International Journal of Emerging Technologies in Computational and Applied Sciences (IJETCAS)

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1 Internatinal Assciatin f Scientific Innvatin and Research (IASIR) (An Assciatin Unifying the Sciences, Engineering, and Applied Research) Internatinal Jurnal f Emerging Technlgies in Cmputatinal and Applied Sciences (IJETCAS) ISSN (Print): ISSN (Online): CFD Analysis f Air-Cled Heat Exchanger Using Reduced Diameter Inner Grved Cpper Tube Firz Rangrez 1, Prf. S.H.Kulkarni 2 Department f Mechanical Engineering, Mumbai University Veermata Jijabai Technlgical Institute, Mumbai, Maharashtra, India Abstract: Replacing cpper tubes with small tube diameters is the ppular trend t lwer the cst f heat exchanger. This paper presents heat transfer enhancement using inner grved cpper tubes with small tube diameters as a substitutin. Perfrmance cmparisn as well as cst analysis are cnducted fr the tw tubes f ϕ12.7mm and ϕ9.52mm inner grved cpper tube air-cled cndenser. The research results shw that: where the testing cnditins are same, the heat transfer cefficient with ϕ9.52mm inner grved cpper tube is abut 30% higher that within ϕ12.7mm inner grved cpper tube. Where the dimensin and perating cnditins are same, ϕ9.52mm inner grved cndenser can achieve the same heat exchanger perfrmance as the ϕ12.7mm inner grved cndenser, the cpper material can be saved up t 15.2%. Keywrds: small diameter inner-grved cpper tube; fin tube heat exchanger; heat transfer perfrmance; cst analysis I. Intrductin Cmpact heat exchangers are characterized by a large heat transfer surface area per unit vlume f the exchanger. Fin Tube Heat Exchanger is a typical cmpact heat exchanger that is widely used in many industrial pwer generatin plants, chemical, petrchemical and petrleum industries [1]. Inner grve tubes are widely used in refrigerant evapratin and cndensatin. Heat transfer frm a prime surface can be increased by attaching fins r extended surfaces t increase the area available fr heat transfer. Fins prvide a mre cmpact heat exchanger with lwer material csts fr a given perfrmance. Inner grve tube can give a better heat transfer cefficient due t increase cntact surface between the tube wall and the refrigerant. As the grve surrunded the entire inner wall f the tube, it can prduce better flw cnditin cmpare t plain tube. The centrifugal frce inside the tube causes the cling fluid t flw in the pattern f the grve, which result t an annular flw inside the tube [2]. Wu Yang et al. [3] substituted the cpper tubes with small diameters. Perfrmance cmparisn as well as cst analysis were cnducted fr tw tube types f ϕ9.52mm and ϕ5mm inner-grved cpper tube, as well as the heat exchangers using these tw types. Kadir Bilen et al. [4] experimentally investigated the surface heat transfer and frictin characteristics f fully develped turbulent air flw in different grve tubes. Tests were perfrmed fr Reynlds number range f t and fr different gemetric grved shape (circular, trapezidal and rectangular) tubes. Gt et al. [5] investigated the cndensatin and evapratin heat transfer f R410A and R22 inside internally grved hrizntal tubes. The measured lcal pressure drp, heat transfer cefficients were cmpared with the predicted values frm previus crrelatins prpsed by the authrs. Wei-kun Ding et al. [6] develped a discretized cmputatinal mdel and general simulatin prgram in which a special circuit data structure was intrduced. The study shwed that increasing circuit branches number, decreasing the single branch length and adding mre tubes in the circuit design, the refrigerant pressure drp were increased while