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Nucleate boiling heat transfer coefficients of flammable refrigerants
International Journal of Refrigeration 27 (2004) 409–414 www.elsevier.com/locate/ijrefrig
Nucleate boiling heat transfer coefficients of flammable refrigerants Dongsoo Junga,*, Heungseok Leeb, Dongsoo Baeb, Sukjae Ohoc a
Department of Mechanical Engineering, Inha University, Incheon 402-751, South Korea b Inha University, Incheon, 402-751, South Korea c TechnoChem Co., Ltd., Seotan-Myun, Pyungtaek-Si, Kyunggi-Do, South Korea
Received 27 January 2003; received in revised form 28 November 2003; accepted 28 November 2003
Abstract Nucleate boiling heat transfer coefficients (HTCs) of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m2 to 10 kW m2 with an interval of 10 kW m2 in the pool temperature of 7 C. Test results exhibited a typical trend that HTCs of flammable refrigerants increase with increasing vapor pressure. Existing nucleate boiling heat transfer correlations showed up to 80% deviation as compared to the present data. Hence a new correlation was developed through a regression analysis taking into account dimensionless variables affecting nucleate boiling heat transfer. The new correlation showed a good agreement with data for flammable refrigerants as well as halogenated refrigerants with a deviation of 5.3%. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Refrigerant; Flammable; Heat transfer; Nucleate boiling; Propylene; Propane; Isobutane; Butane; Dimethylether
Frigorige`nes inflammables : coefficients de transfert de chaleur lors de l’e´bullition nucle´e´e Mots cle´s : Frigorige`nes ; Inflammable ; Transfert de chaleur ; E´bullition nucle´e´e ; Propyle`ne ; Propane ; Isobutane ; Butane ; Dime´thylether
1. Introduction Due to ozone layer depletion, CFCs and HCFCs have been phased out and various alternatives have been proposed for the past years. Traditionally, flammable refrigerants have not been accepted in normal refrigeration and air-conditioning applications due to a safety
* Corresponding author. Tel.: +82-32-860-7320; fax: +8232-868-1716. E-mail address: [email protected] (D. Jung). 0140-7007/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2003.11.007
concern. This trend, however, is somewhat relaxed these days due to an environmental mandate. Therefore, some of the flammable refrigerants have been applied to certain applications as a pure working fluid or as one of the components of mixed working fluids [1,2]. For instance, isobutene (R600a) has dominated the European refrigerator/freezer sector for the past decade [1]. Other countries such as India and China also would like to use it in their own refrigerator/freezer sector [3,4]. Recently, such hydrocarbons as propane (R290) and propylene (R1270) are also proposed and actually used as working fluids in heat pumps for heating applications in Europe
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D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
Nomenclature A C D g h hfg k M n P q Rp T
heat transfer area [m2] constant or exponent diameter [m] gravitational acceleration [m s2] heat transfer coefficient [W m2 K1] heat of evaporation [kJ kg1] thermal conductivity [W m1 K1] molecular weight [kg kmol1] exponent pressure [kPa] heat transfer rate [W] surface roughness [mm] temperature [K or C]
m n r s
viscosity [Pa.s] kinematic viscosity [m2 s1] density [kg m3] surface tension [N m1]
Subscripts b bubble Cooper Cooper’s correlation exp experimental values f saturated liquid g saturated vapor pre prediction r reduced property sat saturation S&A Stephan and Abdelsalam’s correlation
Greek letters a thermal diffusivity [m2 s1] b contact angle [ ]
[5]. In fact, hydrocarbons are known to offer such advantages as a low cost, availability, compatibility with the conventional mineral oil, and environmental friendliness [1,5,6]. Jung et al. [7] demonstrated that dimethylether (DME, RE170) is also a good refrigerant to replace CFC12. DME which is flammable has good thermodynamic properties as well as good compatibility with mineral oil and offers excellent environmental properties of ODP=0 and GWP=5 [8]. Even though some flammable refrigerants are proposed as future working fluids, systematic nucleate boiling heat transfer measurements have not been taken with them on a horizontal tube for refrigeration and airconditioning applications. The objective of this paper is to measure, compare, and correlate nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene, propane, isobutane, butane, and dimethylether. This work is designed mainly to provide reliable data to the refrigeration industry for the design and manufacture of high efficiency evaporators with those fluids. In fact, this is a sequel paper to our previous work for the nucleate boiling heat transfer with eight pure halogenated refrigerants of various vapor pressures including HFC32, HFC125, and HFC134a [9].
