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Experimental analysis of refrigerant mixtures flow through adiabatic capillary tubes
Experimental Thermal and Fluid Science 26 (2002) 499–512 www.elsevier.com/locate/etfs
Experimental analysis of refrigerant mixtures flow through adiabatic capillary tubes Flavio Augusto Sanzovo Fiorelli *, Alex Alberto Silva Huerta, Ot avio de Mattos Silvares Department of Mechanical Engineering, University of S~ ao Paulo, Av. Prof. Mello Moraes, 2231, Cidade Universit aria, 05508-900 S~ ao Paulo, Brazil Received 12 October 2001; received in revised form 12 January 2002; accepted 18 February 2002
Abstract This paper presents an experimental study on HCFC 22 alternative refrigerant mixtures flow through capillary tubes. Results for R-410A and R-407C show that the main operational parameters affect in a similar way the performance of capillary tubes for both refrigerants. The influence of geometry on the bahaviour of capillary tubes in refrigeration systems is characterised. An analysis on the differences in R-410A and R-407C flow through capillary tubes is also performed. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Refrigeration; Air conditioning; Capillary tubes; Experimental analysis; HCFC replacement
1. Introduction Several evidences connecting halogenated compounds to stratospheric ozone layer depletion was collected in the last decades. Such evidences led to the signature of the Montreal Protocol, which main goal is the elimination of such halogenated compounds. One of such substances is the HCFC 22, largely used as refrigerant in equipment for commercial refrigeration, commercial and household air conditioners and heat pumps. Unfortunately up today research indicates that there is no pure substance that could be used as alternative for HCFC 22 small size units without the need of great modifications in existing equipment. The use of zeotropic or near-azeotropic refrigerant mixtures is the most suitable alternative so far. Among the possible alternatives manufacturers are using mainly the R-407C, a zeotropic mixture of HFC 32, HFC 125 and HFC 134a (23%=25%=52% on mass basis) and the R-410A, a near-azeotropic mixture of HFC 32 and HFC 125
*
Corresponding author. Tel.: +55-11-3091-5570x207; fax: +55-113813-1886. E-mail addresses: fi[email protected] (F. Augusto Sanzovo Fiorelli), [email protected] (A. Alberto Silva Huerta), [email protected] (O. de Mattos Silvares).
(50%=50% on mass basis), and the HFC 134a for specific operational ranges. The use of such mixtures demands new experimental and numerical studies in order to evaluate how they affect the performance of refrigeration cycles as well as the design of cycle component. In this way the sizing of adiabatic capillary tubes using zeotropic mixtures is a subject of particular interest for small size air conditioning and heat pump units. Some recent works related to capillary tubes and refrigerant mixtures can be pointed out [1–10]. Nevertheless, these papers present few experimental data, except Motta et al. [10], even though they only present data for subcooled inlet conditions. Therefore, in order to provide a data base (including two-phase inlet conditions) that could be used for obtaining a practical correlation for sizing capillary tubes as well as for experimental validation of capillary tube simulation models/programs under development, this paper presents the results of an extensive experimental survey on R-407C and R-410A flow through capillary tubes. Such survey, which was carried out for both subcooled and two-phase inlet conditions, also characterised the influence of these refrigerants, as well as that of several operating and geometric parameters on the behaviour of capillary tubes used in refrigeration systems.
0894-1777/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 0 8 9 4 - 1 7 7 7 ( 0 2 ) 0 0 1 5 8 - 9
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Nomenclature d L m_ p T W_ e r2
diameter, mm length, m mass flow rate, kg/h pressure, kPa temperature, °C power consumption, w roughness, mm standard deviation, dimensionless
Subscripts cd condenser, condensation
2. Experimental apparatus Fig. 1 presents a schematic of the experimental apparatus implemented for this survey, in which is used a blow-down process. According to Paiva et al. [11], the main advantage of a blow-down unit, when compared to a conventional refrigeration cycle, is to allow an accurate and individualised control of each operational parameter. Its major disadvantage is the requirement of a high refrigerant charge in order to get suitable run times. Besides, it will occur a slight composition variation from test to test, particularly for zeotropic mixtures. Selection of a blow-down process to this study was motivated by authors’ belief that an accurate control of operational parameters is more important than prob-
ct dim exp ev HX i in out sub theo
capillary tube dimensionless experimental evaporator, evaporation heater generic step inlet outlet subcooling theoretical
lems connected with a high refrigerant charge. Concerning to composition variation, as explained later, it was monitored during the tests and the refrigerant composition was within the manufacturer admissible composition range for most of the cases. 2.1. Operation Refrigerant is initially stored upstream the test section in a high-pressure reservoir (two bladder accumulators of 100 l each). Such high pressure is provided by nitrogen filling of the bladders. Test section exit is connected to a low-pressure reservoir. Low pressure is obtained by refrigerant condensation provided by chilled ethylene-glycol/water mixture flowing through a coil inside such reservoir.
Fig. 1. Experimental apparatus schematic.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
At each run, by pressure difference, the refrigerant flows from the high-pressure to the low-pressure reservoir through the test section where the capillary tube is placed. At the end of a run, the refrigerant returns to highpressure reservoir also by pressure difference. Pressure is raised in the low-pressure reservoir by means of two heaters, as long as pressure is lowered in the high-pressure reservoir by releasing nitrogen from the bladders. 2.2. Instrumentation and control Parameters measured in test section are the pressure and temperature profiles along the capillary tube, mass flow rate, electric power consumption of subcooling/ quality final control heater, and mixture composition at capillary tube inlet. Ten strain-gage type pressure transducers (0.1% range value uncertainty) measure pressure profile along the capillary tube. Temperature profile is obtained by 18 T-type thermocouples soldered to the tube wall (0.3 °C uncertainty). It is also used two Pt-100 thermoresistances at capillary tube inlet and subcooling/ quality control system inlet (0.2 °C uncertainty). Mass flow rate measurement is performed by a Coriolis-type flowmeter (0.1% range value uncertainty). Electric power consumption of subcooling/quality final control heater is obtained by a wattmeter (1% reading uncertainty). This power consumption is used to evaluate, by energy balance, the quality for two-phase capillary tube inlet conditions. Due to the unit concept, it is necessary to monitor the mixture composition (in mass percentage) in each run. In this sense it is used a gas chromatograph with 1% uncertainty. The parameters to be controlled in each run are the capillary tube inlet and outlet pressures, and inlet subcooling or quality. Inlet pressure is controlled by a regulator (pre-control) and a PID control/solenoid valve system (final control) on nitrogen line. Outlet pressure is controlled by a PID control/solenoid valve system on ethylene-glycol line. By controlling the glycol flow rate, it is possible to control condenser pressure and, as a result, the capillary tube outlet pressure. For subcooling/quality control a two-step heating process is used. Refrigerant is pre-heated in a coil immersed in a hot water constant-temperature bath. Final adjustment of subcooling or quality is obtained by a 5.0 W/m tape heater and a PID controller (subcooling) or manual control (quality).
501
tubes length was 1.5 m. For dct ¼ 1:372 mm data were also obtained for two other lengths: 1.0 and 1.25 m. All capillary tubes were straight and well insulated. 2.4. Preliminary results First step in obtaining the experimental data was the characterisation of some experimental parameters: actual capillary tube diameters and relative roughness, as well as the heat losses in subcooling/quality control system. Actual capillary tube internal diameter was measured by filling a capillary tube sample with mercury. This procedure provides the average diameter with 1.0% uncertainty. Actual capillary tube diameters obtained were 1.101, 1.394 and 1.641 mm. Relative roughness was evaluated by measuring the pressure losses of an all-liquid R-410A refrigerant flow through the capillary tubes. It was possible to calculate the friction factors from these measurements. From the Colebrook equation and these calculated friction factors relative roughness was evaluated. Obtained values for the above listed diameters were 2:354 104 , 3:604 104 , and 2:193 104 respectively. Fig. 2 shows the calculated friction factors for one capillary tube as function of Reynolds number, as well as the e=dct value that best fits such data. Results show that Colebrook equation and Moody–Rouse diagram are suitable for Reynolds number range and diameters usually used in capillary tubes, as already stated by Mikol [12].
