Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 14.1.5 - Visualization of droplet entrainment in turbulent stratified pipe flow
ALL VOLUMES CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00102023/00342
 Material Information
Title: 14.1.5 - Visualization of droplet entrainment in turbulent stratified pipe flow Droplet Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Lecoeur, N.
Hale, C.P.
Spelt, P.D.M.
Hewitt, G.F.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: stratifying-annular flow
axial images
turbulent flow
entrained droplets
 Notes
Abstract: We report here on the visualization of the detailed processes whereby droplets are entrained from a liquid layer by a highly turbulent gas flow in horizontal stratifying-annular flow. The experimental facility used involves a 30m long, 0.079m diameter pipeline; typical gas and liquid flow rates are of the order of 11.5 and 0.035 m/s, respectively. State-of-the-art high-speed cine cameras an axial viewing are used, allowing one to investigate the flow behaviour in great detail. Different types of droplet entrainment are identified, including ligament and bag-type breakup, as well as intermediate events, all under the same flow conditions. Results will also be presented on the subsequent motion of entrained droplets (also obtained with the axial viewing technique), including droplet deposition at the top of the pipe cross section after the droplets follow a ballistic trajectory directly after entrainment.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00342
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 1415-Lecoeur-ICMF2010.pdf

Full Text


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Visualization of droplet entrainment in turbulent stratified pipe flow


N. Lecoeur, C.P. Hale, P.D.M. Spelt, G.F. Hewitt


Imperial College London, Department of Chemical Engineering,
South Kensington Campus, London SW7 2AZ. UK
n.1ecoeur~imperial.ac.uk



Keywords: Stratifying-annular flow, Axial images, Turbulent flow, Entrained droplets





Abstract

We report here on the visualization of the detailed processes whereby droplets are entrained from a liquid layer by a highly
turbulent gas flow in horizontal stratifying-annular flow. The experimental facility used involves a 30m long, 0.079m diameter
pipeline, typical gas and liquid flow rates are of the order of 11.5 and 0.035 m/s, respectively. State-of-the-art high-speed cine
cameras an axial viewing are used, allowing one to investigate the flow behaviour in great detail. Different types of droplet
entrainment are identified, including ligament and bag-type breakup, as well as intermediate events, all under the same flow
conditions. Results will also be presented on the subsequent motion of entrained droplets (also obtained with the axial viewing


technique), including droplet deposition at the top of the pipe
directly after entrainment.


1.Introduction

Multiphase flow in pipelines occurs widely in the petroleum
industry. In particular, this study focuses on the phenomena
encountered in gas-condensate lines, investigating
gas-liquid stratifying-annular flow. In this regime, which
occurs at high gas flow-rates and low liquid loading, the
liquid flows partly as a film around the wall of the pipe and
partly as droplets within the gas core. Due to the influence
of gravity, this liquid film is very thick at the bottom of the
pipe and it decreases on the upper part of the wall. In some
cases, this liquid film may fail to wet the top of the pipe and
lead to corrosion. The aim of this study is to gain a better
understanding of liquid transport mechanisms in
stratifying-annular flow.
We visualize here the interface behaviour in the
stratifying-annular flow regime, and more particularly the
diverse mechanisms leading to droplet entrainment, using
the in-line axial viewing technique first developed by
Hewitt (1969) and later modified by Badie (2000). The
mechanisms by which the droplets are torn from the waves
in horizontal stratifying-annular flow are investigated and
can be classified as being of the bag break-up and ligament
break-up types, details of the video observations obtained
using the axial viewing technique are given.
Section 2 presents a review on droplet entrainment in
annular flow and stratifying-annular flow. It presents also
the principle of the axial viewing technique and in particular
of the in-line axial viewing technique. Images from
visualisation experiment obtained using this technique are
presented and discussed in Section 3.


