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8ТЕРМОДИНАМИЧЕСКИМ АНАЛИЗ В АЛЬТЕРНАТИВНОЙ ЭНЕРГЕТИКЕ
THERMODYNAMIC ANALYSIS IN RENEWABLE ENERGY
Статья поступила в редакцию 06.12.12. Ред. рег. № 1467 The article has entered in publishing office 06.12.12. Ed. reg. No. 1467
УДК 536.423
ПУЗЫРЬКОВОЕ КИПЕНИЕ ЧИСТОГО ХЛАДАГЕНТА R410A И СМЕСИ R410A С МАСЛОМ НА РАЗВИТЫХ ТЕПЛООБМЕННЫХ ПОВЕРХНОСТЯХ
В. Г. Букин, Хо Вьет Хынг
Астраханский государственный технический университет 414025, Астрахань, ул. Татищева, д. 16 Тел.: +7 9673332994, e-mail: hoviethung81@yahoo.com
Заключение совета рецензентов: 08.12.12 Заключение совета экспертов: 10.12.12 Принято к публикации: 12.12.12
В работе приведены результаты экспериментального исследования теплообмена при пузырьковом кипении хладагента R410A с разными концентрациями масла на стандартно-оребренной трубе и трубах с развитой поверхностью. Было исследовано влияние геометрических параметров и концентрации масла на коэффициент теплоотдачи при кипении на разных поверхностях.
Ключевые слова: озонобезопасный, теплоотдача, кипение, хладагент, потенциал глобального потепления.
NUCLEATE POOL BOILING OF R410A AND R410A/OIL MIXTURES ON HEAT-EXCHANGE ENHANCED SURFACES
V.G. Bukin, Ho Viet Hung
Astrakhan State Technical University 16 Tatisheva str., Astrakhan, 414025, Russia Tel.: +7 9673332994, e-mail: hoviethung81@yahoo.com
Referred: 08.12.12 Expertise: 10.12.12 Accepted: 12.12.12
Investigation of the nucleate boiling heat transfer of R410A with different oil mass fractions for integral-fin tube and enhanced tubes was carried out in this study. The effect of geometric parameters and oil concentration on the boiling heat transfer coefficients of different surfaces were investigated.
Keywords: ozone-safe, heat transfer, boiling, refrigerant, global warming potentials.
Владимир Григорьевич Букин
Сведения об авторе: д-р техн. наук, зав. кафедрой холодильных машин, проф. Астраханского гос. технического университета. Проректор по международному сотрудничеству АГТУ. Председатель регионального отделения международной академии холода, академик МАХ, член редколлегии журнала «Вестник Международной Академии Холода», член диссертационных советов АГТУ и ДагГТУ.
Образование: Астрыбвтуз (1964 г.) с отличием по специальности «Холодильные и компрессорные установки».
Основное направление научных исследований: интенсификация теплообмена при фазовых переходах.
Публикации: 150.
International Scientific Journal for Alternative Energy and Ecology № 01/2 (118) 2013
© Scientific Technical Centre «TATA», 2013
Хо Вьет Хынг
Сведения об авторе: аспирант кафедры холодильных машин Астраханского гос. технического университета.
Образование: Украинский гос. морской технический университет им. адмирала Макарова по специальности «Судовые холодильные машины и установки» (2007 г.). Публикации: 5.
Introduction
It is well-known that Freons widely used in various refrigeration fields will soon be phased out, mainly due to their ozone depletion potentials (ODPs) and global warming potentials (GWPs). As the production of Freons are phased out over the coming years, alternatives such as R407C and R410A will be manufactured to replace Freon R22 [1]. R407C is zeotropic refrigerant, and its temperature glide can reach 4-6 °C. The operation efficiency of refrigeration system using R407C is low due to its heat transfer coefficient 25% smaller than Freon R22. So, as the moment the leading candidate for Freon R22 substitution is HFC R410A, a material that contains no ozone depleting chlorine. R410A is a non-ozone depleting blend of two HFC refrigerants (50% R32, 50% R125). R410A exhibits higher pressures and refrigeration capacity than Freon R22. It has a global warming potential 4.5 times less than Freon R12.
Use of tubes with enhanced surfaces geometries provide an increased heat transfer coefficient significantly. The boiling heat transfer of blended refrigerant is different than of the single component. Vapor compression refrigeration systems use oil-lubricated compressors, and the oil-refrigerant mixture circulates the refrigeration system. Therefore the research of pool boiling of R410A/oil mixtures on tubes with enhanced surfaces geometries is actual.
In recent years significant progress has been made in understanding nucleate boiling heat transfer and oil effects on the integral-fin tubes and tubes with enhanced surfaces geometries at the Astrakhan state university [2, 3]. The objective of this paper is to provide a
comprehensive pool boiling database for R410A and R410A/oil mixtures on enhanced surfaces. Tubes with enhanced surfaces used in this paper were patented by the authors [4].
