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Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study
International Journal of Refrigeration 26 (2003) 823–829 www.elsevier.com/locate/ijrefrig
Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study§ Mitsuhiro Fukutaa,*, Tadashi Yanagisawaa, Toshinori Araib, Yasuhiro Ogia a
b
Department of Mechanical Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432-8561, Japan Graduate school of Science and Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432-8561, Japan Received 27 November 2002; received in revised form 24 March 2003; accepted 27 March 2003
Abstract A capillary tube is widely used as an expansion device for small refrigeration cycles. In a practical refrigeration cycle, some amount of refrigeration oil is discharged from a compressor and refrigerant/oil mixture flows through the capillary tube. This study investigated experimentally the influence of mixing of the refrigeration oil with the refrigerant on the flow through the capillary tube. The experiments are carried out with not only a miscible combination of refrigerant and oil but also an immiscible combination. In both cases, the mass flow rate through the capillary tube and temperature and pressure distributions along the tube are measured under several conditions of subcooled degree and oil concentration. In the case of miscible combination, the mass flow rate of refrigerant decreases with increasing the oil concentration because the viscosity of liquid phase increases by the mixing of viscous oil. Even in the case of the immiscible combination, the oil droplet is so small that it mixes homogeneously in the liquid phase in the capillary tube and the refrigerant mass flow rate decreases by the mixing of immiscible oil. There is no significant influence of the oil concentration on the underpressure, which means pressure difference between saturation pressure and flash inception pressure, in both miscible and immiscible combinations. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Oil; Miscibility; Refrigerant; Two-phase flow; Laminar flow; Capillary
Influence des huiles miscibles et non miscibles sur les caracte´ristiques d’e´coulement dans un tube capillaire. Partie I : e´tude expe´riementale Mots cle´s : Huile ; Miscibilite´ ; Frigorige`ne ; E´coulement diphasique ; E´coulement laminaire ; Capillaire
§
This is an extended version of a paper originally presented at the Ninth International Refrigeration and Air Conditioning Conference at Purdue, IN, USA, 2002. * Corresponding author. Tel.: +81-53-478-1054; fax: +8153-478-1058. E-mail addresses: [email protected] (M. Fukuta).
1. Introduction A capillary tube is commonly used as an expansion device for domestic refrigerators and small air conditioning units because of its low cost and simple structure. In contrast with the simplicity, the flow through the
0140-7007/03/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/S0140-7007(03)00068-9
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M. Fukuta et al. / International Journal of Refrigeration 26 (2003) 823–829
Nomenclature D : m P Pin Psat Tsc
inner diameter, m mass flow rate, kg s1 pressure, Pa inlet pressure, Pa saturation pressure, Pa subcooled degree, C oil concentration
capillary tube is quite complex, and many studies [1–24] have been done both experimentally and theoretically to clarify the flow characteristics and to estimate the flow rate through the capillary tube. Most of the studies discussed the flow characteristics of pure refrigerant, although small amount of refrigeration oil discharged from a compressor circulates with the refrigerant in a practical cycle. Some papers reported the influence of oil on the performance of capillary tube. Bolstad and Jordan [1] and Whitacre et al. [2] reported that the oil increased the mass flow rate through the capillary tube. Wijaya [9] tested the influence of oil circulation and observed no significant difference between data obtained with and without an oil separator. Huerta and Silvares [18] studied the oil influence experimentally and theoretically, and showed that the presence of oil reduces the mass flow rate in the capillary tube. Yana Motta et al. [24] conducted a visualization study in a glass capillary tube to observe a vaporization point and showed that the refrigerant/oil flow has a shorter liquid length than the one of pure refrigerant flow. Concerning short tube orifices, used as expansion devices similar to the capillary tube, there are some studies focusing on the influence of oil [25,26]. The mechanism of flow through the short tube orifice, however, is different from that through the capillary tube, and it is shown that the mixing of oil has little influence on the mass flow rate under large subcooled degree. In the previous studies on the influence of oil mixing, refrigeration oils miscible with refrigerant were used. Recently, however, immiscible oil is sometimes used in the refrigeration cycle using HFC refrigerants [27,28] in order to improve severe lubricating conditions and to prevent blockage of the capillary tube caused by the hydrolysis of ester oil, which is miscible with the HFCs. When some amount of the immiscible oil flows out from the compressor and the refrigerant/oil mixture flows through the capillary tube, the flow pattern, the flow characteristics and the mass flow rate may be different from the case that the pure refrigerant or the refrigerant/miscible oil mixture flows through the capillary tube. In this study, the influence of mixing of the miscible oil with the refrigerant on the flow characteristics
through the capillary tube is investigated in more detail. In addition, the study using the refrigerant/immiscible oil combination is conducted. In both cases, the mass flow rate of refrigerant through the capillary tube is measured with changing the oil concentration quantitatively. Pressure and temperature distributions along the capillary tube are also measured under adiabatic condition. A sight glass and a glass capillary tube are used to observe the flow pattern before the tube and inside the tube respectively. The development of mathematical model both for the miscible and the immiscible combinations and validation of the model will be discussed in a continuous paper.