2.3. Experimental analysis Data were obtained for three nominal capillary tube diameters: 1.067, 1.372 and 1.626 mm. The capillary
Fig. 2. Calculated friction factors and evaluated e=dct as function of Reynolds number for CT-05.
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Table 1 Example of experimental data set for a given run––Run # 1606 Parameter
Point 1 Value
Temperature Pt100-1 Pt100-2 TT-01 TT-02 TT-03 TT-04 TT-05 TT-06 TT-07 TT-08 TT-09 TT-10 TT-11 TT-12 TT-13 TT-14 TT-15 TT-16 TT-17 TT-18 Pressure PT-01 PT-02 PT-03 PT-04 PT-05 PT-06 PT-07 PT-08 PT-09 PT-10 m_ exp CT-1 W_ HX WT-1
27.5 31.5 31.4 31.4 31.4 31.4 31.3 31.2 31.0 30.2 29.2 27.6 26.7 24.4 23.2 21.2 16.8 13.7 9.7 4.8 2166 2041 1956 1897 1860 1802 1708 1586 1313 703 25.03 n/a (subcooled inlet)
Point 2 2
r
Value
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2 3 3 2 2 2 3 2 3 2
Point 3 2
r
30.3 33.5 33.4 33.4 33.4 33.4 33.3 32.7 30.9 29.6 28.9 27.2 26.3 24.0 22.6 20.6 16.2 13.2 9.3 4.5
0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
2167 2051 1970 1917 1871 1798 1694 1565 1287 703
0.17
2 2 2 2 2 2 2 3 3 2
23.90
0.12
n/a (subcooled inlet)
Value 31.64 35.51 35.40 35.41 35.34 35.33 33.75 32.03 30.67 29.26 28.55 26.72 25.85 23.37 21.99 19.97 15.54 12.70 8.77 4.19 2166 2058 1985 1915 1860 1782 1673 1540 1261 701 22.93
r2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2 2 3 2 5 2 2 3 2 2 0.15
n/a (subcooled inlet)
Atmospheric pressure: 93.4 kPa; mixture composition: R-32, 49.2% and R-125, 50.8%.
Evaluation of heat losses in subcooling/quality control system was performed in order to determine how much of the electric power provided to the tape resistance is in fact used to raise refrigerant enthalpy. Similarly to the relative roughness, heat losses were evaluated by measurements of inlet and outlet temperatures, as well as mass flow rate and resistance electric power consumption of an all-liquid refrigerant flow through the heat exchanger. Heat losses were obtained as function of such parameters. Finally, it was evaluated the effect of instrumentation on capillary tube performance. Such instrumentation, particularly the pressure taps, may significantly reduce refrigerant mass flow rate through the capillary tube due to the introduction of an additional local pressure loss or the shift of flashing point, since the pressure taps may
act as active cavities for bubble nucleation. According to literature (cf. [13]), the reduction in mass flow rate may be up to 15%. Another factor that can affect capillary tube behaviour is thermocouples soldering on tube wall. This would locally change heat transfer, once thermocouples act as fins and may consequently create conditions for appearance of active cavities where flashing inception takes place. Such evaluation was performed by comparing a set of 54 experimental points, obtained for a given capillary tube without instrumentation, with a second data set obtained for the same operational conditions after the instrumentation were attached to the capillary tube. Such comparison showed a negligible effect of instrumentation on capillary tube performance (about 1.5% mass flow rate average reduction).
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
503
Table 2 Experimental results for R-410A––subcooled inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
pin
pout
DTsub
m_ exp
34
23.0 23.0 23.0
49.1%
50.9%
2086 2085 2086
793 798 797
5.2 3.6 1.0
23.68 22.87 21.35
20.3 20.5 20.4
49.2%
50.8%
2084 2083 2084
795 795 796
4.7 2.7 0.8
23.63 22.40 21.42
21.2 21.2 21.2
48.8%
51.2%
2085 2085 2085
795 796 796
5.3 3.5 1.5
23.19 22.25 20.74
22.3 22.7 22.8
49.2%
50.8%
2260 2260 2260
796 796 794
5.6 3.6 1.6
25.03 23.90 22.93
22.6 22.7 22.7
49.1%
50.9%
2259 2260 2262
794 793 796
5.5 3.8 1.7
24.91 24.03 22.99
21.5 21.4 21.3
48.8%
51.2%
2259 2259 2257
797 796 797
5.7 3.5 2.1
24.97 23.94 23.15
22.6 22.7 22.7
49.5%
50.5%
2435 2436 2436
796 796 796
5.4 3.5 1.4
25.87 25.05 23.86
18.3 18.6 18.8
49.3%
50.7%
2431 2431 2431
796 794 796
5.4 3.5 1.5
25.95 24.97 24.07
19.4 19.4 19.4
48.5%
51.5%
2433 2434 2431
796 796 797
5.6 3.3 1.4
26.07 24.97 24.05
22.7 22.8 22.6
48.9%
51.1%
2610 2610 2607
796 797 796
5.4 3.3 0.9
26.77 25.90 24.63
19.9 20.3 20.4
48.8%
51.2%
2609 2608 2609
796 797 795
5.3 3.0 1.0
26.85 25.66 24.60
21.3 21.4 21.4
48.8%
51.2%
2612 2610 2608
796 796 796
5.4 3.3 1.4
27.22 26.02 24.90
37
40
43
2.5. Experimental procedure At each run three experimental points were obtained. The first step was to fix inlet and outlet pressure, as well as the initial subcooling or quality, and then start the test/data acquisition. Once the first steady state operational condition was reached, the time and a data set were recorded. Then a new subcooling or quality value was set, and when a steady state operational condition was reached new time and data set were recorded. The same steps were performed to the third point. During the run it was carried out three to five chromatographic
analyses in order to evaluate refrigerant composition for a given run. Steady state verification criterion is the elapsed time since last parameters setting. It was observed during preliminary tests that it was necessary about 50 min to reach the first steady state condition and close to 20 min for subsequent ones. Thus an initial interval of 1 h and subsequent intervals of 30 min were adopted for reaching steady state conditions. Such criterion was adopted in parallel to operator’s monitorship during the tests. Later data analysis showed that such intervals were suitable to all performed tests.
