Advanced Materials Research
ISSN: 1662-8985, Vols. 562-564, pp 363-366
doi:10.4028/www.scientific.net/AMR.562-564.363
© 2012 Trans Tech Publications, Switzerland
Online: 2012-08-30
Investigation on the Properties of Supercritical CO2 Fluid and its Heat
Transfer Characteristics
YANG Zhenjiang1, a, YANG Junlan2, b
1
Department of Overseas Engineering Business, HeBei Electric Power Design and Research
Institute, 050031 Shijiazhuang, Hebei Province, People’s Republic of China
2
Department of Energy and Mechanical Engineering, Tianjin Institute of Urban Construction,
300384 Tianjin, People’s Republic of China
b
[email protected]
Keywords: supercritical CO2 fluid; thermo-physical properties; heat transfer performance
Abstracts: The obvious characteristics of transcritical CO2 cycle are that the heat rejection process
takes place in the supercritical region. The thermophysical properties of supercritical CO2 change
dramatically with the temperature and pressure near the critical region, which results in the
momentum and energy exchange and buoyant force change in the heat flux direction, so it should
be treated as “variable properties”. According to the characteristics of CO2 specific heat, the
correlation of the pseudocritical temperature is obtained and the pseudocritical region is defined.
The heat transfer features of CO2 under supercritical pressure are different from those of the
conventional refrigerants. In order to compare the heat transfer performance of supercritical CO2
fluid and the conventional refrigerants, the contrast investigation on the heat transfer treatment
principle, the heat transfer mechanism and the thermophysical properties are mainly presented in
this paper.
Introduction
With the development of scientific technology, supercritical fluids are widely used in many trades.
In the power engineering fields, it is applied to the supercritical steam power cycle and nuclear
reactor fields, etc. In other trades, it is mainly used in supercritical extraction technology,
biochemistry, pharmacy and dealing with industry castoff and waste. The supercritical CO2 is often
applied to sterilize germ for liquid food. Therefore, many researchers began to study on the
properties and heat transfer performance of supercritical fluids from fifties of the twentieth century.
With growing environmental concerns of global warming and ozone depletion, environmentally
benign natural refrigerant carbon dioxide has attracted considerable attention. In recent years, many
researchers studied the performance of transcritical CO2 cycle. The obvious characteristics of
transcritical CO2 cycle are that the heat rejection process takes place in the supercritical region
(about 8-12Mpa). The heat transfer features of CO2 under supercritical pressure are different from
those of the conventional refrigerants. The main reason is attributed to that the thermo-physical
properties of CO2 change dramatically with the temperature and pressure near the critical region.
The special thermo-physical properties of CO2 determine its unique heat transfer characteristics.
Analysis of CO2 Specific Heat
The specific heat of CO2 is obtained from Engineering Equation Solver software [1], as shown in
Fig.1. It can be seen that at each supercritical pressure, the specific heat changes drastically as the
temperature rises, and reaches a maximum value at a certain temperature. In general, the
temperature at which the specific heat reaches a peak is called pseduocritical temperature for a
given pressure. And the higher the pressure is, the larger the pseduocritical temperature is. The peak
of the CO2 specific heat decreases with the increasing pressure. This can be described by the
following equation.
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364
Materials Engineering and Automatic Control
 ∂c p

 ∂T

 = 0
P
(1)
14
12
cp (kJ/kg.K)
10
8
6
4
2
0
8 .5
o
( C)
80
1 00
7 .5
re
tu r e
P
p e ra
8 .0
60
ss
40
ur
e
20
Tem
(M
P
9.0
a)
1 0.0
9 .5
0
Fig. 1 CO2 specific heat versus temperature and pressure
In fact, it can be seen from the three dimension graph as shown in Fig.1 that the projection of
CO2 specific heat peak value is a curve on the temperature-pressure plane at different supercritical
pressure, which is called pseduocritical curve. The correlation between the pseduocritical
temperature and pressure is obtained as follows.
T pc = −31.40 + 12.15 p − 0.6927 p 2 + 0.03160 p 3 − 0.0007521 p 4
(2)
According to the variation characteristics of CO2 specific heat, pseduocritical region is defined to
a temperature strip near the pseduocritical curve, as shown in Eq. (3).
0.7T pc ≤ T ≤ 1.3T pc
(3)
The range of supercritical pressure corresponding to Eq. (3) is from 7.5MPa to 14.0 MPa .
The heat transfer treatment principle for supercritical CO2
The special properties variation of supercritical CO2 fluid makes its heat transfer performance
different from the low-pressure fluids. At low pressure, the fluids properties are assumed to be
independent of the temperature, that is, treated as constant properties. While when the flow and heat
transfer problems of supercritical CO2 fluid are treated, two points should be paid attention to: (1)
In the mass equation, momentum equation and energy equation, the fluid is not treated as constant
properties, it should be treated as variable properties. (2) Along the flow direction, there is
temperature change between the wall boundary layer and the main fluid, which results in the density
change. In addition, the CO2 viscosity changes dramatically with the temperature near the
pseudocritical region, the natural convection resulted from which may put great effect on the
velocity distribution, shear force distribution of the fluid in the heat exchanger [2].
According to whether the impact of natural convection is taken into consideration or not, the
supercritical fluid heat transfer is classified to simple forced-convection heat transfer and mixed
convection heat transfer. The heat transfer results of Jackson et al [2-3] show that the buoyant force
can be ignored when the following condition is accorded.
Gr
< 10 −3
(for horizontal tube)
(1)
2
Re f
Advanced Materials Research Vols. 562-564
Gr
< 10 −5
Re 2f.7
365
(for vertical tube)
(2)
where, Gr is the Grashof number, Re f is the Reynolds number, subscript f denotes that the mean
temperature is the temperature of the main fluid.
