RESEARCH ARTICLE

PTV investigation of phase interaction in dispersed liquid–liquidtwo-phase turbulent swirling flow

Atsuhide Kitagawa Æ Yoshimichi Hagiwara ÆTakuro Kouda

Received: 23 June 2006 / Revised: 21 February 2007 / Accepted: 28 February 2007 / Published online: 24 May 2007

� Springer-Verlag 2007

Abstract An investigation of dispersed liquid–liquid

two-phase turbulent swirling flow in a horizontal pipe is

conducted using a particle tracking velocimetry (PTV)

technique and a shadow image technique (SIT). Silicone oil

with a low specific gravity is used as immiscible droplets.

A swirling motion is given to the main flow by an impeller

installed in the pipe. Fluorescent tracer particles are applied

to flow visualization. Red/green/blue components extracted

from color images taken with a digital color CCD camera

are used to simultaneously estimate the liquid and droplet

velocity vectors. Under a relatively low swirl motion, a

large number of droplets with low specific gravity tend to

accumulate in the central region of the pipe. With

increasing droplet volume fraction, the liquid turbulence

intensity in the axial direction increases while that in the

wall-normal direction decreases in the central region of the

pipe. In addition, the turbulence modification in the present

flow is strongly dependent on the droplet Reynolds num-

ber; however, the interaction of droplet-induced turbu-

lences is significant due to vortex shedding, particularly at

high droplet Reynolds numbers and higher droplet volume

fraction.

1 Introduction

The active utilization of warm wastewater discharged from

industrial plants and air-conditioning equipment is desir-

able from the perspective of efficient energy use. The

thermal energy of warm wastewater is not so high as to

require high-performance heat exchangers for its reuse.

One type of heat exchanger that can be used for this pur-

pose is a liquid–liquid direct-contact heat exchanger in

which high-temperature droplets contact directly with the

surrounding low-temperature liquid in order to conduct the

heat exchange. This heat exchanger has the following

advantages: (1) the heat loss is sufficiently low because

there is no partition wall for the two fluids in the heat

exchanger. Hence, it simplifies the heat exchange between

two fluids with little difference in temperature. (2) The heat

transfer area is easily increased by using a large number of

small droplets.

Thus far, the flow and heat transfer characteristics of

dispersed liquid–liquid two-phase flows in a vertical pipe

under specific conditions have been investigated. Jacobs

and Golafshani (1989) studied the heat transfer character-

istics of isobutane droplets in water. They showed that

circulation inside the droplets contributes to the heat

transfer enhancement for droplets with radii larger than

1.5–2.0 mm. Kaviany (1994) studied the interactions be-

tween immiscible droplets and a surrounding liquid. Inaba

et al. (2000) conducted experiments on the heat transfer

characteristics of perfluorocarbon droplets in hot water

steam and quantitatively showed the relationship between

thermal efficiency and droplet flow rate. Goto et al. (2005)

measured an upward water flow with low-temperature

hydro-fluoroether droplets, showing the existence of a li-

quid circulation region within the lower part of the droplets

and a liquid low-temperature region behind the droplets.

A. Kitagawa (&) � Y. Hagiwara

Department of Mechanical and System Engineering,

Kyoto Institute of Technology, Goshokaido-cho,

Matsugasaki, Sakyou-ku, Kyoto 606-8585, Japan

e-mail: kitagawa@kit.ac.jp

T. Kouda

Hiroshima Machinery Works, Mitsubishi Heavy Industries, Ltd,

Kannonshinmachi, Nishi-ku, Hiroshima 733-8553, Japan

123

Exp Fluids (2007) 42:871–880

DOI 10.1007/s00348-007-0291-5

Since horizontal flow is dominant in pipelines used to

transport warm wastewater, knowledge regarding the dis-

persed liquid–liquid two-phase flow in horizontal pipes is

necessary. In such flows, droplets migrate toward the pipe

wall depending on specific gravity. Hence, control of

droplet motion is required to efficiently maintain heat

transfer between two phases. One method for effectively

suppressing droplet migration toward the pipe wall is to

give a swirling motion to the main flow. We previously

conducted experiments on turbulent swirling flows with

immiscible droplets in a horizontal pipe, showing that

droplets with low specific gravity are transported to a

specific region in the flow depending on the swirl intensity

and the buoyancy of the droplet (Kouda and Hagiwara

2006a). We also showed that deviations in the droplet

diameter depend on the swirl intensity, and that the liquid

turbulence intensity is increased by the droplet injection

(Kouda and Hagiwara 2006b). However, information on

the two-phase interaction in the flow has not yet been ob-

tained.