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Table 3 Experimental results for R-410A––two-phase inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
pin
pout
xin
m_ exp
34
24.5 25.0 25.6
48.3%
51.7%
2083 2087 2084
798 795 794
0.005 0.022 0.037
20.02 19.51 19.11
25.8 25.9 26.0
48.1%
51.9%
2085 2084 2084
798 797 796
0.004 0.021 0.038
20.21 19.52 19.17
26.2 26.1 26.1
47.8%
52.2%
2084 2084 2084
798 796 796
0.003 0.014 0.037
20.62 19.89 19.16
21.5 21.6 21.6
47.4%
52.5%
2255 2257 2257
795 795 794
0.007 0.028 0.043
21.37 20.78 20.49
21.1 21.0 20.9
48.0%
52.0%
2255 2254 2255
757 803 796
0.006 0.026 0.050
21.41 20.77 20.25
20.5 20.7 20.8
48.1%
51.9%
2255 2258 2253
795 798 796
0.005 0.024 0.048
21.43 20.78 20.19
21.3 21.4 21.6
48.3%
51.7%
2432 2432 2431
796 798 796
0.004 0.020 0.044
22.81 22.28 21.78
21.5 21.4 21.4
48.2%
51.8%
2431 2431 2430
797 795 797
0.004 0.022 0.047
22.89 22.41 21.73
21.0 21.3 21.4
48.3%
51.7%
2431 2432 2431
795 796 797
0.003 0.014 0.040
22.70 22.47 21.79
21.6 21.4 21.2
48.5%
51.5%
2607 2606 2607
796 796 797
0.004 0.023 0.044
23.94 23.21 22.85
19.7 19.7 19.7
48.3%
51.7%
2606 2609 2606
795 795 795
0.004 0.022 0.047
23.69 23.21 22.53
19.8 19.8 19.8
48.3%
51.7%
2607 2606 2607
796 795 795
0.002 0.024 0.051
23.94 23.23 22.50
37
40
43
3. Results Once the experimental apparatus was mounted and adjusted, the data survey was carried out. For R-410A 24 runs were performed, totalling 72 experimental points for dct ¼ 1:101 mm and Lct ¼ 1:5 m. For R-407C, 38 runs were carried out, totalling 113 experimental points for several geometries and operational conditions. Complete data set for each point consists of the measured pressure and temperature profiles, mass flow rate and mixture composition, as well as subcooling/quality control system inlet and outlet temperatures and heater electric power consumption for tests with two-phase flow inlet conditions. Table 1 shows
a typical data set. Standard deviation values presented in such Table 1 are typical for most of the runs. Tables 2–7 present the obtained experimental results. It must be pointed out that for both mixtures used in this work it was fixed as inlet and outlet pressures the bubble pressure at condensation and evaporation temperature. Fig. 3 shows typical experimental temperature profiles for subcooled and two-phase inlet conditions. Pressure profiles are presented in Figs. 4 and 5 for both refrigerant mixtures. In these figures it is also shown the saturation pressure profile evaluated for the given temperature profile and refrigerant composition, assuming zero quality is also presented.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
505
Table 4 Experimental results for R-407C––subcooled inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
R-134a
pin
pout
DTsub
m_ exp
34
23.1 23.1 23.2
21.5%
23.7%
54.8%
1493 1495 1490
558 557 558
5.9 4.0 1.6
19.27 18.35 16.74
24.2 24.7 25.4
21.7%
23.9%
54.4%
1494 1494 1494
552 556 556
5.9 4.0 2.0
19.23 18.02 16.85
24.6 24.5 24.6
21.6%
23.8%
54.6%
1494 1493 1493
555 561 555
5.7 3.5 1.5
19.41 18.30 17.41
23.2 23.3 23.3
21.7%
23.9%
54.4%
1593 1601 1601
557 556 557
5.1 3.3 1.5
19.64 18.79 17.83
25.9 26.1 26.3
21.4%
23.6%
55.0%
1609 1602 1603
558 557 559
6.1 4.0 2.0
19.63 19.02 17.88
24.5 24.6 24.3
21.3%
23.7%
55.0%
1600 1600 1602
556 559 554
5.7 3.6 1.5
20.21 19.10 17.86
23.3 23.5 23.7
21.7%
23.9%
54.4%
1734 1733 1734
551 558 562
6.0 3.7 1.5
21.76 20.21 19.02
26.6 26.6 26.6
21.4%
23.6%
55.0%
1736 1735 1736
559 561 556
6.5 4.4 2.3
21.45 20.50 19.26
23.6 23.8 23.5
21.3%
23.7%
55.0%
1733 1734 1734
557 558 559
5.9 3.8 1.7
21.42 20.36 19.28
24.3 24.3 24.3
21.7%
23.9%
54.4%
1858 1857 1858
558 560 559
5.0 3.5 1.6
21.95 21.09 19.95
27.0 27.0 27.0
21.6%
23.8%
54.6%
1857 1857 1857
558 557 557
6.2 3.8 2.0
22.34 21.22 20.07
23.8 24.0 23.8
21.2%
23.7%
55.1%
1857 1856 1855
555 558 555
5.6 3.6 1.3
22.20 21.35 19.93
37
40
43
These figures show that the temperature, pressure and composition measurements are consistent, once there is a good agreement between measured and calculated pressure profiles for R-410A. On the other hand, for R-407C it can be seen in Fig. 5 that there is a detachment between the measured pressure profile and the calculated bubble pressure profile as the quality increases. It is also noticed that the measured pressure profile approaches the calculated dew pressure profile. Such behaviour is expected once a zeotropic mixture presents composition variation during phase change process, which leads to a saturation temperature glide (as well as a pressure glide) associated to this composition variation. In order to get agreement be-
tween measured and calculated profiles it is required the knowledge of a quality or composition profile along the capillary tube is required. It can also be seen in Figs. 4 and 5 the occurrence of the delay of vaporisation. Such phenomenon occurred for most of the subcooled inlet runs for both fluids. 3.1. Effect of inlet and outlet conditions Figs. 6–9 show the capillary tube performance as function of inlet parameters: condensing temperature, subcooling degree and quality. Such figures were obtained from average experimental mass flow rate values for a given inlet condition.
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Table 5 Experimental results for R-407C––two-phase inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
R-134a
pin
pout
xin
m_ exp
34
25.0 25.1 25.1
21.6
24.1
54.3
1493.4 1490.8 1493.1
561.6 555.4 558.4
0.1 °C 0.024 0.042
15.32 13.79 13.03
25.0 24.9 24.9
21.6
24.1
54.3
1492.9 1492.1 1493.3
558.4 554.1 556.5
0.4 °C 0.020 0.040
15.62 13.94 13.22
26.0 26.1 26.2
22.4
24.8
52.8
1494.3 1493.3 1491.0
558.0 556.8 555.4
0.009 0.029 0.053
14.90 13.72 12.85
24.2 24.4 24.6
21.3
24.0
54.7
1597.7 1601.1 1601.3
558.3 559.7 558.6
0.1 °C 0.019 0.040
15.90 15.00 14.20
24.7 24.8 25.0
21.3
24.0
54.7
1600.6 1600.0 1601.1
562.2 556.4 561.9
0.001 0.018 0.034
16.19 15.18 14.55
25.0 25.1 25.0
21.3
24.0
54.7
1600.5 1598.6 1601.2
558.8 557.6 559.0
0.001 0.019 0.035
16.18 15.05 14.54
24.6 24.6 24.4
21.4
23.9
54.7
1734.8 1734.3 1735.4
556.3 558.5 557.7
0.5 °C 0.016 0.040
17.74 16.64 15.83
24.2 24.0 23.8
21.4
23.9
54.7
1734.2 1736.9 1733.9
557.1 562.3 554.5
0.4 °C 0.015 0.039
17.95 16.85 15.87
23.8 23.9 23.9
21.4
23.9
54.7
1734.0 1734.3 1733.9
550.5 559.4 555.0
0.7 °C 0.019 0.030
17.86 16.52 16.09
24.3 24.5 24.6
21.9
24.3
53.8
1856.4 1855.9 1855.7
557.7 557.6 559.3
0.000 0.021 0.047
18.84 17.86 17.10
24.9 25.1 25.3
21.9
24.3
53.8
1856.2 1858.6 1856.3
556.7 556.3 558.6
0.000 0.025 0.046
18.63 17.83 17.19
25.4 25.4 25.6
21.9
24.3
53.8
1856.3 1855.7 1855.9
557.2 559.2 558.1
0.000 0.025 0.040
18.81 17.71 17.28
37
40
43
For subcooled inlet runs, Figs. 6 and 7, it is verified that capillary tube mass flow rates increase as condensing temperature and subcooling degree increases. Figs. 8 and 9 show that, for two-phase inlet conditions, mass flow rates increase as Tcd increases and the quality decreases. The effect of the outlet pressure is of minor importance on CT performance, as could be verified by an experimental run for R-410A in which a near 180% increase in outlet pressure (from 450 to 1300 kPa) for given inlet conditions reduced the mass flow rate by only 16% (from 21.3 to 18.0 kg/h).