cross section after the droplets follow a ballistic trajectory




2. Background: Techniques and Experimental
FacilIity

In stratifying-annular flow in horizontal pipes, some
mechanism must exist to transport the liquid phase towards
the top of the pipe, this mechanism counteracting the
gravitational force which tends to move the liquid towards
the bottom of the pipe.
Three mechanisms have been suggested for maintaining a
liquid film around the pipe, namely droplet entrainment,
wave spreading (dominant in small diameter pipes) and
secondary flow. Droplet entrainment has been suggested to
be the main mechanism responsible for maintaining a liquid
film in horizontal annular flow in relatively large diameter
pipes (0.079m). In this mechanism, droplets are entrained
from the liquid layer at the bottom of the pipe and are
deposited at the top to form a liquid film. This liquid film
then drains towards the bottom. The liquid film is
continuously renewed by this process. Two modes of droplet
transport for maintaining a liquid film have been proposed.
Large droplets emitted from the bottom passing ballistically
to the upper surfaces (dominant in pipes of medium
diameter) and the turbulent diffusion of small diameter
droplets to upper part of pipe (dominant mechanism in large
diameter pipes).
Large disturbance waves in co-current annular flow are
usually regarded as being the sources of droplet entrainment.
(Cooper et al., 1969, Jacowitz and Brodkey, 1964 and
Arnold and Hewitt, 1967). The formation of droplets from a
liquid layer is a highly complex process and may occur by a
variety of mechanisms. Azzopardi (1983) suggested that the
principle mechanisms are "bag break-up" (reported earlier
for horizontal flow by Woodmansee and Hanratty, 1969)
and "ligament break-up", as illustrated in Figure 1. Bag






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

internal diameter of 1 1/ inch (32 mm). The air is
introduced at the bottom of the tube and a porous wall
section is used to inject water around the periphery so as to
establish air-water flow in the tube.
A short section of the pipe is illuminated. The illuminated
zone is recorded through a window, which is kept free of
liquid by passing an air purge over it and down the
viewing tube. The fluids flowing through the pipe are
diverted into an exit chamber and return to the separation
tank via return pipes.
Through the above-mentioned window, the illuminated
region of the pipe was recorded using a high speed
cine-camera which was focused on the plane of
illumination. This camera was used to capture the axial
view of the flow inside of the tube, framing rates of
2000-4000 per second were employed.


break-up occurs at lower gas flow-rate and ligament
break-up, at higher gas flow-rate. Bag break-up is
considered to be the most important: however it has to be
noted that the bag break-up occurring at lower gas velocities
is easier to observe and that the ligament break-up is
increasingly relevant at higher flow-rates.
In bag break-up (cf. Figure 1), part of the disturbance wave
is undercut and an open ended bubble with a thick filament
rim is created. Gas pressure builds up within the bubble
causing it to expand and finally burst. The thin part of the
bubble forms very small drops. However, most of the liquid
is gathered into the rim by surface tension forces, and this
rim then breaks up shortly afterwards into a smaller number
of larger drops. Ligament break-up is illustrated in Figure 1.
The crests of roll waves are elongated and thin ligaments,
torn from the film, immediately break down into drops.


DROPS IN ANNIJLAR TWOPHASE FLOW
lo)Bag Break-up [b)Ligomerit Break-up


151809 8~OI.UP IO~~PII


.: ....
o0~Q~~


L~

~:1:
o~P~


Idl Ligament Break-up IDrops)






Figure 1: Mechanisms of atomization and drop breakup
(Azzopardi, 1997)

The method used here to study this phenomenon was the
axial viewing technique. It was originally developed for
vertical pipe flow by Arnold and Hewitt (1967) and Hewitt
and Roberts (1969), and applied to horizontal pipe by
Fisher and Yu (1975) and then by Badie (2000). A
schematic of the original device with small depth of field
(Arnold and Hewitt 1967) is shown in Figure 2.
In the axial view method, the flow is viewed along the axis
of the pipe so that the optical system is looking directly
into the flow. This has the advantage that it allows one to
study the circumferential distribution of liquid films in
annular flows, the entrainment of droplets from a liquid
film, and the radial motion of droplets in the gas.
This device is composed of a viewer, a vertical tube and a
camera unit. In the experiments described by Arnold and
Hewitt (1967) and Hewitt and Roberts (1969), the viewer
was fitted on the top of a vertical Perspex tube with an


