International Journal of Refrigeration
36 (2013) 136-144

EFFECT OF LUBRICATING OIL ON FLOW BOILING HEAT TRANSFER
OF CARBON DIOXIDE
Chaobin Danga*, Noburu Haraguchia, Takashi Yamadaa, Minxia Lib, Eiji HiharaaaDepartment of Human and Engineered Environmental Studies, Graduate School of Frontier
Sciences, The University of Tokyo
5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japanb
Key Laboratory of Medium-Low Temperature Energy Efficient Utilization, Ministry of Education,
Department of Thermal Engineering, Tianjin University
No 92, Weijin Road, Nankai District, Tianjin 300072, China*Corresponding author: Fax: +81-47-136-4647. Email: dangcb@k.u-tokyo.ac.jp

ABSTRACTThe flow boiling heat transfer of carbon dioxide with PAG-type lubricating oil entrained from
0% to 5% in horizontal smooth tubes was examined. Experiments were conducted using test tubes
with inner diameter of 2–6 mm at mass fluxes of 360–1440 kg m-2 s-1 and heat fluxes of 4.5–36 kW
m-2. The saturation temperature was 15°C.
At low oil concentrations of 0.5%–1%, the heat transfer coefficient decreased to less than half that
under oil-free conditions. The heat transfer coefficient did not decrease further with increasing oilconcentration up to 5%. The heat flux positively influenced the heat transfer coefficient in low
vapor quality regions, not the high vapor quality regions. The presence of oil caused the mass flux
to significantly influence the heat transfer coefficient at a low heat flux till dryout, while nosignificant influence of the mass flux at a high heat flux was observed. The dryout quality decreased
at a large mass flux. The measured pressure drops increased monotonously because of the
lubricating oil.
Keywords: Carbon dioxide; heat transfer coefficient; pressure drop; lubricating oil; flow boiling;
dryoutNOMENCLATURE
d
ΔP
G
m
P
qw
Q
Tsat
Tw
x
ω

inner diameter (m)pressure drop (kPa)mass flux (kg m-2 s-1)
mass flow (kg s-1)
pressure (MPa)heat flux (W m-2)
heat added to test tube (W)saturation temperature (°C)
wall temperature (°C)
vapor quality
oil concentration (wt%)ααloheat transfer coefficient (W m-2 K-1)
single phase heat transfer coefficient with all flow as liquid (W m-2 K-1)

1. INTRODUCTION
Previous studies on the flow boiling heat transfer of CO2 revealed that the characteristics of
CO2 refrigerant differed from those of other refrigerants due mainly to the former’s high operationpressure and, consequently, unique thermophysical properties (e.g., Hihara and Tanaka, 2000;
Pettersen et al., 2000; Sun and Groll, 2002a, 2002b, 2002c). Hihara and Dang (2007) systematicallyinvestigated the flow boiling heat transfer of CO2 using tube diameters of 1–6 mm under various
saturation temperature, mass flux, and heat flux conditions. Their experimental data revealed that
nucleate boiling is predominant in CO2 flow boiling due to its low liquid-to-vapor density ratio andlow surface tension. Ducoulombier et al. (2008) established a heat transfer database for the flow
boiling of pure CO2 based on recent literature. They discussed the effects of the mass flow rate, heat
flux, and tube inner diameter on the heat transfer of carbon dioxide by reviewing the results ofdifferent studies, including 216 test conditions, and concluded that some phenomena were notclarified, such as the decrease in the heat transfer coefficient with an increase in vapor quality.
Schael and Kind (2005) and Dang et al. (2010b) also conducted experiments on the heat transfer
coefficient of pure CO2 flowing inside a micro-fin tube.
In an actual heat pump cycle, lubricating oil is essential to the compressor for cooling, sealing,
and lubrication purposes. However, when the lubricant flows into other components, it usuallycauses some unexpected negative effects. Hwang et al. (2007) experimentally measured oil
retention in the suction line, evaporator, and gas cooler at various refrigerant mass flow rates and oilconcentrations to investigate the distribution of PAG-type oil in a CO2 transcritical system. They
found that about 50% of the total oil is retained in the heat exchangers and the suction line at an oil
circulation ratio of 5 wt%. The retention of oil in the heat exchangers may result in several