The purpose of this paper is to experimentally investi-

gate the two-phase interaction in a horizontal droplet-laden

turbulent swirling flow. Silicone oil droplets with low

specific gravity are used as immiscible droplets. The

simultaneous two-phase velocity field is estimated using a

particle tracking velocimetry (PTV) technique and a sha-

dow image technique (SIT) (e.g., Kitagawa et al. 2005).

Images of the two phases are obtained using a digital color

CCD camera. The experimental setup is described in

Sect. 2 and the measurement system is introduced in

Sect. 3. Finally, results of the two-phase measurements are

presented in Sect. 4.

2 Experimental setup

2.1 Apparatus

Figure 1 shows a schematic diagram of the apparatus. The

apparatus consists of an upstream chamber, a contraction

nozzle, a transparent acrylic pipe, a downstream chamber,

and a pump. The pipe with a 50 mm ID and a 3,050 mm

total length is set horizontally. A tripping wire is installed

in the inner wall at an inlet of the pipe to enhance the

disturbance of the liquid. The liquid flow rate is controlled

using an inverter connected to the pump. The origin of the

coordinates is the center of the impeller, which is explained

in Sect. 2.2. The x, y, and z axes are the streamwise, ver-

tical, and horizontal directions, respectively.

Table 1 lists the experimental conditions. The Reynolds

number Rec and the droplet volume fraction a in Table 1

are, respectively, given by:

Rec ¼umD

m; ð1Þ

a ¼ Qd

Qd þ Qc

; ð2Þ

where um is the bulk mean liquid velocity, D is the pipe

diameter, and m is the kinematic viscosity of the liquid. Qc

and Qd are the bulk liquid and droplet flow rates, respec-

tively. All the experiments were performed at

Rec = 3.4 · 103. The droplet volume fraction was 0.16 and

0.33%.

2.2 Impeller for swirling motion

A swirling motion is given to the main flow by the use of

an impeller. The impeller is set 2,110 mm downstream

from the inlet of the pipe. Figure 2 shows details of the

impeller. So far there have been a number of ways to define

the swirl intensity in a measurement cross-section. The

swirl number used here is defined as follows (Kitoh 1991):

SwðxÞ ¼2pRD=2

0uhðr;xÞ � uðr;xÞr2dr

pðD=2Þ3u2m

; r¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiy2þ z2

p; ð3Þ

Upstream chamber

Tripping wire

Acrylic circular pipe

Syringe pump Downstream chamber

Stationary impeller

Pump

Fig. 1 Schematic diagram of

the apparatus

Table 1 Experimental conditions

Pipe total length 3,050 mm

Pipe inner diameter 50 mm

Temperature of liquid 21�C

Kinematic viscosity of liquid v = 1.0 · 10–6 m2/s

Bulk Reynolds number ReC = 3.4 · 103

Droplet volume fraction a = 0.16, 0.33%

872 Exp Fluids (2007) 42:871–880

123

where u is the mean liquid velocity in the x direction, and

uh is the mean liquid velocity in the azimuthal direction,

which was obtained in a preliminary experiment. The swirl

number defined in Eq. 3 is the ratio of the angular

momentum due to the swirling motion to the momentum in

the x direction.

