International Communications in Heat and Mass Transfer

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1 Internationa Communications in Heat and Mass Transfer 36 (2009) Contents ists avaiabe at ScienceDirect Internationa Communications in Heat and Mass Transfer journa homepage: Experimenta investigation of heat transfer coefficient of R134a during condensation in vertica downward fow at high mass fux in a smooth tube A.S. Dakiic a,, S. Laohaertdecha b, S. Wongwises b, a Heat and Thermodynamics Division, Department of Mechanica Engineering, Yidiz Technica University, Yidiz, Besiktas, Istanbu 34349, Turkey b Fuid Mechanics, Therma Engineering and Mutiphase Fow Research Lab. (FUTURE), Department of Mechanica Engineering, King Mongkut's University of Technoogy Thonburi, Bangmod, Bangkok 10140, Thaiand artice info abstract Avaiabe onine 23 Juy 2009 Keywords: Condensation Heat transfer coefficient Downward fow R134a Refrigerant The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentay investigated. The test section is a 0.5 m ong doube tube with refrigerant fowing in the inner tube and cooing water fowing in the annuus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between C. The mass fuxes are between 260 and 515 kg m 2 s 1 and the heat fuxes are between 11.3 and 55.3 kw m 2. The quaity of the refrigerant in the test section is cacuated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by appying an energy baance based on the energy transferred from the test section. The effects of heat fux, mass fux and condensation temperature on the heat transfer coefficients are aso discussed. Eeven we-known correations for annuar fow are compared to each other using a arge amount of data obtained from various experimenta conditions. A new correation for the condensation heat transfer coefficient is proposed for practica appications Esevier Ltd. A rights reserved. 1. Introduction Heat exchangers are devices that are commony used to transfer heat between two or more fuids of different temperatures. They are used in a wide variety of appications, e.g., refrigeration and air-conditioning systems, power engineering and other therma processing pants. In a refrigeration equipment, condenser coos and condenses the refrigerant vapor discharged from a compressor by means of a secondary heat transfer fuid such as air and fuid. The heat transfer faiure occurs in the condenser due to design error, effectiveness of condenser shoud be examined and improved carefuy. Since the depetion of the earth's ozone ayer has been discovered, many corporations have been forced to find aternative chemicas to CFCs. Because the thermo-physica properties of HFC-134a are very simiar to those of CFC-12. Refrigerant HFC-134a has received intensive support from the refrigerant and air-conditioning industry as a potentia repacement for CFC-12. However, even though the difference in properties between the two refrigerants is sma, it may resut in significant differences in the overa system performance. Therefore, the properties of HFC-134a shoud be studied in detai before it is appied. Communicated by W.J. Minkowycz. Corresponding authors. E-mai addresses: [email protected] (A.S. Dakiic), [email protected] (S. Wongwises). Heat transfer and pressure drop characteristics of refrigerants have been studied by a arge number of researchers, both experimentay and anayticay, mosty in a horizonta straight tube. The study of the heat transfer and pressure drop of CFCs inside a sma diameter vertica tube for downward condensation has received comparativey itte attention in the iterature. In addition to this, there are few studies on condensation of R134a during downward fow in vertica micro-fin tubes. Briggs et a. [1,2] have used arge diameter tubes of approximatey 20.8 mm with CFC113. Shah correation [3] has been compared by researchers commony for turbuent condensation conditions and is considered to be the most predictive condensation mode for the annuar fow regime in a tube. Up to 40 mm i.d. smooth horizonta, incined and vertica tubes were used in his experiments. Oh and Revankar [4] performed theoretica cacuations using modified Nusset theory [5] for the PCCS condenser, which has 47.5 mm i.d. and 1.8 m ong vertica tube and vaidated by experimenta study. Maheshwari et a. [6] studied on the downward condensation presence of non-condensabe gas in a mm i.d. vertica tube which simuates PCCS condensers used in water-cooed reactors preparing a computer code to predict heat transfer considering mass transfer aong the tube ength. Annuar fow conditions aong the tube ength incude convective condensation which occurs for many appications inside tubes. Annuar two-phase fow is one of the most important fow regimes and is characterized by a phase interface separating a thin iquid fim from the gas fow in the core region. Two-phase annuar fow occurs widey in fim heating and cooing processes, particuary in power /$ see front matter 2009 Esevier Ltd. A rights reserved. doi: /j.icheatmasstransfer