problems, such as an increase in the pressure drop, a decrease in the heat transfer performance, andlower system reliability. Shen and Groll (2008) reviewed the effect of lubricant on the heat transfer
and pressure drop by referring to 142 papers published from 1987 to 2002; these papers examined
more than ten types of refrigerants. Thome’s study (1998) was discussed in this review, particularly
his list of the nine effects of lubricant on refrigerant flow boiling. The five negative effects are oil
viscosity effect, mass transfer resistance, oil holdup, transition from turbulent flow to laminar flow,
and miscibility effect. The four positive effects are nucleate boiling, foaming, increase in the wetted
surface, and flow pattern of the refrigerant–oil mixture.
Bandarra et al. (2009) addressed experimental studies on the flow boiling heat transfer andflow pattern of refrigerant/oil mixtures, including R134a/oil, ammonia/oil, R407C/oil, and CO2/oil.
They suggested that the physical mechanisms of the oil on the flow boiling of refrigerant/lubricant
oil mixtures should be further investigated because there was no agreement on the influence of oilon the flow boiling heat transfer based on present results. Katsuda et al. (2003) measured the flow
boiling heat transfer coefficient of CO2 inside a 3 mm I.D. tube at heat fluxes of 5–15 kW m-2 andmass fluxes of 200–600 kg m-2 s-1. They reported that the heat transfer coefficient decreased sharply
at an oil concentration of 0.3%. Zhao and Bansal (2009) addressed recent studies on the heattransfer of mixtures of CO2 and oil. They summarized the results of these papers (Hassan, 2004;
Katsuta et al., 2002, 2003; Tanaka, 2001; Gao and Honda, 2006; Gao et al., 2007; Dang et al., 2006;
Zhao et al., 2002) and tried to use the properties of CO2 and oil mixtures to predict the heat transfer
coefficient of CO2 with oil through some available correlations.
Although only limited information is available, the aforementioned studies indicate that thepresence of lubricating oil in subcritical CO2 significantly influences the heat transfer performance.
This implies that it is important to systematically investigate the effect of lubricating oil over a wide
range of experimental conditions. This report aims to present a systematic study of flow boiling heattransfer over a wide range of experimental conditions. Experiments were conducted using
horizontal tubes with inner diameters of 2, 4, and 6 mm at oil concentrations of 0.5%–5%, heat
fluxes of 4.5–36 kW m-2, and mass fluxes of 360–1440 kg m-2 s-1. The heat transfer coefficient inboth the pre- and post-dryout regions and the dryout quality were measured. This report presents
only experimental results because it was difficult to correlate them over a wide range ofexperimental conditions; this correlation effort is currently in progress.

2. EXPERIMENTAL APPARATUS AND DATA REDUCTION
2.1 Experimental apparatus
Figure 1 shows a schematic diagram of the experimental apparatus used in this study. The test
loop was comprised of a magnetic gear pump for working fluid circulation, mass flow meter(±0.5% accuracy), heater, oil sampling section, test section, oil separator, and cooler for cooling therefrigerant before it returns to the pump. The vapor quality at the inlet of the test section was
controlled by the heater. The oil concentration was measured in the oil sampling section andcontrolled by the oil separator.
Figure 2 shows the detailed structure of the test section. The test section was made of stainlesssteel and was heated electrically using a DC power source. The electrical resistance of the test tube
was approximately 0.6 . The outer wall temperature of the test section was measured using T-type
thermocouples with an uncertainty of ±0.1C; these thermocouples were attached to the top, bottom,
and sides of the test tube. To avoid the influence of the electric current, the tips of all thermocouples
were electrically insulated from the test tube by a Teflon film of 0.05 mm thickness. The pressure of
the working fluid at the inlet and outlet of the test section was monitored by using a precisionpressure sensor with an uncertainty of ±0.1% F.S.
The specific enthalpy of the working fluid entering the test section was calculated from themeasured temperature and pressure at the inlet of the preheater, and the power input to the heater.
The thermophysical properties of CO2 were determined using REFPROP 7.0 (2003). The inner wall
temperature Tw was calculated from the measured outer wall temperature using an equation for
one-dimensional heat conduction. The local saturation temperature Tsat was determined from thecorresponding saturation pressure, which was interpolated from the measured pressures at the inlet
and outlet. The local heat transfer coefficient α was estimated from the heat flux qw, Tsat, and Tw.
qw
(1)