2. Experiment Fig. 1 shows a schematic diagram of the experimental setup. The experimental refrigeration cycle consists of a compressor, a condenser, a subcooler, a capillary tube and an evaporator. The compressor is a rolling piton type rotary compressor and it has a special oil separator at the top of the casing in order to separate the oil from discharging refrigerant perfectly. The refrigerant delivered from the compressor condenses at the condenser. Then, it enters the capillary tube after its subcooled degree is controlled by the subcooler and an electric heater. The refrigeration oil stored in the compressor casing is fed intentionally through a needle valve to a liquid line at the outlet of subcooler. The mass flow rates of the refrigerant and the oil are measured respectively by positive displacement type flowmeters, which for both the refrigerant and oil have an accuracy of 0.5% of reading. Since some amount of refrigerant dissolves in the oil fed from the compressor, correction of the mass flow rates of refrigerant and oil is made with considering the solubility of refrigerant. The capillary tube is a copper tube of straight, 1 mm nominal I.D. and 0.8 m long. The precise inner diameter is obtained by a preliminary test using water. The pressure
Fig. 1. Schematic diagram of experimental set up.
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gradient and the flow rate under laminar flow condition are measured, and these are substituted in Hagen-Poiseuille equation to calculate the inner diameter. The capillary tube is insulated by a urethane foam and thermocouples, whose accuracy is within 0.1 C, are soldered on the outer surface of the tube to measure the temperature distribution along the tube. Pressure taps of 0.3 mm diameter are machined on the capillary tube wall and Bourdon type pressure gauges with the accuracy of 0.5% of full scale are connected to each pressure tap. Since the pressure and temperature distributions become steep as the flow goes downstream, an interval of the pressure and temperature measuring point is arranged smaller in the downstream. Besides the copper tube, a glass capillary tube whose inner diameter is 1 mm is used for visualization of the flow. A glass pipe (8 mm I.D.) as a sight glass is installed just before the capillary tube to observe the flow pattern in the liquid line. The specifications of capillary tubes are shown in Table 1. In this study, the influence of oil mixing with the refrigerant on the flow characteristics through the capillary tube is investigated by using miscible and immiscible oils. Since replacing the refrigeration oil in the experimental cycle is not an easy task, refrigerant is changed for the same refrigeration oil in this study. The naphthenic type mineral oil (ISO VG32) is selected as the refrigeration oil, and either R22 or R134a is charged in the cycle as the miscible or the immiscible combination with the oil. Fluorescence dye is added to the oil in order to visualize the oil phase by applying ultra-violet light in the case of immiscible combination. In the experiment, the mass flow rate and the temperature and pressure distributions are measured under several conditions of the subcooled degree and the oil concentration. Test conditions are shown in Table 2.
3. Results and discussion 3.1. Instability of flow Fig. 2 shows the refrigerant mass flow rate of R134a versus the subcooled degree under the no-oil mixing condition. The black circles show the results under the condition that the subcooled degree is increased step by step, and the white circles are results obtained with decreasing the subcooled degree. The mass flow rate increases with the subcooled degree because the singlephase region having less pressure drop as compared with the two phase region increases. When the subcooled degree decreases, the mass flow rate shows larger value than that measured with increasing the subcooled degree, and then it becomes same value in the range of subcooled degree smaller than 6 C. The mass flow rate at the subcooled degree of 7 C shown by the black circle, point (a), is 5% smaller than that of the white circle, point (b). Pressure and temperature distributions along the capillary tube at the subcooled degree of 7 C are shown in Fig. 3(a) and (b) corresponding to the point (a) and (b) in Fig. 2. The temperature distribution is represented by saturation pressure corresponding to the temperature. White symbols show the pressures and the temperatures just before and behind the capillary tube. Since the temperature before the capillary tube is measured by a thermocouple inserted in the pipe while the temperatures along the tube are measured on the tube wall, slight temperature difference is shown at the
Table 1 Specifications of capillary tubes Capillary Length Inner Surface Number of Number of tube (mm) diameter roughness thermocouples pressure (mm) (mm) gauges Copper 1 800 Copper 2 800 Glass 800
1.06 1.04 1.00
0.5 0.5 ffi 0
12 12 –
6 8 –
Fig. 2. Mass flow rate of refrigerant versus subcooled degree. The data obtained with increasing or decreasing the subcooled degree step by step show a hysterisis.
Table 2 Experimental condition Combination refrigerant/oil
Capillary tube
Inlet pressure (MPa abs)
Outlet pressure (MPa abs)
Subcooled degree ( C)
Mass flow rate (kg/h)
Reynolds number at inlet
Mass concentration of oil
R22/MO R134a/MO
Copper1, glass Copper2, glass
1.87–2.06 0.81–1.28
0.59–1.16 0.4–1.08
0–26.1 2.4–34.0
18.2–36.5 9.0–25.3
46 000—93 000 7000–49 000
0–0.09 0–0.15
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Fig. 4. Instability of flow through a glass capillary tube.