plain tube of outside diameter of 19.0mm using the same experimental apparatus with the same tube specimen described in Ref. [9]. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane with a sight glass as shown in Fig. 1. The apparatus was composed of a test vessel and a refrigerant circulating loop. The test vessel was made of a stainless steel pipe of 102 mm inside diameter and 230 mm length. In order to observe the boiling phenomenon, a sight glass was installed in the front section of the vessel. The refrigerant vapor boiled off from the heat transfer tube went into the condenser and was condensed there and the liquid
2. Experiments 2.1. Experimental apparatus In this work, nucleate boiling HTCs of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured on a
Fig. 1. Schematic diagram of the experimental apparatus.
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3. Results and discussion In this study, nucleate boiling heat transfer measurements were carried out with five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170). All data were taken at liquid pool temperature of 7 C on a horizontal plain tube of 19.0 mm outside diameter with heat fluxes of 10 kW m2 to 80 kW m2 with an interval of 10 kW m2 in the decreasing order of heat flux. Fig. 2. Heat transfer tube specimen.
3.1. HTCs of flammable refrigerants Fig. 3 and Table 2 illustrate the measured HTCs of the five refrigerants. In Fig. 3, HTCs of HCFC22 were also included for reference. HTCs of various flammable refrigerants of different vapor pressures exhibited a typical trend that nucleate boiling HTCs increase as the vapor pressure increases [9,12]. Specifically, HTCs of propylene and propane are 10.4 and 2.5% higher than those of HCFC22 while those of DME, isobutane, and butane are 32.9%, 44.3%, and 55.2% lower than those of HCFC22. There was no peculiar phenomenon or behavior associated with these flammable refrigerants during experiments as compared to the normally used halogenated refrigerants.
was fed to the bottom of the test tube by gravity. An external chiller with an accurate temperature controller was used to condense the vapor and at the same time to maintain the pool temperature to 7 C. A cartridge heater was used to generate uniform heat flux on the tube as shown in Fig. 2. Data were taken in the order of decreasing heat flux from 80 kW m2 to 10 kW m2 with an interval of 10 kW m2 in the pool temperature of 7 C. Since Ref. [9] contains all the details of the test apparatus, tube specimen and its manufacture, measurements, experimental procedure, data reduction scheme, fouling effect, repeatability of data, data verification etc., they will not be presented again here. An interested reader is referred to Ref. [9] for the details. In this work, only the measurement uncertainty will be reported, which was estimated by the method suggested by Kline and McClintock [10]. In general, the measurement uncertainty was less than 7% at all heat fluxes. The repeatability of the experiment was always within 5% which was within the measurement error.
3.2. Comparison with existing correlations In refrigeration fields, Stephan and Abdelsalam’s [13] and Cooper’s correlations [14] have been widely used in the prediction of nucleate boiling HTCs of refrigerants. In 1980, Stephan and Abdelsalam [13] performed a regression analysis for nearly 5000 pool boiling data and proposed the following correlation for hydrocarbons which is valid in the reduced pressure of 0.0057–0.9.
2.2. Refrigerants In this study, five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were selected for pool boiling tests. Table 1 shows some of the important properties of these fluids at 7 C in the order of vapor pressure. In this paper, all thermodynamic and transport properties are calculated by REFPROP [11].
hS&A
" #0:67 g 0:5 ðq=AÞDb f kf Tsat !0:248 4:33 hfg Db2 f g f 2f
kf ¼ 0:0546 Db
ð1Þ
Table 1 Some properties of tested refrigerants at 7 C Refrigerant
Propylene Propane DME Isobutane Butane
Psat (kPa) 718 584 336 199 134
Pr
Tr
kf
kg 1
(W m 0.1539 0.1375 0.0626 0.0548 0.0353
0.7663 0.7575 0.7002 0.6869 0.6589
0.1197 0.1019 0.1512 0.1036 0.1140
1
K ) 0.01553 0.01664 0.01518 0.01446 0.01447
mu _f
mu _g 6
10 124.0 117.4 208.2 184.1 189.0
(Pa.s) 7.286 8.041 7.817 7.175 7.102
(N m1) 0.00900 0.00923 0.01338 0.01220 0.01397
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D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
Fig. 3. Nucleate boiling HTCs of flammable refrigerants.
