A FLOW PATTERN BASED COMPARISON ON TWO-PHASE PRESSURE DROPS OF REFRIGERANTS HFO-1234yf AND HFC-134a IN MICROCHANNEL HEAT EXCHANGER

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1 The 8 th International Symposium on Transport Phenomena -4 September 17, Peradeniya, Sri Lanka A FLOW PATTERN BASED COMPARISON ON TWO-PHASE PRESSURE DROPS OF REFRIGERANTS HFO-134yf AND IN MICROCHANNEL HEAT EXCHANGER Hamid Nalbandian, Chien-Yuh Yang and Kuan-Tg Chen Department of Mechanical Engeerg, National Central University, Jhong-Li, Taoyuan, 354, Taiwan ABSTRACT Owg to the global warmg effect consideration, the EU has banned the use of refrigerants with Global Warmg Potential (GWP) value higher than 15 all new passenger cars startg from 1 January 17 (EU directive 6/4/EC [1]. The study on the replacement of high GWP refrigerant such as has become an important and urgent issue. Recently, a new refrigerant, HFO-134yf has been developed with similar thermodynamic properties to but much lower GWP value (GWP = 4 comparg to 1,43 of ). It is expected as a good candidate of replacg the refrigerant. However, most of the heat transfer and flow performances are still not very clear up-to-date. For reducg the weight of heat exchangers, most of the vehicles air-condition systems used extruded alumum tubes their evaporators and condensers. The hydraulic diameter of the heat exchangers is generally very small, which called microchannel heat exchanger, to resist the high workg pressure of two-phase refrigerant. This study provides an experimental vestigation on two-phase flow pressure drop of refrigerants HFO-134yf and microchannel heat exchanger with hydraulic diameter of.5 mm. The difference of pressure drops between these refrigerants was compared and terpreted based on the fluids properties and two-phase flow patterns. It is found that for flow annular flow region, liquid viscosity is the major controllg properties for two-phase pressure drop. Sce the viscosity of liquid is approximately 7% higher than those of liquid HFO-134yf, this caused the pressure drops of are up to 6% higher than those of HFO-134yf. For flow dispersed flow region, the major controllg property for pressure drop changed to vapor viscosity. Sce the vapor viscosity of is only 6.6% higher than that of HFO-134yf, the pressure drop of is only 7.8% higher than that of HFO- 134yf. INTRODUCTION HFC refrigerants such as have high global warmg potential (GWP) over 1. Therefore, there is a need to replace these high GWP refrigerants with lowest ones. Vehicle s air conditions are the ma source of greenhouse gas (GHG) pollution. Limit of GWP for mobile air condition system has already been fixed by European Union to less than 15. Now a day s most of the mobile air conditiong systems use HFC -134a as the workg fluid. HFC -134a has GWP as high as 1,43. Recently another good candidate has been developed by Honeywell and DuPont, HFO-134yf, which has GWP as low as 4. In order to replace one refrigerant with another one, several parameters have to be taken to account, such as physical properties, workg pressure and two-phase heat transfer. Similarity of the properties reduces the cost of the system and let it can be easily replaced with the other. Several experiments have been done to evaluate the HFO-134yf two-phase pressure drop compare to the. Two-phase flow patterns are an important parameter heat transfer mechanism and pressure drop []. Visualization experimental system was built order to vestigate and compare the pressure drops and flow regimes between, HFO-134yf and HFC-41A by Padilla et al. [3] horizontal tube with two different diameters. Experimental vides from flow regimes (slug, termittent and annular) demonstrated that there are no significant differences between and HFO-134yf flow pattern. HFO-134yf pressure drop mm diameter test section was vestigated by Saitoh et al. [4] they results were well agree with pressure drops correlation but they didn t compare it with HC-134a. Lu et al. [5] studied the HFO-134yf and heat transfer and pressure drop smooth pipe with 3.9 mm diameter. Experimental pressure drops of HFO-134yf and were compared at different mass fluxes and vapor qualities. Test results showed that very low mass flus pressure drop of those two refrigerants were almost the same. But by creasg the mass flux, at low vapor quality pressure drops of HFO-134yf is similar to but at higher vapor qualities, HFO-134yf shows lower pressure drops compare to. Their results were explaed by the properties differences