2 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) Nomencature A surface area, m 2 CFC coro fuoro carbon Co parameter in Shah correation d interna tube diameter, m F parameter in Travis et a.'s correation Fr Froude number G mass fux of R134a, kg m 2 s 1 Ga Gaieo number g gravitationa constant, m s 2 h convective heat transfer coefficient, W m 2 K 1 HFC hydro four carbon Ja Jakob number k therma conductivity, W m 1 K 1 ength of test tube, m L characteristic ength, m m mass fow rate, kg s 1 Nu Nusset number P pressure, Pa PCCS passive containment cooing system Pr Prandt number Re Reynods number T temperature Q heat transfer rate, W x mean vapor quaity X Lockhart-Martinei parameter Greek symbos ΔT vapor side temperature difference, T ref,sat T wi,k ρ density, kg m 3 δ fim thickness, m δ dimensioness fim thickness μ dynamic viscosity, kg m 1 s 1 ν kinematic viscosity, m 2 s 1 Subscripts corr correation eq equivaent exp experimenta g gas/vapor iquid red reduced ref refrigerant sat saturation sf superficia so Soiman TS test section wi inner wa annuar fow regime which can be repaced by an equivaent a iquid fow. According to this mode, the equivaent a iquid fow produces the same wa shear stress as that of the two phase fow. There are many research studied on this mode, such as Moser et a. [9] and Ma and Rose [10]. Moser [9] deveoped a mode to predict heat transfer coefficient in horizonta conventiona tubes. Ma and Rose [10] investigated heat transfer and pressure drop characteristics of R113 in a 20.8 i.d. vertica smooth and enhanced tubes. Dobson et a. [11] investigated condensation of zeotropic refrigerants over the wide range of mass fux in horizonta tubes. He stated that heat transfer coefficient has increased with mass fux and quaity in annuar fow due to increased shear and thinner iquid fim than other fow regimes. They used a two-phase mutipier approach for annuar fow. Sweeney [12] extended their mode for R407C, using mass fux based modification. Cavaini et a. [13] presented a theoretica anaysis of the condensation process and a critica review of a number of correations for predicting the heat transfer coefficients and pressure drops for refrigerants condensing inside various commerciay manufactured tubes with enhanced surfaces. Recenty, Cavaini et a. [14] reviewed the most recent work in open iterature on the condensation inside and outside smooth and enhanced tubes. Vaadares [15] reviewed in-tube condensation heat transfer correations for smooth and micro-fin tubes and evauated with experimenta data from different researchers considering various experimenta conditions. Wang and Honda [16] compared we-known heat transfer modes with experimenta data beong to various refrigerants from iterature and offered modified annuar and stratified fow modes for micro-fin tubes. Bassi and Bansa [17] presented comparative study of various empirica correations and deveoped two new empirica modes to determine condensation heat transfer coefficients in a smooth tube using R134a with a ubricant oi. Jung et a. [18,19] condensed many refrigerants such as R12, R22, R32, R123, R125, R134a, and R142b inside a pain tube and evauated the experimenta data not ony comparing with various we-known correations but aso proposing a new correation to predict condensation heat transfer coefficients. To the best of the authors' knowedge, there has been insufficient work deaing with condensation heat transfer of HFC-134a in sma diameter tubes during downward fow. However, athough some information is currenty avaiabe, there sti remains room for further research. It can be especiay noted that in a of the experimenta investigations, there are none that are concerned with study in the high mass fux two-phase fow region. Moreover, it shoud aso be noted that