Experiments of water flushing an initially empty undulating pipeline with five upward and downward segments
inclined at 40 are run in a 0.016 m I.D. pipe at atmospheric pressure. Experiments are made by opening a valve
connecting the inlet to a water reservoir at constant height. Four cases with different reservoir heights are investigated.
The first three cases partially fill the pipeline before the water flow stops and in the last case, the pipeline is completely
flushed with water. These end states compare well to simulations from a slug tracking scheme where slugs are
modelled dynamically as moving objects on a coarse grid. Time varying water front positions are obtained from video
analysis of the experiments and compared with similar numerical results.

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

Experiments of water flushing an initially empty undulating pipeline with five upward and downward segments
inclined at 40" are run in a 0.016 m I.D. pipe at atmospheric pressure. Experiments are made by opening a valve
connecting the inlet to a water reservoir at constant height. Four cases with different reservoir heights are investigated.
The first three cases partially fill the pipeline before the water flow stops and in the last case, the pipeline is completely
flushed with water. These end states compare well to simulations from a slug tracking scheme where slugs are
modelled dynamically as moving objects on a coarse grid. Time varying water front positions are obtained from video
analysis of the experiments and compared with similar numerical results.

Introduction

When liquid starts flowing in an initially empty un-
dulating pipeline, liquid can accumulate in low points
and form slugs which completely block the pipe cross-
section. As liquid flows over a bend and then downhill,
stratified flow will occur where the liquid flows down-
wards. If enough liquid accumulates in a low point,
slugs will form and eventually start propagating along
the pipeline. As they flow uphill, the liquid slugs may
decay again as they are penetrated by gas bubbles. If
there is not sufficient inlet pressure to flush the pipe com-
pletely, large gas bubbles between slugs can turn and be-
come trapped at high points in the pipeline. Bubbles will
flow in the direction of lower pressure, usually the outlet.
Laboratory experiments on water flushing in an initially
empty undulating pipeline are conducted and compared
with simulations from a slug tracking scheme.
In the slug tracking scheme, stratified sections be-
tween slugs are modelled on a coarse fixed grid while
slugs are modelled as moving objects. Slugs, or similar
moving objects, have boundaries corresponding to sharp
moving fronts thus avoiding numerical diffusion and the
need for excessive grid refinement (Kj01aas 2007). Front
physics such as bubble nose velocities or gas entrain-
ment can also more easily be implemented in a track-
ing scheme as compared to a capturing scheme. Track-
ing schemes have also been tested for plug simulations
where plugs are treated as rigid moving objects (Kj01aas

2007) and for large roll waves propagating in a simi-
lar way to slugs (De Leebeeck and Nydal 2009; Lee-
beeck and Nydal 2010). The tracking scheme has also
compared well with severe slugging experiments in a S-
shaped riser (Nydal et al. 2001).
The bubble turning process, where a bubble prop-
agating downwards reverses direction and then moves
counter-current to the liquid flow, is a key phenomenon
in the flushing experiments. A turning criteria based on
a critical flow rate balancing friction and gravity is used
in the tracking scheme (Johansen 2006; Nydal 1998;
Reynolds and Yitayew 1995).
Four different experimental cases with inlet pressures
corresponding to the weight of constant water levels at
0.450 m, 0.675 m, 0.750 m, and 0.825 m above the inlet
are run in an undulating pipeline. The end state of wa-
ter flow in the pipeline, whether completely flushed with
liquid or blocked, is compared to simulations with the
slug tracking scheme. Time varying liquid front posi-
tions are obtained from video analysis also for compari-
son with corresponding numerical results.

Experiments

Experiments were conducted in the multiphase flow lab-
oratory at NTNU in a 0.016 m internal diameter (I.D.)
clear acrylic undulating pipeline containing two peaks
and two valleys. A schematic of the experimental setup
is shown in figure 1. Constant pressure was achieved at

Figure 2: Schematic representation of the computa-
tional objects in the slug tracking scheme.
The liquid phase is shown in gray and the gas
phase in white. Dashed lines indicate section
centers while solid lines indicate section bor-
ders. The index notation for section centers
and borders is shown at the top. Section J7 is a
slug between the stratified sections J7 1 and
J + 1. Arrows indicate the movement of slug
borders.