of these tow refrigerants. Anwar et al. [6] vestigated the heat transfer and pressure drops of the HFO-134yf and 1.6 mm diameter under the upward flow boilg condition. They compared HFO- 134yf and one mass flux. Experimental shows by creasg the vapor quality, HFO-134yf shows lower pressure drops compare to those of HFC-13a. EXPERIMENTAL SYSTEM The schematic diagram of the experimental system is shown Figure. It consists of a refrigerant loop, direct heatg test section, preheater water loop, sub cool water loop and relative measurements. Refrigerants loop As shown Figure, a gear pump provided pumpg power to move the liquid refrigerant thought the refrigerant cycle. After pump, a Coriolis flow meter measured the mass flow rate of the refrigerant. A preheater before test section has duty to convert the sgle phase refrigerant to two phase form. The two phase refrigerant flowed to test section and under the direct wall heatg, let vapor quality creased by x=. and left the test section. Afterwards a sub cool heat exchanger converted the two phase refrigerant to the sgle phase form. Sgle phase refrigerant flow to

2 The 8 th International Symposium on Transport Phenomena -4 September 17, Peradeniya, Sri Lanka receiver and refrigerant cycle has been completed. Two RTDs were setup at the let and exit ports of the test section to measure the refrigerant side let and exit temperatures. Because of the high pressure drop at let and let of microchannel text section (Figure 1), two rows of thermocouples are set up on the wall of the test section (no heatg part), where showed by T1 and T5 to measure the changes at temperature of the two phase refrigerant. A pressure transducer was setup prior to the test section to measure the refrigerant saturation pressure for properties evaluation. The pressure drop through the test section was measured by a differential pressure transducer stalled between the let and exit port of the test section. Test section heatg Test section was a parallel microchannel with 37 channel and hydraulic diameter of.5mm. Test section length is 67 mm (from let to let) and heatg length was 6 mm. Two electrical flat heater with length of the 6 mm has been stalled on the top and bottom of the test section to provide the heat flux for boilg side the test section. Voltage and current are adjusted by a DC power supplier. Electrical power for each mass flux to provide enough heat rate to change the let vapor quality by x=.. Preheater water loop A thermostat reservoir was used to provide the heatg water for the preheater. The temperature at the let and exit of the preheater and the flow rate of heatg water were measured two RTDs and a gear type flow meter. By usg the energy balance, the total heat transfer rate to the refrigerant and therefore the exit refrigerant vapor quality can be evaluated. q m c T T (1) w, p w, p p, w w, p, w, p, Vapor quality at entrance of the test tube was calculated based on the water heat transfer to the refrigerant side the preheater: qw, preheater x () mi r lv DATA REDUCTION Pressure drop of the sgle phase flow measured by a differential pressure transmitter Yokogawa EJA (range of the ~5 kpa). Sgle phase measured pressure drop can be express form of summation of several pressure drops: p p p p p (3) exp frictional acceleration let let Sgle phase let and let pressure drop Inlet and let pressure drops are calculated based Keys and London method [7]: G plet (1 Kc ) (4) G plet (1 Ke ) (5) Figure 1. Detail dimensions of the test section Figure. Schematic of the experimental system

3 The 8 th International Symposium on Transport Phenomena -4 September 17, Peradeniya, Sri Lanka Where =A c /A fr is the ratio of the test section crosssectional area to the frontal area of the let and exit. The Kc and Ke are functions of and Reynolds number. Kays and London [7] provide graphs to determe a for several cross sectional shapes. We used their Figure 5., which is for a circular cross-section. Sgle phase acceleration pressure drop Acceleration pressure drop [8] is expressed as: G G pa (6) Temperature at let and let were constant that means density of the liquid were constant along the pipe, therefore acceleration term is neglected. Sgle phase frictional pressure drop Frictional pressure drop was calculated by usg the Darcy friction factor method has been explaed by Munson, Young and Okiishi [8] chapter 8: l G pfrictional f (7) D Rearrange the equation gives: D f pfrictional (8) G l Therefor frictional pressure drop can be evaluated: p