Experiments
In this work, nucleate boiling HTCs R410A and R410A/oil mixtures were measured on enhanced tubes (Fig. 1) using the same experimental apparatus described inRef. [5].
a b
Рис. 1. Эскизы ребер с частично замкнутым объемом экспериментальных труб (s - зазор): а - профиль Г; b - профиль Y Fig. 1. Enhanced boiling surfaces (sg - fin gaps spacing): a - bent fins; b - Y-shaped fins
Tubes with bent fins and Y-shaped fins were made from the integral-fins tubes. To make the bubbles easily form and release, two tunnels have been made on the top and bottom of the tube tests.
The specifications of the test tubes are given in Table.
Геометрические параметры опытных труб The specifications and dimension of enhanced tubes
№ Tubes do di Sf Sf' Sg hf ß F, Ft Rz
1 Integral-fin 21 13,2 2 1,6 - 2,25 3,64 0,0437 0,012 4...5
2 Bent fins 20,5 13,2 2 0,25 2 3,82 0,0458 0,012 4...5
3 Y-shaped fins 21 13,2 2 0,25 2,25 4,2 0,0504 0,012 4...5
do - outside diameter, mm; dj - inside diameter, mm; Fo - outside surface area, m2; Fj - inside surface area, m2; p = FJFt surface ratio; sg - fin gaps spacing, mm; hf - fin heights, mm; Sf - fin spacing, mm; Sf - distance between fins, mm; Rz roughness parameter, ^m.
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Термодинамический анализ
The length of all test tubes are 290 mm. A heater was installed in the test tube. The heater was used to generate uniform heat flux on the tube. Three type-T thermocouples to measure the temperature variations around the tube wall are installed in the tube. The liquid refrigerant temperature is recorded by three type-T thermocouples .All the thermocouples were precalibrated and having a calibrated accuracy of 0.1 °C.
The heat transfer coefficient for each power input was calculated as follows a = NI[F0(twaH - 4)] where N is the electric heating power and ts is the saturation temperature based on the measured system's average liquid refrigerant temperature. The twatt is the mean average wall temperature at the outer surface. The outside surface area (actual wetted area), F0, is evaluated as Prcd,/. Note that di is the inside tube diameter, and l is the effective length of the heater (the length of the heater is 290 mm). The heat flux is based on the actual wetted area of the tube (F0). Data were taken in the order of decreasing heat flux from 6 kWIm2 to 200 WIm2 to avoid a hysteresis effect.
The present data with R22 agree favorably with previous test results [6, 7], which suggests the validity of the present test facility.
Tests were performed at saturation temperatures of -20 °C, -5 °C and +5 °C using refrigerants R410A for oil concentrations of 0, 2%, 5% and 10% by mass. The oil concentrations by mass ^oil , defined as the mass of oil per total mass of liquid refrigerantIoil mixture, is determined by weighting the oil mass and the refrigerant mass before filling. The synthetic oil POE Bitzer BSE32 was used for the tests.
Before charging the system, the test section was cleaned with acetone then was evacuated using a vacuum pump. The vacuum pump was turned on few hours to evacuate the system thoroughly. With no leaks detected over a 24 h period, the test vessel was charged with pure R410A to a level of 20 mm above the top of the tubes. The pool temperature read by the thermocouple was compared to the saturation temperature at the measured pressure to ensures that there are no noncondensibles and no subcooling in the system.
The first experiment carried out was with pure R410A. Once the pure refrigerant test was completed the oil (Bitzer BSE32) was added to bring its concentration to 2% and the experiment repeated. The experiments were then carried out at oil concentrations of 5 and 10% by adding the required amount of oil in steps. When the required oil concentration was attained, the power input was increased to 200W and the system was again allowed to boil and mix for 30 minutes.
Results and discussion
Results for pure R410A Test results for integral-fin and enhanced tubes at saturation temperatures -20 °C are shown in Fig. 2. From the figure we could observe the following features.
Рис. 2. Коэффициент теплоотдачи a в зависимости от плотности теплового потока q при температуре насыщения -20 °C Fig. 2. Heat transfer coefficient a as a function of the heat flux q at saturation temperature -20 °C
First, the boiling heat transfer coefficient of the integral-fin tube is less than that of the two enhanced tubes. The reason for the improvement in heat transfer with bent fins and Y-shaped fins over integral-fins was the fact: with integral-fin tubes liquid from above the fins flow directly onto the fully exposed nucleation sites. To bring this cold liquid to the level required for nucleation require relatively large heat flux. Enhanced tubes, by contrast, due to the narrow fin gap sg which tends to restrict cold liquid from above fins flow directly onto tube, causes superheated liquid to flow onto the active sites on the top surface of tube. Therefore, only a small amount of heat need be added to raise the temperature of the liquid to the incipient boiling point. Second, the boiling heat transfer coefficient increase with an increase in the heat flux.
Results for R410A/oil mixtures For integral-fin tube. The boiling heat transfer coefficients of the integral-fin tube taken at -5 °C saturation temperature are shown in Fig. 3.
