Characteristics of evaporative heat transfer and pressure dropof carbon dioxide and correlation developmentSeok Ho Yoon a ,Eun Seok Cho b ,Yun Wook Hwang b ,Min Soo Kim b,*,Kyoungdoug Min b ,Yongchan Kim caDigital Appliance Research Laboratory,LG Electronics Inc.,Seoul 153-802,South KoreabSchool of Mechanical and Aerospace Engineering,Seoul National University,Seoul 151-742,South KoreacDepartment of Mechanical Engineering,Korea University,Seoul 136-701,South KoreaReceived 31March 2003;received in revised form 15August 2003;accepted 15August 2003AbstractCarbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties.In this study,transcritical refrigeration cycle using carbon dioxide is of great interest,and the evaporation process is investigated by experiment and analysis.This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube.The test section was made of a seamless stainless steel tube with the inner diameter of 7.53mm,and length of 5m.Heat is provided by a direct heating method to the test section.Experiments were conducted at saturation temperatures of À4to 20 C,heat fluxes of 12to 20kWm À2and mass fluxes of 200to 530kgm À2s À1.A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made.Based on the experiments for the evaporation heat transfer,useful correlation is developed.#2003Elsevier Ltd and IIR.All rights reserved.Keywords:Evaporation;Carbon dioxide;Smooth tube;Heat transfer;Mass transfer;Pressure;DropDioxyde de carbone :transfert de chaleur lors de l’evaporation,chute de pression et de veloppement d’une correlation Mots cle ´s :Evaporation ;Dioxyde de carbone ;Tube lisse ;Transfert de chaleur ;Transfert de masse ;Pression ;Perte 1.IntroductionHFC refrigerants,which have been developed asalternatives to CFC and HCFC refrigerants,are known to have a higher global warming potential (GWP),therefore,application of HFC refrigerants as ultimaterefrigerants in a refrigeration system may not be ade-quate.Environmentally benign,‘natural’refrigerants have attracted a considerable attention.The natural refrigerants are the naturally occurring substances,namely,ammonia,hydrocarbons,carbon dioxide,water,air,etc.These natural refrigerants have zero ODP,and the majority of them have zero GWP,how-ever some of them can be flammable and/or toxic.Among these natural refrigerants,carbon dioxide (R744)has been focused on as a refrigerant for automobile0140-7007/$35.00#2003Elsevier Ltd and IIR.All rights reserved.doi:10.1016/j.ijrefrig.2003.08.006*Corresponding author.Tel.:+82-2-880-8362;fax:+82-2-883-0179.E-mail address:minskim@snu.ac.kr (M.S.Kim).air conditioners.Since carbon dioxide is a constituent of the atmosphere,it would be obtained from atmospheric air by fractionation and would have no additional impact on global warming,except for the energy con-sumption associated with the fractionation process.It also has advantages of no toxicity,no inflammability and economical efficiency.In addition to these advan-tages,carbon dioxide has a merit over the existing refrigerants because it has high VCR(volumetric capa-city of refrigeration)with a possibility to make a system compact[1,2],and excellent thermodynamic and trans-port properties.One of the reasons that the application of carbon dioxide to car air conditioners or hot water heaters has been attempted is its high potential as a good alternative refrigerant[3–5].Recent research on carbon dioxide has focused on the development of a transcritical cycle.Many of the recent investigations of carbon dioxide as a refrigerant have included a thermodynamic cycle analysis.However, relatively few investigations have been performed to measure heat transfer coefficients and pressure drop during evaporation of carbon dioxide.Bredesen et al.[6]studied heat transfer and pressure drop for in-tube evaporation of carbon dioxide.The test section is an aluminum tube of the inner diameter7mm, and the tube was heated by indirect heating method. They showed that the heat transfer coefficients of car-bon dioxide are much higher than what is predicted by the correlations reported so far.Based on the experi-mental results of Bredesen et al.[6],Hwang et al.[7] investigated the applicability of six commonly used empirical correlations.They proposed a new empirical model,the modified Bennett–Chen correlation,for car-bon dioxideflow boiling in horizontal smooth tubes. Pettersen et al.[8]developed microchannel heat exchangers for carbon dioxide and experimentally eval-uated the overall heat transfer coefficient.They indi-cated that refrigerant-side heat transfer coefficients are higher than those offluorocarbons and,therefore,the internal surface areas of heat exchangers can be reduced.Zhao et al.