morestable than in the churn flow.

Two phase flow – flow regimes – p. 14

Vertical – spray flow

The liquid is in much smallerquantity than the gas.

The liquid is distributed in the flowas droplets or spray.

Two phase flow – flow regimes – p. 15

Flow regimes and phase equilibrium

Phase equilibrium is obtainable at a constant pressure.

Pressure variations are inevitable in most two phaseflow regimes.

Average pressure changes slowly along the pipe.

Fast fluctuations occur in small sections in the flow.

Conclusion:

Phase changes take place everywhere in the flow, bothfrom steam to water and from water to steam.

In practical cases, fast variation in phase change areneglected, and the slow pressure variations are onlyconsidered.

Two phase flow – flow regimes – p. 16

Flow maps for regimes

Question: Is it possible to determine the type of flow regimefrom available flow data?

Flow maps are attempts to answer this question anddetermine the flow regime type with certain accuracy. Suchmaps involve

Maps for very narrow flow conditions. Typically, specificfluids at specific pressures and temperatures.

General flow maps for various flows and fluids, basedon dimensionless parameters.

Most maps are generated for horizontal flow.

Few maps are available for vertical flow, upwards ordownwards.

Two phase flow – flow regimes – p. 17

Flow regime dependencies

Transport properties of gas and liquid:Density difference.Viscosity, related to the Reynolds number.Surface tension, related to the Weber number.

Geometry scales and pipe roughness.

Mass and volume fractions in pipes.

Velocity ratio between phases.

The flow map will indicate the most likely flow regime for thegiven parameters above.

Two phase flow – flow regimes – p. 18

Flow maps for horizontal flow

Commonly used flow maps are:

Baker map, which is old and widely used.

Hoogendoorn map, which is considered to be moreaccurate than the Baker map.

Mandhane, Gregory, Aziz map, another old map whichis considered better than the two above.

The Mukherjee and Brill map, published in 1985.

The Spedding and Nguyen map, published in 1980.

An universal flow regime map, which is relatively new(2003), published by Spedding and others.

Two phase flow – flow regimes – p. 19

The Baker map

101

102

103

104

100

101

102

Lψ

G/λ

Baker map

Plug

Bubbly

AnnularWavy

Stratified

Slug

NesjavellirKraflaSvartsengiBjarnarflag

λ =

√

ρg

1.2

ρL

1000

ψ =0.0724

σL

(

µL

0.0009

(

1000

ρL

)2)

1

3

Two phase flow – flow regimes – p. 20

The Mukherjee and Brill map

100

101

102

103

10−3

10−2

10−1

100

101

102

Vapor velocity number

Liqu

id v

eloc

ity n

umbe

r

Mukherjee and Brill map

Stradified

Annular

SlugBubble

NesjavellirKraflaSvartsengiBjarnarflag

LVN = VsL

(

ρL

gσ

)1

4

VsL =QL

A

Two phase flow – flow regimes – p. 21

The Spedding and Nguyen map

10−5

10−4

10−3

10−2

10−1

100

101

0

2

4

6

8

10

12

Ql/Q

g

Fr=

(VT/(

gD)1/

2 )1/2Spedding and Nguyen map

Stradified

Slug

Stradified+RW/R/BTS

Annular+BTS

Film+DropletAnnular+RW

Annular+Droplet

Droplet

NesjavellirKraflaSvartsengiBjarnarflag

Two phase flow – flow regimes – p. 22

Flow map for vertical flow

A popular map for vertical flow in the upward direction is theHewitt and Roberts map. Two parameters define the axis

x2G

ρg=mgvsg

A

(1 − x)2G

ρl

=mlvsl

A

which are the momentums of the two phases per pipesection. The map is based on data for steam and water inpipes up to 30 mm in diameter.

Two phase flow – flow regimes – p. 23

Hewitt and Roberts map

104

101

101

102

103

104

105

103

102

1

1

Annular flowwhispy annular

Churn flow

Bubbly slug flow

Bubble flow

liquid

gas

Two phase flow – flow regimes – p. 24

Flow regimes – concluding remarks

Over one hundred flow regime maps exist, based ondifferent classification into regimes.

Most of the existing models are verified and adjusted byusing measurements in small diameter pipes(10 − 100 mm).

Researches in the field do not agree on any universalmodels that can be accepted for general flow situations.

Most likely horizontal flow regime in Icelandicgeothermal power plants is a wavy stratified flow.

Two phase flow – flow regimes – p. 25