Effect of electrohydrodynamic (EHD) on condensation of R-134a in presence of non-condensable gas

International Communications in Heat and Mass Transfer 36 (2009) 286–291

Contents lists available at ScienceDirect

International Communications in Heat and Mass Transfer

j ourna l homepage: www.e lsev ie r.com/ locate / ichmt

Effect of electrohydrodynamic (EHD) on condensation of R-134a in presence ofnon-condensable gas☆

Hamid Omidvarborna, Arjomand Mehrabani-Zeinabad ⁎, Mohsen Nasr EsfahanyDepartment of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

☆ Communicated by W.J. Minkowycz.⁎ Corresponding author.

E-mail address: [email protected] (A. Mehraban

0735-1933/$ – see front matter © 2008 Elsevier Ltd. Aldoi:10.1016/j.icheatmasstransfer.2008.10.014

a b s t r a c t

Available online 17 December 2008

Keywords:

Effects of applying EHD andtubular condensation of refrcoefficient (CHTC), but pres

Condensation heat transferNon-condensable gasHeat transfer coefficientElectrohydrodynamicHeat transfer enhancementHeat exchanger

ence of NC gas in condensing vapour reduces this coefficient. In competition ofthese two effective parameters on condensation, it can be observed that at higher concentration of NC gas, theeffect of electrical field on enhancement of CHTC is greatly reduced. But at lower concentration of NC gas, theeffect of electrical field is more considerable, due to thickness of heat transfer boundary layer.

© 2008 Elsevier Ltd. All rights reserved.

non-condensable (NC) gas contents have been experimentally studied on inter-igerant R-134a flow. Applying of electrical field enhances condensing heat transfer

1. Introduction

Enhancing of heat transfer coefficient is an interesting area for bothindustry and academia. Achieving higher heat transfer rates throughvarious enhancement techniques results in protection of our environ-ment. This is done through substantial energy saving, due to bothincreasing of equipment performance, and designing of smaller systemsto meet required loads.

The beneficial results of heat transfer enhancement by increasingof heat transfer coefficient are:

(a) reduction in size of heat exchangers for specified thermal rates,(b) reduction in temperature difference between the fluids

exchanging heat and thus greater thermal plant efficiency.Alternatively, transfer of greater energy rates through a givensize of heat exchanger while maintaining moderate tempera-ture differences [1].

There are various techniques for heat transfer enhancement. Thesecan be categorized in two groups of active and passive techniques.While the active techniques require application of external power tothe heat transfer surface, such as surface vibration, acoustic or electricfields, passive techniques are based on application of specific surfacegeometries with surface augmentation. The effectiveness of bothtechniques is strongly depends on the mode of heat transfer, singlephase or multiphase.

In the last three decades researchers around world have beenperformed significant works on application of electrohydrodynamic

i-Zeinabad).

l rights reserved.

(EHD), as an active technique for enhancing heat and mass transfer,with a focus on industrial applications, especially for evaporators andcondensers. Advantages of EHD enhancement are:

(a) rapid and smart control of enhancement by varying of appliedelectric field.

(b) Non-mechanical and simple design.(c) Suitable for special environments (space).(d) Applicable to single-phase and multiphase flows.(e) Minimal power consumption [2].

As phase change phenomena of boiling and condensation are veryimportant mode in heat transfer, improvement on enhancing heattransfer in both evaporators and condensers are highly required. Thestudy on the effects of EHD on condensation has begun more than30 years ago. The physical mechanisms occurring during theapplication of EHD on condensation heat transfer enhancement are:

(a) thinning of the condensate film by stripping the liquid form thecondensation surface.

(b) The change of film condensation to pseudo-dropwisecondensation.

(c) Dispersion of the condensate using electrostatic atomization.(d) Disturbing the accumulation of NC gases at the liquid vapour

interface.(e) Introducing of perturbations and waviness into the condensate

film.(f) Are highly practical for various applications [3].

This can lead to a higher heat transfer coefficients that are severaltimes of those achieved by conventional enhancement techniques.