Fig. 3. Pressure and temperature distributions along capillary tube. Temperature distribution is expressed by saturation pressure.
entrance. The point where the saturation pressure, or the temperature, starts to decrease is the flash inception point. The pressure difference between the saturation pressure and the flash inception pressure is termed the underpressure. It is found from Fig. 3(a) and (b) that the underpressure at the point (a) in Fig. 2 is about 50 kPa, whereas that at the point (b) is about 130 kPa although the subcooled degrees for both cases are almost the same. The region where the liquid phase is super-saturated, i.e. single-phase metastable region, for the case (a) is smaller than that for the case (b), and it results in the smaller mass flow rate in the case (a). As a result, the hysterisis shown in Fig. 2 is caused by the change of location of the flash inception point. The similar phenomenon is reported in [20]. This phenomenon is resulted from that once the single phase region is developed to the downstream, the flash inception does not easily occur even when the super-saturated degree increases. Although this instability does not occur always, it should be noted that the result includes the scattering caused by the instability of the flash inception point. Moreover, when the glass capillary tube is used under the no-oil mixing condition, the flows with the flash and without the flash in the capillary tube are repeated periodically and downstream pressure of the capillary tube fluctuates according to the flash occurrence. Fig. 4 shows the periodic change of the downstream pressure
and the output of a photo-sensor attached near the exit of the glass tube. The photo-sensor detects transmittance of light, which is low when the flash occurs in the tube and vice versa. It is found that when the flash does not occur and liquid single-phase region reaches to the exit, the downstream pressure increases. Thus, we should pay attention to the phenomenon when a smooth glass tube is used for an experiment. When the refrigeration oil is mixed into the refrigerant, the hysterisis of mass flow rate against the subcooled degree and the instability flow through the glass capillary tube become hard to occur, and the flash inception became more stable in both the miscible and immiscible combinations. It is supposed that the oil mixing stimulates the flow through the capillary tube and it becomes a trigger of the flash inception. Yana Motta [24] also suggested that the refrigerant/oil flow has a shorter liquid phase length than the one of pure refrigerant flow by the observation through the glass capillary tube. A further study is needed to clear the influences of the specifications of the tube, kinds of refrigerant and oil mixing on the inception of flash quantitatively. 3.2. Influence of miscible oil Fig. 5 shows the influence of oil on the mass flow rate of refrigerant for the miscible combination of R22/
Fig. 5. Influence of miscible oil on mass flow rate of refrigerant.
M. Fukuta et al. / International Journal of Refrigeration 26 (2003) 823–829
mineral oil with a parameter of subcooled degree. The dotted lines in the figure express the imaginary flow rate of refrigerant in the case that the refrigerant flow rate decreases with the same rate of the oil concentration. Since there is the scattering of the data for pure refrigerant and it makes the influence of mixing oil difficult to examine, the data of one or two series with increasing the oil concentration are plotted. The mass flow rate of the refrigerant decreases more than the dotted line and it decreases by about 8% when the oil concentration is 5%. This is because the viscosity of liquid phase increases by mixing the viscous oil. Consideration of the oil mixing is, therefore, important for the design of capillary tube as well as the consideration of the influence of oil mixing on the heat transfer and the pressure drop in heat exchangers. The underpressure is observed in the case of R22/ mineral oil combination similar to the case of R134a shown in Fig. 3. The underpressure for the miscible combination of refrigerant/oil obtained schematically on the pressure and temperature distribution diagrams for each experiment is plotted against the oil concentration in Fig. 6 with the parameter of subcooled degree. In this figure, the underpressure ranges from 120 to 180 kPa. Here, the underpressure of 150 kPa corresponds to about 20 cm in distance and 7 ms in time. The underpressure for the smaller subcooled degree is slightly larger. There seems to be little influence of the concentration of oil on the underpressure, although the oil mixing makes the flash inception stable as described in the previous section. 3.3. Influence of immiscible oil When the immiscible oil is mixed with the refrigerant, the oil phase separates from the refrigerant phase in the liquid line and the flow pattern of immiscible combination in the liquid line is completely different from that of miscible combination. Fig. 7(a) shows a photograph at the glass pipe set horizontally just before the capillary tube. The oil concentration of this case is 1%, and the flow direction is from left to right in this figure. Transparent
Fig. 6. Influence of miscible oil on underpressure.
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Fig. 7. Flow patterns at sight glass before capillary tube (a) and in capillary tube (b).
portion is the refrigerant and droplets are the immiscible oil colored by the fluorescence dye. The oil flows in the upper part of the pipe in the form of the droplet due to its smaller density. Fig. 7(b) shows the flow pattern of refrigerant/oil mixture in the capillary tube flowing from left to right with the concentration of 0.01, which is observed by reducing flow velocity (5 m/s) and using a stroboscope. The white portion is the oil phase and flows in the capillary tube with very small droplets. Although the oil droplet was not identified even with using the stroboscope under a normal operating condition with much higher velocity, it is supposed that the oil is mixed with the refrigerant homogeneously in the form of extremely small droplet under such condition. It was also observed that when the oil concentration increase, a lot of oil accumulates at the upper part of the glass pipe and it blockades the capillary tube when it goes to the capillary tube at one time. It is, therefore, necessary to pay attention to eliminate the portion where the immiscible oil tends to accumulate before the capillary tube. Fig. 8 shows the influence of the immiscible oil on the mass flow rate of R134a for the different subcooled degrees. The dotted line in Fig. 8 shows the imaginary flow rate of refrigerant if the flow rate decreases with the same rate of the oil concentration. It has the same tendency shown in Fig. 5 that the mass flow rate of refrigerant decreases more than the oil mixing ratio shown by the dotted line. It is caused by the same reason as the case of the miscible combination, i.e. the viscosity of liquid phase increases apparently by mixing the immiscible viscous oil. In the case of 0.05 oil concentration, the mass flow rate decreases by about 7%, and it is found that the mixing of the immiscible oil has almost the same influence on the mass flow rate of refrigerant as that of miscible oil.
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Fig. 8. Influence of immiscible oil on mass flow rate of refrigerant.
