JJMIE Jordan Journal of Mechanical and Industrial Engineering

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1 JJMIE Jordan Journal of Meanial and Industrial Engineering Volume 3, mber 1, Mar ISSN Pages Effet of Half Lengt Twisted-Tape Turbulators on Heat Transfer and essure Drop Carateristis inside a Double Pipe U-Bend Heat Exanger Anil Sing Yadav * Department of Meanial Engineering,Maa Kailadevi Institute of Information Tenology and Management,Gwalior , Madya ades, India. Abstrat Influenes of te alf lengt twisted tape insertion on eat transfer and pressure drop arateristis in a U-bend double pipe eat exanger ave been studied experimentally. In te experiments, te swirling flow was introdued by using alflengt twisted tape plaed inside te inner test tube of te eat exanger. Te results obtained from te eat exangers wit twisted tape insert are ompared wit tose witout twisted tape i.e. Plain eat exanger. Te experimental results revealed tat te inrease in eat transfer rate of te twisted-tape inserts is found to be strongly influened by tape-indued swirl or vortex motion. Te eat transfer oeffiient is found to inrease by 40% wit alf-lengt twisted tape inserts wen ompared wit plain eat exanger. It was found tat on te basis of equal mass flow rate, te eat transfer performane of alf-lengt twisted tape is better tan plain eat exanger, and on te basis of unit pressure drop te eat transfer performane of smoot tube is better tan alf-lengt twisted tape. It is also observed tat te termal performane of Plain eat exanger is better tan alf lengt twisted tape by times Jordan Journal of Meanial and Industrial Engineering. All rigts reserved Keywords: Heat transfer oeffiients; eat transfer enanement; eat exanger; turbulenes promoters; Twist tape. Nomenlature * A Area of eat transfer m 2 di Inside tube diameter, m d 0 Outside tube diameter, m d e Equivalent Diameter {(D 2 i -d 2 o )/ d 0 }, m Di Inside diameter of annulus pipe, m D 0 Outside diameter of annulus pipe, m Y Twist ratio Half Pit/tube diameter {P/di} L Lengt of tube Δp essure drop, kg/m 2 M Volume rate of flow, LPM M Mass flow rate, kg/se Q Rate of eat transfer, W C p Speifi eat of fluid, KJ/kg-K T i, T 0 Inlet and outlet temperature of ot fluid, o C T i, T 0 Inlet and outlet temperature of old fluid, o C Inside eat transfer oeffiient for oil, W/m2-k Water side eat transfer oeffiient for oil, W/m2-k θ Log mean temperature differene, K Re Reynolds No. andt1 No. N u sselt No. K Termal ondutivity of fluid, W/m-k f Frition fator U Overall eat transfer oeffiient, W/m 2 -k * Corresponding autor : anilsingyadav@gmail.om. Greek Symbols α Termal diffusivity, m 2 /se Dynami visosity of fluid, N-se/m 2 ρ Density, kg/m 3 ٧ Kinemati visosity of fluid, m 2 /se Subsripts For ot fluid (oil) For old fluid (water) i Inlet o Outlet 1. Introdution Heat exanger is te apparatus providing eat transfer between two or more fluids, and tey an be lassified aording to te mode of flow of fluid or teir onstrution metods. Heat exangers wit te onvetive eat transfer of fluid inside te tubes are frequently used in many engineering appliation. Enanement of eat transfer intensity in all types of termo tenial apparatus is of great signifiane for industry. Beside te savings of primary energy, it also leads to a redution in size and weigt. Up to te present, several eat transfer enanement teniques ave been developed. Twistedtape is one of te most important members of enanement teniques, wi employed extensively in eat exangers. A detailed survey of various teniques to augment onvetive eat transfer is given by Bergles [1].Twisted tape teniques ave been used to augment eat transfer in