the reported mass fuxes, heat fuxes, condensation pressures and dimensions of the test tube do not incude studied parameters apart from authors previous pubications [20 26]. As a consequence, in the present study the main concern is to extend the existing heat transfer data to the high mass fux region of the refrigerant during condensation in a smooth tube-in-tube heat exchanger. A arge amount of coected data is correated and used to predict the heat transfer coefficient of the HFC-134a. The independency of annuar fow heat transfer empirica correations from tube orientation [27] and the genera appicabiity for a vertica short tube was aso shown in the paper. 2. Experimenta apparatus and method generation and especiay in nucear reactors. This fow regime has received the most attention, both anayticay and experimentay, because of its practica importance and the reative ease with which anaytica treatment may be appied. Akers [7] deveoped a two-phase mutipier based correation which assumes that two phase fows are simiar to singe phase fow. His correation predicts frictiona two phase pressure drop by the means of a mutipying factor, which is same rationae as the Lockhart Martinei [8] two-phase mutipier. His mode is known as equivaent Reynods number mode in the iterature. It is used for an Detaied descriptions of the experimenta apparatus for studying condensation of R134a inside a vertica tube can be found in the authors' previous pubications. 3. Data reduction 3.1. Experimenta heat transfer coefficient The data reduction of the measured resuts such as inet and outet vapor quaity can be seen from the authors' previous pubications

3 1038 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) and experimenta heat transfer coefficient can be cacuated as foows: h exp = A wi Q TS ð1þ T ref;sat T wi where h exp is the experimenta average heat transfer coefficient, Q TS is the heat transfer rate in the test section, T wi is the average temperature of the inner wa, T ref,sat is the average temperature of the refrigerant at the test section inet and outet, and A wi is the inside surface area of the test section: A wi = πd where d is the inside diameter of the test tube. is the ength of the test tube Uncertainties The uncertainties of the Nusset number and condensation heat transfer coefficient in the test tube varied from ±7.64% to ±10.71%. The procedures of Kine and McCintock [28] were used for the cacuation of a uncertainties. Various uncertainty vaues of the study can be seen from authors' previous work [21] Heat transfer correations Shah correation The Shah correation [3] has been compared by researchers commony for turbuent condensation conditions especiay in vertica tubes [29] and is considered to be the most comparative condensation mode during annuar fow regime in a tube [30]. The Shah correation [3], which is based on the iquid heat transfer coefficient (Eq. (6)) and is vaid for Re 1 350, was used for the comparison of experimenta data. It has a two-phase mutipier for annuar fow regime of high pressure steam and refrigerants [29,30]. 1:8 h shah = h sf Co 0:8 where the Co parameter is determined from: Co = 1 0:8 x 1 ρ 0:5 g ð4þ ρ where the superficia heat transfer coefficient of the iquid phase is given as foows: h sf = h ð1 xþ 0:8 ð5þ where the iquid heat transfer coefficient is described as foows: h = k d 0:023 Re 0:8 Pr 0:4 1 x reduced pressure P red =P sat /P critic Re = Gdð1 xþ μ Dobson and Chato mode Dobson and Chato [11] deveoped a correation (Eq. (8)) using a twophase mutipier for an annuar fow regime. They have aso provided a correation for a wavy fow regime. Their correations are commony ð2þ ð3þ ð6þ ð7þ used in the iterature for zeotropic refrigerants. It is recommended for GN500 kg m 2 s 1 for a quaities in horizonta tubes. Nu =0:023Re 0:8 Pr 0:4 1+ 2:22 ð8þ X 0:89 where X = 1 x 0:9! ρ 0:5 0:1 g μ ð9þ x ρ μ g Re has been defined in Eq. (7) and Fr so has been defined in Eq. (17). For Re N1250 and Fr so N18 Nu =0:023Re 0:8 2:61 Pr 0:3 X 0:805 ð10þ Chato correation Chato [31] deveoped Dittus Boeter's correation [32] using a twophase mutipier for an annuar fow regime: Nu =0:023Re 0:8 Pr 0:4 2:47 X 1:96 ð11þ Sweeney correation Sweeney [12] provided a modification of the Dobson and Chato mode [11] (Eq. (8)). He studied zeotropic mixtures for annuar fow. He has aso offered a correation for wavy fow: G 0:3 Nu =0:7 Nu 300 Dobson; Chato