Рис. 3. Влияние плотности теплового потока на коэффициенты теплоотдачи при кипении интегрально-оребренной трубы при различных концентрациях масла для ts = -5 °C
Fig. 3. Effect of heat flux on boiling heat transfer coefficients of the integral-fin tube at various oil concentrations for ts = -5 °C
With a small amount of oil added to a refrigerant (^oii = 2%), the heat transfer coefficient increases of up to 10% for integral-fin tube when compared with identical tests carried out with pure refrigerant. The foaming action of the oil around the heating surface is sometimes beneficial to the pool boiling heat transfer. The foam seems to remove the oil from heating surface, which causes some enhancement. So the heat transfer
International Scientific Journal for Alternative Energy and Ecology № 01/2 (118) 2013
© Scientific Technical Centre «TATA», 2013
coefficient increase a little bit. For further oil addition (4oii > 2%), the heat transfer coefficient decreases. Significant degradation of heat transfer coefficient is observed at 10% oil concentration. The degradation of the heat transfer coefficient (1 - aoil/a) is 7... 15% at 4oil = 5%, 15.30% at 4oil = 10%. At higher oil concentration, due to its large viscosity and mass transfer resistance effect, the oil tends to decreases the heat transfer coefficient.
For enhanced tubes. The boiling heat transfer coefficients of the bent fins tube taken at -5 °C saturation temperature are shown in Fig. 4.
Рис. 4. Влияние плотности теплового потока на коэффициенты теплоотдачи при кипении трубы с Г-профилем при различных концентрациях масла для ts = -5 °C
Fig. 4. Effect of heat flux on boiling heat transfer coefficients of bent fins tube at various oil concentrations for ts = -5 °C
For the enhanced tubes, any addition of oil leads to a drop-off in performance, probably due to the accumulation of oil in sub-tunnels. This is especially significant at high oil concentrations (^oil = 10%). The degradation of the heat transfer coefficient is 50.62% at 4oii = 10% for bent fins tube. The present results show that enhanced tubes are more sensitive to the degradation of the heat transfer coefficient for pool boiling than integral-fin tube. This may be because the re-entrant cavity structure of the enhanced surfaces acts to retain the oil-rich mixture at the boiling surfaces.
Рис. 5. Влияние концентрации масла на коэффициенты теплоотдачи при кипении Y-образной формы ребра при различной плотности теплового потока для ts = -5 °C Fig. 5. Effect of oil concentrations on boiling heat transfer coefficients of Y-shaped fins tube at various heat flux for ts = -5 °C
Fig. 5 shows the experimental results of the pool boiling R410A/oil mixture on the Y-shaped fins tube in the a - ^oil diagrams for heat flux 600, 2380, 4760 W/m2. The heat transfer increases with increasing heat flux, decreases with increasing oil concentrations. The degradation of the heat transfer coefficient is 54...67% at с,,, = 10% for Y-shaped fins tube.
Conclusions
The pool boiling heat transfer coefficients of one integral-fin and two enhanced tubes were measured for pure R134A and R410A/oil mixtures mixed with one. The oil mass fraction varies from 2% to 10%, and the surface heat flux from 200 W/m2 to 6 kW/m2. The following conclusions can be made:
Pure R410A. The enhanced tubes provide better performance due to the higher density of active nucleation sites and perhaps better fluid pumping within the channels. The boiling heat transfer coefficient increase with an increase in the heat flux.
R410A/oil mixtures. The addition of oil to boiling R410A for enhanced tubes cause degradation in the heat transfer coefficient. Oil-rich layer is formed at the boiling surface, which adds additional resistance to boiling heat transfer. The heat transfer degradation is larger for enhanced tubes compared to integral-fin tubes. The heat transfer coefficients for R410A/oil mixtures for integral-fin tube larger than those of pure R410A at low oil concentration ^oil = 2%. The increase of heat transfer coefficients at low oil concentration may be related to the foaming process.
References
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2. Кузмин А.Ю., Букин А.В. Экспериментальные исследования энергоэффективности ретрофита холодильной машины на альтернативные озонобезопас-ные смесевые холодильные агенты // Юг России, экология, развитие. 2010. № 4. С. 119-120.
3. Букин В.Г., Ежов А.В. Обобщение экспериментальных данных по теплообмену при кипении смесевых холодильных агентов с маслом в испарителях судовых холодильных машин // Вестн. АГТУ. Сер. «Морская техника и технология». 2009. № 1. С. 161-164.
4. Патент РФ № 89680. Испаритель / Букин В.Г., Кузьмин А.Ю., Васильев В.Н., Бирюлин И.В. // Опубл. 10.12.2009.
5. Букин В.Г., Кузмин А.Ю., Васильев В.Н. Экспериментальное исследование интенсификации теплоотдачи при кипении многокомпонентного хладагента R407C // Изв. Калининградского гос. технического университета. 2004. № 6. С. 177-185.
6. Webb, R.L., and C. Pais. 1992. Nucleate pool boiling data for five refrigerants on plain, integral-fin and enhanced tube geometries // Int. J. Heat Mass Transfer 35(8): 1893-1904.
7. Wang, C.-C., Shieh, W.-Y., Chang, Y.-J, and Yang, C.-Y. Nucleate Boiling Performance of R-22, R-123, R-134a, R-410A and R-407C on Smooth and Enhanced Tubes // ASHRAE Transaction. 1998. Vol. 104, Part. 1, No. SF-98-15-4. P. 1-7.
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