2 Jordan Journal of Meanial and Industrial Engineering. All rigts reserved - Volume 3, mber 1 (ISSN ) double pipe eat exanger. Kumar et al. [2] found tat oiled type of swirl generator auses te inrease of 72% in eat transfer, but fritional power also inreases 90%. For water flowing troug a vertial stainless steel tube, pit to diameter ratio of 1.0 to 5.5 was used. Hong et al. [3] found tat in te ase of ig andtl mber fluid in laminar flow, te eat transfer rate inreases onsiderably for a moderate inrease in pressure drop. Saa et al. [4] ave onsidered twisted tape element onneted by tin irular rods. Tey observed tat te pressure drop assoiated wit te full-lengt twisted tape ould be redued witout impairing te eat transfer augmentation rates in ertain situation. Smit Eiamsa-ard et al. [5] onduted experiments on a onentri tube eat exanger. Hot air passed troug inner tube wile te old water was flown troug te annulus. A maximum perentage gain of 165% in eat transfer rate was obtained by using te elial insert in omparison wit te plain tube.saa et al. [6] investigated te use of turbulenes promoters wit sort lengt Twist tape, and regularly spaed Twist tape element. Tey aieved better termodynamis performane wit sort lengt Twist tape, and regularly spaed Twist tape element instead of fulllengt Twist tape wile working wit twisted tape. Saa et al. [7] ave onsidered different types of strips. Te strips are of longitudinal retangular, square and rossed rosssetion, full lengt and sort lengt, as well as regularly spaed types. Tey observed tat sort lengt strips perform better tan full-lengt strips. Frition fator redues by 8-58% and sselt no redues by 2-40% for sort lengt strips. For regularly spaed strips elements, Frition fator inreases by 1-35% and sselt no inrease by 15-75%.Smit et al. [8] ompared tree different eat transfer enanement metods namely: Mirofins, Twist tape, and Hig fins to tat of smoot tube. Tey observed tat te eat transfer oeffiients are inreased by approximately 46%, 87% and 113% on average ompared to tose of smoot tube, using respetively Twist tape, Hig fins, and Miro fins. Tey also observed tat on average te pressure drop of Miro fins tube is 38% iger tan tose of smoot tube. Hig fins tube inreases te pressure drop by 81% in omparison to smoot tube, and Twist tape inreases te pressure drop by 148%. Heat transfer and pressure drop arateristis of laminar water flow troug a irular tube wit longitudinal inserts were experimentally studied by Hsie et al. [9]. Testing was performed on tubes wit square and retangular as well as rossed longitudinal strip inserts wit Aspet ratio AR1 and 4. Solanki et al. [10] onduted experimental and teoretial studies of laminar fored onvetion in tubes wit polygon inner ores. Ray et al. [11] investigated experimentally orrelations of eat transfer and flow fritions in a square dut wit twisted-tape insert.sivasanmugam et al. [12] investigated experimentally te eat transfer and frition fator arateristis of irular tube fitted wit Rigt-Left elial srew inserts of equal and unequal lengt of different twist ratio.napon [13] investigated eat transfer and te pressure drop arateristis in a orizontal double miro-fin tube wit twisted tape inserts. An overview of undred resear works of enanement eat transfer rate by using twisted tape was reported by Dewan et al. [14]. Anil Sing Yadav [15] investigated eat transfer and te pressure drop arateristis in a double pipe eat exanger wit full lengt twisted tape inserts. Te objetive of tis paper is to study eat transfer and pressure drop arateristis of te u bend double pipe eat exanger wit and witout alf lengt twisted tape insert. Te effets of various relevant parameters on eat transfer arateristis and pressure drop are also investigated. New data gatered during tis work for eat transfer and pressure drop arateristis for te u bend double pipe eat exanger wit alf lengt twisted taped insert are proposed for pratial appliations. 2. Experimental Setup Figures 1 and 2 sow a semati view of te experimental setup and a onentri double pipe eat exanger fitted wit a alf lengt twisted tape. Te set-up onsists of: 1. An oil tank wit eater of 0.64 m 3 apaity plaed on floor. 2. An overead water tank 0.5 m 3 apaity loated at an elevation of 2.75 meters. 3. Double pipe u bend eat exanger. 4. Measuring devies like Rotameter, temperature indiator, and pressure gauge. 5. Twisted tape. 6. Gear pump. Te oil tank is plaed on te floor; and is provided wit eating oil of variable input. Te tank dimensions are 0.8 m x 0.8 m x 1 m. Te tank is provided wit PVC tube of 1.85 m long and 5m diameter, wi is onneted to 1.5HP motor. Te motor outlet is onneted to te inlet of eat exanger troug pipe to irulate ot oil in kt. Tis pipe is onneted to inner tube of eat exanger troug flange oupling. Tis pipe is provided wit different measuring devies like rotameter, temperature indiator, and pressure gauge. An overead tank is a Sintex tank of 0.5 m 3 loated at a eigt of 2.75 m from te floor. Te flow rate of water is kept onstant at te rate of 15 Lit/min. Test setion is double pipe eat exanger of u bend type as sown in Figure 1. Te Heat Exanger onsists of 2 m lengts in ea arm and m lengt of u-bend setion. Te eat exanger is made up of stainless steel tubes. Te inner diameter of inner tube is 2.11m, and outer diameter of inner tube is 2.5 m. Inner diameter of annulus pipe is 5 m. Te two straigt legs of inner tube are onneted to U-bend setion wit te elp of flange oupling. Te test setion was eavily insulated by asbestos rope insulation. Rotameter is used to measure te flow rate of oil in te inner tube. Rotameter is onneted at inlet to inner pipe of eat exanger. Te range of rotameter is 0-50 Lit/minute. Two Burdon pressure gauges are used at inlet setion and anoter at outlet setion of ot oil. Te range of essure gauge is 0-5 kg/m 2. Te differene in reading of inlet and outlet pressure gauge gives te pressure drop in eat exanger. Four Digital Termometers are used at inlet and outlet setion of ea ot and old fluid. In all experiments twisted tapes were made out of 0.8 mm tik stainless steel strip. Te widt of wi was 1 mm less tan te inside diameter of test setion.