3.2. Effect of geometry Fig. 10 presents the effect of diameter on capillary tube performance for several subcooling degrees. As expected, it is verified an increase in mass flow rate as diameter increases, due to the minor pressure drop imposed by bigger diameters. On the other hand Fig. 11 shows mass flow rate increases as capillary tube length diminishes. Similarly to diameter, a bigger capillary tube length means a bigger pressure drop and a smaller mass flow rate.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
507
Table 6 Experimental results for R-407C––(Lct ¼ 1:50 m and Tcd ¼ 37 °C) dct
Tamb
R-32
R-125
R-134a
pin
pout
DTsub
m_ exp
1.101
23.2 23.3 23.3
21.7%
23.9
54.4
1593 1601 1601
557 556 557
5.1 3.3 1.5
19.64 18.79 17.83
25.9 26.1 26.3
21.4
23.6
55.0
1609 1602 1603
558 557 559
5.9 4.0 2.0
19.63 19.02 17.88
24.5 24.6 24.3
21.3
23.7
55.0
1600 1600 1601
556 559 554
5.7 3.6 1.5
20.21 19.10 17.86
28.6 28.4 28.5
21.4
24.3
54.3
1602 1603 1602
559 558 558
6.0 4.5 2.4
37.55 36.09 33.75
28.0 28.3 28.5
21.4
24.5
54.1
1605 1603 1606
558 560 559
6.0 3.9 2.0
37.42 35.49 33.20
29.0 28.7 28.5
21.7
24.5
53.8
1603 1602 1602
558 558 558
5.9 4.1 1.9
37.60 35.79 33.71
27.6 27.4 27.4
21.7
24.5
53.8
1604 1602 1601
558 559 559
5.6 4.2 1.7
56.43 54.42 51.01
27.1 27.5 27.7
20.8
24.2
55.0
1604 1602 1603
558 559 559
6.1 4.2 2.2
56.39 53.33 50.51
28.5 28.6 28.6
20.7
23.9
55.4
1601 1604 1602
558 558 558
5.7 4.9 2.8
55.38 54.54 51.79
1.394
1.641
Table 7 Experimental results for R-407C––(dct ¼ 1:394 mm and Tcd ¼ 37 °C) R-32
R-134a
pin
pout
DTsub
m_ exp
24.3
54.3
1602 1603 1602
559 558 558
6.0 4.5 2.4
37.55 36.09 33.75
21.4
24.5
54.1
1605 1603 1606
558 560 559
6.0 3.9 2.0
37.42 35.49 33.20
29.0 28.7 28.5
21.7
24.5
53.8
1603 1602 1602
558 558 558
5.9 4.1 1.9
37.60 35.79 33.71
27.7 27.8 27.8
20.7
24.4
54.9
1603 1601 1602
556.7 557.3 556.7
5.9 4.6 2.7
40.48 39.29 37.38
24.5 24.4 24.3
21.1
24.4
54.5
1600 1600 1599
557.1 557.1 557.1
4.9 3.8 2.1
40.51 39.31 37.49
24.4 24.4 24.3
21.0
24.4
54.6
1601 1602 1600
557.3 555.9 557.8
5.7 3.9 2.4
41.08 36.63 37.47
21.9
25.1
53.0
1601 1600
557.2 558.3
5.6 44.64 3.3 41.77 (continued on next page)
Lct
Tamb
1.50
28.6 28.4 28.5
21.4
28.0 28.3 28.5
1.25
1.00
25.0 25.0
R-125
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Table 7 (continued) Lct
Tamb
R-32
R-125
R-134a
25.1
pin
pout
DTsub
m_ exp
1601
557.3
1.4
39.28
22.5 22.7 22.7
21.7
25.2
53.1
1600 1599 1594
556.3 557.1 555.1
5.4 3.0 0.8
44.63 41.99 39.51
22.8 22.7 23.0
21.3
24.2
54.5
1600 1602 1592
556.7 557.3 556.7
5.5 4.0 1.5
44.59 42.50 39.76
Fig. 3. Typical temperature profiles for subcooled and two-phase inlet conditions.
Fig. 4. Pressure profiles for R-410a––subcooled and two-phase inlet conditions.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
509
Fig. 5. Pressure profiles for R-407C––subcooled and two-phase inlet conditions.
Fig. 6. Effect of inlet conditions on CT performance for R410A––subcooled inlet.
3.3. Comparison of R-410A and R-407C flow through capillary tubes Figs. 12 and 13 present a comparison of experimental profiles for both refrigerants. In such comparison, pressure profiles are nondimensionalised by pdim ¼
Fig. 7. Effect of inlet conditions on CT performance for R407C––subcooled inlet.
pi =pin , where pi is the pressure at a given point and pin the inlet pressure. A common point for such figures is that both fluids present a similar ratio between inlet and outlet pressure. When it is considered the capillary tube pressure difference (Dpct ¼ pin pout ), however, values for R-410A are near 40% bigger than for R-407C. This
510
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Fig. 8. Effect of inlet conditions on capillary tube performance for R-410A––two-phase inlet.
Fig. 10. Effect of diameter on capillary tube performance for R-407C.
Fig. 9. Effect of inlet conditions on capillary tube performance for R-407C––two-phase inlet.
Fig. 11. Effect of length on capillary tube performance for R-407C.
should be one of the reasons why the R-410A mass flow rate is bigger. In fact it may be the most important reason.
Such figures show that for subcooled inlet conditions R-407C flow presents a bigger liquid region than R410A one, and that in beginning of two-phase region pressure drop is smaller for R-407C than for R-410A. A possible explanation for the first aspect is the bigger
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
511
ferent fluid properties, which lead to a bigger pressure drop as quality grows for R-407C.
4. Conclusion This work presents the results of an extensive experimental survey on R-407C and R-410A flow through capillary tubes. Such survey, which was carried out for both subcooled and two-phase inlet conditions, characterised the influence of these refrigerants, as well as the several operating and geometric parameters on the behaviour of capillary tubes used in refrigeration systems. It could be seen that the main operational parameters (inlet and outlet conditions) affects in a similar way the performance of capillary tubes for both refrigerants. The study characterised the influence of geometry (diameter and length) on the behaviour of capillary tubes used in small refrigeration systems, and analysed the differences in R-410A and R-407C flow through capillary tubes. Fig. 12. Comparison of temperature profiles for R-410A and R407C––subcooled inlet.
Acknowledgements The authors would like to acknowledge FAPESP (Research Support Foundation of the State of S~ ao Paulo, Brazil) for financial support to this study.
References
Fig. 13. Comparison of dimensionless pressure profiles for R-410A and R- 407C––two-phase inlet.
mass flow rate of R-410A, which causes the fluid to reach the saturation condition earlier. The behaviour differences in two-phase region may be related to dif-
[1] R.R. Bittle, M.B. Pate, A theoretical model for predicting adiabatic capillary tube performance with alternative refrigerants, ASHRAE Trans. 102 (2) (1996) 52–64. [2] S.D. Chang, T. Ro, Pressure drop of pure HFC refrigerants and their mixtures flowing in capillary tubes, Int. J. Multiphase Flow 22 (3) (1996) 551–561. [3] R.R. Bittle et al., A generalized performance prediction method for adiabatic capillary tubes, HVAC R Res. 4 (1) (1998) 27–43. [4] S.M. Sami et al., Modelling of capillary tubes behaviour with HCFC 22 ternary alternative refrigerants, Int. J. Energy Res. 22 (1998) 843–855. [5] S.M. Sami, C. Tribes, Numerical prediction of capillary tube behaviour with pure and binary alternative refrigerants, Appl. Therm. Engng. 18 (6) (1998) 491–502. [6] F.A.S. Fiorelli et al., Numerical study on refrigerant mixtures flow in capillary tubes, in: Proc. 1998 Int. Refrigeration Conf. Purdue, 1998, pp. 413–418. [7] F.A.S. Fiorelli et al., Analysis of R-410A and R-407C flow through capillary tubes using a separated flow model, in: Proc. 20th Int. Congr. Refrigeration, 1999, Cd-rom. [8] C.Z. Wei et al., An experimental study of the performance of capillary tubes for R-407C refrigerant, ASHRAE Trans. 105 (2) (1999) 634–638. [9] D. Jung et al., Capillary tube selection for HCFC 22 alternatives, Int. J. Refrig. 22 (1999) 604–614. [10] S.Y. Motta et al., Critical flow of refrigerants through adiabatic capillary tubes: experimental study of zeotropic mixtures R-407C and R-404A, ASHRAE Trans. 106 (1) (2000) 534–549.