Fig. 10. Influence of immiscible oil on underpressure.
Fig. 9 shows the pressure and temperature distributions along the capillary tube with and without the mixing of immiscible oil. In this figure, the distributions around the flash inception point are shown to exaggerate the difference between both cases. When the oil is mixed with refrigerant, the mass flow rate decreases as shown in Fig. 8, while the frictional loss per unit length increases because the viscosity of liquid phase increases. Consequently, the pressure distributions are not different with each other. The flash inception points, where the temperature or the saturation pressure starts to decrease, are almost the same. After the flashing, the temperature in the case of the no oil mixing seems to decrease more rapidly than that in the case of oil mixing. This is because the heat capacity of the fluid becomes larger when the oil is mixed with refrigerant and the two-phase metastable region where the temperature is not equal to the saturation temperature tends to increase under the oil mixing condition. This phenomenon is less notable in the case of miscible combination. Farther study is needed to examine the heat transfer between the refrigerant and oil and to clarify the influence of the oil mixing on the two-phase metastable region quantitatively.
The underpressure for the immiscible combination is plotted versus the oil concentration in Fig. 10 with the parameter of subcooled degree. The underpressure for the immiscible combination has almost the same range as the miscible combination shown in Fig. 6. There also seems to be little influence of the concentration of oil on the underpressure in the experimental range of the oil concentration.
Fig. 9. Influence of immiscible oil on pressure and temperature distributions along capillary tube near flash point.
4. Conclusions The quantitative influence of refrigeration oil on the flow characteristics through a capillary tube is investigated experimentally. The experiments are carried out by using R22 and mineral oil as a miscible combination and R134a and the mineral oil as an immiscible one. In the experiment, the mass flow rate of refrigerant sometimes shows a hysterisis against the subcooled degree because of a change of flash inception point, and the inception of flash seems to become stable by the mixing of oil in both combinations. In the miscible combination, the mass flow rate of refrigerant decreases more than the oil concentration because the viscosity of refrigerant/oil mixture increases. The influence of the oil concentration on the underpressure is relatively small. Even in the case of the immiscible combination, the oil droplet is pretty small and it is mixed with the liquid refrigerant homogeneously in the capillary tube. As the result, apparent viscosity of liquid phase increases with the oil concentration and the influence of the oil mixing on the mass flow rate of refrigerant for the immiscible combination is almost the same as the miscible combination. The influence of the concentration of immiscible oil on the underpressure is small as was in the miscible combination. When the refrigeration oil is mixed with refrigerant, two-phase metastable region seems to increases because of the heat capacity of the oil. Further study is needed to clarify the heat transfer phenomenon between the refrigerant and oil and to get quantitative result for two-phase metastable region.
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References [1] Bolstad NM, Jordan RC. Theory and use of the capillary tube expansion device. Refrigerating Engineering 1949; 56(6):572–83. [2] Whitacre, Stein, Boyd, Eibling. Analysis of the potentialities of using analog computer in the development of residential refrigerators. Report to Whirlpool Corporation, Columbus, OH, 1963. [3] Koizumi H, Yokoyama K. Characteristics of refrigerant flow in a capillary tube. ASHRAE Trans 1980;86(2):19–27. [4] Li RY, Lin S, Chen ZY, Chen ZH. Metastable flow of R12 through capillary tubes. Int J Refrigeration 1990; 13(3):181–6. [5] Chen ZH, Li RY, Lin S, Chen ZY. Correlation for metastable flow of refrigerant 12 through capillary tubes. ASHRAE Trans 1990;96(1):550–4. [6] Li RY, Lin S, Chen ZH. Numerical modeling of thermodynamic non-equilibrium flow of refrigerant through capillary tubes. ASHRAE Trans 1990;96(1):542–9. [7] Kuehl SJ, Goldschmidt VW. Modeling of steady flows of R-22 through capillary tubes. ASHRAE Trans 1991;97(1): 139–48. [8] Lin S, Kwok CCK, Li RY, Chen ZH, Chen ZY. Local frictional pressure drop during vaporization of R12 through capillary tube. Int J Multiphase Flow 1991;17(1): 95–102. [9] Wijaya H. Adiabatic capillary tube test data for HFC134a. In: Proceedings of 1992 Int. Refrigeration Conf. at Purdue, 1992. p. 63–71. [10] Peixoto RA, Bullard CW. A simulation and design model for capillary tube—suction line heat exchangers. In: Proceedings of 1994 Int. Refrigeration Conf., 1994. p. 335–40. [11] Wong TN, Ooi KT. Refrigerant flow in capillary tube: an assessment of the two-phase viscosity correlations on model prediction. Int Comm Heat Mass Transfer 1995; 22(4):595–604. [12] Escanes F, Segarra CDP, Oliva A. Numerical simulation of capillary-tube expansion devices. Int J Refrigeration 1995;18(2):113–22. [13] Bittle RR, Pate MB. Theoretical model for predicting adiabatic capillary tube performance with alternative refrigerants. ASHRAE Trans 1996;102(2):52–64. [14] Chang SD, Ro ST. Pressure drop of pure HFC refrigerants and their mixtures flowing in capillary tube. Int J Multiphase Flow 1996;22(3):551–61. [15] Yilmaz T, U¨nal S. General equation for the design of capillary tubes. Trans ASME, J Fluid Eng 1996;118(1):150–4.
