Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 2.3.2 - Influence of surface tension on downward gas transport
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 Material Information
Title: 2.3.2 - Influence of surface tension on downward gas transport Industrial Applications
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Pothof, I.W.M.
Schuit, A.D.
Clemens, F.H.L.R.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: gas pocket
two-phase flow
hydraulic jump
bubbly flow
surface tension
inclined pipe
 Notes
Abstract: Experimental data on the co-current two-phase flow in downward sloping pipelines is scarce (Pothof 2008), despite the numerous off-design conditions in the water industry, process industry, hydropower and nuclear industry that are dominated by the co-current two-phase flow. Lubbers (2007a, Lubbers 2007b) has shown that a volumetric air discharge in the order 0.1% of the water discharge may cause a significant additional head loss due to the air accumulation in the downward slope of an inverted siphon. A discrepancy between results of laboratory tests and field tests has been observed in a previous study (Lubbers 2007a). Lubbers concluded that fluid properties and/or pipeline conditions must have caused the differences between the lab and the field. To bridge this gap, an industrial scale field test rig (D = 192 mm, L = 40 m, 􀁔 = 10°) was built. This facility was located at the wastewater treatment plant in Hoek van Holland, Netherlands, which enabled experiments with three different fluids: clean water, clean water with surfactants and raw wastewater. This paper will present and analyse the effect of surface tension on the air accumulation and air transport. The addition of surfactants to clean water significantly improves the gas transport. Nevertheless, the lower surface tension of wastewater does not enhance the gas transport due to the dynamic character of the surface tension.
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
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Bibliographic ID: UF00102023
Volume ID: VID00049
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 232-Pothof-ICMF2010.pdf

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


Influence of surface tension on downward air-water transport


I.W.M. (Ivo) Pothof*, A.D. (Tonny) Schuit t F.H.L.R. (Francois) Clemenst

Deltares I Delft Hydraulics, department Industrial Hydrodynamics, Delft, Netherlands
t Department of Water Management, Delft University of Technology, Delft, Netherlands
Ivo.PothofO@deltares.nl


Keywords: gas pocket, two-phase flow, hydraulic jump, bubbly flow, surface tension, inclined pipe


Abstract

Experimental data on the co-current two-phase flow in downward sloping pipelines is scarce (Pothof 2008), despite the
numerous off-design conditions in the water industry, process industry, hydropower and nuclear industry that are dominated by
the co-current two-phase flow. Lubbers (2007a, Lubbers 2007b) has shown that a volumetric air discharge in the order 0.1% of
the water discharge may cause a significant additional head loss due to the air accumulation in the downward slope of an
inverted siphon. A discrepancy between results of laboratory tests and field tests has been observed in a previous study
(Lubbers 2007a). Lubbers concluded that fluid properties and/or pipeline conditions must have caused the differences between
the lab and the field. To bridge this gap, an industrial scale field test rig (D = 192 mm, L = 40 m, 0 = 100) was built. This
facility was located at the wastewater treatment plant in Hoek van Holland, Netherlands, which enabled experiments with three
different fluids: clean water, clean water with surfactants and raw wastewater.
This paper will present and analyse the effect of surface tension on the air accumulation and air transport. The addition of
surfactants to clean water significantly improves the gas transport. Nevertheless, the lower surface tension of wastewater does
not enhance the gas transport due to the dynamic character of the surface tension.


Introduction

Gas pockets are an important cause of capacity reduction in
existing sewerage pressure mains with a negligible static
head, which is typical for drainage systems in highly
urbanized deltas. The traditional design approach for
wastewater mains allowed for admitting gas (mainly air) in
the system via the pumps and via air valves. The pipeline
was designed to transport the gas pockets through the line in
order to prevent the installation of air valves. The following
design equation was used to determine the required water
velocity us to transport gas pockets to the bottom of a
downward slope with inclination 0.