heat exchange rate remained almst the same. Selvaraj P. et al. [7] presented cmputatinal fluid dynamics studies n heat transfer, pressure drp, frictin factr, Nusselt number f a plain tube and tube equipped with three types f internal grves (circular, square and trapezidal). Water was used as the wrking fluid. Tests were perfrmed fr Reynlds number ranges frm 5000 t fr plain tube and different gemetry inside grved tubes. II. Mdel Descriptin A. Physical mdel: The typical fin-tube heat exchanger sectin with staggered arrangement is shwn in Figure 1. The inner-grved tube is a kind f high efficiency heat exchange cmpnent. Because it pssesses 2~3 times the heat exchange capacity f the nrmal bare tube, it has received extensive attentin. The high efficiency f the inner-grved tube stimulates and prmtes the develpment f energy saving and miniaturizatin fr air-cnditining systems. The typical inner-grved tube tth-type parameters are shwn in Figure 2. Of them, d is the uter diameter, is the tube wall thickness, h is the tth height, β is the helix angle, γ is the tth vertex angle and n is the tth number. IJETCAS ; 2014, IJETCAS All Rights Reserved Page 421

2 Fig. 1: Typical Fin-Tube Heat Exchanger sectin with staggered arrangement. Fig. 2: Inner grve tube structure B. Cmputatinal mdel and bundary cnditins fr ϕ9.52mm heat exchanger: B.1. Fin The Gambit Sftware is used t create and mesh the cmputatinal mdel. A diagram f the studied mdel is shwn in Figure 3, and cnsists f air flw area between tw fins f plain fin gemetry and arund the surfaces f fur rws f tubes. Fig. 3: Cmputatinal dmain and bundary cnditins f mdel Fig. 4: Cmputatinal dmain and extended surface f mdel The cmputatinal dmain cntains the bundary cnditins as shwn in Figure 3 with the fllwing cnditins: Tube surfaces as wall, Dirichlet Bundary cnditin T = T w = 54.5 C = Air velcity u = v = w = 0, that is n-slip cnditin at tube surfaces. Unifrm velcity u = u in = 3.8 m/s v = w = 0 T = 40 C = 313K Outlet as a pressure utlet, Neumann Bundary cnditin IJETCAS ; 2014, IJETCAS All Rights Reserved Page 422

3 Zer gradients, u, v, w, pressure and temperature. Side planes as a symmetry, symmetry planes ( The entire cmputatinal dmain was made up f apprximate finite elements with structure grid thrughut mst f the dmain, while the areas arund the tubes are mre unstructured. B.2. Inner grve tube: The Slidwrks sftware is used t create the fluid part f the inner grve tube as shwn in Figure 5. The gemetry is then exprted in Gambit in ACIS frmat t mesh the cmputatinal dmain as shwn in Figure 6. Fig. 5: Cmputatinal dmain f inner grve tube in Slidwrks Fig. 6: Cmputatinal dmain f inner grve tube in gambit The cmputatinal dmain cntains the fllwing bundary cnditin: Inlet as mass flw inlet, Dirichlet Bundary cnditins m = m r = kg/s, alng the z-directin Outlet as pressure utlet, Neumann Bundary cnditin Zer gradients, u, v, w, pressure and temperature. Tube surface as a wall, Dirichlet Bundary cnditin Heat transfer cefficient and free stream temperature f air were applied n wall surface. The entire dmain was made up f apprximate finite vlumes, with a structured grid thrughut mst f the dmain. III. Heat Transfer Calculatins Since the cndenser is a heat exchanger, ne f the heat transfer and pressure drp calculatin methds, LMTD (Lg Mean Temperature Difference) methd is used in this study. Ttal heat transfer capacity is calculated frm Eq. 1, (1) where, Q is the heat transfer capacity, is the verall heat transfer cefficient, is the ttal utside heat transfer area, is the Lg Mean Temperature Difference (LMTD) fr crss flw. Overall heat transfer cefficient is calculated frm Eq. 2 (Kays and Lndn, 1984), (2) where, is the tube inside surface area, is the refrigerant side heat transfer cefficient, k is the tube thermal cnductivity, is the tube utside diameter, is the tube inside diameter, is air-side heat transfer cefficient, is the fin efficiency, is the finned surface area, is the tube utside surface area. IJETCAS ; 2014, IJETCAS All Rights Reserved Page 423