Tw  Tsat
It is essential to reduce the heat loss from the test tube to its surroundings in order to obtaincorrect measurements of qw and Tw. The test tube was heavily insulated and placed inside a windtunnel; the air temperature in the tunnel was maintained at a value close to the outer wall
temperature of the test tube. The electrical input power was measured using a voltmeter andammeter to confirm that the heat generated by direct electrification was transferred effectively wellto the fluid. A preliminary test using water showed that the heat loss to surroundings was within 3%
and the maximum deviation of measured heat transfer coefficient to the calculated results by
Gnielinski equation was less than 10%.
The oil concentration ω was determined by the sampling method described in previous studies(Dang et al., 2007, 2010a). A sampling tube with an I.D. of 4.35 mm and length of 1.5 m was placed
ahead of the test section. The overall weight of the sampled CO2–oil mixture was measured; theCO2 inside the sampling tube was then carefully released to obtain the weight of the retained oil. ω
was measured thrice for each experimental condition, and the difference in the measured ω values
was confirmed to be less than 0.1%; this was considered as the uncertainty of oil concentration.
2.2 Experimental conditions
The experimental conditions are summarized in Table 1. The inner diameters of the smoothtubes were 2, 4, and 6 mm. PAG-type lubricating oil, which is partially miscible with the liquid CO2,
was used at concentrations of 0.5–5.0 wt%; the kinematic viscosity of the oil was 105 mm2 s-1 at
40°C and 20 mm2 s-1 at 100°C. Experiments were conducted by varying the heat and mass fluxes inranges of 4.5–36 kWm-2 and 360–1440 kg m-2s-1, respectively.
2.3 Experimental uncertainty

The uncertainty of the experimental results was calculated by means of the following equation:
1/ 2

2
 k  R 
 R   
 yn   n 1  yn
 

(5)

where δR is the total uncertainty associated with the dependent variable R, y is the independent
variable which affects the dependent variable R, δy is the uncertainty of y. The relative uncertaintyof α can be calculated as follows.
22  q    Tw    Tsat   w   
 qw   Tw  Tsat   Tw  Tsat 2(6)

The uncertainty for qw and Tw follows the same method by using the equation (5). The total
measurement uncertainty varies with the operating conditions but mainly depends on the accuracyof the wall superheat, inner wall temperature and the saturation temperature. The uncertainty of
inner wall temperature is determined from the outer wall temperature Tw, and is approximately
equal to ±0.1oC in spite of effects of qw and Q. The uncertainties of qw and Q is less than 3% due tothe heat loss to environment. The uncertainty in the saturation temperature is related to theuncertainty in the equation of state as well as the accuracy of the local pressure. Measurement
uncertainty of oil concentration is estimated to be 0.1wt%. Uncertainties of main measurement
devices are shown in Table 3. According to uncertainty analysis, the maximum measurement
uncertainty of the heat transfer coefficient was between 8.9% and 13%.

3. EXPERIMENTAL RESULTS AND DISCUSSION
3.1 Effect of oil on heat transfer coefficient
The presence of lubricating oil may enhance or deteriorate the heat transfer coefficientaccording to the miscibility of oil under various refrigerant and flow conditions. For a miscible oiland refrigerant mixture, i.e., HFO1234yf + PAG oil (Saitoh, et al, 2012), usually the heat transfer
coefficient is enhanced in the low vapor quality region, which is attributed to foaming and theincrease in wetted surface due to increased surface tension, but the heat transfer coefficient in thehigh vapor quality region is found decreasing monotonically with oil concentration. However, when
the oil is immiscible with a refrigerant, normally the heat transfer coefficient decreases with theincrease of oil concentration at the whole vapor quality region. Figures 3 and 4 compare themeasured heat transfer coefficients with and without the presence of oil at oil concentrations of
0%–5% inside the 2, 4 and 6 mm tubes, respectively. Since the partially miscible PAG-type oil istested, in general, the presence of a small amount of lubricating oil resulted in a dramatic decreasein the heat transfer coefficient. Figure 3 compares the heat transfer coefficients of the 2 mm tube atoil concentrations of 0%, 0.5%, 1%, and 5% at a heat flux of 9 kW m-2 and mass flux of 360 kg
m-2s-1. In comparison with the average value of 8–9 kW m-2K-1 in the pre-dryout region under theoil-free condition, the heat transfer coefficient decreased to 3–5 kW m-2K-1 at an oil concentration