[9]investigated the forced con-vective boiling heat transfer of carbon dioxide in a smooth tube.The tube has an inner diameter of6mm, and heated by the heating wire wrapped around the tube.They found the vapor quality had an adverse effect on the heat transfer coefficient.Hihara and Tanaka[10] measured boiling heat transfer coefficients and pressure drop in a horizontal smooth tube,of which inner dia-meter is1mm.Their experiment was conducted at only 15 C which is the evaporating temperature.They found that the existing correlations did not agree well with their experimental results.However,any of these investigations takes into care-ful account for their correlation with the partial dryout conditions near the critical temperature(carbon dioxide: 31.06 C).Previous works on the evaporative heat transfer of carbon dioxide have focused on the demon-stration of different trend of heat transfer of carbon dioxide as vapor quality increases.In this study,new correlation of evaporation heat transfer for carbon dioxide has been developed with non-dimensional para-meters to consider the effect of partial dryout near the critical temperature.Therefore,this study is aiming at providing the char-acteristics of heat transfer and pressure drop during the evaporation to help design heat exchangers as part of112S.H.Yoon et al./International Journal of Refrigeration27(2004)111–119refrigeration system using carbon dioxide as a refriger-ant.The primary objective of this study is to explain the characteristics of heat transfer and pressure drop during evaporation for carbon dioxide.2.Experiments2.1.Experimental apparatus and measurementsThe schematic diagram of experimental apparatus is shown in Fig.1.The test rig is composed of magnetic gear pump for refrigerant circulation,liquid receiver, chiller,heat exchanger for subcooling thefluid,pre-heater,massflow meter,and heat transfer test section. The subcooling heat exchanger and the pre-heater are installed to adjust the inlet quality of the refrigerant to a desired value.The subcooled liquid is heated by an electric current in the test tube supplied by a low vol-tage,and high current power supply(0–20V,0–120A). Seamless stainless steel tube(Type316)was used for heat transfer test section in order to maintain electrical and thermal uniformity in the test section.The thermal conductivity of the tube is13.47W mÀ1KÀ1at room temperature.Inner and outer diameters of the stainless steel tube are7.73and9.53mm,respectively.The heated length of the test section is5.0m.In order to reduce heat gain or loss from the surroundings,the test section was covered withfiberglass insulation material.2.2.Data reductionHeat transfer coefficient is defined as in Eq.(1).h¼q00wi satð1Þwhere T wi is calculated by one dimensional heat con-duction equation from measured outer wall tempera-tures,T wo.q00indicates the heatflux to the test section and T sat is calculated from the measured evaporation pressure of carbon dioxide.At each thermocouple location,the average wall temperature was calculated by the measured tempera-tures at the top,bottom,and two sides.The saturation temperatures of refrigerants were calculated from the measured pressure by using a REFPROP[11].3.Results and discussion3.1.Test conditionsExperiments were conducted for various heatfluxes, massfluxes and saturation temperature of refrigerant. Heatfluxes were controlled at12.3–18.9kW mÀ2,and massfluxes were controlled at212–530kg mÀ2sÀ1by a variable speed gear pump.The saturation temperatures were adjusted atÀ4to20 C.In order to prevent a sud-den increase in the tube wall temperature due to the dry-out at high qualities,test was conducted at the quality ranges below0.7.Table1shows experimental conditions.3.2.Heat transfer characteristics of carbon dioxideVariation of heat transfer coefficients versus quality with respect to heatflux is shown in Fig.2when mass flux is318kg mÀ2sÀ1,and saturation temperatureis Fig.1.Schematic diagram of experimental apparatus for evaporative heat transfer test with carbon dioxide.S.H.Yoon et al./International Journal of Refrigeration27(2004)111–1191135 C.At a low mass quality region,heat transfer coeffi-cient has a tendency to increase slightly as quality increase.It is because as quality increases,void fraction increases and liquidfilm thickness becomes thin and accordingly wall superheat decreases.In the evaporation process in a horizontal tube,liquidfilm at the top of the tube becomes thinner and thinner as the refrigerant evaporates andfinally thefilm disappears.As the con-tact area of vapor state on the tube wall is larger,wall temperature at the top rises dramatically and heat transfer coefficient falls down.The wall temperature at the top of the tube rises significantly at some point as quality increases(see Fig.5).Bredesen et al.