Fig. 4. Comparison between various correlations and the present data.
where the bubble departure diameter, Db, is defined as Db=0.0146b[2s/g (fg)]0.5 with a contact angle, , of 35 . On the other hand, in 1984 Cooper [14] developed a simple correlation based upon reduced properties, which is valid in the reduced pressure of 0.001–0.9 with molecular weight of 2–200 kg/kmol. q 0:67 hCooper ¼ 90 M 0:5 Prn ðlog10 Pr Þ0:55 ð2Þ A
On the other hand, Cooper’s correlation overpredicted the data as much as 70% for all fluids tested. In fact, Cooper’s correlation was reduced mainly using the data of water, halogenated refrigerants, and cryogens and ethanol. Therefore, it is not surprising that Cooper’s correlation does not predict the HTCs of hydrocarbons and DME. Both Stephan and Abdelsalam and Cooper used various data set obtained from various geometries such as a plate, wire, and cylinder of different sizes. Hence, it would be difficult to obtain good prediction particularly suited for a horizontal tube used in refrigeration and air-conditioning applications. Recently, Jung et al. [9] developed a correlation based upon the data of eight pure halogenated refrigerants of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125, and HFC32 using the same experimental apparatus as the present one. This correlation showed 7% deviation for the tested halogenated refrigerant data.
where the exponent, n, is defined as n=0.12–0.2log10 Rp with Rp being a surface roughness (mm). Since Eqs. (1) and (2) are the most popular correlations, HTCs measured in this work were compared against these correlations. For Cooper’s correlation, a surface roughness of 3 mm was used in the calculation. Fig. 4 shows the comparison. In general, Stephan and Abdelsalam’s correlation underpredicted the HTCs of flammable refrigerants up to 80%. One of the possible reasons for this large deviation is that they used in their regression analysis only the data of benzene, n-pentane, n-hexane, n-heptane and ethanol. None of the popular hydrocarbon data were employed in their analysis.
h ¼ 10
0:25 kf ðq=AÞDb C1 0:1 f Pr ð1 Tr Þ1:4 Db kf Tsat f
ð3Þ
Table 2 Measured heat transfer coefficients of various refrigerants Fluids
Propylene Propane DME Isobutane Butane
Heat flux (kW m2) 10
20
30
40
50
60
70
80
3206 2709 1696 1324 915
5008 4411 2898 2276 1720
6492 5880 3811 3095 2414
7845 7230 4607 3859 3101
9064 8514 5462 4623 3770
10212 9750 6371 5367 4417
11,435 10,923 7285 6130 5060
12,778 12,056 8215 6887 5698
D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
0:309 g C1 ¼ 0:855 Pr0:437 f
ð4Þ
Jung et al.’s correlation showed 15% deviation for flammable refrigerants tested in this study as illustrated in Fig. 4. Even though their correlation is better than the others, there is still room for improvement. Jung et al. developed their correlation using halogenated refrigerants data only and hence certain characteristics of hydrocarbons and DME were not taken into consideration in their analysis. 3.3. Correlation development Since consistent measurements were taken for halogenated refrigerants [9] and flammable refrigerants in this study, a more thorough regression analysis was made using dimensionless variables suggested by Cooper as done in our previous study [9]. Since Ref. [9] contains all the details of the regression analysis and related dimensionless variables etc., they will not be presented again here. An interested reader is referred to Ref. [9] for the details. Eq. (5) is the final correlation developed both for flammable refrigerants and halogenated refrigerants.
kf ðq=AÞDb C2 h ¼ 41:4 ðlog10 Pr Þ1:52 Db kf Tsat g 0:53 1 ð5Þ f C2 ¼ 0:835ð1 Pr Þ1:33
ð6Þ
413
Fig. 5 shows the comparison between the new correlation and experimental data for various fluids including flammable refrigerants. An average deviation for all fluids shown in Fig. 5 is 5.3%.