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[11] M.A.S. Paiva et al., Experimental and numerical study of the flow through non adiabatic capillary tubes with lateral and concentric capillary tube-suction line heat exchanger configuration, in: Proc. 19th Int. Congr. Refrigeration, 1995.
[12] E.P. Mikol, Adiabatic single and two-phase flow in small bore tubes, ASHRAE J. 11 (5) (1963) 75–86. [13] J.J. Meyer, W.E. Dunn, New insights into the behavior of the metastable region of an operating capillary tube, HVAC R Res. 4 (1) (1998) 105–115.
Experimental analysis of refrigerant mixtures flow through adiabatic capillary tubes Flavio Augusto Sanzovo Fiorelli *, Alex Alberto Silva Huerta, Ot avio de Mattos Silvares Department of Mechanical Engineering, University of S~ ao Paulo, Av. Prof. Mello Moraes, 2231, Cidade Universit aria, 05508-900 S~ ao Paulo, Brazil Received 12 October 2001; received in revised form 12 January 2002; accepted 18 February 2002
Abstract This paper presents an experimental study on HCFC 22 alternative refrigerant mixtures flow through capillary tubes. Results for R-410A and R-407C show that the main operational parameters affect in a similar way the performance of capillary tubes for both refrigerants. The influence of geometry on the bahaviour of capillary tubes in refrigeration systems is characterised. An analysis on the differences in R-410A and R-407C flow through capillary tubes is also performed. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Refrigeration; Air conditioning; Capillary tubes; Experimental analysis; HCFC replacement
1. Introduction Several evidences connecting halogenated compounds to stratospheric ozone layer depletion was collected in the last decades. Such evidences led to the signature of the Montreal Protocol, which main goal is the elimination of such halogenated compounds. One of such substances is the HCFC 22, largely used as refrigerant in equipment for commercial refrigeration, commercial and household air conditioners and heat pumps. Unfortunately up today research indicates that there is no pure substance that could be used as alternative for HCFC 22 small size units without the need of great modifications in existing equipment. The use of zeotropic or near-azeotropic refrigerant mixtures is the most suitable alternative so far. Among the possible alternatives manufacturers are using mainly the R-407C, a zeotropic mixture of HFC 32, HFC 125 and HFC 134a (23%=25%=52% on mass basis) and the R-410A, a near-azeotropic mixture of HFC 32 and HFC 125
*
Corresponding author. Tel.: +55-11-3091-5570x207; fax: +55-113813-1886. E-mail addresses: fi[email protected] (F. Augusto Sanzovo Fiorelli), [email protected] (A. Alberto Silva Huerta), [email protected] (O. de Mattos Silvares).
(50%=50% on mass basis), and the HFC 134a for specific operational ranges. The use of such mixtures demands new experimental and numerical studies in order to evaluate how they affect the performance of refrigeration cycles as well as the design of cycle component. In this way the sizing of adiabatic capillary tubes using zeotropic mixtures is a subject of particular interest for small size air conditioning and heat pump units. Some recent works related to capillary tubes and refrigerant mixtures can be pointed out [1–10]. Nevertheless, these papers present few experimental data, except Motta et al. [10], even though they only present data for subcooled inlet conditions. Therefore, in order to provide a data base (including two-phase inlet conditions) that could be used for obtaining a practical correlation for sizing capillary tubes as well as for experimental validation of capillary tube simulation models/programs under development, this paper presents the results of an extensive experimental survey on R-407C and R-410A flow through capillary tubes. Such survey, which was carried out for both subcooled and two-phase inlet conditions, also characterised the influence of these refrigerants, as well as that of several operating and geometric parameters on the behaviour of capillary tubes used in refrigeration systems.
0894-1777/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 0 8 9 4 - 1 7 7 7 ( 0 2 ) 0 0 1 5 8 - 9
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Nomenclature d L m_ p T W_ e r2
diameter, mm length, m mass flow rate, kg/h pressure, kPa temperature, °C power consumption, w roughness, mm standard deviation, dimensionless
Subscripts cd condenser, condensation
2. Experimental apparatus Fig. 1 presents a schematic of the experimental apparatus implemented for this survey, in which is used a blow-down process. According to Paiva et al. [11], the main advantage of a blow-down unit, when compared to a conventional refrigeration cycle, is to allow an accurate and individualised control of each operational parameter. Its major disadvantage is the requirement of a high refrigerant charge in order to get suitable run times. Besides, it will occur a slight composition variation from test to test, particularly for zeotropic mixtures. Selection of a blow-down process to this study was motivated by authors’ belief that an accurate control of operational parameters is more important than prob-
ct dim exp ev HX i in out sub theo
capillary tube dimensionless experimental evaporator, evaporation heater generic step inlet outlet subcooling theoretical
lems connected with a high refrigerant charge. Concerning to composition variation, as explained later, it was monitored during the tests and the refrigerant composition was within the manufacturer admissible composition range for most of the cases. 2.1. Operation Refrigerant is initially stored upstream the test section in a high-pressure reservoir (two bladder accumulators of 100 l each). Such high pressure is provided by nitrogen filling of the bladders. Test section exit is connected to a low-pressure reservoir. Low pressure is obtained by refrigerant condensation provided by chilled ethylene-glycol/water mixture flowing through a coil inside such reservoir.
Fig. 1. Experimental apparatus schematic.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
At each run, by pressure difference, the refrigerant flows from the high-pressure to the low-pressure reservoir through the test section where the capillary tube is placed. At the end of a run, the refrigerant returns to highpressure reservoir also by pressure difference. Pressure is raised in the low-pressure reservoir by means of two heaters, as long as pressure is lowered in the high-pressure reservoir by releasing nitrogen from the bladders. 2.2. Instrumentation and control Parameters measured in test section are the pressure and temperature profiles along the capillary tube, mass flow rate, electric power consumption of subcooling/ quality final control heater, and mixture composition at capillary tube inlet. Ten strain-gage type pressure transducers (0.1% range value uncertainty) measure pressure profile along the capillary tube. Temperature profile is obtained by 18 T-type thermocouples soldered to the tube wall (0.3 °C uncertainty). It is also used two Pt-100 thermoresistances at capillary tube inlet and subcooling/ quality control system inlet (0.2 °C uncertainty). Mass flow rate measurement is performed by a Coriolis-type flowmeter (0.1% range value uncertainty). Electric power consumption of subcooling/quality final control heater is obtained by a wattmeter (1% reading uncertainty). This power consumption is used to evaluate, by energy balance, the quality for two-phase capillary tube inlet conditions. Due to the unit concept, it is necessary to monitor the mixture composition (in mass percentage) in each run. In this sense it is used a gas chromatograph with 1% uncertainty. The parameters to be controlled in each run are the capillary tube inlet and outlet pressures, and inlet subcooling or quality. Inlet pressure is controlled by a regulator (pre-control) and a PID control/solenoid valve system (final control) on nitrogen line. Outlet pressure is controlled by a PID control/solenoid valve system on ethylene-glycol line. By controlling the glycol flow rate, it is possible to control condenser pressure and, as a result, the capillary tube outlet pressure. For subcooling/quality control a two-step heating process is used. Refrigerant is pre-heated in a coil immersed in a hot water constant-temperature bath. Final adjustment of subcooling or quality is obtained by a 5.0 W/m tape heater and a PID controller (subcooling) or manual control (quality).
