Influences of pitch-length louvered strip insert on thermal characteristic in concentric pipe heat exchanger

Transcription

1 MATEC Web f Cnferences 0, 0304 (07) DOI: 0.05/ mateccnf/ Influences f ptch-length luvered strp nsert n thermal characterstc n cncentrc ppe heat exchanger Indr Yanngsh,*, and Agung Tr Wjayanta Mechancal Engneerng Department, Faculty f Engneerng, Sebelas Maret Unversty, 576 Surakarta, Indnesa Abstract. The effects f ptch-length luvered strp nserts wth a frward arrangement n heat transfer and frctn factr characterstcs n a cncentrc ppe heat exchanger were expermentally nvestgated. Luvered strp nsert was nstalled nsde the tube. Fr cmparsn, the experment wthut luvered strp (plan tube) was cnducted. All cases were tested under steady state cndtn fr ynlds number rangng frm ,500. The results shw that the utlzatn f luvered strp nsert prduced a hgher heat transfer rate cmpare t a plan tube. Nusselt number, frctn factr, and heat transfer ceffcent rat ncreased wth decreasng ptch length. The Nusset number fr luvered strp nsert wth S f 40, 50, and 60 mm ncreasng n the value f 67% - 77%; 48% - 53%; and % - 4% than plan tube, respectvely. Whle the tube ftted wth luvered strp nsert by S f 40, 50, and 60 mm, frctn factr ncreased ;.4 -.9; and tmes greater than a plan tube. The addtn f luvered strp nsert wth S f 40, 50, and 60 mm prducng heat transfer ceffcent rat n the range f.0 -.; and Intrductn Enhanced heat transfer s a methd t ncrease the thermal perfrmance system by enhancng heat transfer and flw characterstcs. In the last years, enhanced heat transfer technlgy has been develped and extensvely appled t heat exchanger applcatns; such as clng devces, autmtve, chemcal ndustres and thers. Sme nvestgatns have been dne t ncrease the heat exchanger perfrmance wth the am t dmnsh the sze and cst f a heat exchanger, whch can lead t mre ecnmcal desgn. Ths ecnmcal desgn can help t make energy savng related t heat exchange prcess. The requred pumpng pwer nfluences energy savng n ths case. The sgnfcant effect f usng heat transfer enhancement technlgy s the rse f pressure drp that leads t the ncrease f requred pumpng pwer. Pressure drp n the tube flw ccurs due t the frctnal lsses wth the sld surface. Therefre, whle selectng the enhancement methd shuld be ptmal wth the cnsequence f the rsng heat transfer ceffcent and the pumpng pwer because f the rsng pressure drp. Amng three enhancement methds, passve, actve, and cmpund methds, the passve methds are mstly used t acheve the hghest heat transfer rate. There are many knds f mechansm f enhanced heat transfer by usng passve methds such as mdfed surfaces r gemetres, extended surfaces, and nsertn. The nsertn cnsdered as the mst advantageus passve methd. There are several advantageus f the use f nsertn () can be ftted n the smth tube; () can be frmed n varus shapes; (3) easy t nstall and perate. searchers have been cnducted a large numbers f expermental nvestgatn usng nsertn methds. Includng luvered strp, twsted tape, wre cl and helcal wre cl t examne the thermal characterstcs n each nsertn. The experment related t a mdfcatn f twsted tape nsert, Yanngsh et al. [] expermentally nvestgated the heat transfer and flw characterstcs f heat exchanger by emplyng perfrated twsted tape nserts wth a dfferent axal ptch rat. At the gven ynlds number, heat transfer rate, frctn factr and thermal perfrmance factr ncreases wth decreasng axal ptch rat. Sbel et al. [] develped a study by usng equlateral trangle crss-sectn cled wre emplyed nsde the tube. The use f nsertn guded t a sgnfcant ncrease n heat transfer and pressure drp ver the smth tube. Mhammed [3] and Fan [4] studed numercally n the cnvectve heat transfer characterstcs f heat exchanger fxed wth luvered strp nsert wth varus slant angle and ptch length. In the tested range, the Nusselt number ncreases wth the ncreasng slant angle and tend t decrease wth rsng ptch length. Eswaraah [5] studed numercally n heat transfer enhancement usng luvered strp nserts n the l cler. Numercal mdellng was usng cmputatnal flud dynamcs (CFD). These experments were carred t cmpare the theretcal crrelatns and CFD smulated result. The result shws that a gd agreement was fund between the theretcal crrelatns and CFD smulated result. Emsa-ard [6] expermentally studed the nfluenced f luvered strp nsert wth backwards and frward arrangement n the * Crrespndng authr: ndryanngsh@staff.uns.ac.d The Authrs, publshed by EDP Scences. Ths s an pen access artcle dstrbuted under the terms f the Creatve Cmmns Attrbutn Lcense 4.0 (