F = = 1.23 sin0 (1)


The flow number F,] (also known as pipe Froude number) is
the most appropriate dimensionless quantity to characterize
two-phase flow with elongated gas pockets and associated
flow regime transitions, like slug-plug, stratified-slug and
blow-back (Bendiksen 1984, Benjamin 1968, Montes 1997,
Ruder and Hanratty 1990), provided that the scale is large
enough to neglect the influence of surface tension. Pothof
(2010) has shown that the transition from blow-back to plug
flow becomes independent of the surface tension if the
Edtvos number Eo pogD2/ > 5000. Since the air
transport in the blow-back regime is based on turbulent
bubble transport in a series of hydraulic jumps (Figure 1),
the lower surface tension of wastewater may have a positive
effect on the air transport capacity in this flow regime.


The effect of reduced surface tension (0.072 to 0.045 N/m)
has been studied in an industrial scale experimental facility
at a wastewater treatment plant (Figure 1 to Figure 3). Also
the behavior of the accumulated air pocket in clean water
and wastewater (at dry weather flow) has been compared.
This paper addresses the question to which extent the gas
transport in the blow back flow regime is enhanced by
reduced surface tension.


figure 1: Subsequent hydra
wastewater treatment plant


Nomenclature


Pipe diameter (m)
E6tv6s number (-)
Flow number (also known as Pipe Froude number, -)
Gravitational constant (m s-2)
Hydraulic liquid head (m)
Length of pipe section (m)
Discharge (m3 s 1)
Absolute pressure (bar)









Velocity (m s-')
Weber number (-)


Greek letters
0 Downward pipe angle (0)
p Density (kg m 3)
o- Surface tension (N m 1)

Subscripts
1 Upstream of sloping section
2 Downstream of sloping section
f Friction
g Gas, air
max Maximum
turb Turbulent
p Gas pocket
s Superficial
up Upstream
w Water


Experimental Facility

The experimental facility at the treatment plant
(D = 192 mm) included an upstream horizontal section with
a length diameter ratio L, /D > 10, a mitre bend into the
downward slope (LD = 209, 0 = 100), a second mitre bend
to a downstream horizontal section, followed by horizontal
and rising pipework back to a reservoir with a separation
function (Figure 2). This lay-out guarantees that none of the
injected air can escape in the upstream direction. The pipe
material of the two horizontal sections and the downward
slope was transparent PVC.
Air was injected in the upstream horizontal section. The air
mass flow rate was automatically controlled at a pre-set
volumetric discharge Qg. The volumetric air discharge was
expressed as an air mass flow rate using the water
temperature and pressure at the location of the upstream
absolute pressure transducer. The upstream absolute
pressure pi was measured in the riser pipe towards the
horizontal section. The downstream pressure tapping was
located in the downstream horizontal section. This tapping
was connected to a second absolute pressure transmitter p2.
A differential pressure transmitter Ap was also installed,
connected to pi and the return line.


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

The water discharge Q, was measured with an
Electro-Magnetic Flow meter (EMF), positioned in the
upstream pipe prior to the air injection point. A flow control
valve controlled the liquid discharge to a pre-set value.
The surface tension was adjusted by adding non-foaming
surfactants to the clean water tank, so that surface tension
values between 0.04 and 0.07 N/m were obtained. After the
measurements with clean water and surfactant-added water
were completed, the facility was modified. The intake and
return pipe were connected to the outflow of the sand trap,
such that raw wastewater could enter the facility (Figure 3)
and experiments with wastewater could be performed. The
surface tension was measured with the Kibron AquaPi
instrument as static surface tension according to the
Langmuir principle. The samples for the surface tension
measurements were extracted at 15 minute intervals and
analysed immediately.

Air-water discharge ratios in the order of 0.001 already
cause gas pocket formation and gas pocket accumulation in
the downward slope. The air pocket head loss AHg at a large
number of air-water discharge combinations was derived
from the measured differential pressure Ap and corrected for
frictional losses without air Apf; see also Figure 5.


A Ap Ap
A. =
Pwg


The accumulated gas pocket length L, is closely related to
the gas pocket head loss (Pothof and Clemens 2010):

Lg AH
(3)
L LsinO
where L is the length of the downward sloping section.

