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Measurements of void fraction, pressure drop andheat transfer in downward inclined gas-liquid

stratified flow

Swanand M. Bhagwat and Afshin J. Ghajar

School of Mechanical and Aerospace Engineering

Oklahoma State University,

Stillwater, OK – 74078.

May 10, 2016 1

9th International Conference on Multiphase Flow, Italy

8/15/2019 Icmf 2016 Italy


Background and Motivation

● Very few studies dedicated to measurements of stratified flow in the entire range

of downward pipe inclinations -90o < θ ≤ 0o.

– Beggs (1972), Nguyen (1975), Mukherjee (1979)

● Most of the existing work focus on near horizontal (-10o < θ ≤ 0o) and wavy

stratified (inertia driven) flow while less attention given to stratified flow at low

gas flow rates.

– Hamersma and Hart (1987), Kokal and Stanislav (1989), Speeding et al.

(1998), Fan (2007) and more (experimental)

– Taitel and Dukler (1976) and its variations, Hart et al. (1989), Chen (1997)

(modeling). Accuracy of these models for downward pipe inclinations is not

yet verified.

● Lack of information and understanding on non-boiling heat transfer

characteristics of two phase stratified flow in downward pipe inclinations.

● Literature reports the existing flow pattern independent correlations to fail to

model stratified flow correctly especially at downward pipe inclinations.2

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Experimental Setup

3

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Experimental Setup …

● Void fraction is measured using quick closing valves (QCV)

● Pressure drop is measured using Validyne differential transducer

● Heat transfer coefficient based on local temperature measurements

4

,1

1 1where

2 ( )

ST N

tp j jw i b j

qh hdz h z h

L L D z T T π =

= = ∆ =∆ −

∑∫

t h f a

dp dp dp dp

dz dz dz dz

= + +

( )

0.25% of F.S.

/ sin [ (1 ) ] sin

t

m l gh

dp

u dz

dp dz g g ρ θ ρ α ρ α θ

→ = ±

= = − +

( ) 0.01 to 0.0151 l

tot

umm

α α = ± ±= − →

,6% to 30% [depending upon magnitude of ( )( ) ]tp w i bT T u h = ± ± −

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Heat Transfer Measurements

5

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Flow Visualization

Falling film flow (c) is a

special type of stratified

flow at θ = -90o

Inertia driven

stratified flow

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Stratified Flow on Flow Map

7

Transition from stratified to non-stratified flow pattern occurs over a certain range of

gas and liquid flow rates. For simplicity, the transition band is represented as

transition lines.

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Transient Behavior During Transition

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• Coexistence of stratified, slug and bubbly flow patterns in the vicinity of

stratified/non-stratified transition line.

• Occurs only at steeper downward pipe inclinations.

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How to Predict Existence of Stratified Flow?

● Two phase flow literature reports following methods/correlations to predict

existence of stratified flow over a wide range of flow conditions.

– Taitel and Dukler (1976) + Barnea (1987) model (mechanistic)

– Crawford and Weinberger (1985) (empirical model)

– Bhagwat and Ghajar (2015) (empirical model)

● Mechanistic model is implicit, iterative and/or uses graphical solution.

● Empirical models are easy to use and are tested over a wide range of flow

conditions listed below.

Bhagwat, S. M. and Ghajar, A. J., “An Empirical Model to Predict the Transition between

Stratified and Non-Stratified Gas-Liquid Two Phase Flow in Horizontal and Downward

Inclined Pipes”, Heat Transfer Engineering, Vol. 36, No. 18, pp. 1485-1494, 2015.9

Parameter Range Parameter Range

Pipe diameter 8.9 – 300 mm Pipe orientation -90o < θ ≤ 0o

Liquid density 780 – 1420 kg/m3 Gas density 1.2 – 35 kg/m3

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Effect of Phase Flow Rates

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● Void fraction and pressure drop remain insensitive to change in in buoyancy

dominated stratified flow (small values of Usg/Usl).

● Inertia driven stratified flow (larger values of Usg/Usl) shows rapid increase in

pressure drop as well as heat transfer. Qualitatively similar trends are

observed at other inclinations.

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Void Fraction

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Source: Fabre (2003)

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Effect of Pipe Inclination

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● Total two phase pressure drop is balanced by pressure loss (frictional component)and pressure recovery (hydrostatic component).

● Two phase heat transfer in stratified flow significantly depends on the pipe wetted

perimeter which in turn depends on pipe inclination.

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Circumferential Heat Transfer Coefficient

0

1

2

3

4

1 2 3 4

h t p ( k W / m

2 K )

Circumferential location

θ = -60o

14

0

1

2

3

4

5

1 2 3 4

h t p ( k W / m 2 K )


θ = -5o

0

1

2

3

4

5

1 2 3 4

h t p ( k W / m 2

K )


θ = -90o

1

2

3

4

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Conclusions

● New data on stratified/non-stratified flow transition, void fraction, pressure drop

and heat transfer in entire range of downward pipe inclinations is presented.

● A transient two phase flow behavior during stratified flow transition is identified.

● A change in pipe inclination is found to significantly affect transition boundary,

void fraction and two phase heat transfer coefficient in stratified flow.

● Significant change in void fraction is essentially due to the residence time of

the gas phase in test section which is a function of its inclination.

● Significant variation in heat transfer coefficient is mainly due to change in

wetted perimeter of pipe as a function of its inclination from horizontal.

● Study of stratified flow in downward pipe inclinations with varying fluid

properties (liquid density) and pipe diameters (bigger than 1 inch) is needed for

better insight, verification and modeling of the two phase flow variables in this

peculiar flow regime.

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Thanks for your attention!

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Icmf 2016 Italy