2 MATEC Web f Cnferences 0, 0304 (07) DOI: 0.05/ mateccnf/ characterstcs heat transfer and frctn factr f cncentrc ppe heat exchanger. The results reveal that the usng f luvered strp nsert yelded the hgher heat transfer rate cmpared wth a plan tube. Pethkl [7] expermentally researched thermal characterstcs f cncentrc ppe heat exchanger ftted wth luvered strp nsert wth the dfferent nclnatn angle. The results shw that the Nusselt number and frctn factr tend t ncrease wth ncreasng nclnatn angle. Sarada et al. [8] usng luvered square leaf nserts nsde a hrzntal tube heat exchanger t estmate the enhancement heat transfer rate fr ar n the presence f nsert. The expermental nvestgatn carred ut by cmbnatn dfferent arrangement f the drectn f the arflw and nclned angle. The results shw that Nusselt number and pressure drp ncreased. The verall enhancement rat s hghest fr the hghest nclned angle. Varatn f the gemetry luvered strp nsert ncludes ptch and slant angle were nvestgated by Husseune et al. [9]. They nvestgate the thermal and hydraulc perfrmance f such a cmpund heat exchanger. They fund that the cmbnatn between small fn ptch and large slant angle cause the best perfrmance f the cmpund heat exchanger. Other experment cnducted by Raut and Farkade [0], they studed cnvectve heat transfer enhancements n a tube wth three arrangements f luvered strp nsert (frward, sem-frward, sembackward, and frward-backward arrangements). The results shw that the that the dfferent arrangement has a varus value f Nusselt number and frctn factr. The lteratures abve shw that the usng f nsertn can ncrease the thermal perfrmance f the heat exchanger. Frm lteratures [3-0], there are many aspects that nfluence the use f luvered strp nsert (gemetry, the arrangement t the flw drectn, and shape f luvered strp nsert). The man mechansm f heat transfer enhancement due t nsertn nclude the reductn f the hydraulc dameter and nduced swrlng flw makes a better flud mxng []. Based n the lterature ths experment wll be carred ut t nvestgate the thermal characterstcs f cncentrc ppe heat exchanger nstalled wth luvered strp nsert. The nnvatve desgn and new materals f luvered strp nsert were present n ths experment. Expermental detals The schematc dagram f test rg s shwn n Fg.. Fg.. Schematc dagram f cncentrc tube heat exchanger rg The expermental cncentrc tube heat exchanger rg cnssts f cncentrc tube heat exchanger, cld water crculatn system, ht water crculatn system and nstrumentatn system t measure cld water and ht water flw rate, pressure and temperature. Cncentrc tube heat sectn made frm the straght alumnum ppe cnssts f an uter ppe and nner ppe. The dmensns f the nner and uter tubes f the heat exchanger are: d = 4.3 mm, d = 5.8 mm, L = 500 mm, D = 3.4 mm, D = 5.4 mm and L = 950 mm. The heat exchanger s rented hrzntally, and the fluds nsde the ppe are n cunter flw drectns. Ht water was passed thrugh the nner tube, whle cld water was flwng thrugh the annulus. The uter wall f the ppe s wrapped wth the glass wl as nsulatn t mnmze the heat lsses t ambences. Cld-water crculatn system cnssts f an verhead cld-water tank, cld-water tank, and cld-

