Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Visual study on flow patterns during convective boiling inside Vertical smooth and
microfin tubes


M.A. Akhavan-Behabadi, V.D. Hatamipour *

School of Mechanical Engineering, College of Engineering, University of Tehran
Amir-Abad St., Tehran, 1439957131, Iran
E-mail: v.hpd84@gmail.com



Keywords: Flow boiling, Flow pattern, Heat transfer, Vertical, Smooth tube, Microfin tube




Abstract

Evaporator is an important and widely used heat exchanger in air conditioning and refrigeration industries. Different methods
have been used by investigators to increase the heat transfer rates in evaporators. One of the passive techniques to enhance
heat transfer coefficient is the application of microfin tubes. On the other hand, the mechanism of heat transfer augmentation
in microfin tubes is dependent on the flow regime of two-phase flow. Therefore many investigations of the flow patterns for
in-tube evaporation have been reported in literatures. The gravitational force, surface tension and the vapor-liquid interfacial
shear stress are known as three dominant factors controlling the vapor and liquid distribution inside the tube. A review of the
existing literature reveals that the previous investigations were mostly concerned with the two-phase flow pattern for flow
boiling in horizontal tubes (Mling-huei Yu et al., 2002; N. Kattan et al., 1998). Therefore, the objective of the present
investigation is to obtain information about the two-phase flow patterns for evaporation of R-134a inside vertical smooth and
microfin tubes. Also Investigation of heat transfer during flow boiling of R-134a inside vertical microfin and smooth tube have
been carried out experimentally The heat transfer coefficients for annular flow in the smooth tube is shown to agree well with
Gungor and Winterton(1987)'s correlation. All the flow patterns occurred in the test can be divided into three dominant
regimes, i.e., chum flow, wavy-annular flow and annular flow. Experimental data are plotted in two kinds of flow maps, i.e.,
Weber number for the vapor versus weber number for the liquid flow map and mass flux versus vapor quality flow map. The
transition from wavy-annular flow to annular or chum flow is identified in the flow maps.


Introduction

Heat exchangers in air conditioning and heat pump
applications play an important role on system efficiency
and physical size. Therefore, the design of an efficient heat
exchanger, especially evaporators and condensers, has
always been significant to equipment designers and several
enhancement techniques, e.g., twisted-tape inserts,
corrugated tubes and microfin tubes, have been tested
Agrawal, K.N. et al. (1986); Thors, P. et al. (1994);
Chamra, L. et al. (1996). These techniques can be broadly
classified into two main categories, viz., active techniques
and passive techniques Webb, R.L. (1994). Active
techniques require external energy to enhance heat
transfer; however, passive techniques are such one, which
need no external power for heat transfer enhancement. One
of these passive techniques is the use of microfin tubes.
Because of importance of microfin tubes in present work
we experimentally investigated microfin tubes.
On the cases of in tube convective boiling, accurate
modeling and trustworthy evaluation of heat transfer and
pressure drop characteristics require precise predictions of
the local two-phase flow patterns, the heat transfer
coefficient generally keeps changing as the flow pattern
changes along an evaporator tube Carey VP. (1992). since


distinct flow regimes can be characterized by quite
different flow and heat transfer mechanisms. Therefore,
studies of two-phase flow patterns and their transitions
during in tube flow boiling have gained increasing interest
for several decades. Kattan et al. (1998) proposed a
diabetic flow pattern map for evaporation (boiling) in
horizontal straight smooth tube. They stated that their flow
pattern map was developed based on flow pattern data for
five different refrigerants, including R134a. Muzzio et al.
(1998) investigated the flow patterns in flow boiling and
convective condensation of refrigerant R22 in a microfin
tube.
The flow patterns could be characterized as a function of
mass flow rate, vapor quality and fluid properties.
Baker (1954) classified the two-phase flow pattern of
air-water mixture in tubes of small diameters (<0.05 m) by
using the mass velocity. Bergles and Suo (1966) used flow
rate and exit quality to plot the flow pattern maps. So the
objective of the present investigation is to obtain
information about the two-phase flow patterns for
evaporation of R-134a inside vertical smooth and microfin
tubes.