ð12þ Cavaini et a. correation Cavaini et a. [13] deveoped a semi empirica correation for the condensation of various organic refrigerants in both vertica and horizonta orientations. It can be cacuated as foows: Nu =0:05Re 0:8 eq Pr 0:33 ð13þ Equivaent Reynods number can be evauated as foows: 0:5Re Re eq = Re g μ g = μ ρ =ρ g ð14þ Re has been defined in Eq. (7) Fujii correation Fujii [33] deveoped the foowing correation for smooth tubes. He has aso offered a correation for gravity controed regimes. Eq. (15) is used for shear-controed regimes: qffiffiffiffiffiffiffiffiffiffiffiffi 0:9 x 0:1x +0:8Pr 0:63 Nu =0:0125 Re ρ =ρ g 1 x ð15þ Tang et a. correation Tang et a. [34] modified the Shah [3] equation for annuar fow condensation of R410A, R134a and R22 in 8.81 mm i.d. tube with Fr so N7 as foows: Nu =0:023Re 0:8 Pr 0:4 x 0: :863 nðp red Þ ð16þ 1 x The imit of his equation is defined by a modified Froude number as foows: Fr so = c 3 Re c 4 1+1:09X 0:039 X! 1:5 1 Ga 0:5 ð17þ

4 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) Re V1250 c 3 =0:025 c 4 =1:59 Re N 1250 c 3 =1:26 c 4 =1:04 Gaieo number Ga = ρ ρ g gd 3 ρ μ 2 ð18þ Tandon et a. correation Tandon et a. [36] have modified the Akers and Rosson [37] correation for shear controed annuar and semi-annuar fows with Re g N30000 as foows: Nu =0:084Re 0:67 g Pr 1 = = 6 ð21þ Ja X has been defined in Eq. (9) Bivens and Yokozeki correation A modified Shah correation [3] was deveoped for various fow patterns of R22, R502, R32/R134a, R32/R125/R134a by Bivens and Yokozeki [35] as foows: Nu = Nu Shah 0: :89 G 2 where " # Nu Shah =0:023Re 0:8 Pr 0:4 1+ 3:8 x 0:76 P 0:38 1 x red ð19þ ð20þ There is another correation for gravity-controed wavy fows with Re g b30000 which beongs to Tandon et a. [36] Traviss et a. correation Traviss et a. [38] investigated fow regime maps for condensation inside tubes. Their correation was suggested for the condensation of R134a inside tubes specificay. Variation in the quaity of the refrigerant were considered using Lockhart Martinei parameter: Nu = Re 0:9 F Pr 1 ðxþ F 2 ðre ; Pr Þ where F 1 ðxþ =0:15 1 X + 2:83 X 0:476 ð22þ ð23þ Fig. 1. Experimenta condensation heat transfer coefficient vs. average vapor quaity for G=260 kg m 2 s 1 and T sat =40 C (a) and T sat =50 C (b). Fig. 2. Experimenta condensation heat transfer coefficient vs. average vapor quaity for G=515 kg m 2 s 1 and T sat =40 C (a) and T sat =50 C (b).

5 1040 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) F 2 =5Pr +5nð1+5Pr Þ +2:5n 0:00313Re 0:812 for Re N 1125 ð24þ X has been defined in Eq. (9) and Re has been defined in Eq. (7) Akers and Rosson correation Akers and Rosson [37] modified the Dittus Boeter [32] singe-phase forced convection correation. Their correation was recommended for turbuent annuar fow in sma diameter circuar tubes and rectanguar channes: Nu =0:0265Re 0:8 eq Pr 1 = 3 where Re eq = G eqd μ ð25þ ð26þ aminar and turbuent fows under the our experimenta conditions. Vaidation process was performed with five different mass fuxes and two different condensing temperatures. Comparison between experimenta and cacuated heat transfer coefficients are shown in a 20% deviation ine in Fig. 7 to give a genera correation for the tested various experimenta conditions. The proposed correation is shown in Eq. (28). for 8:1mm i:d: smooth copper tube δ4 =1:657:Re 0:07 1 ð28þ Liquid heat transfer coefficient h 1 = ðk 1 = LÞ: ð1 = δ4þ ð29þ Liquid Reynos number Re = m = ðπ:d:μ Þ ð30þ Liquid Nusset number Nu =1= δ4 ð31þ Dimensioness fim thickness δ4 = δ = L ð32þ Characteristic ength L = m 2 1 = 3 =g ð33þ 0:5 G eq = G ð1 xþ + x ρ =ρ g ð27þ Nusset number Nu = h :L = k ð34þ 4. Resuts and discussion Correation deveopment concerning heat transfer coefficient Beinghausen and Renz's correation [39] was modified to improve the predictabiity with the 344 smooth tube data points incuding Based on Hewitt and Robertson's [40] fow pattern map, the data shown in a figures were coected in an annuar fow regime and aso checked by sight gass at the inet and outet of the test section. In Fig. 3. Effect of condensation temperatures of R134a on experimenta heat transfer coefficients for x i =1 and G=260 kg m 2 s 1 (a) and G=515 kg m 2 s 1 (b). Fig. 4. Effect of mass fuxes of R134a on experimenta heat transfer coefficients for various temperature differences at x i =1 and T sat =40 C (a) and T sat =50 C (b).

