1. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
80
EXPERIMENTAL STUDY OF HEAT TRANSFER CHARACTERISTICS
OF R744/R134a IN A SMOOTH HORIZONTAL TUBE
A.Ramanan1*
, P.Senthilkumar2
1
Research Scholar, Dept. of Mechanical Engineering, Sathyabama University, Chennai-
600119, TAMILNADU, INDIA
2
Professor, Dept. of Mechanical Engineering, KSR College of Engineering, Tiruchengode-
637215, TAMILNADU, INDIA
ABSTRACT
This paper presents the heat transfer characteristics of the refrigerant mixture of
R744/R134a flowing through the horizontal smooth tube. The refrigerant mixture is studied
in different mass, heat flux and inlet temperature conditions. Experimental results on the heat
transfer coefficient, inner wall temperature and exergy of mass flux from 40 to 80 kg/ m2
s in
a horizontal smooth tube of 4 mm inner diameter are presented. It is found that the mixture
combination of R744/R134a in 25/75 at a mass flux of 80 kg/ m2
s gives maximum heat
transfer.
Keywords: heat flux, mass flux, refrigerant mixture
1. INTRODUCTION
Conventional refrigerants, such as the CFCs and their alternatives the HFCs, have
potential environmental problems, so their use is being curtailed. CO2 is non-flammable and
nontoxic with a zero ozone depletion potential (ODP), and a global warming potential (GWP)
that is very small compared with other conventional refrigerants such as R134a; therefore,
CO2 is a promising refrigerant for environmental, economical and safety reasons, and is
being applied in automobile air-conditioning, heat pump or other low temperature
refrigeration systems, as suggested by Lorentzen and Pettersen(1993) and Riffat et al. (1997).
JOURNAL OF MECHANICAL ENGINEERING AND
TECHNOLOGY (JMET)
ISSN 2347-3924 (Print)
ISSN 2347-3932 (Online)
Volume 1, Issue 1, July-December (2013), pp. 80-92
© IAEME: http://www.iaeme.com/JMET.asp
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2. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
81
2. EXPERIMENTAL APPARATUS AND PROCEDURE
2.1. Experimental apparatus
Fighure (1) represents the experimental system used to investigate the heat transfer of
R744/R134a in a horizontal tube during evaporation and it was used similar to the set up and
working as mentioned by Cho et al (1). The refrigerant loop consists of a pump, test section, a
Coirolis-type mass flow meter, a pre-heater and a condenser. The liquid refrigerant is pumped
via pump. Then the refrigerant passes through a Coirolis-type mass flow meter before
entering the pre-heater. The pre-heater is used to control the vapor quality at the test
section inlet. The refrigerant enters the test section in two-phase state. The test section
consists of 5 mm outer diameter with 0.25 mm thick copper tube having length of 1.44 m.
The wall temperature is measured using type-T, thermocouples, positioned on the
surface. The refrigerant leaves the test section in two-phase or superheated state.
It enters then a counter-current condenser where it is sub-cooled before entering the
pump. Pressure is measured at the test section inlet and outlets. Flow boiling tests
were then performed at different mass fluxes, heat fluxes and inlet temperatures.
Fig.1. Schematic experimental set up
2.2 Data reduction
The thermo physical properties are calculated based on the measured temperature and
pressure. The local heat transfer coefficient at each thermocouple is calculated based on the
following equation
h = q / (Tw -Tsat)
Where, q- heat flux, Tw is the inner wall surface temperature and Tsat is the saturated
temperature of the refrigerant deduced from the fluid pressure. The variations of the
refrigerant thermo-physical properties in the test section were calculated with REFPROP 8.0.
3. RESULTS AND DISCUSSIONS
Heat transfer coefficients (HTCs) are found to depend on some or all of the following
parameters: heat flux, reduced pressure, vapor quality and often mass velocity; furthermore
they might depend on surface roughness and channel geometry. Miyata et al. (2011) present a
correlation to predict heat transfer coefficients with vaporization which takes into account
nucleate boiling, forced convection evaporation and evaporation heat transfer through thin
liquid film around vapor plugs in slug flow. Several equations have been proposed, but none
is widely accepted.
MF
P
PREHEATER
Test section CONDENSER
LIQUID
RECEIVER
R

3. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
82
3.1 Behaviour of R744/R134a mixture at different mass flux conditions
The variation of heat transfer co efficient, inner wall temperature and exergy on the
quality of refrigerant mixture flowing through the horizontal tube at different mass flux
conditions of the refrigerant mixture of R744/R134a in combinations of 25/75,50/50 and
75/25 is shown in fig. 2-4(a-d).
a. Mass flux-40 kg/ m2
s b. Mass flux-60 kg/ m2
s
c. Mass flux-70 kg/ m2
s d. Mass flux-80 kg/ m2 s
Fig.2 Variation of heat transfer coefficient at different mass fluxes
The heat transfer coefficient of refrigerant mixture in three combination at the mass
fluxes of 40, 60,70and 80 kg/ m2
s is shown in the above figure2 (a-d).In all the cases the heat
transfer coefficient of mixture of 25/75 is higher than the mixture of 50/50 at the same time
the highest value is for the mixture combination 75/25.The heat transfer coefficient at the
mass flux 40,the mixture of 25/75 and 50/50 combination is almost same but for the mixture
of 75/25 is well above and reduces drastically.
The heat transfer coefficient for the mass fluxes 60, 70 and 80 follows the similar
pattern in the flow. In all the cases the heat transfer coefficient is maximum for the refrigerant
mixture of 75/25.
The inner wall temperature along the test section at different mass flux conditions of
40, 60, 70 and 80 are shown in the following figure 3(a-d).
0
5
10
15
20
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
2
4
6
8
10
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
5
10
15
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
5
10
15
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25

4. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
83
a. Mass flux-40 kg/ m2
s b. Mass flux-60 kg/ m2
s
c. Mass flux-70 kg/ m2
s d. Mass flux-80 kg/ m2
s
Fig.3 Variation of inner wall temperature vs quality at different mass fluxes
Inner wall temperature of the test section for three refrigerant mixtures namely 25/75,
50/50 and 75/25 is following same pattern for all the mass flux conditions. The inner wall
temperature increases along the test setion, lower value for 25/75 mixture followed by 50/50
mixture with the maximum value is for 75/25 mixture as evident from the above figure.
The exergy of the refrigerant mixture flowing through the test section for three
combinations at different mass fluxes is shown in thefig4( a-d).
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwall
temperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwall
temperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/
75
50/
50
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25
/7
5

5. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
84
a. Mass flux-40 kg/ m2
s b. Mass flux-60 kg/ m2
s
c. Mass flux-70 kg/ m2
s d. Mass flux-80 kg/ m2
s
Fig.4 Variation of exergy vs quality at different mass fluxes
Exergy variation of mixtures in all the three combinations at four different mass
fluxes 4, 60, 70 and 80 are following the same pattern in general. The exergy of fluid
decreases towards the end of tube. The exergy value of 25/75 mixture refrigerant lies in
between the higher value of 50/50 and lower value of 75/25 mixtures. The exergy of the
mixture approaches close value before end point of the tube for the mass fluxes 40,60and 70
kg/ m2
s.
3.2 Behaviour of R744/R134amixture at different heat flux conditions
The variation of heat transfer co efficient, inner wall temperature and exergy on the
quality of refrigerant mixture flowing through the horizontal tube at different heat flux
conditions is shown in fig. 5-7.
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6 0.8 1 1.2
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
30
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
30
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25

6. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
85
a. heat flux-15 Kw/m2
s b. heat flux-18 Kw/m2
s
c. heat flux-24 Kw/m2
s d. heat flux-24 Kw/m2
s
Fig.5 Variation of heat transfer coefficient vs quality at different heat flux
The heat transfer co efficient of the mixture is high at the beginning and then starts
decreasing sharply towards the length of the tube. The heat transfer coefficient is lowest for
50/50 mixture and slightly higher value for 25/75 mixture. The maximum value occurs for
the mixture combimation of 75/25 in the beginning of the section and it starts decreasing
towards end of the tube but the value reaches low at the end section of the tube. But for 24
Kw/m2
s heat flux condition, the heat transfer coefficient has the lower value for 25/75
mixture followed by 50/50 and is higher value is for 75/25 ie at this mass flux blend behaves
differently.
Inner wall temperature of the test section increases steadily from the beginning for all
the heat flux conditions and for all blends as depicted in figure6 (a-d)below.
0
5
10
15
20
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
0
5
10
15
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
5
10
15
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
5
10
15
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25

7. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
86
a. heat flux-15 Kw/m2
s b. heat flux-18 Kw/m2
s
c. heat flux-21 Kw/m2
s d. heat flux-24 Kw/m2
s
Fig. 6 Variation of inner wall temperature vs quality at different heat flux
Variation of inner wall temperature of the test section for all the heat flux conditions
are behaves in different way. At 15Kw/m2
s the inner wall temperature is maximum for
25/75mixture and lower value is for 50/50 mixture between these two 75/25 mixture lies as
the temperature increases towards the end of the tube as in fig a above.
At 18Kw/m2
s the inner wall temperature is maximum for 75/25mixture and lower
value is for 25/75 mixture between these two 50/50 mixture lies as the temperature increases
towards the end of the tube as in fig b above.
At 21Kw/m2
s the inner wall temperature is maximum for 75/25mixture and lower
value is for 50/50 mixture between these two 25/75 mixture lies after first quarter of the tube
as the temperature increases towards the end of the tube as in fig c above.
At 24Kw/m2
s the inner wall temperature is maximum for 25/75mixture and lower
value is for 75/25 mixture between these two 50/50 mixture lies as close as to75/25 as in fig d
above.
The variation of exergy of the mixture on the quality along the tube at different heat
fluxes for three blends is shown in fig7(a-d).
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25

8. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
87
0
5
10
15
20
25
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
a. heat flux-15 Kw/m2
s b. heat flux-18 Kw/m2
s
c. heat flux-21 Kw/m2
s d.heat flux-24 Kw/m2
s
Fig.7 Variation of exergy vs quality at different heat flux
Exergy of blends at 15 and 24 Kw/m2
s vary in similar way as it decreases from
beginning to end of tube. The maximum value is for 50/50 blend and approaches to close
values for 25/75 and 75/25 blends, lower than equal refrigerant blend.
At 18 and 21Kw/m2
s the exergy is maximum for 50/50mixture and lower value is for
75/25 mixture between these two lies 25/75 mixture as above.
3.3 Behaviour of R744/R134amixture at different inlet temperature conditions
The heat transfer coefficient at different inlet temperatures of the test section
decreases from the beginning for all the blends as depicted in figure8 (a-d) below.
0
5
10
15
20
25
30
35
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
30
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
30
35
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25

9. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
88
a. inlet temperature -4o
C b. inlet temperature 0o
C
c. inlet temperature4o
C d. inlet temperature 8o
C
Fig.8 Variation of heat transfer vs quality at different inlet temperature conditions
The heat transfer co efficient of the refrigerant mixture is initially high and start
decreasing towards the end of the test section .The heat transfer coefficient is high for 75/25
mixture and lower value is for 25/75 for the inlet temperatures of -4o
C and 0o
C. In case of
4o
C and 8o
C the higher heat transfer coefficient is for 50/50 mixture and low for 25/75
mixture.
Inner wall temperature of the test section increases steadily from the beginning for all
the inlet temperatures and for all blends as depicted in figure9 (a-d) below.
0
1
2
3
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
1
2
3
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
1
2
3
4
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25
0
1
2
3
4
0 0.5 1 1.5
Heattransfer
coefficient
Quality
25/75
50/50
75/25

10. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
89
a. Inlet temperature -4o
C b. inlet temperature 0o
C
c. inlet temperature4o
C d.inlet temperature 8o
C
Fig.9 Variation of inner wall temperature vs quality at different inlet temperature conditions
The inner wall temperature of the tube increases steadily towards the end. The inner
wall temperature is high for 25/75 followed by 50/50 and low for 75/25 mixture at the inlet
temperature of -4o
C.In case of 0o
C, the inner wall temperature is lowest for 75/25mixture and
maximum is for 50/50 in the first half of the section and in remaining section is for 25/75
blend.In case of 4o
C and 8o
C, the inner wall temperature is lowest for 50/50mixture and
maximum is for 25/75 in between these two lies the 75/25 mixture.
Exergy of the test section decreases from the beginning for all the inlet temperatures
and for all blends as depicted in figure10 (a-d) below.
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25
270
275
280
285
290
295
300
305
310
0 0.5 1 1.5
Innerwalltemperature
Quality
25/75
50/50
75/25

11. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)
90
a. inlet temperature -4o
C b. inlet temperature 0o
C
c. inlet temperature4o
C d. inlet temperature 8o
C
Fig.10 Variation of exergy vs quality at different inlet temperature conditions
In all inlet temperatures the maximum exergy appears at 50/50 blend. The lower
exergy is for 25/75 blend at the inlet temperatures -4 and 0o
C. In case of 4o
C inlet
temperature the low exergy is for 25/75 in the first half of the section and in second half of
section it is for 75/25 mixture. In case of 8o
C inlet temperature the low exergy is for 25/75 in
the first half of the section and in second half of section it is for 75/25 mixture but the values
approaches very close .
4. CONCLUSIONS
Experimental results for the flow boiling of R744/R134a as 25/75,50/50 and 75/25
mixture combination in a horizontal tube under variations in the mass flux , heat flux and
inlet temperature were presented. The behaviours of the local heat transfer coefficient, inner
wall temperature and exergy of different blends were investigated and the following
conclusions could be drawn from this study:
The heat transfer coefficient initially high and starts decreases towards the end of the
in all cases experiment in general. The blend variation influences the heat transfer coefficient
as it is clearly evident from the plots.- In the low heat flux conditions, it was possible to
observe a significant influence of heat flux on the heat transfer coefficient. In the high heat
flux conditions, this influence tended to disappear.
0
10
20
30
40
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
0
5
10
15
20
25
30
35
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
30
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25
0
5
10
15
20
25
30
35
0 0.5 1 1.5
Exergy
Quality
25/75
50/50
75/25

12. Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
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The inner wall temperature increases in all conditions for all blends towards end of
the tube at the same time variations in blend influences and similarly the exergy decreases
from the beginning to towards end of tube with the influence of blend.
To fully exploit the opportunity with natural refrigerants, it is necessary to rely on acceptable
general predicting procedures are still far from satisfactory, and an increased research effort
on this matter definitely desirable.
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