4. Conclusions In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 C on a plain tube of 19.0 mm outside diameter. All data were taken from 80 kW m2 to 10 kW m2 with an interval of 10 kW m2 in the decreasing order of heat flux. Based upon the test results and correlation development, following conclusions can be drawn. 1. Flammable refrigerants’ data showed a typical trend that nucleate boiling HTCs increase with the vapor pressure. No unusual behavior or phenomenon was observed for these fluids during experiments. 2. Stephan and Abdelsalam’s correlation underpredicted the present data up to 80% while Cooper’s correlation overpredicted them as much as 70%. Jung et al.’s correlation developed for halogenated refrigerants showed 15% deviation for the flammable refrigerants. 3. Some dimensionless groups affecting nucleate boiling heat transfer were identified and they were correlated by a regression analysis to yield a new correlation valid for both halogenated refrigerants and flammable refrigerants. Thus developed correlation predicted the present data with a deviation of 5.3% for all refrigerants including CFCs, HCFCs, HFCs, hydrocarbons, and DME. The new correlation takes into account that the exponent to the heat flux term varies significantly among fluids and also is a strong function of reduced pressure.
Acknowledgements This work is partially supported by TechnoChem Co., Ltd., Seotan-Myun, Pyungtaek-Si, Kyunggi-Do, Korea.
References
Fig. 5. Comparison between the new correlation and the data of various fluids.
[1] Kruse H. The state of the art of the hydrocarbon technology in household refrigeration. Proc. of the Int. Conferences on Ozone protection technologies, Washington (DC), 1996, pp. 179–188.
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[2] Jung D, Kim C, Song K, Park B. Testing of propane/isobutane mixture in domestic refrigerators. Int J Refrigeration 2000;23:517–27. [3] Devotta S, Patil SR, Sawant NN, Kulkarni MM. Comparative life cycle testing of hermetic compressors with CFC-12 and HC-blend. Proc. of the Int. Conferences on Ozone protection technologies, Washington (DC), 1997, pp. 194–202. [4] Fine A. Field testing of CFC-free, energy efficient refrigerators in China. Proc. of the Int. Conferences on Ozone protection technologies, Washington (DC), 1997, pp. 203– 212. [5] Int. Energy Agency’s Heat Pump Center, Informative fact sheet: Hydrocarbons as refrigerants in residential heat pumps and air-conditioners, 2002. [6] Paul J. A fresh look at hydrocarbon refrigeration: experience and outlook. Proc. of the Int. Conferences on Ozone protection technologies, Washington (DC), 1996, pp. 252–259. [7] Jung D, Park B, Lee H. Evaluation of supplementary/retrofit refrigerants for automobile air-conditioners charged with CFC12. Int J Refrigeration 1999;33:558–68. [8] Calm JM. Property, safety, and environmental data for
[9]
[10]
[11]
[12]
[13]
[14]
alternative refrigerants. Proc. of the Earth Technologies Forum, Washington (DC), 1998, pp. 192–205. Jung D, Kim Y, Ko Y, Song K. Nucleate boiling heat transfer coefficients of pure halogenated refrigerants. Int J Refrigeration 2003;26(2):240–8. Kline SJ, McClintock FA. Describing uncertainties in single-sample experiments. Mechanical Engineers 1953;75: 3–9. McLinden MO, Klein SA, Lemmon EW, Peskin AP. NIST thermodynamic and transport properties of refrigerants and refrigerant mixtures—REFPROP version 6.0, 1998. Rohsenow WM, Hartnett JP, Ganic EN. Handbook of heat transfer fundamentals. Second edition. McGraw-Hill; 1985 pp. 12–22. Stephan K, Abdelsalam M. Heat transfer correlations for natural convection boiling. Int J Heat Mass Transfer 1980; 23:73–87. Cooper MG. Heat flow rates in saturated nucleate pool boiling—a wide-ranging examination using reduced properties. Advances in Heat Transfer, Academic Press 1984; 16:157–239.