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

The set-up included a test- evaporator, a pre-evaporator,
and an after-evaporator. These tubes were heated by
flexible electrical heating tapes of 2kW capacity, wrapped
around them.
In order to change the inclination angle of the
test-evaporator the connections to this evaporator were
made by special flexible pressure hoses for R-134a. In
order to cover the whole domain of vapor quality, a
pre-evaporator was used. The fluid emerging from
test-evaporator was passed through the after-evaporator for
the complete evaporation of R-134a liquid. The electrical
power of heater in each three evaporators was regulated in
order to control the quality of refrigerant vapor entering
the test evaporator. Then the refrigerant vapor was
circulated inside the condenser with the help of a
reciprocatmng compressor.
The condensed R-134a enters the rotameter, expands in
expansion valve and enters the pre-evaporator again. The
test section is a 1.1 m long vertical mounted copper tube.
Two tubes were tested in the study. A microfin tube with
9.52 OD, 55 fins, and 15" helix angle and the nominal
inside diameter of the microfin tube is 8.92 mm with ridge
height of 0.25 mm, and a smooth tube with same inside
tube diameter. The microfin tube was a copper tube having
internal micro-fins with triangular fin cross-section. The
geometrical parameters of microfin tube are shown in
figure 2.


Paper No


Nomenclature

A tube inside cross-sectional area (m2)
j superficial velocity (m s )
D Tube diameter (m)
Do outside diameter of test-section(m)
G mass velocity (kg. s .m-2)
M mass fow rate (kg.s )
Q heat transfer rate (W)
T temperature (K
Ts average saturation temperature of refrigerant
(K)
Twi average inside test-section wall temperature
(K)
Two average outside test-section wall
temperature (K)
We Weber number
vapor quality
h heat transfer coefficient (Wm-2 K~')
L length of test-section (m)

Greek letters
p density (kg. m-3)
0- surface tension(N. m')

Subsripts
g vapor phase
1 liquid phase

Experimental Facility

The experimental set-up was a well instrumented vapor
compression refrigeration system. The schematic diagram
of experimental set-up is shown in fig 1.


Df-t-------- ------~i~''~~-'-Y


fin tip angle
helix angle
outside diameter
inside diameter
fin height
wall thickness
fin pitch
number of fins


250
150
9.52 mm
8.92 mm
0.25 mm
0.30 mm
0.48mm
55


Figure 2: Dimensions of standard microfin tube

The two-phase flow patterns were observed visually and
the transition locations between different flow patterns
were measured in a transparent test section. A pyrex glass
window is mounted at end of test section for flow pattern
visualization. The sight glass has a length of 100 mm and
an inner diameter identical to that of the test section. A
digital camera is used to record flow patterns in the sight
glass. At the same time in test procedure, naked eye
observations are also written down for references.
The inner diameter of the sight glass is equal to that of
both test tubes and the connections. A camera based flow
visualization technique was used in this study is shown in
figure 3. A canon camera with adjustable focal length and
shutter speed is used to capture the images. Best images


( 14. Sight glass


1- COMPRESSOR
2- CONDENSER
3- ROTAMETER
+ RECIEVER
5- FILTER DRIER
6- EXPANSION VALVE
7- PREEVPORATOR
8- TEST-EVAPORATOR
9- AFTER-EWAPORATOR
10- ACCUMULA~TO R


11- FLEIBLE HOS E~for R134a)
12- ROTATING TRESTLE
13- SIGHT GLASS
TC- THEFW10COUPLE
P-PRESSUREGUAGE
T-THRMOMETER
O SHUT OFF VAl..E
--)WATER FLOW
W REFR I GERANT FLOW
O ELECTRICAL POWER SUPP LY


Figure 1: Schematic diagram of experimental set-up






Paper No


were captured in 8000 fps shutter speed.
A diffuse white film pigmented with evenly spaced black
stripes are placed in the background (behind the glass tube)
and illuminated with a stroboscope directed towards the
camera. The stripe width, spacing, and distance from the
centerline of the test sections are presented by Jassim
(2006).
At the same time in test procedure, naked eye observations
are also written down for references. It is noteworthy to
point out that flow patterns are observed through sight
glasses which are not microfin, but nevertheless they are
assumed as fully representatives of the flow regimes
actually occurring at the test section exit. Indeed, Wenzhi
Cui et al. (2007) conjectured that the flow structure should
experience only a minor disruption in passing to the glass
tube, and the flow should not redevelop through the sight
glass because of its short length,


Glass test










Camera


Striped
Diffuse
Translucent
Background


h:


