FLOW PATTERN IN CO 2 -LUBURICANT TWO-PHASE FLOW AT SUPERCRITICAL PRESSURE

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1 ISTP-16, 2005, PRAGUE 16 TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA FLOW PATTERN IN CO 2 -LUBURICANT TWO-PHASE FLOW AT SUPERCRITICAL PRESSURE Koji Mori and Kunihiro Shimoki Osaka Electro-Communication University Corresponding author: k-mori@isc.osakac.ac.jp, Keywords: Flow Pattern, CO 2, Lubricant, Two-phase, Heat Transfer Coefficient Abstract In order to obtain the data for designing and optimizing air conditioning systems using a CO 2 refrigerant, the cooling heat transfer characteristics of CO 2 were examined at a supercritical pressure condition 9.5 MPa. The experiments were carried out at heat fluxes between kg/(m 2 s) and temperatures between C. The obtained data revealed that heat transfer coefficients increased with mass flux and the maximum heat transfer coefficients were obtained at about 45 C. The CO 2 -Oil mixtures were also investigated and it was found that the heat transfer coefficients of the mixtures were approximately times as much as those of only CO 2. Flow visualization experiments revealed that the oil film existed along the tube wall and became an obstacle for heat transfer. Flow pattern in the tube was found 2 types, stratified and annular, and the pattern changed according to the CO 2 density and velocity variations due to its temperature variation. The well-known pattern maps were examined whether those maps were consistent with the measured pattern data. It was found that no map was available and a new pattern map was proposed for CO 2 -oil mixtures. 1 Introduction A lot of studies on air conditioning systems using a CO 2 refrigerant have been carried recently in order to establish a new refrigerant technology with lower global warming potential, GWP [1][2]. The use of CO 2 in heat pump-type hot-water supply systems has also become of interest because the systems can save energies compared with gas-type hot water supply systems [3]. For such systems, precise and detailed knowledge of the heat transfer characteristics of CO 2 in heat exchangers should be required. However, as CO 2 in these systems is maintained at supercritical conditions, experimental research is not easy. As an added complication, the refrigerant in general-purpose heat pumps is contaminated with the oil used to lubricate the compressor. Therefore, it is also necessary to understand the heat transfer characteristic of CO 2 -oil mixtures in order to design and optimize heat exchangers. However, the heat transfer characteristics of CO 2 -oil mixtures have yet to be investigated in detail. This situation seems to be due to the difficulty in experiments. In the present study, an experimental system was constructed to examine these fluids under supercritical pressure conditions. Heat transfer experiments were carried out using CO 2 at various rates and temperatures, and the obtained heat transfer coefficients are presented. Similar experiments were then performed for CO 2 -oil mixtures and the heat transfer coefficients of the mixtures are also represented. Based on visualizations of the, the relationship between heat transfer coefficients and characteristics about CO 2 -oil mixtures are also discussed. Based on the visual observation of the, pattern was determined. The measured patterns are compared with the well-known pattern maps and their applicability is examined. Moreover, a new pattern map is proposed. 1

2 Oil supply section Fig. 1 Experimental setup 2 Experimental The experimental setup is shown in Fig. 1. CO 2 of wt% purity was employed in this study and 100cSt polyalkylene glycol (PAG) oil was used as the lubricant. A commercial heat pump compressor was used to pressurize CO 2 vapor. The CO 2 refrigerant was accordingly contaminated with the oil. For experiments on CO 2 only, three oil separators were installed in series immediately downstream of the compressor, providing perfect oil removal. The rates of CO 2 were measured using a mass meter at the inlet to the first cooling stage. In the cooling stage, the of CO 2 passed through pipes fitted with water sheaths. The CO 2 temperature was monitored at the outlet of the first cooling stage and controlled by varying the temperature of water coolant or its rate. The CO 2 then ed into -stabilizing section and into the test section where the heat transfer coefficients were measured. The then passed past a viewing section, through the second cooling stage, an expansion valve, and then to a heating stage. Two cooling stages were employed for greater control of the CO 2 Fig.2 Detail of test section Fig. 3 Detail of viewing section temperature in the test section without requiring a temperature- regulating stage at the inlet of the expansion valve. In the heating zone, CO 2 was heated to a vapor and passed to the compressor. 2