3 MATEC Web f Cnferences 0, 0304 (07) DOI: 0.05/ mateccnf/ water pump. The verhead water tank s used fr supplyng cld water usng gravty flw wth cnstantly flw rate at 0.03 kg/s. The nlet cld-water temperature was the ambent temperature that relatve cnstant at ± 8 C. After passed thrugh the system, cld water expsed t the surrundng. The ht water crculatn system cnssts f ht water tank, flw regulatn valves, heatng system, and ht water pump. The heatng system cnssts f a water tank wth eght electrc heaters, 4000 W fr each electrc heater. The nlet ht water temperature was kept cnstant at 60 C. Rtameter was used t measure the flw rates. The test runs are dne at the ht water mass flw rates rangng between kg/s and kg/s. Data were cllected at steady state cndtns. Temperature measurement was dne by K-type thermcuples. K-type thermcuples were used fr the measurement f nlet and utlet temperatures f cld and ht water fluds. Smlar t the flud, ten thermcuples were used t measurement f nner ppe uter wall. The temperatures were read by usng a mult-channel dgtal thermmeter. Pressure lss f the nner ppe sde was determned by usng U-tube manmeter. The wrkng flud nsde manmeter was water. At present wrk, t s ntended t fnd the changes n the cnvectn heat transfer ceffcents f the nner ppe sde turbulent flw by affectng the regns near the wall f the ppe flw. Fr ths bjectve, luvered strp nsert was nstalled n the nner ppe sde f the cncentrc duble ppe heat exchanger as a tubulatr. Fr cmparsn, ths study als tested nner ppe wthut nsert (plan tube). The luvered strps that used n ths expermental study were shwn n Fgs. and mm Frnt vew a = 5 S Tp vew 6 mm 0 mm 0 mm Fg.. Luvered strp wth frward arrangements Luvered strp nsert, S = 40 mm, α = 5 Luvered strp nsert, S = 50 mm, α = 5 Luvered strp nsert, S = 60 mm, α = 5 Fg. 3. Luvered strp wth dfferent ptch length (S) Cre-rd Luvered strp nserts made frm mld steel, mm n thckness wth an ellptcal shape. The dmensn f ellptcal luvered strp nsert was 6 mm, 0 mm respectvely fr mnr and mayr axs. The luvered strp nsert was cnnected t a central rd f mm dameter as shwn n the Fg.. The nfluenced f Inflw dfferent ptch length (S = 40, 50, and 60 mm) are studed n the present study. The slant angle () was kept cnstant at 5. Luvered strps nsert s n a frward arrangement wth the drectn f flud flw. Steady state values f the flud and wall temperatures f the experments were btaned fr gven mass flw rate f the ht water flud. These values were used t cmpute heat transfer rate t the flud flwng nsde a tube. Nusselt number and frctn factr were als calculated t knw the effect f luvered strp nserts n heat transfer and frctn characterstcs. The fllwng expressn were used fr calculatn f heat transfer rate f ht and cld water (), heat transfer ceffcent (h), Nusselt number (Nu), frctn factr (f), ynlds number () and thermal perfrmance factr (). Heat transfer rate frm ht water n the nner tube: h. m. C p, h.( Th, Th, Heat transfer rate frm cld water n the annulus: where T c b, T w,. ) m. C p, c.( Tc, Tc, ) h. A.( T w, Tb, T c, T c, T T w, and 0 w, s the average f the uter wall nner tube temperature. There are ten pnt measurements lcated n the uter wall nner tube. T b, s the bulk cld temperatures, cmputed frm the average f nlet and utlet cld water temperature. The dfferences between equatns () and () ndcate the cnvectve heat lss, whch may be assumed t be neglgble: lss = h - c (4) An average heat transfer rate between ht and cld water, ave h c Overall heat transfer s: U A. T ave LMTD ) () () (3) (5) whle an average cnvectn heat transfer ceffcent, h, btaned frm the verall thermal resstance cnsst f three resstances n seres: cnvectve thermal resstance f the nner tube, cnductance thermal resstance f the nner tube wall, and cnvectve thermal resstance n the annulus sde: ln ( d d) U. A h. A k L h. A Thus h can be cmputed frm equatn (7) as: p (6) (7) 3