[6]obtained the same tendency at5 C,and explained that this effect is due to the high pressure and low liquid/vapor density ratio near the critical point.In general,the heat transfer coefficients of refrigerants get higher as the saturation temperature increases at low quality region,which is because the nucleate boiling is more active in the higher saturation temperature.Cooper[12]proposed that the nucleate boiling heat transfer coefficient is a function of reduced pressure,and reduced pressure increases as saturation temperature increases.However,carbon dioxide shows a different tendency in the low quality region.Variation of heat transfer coefficient versus mass quality with respect to saturation temperature is shown in Fig.3.The heat transfer coefficient of carbon dioxide increases as the saturation temperature becomes higher,up to5 C,but it decreases as the saturation temperature exceeds5 C. This is because as the temperature is closer to the critical temperature,the liquidfilm at the upper side of the horizontal tube can be easily broken as the viscosity of the liquid phase is quite reduced.The saturation tem-perature has minor effect on the convective heat transfer due to the suppression of the nucleate boiling as quality increases at a high quality region in Fig.3.3.3.Modeling and correlationparison of experimental data and correlations Table2lists the results of comparison between the various correlations and the present results for carbon dioxide with the average deviation,absolute average deviation,and RMS(root-mean-square)deviation. When the heat transfer coefficient from the experiment for carbon dioxide and the existing correlations are compared for some cases,all the correlations under-predicted the experimental heat transfer coefficients. Among the correlations,the Liu and Winterton’s correlation[13]predicts the highest value of heat trans-fer coefficient.The correlations of Jung et al.[14]and Kandlikar[15]showed a decreasing tendency of heat transfer coefficient with respect to quality and those of Gungor and Winterton[16]and Liu and Winterton[13] provide lower values.3.3.2.Correlation developmentThe characteristics of heat transfer in a horizontal tube are influenced byflow patterns.It must be investi-gated also for theflow pattern of carbon dioxide during evaporation process.First of all,the circumferential distribution of inner wall temperature is investigated to understand theflow pattern at each experimental condition.Fig.4shows the flow pattern during evaporation process for carbon dioxide and a critical quality at the top and bottom of the tube.It is observed that the topside wall temperature at the top of the tube increases suddenly at specific location,because the liquidfilm at the top of the tube becomes thinner and thinner as the refrigerant evapo-rates andfinally thefilm disappears at this location. After thefilm disappears the heat transfer decreases significantly,because the vapor contacts the wall of the tube directly.As the contact area of vapor state is lar-ger as quality increases,wall temperature at the topTable1Test conditions of the evaporative heat transfer experimentMassflux,G(kg mÀ2sÀ1)Heatflux q00(kW mÀ2)at evaporating temperature T satÀ4 C0 C5 C10 C15 C20 C21212.312.312.312.331812.312:316:412:316:418:912:316:418:912:316:442412.312:316:412:316:418:912:316:418:916.412:316:418:953012.312:316:412:316:418:912:316:418:916.412:316:418:9rises dramatically and heat transfer coefficient falls down.This phenomenon is observed with halocarbon refrigerants at the quality over 0.9[17].Since the visc-osity and surface tension of carbon dioxide is very lower than halocarbon refrigerants,the rupture of liquid film takes place at very low quality region.Fig.4shows the cross sectional view with partial dryout con-figurations.For the vapor contacting area of the tube,the single phase vapor heat transfer mechanism is dominant and for the wetted area of the tube,two phase evaporative heat transfer mechanism is dominant.The single phase heat transfer coefficients are very lower than that of two phase.Kattan et al.[18]proposed the similar concept for halocarbon refrigerants at over the critical heat flux.The quality which liquid film breaks down is defined as ‘critical quality’[19].In the horizontal tube,the cri-tical quality at the top of the tube is different with thatat bottom of the tube because of gravitational force.Therefore,the critical quality where the top of the tube is dried out is important in analyzing the behavior of the heat transfer coefficient change.The mass flux,heat flux,and saturation temperature are considered to determine the critical quality at the top of the tube.When the correlation is developed,it is reasonable to use dimensionless numbers than to use these parameters directly.Dimensionless numbers