The migration of droplets with a low specific gravity in

the radial direction is strongly dependent on the swirl

number. For instance, droplets at relatively low swirl

numbers do not accumulate in the central region of the

pipe because the pressure gradient in the radial direction

is weak. In contrast, droplets at sufficiently high swirl

numbers accumulate significantly in the central region of

the pipe and consequently the possibility for the coales-

cence of droplets becomes high. According to our previ-

ous studies, droplets with a low specific gravity at

Sw = 0.47 accumulate in the second quadrant in the y–z

plane (Kouda and Hagiwara 2006a), while those at

Sw = 1.34 accumulate in the central region of the pipe

(Kouda and Hagiwara 2006b). In the present study,

therefore, we focus on the two-phase turbulent swirling

flow in which the swirl number at the center of the

measurement region (i.e., x/D = 4.0) is 1.0.

2.3 Silicone oil droplets

Silicone oil (Shin-Etsu chemical, KF995) droplets were

used as immiscible droplets in the experiments. The

physical properties of the silicone oil at 25�C are as fol-

lows: the specific gravity is 0.96, the kinematic viscosity is

4.0 · 10–6 (m2/s), the surface tension coefficient is

40 · 10–3 (N/m), and the refractive index is 1.40. The

surface tension coefficient was obtained through the

Wilhelmy plate method using a surface tensiometer

(Kyowa Interface Science, CBVP-Z). The silicone oil is

injected into the flow from four stainless steel tubes. The

locations of the exits of the tubes are (x = 3.0D, y = 0,

z = 0.25D), (x = 3.0D, y = 0.25D, z = 0), (x = 3.0D, y = 0,

z = –0.25D), and (x = 3.0D, y = –0.25D, z = 0), respec-

tively. All the tubes are connected to the syringes in a

parallel syringe pump, and the flow rate of the silicone oil

is controlled using the syringe pump. The temperature of

the generated droplets is the same as that of the liquid.

Figure 3a–b shows the images of droplets close to the

injection tubes. The figure shows that the generation

pattern of droplets is of a single-droplet type for both

droplet volume fractions. The droplets are collected at the

downstream chamber.

3 Measurement techniques

3.1 Simultaneous velocity measurement technique

The PTV technique is used to simultaneously measure the

velocities of both phases. For the liquid velocity mea-

surement, fluorescent tracer particles are used to prevent

particles from becoming invisible due to the light scattered

from droplets. The droplet interface is clearly obtained

using the SIT.

Syringe pump Motor Syringe

Droplet injection tube Impeller

25 mm 151.5 mm

20 mm 50 mm 25 mm

Fig. 2 Details of impeller and

droplet injection tubes

(a) = 0.16 %

x

(b) = 0.33 % y

Fig. 3 Images of droplets close to the injection tubes

Exp Fluids (2007) 42:871–880 873

123

3.2 Fluorescent tracer particles

Fluorescent tracer particles that absorb Rodamine-B are

made of a porous material with a diameter of 75–100 lm

and a specific gravity of 1.02. The absorption wavelength

of the Rhodamine-B is 460–600 nm, and its emission

wavelength is 550–700 nm. According to our estimation,

the particle diameter is 0.31–0.51 times larger than the

Kolmogorov length scale and the particles respond to a

sinusoidal flow oscillation of up to 533 Hz.

3.3 Measurement system

Figure 4 shows a schematic diagram of the measurement

system used in this study. A transparent water-filled acrylic

box surrounds the test section of the pipe in order to sup-

press the distortion of images resulting from refraction at

the pipe wall. A digital color CCD camera (IMPERX,

VGA210-LC) with a resolution of 640 · 480 pixels is used

to capture the images. The frame rate of the camera is set at

150 fps, and the spatial resolution is 0.091 mm/pixel.

The light source for the fluorescent tracer particles is a

Nd:YVO4 laser (JENOPTIK, k = 532 nm), while that for

the SIT technique consists of blue LED arrays (IMAC,

k = 470 nm). A laser light sheet of 5 mm thickness and

30 mm width is produced with two kinds of cylindrical

lenses and illuminates from the top and bottom of the pipe.

The blue LED illuminates from behind the pipe.