of 0.5%. However, there were only slight differences in the measured heat transfer values for oilconcentrations of 0.5%–5%. It appears that the effect of oil on the heat transfer coefficient issaturated at a critical oil concentration, which was 0.5% for the 2 mm tube. Previous studies also
confirmed similar saturation phenomena for microfin tubes with an average diameter of 2 mm
(Dang et. al., 2010b). Figure 4 shows similar results for the 4 mm and 6 mm tubes at a heat flux of
18 kW m-2 and mass flux of 720 kg m-2 s-1. As for the 4 mm tube, in the presence of lubricating oil,the heat transfer coefficient in the pre-dryout region decreased from 9–10 kW m-2K-1 toapproximately 4 kW m-2K-1. The critical oil concentration at which the heat transfer coefficientsignificantly decreased was approximately 1%. Similar results can be found for 6 mm tube as
shown in Fig.4 (b). In addition, it is seen from Figs. 3 and 4 that the dryout quality and post-dryoutheat transfer were not influenced by the presence of oil up to concentrations of 5% for both the 2, 4
and 6 mm tubes.
3.2 Effect of heat flux on heat transfer coefficient
Figures 5 and 6 show the effect of the heat flux on the heat transfer coefficient for pure CO2
and at an oil concentration of 1% inside the 2 mm tube at mass fluxes of 360 and 1440 kg m-2 s-1,
respectively. In general, for pure CO2, the heat transfer coefficient is proportional to heat flux. With
the presence of oil, the heat flux positively influences the heat transfer coefficient in the low-quality
region at small mass fluxes. However, in the high-quality region or at a large mass flux, the heat
flux was not observed to have an obvious influence on the heat transfer coefficient.
Because CO2 has a low liquid-to-vapor density ratio, nucleate boiling is considered to be thedominant mechanism in CO2 flow boiling. As shown in Fig.5 (a), the heat transfer coefficientremains constant with the increase of quality in the pre-dryout region and the heat transfer
coefficient at a higher heat flux is much higher than that at a lower heat flux. However, the presenceof lubricating oil in CO2 results in the suppression to some extent of nucleate boiling depending on
the mass flux. Figure 5 (b) shows the measured heat transfer coefficient at heat fluxes of 4.5–36 kW
m-2 and a small mass flux of 360 kg m-2s-1. The heat transfer coefficient increased with the quality
in the pre-dryout region at heat fluxes of 4.5 and 9 kW m-2, indicating the influence of convective
heat transfer. With the generation of vapor constantly, the velocity of liquid is accelerated. That isbeneficial for the increment of convective heat transfer and cause suppression for nucleate boiling.
When heat flux is not so strong, the convective heat transfer is main contribution for heat transfer
comparing to nucleate boiling, then the tendency of heat transfer coefficient virus vapor quality is
upwards gradually. In contrast, at heat fluxes of 18 and 36 kW m-2, the pre-dryout heat transfer
coefficient did not change with the quality. In the low vapor quality region, as the heat flux wasincreased from 4.5 kWm-2 to 36 kWm-2, the heat transfer coefficient increased by approximately
100%. However, no obvious influence was observed in the high vapor quality region. This implies
that nucleate boiling dominates in the low vapor quality region, while it is suppressed in thehigh-quality region. Moreover, both the dryout quality and post-dryout heat transfer coefficient did
not change with the heat flux.

Figure 6 shows the effect of the heat flux at a large mass flux of 1440 kg m-2 s-1. For pure CO2,
it is found that the heat transfer coefficient in the pre-dryout region is mainly dependent on the heat
flux and independent of the mass flux. With the presence of oil, the heat transfer coefficient in boththe pre- and post-dryout regions and the dryout quality were not influenced by the heat flux. The
dryout quality for pure CO2 and with the presence of oil took a small value of 0.4 when the massflux was 1440 kg m-2 s-1. In the post-dryout region, the heat transfer coefficient increased with thequality.
3.3 Effect of mass flux on heat transfer coefficient
Figure 7 shows the measured heat transfer coefficients of the 2 mm tube at mass fluxes of
360–1440 kg m-2 s-1 and a heat flux of 18 kWm-2 for the pure CO2 and with the presence of oil.
Similar results at a higher heat flux of 36 kW m-2 are shown in Fig. 8. Under the oil-free condition,
as shown in Fig. 7(a) and Fig.8 (a), the heat transfer coefficient at a large mass flux was slightly lessthan that at a small mass flux, and dryout occurred at a low quality for a large mass flux. In thepresence of lubricating oil, the heat transfer coefficient increased significantly with the mass flux inthe pre-dryout region at a low heat flux. This increase was attributed to the influence of convective
boiling because nucleate boiling was suppressed by the oil. However, at a higher heat flux of 36 kW
m-2, no obvious difference was observed in the heat transfer coefficients at different mass fluxes in
the pre-dryout region, as shown in Fig. 8(b). Dryout occurred early at the high mass flux and was
not influenced by the heat flux.
The tendency of the heat transfer coefficient to vary with the quality in the post-dryout region
differed according to the mass flux. For both the pure CO2 and with the presence of oil, at a large
mass flux, the heat transfer coefficient increased with the quality, while at a low mass flux, thereverse was observed. As explained by Hihara et al. (2008), this decrease in the heat transfer
coefficient at a low mass flux is related to a thermodynamic non-equilibrium phenomenon, in which
the vapor becomes superheated in order to evaporate liquid droplets suspended in the bulk vapor
phase. With an increase in mass flux, the vapor-to-droplet heat transfer coefficient increases, whichleads to the alleviation of the thermodynamic non-equilibrium and to an increase in the heat transfer
coefficient with the vapor quality.
3.4 Effect of tube diameter on heat transfer coefficient
Figure 9 shows the measured pre-dryout heat transfer coefficients for pure CO2 and with thepresence of oil inside tube inner diameters of 2–6 mm (1-6 mm for pure CO2) at heat fluxes from
4.5 to 36 kW m-2 and mass flux of 360 kg m-2 s-1. The influence of tube diameters for pure CO2 andthe CO2-oil mixture are similar. For all tube diameters, the heat transfer coefficient increased withthe heat flux. This tendency was more significant for small tubes. Due to the small viscosity of CO2,
the flow is turbulent flow even for 1 mm tube. Normally the two-phase heat transfer coefficient is asuperposition of convective heat transfer and nucleate boiling. The single phase convective heattransfer coefficient αlo is proportional to d-0.2, with the decrease in tube diameter, the convective