501
tubes length was 1.5 m. For dct ¼ 1:372 mm data were also obtained for two other lengths: 1.0 and 1.25 m. All capillary tubes were straight and well insulated. 2.4. Preliminary results First step in obtaining the experimental data was the characterisation of some experimental parameters: actual capillary tube diameters and relative roughness, as well as the heat losses in subcooling/quality control system. Actual capillary tube internal diameter was measured by filling a capillary tube sample with mercury. This procedure provides the average diameter with 1.0% uncertainty. Actual capillary tube diameters obtained were 1.101, 1.394 and 1.641 mm. Relative roughness was evaluated by measuring the pressure losses of an all-liquid R-410A refrigerant flow through the capillary tubes. It was possible to calculate the friction factors from these measurements. From the Colebrook equation and these calculated friction factors relative roughness was evaluated. Obtained values for the above listed diameters were 2:354 104 , 3:604 104 , and 2:193 104 respectively. Fig. 2 shows the calculated friction factors for one capillary tube as function of Reynolds number, as well as the e=dct value that best fits such data. Results show that Colebrook equation and Moody–Rouse diagram are suitable for Reynolds number range and diameters usually used in capillary tubes, as already stated by Mikol [12].
2.3. Experimental analysis Data were obtained for three nominal capillary tube diameters: 1.067, 1.372 and 1.626 mm. The capillary
Fig. 2. Calculated friction factors and evaluated e=dct as function of Reynolds number for CT-05.
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Table 1 Example of experimental data set for a given run––Run # 1606 Parameter
Point 1 Value
Temperature Pt100-1 Pt100-2 TT-01 TT-02 TT-03 TT-04 TT-05 TT-06 TT-07 TT-08 TT-09 TT-10 TT-11 TT-12 TT-13 TT-14 TT-15 TT-16 TT-17 TT-18 Pressure PT-01 PT-02 PT-03 PT-04 PT-05 PT-06 PT-07 PT-08 PT-09 PT-10 m_ exp CT-1 W_ HX WT-1
27.5 31.5 31.4 31.4 31.4 31.4 31.3 31.2 31.0 30.2 29.2 27.6 26.7 24.4 23.2 21.2 16.8 13.7 9.7 4.8 2166 2041 1956 1897 1860 1802 1708 1586 1313 703 25.03 n/a (subcooled inlet)
Point 2 2
r
Value
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2 3 3 2 2 2 3 2 3 2
Point 3 2
r
30.3 33.5 33.4 33.4 33.4 33.4 33.3 32.7 30.9 29.6 28.9 27.2 26.3 24.0 22.6 20.6 16.2 13.2 9.3 4.5
0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
2167 2051 1970 1917 1871 1798 1694 1565 1287 703
0.17
2 2 2 2 2 2 2 3 3 2
23.90
0.12
n/a (subcooled inlet)
Value 31.64 35.51 35.40 35.41 35.34 35.33 33.75 32.03 30.67 29.26 28.55 26.72 25.85 23.37 21.99 19.97 15.54 12.70 8.77 4.19 2166 2058 1985 1915 1860 1782 1673 1540 1261 701 22.93
r2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2 2 3 2 5 2 2 3 2 2 0.15
n/a (subcooled inlet)
Atmospheric pressure: 93.4 kPa; mixture composition: R-32, 49.2% and R-125, 50.8%.
Evaluation of heat losses in subcooling/quality control system was performed in order to determine how much of the electric power provided to the tape resistance is in fact used to raise refrigerant enthalpy. Similarly to the relative roughness, heat losses were evaluated by measurements of inlet and outlet temperatures, as well as mass flow rate and resistance electric power consumption of an all-liquid refrigerant flow through the heat exchanger. Heat losses were obtained as function of such parameters. Finally, it was evaluated the effect of instrumentation on capillary tube performance. Such instrumentation, particularly the pressure taps, may significantly reduce refrigerant mass flow rate through the capillary tube due to the introduction of an additional local pressure loss or the shift of flashing point, since the pressure taps may
act as active cavities for bubble nucleation. According to literature (cf. [13]), the reduction in mass flow rate may be up to 15%. Another factor that can affect capillary tube behaviour is thermocouples soldering on tube wall. This would locally change heat transfer, once thermocouples act as fins and may consequently create conditions for appearance of active cavities where flashing inception takes place. Such evaluation was performed by comparing a set of 54 experimental points, obtained for a given capillary tube without instrumentation, with a second data set obtained for the same operational conditions after the instrumentation were attached to the capillary tube. Such comparison showed a negligible effect of instrumentation on capillary tube performance (about 1.5% mass flow rate average reduction).
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
503
Table 2 Experimental results for R-410A––subcooled inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
pin
pout
DTsub
m_ exp
34
23.0 23.0 23.0
49.1%
50.9%
2086 2085 2086
793 798 797
5.2 3.6 1.0
23.68 22.87 21.35
20.3 20.5 20.4
49.2%
50.8%
2084 2083 2084
795 795 796
4.7 2.7 0.8
23.63 22.40 21.42
21.2 21.2 21.2
48.8%
51.2%
2085 2085 2085
795 796 796
5.3 3.5 1.5
23.19 22.25 20.74
22.3 22.7 22.8
49.2%
50.8%
2260 2260 2260
796 796 794
5.6 3.6 1.6
25.03 23.90 22.93
22.6 22.7 22.7
49.1%
50.9%
2259 2260 2262
794 793 796
5.5 3.8 1.7
24.91 24.03 22.99
21.5 21.4 21.3
48.8%
51.2%
2259 2259 2257
797 796 797
5.7 3.5 2.1
24.97 23.94 23.15
22.6 22.7 22.7
49.5%
50.5%
2435 2436 2436
796 796 796
5.4 3.5 1.4
25.87 25.05 23.86
18.3 18.6 18.8
49.3%
50.7%
2431 2431 2431
796 794 796
5.4 3.5 1.5
25.95 24.97 24.07
19.4 19.4 19.4
48.5%
51.5%
2433 2434 2431
796 796 797
5.6 3.3 1.4
26.07 24.97 24.05
22.7 22.8 22.6
48.9%
51.1%
2610 2610 2607
796 797 796
5.4 3.3 0.9
26.77 25.90 24.63
19.9 20.3 20.4
48.8%
51.2%
2609 2608 2609
796 797 795
5.3 3.0 1.0
26.85 25.66 24.60
21.3 21.4 21.4
48.8%
51.2%
2612 2610 2608
796 796 796
5.4 3.3 1.4
27.22 26.02 24.90
37
40
43
2.5. Experimental procedure At each run three experimental points were obtained. The first step was to fix inlet and outlet pressure, as well as the initial subcooling or quality, and then start the test/data acquisition. Once the first steady state operational condition was reached, the time and a data set were recorded. Then a new subcooling or quality value was set, and when a steady state operational condition was reached new time and data set were recorded. The same steps were performed to the third point. During the run it was carried out three to five chromatographic
analyses in order to evaluate refrigerant composition for a given run. Steady state verification criterion is the elapsed time since last parameters setting. It was observed during preliminary tests that it was necessary about 50 min to reach the first steady state condition and close to 20 min for subsequent ones. Thus an initial interval of 1 h and subsequent intervals of 30 min were adopted for reaching steady state conditions. Such criterion was adopted in parallel to operator’s monitorship during the tests. Later data analysis showed that such intervals were suitable to all performed tests.