Therefore, the gas pocket head loss is scaled to the vertical
elevation of the downward slope L-sinO. Figure 5 also
confirms that the air transport increases significantly from
Fg = 0.4-10- to Fg = 6-10-, if the gas pocket length (and
thus gas pocket head loss) increases, at a certain water
discharge.


L0 192 mm


Figure 2: Schematic setup for experiments with clean water and reduced surface tension.






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


L 40 M 192
- 192 rmm


Intake Return
wastewater
Figure 3: Schematic setup for experiments with wastewater.


In this way, 66 air-water discharge combinations were
measured with clean water, 27 with surfactants and 38 with
wastewater. Each combination was measured at least 3 times
with 5 minute intervals to verify whether the gas pocket
head loss had stabilised. It could take up to 8 hours before
the gas pocket head loss was stable.

A couple of wastewater samples and the surfactant-added
water was also analysed on the dynamic surface tension
with the Wilhelmy plate method. The interface
concentration of surfactants is suddenly decreased by
sucking away the top layer from the liquid. The surfactant
concentration at the interface increases by Brownian motion
of the surfactant particles in the liquid and the surface
tension drops accordingly until an equilibrium concentration
is reached. A typical recording is shown in Figure 4.


40 -


-Surfactant. 10-5


20


0 100 200 300 400
Time, t (s)
Figure 4: Time series of dynamic surface
(10-' solution of surfactant)


tension


A plot of the surface tension versus tf'2 should be linear, if
adsorption is diffusion-controlled. This plot permits
evaluation of the apparent diffusion coefficient from the
slope of the plot. Furthermore, the static surface tension can
be estimated from the Y-axis intercept (Rosen 2004).

Theory

The transport of gas through downward sloping pipes has
been investigated by a number of researchers in the previous
century (Escarameia 2006, Gandenberger 1957, Kalinske
and Bliss 1943, Kent 1952). These researchers focused on
the air entrainment in a hydraulic jump at the end of a gas
accumulation, which is the dominant transport mechanism
in the blow-back flow regime. Unfortunately, the
investigators neglected the importance of the gas pocket


length in relation to the slope length. Lubbers was the first
to investigate this influence in detail (Lubbers 2007a,
2007b). He (Lubbers 2007a) studied the behavior of
co-current air-water flows at different configurations
(diameter, inclination, slope length, liquid and gas
discharges) in a laboratory environment using clean water.
Lubbers and Clemens (2006) have shown experimentally
that Kent's equation (1) is too optimistic at downward pipe
angles 0< 25.

0 75 -
-Fg=0
--Fg*1000=0 4
S"- Fg*1000=0 8
S05 -- Fg'1000=2
- Fg*1000=4
S--Fg*1000=6

S0 25




0 03 06 09 12
Liquid flow number, F. [-]
Figure 5: Measured differential pressures with clean water

Gas can be transported through a downward sloping pipe in
a flowing liquid by several transport mechanisms (Pothof
and Clemens 2010):
1. Stratified flow in entire slope. A single gas pocket fills
the entire slope, causing the maximum gas pocket head
loss. The gas entraining hydraulic jump occurs in the
downstream horizontal section. The transition to the
blow-back flow regime depends on the gas flow
number.
2. Blow-back flow regime. The gas pocket fills a part of
the slope with a liquid film underneath. The downward
sloping pipe contains one or more gas-entraining
hydraulic jumps (Figure 1). The entrained gas bubbles
rise to the pipe invert, coalesce to larger bubbles and
secondary gas pockets. These secondary gas pockets
have their own hydraulic jumps that pump the gas
further downward. The larger bubbles and gas pockets
blow back into the top gas accumulation. The larger the
liquid flow rate, the smaller the upward velocity of the
secondary gas pockets. The gas pocket head loss
reduces gradually at increasing water discharge (Figure
5). Only a fraction of the entrained air reaches the
bottom of the slope. A fraction of the air dissolves into
the water phase due to the large interface area in the
hydraulic jumps, the rest of the air is transported as
bubbles by turbulence.