heat transfer is enhanced. Although the nucleate boiling heat transfer coefficient is not explicitly
related to tube diameter, the ratio of amount of heat added to mass flow (Q/m = qw/G*4/d) isinversely proportional to tube diameter when compared at the same qw and G, which implies withthe decrease in tube diameter, the superheat increase, thus promotes the nucleate boiling.
As shown in Fig. 9(a), for pure CO2, the heat transfer coefficient increases with the decrease intube diameter, and this tendency becomes more distinct at higher heat flux. The heat transfer
coefficient for 6 mm tube does not change with heat flux because the flow pattern in 6 mm tube at alow mass flux of 360 kg m-2s-1 is stratified flow, the local heat transfer at the top of the tube is much
lower than bottom side (Hihara and Dang, 2007). At a heat flux of 36 kW m-2, the heat transfer
coefficient takes value of 5.2, 12, 16 and 20 kW m-2K-1 for 6, 4, 2 and 1 mm tubes, respectively.
With the presence of oil, as shown in Fig.9(b), due to the suppression of nucleate boiling by the oil,the increase tendency of heat transfer coefficient against tube diameter becomes moderate. At a heat
flux of 36 kW m-2, the heat transfer coefficient increased by 15% when the tube diameter was
decreased from 4 mm to 2 mm. However, when the tube diameter was decreased from 6 mm to 2
mm, the heat transfer coefficient increased by 100%.
3.5 Effect of oil concentration on pressure dropFigure 10 shows the effect of oil concentration on the measured pressure drop. The pressure
drop presented is the total pressure drop which includes the fraction pressure drop and momentum
pressure drop. The pressure drop was compared for tube diameters of 2–4 mm, a heat flux of 18 kW
m-2, and a mass flux of 720 kg m-2 s-1. The pressure drop increased with the tube diameter because alonger tube is needed to evaporate the CO2 from the saturation liquid state (x = 0) to the saturation
vapor state (x = 1). With the same tube diameter, the pressure drop monotonously increased with oilconcentration. At an oil concentration of 5%, the pressure drop increased by 80% of its value under
the oil-free condition. This result was attributed to the formation of an oil layer along the inner wallof the tube and an increase in viscosity due to the entrainment of lubricating oil in CO2.

4. CONCLUSIONS
The flow boiling heat transfer of CO2 with a small amount of PAG-type lubricating oil wasstudied experimentally. The main conclusions are summarized as follows:(1) The heat transfer coefficient decreases sharply at the critical oil concentration. The critical oilconcentration for a 2 mm tube was approximately 0.5%, while it was approximately 1% for the 4
and 6 mm tubes. However, the heat transfer coefficient does not decrease further at higher oilconcentrations.
(2) The heat flux positively influences the pre-dryout heat transfer coefficient at a small mass fluxin the low vapor quality region due to the nucleate boiling effect. At a large mass flux or in the highvapor quality region, no obvious difference was observed when the heat flux was varied. Moreover,the heat flux does not influence the dryout quality or post-dryout heat transfer coefficient.