p p p p (9) frictional exp a let let Two phase pressure drops Pressure drop of the two phase flow measured by a differential pressure transmitter Yokogawa EJA (range of the ~5 kpa). Two phase measured pressure drop is combation of: p p p p p (1) exp frictional a c e Two phase let and let pressure drop Collier and Thome [9] (chapter and 3.8.) recommend use of a separated flow model for calculation of p c and p e two-phase flow. Their recommended equations for a sudden contraction: 3 3 x 1 x 1C c G g 1 f pc 1 C c Cc x 1 x g (11) f 1 x 1 x c g f C Collier and Thome [9] also provided a table for Cc based on the area ration () chapter Sudden expansion is expressed as: 1 1 x f x pe G 1 (1) f 1 g where α is the void fraction which has been calculated from Zivi [1] equation: 1 x g 1 x f /3 1 (13) Two phase acceleration pressure drop Acceleration pressure drop also explaed by Collier and Thome [9] (chapter.4.). G x f 1 x pa, 1 (14) f g 1 G x f 1 x pa, 1 (15) f g 1 Acceleration pressure drop is express as: p p p (16) a a, a, Two phase frictional pressure drop: By knowg the pressure drops parameter, Frictional pressure drop has been calculated: p frictional pexp ( pa pc pe ) (17) Uncertaty of the system apparatus is listed on the Table 1. Table 1 system uncertaty Apparatus Uncertaty Temperature.1K Water flow rate.5% Refrigerant mass flow rate.1% Pressure.5% Pressure difference.7% Derive parameter Mass flux, G ±.% ~ 1.% Vapor quality, x ±3.9 ~ 7.3% EXPERIMENTAL RESULTS AND DISCUSSION Sgle-phase pressure drop Figure 3 shows the HFO-134yf and sglephase friction coefficients based on the Reynolds numbers. In lamar region sgle phase friction factor is well predicted by the Poiseuille theory. The predictions by usg Blasius [11] and Filonenko [1] equations were also plotted for comparison. Figures 3 also shows that Blasius equation [1] is a good prediction for turbulent region with lower Reynolds number. Two phase pressure drop and flow pattern flow patter side a tube with.5mm diameter has been provided by Revell et. al [13] Black solid les are the transition le of the flow pattern changes. Black Dash le are the added to origal map to show the extended transition le of the flow pattern. Revell et. al [13] divided the flow pattern to bubbly flow (B), slug flow (S), semi-annular flow (S-A) and annular flow (A) (figure 7). Experimental flow boilg pressure drop has been potted on the given flow pattern. In figure 7 Colors and geometries pot dicate the different mass fluxes. Figure 4~7 shows the two-phase pressure drops versus vapor quality durg the flow boilg of at mass velocities G=1,, 4 and 6 kg/m s. Friedel [14] correlation from two phase pressure drop has been plotted to compare with the experimental data. Table shows the HFO-134yf and properties at saturation temperature of 31. These properties are the most important properties at pressure drops mechanism.

4 The 8 th International Symposium on Transport Phenomena -4 September 17, Peradeniya, Sri Lanka..1.5 HFO-134yf Lamar Blasius Filonenko G=1(kg/m s) HFO-134yf Friedel, HFO-134yf Friedel, Friction factor p (kpa) Re Figure 3 Sgle phase frictional coefficient Table and HFO-134yf properties (T sat = 31 o C) HFO- HFC- Properties Units 134yf 134a Liquid Viscosity 1-6 N/m s Vapor Viscosity 1-6 N/m s Liquid Density kg/m Vapor Density kg/m Surface Tension 1-3 N/m In low mass fluxes (G=1 kg/m s, Figure 4) flow patterns of HFO-134yf and (figure 7) are happened bubbly flow, therefore pressure drop creasg follow the same trend (Figure 4) and this bubbly region pressure drops of HFO-134yf and are similar. Dry has happened at vapor quality over x>.65. Experimental pressure drop shows that at G= kg/m s (figure 5), the range of vapor quality lower than x<.45, pressure drops of HFO-134yf is similar to those of HFC- 134a. Based on the flow pattern map (Figure 7), flow pattern of both refrigerant is the Bubble/Slug, which is the reason for similarity of the pressure drops of HFO- 134yf and. By creasg the vapor quality, x>.45, flow pattern of HFO-134yf and also move to Slug/Semi-Annular region, therefore HFO-134yf shows lower pressure drop compare to this flow region. At medium mass flux (G=4 kg/m s) Flow pattern shows that, the early vapor qualities (x,.5) are the slug region, which means pressured drop has different slop of creasg. By gog to higher vapor qualities (x..5) where the flow pattern is slug/semiannular and semi-annular, the slop of the pressure drops data are creased. At high mass flux (G=6 kg/m s) flow pattern of the pressure drops are slug/semi-annular and semi-annular, therefor pressure drops are crease flow by the similar trend. p (kpa) x ave Figure 4 Flow boilg pressure drop (G=1 kg/m s) G=(kg/m s) HFO-134yf Friedel, HFO-134yf Friedel, x ave Figure 5 Flow boilg pressure drop (G= kg/m s)