Comparison to the conventional refrigerants
Compared to the conventional vapor compression refrigeration cycle, the function of the gas cooler
in the transcritical CO2 cycle is similar to the condenser. But in the condenser the phase-change
condensing heat transfer is undergoing, while in the gas cooler the single-phage forced-convection
heat transfer is taking place. So the heat transfer mechanism for the two processes and their heat
transfer performances are different [5-7]. The explanation is given in the following by means of
thermophysical properties analog analysis.
When supercritical CO2 fluid is cooled in the gas cooler, its density is increased. Fig.2 gives the
density variation trends for supercritical CO2 fluid, CO2 saturated liquid and conventional refrigerant
R134a and R22. It is found that the density of supercritical CO2 is closer to that of the CO2 saturated
liquid near the critical point, which shows that the distance between molecules for supercritical CO2
is correspond to its liquid. It also can be seen from Fig.2 that the density of supercritical CO2 is
lower than that of the R134a and R22 saturated liquid, and higher than that of the R134a and R22
saturated gas.
The specific heat of supercritical CO2 is greater than that of the R134a and R22 saturated liquid
and saturated gas, as shown in Fig.3, especially in the pseudo-critical region, the specific heat of
CO2 is far greater than that of the R134a and R22 saturated liquid and saturated gas.
12
1400
R134a saturated liquid
R22 saturated liquid
p=9.0MPa (CO2)
CO2 saturated liquid
600
p=9.0MPa (CO2)
10
cp (kJ/kg.K)
800
R134a saturated liquid
8
R22 saturated liquid
R134a saturated gas
6
R22 saturated gas
R22 saturated gas
4
R134a saturated gas
400
2
200
0
0
10
20
30
40 50 60 70 80
Temperature (oC)
0
30
90 100 110
Fig.2 Density comparison of supercritical
CO2 to conventional refrigerant
40
50
60
70
80
Temperature (oC)
90
100
110
Fig.3
Specific
heat
comparison
of
supercritical CO2 to conventional refrigerant
2
0.08
Viscosity
0.06
0.04
p=9.0MPa (CO2)
R134a saturated liquid
R22 saturated liquid
R22 saturated gas
R134a saturated gas
1.6
4
p=9.0MPa (CO2)
R22 saturated liquid
R134a saturated liquid
R134a saturated gas
R22 saturated gas
0.1
(10 kg/m.s)
0.12
(W/m.K)
3
1000
λ
Density (kg/m )
1200
1.2
0.8
0.4
0.02
0
30
40
50
60
70
80
90
Temperature (oC)
100
110
Fig.4
Conductivity
comparison
of
supercritical CO2 to conventional refrigerants
0
30
40
50
60
70
80
Temperature (oC)
90
100
110
Fig.5 Viscosity comparison of supercritical
CO2 to conventional refrigerants
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Materials Engineering and Automatic Control
Fig.4 presents the conductivity comparison for supercritical CO2 and R134a and R22 saturated
liquid and saturated gas. It is found that CO2 conductivity drops quickly near the critical point, and
it is lower than that of R134a and R22 saturated liquid and larger than their saturated gas.
The viscosity comparison of supercritical CO2 to R134a and R22 saturated liquid and saturated
gas is given in Fig.5. It is obvious that the viscosity of supercritical CO2 is far lower than that of
R134a and R22 saturated liquid, and a little higher than that of their saturated gas.
From the view of properties analysis, it is found from the above comparison for supercritical CO2
to R134a and R22 saturated liquid and gas that the characteristics of supercritical CO2 fluid are
equivalent to those of the conventional refrigerants.
Conclusions
The specific heat of supercritical CO2 changes dramatically with the temperature and pressure near
the critical region. According to the characteristics of CO2 specific heat, the correlation of the
pseudo-critical temperature is obtained and the pseudo-critical region is defined. In this paper, the
supercritical CO2 fluid heat transfer treatment principle, the thermo-physical properties of
supercritical CO2 fluid comparison to the conventional refrigerants are mainly analyzed. The special
properties variation of supercritical CO2 fluid makes its heat transfer performance different from the
conventional fluids. From the view of properties analysis, it can be seen that the heat transfer
performance of supercritical CO2 is equivalent to the condensation heat transfer of conventional
refrigerants. The reason may be that there is no liquid film in existence and the thickness of the
boundary layer is very thin in the supercritical CO2 cooling process. The heat transfer temperature
difference is very large, so the heat transfer coefficient for supercritical CO2 cooling process is
equivalent to that of the condensation heat transfer.
Acknowledgements
The authors acknowledge the support by the Natural Science Foundation of Tianjin, China under
Grant 10JCYBJC08300.
Nomenclature
c p specific heat (kJ/kg.K)
λ conductivity (W/m.K)
p pressure (Mpa)
T temperature (oC)
References
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[2] Zhou Qiangtai.
Two-phase flow and heat exchange. Water and electricity power Press. (1987)
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A.E.R.E.-R 8158, DESIGN report 34. (1975)
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Materials Engineering and Automatic Control
10.4028/www.scientific.net/AMR.562-564
Investigation on the Properties of Supercritical CO2 Fluid and its Heat Transfer Characteristics
10.4028/www.scientific.net/AMR.562-564.363
DOI References
[6] Dongsoo Jung, Kil-hong Song, Youngmok Cho, et al: Flow condensation heat transfer coefficients of pure
refrigerants. International Journal of Refrigeration, Vol. 26 (2003), pp.4-11.
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