*Water level at 0.825 m where the pipeline was
flushed completely with water.

Simulations

Simulations of filling the undulating pipeline were run
with a slug tracking code (Kj01aas 2007; De Leebeeck
and Nydal 2009; Leebeeck and Nydal 2010) which was
written in C++ using object oriented programming tech-
niques. The slug tracking scheme uses a one dimen-
sional finite volume method with a moving grid. Slug
flow is represented with alternating slug objects that
completely fill the pipe cross-section with liquid and
bubble regions in between slugs, where the gas flows
over the liquid phase. The two-fluid model of stratified
flow is solved in the bubble regions on a stationary stag-
gered grid where phase velocities are determined at sec-
tion borders, and pressure and masses are determined at
section centers. Slug sections are modelled as moving
objects where liquid phase velocity, slug length, front
and tail velocities are determined from mass and mo-
mentum balances. A schematic of the computational ob-
jects, slugs and stratified sections, is shown in figure 2.
The index notation shown at section centers and borders
is used in later equations and the variables are defined
below.
For the purposes of these simulations, flow is assumed
isothermal so the energy equations can be neglected. It
is also assumed that there is no mass transfer between
the phases through evaporation or condensation. Gas
entrainment and droplets in the gas phase are also ne-
glected.
Stratified flow sections. The two-fluid model is ap-
plied in stratified sections on a stationary grid. Phase
velocities are determined at section borders by solving

107S

Figure 1: Schematic of the undulating pipeline setup.
The test section consists of five pipe segments
labeled P1 through P5 inclined at 40 The
magnetic valve allows liquid to flow into the
pipeline from the open tank. Water filled areas
are colored gray.

the inlet by attaching a large open water tank which was
elevated with a manual jack above the inlet. The inlet
pressure was determined by the weight of the liquid col-
umn above it. The tank volume was large compared to
the volume of the undulating pipeline so that the tank
level was essentially constant during pipe flushing. The
water was allowed to flow through the initially empty
pipeline by opening a magnetic valve (ASCO Magnetic
Diafragma) until it came to rest. The different straight
rigid pipe segments were inclined at 40 degrees upward
or downward after each bend. The first segment was
0.91 m long and inclined upward to the first peak. This
was followed by four alternating downward and upward
segments each 0.83 m long. The rigid segments were
connected with 0.16 m long clear flexible hoses at the
bends. The outlet was at the top of the last upward seg-
ment. In preparation for the next experiment, pressur-
ized air was used to empty the pipeline and the water
was pumped back into the tank.
Fluorescent green dye (Merck natrium and sodium)
was added to the water so that the propagating liquid
front could be recorded on video with a high definition
video camera (Sony HDR-UX7E) at 25 fps with a reso-
lution of 1920 x 1080 pixels. The position of the green
liquid front in each frame and the corresponding time
stamp was then extracted from the video using image
analysis scripts developed in Matlab. The video was
taken for about 12 see in each experiment, which was
long enough for the water in the pipe to come to rest or
for the pipe to be flushed completely with water. The end
state water height in the pipe segments was also mea-
sured.
Experiments with four different water levels in the
tank above the inlet were run:

Water level at 0.450 m where liquid settled in the
first upward pipe segment Pl.

Water level at 0.675 m where liquid settled in the
first three pipe segments.