Condensation heat transfer rates reduce in presence of NC gassesor inert gases in the condensing vapour–gas mixture. In the mixtures,

Table 1Properties of R-134a at 25 °C

Boiling point at 1 atm (°C) −26.1Liquid density (kg/m3) 1207Molecular weight 102.03Vapour pressure (MPa) 0.665Ozone depletion potential 0% Volatiles 100

Fig. 1. Schematic of the closed loop of the experimental rig.

Nomenclature

Cg [-] Relative volume concentration of component,(Pmix-P)/Pmix

Cp [J/kg.K] Specific heat of liquidD [m] DiameterE [V/m] Applied voltageH [W/m2.K] Condensation heat transfer coefficientL [m] Lengthm [kg/sec] Mass flow rateP [kpa] Pressureq [C/m3] Electric field space charge densityQ [W] Heat transfer rateS [m2] Heat transfer surfaceT [K] Temperature

Greek Symbolsε [F/m] Electric permittivity of liquidρ [kg/m3] Density

Subscriptsg Gasi Inmix Vapour-gas mixtureo Outsat SaturateSavg Average wallw Wall

287H. Omidvarborna et al. / International Communications in Heat and Mass Transfer 36 (2009) 286–291

the condensable component is called vapour and the NC component isnamed gas. Gases, other than the refrigerant, are a form of contami-nation that is frequently found in refrigerating systems. Sources ofthese gases are:

(a) incomplete evacuation.(b) When functional materials release absorbed gases or decom-

pose to form gases at an elevated temperature during systemoperation.

(c) Through low-side leaks.(d) From chemical reaction during system operation [4].

These gases, which do not liquefy in condensers, reduce coolingefficiency. The quantity of inert, NC, gas depends on the design, size ofthe refrigerating unit, and nature of the refrigerant.

2. Background

There have been several studies on EHD heat transfer enhancementduring condensation and detailed reviews are given by Laohalertdechaet al. [3], Allen andKarayiannis [1], and Seyed-Yagoobi andBryan [2]. Themajority of the early studies on EHD condensation heat transfer havefocused on the effect of applying a high voltage electric field on freefalling condensate liquid films on vertical plates and tubes. Velkoffand Miller [5], Choi [6], Didkovsky and Bologa [7], Bologa et al.[8],Yabe et al. [9] Yamashita et al. [10], Wawzyniak and Seyed-yagoobi[11], Yamashita and Yabe [12], and Chung et al. [13] investigated EHDaugmented condensation on vertical surfaces. There has beenrelatively less investigation on the effect of EHD on falling liquidfilms during condensation on horizontal or inclined heat transfersurfaces, Brand and Seyed-Yagoobi [14], Sadek et al. [15], Laohalert-decha and Wongwises [16]. Holmes and Chapman [17], Copper and

Allen [18], Xu et al. [19], Chung et al. [13] and Ohadi et al. [20], amongothers, have contributed to knowledge on area of EHD enhanced heattransfer.

The more recent work of Singh [21], Gidwani et al.[22], and Fengand Seyed-Yagoobi [23], investigated the effect of EHD on in-tubecondensation heat transfer. Investigations undertaken by thepresent authors and those described in before proved the advisa-bility of the use of EHD effect on film condensation heat transfer inthe case of pure vapour. But only a few papers to cover the effect ofboth NC gas, and EHD on condensation were published. Seth et al.[24] found that the presence of NC gases in the bulk Freon vapour inpresence and absence of electric field decreases the film condensa-tion heat transfer in a horizontal tube. Bologa et al. [8] presentedexperimental results of heat transfer in film condensation of avapour–gas mixture on a vertical plate condenser under theinfluence of an electric field. They discussed the effects of NC gasconcentration in the vapour, media pressure, temperature differencebetween the vapour–gas mixture and the wall, applied potentials,electric current strength, physical properties of media on heattransfer enhancement.