4 IV. Results and Discussins A. Cmputatinal Fluid Dynamics Results: A.1. Fin A characteristic f flw describes the initial bservatin fund using Cntur Display after running the CFD simulatin in Fluent. This gives the behavir f the air flw velcity inside the cmputatinal dmain. a) Velcity Observatins: Fig. 7: Cnturs f velcity field, k-epsiln mdel, inlet air flw 3.8 m/s f ϕ12.7mm heat exchanger Fig. 8: Cnturs f velcity field, k-epsiln mdel, inlet air flw 3.8 m/s f ϕ9.52mm heat exchanger The air enters at the inlet and exits at the utlet as shwn in the abve figures. The utlet is extended twice the height in rder t reduce reversed flw. Nw the flw characteristic f air in heat exchanger can be easily understd. In the abve cases, as the air flw arund the first tube, it begins t speed up and then the air velcity increases again as it ges arund the secnd tube. Frm calculatin, it can be seen that, the minimum free-flw area f ϕ12.7mm heat exchanger is less than that f ϕ9.52mm heat exchanger, which shwed that the velcity ging arund the secnd tube is faster than that ging arund the first tube. The minimum free flw area is the area f the heat exchanger between tw transverse tubes, s the area just abve tube ne r just belw the secnd tube are the minimum free-flw areas. The flw is frced t speed up, as the tubes act as a type f pipe cntractin in the air flw channel. As the velcity increases alng the flw the Pressure f air flw is decreases accrdingly. It is bserved that the size f the tubes impact the Reynlds number f the air flwing arund them, since with larger tubes there wuld be an even smaller minimum free-flw area. In this study, the characteristic length fr the Reynlds number is the tube cllar diameter, and it can be seen here, that increases in this parameter can induce higher velcities and with it a higher turbulence and Reynlds number. b) Temperature Observatin: Fig. 9: Cnturs f temperature field, k-epsiln mdel, inlet air flw 3.8 m/s f ϕ12.7mm heat exchanger. Fig. 10: Cnturs f temperature field, k-epsiln mdel, inlet air flw 3.8 m/s f ϕ9.52mm heat exchanger. The temperature cnturs fr the tw kinds f heat exchangers is shwn in the figure 9 and 10. Fr high Reynlds number as in case f air flw fr ϕ12.7mm heat exchanger, shwn in figure 9 the air has nt sufficient IJETCAS ; 2014, IJETCAS All Rights Reserved Page 424

5 time t absrb the heat frm the tube and fin. S the air temperature at the utlet is lwer than as in lw Reynlds number flw. The largest temperature changes fr the cases are ccurring in the recirculatin and slw velcity znes just after each f the tubes. As in the slw-mving flw in the case with ϕ9.52mm heat exchanger, the slw-mving areas f the heat exchanger are als better able t absrb heat. A.2. Inner Grve Tube a) Velcity Observatin: Fig. 11: Cnturs f velcity field, k-epsiln mdel and mass flw rate 0.086kg/s f ϕ12.7mm heat exchanger Fig. 12: Cnturs f velcity field, k-epsiln mdel and mass flw rate 0.053kg/s f ϕ9.52mm heat exchanger. The velcity cnturs f inner grve tube f bth kinds f heat exchangers are shwn in Figures 11 and 12. It can be seen that the refrigerant clsest t the wall tends t mve slwer than the fluid at the center. The internal enhancement in Inner-Grve tube reduces bundary layers and increase heat transfer thrugh