5 The 8 th International Symposium on Transport Phenomena -4 September 17, Peradeniya, Sri Lanka p (kpa) G=4(kg/m s) x vs P Exp x Correleation vs P Friedel CONCLUSION Pressure drop of the HFO-134yf and are experimentally vestigated this paper. Sgle phase friction factor of HFO-134yf and are well predicted by lamar theory and Blasius [11] correlations. Two phase pressure drops are vestigated mass fluxes from G=1 to G=6 kg/m s. Flow pattern of the data are the keys to expla the difference between pressure drops of HFO-134yf and differences mass fluxes and vapor qualities. Flow pattern also can expla the reason of the different slop pressure drop creasg. p (kpa) x avg Figure 6 Flow boilg pressure drop (G=4 kg/m s) G=6(kg/m s) Friedel, x avg Figure 7 Flow boilg pressure drop (G=6 kg/m s) Figure 7 and HFO-134yf Flow pattern [13] NOMENCLATURE c p specific heat, J/kgC D hydraulic diameter, m f friction factor G mass flux, kg/m i lv enthalpy of evaporation, kj/kg h heat transfer coefficient, W/m C l length, m ṁ mass flow rate, kg/s p pressure, kpa p pressure drop, kpa x vapor quality void fraction area ration density, kg/m 3 Subscripts a acceleration c contraction e expansion exp experimental f liquid g vapor p preheater w water r refrigerant REFERENCES (1) Union, E. (6): Directive 6/4/EC of the European Parliament and of the Council of 17 May 6 relatg to emissions from air-conditiong systems motor vehicles and Amendg Council Directive 7/156/EEC. Official Journal of the European Union, L161, pp () Yang, C.-Y. and Shieh, C.-C. (1): Flow pattern of air water and two-phase R-134a small circular tubes. International Journal of Multiphase Flow, Vol. 7, pp (3) Padilla, M., Revell, R., Haberschill, P., Bensafi, A. and Bonjour, J. (11): Flow regimes and two-phase pressure gradient horizontal straight tubes: Experimental results for HFO-134yf, R-134a and R- 41A. Experimental Thermal and Fluid Science, Vol. 35, pp (4) Saitoh, S., Dang, C., Nakamura and Y. and Hihara, E. (11): Boilg heat transfer of HFO-134yf flowg a smooth small-diameter horizontal tube. International Journal of Refrigeration, Vol. 34, pp

6 The 8 th International Symposium on Transport Phenomena -4 September 17, Peradeniya, Sri Lanka (5) Lu, M.-C., Tong, J.-R. and Wang, C.-C. (13): Investigation of the two-phase convective boilg of HFO-134yf a 3.9 mm diameter tube. International Journal of Heat and Mass Transfer, Vol. 65, pp (6) Anwar, Z., Palm, B. and Khodabandeh, R. (15): Flow boilg heat transfer, pressure drop and dry characteristics of R134yf: Experimental results and predictions. Experimental Thermal and Fluid Science, Vol. 66, pp (7) Kays, W.M. and London, A.L. (199): Compact heat exchangers. McGraw-Hill, New York, NY, United States. (8) Munson, B.R., Young, D.F. and Okiishi, T.H. (199): Fundamentals of fluid mechanics. New York, United States. (9) Collier, J.G. and Thome, J.R. (1994): Convective boilg and condensation. Oxford university press. (1) Zivi, S. (1963): Estimation of steady-state steam void-fraction by means of the prcipal of mimum entropy production. ASME reprt 63-HT-16, 6th Nat, : Heat Transfer Conf., AIChE-ASME, Boston. (11) Blasius, H. (1913): Das Aehnlichkeitsgesetz bei Reibungsvorgängen Flüssigkeiten, : Mitteilungen über Forschungsarbeiten auf dem Gebiete des Ingenieurwesens: sbesondere aus den Laboratorien der technischen Hochschulen. Sprger Berl Heidelberg, Berl, Heidelberg, pp. 1-41, quoted Bejan, A. (13) Convection heat transfer. John wiley & sons. (1) Filonenko, G. (1948): On friction factor for a smooth tube. All Union Thermotechnical Institute, quoted Kedzierski, M.A. and Kim, M.S. (1996) Sglephase heat transfer and pressure drop characteristics of an tegral-spe f with an annulus. Journal of Enhanced Heat Transfer, Vol. 3, pp (13) Revell, R., Dupont, V., Ursenbacher, T., Thome, J.R. and Zun, I. (6): Characterization of diabatic two-phase flows microchannels: flow parameter results for R-134a a.5 mm channel. International Journal of Multiphase Flow, Vol. 3, pp (14) Friedel, L. (1979): Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow. : European two-phase flow group meetg, Paper E, Vol., quoted Collier, J.G., Thome, J.R. (1994): Convective boilg and condensation. Oxford university press.