3. EHD mechanism

The physical basis of the electrically enhanced condensation isdue to existence of the EHD force generated by application of anelectric field. Its value and involved parameters can be shown byEq. (1):

fe = qE −12E2rɛ +

12r ρE2

@ ɛ

@ ρ

� �T

� �ð1Þ

The first term on the RHS of the equation is the coulomb orelectrophoretic force. This is due to existence of the net free chargeswithin the fluid. The second and the third terms are, dielectrophore-tic, and electrostrictive forces that contribute substantially in phasechange processes such as boiling and condensation. The amounts ofthese terms are directly proportional to square power of the appliedelectric field. The value of the second term is a measure of the

Fig. 2. Schematic of the attached thermocouples' locations in test section.

Table 2Properties of the test section

Inner-tubeMaterial CopperEffective length (m) 1ID (mm) 25.4Thickness (mm) 3

ElectrodeMaterial BrassLength (m) 1Diameter (mm) 4 and 12Position Central

288 H. Omidvarborna et al. / International Communications in Heat and Mass Transfer 36 (2009) 286–291

difference in permittivity of the fluid and gas phases,∇ε. Therefore,for higher density or permittivity difference between two phases,the effect of electric field is greater. The third term represents theacting force due to heterogeneity of the electric field within the fluid[24].

4. Apparatus and experimental procedures

Experimental apparatus is mainly composed of two sections, acondensation heat transfer loop, and a high voltage power supply. It isdesigned tomeasure the heat transfer coefficient of R-134a vapour in atest tube in two various orientations, horizontal and vertical. Thephysical properties of the fluid at 25 °C are given in Table 1.

As shown in Fig. 1, the major components of the test facility areconsisting of: main condenser, final condensers, evaporator, com-pressor, hot water loop, cooling water supply, sight glass, electrode,and a connection to R-134a gas source. Fig. 2, shows the schematic of adouble pipe heat exchanger with a concentric internal electrode, andlocations of attached thermocouples are shown in the figure.

In the refrigeration loop, the liquid refrigerant after passingthrough a rotameter for measurement of its flow rate was vaporizedby passing through the double pipe evaporator. The moving liquidin its internal tube absorbs thermal energy from a circulated hotwater passing through annulus section of the evaporator. The cir-culated hot water was produced by a heater and an inline centr-ifugal pump. The vaporized R-134a liquid was passed through asight glass for showing and ensuring its quality, a single phase flow.Then the vaporized refrigerant was passed through a compressor inorder to increase its pressure and consequently its temperature.Furthermore, the vapour was flowed through the test section andthe second condenser. The main objective of the condenser afterthe test section is total condensation of the vapour. In the con-denser, an open loop flow of cooling water takes the vapour thermalenergy.

The test section is a horizontal double pipe countercurrent flowheat exchanger. Surface temperatures of the test section tube aremeasured by six thermocouples along its axial direction. These mea-surements are used to estimate average heat transfer coefficient onthe refrigerant side of the condenser. A specially calibrated rotameterfor R-134a is used to measure the refrigerant flow rate. In the allexperiments a concentric cylindrical electrode, is placed inside ofinternal tube. It is fixed at both ends of the test section by polymericspacer insulator.

Condensation heat transfer coefficient (CHTC) in the internal tubeof the test section can be approximated by

h =Q

S Tsat−TSavg� � ð2Þ

Where

Q =:mCp To−Tið Þ ð3Þ

The arithmetic average of the internal tube surface temperature,was obtained according to

TSavg =16

∑6

i = 1TSi ð4Þ

inner surface area of the tube is:

S = πDL ð5Þ

Table 2 shows specifications of the test section condenser.

5. Results

In this experiment, enhancement of the heat transfer CHTC basedon application of EHD is introduced. The phenomenon of EHD resultsin waves on the vapour–liquid interface and reduces the condensatethickness.

Results are presented in three parts. In the first part, obtainedresults from the system under effects of applied EHD or presence of

Fig. 3. Effect of applied EHD and electrode diameter on CHTC, in absence of NC gas.