Water level at 0.750 m where liquid settled in all
five of the pipe segments.

the momentum balance equations. The following mo-
mentum balance equation applies to either the gas or the
liquid phase at a border j:

A ~uM M u -
As U (Up

Ubj)

h; h; 1

S1 SL,
2 4
1 SL,
An ;|p s4|( p s4 (1)
2 4
where M~ and as are the averaged values of the phase
mass and area fraction at the border. a represents a
change in a quantity e.g. at is the time step. U is the
phase velocity, Ub is the border velocity, A is area, L
is length, P is pressure, a is the angle of inclination, hi
is the liquid height, S is the perimeter, A is the friction
factor, p is density, and g is the acceleration of gray-
ity. Subscripts are i for interface quantities and n for the
neighboring phase.
Pressure is determined from the volumetric flow bal-
ance using a combination of mass conservation and the
equations of state in both phases, as follows:

(a; py~a 1 a; 8pl LJAAP ,
i3p pt~ d pl at
+ A(Ub,j 1 Ub,j

+ 1 ; A (Uk,y I Ub,j 1)

-C -;A (Us, Ub,j)] = 0 (2)

In the above equation, m = M/AL is the mass per
pipe volume and subscripts are g for gas phase, I for
liquid phase, and k representing either phase. As equa-
tion (2) is not formulated in a mass-conservational man-
ner, a source term is added in the following time step to
ensure consistency with the mass conservation equation
over time. This source term is as follows:

ALJ mi, J I. r
+ --1 (3)
at p, yJ

Next, the mass balance equations are solved. The
mass balance equation in a section J7 for either phase
is given below:

As the slug grid moves, stratified sections are adjusted
dynamically according to the movement of slugs. Large
sections are split while short ones are merged together.
Slug sections. The dynamics of the flow in a slug
can be determined from a mixture momentum equation
and a slip relation. A simplified version of the mixture
momentum equation is the liquid momentum equation
without the gas interaction term. The liquid momentum
equation in a slug J7 takes the following form:
AUIJMJ I,
at L IJ(U,-Ubj

1 S1tJL
2hJptJ |U1,J| (U1,J) (5)
The above equation has a similar form to equation (5),
except interface friction can be neglected (no interface).
Gas velocity in a slug is determined using a slip rela-
tion or assuming no slip, by the following general slip
equation:

Up, J = Sd (U1, J + 00) (6)
where Sd is the distribution slip ratio and vo is the aver-
aged drift velocity.
The change in mass in a slug for either phase in a
given time step is the difference in mass flux in and out,
determined from the same mass balance equation (4) as
in stratified sections.
Slugs are modelled as objects with moving bound-
aries, which have either a front velocity or a bubble nose
(tail) velocity. If a slug's left boundary velocity is greater
than its right boundary velocity, then the slug length will
grow. The front velocity is determined from a mass bal-
ance across the front as follows:

T/ front (7)
frontHslug Hstrat
where H is the liquid holdup and subscripts stral relates
to stratified sections while slug relates to slug sections.
The bubble nose velocity is as follows (Bendiksen et al.
1996; Bendiksen 1984):
Unose = CoUmix + Uo (8)
where Umiz HslugUl,slug + (1 Hslug)Ug,stg is
the local mixture velocity in the slug. Values for Co
and Uo which give the largest Unose are applied: if
|Umix| < 3.60/cos0 then Co 1.05 + 0.15sin 0
and Uo Uov + Uoh, otherwise Co = 1.2 and Uo
Uo,. Uo, and Ush are Uov = 0.350sin0 and Us =
f0.54@gcosB.
The direction of slug movement and therefore
whether a border is a slug front or a bubble nose is de-
termined from a bubble turning criterion under the as-
sumption that large bubbles between slugs will move

aMt (My

Ubt,j)

Upstream values of My are used in the above equation.