the tube wall. The additinal mixing f the refrigerant that ccurs inside the tube because f these grves increases the amunt f refrigerant that cmes in cntact with the tube wall. Frm the abve figure, it can be seen that, the high velcity increases in ϕ9.52mm tube due t the hmgeneity f the refrigerant. b) TemperatureObservatins: Fig. 13: Cnturs f temperature field, k-epsiln mdel and mass flw rate 0.053kg/s f ϕ12.7mm heat exchanger. Fig. 14: Cnturs f temperature field, k-epsiln mdel and mass flw rate 0.053kg/s f ϕ9.52mm heat exchanger. IJETCAS ; 2014, IJETCAS All Rights Reserved Page 425

6 It is speculated that the grved tubes prvide mre increase in heat transfer augmentatin, due t nt nly prducing mre turbulence and gd fluid mixing than the smth tube but als increase in heat transfer area. They als prvide peridic redevelpment f the bundary layers and cause a mre effective heat transfer. The temperature cnturs f the tw kind f inner grve tube are shwn in figures 13 and 14. Higher pressures are typically required t cndense the envirnmentally friendly refrigerants. The temperature drp f the refrigerant is unifrm in ϕ9.52mm tube as cmpared t ϕ12.7mm tube. This is because the wrking pressure is mre in ϕ9.52mm tube. B. Perfrmance and cst analysis f heat exchangers: Gemetric Parameter Symbl Value Unit Heat Duty Q 65 KW Air Vlume m 3 /s Air Inlet Velcity V 3.8 m/s Air inlet Temperature T a,i 40 C Cndensing Temperature T c 54.5 C Refrigerant - R22 - Table 1: The perating cnditins f Air-Cled Cndenser Gemetric Parameter Symbl Value Unit Finned Length L mm Finned width L mm Finned height L mm Fin Spacing N f mm Vertical Tube Pitch X t mm Hrizntal Tube Pitch X l 27.5 mm Fin Thickness 0.15 mm Tube Outer Diameter d 12.7 mm Tube Inner Diameter d i mm Cllar Diameter D c 13 mm Bttm wall thickness 0.36 mm Helix angle β 18 Tube Fin Height h 0.25 mm Tube Fin angle γ 40 Number f grves n 70 - Number f tubes N t Tube weight/meter g/m Table 2: Structure parameters fr ϕ12.7mm heat exchanger Gemetric Parameter Symbl Value Unit Finned Length L mm Finned width L 2 88 mm Finned height L mm Fin Spacing N f mm Vertical Tube Pitch X t 25.4 mm Hrizntal Tube Pitch X l 22 mm Fin Thickness 0.15 mm Tube Outer Diameter d 9.52 mm Tube Inner Diameter d i 8.72 mm Cllar Diameter D c 9.82 mm Bttm wall thickness 0.3 mm Helix angle β 18 Tube Fin Height h 0.2 mm Tube Fin angle γ 53 Number f grves n 60 - Number f tubes N t Tube weight/meter - 94 g/m Table 3: Structure parameters fr ϕ9.52mm heat exchanger Output variables Unit Baseline mdel 3/8 heat exchanger Difference (%) Refrigerant-side Pressure drp KPa Refrigerant-side Nusselt number Refrigerant heat transfer cefficient W/m 2 K Ttal mass flw rate kg/s Cpper tube weight kg Table 4: The cmparisn f mdeling utput and their crrespnding results frm baseline mdel T verify the mdeling results, the simulatin utput ϕ9.52mm heat exchanger were cmpared with the crrespnding ϕ12.7mm heat exchanger. The tw kinds f heat exchangers are simulated using the same entry parameters as shwn in Table 1. Table 2 and Table 3 shw the structure parameters f the ϕ12.7mm heat IJETCAS ; 2014, IJETCAS All Rights Reserved Page 426