Fig. 4. Effect of NC gas concentration and electrode diameter on CHTC, in absence ofEHD.

Fig. 6. Effect of NC gas and electrode diameter on CHTC, applied voltage=8 kV.

Fig. 7. Effect of NC gas concentration on temperature difference at various sections ofthe condensers, in absence of EHD.

289H. Omidvarborna et al. / International Communications in Heat and Mass Transfer 36 (2009) 286–291

NC gas are shown. In the second part, results from both, applyingelectrical field, and effect of NC gas are presented, and in the last part,effect of EHD and NC gas on temperature gradient are investigated.

In results, notation of Cg% is used to represent relative volumetricconcentration of gas in mixture and it can be shown as:

Cg% = Pmix− Pð Þ=Pmix ð6Þ

Uncertainty of experimental results can be estimated by carefulspecifications and analysis of involved measurements. Maximumuncertainties of experimental results are estimatedwithin range of ±8%.

5.1. Effects of applied electrical field/presence of NC gas on condensation

In presentation of results a parameter namely, heat transfer ratio,h/ho,is defined.Where, h is the average of heat transfer coefficient in presenceof EHD, NC, or both of them, and ho is the one in absence of theseaffecting condensation parameters, but in presence of electrodes.

Fig. 5. Effect of NC gas and electrode diameter on CHTC, applied voltage=4 kV.

Enhancement heat transfer coefficient for two applied voltages of4 kV and 8 kV are compared with the corresponding results withoutapplying electrical field in Fig. 3. As shown, a maximum enhancementratio of 85% is experienced for the 12 mm diameter electrode, whilethe maximum enhancement ratio is 10% for the 4 mm diameterelectrode. The enhancements are mainly due to the effect of EHD onliquid extraction, the second and third terms on RHS Eq. (1). Theaction of applying EHD force is destabilization of the formedcondensate layer and can convert film condensation to pseudo-dropwise condensation which reduces thermal resistance. Resultsshow that increasing in applied voltage leads to a higher CHTC. Butthere is an upper limit on applying voltage which is due to ionizing ofthe refrigerant. Ionizing of molecules in not desired as it may changethe property of refrigerant.

Fig. 8. Effect of both EHD and NC gas concentration on temperature difference at varioussections of the condenser, applied voltage=8 kV.

Fig. 9. Effect of temperature difference on local CHTC in presence of EHD and NC gas.

290 H. Omidvarborna et al. / International Communications in Heat and Mass Transfer 36 (2009) 286–291

Presence of NC gas in refrigerant media has a negative effect oncondensation process. Concentration of NC gas in refrigerationsystems is reduced by performing of suitable evacuation of themfrom the systems. Inspection of pipes, fittings, and connections priorto filling of the systems with refrigerant media will reduce theamount of NC gas in them. However another source of these gasesis oxidation of leaked oil in condensation system during filling of thesystem.

The effect of NC gas on heat transfer coefficient is shown in Fig. 4.The CHTC decreases by increasing concentration of NC gas in thecondensation media. Since the NC gas is impermeable in thecondensate film, it is accumulated at condensate vapour interface.So, the condensate boundary layer and the concentrated NC gas layerthicknesses are increased along the condenser tube length. Thedeveloping NC gas layer acts as a strong resistance to condensationphenomenon and thermal heat transfer. This leads to a lower heattransfer coefficient. Experimental data shows, by increasing ofelectrode diameter, there is a decrease in refrigerant vapour flowcross section and consequently a small increase in vapour Reynoldsnumber, Re≈1700 for electrode diameter of 4 mm, and Re≈1800 forelectrode diameter of 12 mm. For all of the NC gas concentration, theheat transfer coefficient for the thicker electrode has a lower decreasein comparison to the corresponding ones for the thinner electrode.

5.2. Study of simultaneous effects of both EHD and NC on condensation

Presence of NC gas in condensing media has undesirable effect oncondensation phenomenon. By increasing of air fraction as a NC gas incondensing media, reduction in CHTC is experienced (Figs. 5 and 6). Innext step, experimental study of this negative factor on enhancing ofCHTC in conjunction with the positive feature of EHD is performedsimultaneously.