Stroboscope


Figure 3: Flow visualization schematic

Measurements

The average outside wall temperatures of the tube was
measured at six axial locations. At each location four
thermocouples were fixed at top, two sides and bottom
positions (when microfin tube is in horizontal position).
The refrigerant temperatures at the inlet and outlet of the
test-evaporator were also measured. All the above
temperature measurements were done by T type
(Cu-Constantan) thermocouples with a calibrated accuracy
of 0.1 oC. The thermocouples were carefully soldered on
the outer surface of microfin tube at six sections with each
20 cm distance along the tube. The arrangements were also
made for the measurement of refrigerant pressure at inlet
and outlet of the test-evaporator, pre-evaporator and
after-evaporator. The refrigerant mass flow rate was
measured by a rotameter installed down stream of
condenser.
It was ensured that the refrigerant enters the rotameter was
completely liquid, whenever it was not seen any bubble in
the condensed refrigerant liquid. The whole of
test-evaporator, pre-evaporator and after-evaporator were
insulated by glass wool and AFLEX tube insulations to
prevent any heat loss to the surroundings.
A total of 8 test runs with four different refrigerant mass
velocities of 53.66, 78.32, 107.26 and 153.74 kg.m-2.S-1
were performed for vertical smooth tube and four
refrigerant mass velocities of 58.3, 74.46, 97.54 and
157.17 kg.m-2.S-1 were performed for vertical microfin


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

tube. The range of operating parameters is given in table 1.

Working fluid : R-134a
Refrigerant mass velocity : 56 to 157 kg/m2.S
Average evaporating temperature: -24.4 to -5.1 OC
Average heat flux: 2.2 to 6.2 kw/m2
Average vapor quality: 0.1 to 0.96
Tab. 1: The ranges of operating parameters

The vapor quality at the inlet of pre-evaporator was
computed by considering the isoenthalpic expansion of
refrigerant in the needle valve. An energy balance
technique was used to get the vapor quality of the inlet and
the exit of test-evaporator. The mean vapor quality was
taken as the average of inlet and outlet vapor qualities of
test-section. The heat transfer coefficient of test-section
was determined using the heat gain from electrical heater
and the temperature difference between the evaporator
inside wall surface and the boiling refrigerant.
The average outside tube wall temperature of
test-evaporator at a particular station, Two, was calculated
by using Equation 1.

T =T, +2Ts +Tb

The avera e outside tube wall temperature of
test-evaporator, Two, was computed by taking the average
temperatures of six axial stations as:



(2)
Two

The radial heat flux, q for test-evaporator was calculated
by Equation 3.

q = Q/(arDL) 3
Temperature inside the tube wall, ATw,, was determined
using Equation 4 for radial heat flux.

qdl In(do /dl)
AT, = 2k(4)
2k
The average static pressure in test-evaporator was taken to
be the mean of the inlet and outlet pressure. The vapor
saturation temperature in test-evaporator, T,, was taken as
the saturation temperature corresponding to this average
static pressure. The heat transfer coefficient of
test-evaporator was calculated by the following Equation


h = (5)
(T,, -T)
The thermo-physical properties of R-134a were taken from
Stoecker, W.F. (1998) and Sonntag, R.E., Borgnakke, C.,
and Van Wylen, GJ. (2003). The uncertainty analysis of
experimental results has been carried out by the method
proposed in Schultz, R.R., and Cole, R. (1979). It was
found that the expected experimental uncertainty was
within a band of 19.33% for all the test runs.






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


G =- (6)

The superficial liquid velocity and the superficial vapor
velocity are


Paper No


Results and Discussion

Flow Pattern

In the test, all the flow patterns observed in the vertical
microfin and smooth tubes can be classified into chum
flow, wavy-annular flow and annular flow. Fig. 4 shows
the photographs of three main flow patterns captured by
digital camera.


G (1-x)
JL
PL
Gx
G,
PG
The corresponding
respectively as:


Weber numbers are defined


WeL JJD PL


WeGJ PG


(b) Wavy-annular
Flow


(a) Chum Flow


(c) Annular Flow


Figure 4: Flow patterns in vertical microfin and plain tube.