3 FLOW PATTERN IN CO 2 -LUBURICANT TWO-PHASE FLOW AT SUPERCRITICAL PRESSURE Lubricant oil was supplied from the pump in the oil supply section. The rate of the oil was able to be varied by adjusting the diaphragm stroke of the pump and measured by the mass meter. The CO 2 and the oil were mixed upstream the first cooling stage. The oil was collected at the separator mounted downstream of the heating zone and returned to the pump. Detail of the test section is shown in Fig. 2. The test section was fitted with water sheaths, and the water temperature was controlled precisely by the chiller. The inner tube was made of copper and the outer tube was made of stainless steel. The outer tube was insulated to minimize heat loss. Four K-type thermocouples with outer diameter of 1 mm were fixed on the outer surface of the inner tube. Detail of the viewing section is shown in Fig. 3. The viewing section was a quartz glass tube with the same inner diameter as the test section. The tube was inserted into a stainless steel tube for reinforcement considered the high pressures involved, and two opposing viewing windows were cut in the outer steel pipe to observe the. The was illuminated with a strobe light from one window and observed at the opposing window. Heat transfer coefficients were determined as follows. The heat transfer rate Q was obtained from the rates of the cooling water and the temperature difference of the cooling water between the inlet and outlet of the test section. The CO 2 temperature, T b, was calculated as the average temperature at the inlet and outlet of the test section. The differences between inlet and outlet temperatures are in the range from 2 to 5 C. Due to the high thermal conductivity of copper, the temperature of the inner surface of the inner tube was considered to be equal to the measured temperature at the outer surface of the inner tube. The representative temperature T w was obtained by averaging the temperatures measured by the four thermocouples. The heat transfer coefficient was then obtained by the following equation. Q h = (1) A( T b Tw) where A is the area of the inner surface of the inner tube. The experimental conditions were as follows. In the heat transfer experiments for CO 2, three inner tubes with inner diameters of 4, 6 and 8 mm were used to examine the effects of tube diameter on the heat transfer characteristics of CO 2. Mass fluxes of CO 2, G r, were varied in the range kg/(m 2 s) in increments of 100 kg/(m 2 s). The temperature of CO 2 in the test section was varied from 20 to 70 C in increments of 5 C. The pressure in the test section was controlled at 9.5MPa. In the experiments of CO 2 -oil mixtures, only the 6mm diameter tube was used and the mass flux of CO 2 was 300 kg/(m 2 s). Mass flux of oil, G o, was varied in the range 6-45 kg/(m 2 s). The range of oil concentration in the mixtures, ξ, was 2 15 %. The temperature and pressure conditions were the same as those for CO 2 experiments. 3. Results and Discussion 3.1 Heat Transfer Characteristics of CO 2 Figure 4 shows the heat transfer coefficients obtained for CO 2 using a 6 mm tube. The heat transfer coefficients for 4mm and 8mm tubes were nearly the same as that for 6mm. The heat transfer coefficients increase with the mass flux, and become maximal aroud 45 C. The temperatures that heat transfer coefficients are maximal are nearly consistent with the pseudocritical temperature, shown as a broken line in Fig. 4. In the calculation of heat transfer coefficients, physical properties of CO 2 were obtained from NIST Refprop ver [4]. The physical properties of CO 2 were also obtained from NIST Standard Reference Database 12, Version 5.0 (Thermo-dynamics and Transport Properties of Pure Fluids) [5]. It was confirmed that the differences between them were less than 3% at the present experimental conditions. 3

4 3.2 Heat Transfer Characteristics of CO 2 -Oil Mixture The experimental results of the heat transfer coefficients of the CO 2 -oil mixtures are shown in Fig. 5 with open symbols. In addition, the heat transfer coefficients of CO 2 are also shown in this figure with close circles. The heat transfer coefficients of the mixtures are smaller than that of CO 2. The relationship between the heat transfer coefficients of the mixtures and the temperature is similar to that for CO 2 in lower oil concentration conditions. However, the heat transfer coefficients of the mixtures at the oil concentration of 13% are almost independent of the temperature. Figure 6 shows the ratios of the heat transfer coefficients of the mixtures to those of CO 2. This figure also shows patterns classified by visual observation at the viewing section. The observed patterns were stratified and annular. The photos of those patterns are shown in Fig. 7. Figure 6 reveals that the heat transfer coefficients of the mixtures are approximately times as much as those of CO 2. The ratios depend not only on temperature but also on the oil concentration. The relationships between the ratios and temperature differ among the oil concentration conditions. At the oil concentration of 2 %, the ratios are about 0.6 in the temperature range of C. The is stratified and it is h W/m 2 K ξ % Gr = 300 kg/m 2 s p=9.5mpa D=6mm T Fig. 5 Heat transfer coefficients of CO 2 -oil mixtures h mix /h CO Separated (Fig. 9(a)) Annnular (Fig.9(b)) p=9.5mpa D=6mm G=300 kg/m 2 s ξ % T Fig. 6 Comparison in heat transfer coefficients between CO 2 -oil mixtures and CO 2. h W/m 2 K Gr kg/m 2 s Pseudo-critical temperature p=9.5mpa D=6mm (a) Stratified (G r =100kg/m 2 s, ξ =5%, T =35 C) T Fig. 4 Heat transfer coefficients of CO 2 gas (b) Annular (G r =500kg/m 2 s, ξ =2%, T =65 C) Fig.7 Flow patterns 4