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Table 3 Experimental results for R-410A––two-phase inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
pin
pout
xin
m_ exp
34
24.5 25.0 25.6
48.3%
51.7%
2083 2087 2084
798 795 794
0.005 0.022 0.037
20.02 19.51 19.11
25.8 25.9 26.0
48.1%
51.9%
2085 2084 2084
798 797 796
0.004 0.021 0.038
20.21 19.52 19.17
26.2 26.1 26.1
47.8%
52.2%
2084 2084 2084
798 796 796
0.003 0.014 0.037
20.62 19.89 19.16
21.5 21.6 21.6
47.4%
52.5%
2255 2257 2257
795 795 794
0.007 0.028 0.043
21.37 20.78 20.49
21.1 21.0 20.9
48.0%
52.0%
2255 2254 2255
757 803 796
0.006 0.026 0.050
21.41 20.77 20.25
20.5 20.7 20.8
48.1%
51.9%
2255 2258 2253
795 798 796
0.005 0.024 0.048
21.43 20.78 20.19
21.3 21.4 21.6
48.3%
51.7%
2432 2432 2431
796 798 796
0.004 0.020 0.044
22.81 22.28 21.78
21.5 21.4 21.4
48.2%
51.8%
2431 2431 2430
797 795 797
0.004 0.022 0.047
22.89 22.41 21.73
21.0 21.3 21.4
48.3%
51.7%
2431 2432 2431
795 796 797
0.003 0.014 0.040
22.70 22.47 21.79
21.6 21.4 21.2
48.5%
51.5%
2607 2606 2607
796 796 797
0.004 0.023 0.044
23.94 23.21 22.85
19.7 19.7 19.7
48.3%
51.7%
2606 2609 2606
795 795 795
0.004 0.022 0.047
23.69 23.21 22.53
19.8 19.8 19.8
48.3%
51.7%
2607 2606 2607
796 795 795
0.002 0.024 0.051
23.94 23.23 22.50
37
40
43
3. Results Once the experimental apparatus was mounted and adjusted, the data survey was carried out. For R-410A 24 runs were performed, totalling 72 experimental points for dct ¼ 1:101 mm and Lct ¼ 1:5 m. For R-407C, 38 runs were carried out, totalling 113 experimental points for several geometries and operational conditions. Complete data set for each point consists of the measured pressure and temperature profiles, mass flow rate and mixture composition, as well as subcooling/quality control system inlet and outlet temperatures and heater electric power consumption for tests with two-phase flow inlet conditions. Table 1 shows
a typical data set. Standard deviation values presented in such Table 1 are typical for most of the runs. Tables 2–7 present the obtained experimental results. It must be pointed out that for both mixtures used in this work it was fixed as inlet and outlet pressures the bubble pressure at condensation and evaporation temperature. Fig. 3 shows typical experimental temperature profiles for subcooled and two-phase inlet conditions. Pressure profiles are presented in Figs. 4 and 5 for both refrigerant mixtures. In these figures it is also shown the saturation pressure profile evaluated for the given temperature profile and refrigerant composition, assuming zero quality is also presented.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
505
Table 4 Experimental results for R-407C––subcooled inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
R-134a
pin
pout
DTsub
m_ exp
34
23.1 23.1 23.2
21.5%
23.7%
54.8%
1493 1495 1490
558 557 558
5.9 4.0 1.6
19.27 18.35 16.74
24.2 24.7 25.4
21.7%
23.9%
54.4%
1494 1494 1494
552 556 556
5.9 4.0 2.0
19.23 18.02 16.85
24.6 24.5 24.6
21.6%
23.8%
54.6%
1494 1493 1493
555 561 555
5.7 3.5 1.5
19.41 18.30 17.41
23.2 23.3 23.3
21.7%
23.9%
54.4%
1593 1601 1601
557 556 557
5.1 3.3 1.5
19.64 18.79 17.83
25.9 26.1 26.3
21.4%
23.6%
55.0%
1609 1602 1603
558 557 559
6.1 4.0 2.0
19.63 19.02 17.88
24.5 24.6 24.3
21.3%
23.7%
55.0%
1600 1600 1602
556 559 554
5.7 3.6 1.5
20.21 19.10 17.86
23.3 23.5 23.7
21.7%
23.9%
54.4%
1734 1733 1734
551 558 562
6.0 3.7 1.5
21.76 20.21 19.02
26.6 26.6 26.6
21.4%
23.6%
55.0%
1736 1735 1736
559 561 556
6.5 4.4 2.3
21.45 20.50 19.26
23.6 23.8 23.5
21.3%
23.7%
55.0%
1733 1734 1734
557 558 559
5.9 3.8 1.7
21.42 20.36 19.28
24.3 24.3 24.3
21.7%
23.9%
54.4%
1858 1857 1858
558 560 559
5.0 3.5 1.6
21.95 21.09 19.95
27.0 27.0 27.0
21.6%
23.8%
54.6%
1857 1857 1857
558 557 557
6.2 3.8 2.0
22.34 21.22 20.07
23.8 24.0 23.8
21.2%
23.7%
55.1%
1857 1856 1855
555 558 555
5.6 3.6 1.3
22.20 21.35 19.93
37
40
43
These figures show that the temperature, pressure and composition measurements are consistent, once there is a good agreement between measured and calculated pressure profiles for R-410A. On the other hand, for R-407C it can be seen in Fig. 5 that there is a detachment between the measured pressure profile and the calculated bubble pressure profile as the quality increases. It is also noticed that the measured pressure profile approaches the calculated dew pressure profile. Such behaviour is expected once a zeotropic mixture presents composition variation during phase change process, which leads to a saturation temperature glide (as well as a pressure glide) associated to this composition variation. In order to get agreement be-
tween measured and calculated profiles it is required the knowledge of a quality or composition profile along the capillary tube is required. It can also be seen in Figs. 4 and 5 the occurrence of the delay of vaporisation. Such phenomenon occurred for most of the subcooled inlet runs for both fluids. 3.1. Effect of inlet and outlet conditions Figs. 6–9 show the capillary tube performance as function of inlet parameters: condensing temperature, subcooling degree and quality. Such figures were obtained from average experimental mass flow rate values for a given inlet condition.
506
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Table 5 Experimental results for R-407C––two-phase inlet (dct ¼ 1:101 mm and Lct ¼ 1:5 m) Tcd;theo
Tamb
R-32
R-125
R-134a
pin
pout
xin
m_ exp
34
25.0 25.1 25.1
21.6
24.1
54.3
1493.4 1490.8 1493.1
561.6 555.4 558.4
0.1 °C 0.024 0.042
15.32 13.79 13.03
25.0 24.9 24.9
21.6
24.1
54.3
1492.9 1492.1 1493.3
558.4 554.1 556.5
0.4 °C 0.020 0.040
15.62 13.94 13.22
26.0 26.1 26.2
22.4
24.8
52.8
1494.3 1493.3 1491.0
558.0 556.8 555.4
0.009 0.029 0.053
14.90 13.72 12.85
24.2 24.4 24.6
21.3
24.0
54.7
1597.7 1601.1 1601.3
558.3 559.7 558.6
0.1 °C 0.019 0.040
15.90 15.00 14.20
24.7 24.8 25.0
21.3
24.0
54.7
1600.6 1600.0 1601.1
562.2 556.4 561.9
0.001 0.018 0.034
16.19 15.18 14.55
25.0 25.1 25.0
21.3
24.0
54.7
1600.5 1598.6 1601.2
558.8 557.6 559.0
0.001 0.019 0.035
16.18 15.05 14.54
24.6 24.6 24.4
21.4
23.9
54.7
1734.8 1734.3 1735.4
556.3 558.5 557.7
0.5 °C 0.016 0.040
17.74 16.64 15.83
24.2 24.0 23.8
21.4
23.9
54.7
1734.2 1736.9 1733.9
557.1 562.3 554.5
0.4 °C 0.015 0.039
17.95 16.85 15.87
23.8 23.9 23.9
21.4
23.9
54.7
1734.0 1734.3 1733.9
550.5 559.4 555.0
0.7 °C 0.019 0.030
17.86 16.52 16.09
24.3 24.5 24.6
21.9
24.3
53.8
1856.4 1855.9 1855.7
557.7 557.6 559.3
0.000 0.021 0.047
18.84 17.86 17.10
24.9 25.1 25.3
21.9
24.3
53.8
1856.2 1858.6 1856.3
556.7 556.3 558.6
0.000 0.025 0.046
18.63 17.83 17.19
25.4 25.4 25.6
21.9
24.3
53.8
1856.3 1855.7 1855.9
557.2 559.2 558.1
0.000 0.025 0.040
18.81 17.71 17.28
37
40
43
For subcooled inlet runs, Figs. 6 and 7, it is verified that capillary tube mass flow rates increase as condensing temperature and subcooling degree increases. Figs. 8 and 9 show that, for two-phase inlet conditions, mass flow rates increase as Tcd increases and the quality decreases. The effect of the outlet pressure is of minor importance on CT performance, as could be verified by an experimental run for R-410A in which a near 180% increase in outlet pressure (from 450 to 1300 kPa) for given inlet conditions reduced the mass flow rate by only 16% (from 21.3 to 18.0 kg/h).