in the direction opposite to pressure gradient (Kj01aas
2007). The turning criterion is the point where friction
and gravity balance each other and pressure gradient is
zero (Nydal 1998; Johansen 2006). It is particularly im-
portant in downward flow where bubbles can reverse di-
rection relative to the liquid flow.
The initiation of slugs occurs when the liquid level in
a low point exceeds a user specified maximum holdup,
for example H 0.99. In that case, the stratified section
will be converted to a slug section. Slugs are removed
either when they exit the pipe or when their length goes
below a user specified minimum value.
Computational Sequence. The computational se-
quence in a given time step begins with determining
the equation coefficients on moving borders in slug sec-
tions. Next the system of pressure and momentum bal-
ance equations (equations (5) and (2)) is solved simul-
taneously by inverting a banded matrix. The solution
is found using direct Gauss elimination and gives phase
velocities and pressures. The border velocities and po-
sitions of slugs are then updated. This is followed by
solving the mass balance equations, with implicit time
integration in the stratified sections. In slug sections,
the mass balance equations are solved explicitly. Next,
phase masses, phase densities from equations of state,
and volume errors are updated. Slugs are inserted or
deleted and sections are merged and split according to
the the movement of slug borders. Lastly, the time step
is incremented and the sequence repeated.
Simulation setup. The undulating pipeline was simu-
lated as a series of five straight pipe segments inclined
at 40" degrees alternating upward and downward. Each
pipe had a 0.016 m I.D. and was 0.98 m long to account
for the additional length of the flexible hose at the bends.
A vertical column filled with the same water height as in
the inlet tank in the experiment was modelled at the in-
let. A constant level of water at the top of the vertical
pipe was assumed. The pipeline outlet was set at atmo-
spheric pressure. The pipeline was initially empty and
the propagating front coming in from the liquid column
was simulated for a total of 60 sec. Water properties
were constant and the air was treated as an ideal gas.
Effect of grid sizes. Grid sizes are adjusted dynami-
cally according to the movement and growth of slugs.
Minimum and maximum grid lengths in numbers of pipe
diameters were: one diameter for the minimum slug
length at which point slugs are killed, four diameters for
the minimum stratified section length when the section
is merged with its neighbor, and 15 diameters for the
maximum stratified section length when the section is
split.
The maximum grid size will determine how quickly
slugs form at low points. It takes longer for a large com-
putational section to fill with liquid and become a slug.

Figure 3: Images of the simulated end state in the undu-
lating pipeline with different maximum grid
sizes in multiples of pipe diameter D. The
water height above the inlet is 0.675 m. Wa-
ter filled areas are shown in black, white areas
represent the air phase.

As an example simulations with an inlet liquid column
height of 0.675 m were run with different maximum grid
lengths while the minimum slug length and stratified
section length were fixed at 1 and 4 diameters respec-
tively. The end state of simulations with a maximum
grid length of 5, 10, 15, and 30 diameters are shown
in figure 3. The results were qualitatively similar but
there was more liquid in the low point as the grid size
increased to 30 diameters.
Pipe friction and additional losses. Frictional losses
were larger in the experiments than in the first compu-
tations. Therefore an attempt was made to approach the
experimental conditions by adding the component losses
of bends and a valve to the simulated system. These
were introduced into the simulation assuming loss coef-
ficients K for a single phase pipeline from (White 2005).
Table 1 lists the components causing additional losses in
the experimental setup and their estimated loss coeffi-
cients assuming the pipeline is completely flushed with
liquid. The total loss coefficient EK 13.5 was as-
sumed in simulations.
Pipe roughness was also specified, t 1 x 10-s m
for acrylic pipe, and used in determining the pipe friction
factor, as follows (Haaland 1983):

1 ~6.9
=-1.81og +ee !~1 1 (9)

where Re is the Reynolds number and D;, is the hy-
draulic diameter. Head loss from pipe wall friction is as

Figure 4: Images showing the effect of the total loss co-
efficient EK on the end state of simulations.
The water level above the inlet is 0.675 m.
Water filled areas are shown in black, air filled
areas in white.

hf = A X ;, |"
S D '>-9

where Lpipe is the length of the pipeline. The head loss
from additional pipe components is as follows (White
2005):

IUIU
h, = CK
2g

Figure 5: Experimental and simulated end states for wa-
ter height 0.450 m above the inlet.