of Reynolds number,boiling number,and Bond number were selected since Reynolds number reflects the effect of mass flux,boiling number reflects the influence of heat flux,and Bond number includes surface tension contribution.The influence of the saturation temper-ature is substantially included in the above dimension-less numbers because thermophysical properties such as density,viscosity,and surface tension are strongly dependent on the saturation temperature.So the follow-ing equation is proposed to predict the critical quality at the top of the tube.x cr ;t ¼38:27Re 2:12l1000Bo ðÞ1:64Bd À4:7ð2ÞFig.5shows the inner wall temperature variation and predicted critical quality at the top of the tube.It is observed that the inner wall temperature at the top of the tube rises at the predicted critical quality.In developing a new heat transfer correlation during evaporative process for carbon dioxide,it is one way to use different correlations for each region before and after the critical quality.Before the critical quality at the top of the tube,it is thought that heat transfer mechan-ism is the same as the existing correlations for annular flow.As a correlation applicable to the low quality region,the Liu and Winterton’s correlation [14]is selected.Then the coefficients in this correlation are modified such that the deviation between themeasuredFig. 3.Variation of heat transfer coefficients with different saturation temperature for constant heat and mass flux.(q 00=16.4kW m À2;G =318kg m À2s À1).Table 2The errors between predicted and experimental heat transfer coefficientsKandlikar (1990)[15]Gungor and Winterton (1987)[16]Jung et al.(1989)[14]Liu and Winterton (1991)[13]Modified Hwang et al.(1997)[7]Average deviation (%)À33.9À21.20.422.7À24.7Absolute average deviation (%)39.934.837.144.748.5Root-mean-square deviation (%)43.138.948.376.158.7Average deviation %ðÞ¼P N i ¼1h pred ;i Àh meas ;imeas ;i!Â100=N :Absolute average deviation %ðÞ¼P N i ¼1h pred ;i Àh meas ;i h meas ;i!Â100=N :RMS deviation %ðÞ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP N i ¼1h pred ;i Àh meas ;i h meas ;i2"#Â100=N v u u t :S.H.Yoon et al./International Journal of Refrigeration 27(2004)111–119115and calculated data is minimized and h l is calculated by the Dittus–Boelter’s equation [20].As a result of this fitting,the final modified equation is suggested as shown in Eq.(3).These equations are applicable to the region of x <x cr,t .h tp ¼S Áh nb ðÞ2þE Áh l ðÞ2ÂÃ1=2ð3a Þh nb ¼55PÃ0:12Àlog PÃÀÁÀ0:55M À0:5q 000:67ð3b ÞS ¼11þ1:62Â10À6E 0:69Re 1:11l ð3c ÞE ¼1þ9:36Â103x Pr l lgÀ1!0:11ð3d ÞBecause the existing correlation did not consider the flow pattern and the rupture of liquid film,these corre-lations cannot predict the heat transfer coefficient for carbon dioxide after the critical quality.Therefore,over a cross section of the evaporator tube,the heat transfer coefficient,h tp ,is estimated as a superposition of the liquid and vapor heat transfer coefficients for the wet and dry portion.The heat transfer correlation applic-able for the quality range of x 5x cr,t is proposed as fol-lows:h tp ¼ dry h g þ2 À dry ÀÁh wet2 ð4Þwhere h g is the vapor heat transfer coefficients,and h wet is the heat transfer coefficients on the wetted portion of the tube.Correlations for h wet are shown in Eq.(5).h wet ¼E Áh lð5a ÞE ¼1þ3000Bo0:86þ1:12x 1Àx 0:75 lg0:41ð5b ÞThe formulation of the Gungor and Winterton’s correlation [16]is selected to develop the correlation of heat transfer coefficients on the wetted portion of the tube,h wet ,because this correlation shows the smallest absolute average deviation.Enhancement factor,E ,is modified in the power of density ratio.dry is the angle of the dry portion as shown in Fig.5.The vapor heat transfer coefficients,h g ,is determined with the Dittus–Boelter’s correlation [20].Kattan et al.[17]suggested the new flow pattern.They used this new flow pattern map to determine the dry angle, dry .But carbon dioxide is not included as a working fluid in this flow pattern map.Therefore,the new model is needed to determine the dry angle.The dry angle is closely related with the flow pattern of carbon dioxide.It is preferential to introduce the dimensionless numbers(Reynoldsparison of prediction of critical quality with innerwall temperature of test section upper side.(q 00=16.4kW m À2;G =318kg m À2s À1;T sat =5.0 C).116S.H.Yoon et al./International Journal of Refrigeration 27(2004)111–119number,boiling number,and Bond number)to deter-mine the critical quality.Martinelli parameter is added to the dry angle correlation because the contact area between vapor and tube wall increases as quality increases.Therefore,the equation of the dry angle, dry,is sug-gested as follows:dry¼36:23Re3:47Bo4:84BdÀ0:2712:6ð6ÞThe coefficients in Eq.