Nucleate boiling heat transfer coefficients of flammable refrigerants Dongsoo Junga,*, Heungseok Leeb, Dongsoo Baeb, Sukjae Ohoc a
Department of Mechanical Engineering, Inha University, Incheon 402-751, South Korea b Inha University, Incheon, 402-751, South Korea c TechnoChem Co., Ltd., Seotan-Myun, Pyungtaek-Si, Kyunggi-Do, South Korea
Received 27 January 2003; received in revised form 28 November 2003; accepted 28 November 2003
Abstract Nucleate boiling heat transfer coefficients (HTCs) of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m2 to 10 kW m2 with an interval of 10 kW m2 in the pool temperature of 7 C. Test results exhibited a typical trend that HTCs of flammable refrigerants increase with increasing vapor pressure. Existing nucleate boiling heat transfer correlations showed up to 80% deviation as compared to the present data. Hence a new correlation was developed through a regression analysis taking into account dimensionless variables affecting nucleate boiling heat transfer. The new correlation showed a good agreement with data for flammable refrigerants as well as halogenated refrigerants with a deviation of 5.3%. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Refrigerant; Flammable; Heat transfer; Nucleate boiling; Propylene; Propane; Isobutane; Butane; Dimethylether
Frigorige`nes inflammables : coefficients de transfert de chaleur lors de l’e´bullition nucle´e´e Mots cle´s : Frigorige`nes ; Inflammable ; Transfert de chaleur ; E´bullition nucle´e´e ; Propyle`ne ; Propane ; Isobutane ; Butane ; Dime´thylether
1. Introduction Due to ozone layer depletion, CFCs and HCFCs have been phased out and various alternatives have been proposed for the past years. Traditionally, flammable refrigerants have not been accepted in normal refrigeration and air-conditioning applications due to a safety
* Corresponding author. Tel.: +82-32-860-7320; fax: +8232-868-1716. E-mail address: [email protected] (D. Jung). 0140-7007/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2003.11.007
concern. This trend, however, is somewhat relaxed these days due to an environmental mandate. Therefore, some of the flammable refrigerants have been applied to certain applications as a pure working fluid or as one of the components of mixed working fluids [1,2]. For instance, isobutene (R600a) has dominated the European refrigerator/freezer sector for the past decade [1]. Other countries such as India and China also would like to use it in their own refrigerator/freezer sector [3,4]. Recently, such hydrocarbons as propane (R290) and propylene (R1270) are also proposed and actually used as working fluids in heat pumps for heating applications in Europe
410
D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
Nomenclature A C D g h hfg k M n P q Rp T
heat transfer area [m2] constant or exponent diameter [m] gravitational acceleration [m s2] heat transfer coefficient [W m2 K1] heat of evaporation [kJ kg1] thermal conductivity [W m1 K1] molecular weight [kg kmol1] exponent pressure [kPa] heat transfer rate [W] surface roughness [mm] temperature [K or C]
m n r s
viscosity [Pa.s] kinematic viscosity [m2 s1] density [kg m3] surface tension [N m1]
Subscripts b bubble Cooper Cooper’s correlation exp experimental values f saturated liquid g saturated vapor pre prediction r reduced property sat saturation S&A Stephan and Abdelsalam’s correlation
Greek letters a thermal diffusivity [m2 s1] b contact angle [ ]
[5]. In fact, hydrocarbons are known to offer such advantages as a low cost, availability, compatibility with the conventional mineral oil, and environmental friendliness [1,5,6]. Jung et al. [7] demonstrated that dimethylether (DME, RE170) is also a good refrigerant to replace CFC12. DME which is flammable has good thermodynamic properties as well as good compatibility with mineral oil and offers excellent environmental properties of ODP=0 and GWP=5 [8]. Even though some flammable refrigerants are proposed as future working fluids, systematic nucleate boiling heat transfer measurements have not been taken with them on a horizontal tube for refrigeration and airconditioning applications. The objective of this paper is to measure, compare, and correlate nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene, propane, isobutane, butane, and dimethylether. This work is designed mainly to provide reliable data to the refrigeration industry for the design and manufacture of high efficiency evaporators with those fluids. In fact, this is a sequel paper to our previous work for the nucleate boiling heat transfer with eight pure halogenated refrigerants of various vapor pressures including HFC32, HFC125, and HFC134a [9].
plain tube of outside diameter of 19.0mm using the same experimental apparatus with the same tube specimen described in Ref. [9]. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane with a sight glass as shown in Fig. 1. The apparatus was composed of a test vessel and a refrigerant circulating loop. The test vessel was made of a stainless steel pipe of 102 mm inside diameter and 230 mm length. In order to observe the boiling phenomenon, a sight glass was installed in the front section of the vessel. The refrigerant vapor boiled off from the heat transfer tube went into the condenser and was condensed there and the liquid
2. Experiments 2.1. Experimental apparatus In this work, nucleate boiling HTCs of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured on a
Fig. 1. Schematic diagram of the experimental apparatus.
411
D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
3. Results and discussion In this study, nucleate boiling heat transfer measurements were carried out with five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170). All data were taken at liquid pool temperature of 7 C on a horizontal plain tube of 19.0 mm outside diameter with heat fluxes of 10 kW m2 to 80 kW m2 with an interval of 10 kW m2 in the decreasing order of heat flux. Fig. 2. Heat transfer tube specimen.