Figure 2: Principle of the axial view technique applied to
9Nuicl flow. Axial viewer used by Arnold & Hewitt


The In-line Axial Viewing System designed by Badie
(2000) for horizontal gas-liquid flow with low liquid
loading was based on the axial viewer for horizontal flows
investigations developed by Fisher and Yu (1975) and
Fisher et al.(1978) based on the general idea reported by
Hewitt and Roberts (1969).
This viewer, shown schematically in Figure 3, is less
complex to machine and more flexible. Moreover, a part of
the main design is a drastically improved air purge system
that is used to keep the flat viewing window clear of all
impinging droplets.


Main now






Figure 3: The basic design of the In-Line Axial Viewing
System (Badie, 2000).











A protrusion of the viewer provides a platform so that any
liquid on the upper wall of the main section drained around
the viewing tube, rather than into it. Flow is diverted into
two tubes to let space for the viewing and air purge
systems. The air purge system (cf. Figure 4) is blowing
against the direction of the flow to maintain the window of
the viewing tube free of liquid. A camera system is
focused on an illumination plane much further upstream
of the diverted flow than in previous studies.







v--




Figure 4: Side view and front view of the air distributor
(Badie, 2000).

The WASP (Water, Air, Sand and Petroleum) high pressure
facility used for the present experimental investigation of
two-phase air-water stratifying-annular flow has a 78mm
test section. A schematic diagram of the facility is shown in
Figure 5.


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


LY
"'""'


Figure 6: Images of air-oil flows (superficial gas velocity of
15 m.s' and superficial liquid velocity of 0.01 m.s' (Badie
et al, 2001)).


Figure 7: Successive frames of air-oil flows (superficial gas
velocity of 15 m.s' and superficial liquid velocity of 0.01
m.s' (Badie et al, 2001)).

Increasing the liquid superficial velocity at constant gas
velocity increases the amount entrained. This is most
probably due to the increased liquid layer thickness at the
bottom of the channel giving rise to increased interfacial
wave activity and, hence, an increase in entrainment rate.
An increase in superficial gas velocity at constant liquid
superficial velocity leads to an increase in the flow rate of
entrained droplets. The more the superficial gas velocity is
increased, the more the activity in the form of waves is
increased and, since the waves are the source of entrainment
events, this leads to more droplet entrainment. For a liquid
superficial velocity of 0.02 m.s- the onset of entrainment
occurs at a gas velocity of around 19 m.s' for air-water flow
and at around 17 m.s' for air-oil flow(Badie, 2000).

The experiments reported here also used the WASP facility
and the axial view section developed by Badie (2000).
However, a new camera (an i Speed 2 monocolor Olympus
camera) was employed which allowed images to be
obtained at much higher frequencies and resolution
(typically 500 fps with an active sensor area of 800 X 600)
and this allowed much more detailed visualization of the
entramment events.


3. Results and Discussion

Key results of the new visualisations of the entrainment
phenomena occurring at the air-water interface are presented
here.
A typical bag breakup sequence is shown in Figure 8. A
perturbation is formed on the interface and this is undercut
by the gas to form a bag. This bag has a thick filament of
liquid around its base and, when the bag bursts, this filament
breaks into an are of droplets. The bag is initiated by the gas
phase undercutting a large wave on the interface. Thus, the
pressure in the bag is higher than the pressure above it and
this leads to bag growth. Eventually, the bag bursts and this
leads to radial release of the air in the bag and the
acceleration away from the interface of the are of droplets
formed from the filament


Figure 5: Schematic diagram of the WASP facility (Badie
et al, 2001).

Badie (2001) investigated various aspects of the process of
entrainment and reported the effect of parameters such as
the superficial liquid velocity, the superficial gas velocity
and the fluid viscosity. He also measured liquid holdup
and droplet mass flux.