H=3.30 m


H=5.20 m


Interface refreshed










3. Plug flow regime. Stratified flow conditions and blow
back phenomena do not occur anymore. A series of gas
plugs slowly moves in downward direction along the
pipe invert. The liquid hardly entrains gas from the gas
pockets, which means that the Froude number of the
liquid film must be smaller than 2 (Fr < 2). The gas
pocket head loss has become marginal. The transition
from flow regime 2 to 3 marks the dimensionless
clearing velocity or required flow number Freq to keep
elongated air pockets stationary, as investigated by
other researchers (Escarameia 2006, Gandenberger
1957, Kent 1952, Wisner et al. 1975). Dissolving of air
into the water phase is not important anymore. Details
on this plug flow regime are found in (Pothof et al.
2010). This flow regime has many similarities with
plug flow in horizontal pipes (Ruder and Hanratty
1990).
4. Dispersed bubble flow regime. All gas is transported
downward as dispersed bubbles in the liquid and along
the pipe invert. The gas pocket head loss has become
negligible.

It is anticipated that the surface tension plays an important
role in the blow back flow regime, because of its effect on
bubble size and interface area.



Results and Discussion

Clean water and surfactant-added water. Figure 6
compares data points with surfactant-added clean water (o-<
72 mN/m) with the clean water data points (o= 72 mN/m)
and clearly shows a positive effect of a lower surface
tension on the gas pocket head loss. The trendlines suggest
that the gas pocket head loss becomes zero at a theoretical
surface tension equal to 0.


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

because the air discharge only varies with the pipe Weber
number at a certain air pocket head loss and air pocket
length. The air accumulation is accurately predicted by the
following experimental correlations (Figure 7):


AH
g

LsinO

AHs

Lsin O


3.4 0.415 In(We)



7.5- 0.88 ln(We)


,Fg 1000



,F 1000
g


- a- tapwater, Fg*1000=0 4
A surfactant, Fg*1000=04
----tapwater, Fg*1000=4
* surfactant, Fg*1000=4
correlation, Fg*1000=0 4
--- correlation, Fg*1000=4


10 100 1000
Pipe Weber number, We, [-]
Figure 7: Gas pocket head loss scales
blow-back flow regime


10000


to We number in


Wastewater. It is anticipated that the different constituents
in wastewater will influence the gas transport processes.
Initial tests with the wastewater showed that the surface
tension of dry-weather-flow wastewater hardly varied
during the day.





075-

P


* Fw=063, Fg*1000=4


075 Fw=O 72,Fg*1000=04
0)7


05


o Fw=0 45, Fg*1000=0 4
* Fw=063, Fg*1000=0 4


05
0


- 025
CD


SFw=0 63, Fg*1000=4VWV

* Fw=063, Fg*1000=4


0 20 40 60 80
surface tension, o [mN/m]
Figure 8: Influence of wastewater surface tension on gas
pocket head loss


40 50 60 70 80
surface tension, Figure 6: Influence of surfactants on gas pocket head loss.

The multiple curves in Figure 6 at a constant air flow
number collapse to a single line, if the gas pocket head loss
is plotted as a function of the pipe Weber number WeD
(Figure 8).


WeD


pu D


Apparently, a lower surface tension can be compensated
with a lower water discharge following a Weber scaling,


oP
0 05

Q.


--Fg*1000=04
--- Fg*1000=0 4WW
Fg*1000=2
-0- Fg*1000=2 WW
-m-Fg*1000=4
-0-- Fg'1000=4 WW








03 06 09
Liquid flow number, F [-]


Figure 9: Gas pocket head loss in clean water and
wastewater


o



05
o


025










During all measurements in dry-weather-flow conditions the
wastewater surface tension varied between 45 and 55 mN/m.
Therefore, measurements could be performed at constant
liquid and gas flow rates. Figure 8 shows that the gas pocket
head loss in wastewater is comparable with the head loss in
clean water at Fw = 0.63 and Fg=0.004, despite the smaller
surface tension of wastewater. Similar results were obtained
at the other water and air discharges, as illustrated in Figure
9.