As might be expected, the local two-phase interaction

becomes weaker with the increase of x due to decay in the

swirl motion. Since the purpose of this paper is to inves-

tigate the two-phase interaction in detail, measurements are

carried out at x/D = 4.0 and z = 0 where the swirl motion is

hardly attenuated. However, the flow has a possibility to be

a non-fully developed flow because the wake flow of the

boss of the impeller remains in the measurement region.

According to our estimation for two different measurement

lengths in the streamwise direction, Lx, i.e., Lx = 6 and

Lx = 20 mm, the relative mean difference of the stream-

wise liquid mean velocities and that of the streamwise li-

quid turbulence intensities were 0.44 and 0.98%,

respectively. This means that the change in flow statistics

of the liquid phase in the streamwise direction can be ig-

nored. Therefore the measurement length in the streamwise

direction is limited to 20 mm.

3.4 Image processing technique for two-phase velocity

estimation

Figure 5 shows a flow chart of the image processing

implemented to estimate the velocities of the liquid and

droplets. The RGB components extracted from the color

image are used. The tracer particles are captured in the red

images because the wavelength band of the fluorescence of

the fluorescent particles is almost the same as that of red

light. Droplets are captured in the blue images. The pro-

cedure used to estimate the liquid velocity is described

below (see Fig. 5a).

1. The particle centroids are estimated from the red

images using a particle mask correlation method

(Takehara and Etoh 1999).

2. Some of the data in Step 1 are obtained from particle

images that overlap with droplet images. At the very

least, these data are affected by reflection or refraction

at the droplet interface and thereby might lead to errors

with respect to the liquid velocity estimation. These

data are eliminated using binarized droplet images.

3. The liquid velocity vectors are estimated using a velocity

gradient tensor method (Ishikawa et al. 2000). In this

method, the velocity gradient tensor is taken into ac-

count to obtain the velocity vectors with high accuracy.

Note that Steps 1 and 3 are followed when estimating

the liquid velocity without droplets. For the flow statistics

of the liquid, the discrete liquid velocity vectors are relo-

cated to 40 · 40 grid elements in each direction. We re-

moved velocities that deviate more than three times the

standard deviation from the mean velocity for each of the

grid elements. The uncertainty in the liquid velocity asso-

ciated with the particle centroid detection is estimated to be

4.1 mm/s, and consequently the relative error of this value

to the maximum liquid velocity in the x direction (88 mm/s)

is 4.7%.

The procedure used to estimate the droplet velocity is as

follows (see Fig. 5b).

1. The droplet centroids are estimated from the

blue images using a binary labeling method (e.g.,

Yamamoto et al. 1996). The threshold for the image

binarization is determined using the method developed

by Otsu (1979). When the aspect ratio of a droplet

is higher than the threshold, the droplet image is

Plano-convex lens

Plano-convex lens

Blue LED array

Flow direction

Digital color CCD camera Rectangular box

Fig. 4 Schematic diagram of the measurement system

874 Exp Fluids (2007) 42:871–880

123

considered to be overlapped with other droplet images,

and the image is eliminated.

2. In Step 1, the centroid detection is performed for

droplets in and out of the plane of the laser sheet. This

is because in the SIT, droplet images are obtained as

projection images. In this step, only droplets in the

plane of the laser sheet are extracted using Green

images in which the laser light scattered at the droplet

interface is captured.

3. The droplet velocity vectors are estimated by dividing

the displacements of droplets between two sequential

images with a time interval of Dt = 1/150 s.

The uncertainty in the droplet velocity associated with

the droplet centroid detection is estimated to be 2.7 mm/s.

As a result, the relative error of this value to the droplet

velocity in the x direction at 2y/D = 0.5 (52.8 mm/s) is

5.1%. In the image processing, more than 32,000 images

for each of the experimental conditions are used to obtain

reliable measured results.

4 Results and discussion

4.1 Instantaneous two-phase velocity field

Figure 6 illustrates a typical instantaneous two-phase

velocity field for a = 0.33%. In this figure, arrows repre-

sent velocity vectors, solid black circles are droplets whose

centroids are in the measurement plane, and solid gray

circles are droplets that are out of the measurement plane.