Results and Discussion

Results from the experiments included videos, transient
plots of the liquid front position, measurements of the
end state liquid column heights in each pipe segment if
the test section was not completely flushed, and times to
reach the final state. The results from the four experi-
mental cases run have been compared to simulations.
The simulations gave qualitatively similar results to
the experiments when the liquid in the test section
reached equilibrium or when the pipe was flushed com-
pletely. Snapshots of the end states from experiments
and simulations are shown together for each experimen-
tal case in figures 5 through 8.
For the largest liquid level at 0.825 m, the pipeline was
flushed completely with water. The measured height of
the water column in each pipe segment at the final state
and the time to reach the final state are listed in table 2
for experiments and simulations. The end states and fi-

Head losses are related to shear stressT 7s follows:

qLI ip
PUD

hi,,,,, = hf + h,

Rearranging equations 10, 11, and 12 gives the
frictional shear stress including both pipe wall fric-
tion losses and losses from additional pipe components
which will appear in the momentum balance equations:

Taking an example with the water height above the in-
let equal to 0.675 m, the effect of additional component
losses was investigated in simulations. The end state
images corresponding to different total loss coefficients
are shown in figure 4. The expected behavior from the
experiment was that the first pipe filled completely and
liquid collected in the first low point. Without the com-
ponent losses or if the losses were approximately half
of the estimated total EK = 13.5, liquid completely
filled the first three pipes and accumulated in the second
low point, behavior which differed from the experiment.
Once component losses approached the estimated value
EK = 13.5, the simulations produced the expected re-
sults.

Figure 7: Experimental and simulated end states for wa-
ter height 0.750 m above the inlet.

nal water heights were similar between experiments and
simulations, but the time to reach the final state differed
by 1 to 4 sec.
The transient behavior during pipe flushing was also
investigated experimentally and numerically. Figure 9
shows snapshots at different times during filling when
the liquid was 0.825 m above the inlet. In the first snap-
shot, the liquid has reached the top of Pl. This segment
was filled faster in the simulation than the experiment
where the liquid reached the first peak in 1.05 sec as op-
posed to 1.80 sec experimentally. The liquid flowed as a
stratified film down P2 into the first low point where liq-
uid collected and formed a slug. The film traveled faster
than the liquid front which continued to propagate down
P2 as well. The second snapshot shows a slug forming

Table 2: Measurements of the liquid height in pipe seg-
ments at the final state and the time to reach
the final state (end time) in experiments (expr)
and simulations (sim). Segments P1 to P5 cor-
respond to the first through fifth pipe segments
labeled in figure 1.
Level Water height in segment (m) End
above inlet time
(m) (s)
P1 P2 P3 P4 P5

Exp 0.825 full full full full full 7.30
Sim 0.825 full full full full full 11.5

in the first low point at 2.68 sec experimentally and 2.42
see in the simulation. The slug then flowed up P3 where
it decayed. The liquid front followed the decaying slug
up P3 and completely filled the segment (third snapshot).
The simulation was slower to reach this point in 4.32
sec as opposed to 3.80 sec experimentally. Just as in P2,
the liquid flowed as a stratified film down P4 and slugs
formed in the second low point. The first slug is shown
in the fourth snapshot. It formed later in the simulation
at 5.96 sec compared to 4.84 sec in the experiment. In
total, five slugs formed at the second low point in the
simulation while four formed in the experiment. Slugs
going up the last pipe segment P5 are shown in the fifth
snapshot. The last snapshot shows the end state where
the pipeline is completely flushed with liquid.
Figure 10 shows snapshots at different times during
filling when the liquid was 0.675 m above the inlet. In
the first snapshot, the liquid has reached the top of P1
at the same time in the simulation and the experiment.
The liquid film then flowed down into the first low point
where a slug formed after 2.96 sec in the simulation and
after 2.24 sec in the experiment. The liquid front has also
entered into the downward segment P2. The low point
then continued filling until an end state was reached, the
last snapshots in figure 10. The liquid front reversed di-
rection (bubble turning) and eventually came to rest at
the top of the first peak.
More quantitatively, the plots in figure 11 show the
position of the first liquid front plotted against time. The
video analysis of the experimental data was not sensi-
tive enough to pick out the front (or tail) of individual

Figure 8: Experimental and simulated end states for wa-
ter height 0.825 m above the inlet.