(6)were determined by non-linear curvefitting using the experimental results at qualities greater than the critical quality at the top of the tube.In this study,the dry angle, dry,is the weighting factor that shows the ratio of the contact area between vapor and tube wall to the contact area between liquid film and tube wall.The average deviation,absolute average deviation, and RMS deviation of the measured heat transfer coef-ficients from those predicted by this study were1.5%, 15.2%,and21.1%,respectively.3.4.Two phase pressure drop of carbon dioxide Several correlations on the two phase pressure drop were compared with the experimental results from this study.The B-coefficient method[21]predicts the same data with a mean deviation of up to87.0%and Fig.6 shows the same tendency for different saturation tem-perature.But this method does not contain a parameter that accounts for the effect of tube dimensions andfluid surface tension.The C-coefficient method[21]over-predicted the experimental data with an absolute aver-age deviation of125.9%.This method shows a different tendency when massflux increases.Jung and Raderma-cher’s correlation[22]overpredicted the experimental data with an absolute average deviation of128.9%.It is found that the change of pressure drop for different saturation temperatures is very smaller than the experi-mental data.In the following,a new pressure drop correlation is developed on the basis of the B-method[21].Surface tension is expected to be an importantfluid property that needs to be included in the two-phaseflow pressure drop correlation,particularly for carbon dioxide.It is known that there are two dimensionless numbers that include surface tension.Weber number is the ratio of inertia forces and surface tension forces and Bond number is the ratio of gravitational forces and surface tension forces.The new frictional two-phase multiplier can then be expressed as2f;lo¼1þa G2À1ÀÁBDx0:8751ÀxðÞ0:875þx1:75!ð7ÞThe constant a is evaluated as a valued of4.2by a least square curvefitting.New correlation is applicable for smooth tube,for carbon dioxide duringflow boiling. The pressure drop calculated with the new frictionaltwo-phase multiplier 2f;lois compared with all experi-mental data and shown in Fig.7.An analysis shows that most of the data are within 1.8%of average deviation of prediction.The proposed correlation has an absolute average deviation of16.2%; This represents a significant improvement of the pro-posed correlation over theB-method.Fig.7.Predicted frictional two-phaseflow pressure drop versusexperimental data.S.H.Yoon et al./International Journal of Refrigeration27(2004)111–1191174.Concluding remarksIn this work,the heat transfer coefficients and pres-sure drop during the evaporation process of carbon dioxide in a horizontal tube have been investigated and the followings are thefindings of this study.1.At a low mass quality region during evapor-ation,heat transfer coefficient increases as massquality increases because convective boilingbecomes more dominant.But when mass qualityis greater than a certain value,heat transfercoefficient tends to decrease.It is because surfacetension and viscosity of carbon dioxide is farsmaller than those of conventional refrigerantsso that the liquidfilm breaks down quite easily.2.The existing correlations for heat transferunderpredicted the experimental data because ofthe differentflow patterns of carbon dioxide.Anew correlation for carbon dioxide during eva-poration was developed by considering the cri-tical quality.Atfirst,it is proposed that thecorrelation can predict the critical quality atwhich the liquidfilm breaks down at top of tube.Before the critical quality,the Liu and Winter-ton’s correlation[13]is used to predict the heattransfer coefficient.After the critical quality,theDittus–Boelter’s equation[20]for vaporflowand the Gungor and Winterton’s correlation[16]for liquidflow is superposed to predict the heattransfer coefficient of carbon dioxide.This newcorrelation predicts the experimental data withinan absolute average deviation of15.3%.3.The measured pressure drop of carbon dioxideincreases as massflux increases and decreases assaturation temperature increases because of thevariation of thermophysical properties.Thepressure drop data obtained in the experimentwere compared with the several existing pressuredrop correlations.The absolute average devi-ations of the Chisholm’s B-coefficient method,the C-coefficient method,and the Jung andRadermacher’s correlation are87%,125.9%,and128.9%,respectively.A new correlation issuggested that includes Weber number,and thisnew correlation predicts the experimental datawithin16.2%of absolute average deviation. 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