2009 Jordan Journal of Meanial and Industrial Engineering. All rigts reserved - Volume 3, mber 1 (ISSN 1995-6665) 19 Figure 1: Semati Diagrams of Experimental Setup.

3 2009 Jordan Journal of Meanial and Industrial Engineering. All rigts reserved - Volume 3, mber 1 (ISSN ) 19 Figure 1: Semati Diagrams of Experimental Setup. Figure 2: Te inner tube fitted wit alf lengt twisted tape. Te strip were at first pused into a tube, and ten one end of te strip was tigtened in a vie keeping tube in perpendiular position, and oter end was twisted by tong. Twisted tapes were manufatured in te Amsler torsiontesting maine to te desired twist ratio; and were later inserted in te test setion. 3. Experimental oedure and Data Redution First te plain tube double pipe eat exanger (i.e. witout turbulator) was tested. At te beginning of series of tests, te ot oil was irulated troug inner tube and ooling water troug annulus tube in ounter-flow onfiguration. Te air was bled at various loations. Te flow rate of water was fixed to 15 Lit./min. Te ooling water oming in eat exanger was at room temperature. First te oil flow rate was fixed to 2 Lit/min. A presribed eat input was given to te oil in oil tank in suffiient state. Usually 1/2 our was required for te attainment of steady state for a run. One te steady state was reaed te flow rates of ot and old fluid, temperature reading at inlet and outlet setion of ot and old fluid and burden pressure gauge readings were taken. Te flow rate of old water was kept onstant, and above proedure was repeated for different flow rates of ot fluid. After ompleting te test wit plain eat exanger (i.e. witout turbulator), te u bend double pipe eat exanger was removed from loop. Ten alf-lengt twisted tape was inserted into te bot straigt legs (2m ea) of te u- tube. Te tape was inserted from one side and pulled from oter end by tread or tin wire. Ten te eat exanger was onneted in loop; and took various readings. Transformer oil was irulated inside tube and old water troug annulus in ounter flow arrangement. Te range of values of various parameters onsidered in te present investigations is given in table 1.Heat input was determined from te entalpy rise of te fluid. A linear variation in te bulk temperature was assumed over te test lengt. Te tube wall inside temperature was alulated by one-dimensional ondution equation. Te average wall temperature and te bulk mean temperature were ombined wit eat flux to give te sselt No. all te fluid properties were evaluated at te mean film temperature. essure drop data was obtained under isotermal ondition, and te fanning fration fator was alulated.