(3) At a low heat flux, the heat transfer coefficient in the pre-dryout region increases with the massflux. However, at a large heat flux, the influence of the mass flux on the pre-dryout heat transfer
coefficient is not significant. The dryout quality decreases at a large mass flux.
(4) As the tube diameter decreases, the increase in the heat transfer coefficient due to the heat fluxin the pre-dryout region becomes significant.
(5) The pressure drop monotonously increases with oil concentration. This was attributed to anincrease in the viscosity of the CO2–oil mixture and the formation of an oil layer along the innerwall of the tube.

ACKNOWLEDGMENTS
The authors are grateful to Matsushita Electric Industrial Co., Ltd., for financial support. The fourthauthor gratefully acknowledges the financial support provided by the Japan Society for Promotionof Science (JSPS).

REFERENCES
Bandarra, F., Enio, P., Cheng, L., Thome, J.R., 2009. Flow boiling characteristics and flow patternvisualization of refrigerant/lubricant oil mixtures. International Journal of Refrigeration, 32,
185-202.
Dang, C., Haraguchi, N., Yamada, N., Hihara, E., 2006. Effect of lubricant oil on boiling heattransfer of carbon dioxide, in: Proceedings of 7th IIR-Gustav Lorentzen Conference, Trondheim,
Norway, 495-498.
Dang, C., Iino, K., N., Fukuoka, K., Hihara, E., 2007. Effect of lubricating oil on cooling heattransfer of supercritical carbon dioxide. International Journal of Refrigeration, 30, 724-731.
Dang, C., Iino, K., N., Hihara, E., 2010a. Effect of PAG-type lubricating oil on heat transfer
characteristics of supercritical carbon dioxide cooled inside a small internally grooved tube.
International Journal of Refrigeration, 33, 558-565.
Dang, C., Haraguchi, N., Hihara, E., 2010b. Flow boiling heat transfer of carbon dioxide inside asmall-sized microfin tube. International Journal of Refrigeration, 33, 655-663.
Ducoulombier, M., Colasson, S., Haberschill, P., 2008. A review on carbon dioxide heat transfer
characteristics during flow boiling. Proceedings of 8th IIR Gustav Lorentzen Conference on Natural
Working Fluids, Copenhagen, Denmark, M4-03.
Hihara, E., Dang C., 2007, Boiling heat transfer of carbon dioxide in a horizontal tube, 2007
ASME-JSME Thermal Engineering Summer Heat Transfer Conference, Vancouver, British
Columbia, CANADA, May.
Gao, L., Honda, T., Koyama, S., 2007. Experiments on flow boiling heat transfer of almost pure
CO2 and CO2–oil mixtures in horizontal smooth and microfin tubes, HVAC&R Res. 13 (3),415–425.

Gao, L., Honda, T., 2006, Flow and heat transfer characteristics of refrigerant and PAG oil in theevaporator of a CO2 heat pump system, in: Proceedings of 7th IIR Gustav Lorentzen Conference,
Trondheim, Norway, 491–494.
Hassan, M., 2004. Flow boiling of pure and oil contaminated carbon dioxide as refrigerant, Ph.D.
Thesis, Technical University of Denmark.
Hihara, E., Tanaka, S., 2000. Boiling heat transfer of carbon dioxide in horizontal tubes.
Preliminary Proceedings of the 4th IIR-Gustav Lorentzen Conference on Natural Working Fluids at
Purdue, 279-284.
Hihara, E., Haraguchi, N., Yamada, T., Dang, C., 2008. Investigation of post-dryout heat transfer ofcarbon dioxide, Preliminary Proceedings of the 8th IIR-Gustav Lorentzen Conference on Natural
Working Fluids at Purdue, T3-12.
Hwang, Y., Radermacher, R, Hirata, T., 2008. Oil mass fraction measurement of CO2/PAG mixture.
International Journal of Refrigeration 31, 256-261.
Katsuta, M., Kinpara, H., Aoyagi T., Yagi S., Mukaiyama H., Kamimura I. 2003, Evaporative heattransfer characteristics of CO2 refrigeration cycle and comparison with R134a: The change ofthermal and hydraulic characteristics due to evaporation temperature. Proc. of the 2003 JSRAE
Annual Conference, Tokyo, Japan: Japan Society of Refrigeration and Air Conditioning Engineers,
Oct, A309.
Katsuta, M., Takeo, N., Kamimura, I., Mukaiyama, H. A, 2002. study on the evaporator of CO2
refrigerant cycle: characteristics of heat transfer coefficient and pressure drop on mixing CO2 andoil (PAG), ACRA 2000 Lecture dissertation selected work, 20(3) 67–74.
Pettersen, J., Rieberer, R., Munkejord S.T., 2000. Heat transfer and pressure drop characteristics ofevaporating carbon dioxide in micro-channel tubes. Preliminary Proceedings of the 4th IIR-Gustav
Lorentzen Conference on Natural Working Fluids at Purdue, 107-114.
REFPROP. NIST Standard Reference Database 23, Version 7.0.
Saitoh, S., Dang, C., Hihara, E., 2012. Effect of lubricating oil on flow boiling heat transfer of low
GWP refrigerant HFO-1234yf in a horizontal small-diameter tube, Proceedings of 3rd International
forum on heat transfer, November 13-15, Nagasaki, Japan, paper No. IFHT2012-111.
Schael, A.E., Kind, M., 2005. Flow pattern and heat transfer characteristics during flow boiling of
CO2 in a horizontal micro fin tube and comparison with smooth tube data. International Journal of
Refrigeration 28, 1186-1195.
Shen B., Groll, E.A., 2008. Critical literature review of lubricant influence on refrigerant heattransfer and pressure drop. Final Report, ARTI-21CR/611-20080.
Sun, Z., Groll, E.A., 2002b. CO2 flow boiling heat transfer in horizontal tubes, Part 2: Experimental
results. Preliminary Proceedings of the 5th IIR-Gustav Lorentzen Conference on Natural Working
Fluids at Guangzhou, China, September 17-20, 126-132.
Sun, Z., Groll, E.A., 2002c. CO2 flow boiling heat transfer in horizontal tubes, Part 3: Prediction ofheat transfer coefficient. Preliminary Proceedings of the 5th IIR-Gustav Lorentzen Conference on
Natural Working Fluids at Guangzhou, China, September 17-20, 133-142.