3.1. HTCs of flammable refrigerants Fig. 3 and Table 2 illustrate the measured HTCs of the five refrigerants. In Fig. 3, HTCs of HCFC22 were also included for reference. HTCs of various flammable refrigerants of different vapor pressures exhibited a typical trend that nucleate boiling HTCs increase as the vapor pressure increases [9,12]. Specifically, HTCs of propylene and propane are 10.4 and 2.5% higher than those of HCFC22 while those of DME, isobutane, and butane are 32.9%, 44.3%, and 55.2% lower than those of HCFC22. There was no peculiar phenomenon or behavior associated with these flammable refrigerants during experiments as compared to the normally used halogenated refrigerants.
was fed to the bottom of the test tube by gravity. An external chiller with an accurate temperature controller was used to condense the vapor and at the same time to maintain the pool temperature to 7 C. A cartridge heater was used to generate uniform heat flux on the tube as shown in Fig. 2. Data were taken in the order of decreasing heat flux from 80 kW m2 to 10 kW m2 with an interval of 10 kW m2 in the pool temperature of 7 C. Since Ref. [9] contains all the details of the test apparatus, tube specimen and its manufacture, measurements, experimental procedure, data reduction scheme, fouling effect, repeatability of data, data verification etc., they will not be presented again here. An interested reader is referred to Ref. [9] for the details. In this work, only the measurement uncertainty will be reported, which was estimated by the method suggested by Kline and McClintock [10]. In general, the measurement uncertainty was less than 7% at all heat fluxes. The repeatability of the experiment was always within 5% which was within the measurement error.
3.2. Comparison with existing correlations In refrigeration fields, Stephan and Abdelsalam’s [13] and Cooper’s correlations [14] have been widely used in the prediction of nucleate boiling HTCs of refrigerants. In 1980, Stephan and Abdelsalam [13] performed a regression analysis for nearly 5000 pool boiling data and proposed the following correlation for hydrocarbons which is valid in the reduced pressure of 0.0057–0.9.
2.2. Refrigerants In this study, five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were selected for pool boiling tests. Table 1 shows some of the important properties of these fluids at 7 C in the order of vapor pressure. In this paper, all thermodynamic and transport properties are calculated by REFPROP [11].
hS&A
" #0:67 g 0:5 ðq=AÞDb f kf Tsat !0:248 4:33 hfg Db2 f g f 2f
kf ¼ 0:0546 Db
ð1Þ
Table 1 Some properties of tested refrigerants at 7 C Refrigerant
Propylene Propane DME Isobutane Butane
Psat (kPa) 718 584 336 199 134
Pr
Tr
kf
kg 1
(W m 0.1539 0.1375 0.0626 0.0548 0.0353
0.7663 0.7575 0.7002 0.6869 0.6589
0.1197 0.1019 0.1512 0.1036 0.1140
1
K ) 0.01553 0.01664 0.01518 0.01446 0.01447
mu _f
mu _g 6
10 124.0 117.4 208.2 184.1 189.0
(Pa.s) 7.286 8.041 7.817 7.175 7.102
(N m1) 0.00900 0.00923 0.01338 0.01220 0.01397
412
D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
Fig. 3. Nucleate boiling HTCs of flammable refrigerants.
Fig. 4. Comparison between various correlations and the present data.
where the bubble departure diameter, Db, is defined as Db=0.0146b[2s/g (fg)]0.5 with a contact angle, , of 35 . On the other hand, in 1984 Cooper [14] developed a simple correlation based upon reduced properties, which is valid in the reduced pressure of 0.001–0.9 with molecular weight of 2–200 kg/kmol. q 0:67 hCooper ¼ 90 M 0:5 Prn ðlog10 Pr Þ0:55 ð2Þ A
On the other hand, Cooper’s correlation overpredicted the data as much as 70% for all fluids tested. In fact, Cooper’s correlation was reduced mainly using the data of water, halogenated refrigerants, and cryogens and ethanol. Therefore, it is not surprising that Cooper’s correlation does not predict the HTCs of hydrocarbons and DME. Both Stephan and Abdelsalam and Cooper used various data set obtained from various geometries such as a plate, wire, and cylinder of different sizes. Hence, it would be difficult to obtain good prediction particularly suited for a horizontal tube used in refrigeration and air-conditioning applications. Recently, Jung et al. [9] developed a correlation based upon the data of eight pure halogenated refrigerants of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125, and HFC32 using the same experimental apparatus as the present one. This correlation showed 7% deviation for the tested halogenated refrigerant data.