4 Jordan Journal of Meanial and Industrial Engineering. All rigts reserved - Volume 3, mber 1 (ISSN ) Table 1: Range/values of parameters. Parameters Range/values Te flow rate of oil (M H ) 4, 8, 12,18,24,30 (Lit/min) Te flow rate of water (M C ) ID of inner tube (di) OD of inner tube(do) ID of outer tube(di) Te water temperature at inlet temperature) Twist ratio for alf lengt twisted tape 7 Tikness of Twisted tape Lengt of Twisted tape 15 Lit/min (Constant) m m 0.05 m 25 C (ambient 0.8mm 2m (2-piee) Heat exanger area(a 0 ) 0.628m 2 Test lengt of eat exanger: For eat transfer 4. Data Redution Equations 8m For pressure drop 8.46m Te data redution of te measured results is summarized in te following proedures: For fluid flows in a onentri tube eat exanger, te eat transfer rate of te ot fluid (oil) in te inner tube an be expressed as: q m Were m is te mass flow rate of ot oil, is te speifi eat of ot oil, T i and T o are te inlet and outlet ot oil temperatures, respetively.heat transfer oeffiient on ot oil side ( ) an be alulated from Were is te sselt number for ot oil side and is given by: 1. For turbulent flow 2. For laminar flow ( T T ) i o Re d i 1.86 L 0. 4 Wile te eat transfer rate of te old fluid (water) in te annulus is ( ) Re q m T o T i Were m is te mass flow rate of old fluid, is te speifi eat of old fluid, T i and T o are te inlet and outlet old fluid temperatures, respetively.heat transfer oeffiient on old fluid (water) side ( ) an be alulated from k d e 0.33 w 0.14 Were is te sselt number for old fluid (water) side and is given by 1. For turbulent flow 2. For laminar flow Re d e 1.86 L Re 0.33 w 0.14 Te overall eat transfer oeffiient based on Outer surfae (U ) an be determined from 1 U O 1 d o d o d o + ln + k d i d i Frition fator, f, an be alulated from f ρu 2 Δp L 2 D i 1 Were ΔP is te pressure drop aross te test setion, ρ is te density of oil, d i is te inner diameter of tube, u is te veloity of oil, and L is te lengt of tube. 5. Result and Disussion After aving studied eat transfer and pressure arateristi, it beomes neessary to ombine tese to evaluate te performane of alf-lengt tapes. For tis purpose, teir performane was studied, for ea eat flux separately, for equal mass flow rates and unit pressure drop Equal Mass Flow Rate Basis Figure 3 sows te performane evaluation for te alflengt tapes on equal mass flow rates basis. Tis is a simple riterion for performane evaluation. Heat Transfer Coeffiient (w/m2k) Smoot Heat Exanger Heat Exanger w it Half Lengt Tw isted tape Flow Rate (LPM) Figure 3: Flow rate vs. eat transfer oeffiient. Figure 3 sows tat te average eat transfer oeffiient inside tube inreases wit inrease in te flow rate of fluid in ea ase. On omparing te different urves, it as been observed tat eat transfer performane of alf-lengt twisted tape is maximum followed by smoot tube. Te eat transfer oeffiient is inreased by approximately 40% on average ompared to tose of smoot tubes using alf-lengt twisted tape. Te inrease in eat transfer oeffiient from smoot tube to twisted tape an be well understood by boundary layer penomenon. In smoot tubes, te flow is