Tanaka, S., Daiguji, H., Takemura, F., Hihara, E., 2001. Boiling heat transfer of carbon dioxide inhorizontal tubes, in: 38th National heat transfer symposium of Japan, Saitama, Japan, 899–900.
Thome, J.R., 1998. Boiling and evaporation of fluorocarbon and other refrigerants: A state-of the-art
review. ARTI internal report.
Zhao, X., Bansal, P., 2009. Critical review of flow boiling heat transfer of CO2-lubricant mixtures.
Int. J. of Heat and Mass Transfer 52, 870-879.
Zhao, Y., Molki, M., Ohadi, M.M., Fanca, F.H.R., Radermacher, R., Cho, K., 2002. Flow boiling of
CO2 with miscible oil in microchannels. ASHRAE Trans. 108(1), 135-144.

P

T

Gear Pum p

Refrigerant
Charge
P

Mass
Flow
Meter

T

CoolerHeaterOil Sam pling
Section

Sight
Glass
Oil

TEST SECTION

P

T

P

T

P

T

Oil
Separator

Figure 1. Schematic diagram of experimental apparatus

Pressure indicator

PWind tunnel

Teflon packing
Electrode
Test tube

Thermocouple

F low of CO 2

Insulation layer

FlangeT
Thermocouple

DC power supply

Figure 2. Detailed structure of test section

Heat transfer coefficient

Heat transfer[kW/m2K]
coefficient [kW m-2K-1]

14
12
10
8
6
4
2
0
0

0.2

0.4
0.6
Quality

0.8

1

Figure 3. Comparison of heat transfer coefficient with and without the presence of lubricating oil. d= 2 mm, ω = 0%–5%. qw = 9 kW m-2, and G = 360 kg m-2 s-1

Heat transfer coefficient [kW m-2K-1]

Heat transfer coefficient-2 -1
Heat transfer coefficient[kW/m2K][kW m K ]● 0% ○ 1% △ 3% □ 5%

10

12
10
8
6
4
2
0
0

0.2

0.4
0.6
Quality(a) d = 4 mm

0.8

1

8
6
4
2
0
0

0.2

0.4
0.6
Quality

0.81(b) d = 6 mm

Figure 4. Comparison of heat transfer coefficient with and without the presence of lubricating oil. ω= 0%–5%, qw = 18 kW m-2, and G = 720 kg m-2 s-1

4.5 kW m-2
18 kW m-2

9 kW m-2
36 kW m-2

0.2

0.4
0.6
Quality

Heat transfer coefficient [kWm[kWm -2-2K]
K-1]-1Heat transfer coefficient [kWm-2-2KK]
]

