CC BY
5Статья поступила в редакцию 21.12.12. Ред. рег. № 1488
The article has entered in publishing office 21.12.12. Ed. reg. No. 1488
УДК 536.423
ВЛИЯНИЕ ГЕОМЕТРИЧЕСКИХ ПАРАМЕТРОВ ПОВЕРХНОСТИ ТЕПЛООБМЕНА И КОНЦЕНТРАЦИИ МАСЛА НА ТЕПЛООТДАЧУ ПРИ ПУЗЫРЬКОВОМ КИПЕНИИ
ХЛАДАГЕНТА R410A
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: 28.12.12 Заключение совета экспертов: 29.12.12 Принято к публикации: 02.01.13
В работе приведены результаты экспериментального исследования теплообмена при пузырьковом кипении хладагента R410A с разными концентрациями масла на стандартно-оребренной трубе и четыре трубах с развитой поверхностью при разных насыщенных температурах. Концентрация масла, соответственно, 0, 2, 5 и 10%. В исследуемом диапазоне теплового потока концентрация масла оказывает влияние на коэффициент теплоотдачи.
Ключевые слова: озонобезопасный, теплоотдача, кипение, хладагент, испаритель, чиллер.
EFFECTS OF GEOMETRICAL PARAMETRIC HEAT TRANSFER SURFACE AND OIL CONCENTRATION ON NUCLEATE BOILING HEAT TRANSFER OF
REFRIGERANT R410A
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: 28.12.12 Expertise: 29.12.12 Accepted: 02.01.13
Pool boiling heat transfer coefficients of R410A with different oil mass fractions for integral-fin tube and four enhanced tubes were tested at various saturation temperatures. The lubrication mass fractions were 0, 2, 5, and 10.0%, respectively. Within the tested heat flux range, the oil generally has a different influence on pool boiling heat transfer.
Keywords: ozone-safe, heat transfer, boiling, refrigerant, evaporator, chiller.
Introduction
In chillers with reciprocating, rotary and screw compressors the working substance is refrigerant/oil mixture. Oil concentration can be up to 10% on systems with screw compressors.
The influence of oil on the efficiency of the evaporator has been studied for a long time. The researchers tried to understand the effects of low concentrations of oil in the boiling heat transfer coefficient of refrigerant on smooth, fins surfaces and
more recently in the enhanced surfaces. Boiling heat transfer on tubes is strongly influenced by the presence of even small amounts of oil. All this requires obtaining data on the effect of the concentration of oil in the intensity of the boiling process. Experimental studies of heat transfer on pool boiling refrigerant/oil mixtures were conducted by several authors [1-3]. But there are still not enough, and no systematic studies on boiling heat transfer of HFC/oil mixtures in a wide temperature range. Works devoted to the study of boiling on ozone-
International Scientific Journal for Alternative Energy and Ecology № 01/2 (118) 2013
© Scientific Technical Centre «TATA», 2013
В.Г. Букин, Х.В. Хынг. Влияние геометрических параметров поверхности теплообмена и концентрации масла на теплоотдачу
friendly refrigerants with oil on enhanced tubes of the heat transfer to the present time is very limited.
In a flooded cooler the refrigerant surrounds the tubes in the shell and the water to be cooled flows through the tubes, in which boiling occurs on the surface of the tubes. Evaporator tubes immerse entirely in the boiling refrigerant liquid. Thus, the total heat transfer rate is very high, especially when using enhanced tubes.
Analysis of published data shows that the effect of oil on the heat transfer coefficient during pool boiling on enhanced tubes is not resolved yet. Therefore, the objective of this study is to provide more information about the effect of oil concentration and the related heat transfer performance, measure and compare nucleate boiling heat transfer coefficients (HTCs) of HFC 410A on four different enhanced tubes and integral-fin tube.
Experimental apparatus and test procedures
Since Ref. [4, 5] contains all the details of the test apparatus, specifications and dimension of integral-fin tube and two enhanced tubes (bent fins tube №1,fin gaps spacing sg = 0,25 mm, Y-shaped fins tube №1 sg = 0,25 mm), manufacture of tube specimens, measurements, experimental procedure, they will not be presented again here. In this study bent fins tube №2, sg = 0,50 mm and Y-shaped fins tube №2 sg = 0,50 mm will be added to understand the effect of fin gaps spacing sg on the heat transfer coefficient.
Tubes with bent fins and Y-shaped fins were made from the integral-fins tubes. Reentrant grooves formed by bending fin tips to form narrow cavity opening (bent fins tube). Fins on integral fin deformed to make Y-shaped cross section, resulting in reentrant grooves (Y-shaped fins tube).
Experimental results and discussion
integral-fin tube is significantly less than that of the four enhanced tubes. Enhanced tubes augment nucleate pool boiling heat transfer in two different ways; by increasing the wetted surface area and by modifying the boiling process. Second, from all four enhanced tubes with reentry cavity, the performances of Y-shaped fins tube №1 is the best. Performances of tubes with smaller fin gaps spacing are better than tubes with larger fin gaps spacing. So, the gap spacing has a significant effect on the heat transfer. Third, the effect of the heat flux and pressure on a is weaker for enhanced tubes than for integral-fin tube. This phenomenon can be explained by the fact that: the increase in a can be attributed to the additional convection caused by the sub-surfaces channel of the enhanced tubes. The influence increases heat transfer at low heat fluxes more than at higher, the augmentation of a by convection is (almost) independent of pressure so the effect of the heat flux and pressure on a are weaker for enhanced tubes than for integral-fin tube.