5 2009 Jordan Journal of Meanial and Industrial Engineering. All rigts reserved - Volume 3, mber 1 (ISSN ) 21 stream lined flow. Due to slip ondition, te fluid in ontat wit tube (wetted perimeters) flow at very slow speed tan inner ore of tube. Due to tis, boundary layer tikness is ig, and eat transfer is retarded. Te boundary layer tikness may be redued by fitting turbulators to eat transfer surfaes. Tese twisted tape tabulators interrupt te fluid flow, so tat a tik boundary layer annot form. essure drop (Kg/m2) Flow Rate (LPM) Figure 4: essure drop (ΔP) Vs. Flow rate Unit pressure drop basis Unit pressure drop basis is an important riterion in eat exange equipment design. An augmentative tenique, wi is effetive from te eat transfer point of view, may fail in ase it results in a pressure drop penalty greater tan wat te equipment an andle. Te inrease in pressure drop is ertainly a disadvantage resulting out of te use a turbulene promoter. Te advantage gained in terms of inrease of average eat transfer oeffiient by using a turbulene promoter is partially offset by te inreased pumping power requirements. In order to study te relative advantage of turbulene promoter vis-à-vis its disadvantage, te study of te parameter eat transfer oeffiient per unit pressure drop appears to be appropriate. Figure 4 sows te plots of essure drop (ΔP) against Flow rate. Figure 5 sows te plots of i/δp against Flow rate. Termal performane ratio of te eat exanger is ratio of eat transfer oeffiient to pressure drop. Termal performane ratio i /ΔP (mk -1 s -1 ). On omparing te different urves of figure 5, it as been observed tat Termal performane ratio of smoot tube is maximum followed by alf-lengt twisted tape. It as been observed tat termal performane of smoot tube is better tan alf lengt twisted tape by times. Termal performane ratio dereases wit use of turbulators beause of inrease in pressure drops more tan inrease in eat transfer oeffiient. 6. Conlusion Smoot Heat Exanger Heat Exanger w it Half Lengt Tw isted tape From te present investigation on double pipe eat exanger wit and witout twisted tapes inserts at different mass flow rate of oil, it was found tat: 1. As ompared to onventional eat exanger, te augmented (wit turbulator) eat exanger as sown a signifiant improvement in eat transfer oeffiient by 40% for alf-lengt twisted tape. 2. On equal mass flow rate basis, te eat transfer performane of alf-lengt twisted tape is maximum followed by smoot tube. 3. On unit pressure drop basis, te eat transfer performane of smoot tube is maximum followed by alflengt twisted tape. It as been observed tat termal performane of smoot tube is better tan alf lengt twisted tape by times. Termal Performane Smoot Heat Exanger 4500 Heat Exanger w it Half Lengt Tw isted tape Flow Rate (LPM) Figure 5: Flow rate Vs. Termal Performane Ratio. Aknowledgement Te autor would like to express is appreiation and gratitude to Mr. K.C.Arora-IPSCTM, Gwalior, Mr. N.H.Kela-CSIT, Durg, Mr. Rajes Kumar-CSIT, Durg, and Mr. Sanjay Sarma-CSIT, Durg, for teir fruitful suggestions, wi led to improving tis paper and to opening te way for furter resear relevant to it. Referenes [1] Bergles A E. Teniques to augment eat transfer readings. Handbook of eat transfer appliations, NewYork: MGraw Hill; 1995, Capter 3. [2] P. Kumar, R.L.Judd, Heat transfer wit oiled wire turbulene promoters. Te Canadian Journal of Cemial Engineering, Vol. 48, 1970, [3] S.W. Hong, A.E. Bergles, Augmentation of laminar flow Heat Transfer in tube by means of twist tape insert. Journal of Heat Transfer, Trans.ASME, Vol.98, No.2, 1976, [4] S.K Saa, U.N. Gaitonde, A.W.Date, Heat transfer and pressure drop arateristis of Laminar flow troug a irular tube fitted wit regularly spaed twisted tape elements wit multiple twists. Exp.Term. Fluid Si., Vol. 2, No.3, 1989, [5] E. Smit, P. Yuttana, An Experimental study of eat transfer and frition fator arateristis in a irular tube fitted wit a elial tape.18 t Conferene of Meanial Engineering network of Tailand, Kon Kaen,Tailand, [6] S.K. Saa, A.Dutta, Termodynami study of laminar swirl flow troug a irular tube fitted wit twisted tape elements. Journal of Heat Transfer, ASME, Vol. 123, No.3, 2001, [7] S.K. Saa, P. Langile, Heat transfer and pressure drop arateristis of Laminar flow troug a irular tube wit longitudinal strip inserts under uniform wall eat wall. Journal of Heat Transfer, ASME, Vol. 124, 2002, [8] F.J. Smit, J.P. Meyer, R-22 and Zeotropi R-22/R-142b Mixture ondensation in Miro fin, Hig fin and Twisted tape insert tube. Transations of te ASME, Vol. 124, 2002,

6 Jordan Journal of Meanial and Industrial Engineering. All rigts reserved - Volume 3, mber 1 (ISSN ) [9] S.S. Hsies, I.W. Huang, Heat transfer and pressure drop arateristis of Laminar flow troug a irular tube wit/witout longitudinal strip inserts.asme J.Heat Transfer, Vol. 122, 2000, [10] S.C. Solanki, S.akas, C.P.Gupta, fored onvetion eat transfer in doubly onneted dut. Int. J. Heat Fluid Flow, Vol. 8, 1987, [11] S. Ray, A.W. Date, Frition and Heat Transfer Carateristis of Flow troug Square Dut wit Twisted Tape Insert. International Journal of Heat and Mass Transfer, Vol. 46, 2003, [12] P. Sivasanmugam, P.K. Nagarajan, Studies on eat transfer and frition fator arateristis of laminar flow troug a irular tube fitted wit rigt and left elial srew-tape inserts. Experimental Termal and Fluid Siene, Vol. 32, No.01, 2007, [13] P. Napon, Heat transfer and pressure drop in te orizontal double pipes wit and witout twisted tape insert. Int. Commun. Heat Mass Transf. Vol. 33, 2006, [14] A. Dewan, P. Maanta, K. Sumitra Raju, P.Sures Kumar, Review of passive eat transfer augmentation teniques. J. Power Energy, Vol. 218, 2004, [15] A. S. Yadav, Experimental investigation of eat transfer performane of double pipe U-bend eat exanger using full lengt twisted tape. International Journal of Applied Engineering Resear (IJAER), Vol.3, No.3, 2008,

CHAPTER 3 FABRICATION AND EXPERIMENTS OF MICROCHANNEL HEAT SINKS

66 CHAPTER 3 FABRICATION AND EXPERIMENTS OF MICROCHANNEL HEAT SINKS 3.1 OVERVIEW In this hapter, fabriation of slitting saw type plain milling utters (of various dimensions) and form type slotter shaping