5 FLOW PATTERN IN CO 2 -LUBURICANT TWO-PHASE FLOW AT SUPERCRITICAL PRESSURE considered that the ratios are determined by the ratios of the contact length between CO 2 and the tube wall to the tube inner periphery length. At the temperature of C, the ratio increases with temperature. This seems to be due to the mechanism that the decrease of CO 2 density, i.e. increase of CO 2 velocity, brings the oil film ing near the bottom of the tube and the contact area between CO 2 and tube wall becomes wider. The temperature increasing further, the ratio, however, decreases once and increases after having a minimum value. This is considered to be due to the following mechanism. The temperature increasing, increase of CO 2 gas velocity makes the contact area between the oil film and the tube wall wider and thus the heat transfer coefficients become smaller. At the temperature of 55 C, the pattern changes to annular and heat transfer coefficient decreases more. The temperature increasing further, velocity of the oil film increases due to the increase of CO 2 velocity, the oil film thickness becomes thinner, and thus the heat transfer coefficient becomes larger. In cases that the oil concentration is 5% or 9%, the ratios decrease before having a relative maximum value, because the changes from stratified to annular. The relationships between the ratio and the temperature in other temperature conditions are almost the same as those at the oil concentration of 2 %. At the oil concentration of 15 % the pattern is annular over the temperature of 35 C, so that the ratio has no maximum value. 4. Flow Pattern Figures show the comparisons between measured patterns and wellknown published pattern maps. Figure 10 shows the comparison between measured patterns and Mandhane s map [6]. Open and close circles in the figure denote stratified and annular, respectively. The data of stratified lie in the stratified region. However, the data of annular lie in the stratified region, there appears great discrepancy between them. j o m/s Plug Stratified m/s j r Slug Wavy Stratified Annular Annular Fig.10 Comparison between measured patterns and Mandhane s map G r /λ 10 5 Wavy Annular Stratified Stratified 10 4 Annular Slug (G o /G r )λψ Fig.11 Comparison between measured patterns and Baker s map G r /λ Annular Semiannular Wavy Stratified Annular (G o /G r )λψ Fig.12 Comparison between measured patterns and Soliman s map 5

6 Figure 11 shows the comparison between measured patterns and Baker s map [7]. In this case the data of stratified lie in both stratified and annular regions. The data of annular lie in both regions as well. Even in the baker s map there appears great discrepancy between measured data and baker s map. Figure 12 shows the measured data on Soliman s map [8]. The data of stratified lie from wavy to semi-annular regions, while the data of annular lie from semiannular to annular regions. Considering the semi-annular to be the transition from wavy to annular, Soliman s map seems to be a bit consistent with the measured data. In the present study a lot of examinations were carried out to obtain a pattern map suitable for CO 2 -oil mixtures, it was revealed that the way of using gas and oil mass fluxes brought the best result. Figure 13 shows the new pattern map. The bold line shows the boundary between stratified and annular regimes. All data are well discriminated into stratified and annular regions. 4. Concluding Remarks Heat transfer measurements were conducted for supercritical CO 2 and a CO 2 -oil mixture. The obtained data revealed the following findings. (1) Heat transfer coefficient of CO 2 becomes highest at about 45 C at 9.5 MPa, which is close to the pseudo-critical temperature. The heat transfer coefficients increase with mass rate of CO 2. (3) The heat transfer coefficients of the CO 2 - oil mixture are times as much as those of only CO 2. The heat transfer coefficient of the mixture depends on both the temperature and oil concentration. By visualizing the, it was found that two types of patterns, i. e. stratified and annular, were observed in the present experimental conditions. In comparison between the pattern and heat transfer coefficient, the heat transfer coefficients in stratified is greater G o kg/m 2 s than that in annular at the conditions with same temperature. (3) A new pattern map using mass flux of CO 2 and oil was proposed for CO 2 -oil mixtures. References Stratified Stratified Annular Annular G r kg/m 2 s Fig.13. New pattern map for present data [1] Proceedings of Heat Transfer Issues in Natural Refrigerants, College Park, (1997). [2] Proceeding of Natural Working Fluids 98, Oslo, pp , (1998). [3] M. Saikawa, K. Hashimoto, T. Kobayakawa, K. Kusakari, M. Itoh and H. Sakakibara, Development of CO 2 heat pump water heater for residential use, Transactions of Japan Society of Refrigerating and Air Conditioning Engineers, Vol. 18, No. 3, pp (2001) (in Japanese). [4] Thermodynamics Properties of Refrigerant and Refrigerant Mixtures, Version 6.01, National Institute of Standards and Technology, (1998). [5] Thermo-dynamics and Transport Properties of Pure Fluids, Version 5.0, National Institute of Standards and Technology, (2000). [6] Mandhane, J. M., Gregory, G. A., and Aziz, K., A pattern map for gas-liquid in horizontal pipes, Int. J. Multiphase Flow, 1-4, pp , (1974). [7] Baker, O., Simultaneous of oil and gas, Oil Gas J., 53, pp , (1954). [8] Soliman, H. M. and Azer, N. Z., Flow patterns during condensation inside a horizontal tube, Trans. ASHRAE, 77-1, pp , (1971). 6