However by application of EHD the heat transfer is enhanced bythinning of the condensate film, but amount of compensation does notcompensate the reduction due to presence of NC gas. The results showthat at a constant NC gas concentration, increasing of applied voltageenhance condensation rate. A similar result is gained by increasingelectrode diameter.

5.3. Effects of EHD and NC gas on temperature gradient

One of the main parameters that affects CHTC is condensationdriving force, i.e. temperature difference between saturated vapourand the cold wall temperatures.

The temperature differences at 4 positions of the condensing systemwith 12 mm diameter electrode for different values of applied voltageand existingNC gas concentration are depicted in Figs. 7 and 8. In absenceof electric field and NC gas, the temperature difference is increasedalong condensation tube due to gradual increase of the condensed film

thickness. This issue increases the amount of thermal resistance allthe way through the condensing process in the tube which as a resultCHTC is decreased.

Furthermore, presence of NC gas in condensing media introducesan extra heat transfer resistance to heat transfer path. This causes alower heat transfer coefficient and consequently increases tempera-ture difference. Application of electric field to condensation systemreduces the condensate film thickness which results in lower tem-perature difference.

Fig. 9 shows the dependence of local CHTC in presence of electricfield and NC gas. Results show that increasing of electric field for thedifferent concentrations of NC gas; reduces temperature difference,and increases local CHTC.

6. Conclusion

The experiments conducted for condensing of a two-phase flowunder influence of an applied electric field show that the EHD force isstrong enough to extract sufficient liquid from condensing surface.Increasing of applied voltage decreases the liquid layer thickness, dueto pulling of the condensate into the flowing vapour.

The heat transfer resistance on the condensing side is increased bypresenceof air in condensingvapour. This is due toaccumulationof theNCgas at condensing part which decreases the condensation rate of R-134a.

For the system under two competitive effects due to application ofhigher voltages, and higher concentrations of the NC gas content, resultsshow that at high concentration ofNCgas, increasingof electricalfield haslower effect on enhancement of condensation heat transfer. This issue isdue to considerable effect of applied voltage on refrigerant of R-134a incomparison with its negligible effect on NC gas, the content air.Consequently, increasing of applied voltage cannot absorb the air fromactive heat transfer section, and leave the surface for condensing vapour.However, itmust bementioned that there is anupper limit on the amountof applied voltage due to existence of a breaking voltage in the system.

References

[1] P.H.G. Allen, T.G. Karayiannis, Electrohydrodynamic enhancement of heat transferand fluid flow, Heat Recovery Syst. CHP 15 (5) (1995) 389–423.

[2] J. Seyed-Yagoobi, J.E. Bryan, Enhancement of heat transfer and mass transport insingle-phase and two-phase flows with electrohydrodynamics, Academic Press,1999, pp. 95–186.

[3] S. Laohalertdecha, P. Naphon, S. Wongwises, A review of electrohydrodynamicenhancement of heat transfer, Renew. Sustain. Energy Rev. 11 (2006) 858–876.

[4] R.A. Parsons, ASHRAE Handbook: Refrigeration, [SI Edition], 1998 Chapter 6, Part 7.[5] H.R. Velkoff, J.H. Miller, Condensation of vapor on a vertical plate with a transverse

electrostatic field, J. Heat Transfer, Trans. ASME Series C87 (1965) 197–201.[6] H.Y. Choi, Electrohydrodynamic condensation heat transfer, J. Heat Transfer, Trans.

ASME C90 (1968) 98–102.[7] A.B. Didkovsky, M.K. Bologa, Vapor film condensation heat transfer and

hydrodynamic under the influence of an electric field, Int. J. Heat Mass Transfer24 (5) (1981) 811–819.