The left- and right-hand sides of the image are the lower

and upper pipe walls, respectively. It is apparent from this

figure that the liquid and droplet velocity vectors and the

droplet locations in and out of the measurement plane are

obtained satisfactorily.

When estimating high-order differential quantities from

the spatial interpolation of discrete velocity vectors, the

reliability of the obtained results is low. This is because the

spatial interpolation leads to a smoothing of the high-order

fluctuation of the quantities. In contrast, the discrete

velocity vectors provide reliable low-order moments. In

Sect. 4.3, we estimate low-order moments for the flow

statistics of the liquid phase.

4.2 Diameter and location of droplets

Figure 7 shows the probability distribution of the droplet

equivalent diameter d in the measurement plane at around

x/D = 4.0. In this figure, n is the number of droplets for

each equivalent diameter, and N is the total of n. It is clear

that the droplet diameter ranges from 3.5 to 6.0 mm and

that the probability distribution has a peak at d = 4.4 mm.

Deviation of the droplet diameter results from the balance

of forces (in particular, surface tension force, drag force,

(a) Liquid velocity estimation (b) Droplet velocity estimation

a1

a1: PMC a2: Elimination of overlappingparticlea3: VGT

b1: Binary labeling b2: Droplet extraction b3:Two-time-step tracking

a2

a3 b3

b2

b1

Color image Red image Blue image Green image

x

y

Fig. 5 Flow chart of the image

processing for liquid and droplet

velocity estimations

Exp Fluids (2007) 42:871–880 875

123

buoyancy force, and pressure gradient force) acting on

droplets at the exit of the injection tube (see Kouda and

Hagiwara 2006b). It is also clear that the probability dis-

tribution is independent of the droplet volume fraction.

This depends strongly on the generation pattern of droplets

(e.g., Inaba et al. 2000).

The probability distribution of droplet location in the

measurement plane at around x/D = 4.0 is shown in Fig. 8.

In this figure, n is the number of droplets at each y location.

The probability distribution for both droplet volume frac-

tions is high, in the range of –0.6 < 2y/D < 0.6. The

droplet locations are strongly dependent on the balance of

forces (especially, the pressure gradient force, the centrif-

ugal force, the lift force, and the drag force) acting on the

droplets in the radial direction. This tendency was also

observed in similar measurements for the x–z plane as well

as our previous measurements at Sw = 1.34. Therefore, it

is possible to accumulate a large number of droplets in the

central region of the pipe under a relatively low swirl

motion. The decrease in the swirl number causes a reduc-

tion in the pressure loss and consequently enhances the

transport efficiency in the axial direction. Note that in the

experiments, droplet coalescence did not occur in the

central region of the pipe.

4.3 Flow statistics of liquid phase

Figure 9 shows a profile of the mean liquid velocity for

different droplet volume fractions. In this figure, v is the

mean liquid velocity in the y direction. It is found that the

mean velocity in the x direction decreases in the central

region of the pipe and that the mean velocity in the y

direction becomes negative in the range of –0.4 < 2y/

D < 0.5. This decrease in axial velocity is peculiar to a

swirling flow in a pipe. It is also found that the mean

velocities in both directions do not vary with droplet vol-

ume fraction. In other words, the velocity profile is unaf-

fected by the increase in the bulk liquid velocity due to the

droplet injection. In addition, there is no asymmetry in the

profile of the mean velocity which was seen in our previous

measurements at Sw = 1.34. This symmetry results from

the stability of the wake flow of the boss of the impeller

Lower wall Upper wall

x

y

Scale definition 5 mm 80 mm/s

Fig. 6 Typical instantaneous

two-phase velocity field for

a = 0.33%

Fig. 7 Probability distribution of droplet equivalent diameter in the

measurement plane at around x/D = 4.0

Fig. 8 Probability distribution of droplet location in the measurement

plane at around x/D = 4.0

876 Exp Fluids (2007) 42:871–880

123

and the attenuation of the meandering of the axis for the

swirl motion due to the decrease in the swirl number at the

center of the measurement region.