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[16] Seixlack AL, Prata AT, Melo C. Modeling the HFC134a flow through capillary tubes using a two-fluid model. In: Proceedings of 1996 Int. Refrigeration Conf. at Purdue, 1996. p. 89–94. [17] Wong TN, Ooi KT. Adiabatic capillary tube expansion devices: a comparison of the homogeneous flow and the separated flow models. Applied Thermal Engineering 1996;16(7):625–34. [18] Huerta AAS, Silvares OM. Simulation of the effects of oil in capillary tubes considering a separated flow model. In: Proceedings of 1998 Int. Refrigeration Conf. at Purdue, 1998. p. 443–8. [19] Katsuta M, Tubaki N, Ohno M, Komoto N, Ishigaki S, Sekitani H. Basic performance of adiabatic capillary tube for HCFC22 and its prediction. In: Proc. of 1998 JSRAE Annual Conf., 1998. p. 85–8 [in Japanese]. [20] Melo C, Neto CB, Ferreira RTS. Empirical correlations for the modeling of R-134a flow through adiabatic capillary tubes. ASHRAE Trans 1999;105(2):51–9. [21] Chen SL, Liu CH, Jwo CS. On the development of rating correlations for R-134a flowing through adiabatic capillary tubes. ASHRAE Trans 1999;105(2):75–86. [22] Wonwises S, Chan P, Luesuwanatat N, Purattanarak T. Two-phase separated flow model of refrigerants flowing through capillary tubes. Int Comm Heat Mass Transfer 2000;27(3):343–56. [23] Chen D, Lin S. Underpressure of vaporization of refrigerant R134a through a diabatic capillary tube. Int J Refrigeration 2001;24(3):261–71. [24] Yana Motta S, Parise JAR, Braga SL. A visual study of R-404A/oil flow through adiabatic capillary tubes. Int J Refrigeration 2002;25(5):586–96. [25] Kim Y, O’Neal DL. The effect of oil on the two-phase critical flow of Refrigerant-134a through short tube orifices. Int J Heat Mass Transfer 1994;37(9):1377–86. [26] Payne WV, O’Neal DL. Multiphase flow of Refrigerant 410A through short tube orifices. ASHRAE Trans 1999; 105(2):66–74. [27] Kawaguchi S, Shimizu T, Mochizuki T, Masuda N, Konishi H, Sumida Y. Development of HFC-134a rotary compressor for household refrigerator. In: Proceedings of 1994 Int. Compressor Eng. Conf. at Purdue, 1994. p. 535– 41. [28] Sekiya S, Shirafuji Y, Kwaguchi S, Kato T, Izawa T. Alkylbenzenes for split air-conditioners with R410A, part 1; reliability evaluation of compressors. In: Proceedings of 1998 Int. Compressor Eng. Conf. at Purdue, 1998. p. 465– 70.
Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study§ Mitsuhiro Fukutaa,*, Tadashi Yanagisawaa, Toshinori Araib, Yasuhiro Ogia a
b
Department of Mechanical Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432-8561, Japan Graduate school of Science and Engineering, Shizuoka University, 3-5-1, Johoku, Hamamatsu, 432-8561, Japan Received 27 November 2002; received in revised form 24 March 2003; accepted 27 March 2003
Abstract A capillary tube is widely used as an expansion device for small refrigeration cycles. In a practical refrigeration cycle, some amount of refrigeration oil is discharged from a compressor and refrigerant/oil mixture flows through the capillary tube. This study investigated experimentally the influence of mixing of the refrigeration oil with the refrigerant on the flow through the capillary tube. The experiments are carried out with not only a miscible combination of refrigerant and oil but also an immiscible combination. In both cases, the mass flow rate through the capillary tube and temperature and pressure distributions along the tube are measured under several conditions of subcooled degree and oil concentration. In the case of miscible combination, the mass flow rate of refrigerant decreases with increasing the oil concentration because the viscosity of liquid phase increases by the mixing of viscous oil. Even in the case of the immiscible combination, the oil droplet is so small that it mixes homogeneously in the liquid phase in the capillary tube and the refrigerant mass flow rate decreases by the mixing of immiscible oil. There is no significant influence of the oil concentration on the underpressure, which means pressure difference between saturation pressure and flash inception pressure, in both miscible and immiscible combinations. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Oil; Miscibility; Refrigerant; Two-phase flow; Laminar flow; Capillary
Influence des huiles miscibles et non miscibles sur les caracte´ristiques d’e´coulement dans un tube capillaire. Partie I : e´tude expe´riementale Mots cle´s : Huile ; Miscibilite´ ; Frigorige`ne ; E´coulement diphasique ; E´coulement laminaire ; Capillaire
§
This is an extended version of a paper originally presented at the Ninth International Refrigeration and Air Conditioning Conference at Purdue, IN, USA, 2002. * Corresponding author. Tel.: +81-53-478-1054; fax: +8153-478-1058. E-mail addresses: [email protected] (M. Fukuta).