291H. Omidvarborna et al. / International Communications in Heat and Mass Transfer 36 (2009) 286–291

[8] M.K. Bologa, I.K. Savin, A.B. Didkovsky, Electric field induced enhancement of vaporcondensation heat transfer in presence of non-condensable gas, Int. J. Heat MassTransfer 30 (8) (1987) 1577–1585.

[9] A. Yabe, K. Kikuchi, T. Taketani, Y. Mori, K. Hijikata, Augmentation of condensationheat transfer by applying electro-hydrodynamical pseudo-dropwise condensation,Proc. of the 8th Int. Heat Trans. Conf., 1986, pp. 2957–2962.

[10] K.Yamashita,M.Kumagai, S. Sekita,Heat transfer characteristicsof anEHDcondenser,Proc. of the 3rd ASME/JSME Joint Thermal Eng. Conf., vol. 3, 1991, pp. 61–67.

[11] M. Wawzynaik, J. Seyed-yagoobi, Experimental study of electrohydrodynamicallyaugmented condensation heat transfer on a smooth and enhanced tube, J. HeatTransfer 118 (1996) 499–502.

[12] K. Yamashita, A. Yabe, Electrohydrodynamic enhancement of falling filmevaporation heat transfer and its long-term effect on heat exchangers, J. HeatTransfer 119 (1997) 339–347.

[13] K. Cheung,M.M. Ohadi, S.V. Dessiatoun, EHD-assisted external condensation of R-134aon smoothhorizontal andvertical tubes, Int. J. HeatMass Transfer 42 (1999)1747–1755.

[14] K. Brand, J. Seyed-Yagoobi, Experimental study of electrohydrodynamic inductionpumping of a dielectric micro liquid film in external horizontal condensationprocess, J. Heat Transfer 125 (2003) 1096–1105.

[15] H. Sadek, A.J. Robinson, J.S. Cotton, C.Y. Ching, M. Shoukri, Electrohydrodynamicenhancement of in-tube convective condensation heat transfer, Int. J. Heat MassTransfer 49 (2006) 1647–1657.

[16] S. Laohalertdecha, S. Wongwises, Effects of EHD on heat transfer enhancement andpressure drop during two-phase condensation of pure R-134a at high mass flux ina horizontal micro-fin tube, Exp. Therm. Fluid Sci. 30 (2006) 675–686.

[17] R.E. Holmes, A.J. Chapman, Condensation of freon-114 in the presence of a strongnonuniform alternating electric field, J. Heat Trans., Trans. ASME, Series C 92 (1970)616–620.

[18] P. Cooper, P.H.G. Allen, The potential of electrically enhanced condensers, Proc. of 2ndInt. Symp. on the Large Scale Application ofHeat Pumps, York, UK,1984, pp. 295–309.

[19] Y. Xu, R.K. Al-Dadah, T.G. Karayiannis, P.H.G. Allen, Incorporation of EHD enhance-ment in heat exchangers, J. Enhanc. Heat Transf. 2 (1995) 87–93.

[20] M.M. Ohadi, J. Darabi, B. Roget, Electrode design fabrication and materials sciencefor EHD-enhanced heat and mass transport, Annual Review of Heat Trans. BegelHouse Inc., 1999, pp. 1–55.

[21] A. Singh, Electrohydrodynamic (EHD) enhanced of in-tube condensation heattransfer of alternative refrigerants R-134a in smooth and microfine tubes, ASHRAETrans. 103 (1) (1995) 813–823.

[22] A. Gidwani, M. Molki, M.M. Ohadi, EHD-enhanced condensation of alternativerefrigerants in smooth and corrugated tubes, HVAC Res. 8 (2002) 219–238.

[23] Y. Feng, J. Seyed-Yagoobi, Mechanism of annular two-phase flow heat transferenhancement and pressure drop penalty in the presence of a radial electric field-turbulence analysis, Trans. ASME 125 (2003) 478–486.

[24] A.K. Seth, L. Lee, The effect of an electric field in the presence of noncondensablegas on film condensation heat transfer, J. Heat Transfer (1974) 257–258.