Oil effects
The boiling heat transfer coefficients of the integral-fin tube taken at q = 20000 W/m2 are shown in Fig. 1.
Рис. 1. Влияние концентрации масла на коэффициенты теплоотдачи при кипении интегрально-оребренной трубы при различных температурах для q = 20000 Вт/м Fig. 1. Effect of oil concentrations on boiling heat transfer coefficients of integral-fin tube at various temperatures
for q = 20000 W/m2
Boiling heat transfer performance for pure R410A The pool boiling tests were conducted from 1000 to 20000 W/m2, at saturation temperatures of -20 °C, -5 °C and +5 °C using refrigerants R410A. Over this range, the pool boiling heat transfer coefficient (in W/m2-K) can be expressed as a function of heat flux q (in W/m2) and pressure p (in kPa) as:
for integral-fin tube for bent fins tube №1
а = 10.05-q°,41-pK039, а = 74.27-q0,34pK0'22,
for Y-shaped fins tube №1 а = 80.46-qa34pH022,
for bent fins tube №2
а = 44.66qa37pH0 25,
for Y-shaped fins tube №2 a = 46.48-qa37pH0 25.
The calculated and measured values agree well. The maximum deviations of the calculated data from the measured values were 15%.
From the results we could observe the following features. First, the boiling heat transfer coefficient of the
For oil below 2 percent by weight in the mixture (^oil < 2%), the foaming action of the oil may actually increase the heat transfer coefficient of integral-fin tube. According to Udomboresuwan and Mesler [6] there are some possible reasons. (1) The distance between the wall and the interface of liquid and vapor is drawn closer by the foam; yielding very large heat transfer coefficients. (2) The foam provides convenient conditions of secondary nucleation. (3) The foam seems to remove the lubricant from heating surface. But the enhancement with a small amount of oil added to a refrigerant is very small , so design or performance calculations based on oil-free heat transfer data can be made without serious errors .For further oil addition (^oil > 2%), the heat transfer coefficient of integral-fin tube decreases.
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%).
Международный научный журнал «Альтернативная энергетика и экология» № 01/2 (118) 2013 © Научно-технический центр «TATA», 2013
Термодинамический анализ
Pressure effects The boiling heat transfer coefficients of the integral-fin, bent fins №1, and Y-shaped fins tube №1 for various pressures are shown in Fig. 2.
Fig. 2 shows that the heat transfer degradation by oil decreased as the saturation temperature decreased. The reason was attributed to the decrease of the surface tension and more intense foaming at lower temperatures. In the present test, visualization of the boiling phenomenon is performed to understand the effect of foaming. As we see, for the same oil concentration, the foaming process increases as saturation temperature is decreased.
Рис. 2. Влияние давления на коэффициенты теплоотдачи при кипении разных трубах для q = 5000 Вт/м2, = 10% Fig. 2. Effect of pressure on boiling heat transfer coefficients for various tubes at q = 5000 W/m2, ^ = 10%
The experimental 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 sub-surfaces structure of the enhanced tubes acts to retain the oil-rich mixture.
Conclusions
The following conclusions are drawn from the present study.
It is found that the pool boiling heat transfer coefficient of enhanced tubes are larger than of integral-fin tube. Performances of tubes with smaller fin gaps spacing are better than tubes with larger fin gaps spacing.
The experimental correlations for the oil-free pool boiling heat transfer on enhanced tubes without and on integral-fin tube are developed with the error bands of ±15%, respectively.
At low oil concentration ^oil = 2%, the foaming action of the oil increases heat transfer on integral-fin tubes but cause the degradation in heat transfer coefficient of enhanced tubes.
At larger oil concentration ^oil > 2%, performances of all tubes decrease, with the heat transfer degradation by oil decreased as the saturation temperature decreased.
References
1. Mohrlok K., Spindler K. and Hahne E. The Influence of a Low Viscosity Oil on the Pool Boiling Heat Transfer of the Refrigerant R507 // Int. J. Refrig. 2001. Vol. 24(1). P. 25-40.
2. Webb R.L. and McQuade W.F. Pool Boiling of R11 and R123 Oil-Refrigerant Mixtures on Plain and Enhanced Tube Geometries // ASHRAE Trans. 1993. Vol. 99(1). P. 1225-1236.
3. 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.
4. 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.
5. Букин В.Г., Саид Ахмед эль Саид, Ахмед эль Рефаи Мохаммед Эмам. Результаты экспериментального исследования интенсификации теплообмена при кипении на трубах смесевого хладагента // Вестник АГТУ. 2008. № 2 (43). С. 179-185.
6. Udomboresuwan A., and Mesler R. The Enhancement of Nucleate Boiling by Foam // Proceedings of the Eighth International Heat Transfer Conference. 1986. P. 2939-2944.
Гх?i
- TATA — LXJ
GRIX - GREEN IT EXPO JAPAN-2013 НАЦИОНАЛЬНАЯ ВЫСТАВКА ЭНЕРГОСБЕРЕЖЕНИЯ И ЗАЩИТЫ ОКРУЖАЮЩЕЙ СРЕДЫ
grixtit L J Время проведения: 09 - 10.05.2013 Место проведения: Tokyo International Exhibition Center (Tokyo Big Sight) Организаторы Reed Exhibitions Japan Web: http://www.reedexpo.co.jp E-mail: info@reedexpo.co.jp Официальный веб-сайт http://www.grix-expo.jp/en/
Reed Exhibitions Companies Web: http://www.reedexpo.com E-mail: rxinfo@reedexpo.co.uk
International Scientific Journal for Alternative Energy and Ecology № 01/2 (118) 2013
© Scientific Technical Centre «TATA», 2013