The structure of chumn flow pattern as shown in fig. 4(a)
becomes unstable with the fluid traveling up and down in
an oscillatory fashion but with a net upward flow. The
instability is the result of the relative parity of the gravity
and shear forces acting in opposing directions on the thin
film of liquid of Taylor bubbles. This flow pattern is in fact
an intermediate regime between the slug flow and annular
flow regimes. But in this experiment we did not observe
slug flow in these ranges of mass flow velocities and vapor
qualities.
In wavy annular as shown in fig. 4(b) When in a constant
mass flow velocity, quality of flow increases shear forces
become more powerful than gravity but not very much.
Because of shear forces, the liquid expelled from the center
of the tube and flows as a thin film on the wall but waves
are formed on the interface of flow because of gravity
effect.
In almost high vapor quality flows, the interfacial shear of
the high velocity gas on the liquid film becomes dominant
over gravity, so the liquid is expelled from the center of the
tube and flows as a thin film on the wall while the gas
flows as a continuous phase up the center of the tube. The
interface is disturbed by high frequency waves and ripples.
Annular flow regime as shown in fig. 4(c) is particularly
stable. Thome J.R. et al I 21 ** 4-21*In r a
Flow pattern maps are often used to depict the transitions
of different flow patterns. Although different flow
characteristics exist between two-phase flow in the straight
tube. Fig. 5 shows the flow pattern map for the present
flow boiling in microfin tube. In Fig. 5, the abscissa and
ordinate are Weber number for the liquid and weber
number for the vapor, respectively.
The refrigerant mass velocity in the tube is defined as:


WeL


1-Churn Flow; 2-Wavy Annular Flow; 3-Annular Flow

Figure 5: We, -WeL flOw pattern map for vertical microfin
tube.

to better identify flow patterns during the evaporation
process at different mass velocities and to make the map a
more useful research and design tool, the axes of the
previous flow pattern map have been converted to mass
flux versus vapor quality (similar to how local flow boiling
coefficients are plotted, i.e., heat transfer coefficient versus
vapor quality) Fig. 6 shows such kind of flow map.













O Chan
0 Wway-Andlar
200 a Anlr








m~ a


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





O Churn
0Wavy-Annular
200 n nnular







I lii l
U15 0. .4 06 .
drX


Heat transfer coefficient

The calculated experimental heat transfer coefficients of
the present vertical up flow in smooth tube were compared
with the correlation of Gungor and Winterton (1987).
The correlation predicts the experimental data for vertical
upward flow in an error band of -20% to +6%. This
agreement of experimental heat transfer coefficlents of
vertical up flows with the predicted values establishes the
integrity of experimental set-up.
The variation of evaporation heat transfer coefficient, h,
with vapor quality of microfmn and smooth tube have
been drawn in figures 9-10 for four mass velocities of
R-134a form 53.66 to 157.17 kg/m2.s.


0 0.1 0.2 Q.3 0.4 0.5 Q.8 0.7 D,8 0,9 1
V apow Quality

Figure 9. Variation of heat transfer coefficient with mass
velocity in vertical up flow smooth tube


I We_

1-Churn Flow; 2-Wavy Annular Flow; 3-Annular Flow
Figure 7: WeG LWe flOw pattern map for vertical smooth
tube.

Fig. 8 shows mass flux versus vapour quality flow map for
vertical smooth tube.


Paper No


Figure 8:
tube.


G x flow pattern map for vertical smooth


Figure
tube.


6: G x flow pattern map for vertical microfin


According to Fig. 6 transition between chumn flow and
wavy annular flow is at x = 0.3 And flow pattern remain
wavy annular till almost x = 0.45 After that in all mass
flow velocities for vapour qualities greater thanx = 0.45
flow pattern is annular flow.
Also in this experiment flow patterns of vertical plain tube
are observed. In almost same conditions as vertical flow in
microfin tube flow maps are plotted. There are two kinds
of flow maps for vertical flow in smooth tube, Weber
number for the vapor versus weber number for the liquid
flow map and mass flux versus vapor quality flow map.
Fig. 7 shows Weber number for the vapor versus weber
number for the liquid flow map in vertical smooth tube.


6500
S5000
4500
S4000
*
"U 3500
o 3000


S2000
'1500
S1000
500


- G~0- oor.2sarm
---A-- G=7832Kginf'.s
- -~- G -53.66 Kglads














Ssooo -
4500 -
E~4000 0_
S3500 -~ GY
E 3000 -Y 6 -
2500 -\
S2000 -/ o
S1500 0
~ loooG=157.17 Kg/m .s
100 G=97.54 Kg/m .s
--A G=74.46 Kg/m .s
500 -O G=58.3 Kgim .s
0.10 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1
Vapour Quality
Figure 10. Variation of heat transfer coefficient with mass
velocity in vertical up flow microfin tube