1. Introduction A capillary tube is commonly used as an expansion device for domestic refrigerators and small air conditioning units because of its low cost and simple structure. In contrast with the simplicity, the flow through the
0140-7007/03/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/S0140-7007(03)00068-9
824
M. Fukuta et al. / International Journal of Refrigeration 26 (2003) 823–829
Nomenclature D : m P Pin Psat Tsc
inner diameter, m mass flow rate, kg s1 pressure, Pa inlet pressure, Pa saturation pressure, Pa subcooled degree, C oil concentration
capillary tube is quite complex, and many studies [1–24] have been done both experimentally and theoretically to clarify the flow characteristics and to estimate the flow rate through the capillary tube. Most of the studies discussed the flow characteristics of pure refrigerant, although small amount of refrigeration oil discharged from a compressor circulates with the refrigerant in a practical cycle. Some papers reported the influence of oil on the performance of capillary tube. Bolstad and Jordan [1] and Whitacre et al. [2] reported that the oil increased the mass flow rate through the capillary tube. Wijaya [9] tested the influence of oil circulation and observed no significant difference between data obtained with and without an oil separator. Huerta and Silvares [18] studied the oil influence experimentally and theoretically, and showed that the presence of oil reduces the mass flow rate in the capillary tube. Yana Motta et al. [24] conducted a visualization study in a glass capillary tube to observe a vaporization point and showed that the refrigerant/oil flow has a shorter liquid length than the one of pure refrigerant flow. Concerning short tube orifices, used as expansion devices similar to the capillary tube, there are some studies focusing on the influence of oil [25,26]. The mechanism of flow through the short tube orifice, however, is different from that through the capillary tube, and it is shown that the mixing of oil has little influence on the mass flow rate under large subcooled degree. In the previous studies on the influence of oil mixing, refrigeration oils miscible with refrigerant were used. Recently, however, immiscible oil is sometimes used in the refrigeration cycle using HFC refrigerants [27,28] in order to improve severe lubricating conditions and to prevent blockage of the capillary tube caused by the hydrolysis of ester oil, which is miscible with the HFCs. When some amount of the immiscible oil flows out from the compressor and the refrigerant/oil mixture flows through the capillary tube, the flow pattern, the flow characteristics and the mass flow rate may be different from the case that the pure refrigerant or the refrigerant/miscible oil mixture flows through the capillary tube. In this study, the influence of mixing of the miscible oil with the refrigerant on the flow characteristics
through the capillary tube is investigated in more detail. In addition, the study using the refrigerant/immiscible oil combination is conducted. In both cases, the mass flow rate of refrigerant through the capillary tube is measured with changing the oil concentration quantitatively. Pressure and temperature distributions along the capillary tube are also measured under adiabatic condition. A sight glass and a glass capillary tube are used to observe the flow pattern before the tube and inside the tube respectively. The development of mathematical model both for the miscible and the immiscible combinations and validation of the model will be discussed in a continuous paper.
2. Experiment Fig. 1 shows a schematic diagram of the experimental setup. The experimental refrigeration cycle consists of a compressor, a condenser, a subcooler, a capillary tube and an evaporator. The compressor is a rolling piton type rotary compressor and it has a special oil separator at the top of the casing in order to separate the oil from discharging refrigerant perfectly. The refrigerant delivered from the compressor condenses at the condenser. Then, it enters the capillary tube after its subcooled degree is controlled by the subcooler and an electric heater. The refrigeration oil stored in the compressor casing is fed intentionally through a needle valve to a liquid line at the outlet of subcooler. The mass flow rates of the refrigerant and the oil are measured respectively by positive displacement type flowmeters, which for both the refrigerant and oil have an accuracy of 0.5% of reading. Since some amount of refrigerant dissolves in the oil fed from the compressor, correction of the mass flow rates of refrigerant and oil is made with considering the solubility of refrigerant. The capillary tube is a copper tube of straight, 1 mm nominal I.D. and 0.8 m long. The precise inner diameter is obtained by a preliminary test using water. The pressure
Fig. 1. Schematic diagram of experimental set up.
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gradient and the flow rate under laminar flow condition are measured, and these are substituted in Hagen-Poiseuille equation to calculate the inner diameter. The capillary tube is insulated by a urethane foam and thermocouples, whose accuracy is within 0.1 C, are soldered on the outer surface of the tube to measure the temperature distribution along the tube. Pressure taps of 0.3 mm diameter are machined on the capillary tube wall and Bourdon type pressure gauges with the accuracy of 0.5% of full scale are connected to each pressure tap. Since the pressure and temperature distributions become steep as the flow goes downstream, an interval of the pressure and temperature measuring point is arranged smaller in the downstream. Besides the copper tube, a glass capillary tube whose inner diameter is 1 mm is used for visualization of the flow. A glass pipe (8 mm I.D.) as a sight glass is installed just before the capillary tube to observe the flow pattern in the liquid line. The specifications of capillary tubes are shown in Table 1. In this study, the influence of oil mixing with the refrigerant on the flow characteristics through the capillary tube is investigated by using miscible and immiscible oils. Since replacing the refrigeration oil in the experimental cycle is not an easy task, refrigerant is changed for the same refrigeration oil in this study. The naphthenic type mineral oil (ISO VG32) is selected as the refrigeration oil, and either R22 or R134a is charged in the cycle as the miscible or the immiscible combination with the oil. Fluorescence dye is added to the oil in order to visualize the oil phase by applying ultra-violet light in the case of immiscible combination. In the experiment, the mass flow rate and the temperature and pressure distributions are measured under several conditions of the subcooled degree and the oil concentration. Test conditions are shown in Table 2.