6 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) order to obtain annuar fow conditions at various high mass fuxes of R134a, vapor quaity range is kept approximatey between in the 0.5 m ong test tube. In the present study, these correations are used to show the simiarity of annuar fow correations which are independent of tube orientation (horizonta or vertica). Chen et a. [26] aso mentioned on this simiarity in their artice. He deveoped a genera correation by reating the interfacia shear stress to fow conditions for annuar fim condensation inside tubes. Figs. 1 and 2 show the variation of the average condensation heat transfer coefficient of R134a with average quaity in the 8.1 mm i.d. smooth copper tube at saturation temperatures of 40 (a) 50 (b) C and mass fuxes of 260 and 515 kg m 2 s 1, respectivey. During the condensation, the iquid fim thickness graduay increases the therma resistance is therefore increased, which resuts in a decrease in the heat transfer rate. According to the comparison of the heat transfer coefficient at higher and ower average vapor quaity, smaer iquid fim thickness together with higher vapor veocity at the vapor iquid interface resut in an increase of the heat transfer coefficient when the average quaity is higher. The resut is that the average heat transfer coefficient increases with increasing average quaity. Fig. 3 shows the variation of the average condensation heat transfer coefficient with condensation temperature difference at saturation temperatures of C and mass fuxes of 260 (a) and 515 kg m 2 s 1 (b), respectivey. It shoud be noted that R134a was entering to the test section as pure saturated vapor in these figures. The fim thickness increases with increasing condensation temperature difference, in other words, heat fux in the tube. For this reason, the average condensation heat transfer coefficient of R134a decreases with increasing condensation temperature difference reated to the Eq. (1). These figures aso show that the average condensation heat transfer coefficient of R134a decreases with increasing condensation temperature. This is due to ateration of the physica properties of R134a by condensation temperature, in other words, condensation pressure. Fig. 4 shows the effect of mass fux on the average condensation heat transfer coefficient with temperature difference of pure R-134a during condensation in the test tube at saturation temperatures of 40 (a) 50 (b) C for a mass fuxes of 260, 300, 340, 400, 456 and 515 kg m 2 s 1. This figure shows that the heat transfer coefficient decreases with decreasing mass fux. This is due to the increase of vapor veocity of R134a. In addition to this, these resuts aso show that the average condensation heat transfer coefficient of R134a decreases with increasing condensation temperature difference, in the same way as in Figs.1 3. According to the condensation temperatures of 40 and 50 C, comparison of experimenta heat transfer coefficients with various annuar fow correations are shown in Figs. 5 and 6 in a 30% deviation ine, in addition to those in Tabe 1 for the minimum mass fux of 260 kg m 2 s 1 and maximum mass fux of 515 kg m 2 s 1.Itiscearyseen from Tabe 1 that the Dobson and Chato [11] correation (Eq. (8)), Cavaini et a. [13] correation (Eq. (13)), Fujii [33] correation (Eq. (15)) have the best predictabiity of experimenta data. In addition to this, the Fig. 5. Comparison of experimenta condensation heat transfer coefficient vs. various correations for G =260 kg m 2 s 1 and T sat =40 C (a) and T sat =50 C (b). Fig. 6. Comparison of experimenta condensation heat transfer coefficient vs. various correations for G = 515 kg m 2 s 1 and T sat =40 C (a) and T sat =50 C (b).