4 MATEC Web f Cnferences 0, 0304 (07) DOI: 0.05/ mateccnf/ h d.ln ( d U k p d ) d d. h The mean Nusselt s expressed as fllws: h. d Nu k The frctn factr s defned as: f ( u P )( L d The ynlds number can be calculated as: ) (8) (9) (0).u.d () The flud therm-physcal prpertes f the water are determned at the mean bulk flud temperature (T b). Thermal perfrmance factr () s defned as the rat f the heat transfer enhancement rat t the frctn rat at the same pressure drp. The thermal perfrmance factr can be wrtten as: 3 ( Nut Nup) ( ft / f p) () The present expermental data wll be cmpared wth standard crrelatns fr Nusselt number and frctn factr under a turbulent flw regme. The Nusselt number btaned frm the present plan tube are cmpared wth thse frm the prpsed crrelatns by Petukhv (3) and Gnelnsk (4) [] and frctn factr prpsed crrelatn by Blasus (5), 3. Nusselt number crrelatn frm Petukhv s: f 8.Pr Nu f 8. Pr fr 0 4 < < 5 x 0 6. Nusselt number crrelatn frm Gnelnsk: f (3).Pr Nu (4) 3. 7f 8. Pr fr 0 3 < < 5 x 0 6. Frctn factr crrelatn frm Blasus: f = (5) fr 4 x0 3 < < sults and dscussn 3. Valdatn The present data (Nusselt number and frctn factr) f plan tube were cmpared wth the emprcal equatns (3-5) t cnfrm the accuracy and relablty f the expermental setup. Fgure 4 demnstrated the cmparsn between expermental data and the standard equatn fr Nusselt number Nu Plan tube expermental data Plan tube crrelatn Gnelnsk Petukhv Fg. 4. Nusselt number valdatn fr plan tube Frm Fg. 4, t appears that the Nusselt number f the present data devated frm thse f Dttus-Belter and Pethukv emprcal expressn wthn ±.78% and ±7.05% respectvely. Fgure 5 shws the cmparsn between frctn factrs frm the present data wth Blasus crrelatn f Plan tube expermental data Plan tube crrelatn Blasus Fg. 5. Frctn factr valdatn fr plan tube The frctn factr devated frm the prpsed crrelatn by Blasus wthn ±.46%. The cmparsns mply that the present data accrd well wth the crrelatns, valdatng the relablty f the present expermental setup and methd. The present results are crrelated wth the Nusselt number and frctn factr fr the plan tube as fllws: Nu = 0,007 0,946 Pr 0,3 (6) f = 0,495-0,98 (7) Equatns (6) and (7) are fund t represent the expermental data wthn ±6.6% fr Nusselt number and ±.6% fr frctn factr, as shwn n Fgs. 3 and 4 respectvely. 4

5 MATEC Web f Cnferences 0, 0304 (07) DOI: 0.05/ mateccnf/ Heat transfer characterstc Fgure 6. depcted the heat transfer characterstcs f the tube n term f Nusselt number (Nu) wth ynlds number () Nu S = 40 mm S = 50 mm S = 60 mm Plan tube Fg. 6. Nusselt number vs ynlds number Fr all cases, the results present that the Nusselt number (Nu) rses wth the ncrease f ynlds number. The results are smlar wth prevus experment []. Fr dfferent ptch-length (40, 50, 60 mm), the Nusselt number tend t ncrease wth decreasng the ptch-length. The expermentatn shwed that ptch-length 40 mm prvded the maxmum value. Hwever, ptch-length 60 mm has the lwest value f Nusselt number. The reasns why the excellent heat transfer was prduced by smallest ptch-length s wth the smaller ptch-length value resultng the hgher cmpactness f the leaves f the luvered strp nsert. The effects f the hgher cmpactness are there was mre streamlned pattern breaks up f flud flws, therefre the flw between the elements f the leaves prduces hgher turbulence ntensty due t rapd flud mxng. Partcularly n a ptch that was gettng clser [3]. At gven ynlds number, the cmparsn between the plan tube and luvered strp nsert wth dfferent ptch-lengths the Nusselt number fund t be 67% - 77%; 48% - 53%; and % - 4%, respectvely. 3.3 Frctn factr Frctn factr characterstc s shwn n Fg. 7. In term f frctn factr wth ynlds number. The frctn factr s nversely prprtnal t ynlds number fr all cndtns. These results are n accrdance wth prevus experment []. The frctn factr becmes greater fr smaller ptch-lengths and decreases fr the hgher ptch length. The hghest value f frctn factr was fund at ptch-length 40 mm, whle the plan tube gave the mnmum. Cncentrc tube ftted wth luvered strp nsert, the values f frctn factr are decreasng wth the rse f ptch-length [4]. In the range f < < frctn factr f the nner tube ftted wth luvered strp nsert wth dfferent ptch length (40, 50, 60 mm) were ;.4.9; and tmes greater than the plan tube f S = 