3.2. Effect of geometry Fig. 10 presents the effect of diameter on capillary tube performance for several subcooling degrees. As expected, it is verified an increase in mass flow rate as diameter increases, due to the minor pressure drop imposed by bigger diameters. On the other hand Fig. 11 shows mass flow rate increases as capillary tube length diminishes. Similarly to diameter, a bigger capillary tube length means a bigger pressure drop and a smaller mass flow rate.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
507
Table 6 Experimental results for R-407C––(Lct ¼ 1:50 m and Tcd ¼ 37 °C) dct
Tamb
R-32
R-125
R-134a
pin
pout
DTsub
m_ exp
1.101
23.2 23.3 23.3
21.7%
23.9
54.4
1593 1601 1601
557 556 557
5.1 3.3 1.5
19.64 18.79 17.83
25.9 26.1 26.3
21.4
23.6
55.0
1609 1602 1603
558 557 559
5.9 4.0 2.0
19.63 19.02 17.88
24.5 24.6 24.3
21.3
23.7
55.0
1600 1600 1601
556 559 554
5.7 3.6 1.5
20.21 19.10 17.86
28.6 28.4 28.5
21.4
24.3
54.3
1602 1603 1602
559 558 558
6.0 4.5 2.4
37.55 36.09 33.75
28.0 28.3 28.5
21.4
24.5
54.1
1605 1603 1606
558 560 559
6.0 3.9 2.0
37.42 35.49 33.20
29.0 28.7 28.5
21.7
24.5
53.8
1603 1602 1602
558 558 558
5.9 4.1 1.9
37.60 35.79 33.71
27.6 27.4 27.4
21.7
24.5
53.8
1604 1602 1601
558 559 559
5.6 4.2 1.7
56.43 54.42 51.01
27.1 27.5 27.7
20.8
24.2
55.0
1604 1602 1603
558 559 559
6.1 4.2 2.2
56.39 53.33 50.51
28.5 28.6 28.6
20.7
23.9
55.4
1601 1604 1602
558 558 558
5.7 4.9 2.8
55.38 54.54 51.79
1.394
1.641
Table 7 Experimental results for R-407C––(dct ¼ 1:394 mm and Tcd ¼ 37 °C) R-32
R-134a
pin
pout
DTsub
m_ exp
24.3
54.3
1602 1603 1602
559 558 558
6.0 4.5 2.4
37.55 36.09 33.75
21.4
24.5
54.1
1605 1603 1606
558 560 559
6.0 3.9 2.0
37.42 35.49 33.20
29.0 28.7 28.5
21.7
24.5
53.8
1603 1602 1602
558 558 558
5.9 4.1 1.9
37.60 35.79 33.71
27.7 27.8 27.8
20.7
24.4
54.9
1603 1601 1602
556.7 557.3 556.7
5.9 4.6 2.7
40.48 39.29 37.38
24.5 24.4 24.3
21.1
24.4
54.5
1600 1600 1599
557.1 557.1 557.1
4.9 3.8 2.1
40.51 39.31 37.49
24.4 24.4 24.3
21.0
24.4
54.6
1601 1602 1600
557.3 555.9 557.8
5.7 3.9 2.4
41.08 36.63 37.47
21.9
25.1
53.0
1601 1600
557.2 558.3
5.6 44.64 3.3 41.77 (continued on next page)
Lct
Tamb
1.50
28.6 28.4 28.5
21.4
28.0 28.3 28.5
1.25
1.00
25.0 25.0
R-125
508
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Table 7 (continued) Lct
Tamb
R-32
R-125
R-134a
25.1
pin
pout
DTsub
m_ exp
1601
557.3
1.4
39.28
22.5 22.7 22.7
21.7
25.2
53.1
1600 1599 1594
556.3 557.1 555.1
5.4 3.0 0.8
44.63 41.99 39.51
22.8 22.7 23.0
21.3
24.2
54.5
1600 1602 1592
556.7 557.3 556.7
5.5 4.0 1.5
44.59 42.50 39.76
Fig. 3. Typical temperature profiles for subcooled and two-phase inlet conditions.
Fig. 4. Pressure profiles for R-410a––subcooled and two-phase inlet conditions.
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
509
Fig. 5. Pressure profiles for R-407C––subcooled and two-phase inlet conditions.
Fig. 6. Effect of inlet conditions on CT performance for R410A––subcooled inlet.
3.3. Comparison of R-410A and R-407C flow through capillary tubes Figs. 12 and 13 present a comparison of experimental profiles for both refrigerants. In such comparison, pressure profiles are nondimensionalised by pdim ¼
Fig. 7. Effect of inlet conditions on CT performance for R407C––subcooled inlet.
pi =pin , where pi is the pressure at a given point and pin the inlet pressure. A common point for such figures is that both fluids present a similar ratio between inlet and outlet pressure. When it is considered the capillary tube pressure difference (Dpct ¼ pin pout ), however, values for R-410A are near 40% bigger than for R-407C. This
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F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
Fig. 8. Effect of inlet conditions on capillary tube performance for R-410A––two-phase inlet.
Fig. 10. Effect of diameter on capillary tube performance for R-407C.
Fig. 9. Effect of inlet conditions on capillary tube performance for R-407C––two-phase inlet.
Fig. 11. Effect of length on capillary tube performance for R-407C.
should be one of the reasons why the R-410A mass flow rate is bigger. In fact it may be the most important reason.
Such figures show that for subcooled inlet conditions R-407C flow presents a bigger liquid region than R410A one, and that in beginning of two-phase region pressure drop is smaller for R-407C than for R-410A. A possible explanation for the first aspect is the bigger
F.A.S. Fiorelli et al. / Experimental Thermal and Fluid Science 26 (2002) 499–512
511
ferent fluid properties, which lead to a bigger pressure drop as quality grows for R-407C.
4. Conclusion This work presents the results of an extensive experimental survey on R-407C and R-410A flow through capillary tubes. Such survey, which was carried out for both subcooled and two-phase inlet conditions, characterised the influence of these refrigerants, as well as the several operating and geometric parameters on the behaviour of capillary tubes used in refrigeration systems. It could be seen that the main operational parameters (inlet and outlet conditions) affects in a similar way the performance of capillary tubes for both refrigerants. The study characterised the influence of geometry (diameter and length) on the behaviour of capillary tubes used in small refrigeration systems, and analysed the differences in R-410A and R-407C flow through capillary tubes. Fig. 12. Comparison of temperature profiles for R-410A and R407C––subcooled inlet.
Acknowledgements The authors would like to acknowledge FAPESP (Research Support Foundation of the State of S~ ao Paulo, Brazil) for financial support to this study.
References
Fig. 13. Comparison of dimensionless pressure profiles for R-410A and R- 407C––two-phase inlet.
mass flow rate of R-410A, which causes the fluid to reach the saturation condition earlier. The behaviour differences in two-phase region may be related to dif-
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