Some axial view photography sequences from videos of
the flow in 79 mm diameter pipe obtained by Badie (2001)
are shown here. An example is a so-called "ballooning"
event, shown in Figure 6. At low gas velocities, drop
formation is rare but, as the gas velocity is increased, drops
may be formed by "bag breakup" (Figure 6), breakup of
larger drops (Figure 7) or breakup of ligaments torn from
the film.





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

breakup. In Figure 10, we can observe a ligament on the
left hand side of the pipe and from this ligament is created
a droplet which has a large initial radial velocity allowing
to be projected ballistically. The ligament is formed from a
small perturbation on the interface. The proximity of the
perturbation to the wall is an important factor in ligament
formation and breakup. The nearer the perturbation to the
wall, the more likely are the ligament and its droplet
daughters to be projected upwards.


Figure 8: Images of air-water flows with Superficial gas
velocity = 11.6 m.s' and Superficial liquid velocity =
0.036 m.s ', 500fps Filament.




Some of the bag breakup events observed involve a complex
evolution and breakup of multiple bags. Such a sequence is
illustrated in Figure 9. The process starts with the
appearance of several deformations of the interface and
these develop to form the complex bag whose outer limit
consists of thick ring of water (a "strap") but which has a
series of compartments separated by filaments whose source
was the original deformations of the interface. For the case
shown in Figure 9, five compartments in the bag are visible
during the growth of the bag, The compartments break, in
succession with the compartment nearest the wall being
ruptured first. The last remaining remnants of the bag are
formed from the strap that delineated the outside of the
compound bag. The strap ruptures into quite large droplets.
One node on the right of the bag evolves as a ring, which
extends to break up into droplets. One of the droplets
formed from this ring reaches the top of the pipe wall.

















Figure 9: Images of air-water flows with Superficial gas
velocity = 11.6 m.s' and Superficial liquid velocity =
0.036 m.s ', 500fps Bag break-up.


Figures 10 and 11 show cases of ligament rather than bag


111


Figure 10: Images of air-water flows with Superficial gas
velocity = 13 m.s' and Superficial liquid velocity = 0.036
m.s ', 500fps Ligament.

Figure 11 shows the formation of multiple ligaments, again
arising from independent small perturbations. In this
particular case, the drops formed from the ligaments did
not have sufficient upwards momentum to reach the top of
the pipe.


Figure 11: Images of air-water flows with Superficial gas
velocity = 11.6m.s' and Superficial liquid velocity = 0.036
m.s' ,500fps -Ligament.

In the above, we have made a distinction between bag
breakup (Figures 8 and 9) and ligament breakup (Figures
10 and 11). However, events may occur which demonstrate
the characteristics of both types of drop formation, as
exemplified by the sequences shown in Figures 12 and 13.
Figure 12 shows the formation of ligaments arising from
perturbations formed at the top of a breaking bag. The
ligament thus formed then breaks up leading to the
entrainment of at least three droplets, one of which reaches


==== Ls l





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

The axial viewing technique has allowed the visualisation of
the complex processes occurring in stratifying annular flow.
The classical processes of bag and ligament breakup are
clearly seen but the breakup processes can often be a
combination of the two processes.
Acknowledgements
This work has been undertaken within the Joint Project on
Transient Multiphase Flows and Flow Assurance. The
Author(s) wish to acknowledge the contributions made to
this project by the UK Engineering and Physical Sciences
Research Council (EPSRC) and the following: Advantica;
BP Exploration; CD-adapco; Chevron; ConocoPhillips;
ENI; ExxonMobil; FEESA; IFP; Institutt for Energiteknikk;
PDVSA (INTEVEP); Petrobras; PETRONAS;
Scandpower PT; Shell; SINTEF; StatoilHydro and TOTAL.
The Author(s) wish to express their sincere gratitude for this
support.