where the exponent, n, is defined as n=0.12–0.2log10 Rp with Rp being a surface roughness (mm). Since Eqs. (1) and (2) are the most popular correlations, HTCs measured in this work were compared against these correlations. For Cooper’s correlation, a surface roughness of 3 mm was used in the calculation. Fig. 4 shows the comparison. In general, Stephan and Abdelsalam’s correlation underpredicted the HTCs of flammable refrigerants up to 80%. One of the possible reasons for this large deviation is that they used in their regression analysis only the data of benzene, n-pentane, n-hexane, n-heptane and ethanol. None of the popular hydrocarbon data were employed in their analysis.
h ¼ 10
0:25 kf ðq=AÞDb C1 0:1 f Pr ð1 Tr Þ1:4 Db kf Tsat f
ð3Þ
Table 2 Measured heat transfer coefficients of various refrigerants Fluids
Propylene Propane DME Isobutane Butane
Heat flux (kW m2) 10
20
30
40
50
60
70
80
3206 2709 1696 1324 915
5008 4411 2898 2276 1720
6492 5880 3811 3095 2414
7845 7230 4607 3859 3101
9064 8514 5462 4623 3770
10212 9750 6371 5367 4417
11,435 10,923 7285 6130 5060
12,778 12,056 8215 6887 5698
D. Jung et al. / International Journal of Refrigeration 27 (2004) 409–414
0:309 g C1 ¼ 0:855 Pr0:437 f
ð4Þ
Jung et al.’s correlation showed 15% deviation for flammable refrigerants tested in this study as illustrated in Fig. 4. Even though their correlation is better than the others, there is still room for improvement. Jung et al. developed their correlation using halogenated refrigerants data only and hence certain characteristics of hydrocarbons and DME were not taken into consideration in their analysis. 3.3. Correlation development Since consistent measurements were taken for halogenated refrigerants [9] and flammable refrigerants in this study, a more thorough regression analysis was made using dimensionless variables suggested by Cooper as done in our previous study [9]. Since Ref. [9] contains all the details of the regression analysis and related dimensionless variables etc., they will not be presented again here. An interested reader is referred to Ref. [9] for the details. Eq. (5) is the final correlation developed both for flammable refrigerants and halogenated refrigerants.
kf ðq=AÞDb C2 h ¼ 41:4 ðlog10 Pr Þ1:52 Db kf Tsat g 0:53 1 ð5Þ f C2 ¼ 0:835ð1 Pr Þ1:33
ð6Þ
413
Fig. 5 shows the comparison between the new correlation and experimental data for various fluids including flammable refrigerants. An average deviation for all fluids shown in Fig. 5 is 5.3%.
4. Conclusions In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 C on a plain tube of 19.0 mm outside diameter. All data were taken from 80 kW m2 to 10 kW m2 with an interval of 10 kW m2 in the decreasing order of heat flux. Based upon the test results and correlation development, following conclusions can be drawn. 1. Flammable refrigerants’ data showed a typical trend that nucleate boiling HTCs increase with the vapor pressure. No unusual behavior or phenomenon was observed for these fluids during experiments. 2. Stephan and Abdelsalam’s correlation underpredicted the present data up to 80% while Cooper’s correlation overpredicted them as much as 70%. Jung et al.’s correlation developed for halogenated refrigerants showed 15% deviation for the flammable refrigerants. 3. Some dimensionless groups affecting nucleate boiling heat transfer were identified and they were correlated by a regression analysis to yield a new correlation valid for both halogenated refrigerants and flammable refrigerants. Thus developed correlation predicted the present data with a deviation of 5.3% for all refrigerants including CFCs, HCFCs, HFCs, hydrocarbons, and DME. The new correlation takes into account that the exponent to the heat flux term varies significantly among fluids and also is a strong function of reduced pressure.
Acknowledgements This work is partially supported by TechnoChem Co., Ltd., Seotan-Myun, Pyungtaek-Si, Kyunggi-Do, Korea.
References
Fig. 5. Comparison between the new correlation and the data of various fluids.
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