Figure 10a–b shows profiles of the liquid turbulence

intensity for different droplet volume fractions. In this

figure, urms and vrms are, respectively, the RMS values of

the liquid fluctuation velocities in the x and y directions,

and uc0 is the mean liquid velocity at the center of the pipe

for each droplet volume fraction. The turbulence intensity

in the x direction generally increases with the droplet

volume fraction. In particular, an increase in the turbulence

intensity is noticeable in the ranges of –0.6 < 2y/D < –0.1

and 0.2 < 2y/D < 0.7. These ranges are approximately

consistent with those of the high droplet probability dis-

tribution (see Fig. 8). This tendency is the same as that

observed in our previous measurements at Sw = 1.34 and

the increasing rate of the turbulence intensity at Sw = 1.34

is larger than that at Sw = 1.0. In contrast, the turbulence

intensity in the y direction decreases in the central region of

the pipe as the droplet volume fraction increases. This

decrease originates from the reductions of a temporal

eccentricity of the axis for the swirl motion and of the

disturbance in wake flow of the boss of the impeller due to

the accumulation of droplets in the central region of the

pipe. These turbulent modifications are considered to be

strongly related to local interaction between the droplets

and the surrounding liquid. In Sect. 4.4, we discuss the

two-phase interaction in the present flow.

4.4 Local two-phase interaction

Figure 11a–b illustrates a typical instantaneous liquid tur-

bulence ratio field at a = 0.33%. In this figure, arrows

represent turbulence ratio vectors and are defined as:

bx ¼u0u0

u2rms

; by ¼v0v0

v2rms

; ð4Þ

where u¢ and v¢ are the liquid fluctuation velocities in the x

and y directions, respectively. urms and vrms are obtained as

values at those grid elements that contain an individual

liquid tracer particle. The definitions for the circle and its

color are the same as those in Fig. 6. Figure 11 shows that

Fig. 9 Profile of liquid mean velocity for different droplet volume

fractions

Fig. 10 Profile of liquid turbulence intensity for different droplet

volume fractions

(a)

(b)

x

y

Lower wall Upper wall Scale definition

5 mm 3

Fig. 11 Typical instantaneous

liquid turbulence ratio field at

a = 0.33%

Exp Fluids (2007) 42:871–880 877

123

turbulence ratio vectors tend to increase in the vicinity of

droplets.

To statistically clarify this tendency, we calculated the

profile of the liquid turbulence ratio for different estimation

domains (Fig. 12). The turbulence ratio is given by:

b ¼ u0u0 þ v0v0

u2rms þ v2

rms

� �

: ð5Þ

The estimation domain is the area of radius L based on the

droplet centroid. Each datum is calculated as the grid-

averaged value in order to clarify the difference between

the data. Note that only data in the region where the

probability distribution of the droplet location is high (i.e.,

–0.5 < 2y/D < 0.5) are presented. Figure 12 shows that the

turbulence ratio increases with a decrease in L. In other

words, the same tendency as in Fig. 11 is statistically

shown again. Figure 12 also shows that the dependence of

the liquid turbulence ratio on droplet volume fraction is

weak. It is therefore expected that the turbulence modifi-

cation in the present flow is strongly dependent on the

droplet Reynolds number. The relationship between the

droplet Reynolds number and the liquid turbulence ratio is

discussed in the following.

Figure 13 shows the probability distribution of the

droplet Reynolds number. The droplet Reynolds number

Red is defined as:

Red ¼ud � uj j � d

m; ð6Þ

where ud and u are the droplet and liquid velocity vectors,

respectively. In this figure, vd is the droplet velocity in the y

direction. It is clear that the probability distribution of rising

droplets (vd > 0) is higher than that of falling droplets

(vd < 0). It is also clear that the probability distribution of

falling droplets has a peak at Red = 0–100, while that of

rising droplets has a peak at Red = 100–200. This means

that the velocity of rising droplets is higher than that of

falling droplets. The difference in velocities is due to the

balance of forces acting on the droplet. Vertical components

of forces acting on the droplet in the regions of upward or

downward swirl flow are mainly the drag force, the buoy-

ancy force, the gravity force, and the inertia force (when the

time variation of the relative velocity is relatively small). In

our experiment, the difference between the buoyancy and

gravity forces is positive because the density of the droplet

is lower than that of the surrounding liquid. Therefore, the

direction of the swirl motion is the same as that of the

difference between the two forces when the droplet exists in

the region of upward swirl flow and as a result the droplet

velocity increases. In contrast, the velocity of the droplet in

the region of downward swirl flow decreases.