It is observed from Figures 9-10, that the heat transfer
coefficient increases with the increase of flow mass
velocity for smooth and microfin tubes. Also it is noted
that the increase of heat transfer coefficient for both tubes
is occurred in the same way.
The heat transfer coefficient increases with the increase of
vapor quality up to a maximum value and then it decreases
with more increases in vapor quality. The reason for such a
phenomenon is the fact that, as the vapor quality increases
the thickness of annular liquid film inside the tube
decreases and subsequently decreases liquid film thickness
offers lower thermal resistance of liquid film to heat flow
from electrical heater to the vapor side of liquid film
surface. Furthermore, as the evaporation progresses, the
vapor phase velocity increases resulting in the raising of
the interfacial shear stresses. The cumulative effect of the
above two factors contributes towards the increase in the
heat transfer coefficient, h. Finally after a maximum value,
there is often a reduction mn heat transfer coefficient due to
the tube circumference is only partially wetted with liquid.
Vertical microfin tube increases the heat transfer
coefficient by 0.75 percent in comparison to that for the
vertical smooth tube at high vapour quality region at the
mass velocities of 100 and 150 kg/m2.S.

Conclusions

Two-phase flow regimes and heat transfer coefficient of
refrigerant R134a boiling in microfin and smooth tube are
experimentally studied in this paper. The flow patterns are
identified using visualization methods and grouped into
three dominant regimes, i.e., Chumn flow, wavy annular
flow and annular flow. Flow map is used to figure out the
transitions of different flow patterns. Two kinds of usually
used flow maps, i.e., Weber number for the vapor versus
weber number for the liquid (WeG -WeL ) flOw map and
mass flux versus vapor quality (G -x ) flow map, are
chosen in this study.
At high vapor quality region annular flow is observed and
in low vapor qualities Chumn flow is captured. And in a
small region between two previous regions wavy annular


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

flow is observed.
It was found microfin tube have a noticeable effect on heat
transfer coefficient. Vertical microfin tube increases the
heat transfer coefficient by 0.75 percent in comparison to
that for the vertical smooth tube at high vapour quality
region at the mass velocities of 100 and 150 kg/m2.S.
It was revealed heat transfer coefficient increases with the
increase of flow mass velocity for smooth and microfin
tubes.

References

Bergles, AE & Suo, M. Investigation of boiling water flow
TegimeS at high pressure. In: Proc. Heat Trans. Fluid Mech.
Institute. Stanford Press, p. 77-79 (1966)

Schultz, R.R. & Cole, R. Uncertainty Analysis in Boiling
Nucleation, AIChE Symp. Series, Vol.75, No.189, pp.
32-38, (1979)

Agrawal, K.N. & Varma, H.K. & Lal, S.N. Heat Transfer
During Forced Convection Boiling of R12 Under Swirl
Flow, ASMIE J. Heat Transfer, Vol. 108, pp. 567-573,
(1986)

Gungor, K.E. & Winterton, R.H. Simplified General
Correlation for Saturated Flow Boiling and Comparison of
Correlations to Data", Industrial &~ Engineering Chemistry
Process Design and Development, Vol. 65, pp.148-156,
(1987)

Carey VP. Liquid-vapor phase-change phenomena.
Washington (DC): Hemisphere Pub. Co, (1992)

Thors, P. & Bogart, J.E. In-Tube Evaporation of HCFC-22
with Enhanced Tubes, J. Enhanced Heat Transfer, Vol. 1,
pp. 365-377, (1994)

Webb, R.L. Principles of Enhanced Heat Transfer, John
Wiley and Sons, New York (1994)


Chamra, L. & Webb, R. & Randlett, M. Advanced
Microfin Tubes for Evaporation, International Journal of
Heat and M~ass Transfer, Vol. 39 (9), pp.1827-1838,
(1996)

Kattan, N. & Thome, J.R. & Favrat, D. Flow boiling in
horizontal tubes: Part 1: development of a diabetic
two-phase flow pattern map. Journal of Heat
Transfer;,120:140-7.( 1998)

Muzzio, A. & Niro, A. & Garavaglia, M. Flow patterns and
heat transfer coefficients in flow-boiling and convective
condensation of R22 inside a microfin tube of new design,
in: Heat Transfer 1998, Proceedings of 11th IHTC,
Kyongju, Korea, vol. 2, pp. 291-296 ( 1998)

Stoecker,W.F. Industrial i, .-,-; .r. an.. handbook,
Mc Graw Hill Companies, Inc. (1998)

Ming-huei, Yu. & Tsun-kuo, Lin & Chyuan-chyi, Tseng
Heat transfer and flow pattern during two-phase flow


Paper No






Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

boiling of R-134a in horizontal smooth and microfin tubes.
International journal of refrigeration (2001)

Sonntag, R.E. & Borgnakke, C. & Van Wylen, G.J.
Fundamentals of Thermodynamics, John wiley and sons,
New York, (2003)

Thome, J.R. Engineering Data Booklll, by Wolverin tube,