40 mm S = 50 mm S = 60 mm Plan tube Fg. 7. Frctn factr vs ynlds number 3.4 Thermal perfrmance factr Thermal perfrmance factr s used t evaluate the qualty f the enhancement technque used. The thermal perfrmance factr gradually ncreased wth the rse f ynlds number as shwn n Fg η S = 40 S = 50 S = Fg. 8. Thermal perfrmance factr vs ynlds number Thermal perfrmance factr s bserved hghest fr ptch-lengths 40 mm. The values f thermal perfrmance factr f the nner tube wth luvered strp nsert at varus ptch-lengths (40, 50, 60 mm) were fund.0.; and tmes greater than the plan tube. 4 Cnclusns The expermental study s cnducted t enhance the perfrmance f cncentrc ppe heat exchanger by 5

6 MATEC Web f Cnferences 0, 0304 (07) DOI: 0.05/ mateccnf/ nstallng luvered strp nsert wth varus ptch-lengths. The results can be cncluded as fllws: ) Luvered strp enhanced the thermal perfrmance f the cncentrc tube heat exchanger. Nusselt number, frctn factr and thermal perfrmance factr tends t be hgher than the plan tube. ) The nfluenced f the dfferent ptch-length n the thermal perfrmance, the smallest ptch length gave the hgher Nusselt number, frctn factr and thermal perfrmance factr. 3) The 40, 50, 60 mm f ptch lengths augmented average heat transfer 67% - 77%; 48% - 53%; and % - 4%, respectvely at ynlds number ,500 cmpared t the plan tube. Nmenclature A nner surface area f nner ppe (m ) A uter surface area f nner ppe (m ) C p specfc heat capacty (J/kg. C) d nner dameter f nner ppe (m) d uter dameter f nner ppe (m) f frctn factr (dmensnless) f p frctn factr f the plan tube (dmensnless) f t frctn factr f the nner ppe wth luvered strp nsert (dmensnless) h average cnvectve heat transfer ceffcent f the nner ppe sde (W/m. C) h average cnvectve heat transfer ceffcent f the annulus sde (W/m. C) h p average cnvectve heat transfer ceffcent f the plan tube (W/m. C) k thermal cnductvty f ht water (W/m. C) k p thermal cnductvty f the nner ppe materal (W/m. C) L length f the nner ppe (m) L length f the uter ppe (m) Nu average Nusselt number f the nner ppe sde (dmensnless) Nu p average Nusselt number f the plan tube (dmensnless) Nu t average Nusselt number f the nner ppe sde wth luvered strp nsert (dmensnless) Pr Prandtl number (dmensnless) h heat transfer rate frm the ht water n the nner ppe (Watt) c heat transfer t the cld water n the annulus (Watt) ynlds number (dmensnless) S ptch length T b bulk water temperature ( C) T c cld water temperature ( C) T h ht water temperature ( C) T w uter wall nner ppe temperature ( C) u velcty f ht water n the nner ppe (m/s) U verall heat transfer ceffcent based n the nner ppe nsde surface area (W/m. C) Greek symbl α slant angle ( ) densty f ht water (kg/m 3 ) dynamc vscsty f ht water (kg/m.s) thermal perfrmance factr (dmensnless) P pressure drp acrss the nner ppe (Pa) T LMTD lgarthmc mean temperature dfference ( C) ferences. I. Yanngsh, T. Istant, A.T. Wjayanta, Prceedngs f Internatnal Cnference and Exhbtn Sustanable Energy and Advanced Materal (ICESEAM), AIP Cnference Prceedngs 737, (05). G. Sbel, O. Veysel, B. Orhan, Exp. Therm Flud Sc (00) 3. H.A. Mhammed, Husam A. Hasan, M.A. Wahd, Int. Cmmun. Heat Mass Transfer (03) 4. A.W. Fan, J.J. Deng, A. Nakayama, W. Lu, Int. J. Heat Mass Tran (0) 5. D. Eswaraah, S.S. Naga, IJERT (7) -9 (0). 6. S. Emsa-ard, S. Pethkl, C. Thanpng, P. Prmvnge, Int J Heat Mass Tran (008) 7. S. Pethkll, S. Emsa-ard, A. Rdluan, P. Prmvnge, The nd Internatnal Cnference n Sustanable Energy and Envrnment (SEE), A-08, Bangkk (006) 8. N. Sarada, R. ddy, G. Rav, IJTAE 3 (8) (03) 9. H. Husseune, C. T Jen, C., P. De Jaeger, B. Ameel, S. De Schamphelere, M. De Paepe, Int. J. Heat Mass Transfer (03) 0. R. Raut, H.S. Farkade, Int. J. Eng. s. Appl. (4) 0-04 (04). C. Zhang, D. Wang, K. n, Y. Han, Y. Zhu, X. Peng, J. Deng, X. Zhang, new. Sust. Energy v (06). Y.A. Cengel, Heat and Mass Transfer 5th ed. (McGraw-Hll, New Yrk, 008) 3. Frank M. Whte, Flud Mechancs 7th ed. (McGraw- Hll, New Yrk, 0) 6