The insensitivity of the air transport for the surface tension
of wastewater may be caused by the lower molecular
diffusion of the o-reducing constituents in wastewater
compared to the detergents used to reduce the surface
tension of the clean water. In fact, the surface tension is a
dynamic variable, because the detergents need time to reach
an equilibrium between the interface concentration and the
bulk concentration. In a lab analysis the dynamic evolution
of the surface tension is dominated by molecular diffusion.
Measurements of the dynamic surface tension of a couple of
samples of wastewater and surfactant-added water were
performed to investigate this dynamic behaviour. A 10-5
solution with surfactants had a static surface tension of 44
mN/m, which is comparable with the surface tension of
wastewater samples (Figure 10). The average composition
of the wastewater in our facility was a 4:1 mixture of
Wastewater and Wastewater2. The straight lines in Figure
10 confirm that the dynamic surface tension is
diffusion-controlled.


S40 Surfactant, 10-5
S- Wastewater 1
35 x Wastewater 2

30
00 04 0 08 0 12
transformedtime, t & (s 0)
Figure 10: Plot of dynamic surface tension versus t12

Unfortunately, the gradients in Figure 10 do not differ by an
order of magnitude. Therefore, the molecular diffusion in
wastewater cannot explain the observed differences in the
gas pocket head loss between wastewater and
surfactant-added water.

Another explanation for the observed differences takes the
turbulent mixing in the water film into account. On one
hand, if a new interface is created in the top of the inverted
siphon, turbulent mixing promotes the motion of surfactant
particles towards the interface. On the other hand,
turbulence may shed particles from the interface. Therefore,
turbulence may affect the time scale at which the
equilibrium surface tension is reached and it may affect the
equilibrium surface tension itself. Figure 11 shows that
manual stirring of the sample accelerates the transport of


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

surfactants towards the interface. The effect of stirring on
the wastewater sample is less pronounced. The influence of
turbulence on the surface has not been investigated in
sufficient detail to draw firm conclusions, but the
exploratory experiments indicate that turbulence has a more
positive influence on the surfactant-added water than on the
wastewater. Probably, the surfactants in wastewater (mainly
proteins, fat molecules) are detached more easily from the
interface than the surfactants added to the clean water.


40 -
0 100 200 300 400 500
Time, t(s)
Figure 11: Dynamic surface tension with manual stirring


Conclusions

Reduction of the surface tension of clean water by means of
surfactants shows a linear reduction of the gas pocket head
loss. The reduced surface tension reduces the bubble size of
entrained bubbles in the hydraulic jump, which enhances
gas transport. Furthermore, more gas will dissolve in the
liquid phase, because of the larger contact surface.
Despite the fact that the static surface tension of the
wastewater is significantly smaller than that of clean
water, the experiments show that the gas pocket head loss is
similar to the gas pocket head loss of clean water. This
discrepancy in test results is caused by the dynamic
character of the surface tension and probably its interaction
with turbulence in the water film. The effective surface
tension of wastewater on the time scale of the transport in
the water film to the hydraulic jump is similar to the surface
tension of clean water without additives. Hence the lower
surface tension of wastewater does not enhance the gas
transport in downward sloping pipes.


Acknowledgements

This research is co-funded by: Dutch ministry of Economic
affairs, Foundation STOWA, the waterboards of Aquafin,
Brabantse Delta, Delfland, FryslAn, Hollands
Noorderkwartier, Hollandse Delta, Reest en Wieden,
Rivierenland, Waternet, Zuiderzeeland, ITT Water and
Wastewater and the engineering consultants Grontmij and
Royal Haskoning. We also acknowledge the staff at
Unilever Research for their support on the measurements of
the dynamic surface tension.


imp --iiiiii..""o-i--i





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

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