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Transcript of Icmf 2016 Italy
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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
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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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6
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
8
• 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
10
● 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
11
Source: Fabre (2003)
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Effect of Pipe Inclination
13
● 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 )
Circumferential location
θ = -5o
0
1
2
3
4
5
1 2 3 4
h t p ( k W / m 2
K )
Circumferential location
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!