7 exchanger and ϕ9.52mm heat exchanger. Als it can be seen that weight f tube is reducing in ϕ9.52mm heat exchanger. The Numerical results shws that after adjusting the tube number and tube spacing, the ϕ9.52mm inner-grved heat exchanger can reach the almst the same heat exchanger perfrmance as that f the ϕ12.7mm inner-grved heat exchanger. The cmparisn f mdeling utput and their crrespnding results are shwn in Table 4. It can be seen that the cpper tube usage amunt is reduced by 15.2%. The length and height are the same fr bth kinds f heat exchangers. Therefre, the aluminum fil usage fr the ϕ9.52mm heat exchanger is reduced by 20%. C. Mdel Validatin The ϕ9.52mm heat exchanger is als cmpared with the fllwing crrelatins: The Nusselt number fr plain fin, staggered arrangement heat exchanger is defined by Zhukaukas (1972) [8], Fr inner-grved tube is defined by Kadir Bilen et al [4], (4) The Nusselt number f ϕ9.52mm heat exchanger fr the plain fin and inner-grved tube, when cmpared with the crrelatin, is reduced by 10% and 21% respectively. V. Cnclusins The cnclusin btained is as belw: (1) Under the same testing cnditins, the refrigerant-side heat transfer cefficient within the Φ9.52mm inner-grved cpper tube is arund 38.49% higher than that within the Φ12.7mm inner-grved cpper tube. The material cst is thus greatly reduced. (2) Fr the same dimensins and under the same perating cnditins, the Φ9.52mm inner-grved cpper tube heat exchanger can achieve almst the same heat exchange perfrmance as the Φ12.7mm inner-grved cpper tube heat exchanger. The cpper tube usage amunt can als be reduced by The aluminum fil cst is reduced by arund 20%. (3) While keeping the windward crss-sectinal dimensins unchanged, thrugh adjusting the refrigerant manifld and tube spacing, as well as structural parameters like the fin spacing, the heat exchanger with small tube diameter can btain the same perfrmance as the cpper tube heat exchanger with big tube diameter. Nmenclature Q Heat Transfer Capacity (W) U Overall heat transfer cefficient (W/m 2 K) A Ttal utside heat transfer area (m 2 ) T m Lg Mean Temperature Difference ( C) A i Tube inside surface area (m 2 ) h i Refrigerant side heat transfer cefficient (W/m 2 K) k Tube thermal cnductivity (W/mK) d Tube utside diameter (m) d i Tube inside diameter (m) h Air-side heat transfer cefficient (W/m 2 K) η f Fin efficiency A f Finned surface area (m 2 ) A t Tube utside surface area (m 2 ) References [1] Ehsan Khrasani Nejad, Mhsen Hajabdllahi and Hassan Hajabdllahi, Mdeling and Secnd Law based ptimizatin f Plate Fin and Tube Heat Exchanger using MOPSO, J Appl Mech Eng [2] Jia Quigxian, Zha Fufeng, Zeng Zahshun, Zhang Zhi and Li Tingxun, Experimental Research fr Lwering Refrigerant Charge with 4mm Tube Heat Exchanger, GD Midea Refrigeratin Equiment C., Ltd., Fshan, Guangdng. [3] Wu Yang, Li Changsheng and Deng Bin, Perfrmance and Cst Analysis and Research f Air-Cled Heat Exchanger Using Small Diameter Cpper Tubes, Secnd IIR Wrkshp n Refrigerant Charge Reductin, Stckhlm, Sweden, June [4] Kadir Bilen, Murat Cetin, Hasan Gul, Tuba Balta., The investigatin f grve gemetry effect n heat transfer fr internally grved tubes, Internatinal Jurnal f Applied Thermal Engineering, 29(2009), [5] M.Gt, N.Inue, N.Ishiwatari, Cndensatin and evapratin heat transfer f R410A inside internally grved hrizntal tubes, Internatinal Jurnal f Refrigeratin 24 (2001), [6] Wei-kun Ding, Ju-fang Fan, Wen-quan Ta, Develpment f Small-Diameter Tube Heat Exchanger: Circuit Design and Perfrmance Simulatin, Cnference n Thermal and Envirnmental Issues in Energy Systems, Srrent, Italy, May [7] Selvaraj.P, CFD Analysis n Heat Transfer and frictin factr characteristics f a Turbulent flw fr Internally Grved Tubes, Natinal Institute f Technlgy, Tiruchipalli. [8] Clark Bullard, Ian Dvidsn, Extended Crrelatins fr Staggered rund Tube Plain Fin Heat Exchangers, Internatinal Refrigeratin and Air Cnditining Cnference, Purdue University(2006), Paper 803. Acknwledgments This wrk was supprted by members f VJTI at Mumbai and Star Clers and Cndensers Pvt Ltd., Jalgan. (3) IJETCAS ; 2014, IJETCAS All Rights Reserved Page 427