Figure 6: Experimental and simulated end states for wa-
ter height 0.675 m above the inlet.

Figure 9: Snapshots from experiment and simulation with a liquid level of 0.825 m above the inlet. The time when
the snapshot was taken is shown to the left of the images. The water phase is dyed green in experimental
images and shown as black in the simulation snapshots.

Figure 10l: Snapshots from experiment and simulation with water 0.675 m above the inlet. The time when the snapshot
was taken is shown to the left of the images. The water phase is dyed green in experimental images and
shown as black in the simulation snapshots.

Figure 12: Plots of simulated slug front and bubble nose
horizontal positions against time. The top
plOt has a water level above the inlet of 0.750
m while it is 0.825 m in the bottom plot.

uid front reached the remaining pipe segments at a later
time than in the experiments. At 0.450 m, the liquid
level oscillated before settling both experimentally and
in the simulation. At 0.675 m and 0.750 m, the simu-
lated front reached almost the same final position as in
experiments. At 0.825 m, the first simulated slug to exit
the pipe reached the outlet much later than in the exper-
iment but the end result was the same: the pipeline was
flushed completely with liquid.
Additional information about slug formation and de-
cay can be obtained from simulations. Slugs form when
liquid collects in the low points and they may decay
as they propagate along the upward inclined pipe seg-
ments. The plots in figure 12 show the positions of slug
fronts and the bubble noses at the slug tail from simu-
lations. When the bubble nose catches up to the slug
front, the slug decays. In the 0.750 m simulation, two
slugs formed at the first low point and eventually de-
cayed. The first one decayed in the upward inclined seg-
ment P3, while the second one decayed in the beginning
of the downward segment P4. In the 0.825 m simulation,
one slug formed in the first low point and it decayed in
P3. After forming, the slugs grew slightly before shrink-
ing and decaying.

Conclusions

Experiments of water flushing an initially empty undu-
lating pipeline have been run in a 0.016 m I.D pipe at
atmospheric pressure. The results were compared with
simulations from a slug tracking scheme investigating
similarities between the final state and the transient liq-
uid front position.

S4 6
Time (sec)

8 10 12

Figure 11: Plots of horizontal front position against
time for experiments and simulations. The
water level above the inlet corresponding to
each plot is from top to bottom: 0.450 m,
0.675 m, 0.750 m, and 0.825 m.

slugs. The position obtained was the position of the liq-
uid front furthest from the inlet. For example, at the end
state with a liquid level 0.675 m above the inlet, this
would be the position of the top of the liquid column in
P3. The video analysis also picked out the position of the
front of the liquid film in downward inclined pipe seg-
ments (P2 and P4). The liquid front in both upward and
downward pipes can be picked out from simulations run
with the slug tracking scheme. Since the position of the
liquid front was not comparable with the position of the
liquid film as it ran downward, positions in downward
inclined pipes were omitted in the plots. The front po-
sitions plotted correspond to the propagation of the first
front in the upward inclined segments against time.
At all liquid levels above the inlet, the simulated re-
sults matched quite closely with the experimental front
positions in filling Pl. After that, the simulated liq-

At first, simulations showed the pipes were com-
pletely flushed when liquid flow should have stopped.
Improved predictions were obtained by including stan-
dard loss terms for valves and bends.
The flow transient includes liquid fronts propagating
upwards, bubble turning in the downwards pipes, slug
initiation in bends and a subsequent slug decay by bub-
ble penetration. These phenomena were observed exper-
imentally and also reproduced in the slug tracking sim-
ulations.
The end state results from the simulations were qual-
itatively similar to the experiments. The simulations,
however, differed from the experiments by up to 4 sec
in the time to reach the final state. The transient liquid
front positions were very similar in simulations and ex-
periments in the first pipe segment, however after that
simulated front positions lagged behind the experimen-
tal results.

Acknowledgements

Financial support from Total E&P Norge is gratefully
acknowledged by A. De Leebeeck