7 1042 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) Tabe 1 Comparison of heat transfer equations for the mass fuxes of 260 and 515 kg m 2 s 1. Correation Deviation G (kg m 2 s 1 ) T sat ( C) Agreed data ratio Predictabiity % Correation Deviation G (kg m 2 s 1 ) T sat ( C) Agreed data ratio Shah [3] (Eq. (3)) ± % / Fujii [21] (Eq. (15)) ± % / / / / / / / Dobson and Chato [11] (Eq. (8)) Dobson and Chato [11] (Eq. (10)) Predictabiity % ± % / Tang et a. [34] (Eq. (16)) ± % / / / / / / / ± % / Bivens and Yokozeki [35] ± % / / (Eq. (19)) / / / / / Chato [31] (Eq. (11)) ± % / Tandon et a. [36] (Eq. (21)) ± % / / / / / / /34 0 Sweeney [11] (Eq. (12)) ± % / Traviss et a. [38] (Eq. (22)) ± % / / / / / / / Cavaini et a. [13] ± % / Akers and Rosson [37] ± % /28 0 (Eq. (13)) / (Eq. (25)) / / / / /34 0 majority of the data cacuated by Shah [3] correation (Eq. (3)), Dobson and Chato [11] correation (Eq. (10)), Tang et a. [34] correation (Eq. (16)), Traviss et a. [38] correation (Eq. (22)) fa within ±30%. On the other hand, The Tandon et a. [36] correation (Eq. (21)) and Akers and Rosson [37] correation (Eq. (25)) are incompatibe with the experimenta data. Nevertheess, the Chato [30] correation (Eq. (11)), the Sweeny [12] correation (Eq. (12)), and the Bivens and Yokozeki [35] correation (Eq. (19)) have poor agreement with experimenta data. There are some correations for gravity-controed regimes in the iterature such as Fujii [33] and Tandon et a. [36] correations. In this study, these kind of correations are not found in good agreement with the data as expected for annuar fow regime. Due to their arge deviations and vaidity for wavy fow, they are not exist in the paper. The vapor shear stress acting on the interface of vapor iquid phases affects the forced convective condensation inside tubes especiay at high vapor fow rates [27]. For that reason, shear-controed correations of Fujii [33] and Tandon et a. [36] were used to predict condensation heat transfer coefficient of R134a. In addition to this, simiar resuts on these expanations in this part of the study have been obtained for condensation of various refrigerants in horizonta tubes by Vaadares [15]. Comparison of the proposed condensation heat transfer coefficient correation (Eq. (28)) and experimenta heat transfer coefficient (Eq. (1)) is show in Fig. 7. ItcanbeseenfromFig. 7 that the majority of the measured data fas within ±20% by the means of proposed correation (Eq. (29))fortheamassfuxes (260, 300, 340, 400, 456, 515 kg m 2 s 1 ) and saturation temperatures (40 C and 50 C) of R134a. A arge number of graphics coud be generated from the output of the cacuations, however, due to space imitation, ony typica resuts are shown. It shoud be aso noted that detaied information on the expanations above and some additiona figures with different experimenta parameters reated to this study can be seen from the authors previous pubications [20 26]. the iterature. For this reason, resuts of this study are expected to fi a gap in the iterature. The foowing resuts were obtained: Firsty, many figures were given for the vaidation of data in the paper. It can be ceary seen from Figs. 1 4 that experimenta heat transfer coefficient increases with the average vapor quaity of R134a and the mass fux of R134a. On the other hand, it decreases with increase of condensation temperature difference (T sat T wi ). Effects of condensation temperatures (40 C and 50 C) on experimenta heat transfer coefficients for the various mass fuxes are shown in Fig. 3. It is ceary seen from the figures that condensation heat transfer coefficients at 40 C are found to be higher than at 50 C. Secondy, various annuar fow heat transfer correations were chosen to compare with experimenta heat transfer coefficients. The comparisons are shown in Figs. 5 and 6 with ±30% deviation ine and in Tabe 1. It can be noted that the Dobson and Chato [11] correation (Eq. (8)), the Cavaini et a. [13] correation (Eq. (13)), the Fujii [33] correation (Eq. (15)) give better resuts than others in an 8.1 mm i.d. 5. Concusion Average heat transfer coefficient of R134a was investigated during condensation in vertica downward fow at high mass fux in a smooth tube. The accurate and repeatabe heat transfer data during annuar fow are obtained. There are few research on the parameters of such cases in Fig. 7. Comparison of experimenta and cacuated (Eq. (29)) heat transfer coefficients for a mass fux of R134a at T sat =40 50 C.

8 A.S. Dakiic et a. / Internationa Communications in Heat and Mass Transfer 36 (2009) copper tube for the mass fuxes of 260, 300, 340, 400, 456, 515 kg m 2 s 1 and condensation temperatures of 40 and 50 C. According to the anaysis in this paper, it is shown that annuar fow modes are independent of tube orientation provided that annuar fow regime exists aong the tube ength and capabe of predicting condensation heat transfer coefficients inside the test tube. Finay, a new heat transfer correation was proposed in Fig. 7 with ±20% deviation ine according to Beinghausen and Renz's method [39] for practica appications by the means of arge experimenta data points. Acknowedgements The authors are indebted to the Department of Mechanica Engineering, King Mongkut's University of Technoogy Thonburi (KMUTT) and the Thaiand Research Fund (TRF) for supporting this study. Especiay, the first author wishes to thank KMUTT for providing him with a Post-doctora feowship. References [1] A. Briggs, C. 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