3. Results and discussion 3.1. Instability of flow Fig. 2 shows the refrigerant mass flow rate of R134a versus the subcooled degree under the no-oil mixing condition. The black circles show the results under the condition that the subcooled degree is increased step by step, and the white circles are results obtained with decreasing the subcooled degree. The mass flow rate increases with the subcooled degree because the singlephase region having less pressure drop as compared with the two phase region increases. When the subcooled degree decreases, the mass flow rate shows larger value than that measured with increasing the subcooled degree, and then it becomes same value in the range of subcooled degree smaller than 6 C. The mass flow rate at the subcooled degree of 7 C shown by the black circle, point (a), is 5% smaller than that of the white circle, point (b). Pressure and temperature distributions along the capillary tube at the subcooled degree of 7 C are shown in Fig. 3(a) and (b) corresponding to the point (a) and (b) in Fig. 2. The temperature distribution is represented by saturation pressure corresponding to the temperature. White symbols show the pressures and the temperatures just before and behind the capillary tube. Since the temperature before the capillary tube is measured by a thermocouple inserted in the pipe while the temperatures along the tube are measured on the tube wall, slight temperature difference is shown at the
Table 1 Specifications of capillary tubes Capillary Length Inner Surface Number of Number of tube (mm) diameter roughness thermocouples pressure (mm) (mm) gauges Copper 1 800 Copper 2 800 Glass 800
1.06 1.04 1.00
0.5 0.5 ffi 0
12 12 –
6 8 –
Fig. 2. Mass flow rate of refrigerant versus subcooled degree. The data obtained with increasing or decreasing the subcooled degree step by step show a hysterisis.
Table 2 Experimental condition Combination refrigerant/oil
Capillary tube
Inlet pressure (MPa abs)
Outlet pressure (MPa abs)
Subcooled degree ( C)
Mass flow rate (kg/h)
Reynolds number at inlet
Mass concentration of oil
R22/MO R134a/MO
Copper1, glass Copper2, glass
1.87–2.06 0.81–1.28
0.59–1.16 0.4–1.08
0–26.1 2.4–34.0
18.2–36.5 9.0–25.3
46 000—93 000 7000–49 000
0–0.09 0–0.15
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Fig. 4. Instability of flow through a glass capillary tube.
Fig. 3. Pressure and temperature distributions along capillary tube. Temperature distribution is expressed by saturation pressure.
entrance. The point where the saturation pressure, or the temperature, starts to decrease is the flash inception point. The pressure difference between the saturation pressure and the flash inception pressure is termed the underpressure. It is found from Fig. 3(a) and (b) that the underpressure at the point (a) in Fig. 2 is about 50 kPa, whereas that at the point (b) is about 130 kPa although the subcooled degrees for both cases are almost the same. The region where the liquid phase is super-saturated, i.e. single-phase metastable region, for the case (a) is smaller than that for the case (b), and it results in the smaller mass flow rate in the case (a). As a result, the hysterisis shown in Fig. 2 is caused by the change of location of the flash inception point. The similar phenomenon is reported in [20]. This phenomenon is resulted from that once the single phase region is developed to the downstream, the flash inception does not easily occur even when the super-saturated degree increases. Although this instability does not occur always, it should be noted that the result includes the scattering caused by the instability of the flash inception point. Moreover, when the glass capillary tube is used under the no-oil mixing condition, the flows with the flash and without the flash in the capillary tube are repeated periodically and downstream pressure of the capillary tube fluctuates according to the flash occurrence. Fig. 4 shows the periodic change of the downstream pressure
and the output of a photo-sensor attached near the exit of the glass tube. The photo-sensor detects transmittance of light, which is low when the flash occurs in the tube and vice versa. It is found that when the flash does not occur and liquid single-phase region reaches to the exit, the downstream pressure increases. Thus, we should pay attention to the phenomenon when a smooth glass tube is used for an experiment. When the refrigeration oil is mixed into the refrigerant, the hysterisis of mass flow rate against the subcooled degree and the instability flow through the glass capillary tube become hard to occur, and the flash inception became more stable in both the miscible and immiscible combinations. It is supposed that the oil mixing stimulates the flow through the capillary tube and it becomes a trigger of the flash inception. Yana Motta [24] also suggested that the refrigerant/oil flow has a shorter liquid phase length than the one of pure refrigerant flow by the observation through the glass capillary tube. A further study is needed to clear the influences of the specifications of the tube, kinds of refrigerant and oil mixing on the inception of flash quantitatively. 3.2. Influence of miscible oil Fig. 5 shows the influence of oil on the mass flow rate of refrigerant for the miscible combination of R22/
Fig. 5. Influence of miscible oil on mass flow rate of refrigerant.
M. Fukuta et al. / International Journal of Refrigeration 26 (2003) 823–829
mineral oil with a parameter of subcooled degree. The dotted lines in the figure express the imaginary flow rate of refrigerant in the case that the refrigerant flow rate decreases with the same rate of the oil concentration. Since there is the scattering of the data for pure refrigerant and it makes the influence of mixing oil difficult to examine, the data of one or two series with increasing the oil concentration are plotted. The mass flow rate of the refrigerant decreases more than the dotted line and it decreases by about 8% when the oil concentration is 5%. This is because the viscosity of liquid phase increases by mixing the viscous oil. Consideration of the oil mixing is, therefore, important for the design of capillary tube as well as the consideration of the influence of oil mixing on the heat transfer and the pressure drop in heat exchangers. The underpressure is observed in the case of R22/ mineral oil combination similar to the case of R134a shown in Fig. 3. The underpressure for the miscible combination of refrigerant/oil obtained schematically on the pressure and temperature distribution diagrams for each experiment is plotted against the oil concentration in Fig. 6 with the parameter of subcooled degree. In this figure, the underpressure ranges from 120 to 180 kPa. Here, the underpressure of 150 kPa corresponds to about 20 cm in distance and 7 ms in time. The underpressure for the smaller subcooled degree is slightly larger. There seems to be little influence of the concentration of oil on the underpressure, although the oil mixing makes the flash inception stable as described in the previous section. 3.3. Influence of immiscible oil When the immiscible oil is mixed with the refrigerant, the oil phase separates from the refrigerant phase in the liquid line and the flow pattern of immiscible combination in the liquid line is completely different from that of miscible combination. Fig. 7(a) shows a photograph at the glass pipe set horizontally just before the capillary tube. The oil concentration of this case is 1%, and the flow direction is from left to right in this figure. Transparent
Fig. 6. Influence of miscible oil on underpressure.