References
Arnold, C. R. and Hewitt, G. F. (1967) Further
developments in the photography of two phase gas-liquid
flow. J. Photogr. Sci., 15, 97-114.
Azzopardi, B.J. (1983). Mechanisms of entrainment in
annular two phase flow. AERE R 11068, UKAEA, Harwell,
UK
Azzopardi, B.J. (1987). Observation of drop motion in
horizontal annular flow. Chem. Eng. Sci., Vol.42(8), pp
2059-2062.
Azzopardi, B.J. (1997). Drops in annular two-phase flow,
Int. J. 1Multiph. Flow 23 (Suppl.): 1-53.
Badie, S. (2000). Horizontal stratifying/annular gas-liquid
flow. Phd Thesis, University of London, London, UK.
Badie, S., Lawrence, C. J. & Hewitt, G. F. (2001). Axial
viewing studies of horizontal gas-liquid flows with low
liquid loading, Int. J. Miultiph. Flow 27, 1259-1269.
Cooper, K.D., Hewitt, G.F., and Pinchin, B. (1964).
Photography of two-phase gas-liquid flow. Journal of
Photographic Science, Vol.12, pp.269.
Fisher, S. A. and Yu, S. K. W. (1975) Dryout in serpentine
evaporators. Int. J. 1Multiphase Flow, 1,p.771.
Fisher, S. A., Harisson, G. S. and Pearce, D. L. (1978)
Instrumentation for localised measurements with fast
response in liquid/vapour flows. Symposium on
1Measurements on Polyphase Flows, ASIME Winter Annual
""i, ;~, San Francisco, California, USA.
Hewitt, G. F. and Roberts, D.N. (1969) Investigation of
interfacial phenomena in annular two-phase flow by means
of the axial view technique. UK AEA Report, AERE -
R6070.


the top of the pipe.


Figure 12: Images of air-water flows with Superficial gas
velocity = 11.6 m.s' and Superficial liquid velocity =
0.036 m.s ', 500fps Ligament formed from growing bag
and breaking up into droplets which may reach the top of
the pipe..
Figure 13 presents the successive frames of a mechanism
of droplet entrainment which may also lead to droplet
deposition at the top of the pipe. Two "lumps"of liquid are
seen. The liquid lumps have small perturbations on them
and collide as shown. A ligament extends from the first
lump and the second lump undergoes bag breakup with the
formation of a ring filament. The ring filament then breaks
up to form two ligaments, one of which has sufficient
kinetic energy to retain its ligament shape and extend again.
This ligament then splits into droplets, the largest droplet
being located at the top end of the ligament. This droplet is
released with sufficient vertical velocity to reach the top of
the pipe. The velocity of this ballistic droplet was around
Im/s.
Here, one may speculate that the energy acquired by the
droplet emitted from the ligament is due to the collision of
the two lumps.


H

5


,I H


Figure 13: Images of air-water flows with Superficial gas
velocity = 11.6m.s' and Superficial liquid velocity = 0.036
m.s ', 500fps Droplets entrainment at the top of the pipe.

Conclusion


I

I 5


H






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Hellitt, G. F. and Whalley, P.B. (1980) Advanced optical
instrumentation methods. Int. J. Miultiph. Flow, 6, 139-156.

Jacollitz and R. S. Brodkey (1964), An analysis of geometry
and pressure drop for the horizontal, annular, tivo-phase
Hlol of water and air in the entrance region of a pipe, Chem.
Engng Sci. 19, pp. 261-274.

Whalley, P. B., Azzopardi, B. J., Pshyk, L. and Hewitt, G. F.
(1977) Axial view photography in annular tivo-phase loly.
European Two-Phase Flow Group, Grenoble, June.

Whalley, P. B., Hewitt, G. F., Azzopardi, B. J. and Pshyk, L.
(1977) Axial view photography of waves in annular
tivo-phase Hlol. UK AEA Report, AERE R8787.

Whalley, P. B., Hewitt, G. F. and Terry, J. W. (1979)
Photographic studies of tivo-phase Hlol using a parallel
light technique. UK AEA Report, AERE R93 89.

Woodmansee, D.E., Hanratty, T.J. (1969) Mechanism for the
removal of droplets front a liquid surface by a parallel air
Hlol. Chent. Eng. Sci. 24, 299-307




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - Version 2.9.7 - mvs