Figure 14 shows a profile of the liquid turbulence ratio

for different droplet Reynolds numbers at L = d. It is found

that the liquid turbulence ratio increases with the droplet

Reynolds number. Specifically, the increasing rate of the

turbulence ratio is highest at 400 < Red < 600. According

to DNS results obtained by Sugioka and Komori (2005),

the unsteady vortex behind a droplet occurs at Red = 300.

In the case of a rigid sphere, Sakamoto and Haniu (1990)

showed that the hairpin vortex sheds from the sphere at

300 < Red < 420. In our experiment, it is possible that the

tap water used as the working liquid contains contaminants

that cause a no-slip condition at the liquid–liquid interface.

Therefore, at high Red (especially at 300 < Red), vortex

shedding similar to the hairpin vortex probably occurs

behind droplets. It is also found that the difference in liquid

turbulence ratios for different droplet volume fractions is

significant at 400 < Red < 600. This difference is due to

the interaction of droplet-induced turbulences.

To investigate the influence region of this interaction,

we calculated the relationship between the estimation do-

main and the liquid turbulence ratio for different droplet

Fig. 12 Profile of liquid turbulence ratio for different estimation

domains

Fig. 13 Probability distribution of droplet Reynolds number

878 Exp Fluids (2007) 42:871–880

123

Reynolds numbers (Fig. 15). Note that each datum is the

value averaged over the range –0.5 < 2y/D < 0.5 and only

data with more than 1,000 samples are presented. At

400 < Red < 600, there is a substantial difference in liquid

turbulence ratios for different droplet volume fractions in

the range of 0.8 < L/d < 2.5. This means that the interac-

tion of droplet-induced turbulences is significant for a wide

range of L/d, and this leads to turbulence enhancement.

This enhancement is closely related to vortex shedding

from high-Red droplets because the large effect still re-

mains at L/d = 2.5. One possible explanation for the tur-

bulence enhancement is as follows: according to numerical

results obtained by Mittal (2000), in the case of 300 < Red,

low-level free-stream fluctuations in the flow enhance the

turbulence in the wake of a sphere. As shown in Fig. 13,

the probability of occurrence of low-Red droplets is much

higher than that of high-Red droplets. This fact enables us

to expect that in the central region of the pipe, low-level

velocity fluctuations are induced by a large number of low-

Red droplets with steady wakes. This configuration in the

flow with droplets is similar to that for the numerical result

obtained by Mittal (2000). Therefore, the turbulence

enhancement described above results mainly from local

interactions between low-level velocity fluctuations in-

duced by low-Red droplets and vortex shedding from high-

Red droplets (see Fig. 16).

In our experiment, the probability of occurrence of

droplets in the range Red = 400–600 is sufficiently low (see

Fig. 13). However, the droplet Reynolds number increases

in the region of upward swirl flow and the concentration of

droplets becomes high in the central region of the pipe.

Therefore, we expect that the turbulence enhancement due

to local two-phase interactions occurs actively in such a

duplicated region.

5 Conclusions

We investigated a dispersed liquid–liquid two-phase tur-

bulent swirling flow in a horizontal pipe using a PTV

technique and a SIT. The conclusions obtained from our

experiment are as follows.

1. The liquid and droplet velocity vectors and the droplet

locations in and out of the measurement plane were

successfully obtained through a measurement tech-

nique using a digital color CCD camera.

2. It is possible to accumulate a large number of droplets

in the central region of the pipe under a relatively low

swirl motion.