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Fig. 7. Flow patterns at sight glass before capillary tube (a) and in capillary tube (b).
portion is the refrigerant and droplets are the immiscible oil colored by the fluorescence dye. The oil flows in the upper part of the pipe in the form of the droplet due to its smaller density. Fig. 7(b) shows the flow pattern of refrigerant/oil mixture in the capillary tube flowing from left to right with the concentration of 0.01, which is observed by reducing flow velocity (5 m/s) and using a stroboscope. The white portion is the oil phase and flows in the capillary tube with very small droplets. Although the oil droplet was not identified even with using the stroboscope under a normal operating condition with much higher velocity, it is supposed that the oil is mixed with the refrigerant homogeneously in the form of extremely small droplet under such condition. It was also observed that when the oil concentration increase, a lot of oil accumulates at the upper part of the glass pipe and it blockades the capillary tube when it goes to the capillary tube at one time. It is, therefore, necessary to pay attention to eliminate the portion where the immiscible oil tends to accumulate before the capillary tube. Fig. 8 shows the influence of the immiscible oil on the mass flow rate of R134a for the different subcooled degrees. The dotted line in Fig. 8 shows the imaginary flow rate of refrigerant if the flow rate decreases with the same rate of the oil concentration. It has the same tendency shown in Fig. 5 that the mass flow rate of refrigerant decreases more than the oil mixing ratio shown by the dotted line. It is caused by the same reason as the case of the miscible combination, i.e. the viscosity of liquid phase increases apparently by mixing the immiscible viscous oil. In the case of 0.05 oil concentration, the mass flow rate decreases by about 7%, and it is found that the mixing of the immiscible oil has almost the same influence on the mass flow rate of refrigerant as that of miscible oil.
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Fig. 8. Influence of immiscible oil on mass flow rate of refrigerant.
Fig. 10. Influence of immiscible oil on underpressure.
Fig. 9 shows the pressure and temperature distributions along the capillary tube with and without the mixing of immiscible oil. In this figure, the distributions around the flash inception point are shown to exaggerate the difference between both cases. When the oil is mixed with refrigerant, the mass flow rate decreases as shown in Fig. 8, while the frictional loss per unit length increases because the viscosity of liquid phase increases. Consequently, the pressure distributions are not different with each other. The flash inception points, where the temperature or the saturation pressure starts to decrease, are almost the same. After the flashing, the temperature in the case of the no oil mixing seems to decrease more rapidly than that in the case of oil mixing. This is because the heat capacity of the fluid becomes larger when the oil is mixed with refrigerant and the two-phase metastable region where the temperature is not equal to the saturation temperature tends to increase under the oil mixing condition. This phenomenon is less notable in the case of miscible combination. Farther study is needed to examine the heat transfer between the refrigerant and oil and to clarify the influence of the oil mixing on the two-phase metastable region quantitatively.
The underpressure for the immiscible combination is plotted versus the oil concentration in Fig. 10 with the parameter of subcooled degree. The underpressure for the immiscible combination has almost the same range as the miscible combination shown in Fig. 6. There also seems to be little influence of the concentration of oil on the underpressure in the experimental range of the oil concentration.
Fig. 9. Influence of immiscible oil on pressure and temperature distributions along capillary tube near flash point.
4. Conclusions The quantitative influence of refrigeration oil on the flow characteristics through a capillary tube is investigated experimentally. The experiments are carried out by using R22 and mineral oil as a miscible combination and R134a and the mineral oil as an immiscible one. In the experiment, the mass flow rate of refrigerant sometimes shows a hysterisis against the subcooled degree because of a change of flash inception point, and the inception of flash seems to become stable by the mixing of oil in both combinations. In the miscible combination, the mass flow rate of refrigerant decreases more than the oil concentration because the viscosity of refrigerant/oil mixture increases. The influence of the oil concentration on the underpressure is relatively small. Even in the case of the immiscible combination, the oil droplet is pretty small and it is mixed with the liquid refrigerant homogeneously in the capillary tube. As the result, apparent viscosity of liquid phase increases with the oil concentration and the influence of the oil mixing on the mass flow rate of refrigerant for the immiscible combination is almost the same as the miscible combination. The influence of the concentration of immiscible oil on the underpressure is small as was in the miscible combination. When the refrigeration oil is mixed with refrigerant, two-phase metastable region seems to increases because of the heat capacity of the oil. Further study is needed to clarify the heat transfer phenomenon between the refrigerant and oil and to get quantitative result for two-phase metastable region.
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