18
16
14
12
10
8
6
4
2
0

8

4.5
m-2
4.5 kW
kW/m2
kW/m2
18 kW
m-2

m-2
9 kW
kW/m2
36
kW/m2
36 kW m-2

6

4

2

0

0

(a) Pure CO2

0.8

1

0

0.2

0.4
0.6
Quality

0.8(b) CO2 with oil, ω = 1%

Figure 5. Effect of heat flux at a small mass flux of 360 kg m-2 s-1, d = 2 mm

1

Heat transfer coefficient [kWm[kWm -2-2K]
K-1]-1Heat transfer coefficient [kWm -2-2KK]
]

18
-2

18 kW m16

36 kW m-2

14
12
10
8
6
4
2
0

10
18 kW
m-2
kW/m2
36 kW
m-2
kW/m2

8
6
4
2
0

0

0.2

0.4
0.6
Quality(a) Pure CO2

0.8

1

0

0.2

0.4
0.6
Quality

0.8(b) CO2 with oil, ω = 1%

Figure 6. Effect of heat flux at a large mass flux of 1440 kg m-2 s-1. d = 2 mm

1

-2
Heat transfer coefficient [kWm[kWm -2
K]
K-1]

Heat transfer coefficient [kWmK]-1]
[kWm-2-2K

18
16
14
12
10
8
6

-2 -1

360 kg m s

4

750 kg m-2s-1

2

-2 -1

1440 kg m s

10
360 kg m-2s-1

8

720kg m-2s-1
1440kg m-2s-1

6
4
2
0

0
0

0.2

0.4
0.6
Quality(a) Pure CO2

0.8

1

0

0.2

0.4

0.6

0.8

Quality(b) CO2 with oil, ω = 1%

Figure 7. Effect of mass flux at a small heat flux of 18 kW m-2. d = 2 mm

1

Heat transfer coefficient [kWmK]-1]
[kWm-2-2K-1
Heat transfer coefficient [kWm-22KK]
]

8

20
18
16
14
12
10
8
6

360 kg m-2s-1

4

720kgm-2s-1

21440kg m-2s-1

0
0

0.2

0.4
0.6
Quality(a) Pure CO2

0.8

1

6

4

360kg
kg/m2s360
m-2s-1

2

750kg m-2s-1
720kg/m2s
1440
kg m-2s-1
1440kg/m2s

0
0

0.2

0.4
0.6
Quality

0.8(b) CO2 with oil, ω = 1%

Figure 8. Effect of mass flux at a high heat flux of 36 kW m-2, d = 2 mm

1

20
15

2

1mm I.D.
2mm I.D.
4mm I.D.
6mm I.D.

10
5
0
0

10

20

30
2
-2

Heat
Heat flux
flux [kW/m[kW m ](a) Pure CO2Heat transfer
coefficient[kW[kW/mHeat transfer
coefficientm-2K]K-1]

Heat transfer coefficient-2 -1
Heat transfer coefficient[kW/m2K][kW m K ]

25

40

6
5
4
3
2

2mm I.D.
4mm I.D.

1

6mm I.D.
0
0

10

20

30
-22

Heatflux
flux[kW
[kW/m
Heat
m ]]

(b) CO2 with oil, ω = 1%

Figure 9. Measured heat transfer coefficients at different tube diameters and heat fluxes. G = 360
kgm-2 s-1

40

80

Pressure drop [kPa]

70
60
50
40
30

0%

20

0.50%

10

5%

0
0

2

4

6

Inner diameter [mm]8

Figure 10. Comparison of measured pressure drop inside 2- 6 mm I.D. tube at different oilconcentrations. qw = 18 kW m-2. and G = 720 kg m-2 s-1

Table 1 Experimental conditions
Tube material

SUS316

Inner diameter of tube [mm]2, 4, 6

Oil type

PAG100Oil concentration [wt%]-2-1

0.5–5.0

Mass velocity [kg m ·s ]360, 720, 1440

Heat flux [kW m-2]

4.5, 9, 18, 36

Evaporation temperature [°C]15

Quality

0.1–1.0

Table 2

Uncertainty of measurement devices

Variable

DeviceRange

Accuracy

Temperature of tube wall

Thermocouples

0–50 °C±0.1 K

Temperature of refrigerantPt 100-50–200 °C±0.03 K
-1

Mass flow rate

Coriolis type flow meter

0–150 g min

Pressurepressure sensor

1–20 MPa±0.1% full scale

Voltage

Voltmeter

0–64 V±0.02% reading value

Inner diameter
Length

±0.5% reading value±0.03 mm*Ruler

* provided by the manufacture

0–6 m±0.001 m