3. As the droplet volume fraction increases, the liquid

turbulence intensity in the axial direction increases

while that in the wall-normal direction decreases in the

central region of the pipe. This decrease originates

from the reductions of a temporal eccentricity of the

axis for the swirl motion and of the disturbance in

wake flow of the boss of the impeller due to the

accumulation of droplets in the central region of the

pipe.

Fig. 14 Profile of liquid turbulence ratio for different droplet

Reynolds numbers at L = d

Fig. 15 Relationship between estimation domain and liquid turbu-

lence ratio for different droplet Reynolds numbers

Low-Red droplet

Vortex shedding

High-Red droplet

Turbulence enhancement

Low-level velocity fluctuation

Main flow

Swirl motion direction

Fig. 16 Sketch of turbulence enhancement mechanism

Exp Fluids (2007) 42:871–880 879

123

4. The probability of occurrence of rising droplets is

higher than that of falling droplets. In addition, the

velocity of the droplet in the region of upward swirl

flow increases and that in the region of downward

swirl flow decreases, depending on the balance of

forces acting on the droplet.

5. In the present flow, turbulence modification is strongly

dependent on the droplet Reynolds number; however,

the interaction of droplet-induced turbulences is sig-

nificant due to vortex shedding, and this leads to the

turbulence enhancement, particularly at high-Red and

higher droplet volume fraction. This enhancement re-

sults mainly from local interactions between low-level

velocity fluctuations induced by low-Red droplets and

vortex shedding from high-Red droplets.

References

Goto M, Matsukura N, Hagiwara Y (2005) Heat transfer character-

istics of warm water flow with cool immiscible droplets in a

vertical pipe. Exp Therm Fluid Sci 29:371–381

Inaba H, Horibe A, Ozaki K, Yokoyama N (2000) Liquid–liquid

direct contact heat exchange using a perfluorocarbon liquid for

waste heat recovery (heat transfer characteristics obtained with

perfluorocarbon droplets descending in a hot water medium).

JSME Int J Ser B 43(1):52–61

Ishikawa M, Murai Y, Wada A, Iguchi M, Okamoto K, Yamamoto F

(2000) Novel algorithm for particle tracking velocimetry using

the velocity gradient tensor. Exp Fluids 29(6):519–531

Jacobs HR, Golafshani M (1989) A heuristic evaluation of the

governing mode of heat transfer in a liquid–liquid spray column.

ASME J Heat Transf 111:773–778

Kaviany M (1994) Principals of convective heat transfer. Springer,

Heidelberg, pp 417–425

Kitagawa A, Hishida K, Kodama Y (2005) Flow structure of microbub-

ble-laden turbulent channel flow measured by PIV combined with

the shadow image technique. Exp Fluids 38(4):466–475

Kitoh O (1991) Experimental study of turbulent swirling flow in a

straight pipe. J Fluid Mech 225:4454–4479

Kouda T, Hagiwara Y (2006a) Turbulent swirling water flow with oil

droplets. Multiphase Sci Technol 18(1):55–72

Kouda T, Hagiwara Y (2006b) An experimental study on turbulent

swirling water flow with immiscible droplets. Int J Heat Fluid

Flow 27:611–618

Mittal R (2000) Response of the sphere wake to freestream

fluctuations. Theor Comp Fluid Dyn 13:397–419

Otsu N (1979) A threshold selection method from gray-level

histograms. IEEE Trans Syst Man Cybern SMC 9(1):62–66

Sakamoto H, Haniu H (1990) A study on vortex shedding from

spheres in a uniform flow. ASME J Fluids Eng 112:386–392

Sugioka K, Komori S (2005) Drag and lift forces acting on a spherical

droplet in homogeneous shear flow (in Japanese). Trans JSME

71B:7–14

Takehara K, Etoh T (1999) A study on particle identification in PTV.

J Vis 1(3):313–323

Yamamoto F, Wada A, Iguchi M, Ishikawa M (1996) Discussion of

the cross-correlation methods for PIV. J Flow Vis Image Process

3(1):65–78

880 Exp Fluids (2007) 42:871–880

123