Convection and Radiation Heat Transfer in a Tube with Core Rod and Multi Duct Inserts at High Temperature. M.R. Alijani 1, A.A.

Transcription

1 Convecion and Radiaion Hea Transfer in a Tube i Core Rod and Muli Duc Insers a Hig Temperaure M.R. Aliani 1, A.A. Hamidi 2 1. Deparmen of Environmen and Energy, Science and Researc Branc, Islamic Azad Universiy, Teran, Iran 2. Deparmen of Cemical Engineering, Teran Universiy, Teran, Iran Corresponding Auor (aliani_1373@yaoo.com) Absrac: Hea ransfer and fricion facor caracerisics in a circular ube fied i core rod and muli duc insers a ig emperaure ave been invesigaed experimenally. In e experimens, ambien air i Reynolds numbers in a range of ,000 is passed roug a circular ube i uniform all emperaure and convecion and radiaion ea ransfer penomena are sudied. Experimens ave been performed a four consan all emperaure ubes i core rod inser. For eac all emperaure considered, convecion and radiaion ea ransfer coefficiens ave been deermined. Te experimenal resuls so a a uniform all emperaure of 373 K, 473 K, 553 K and 633 K e average sare of e radiaion ea ransfer coefficien o e oal ea ransfer coefficien are 11.5, 13.1, 15.3 and 17.8% for core rod and 16.9, 20.0, 24.3 and 27.8% for muli duc inser respecively. In addiion i as noed a for e menioned emperaures, e ea ransfer coefficien increased by 227, 299, 327 and 369% for core rod and 279, 396, 453 and 539% for muli duc inser respecively in comparison o e plain ube. I as also noed a increasing e all emperaure also resuled in increase of fricion facor. Based on e experimenal resuls for Nussel number and fricion facor empirical correlaion ave been derived. Ploing e experimenal findings and e correlaions, i as noed a e maoriy of e daa are iin ±12% and ±8% of e proposed correlaions for ea ransfer coefficien and fricion facor, respecively. Te resuls ere also esed agains available and ell proven correlaions i reasonable agreemen. [M.R. Aliani, A.A. Hamidi. Convecion and Radiaion Hea Transfer in a Tube i Core Rod and Muli Duc Insers a Hig Temperaure. Life Sci J 2013;10(3s): ] (ISSN: ).. 73 Keyords: Hig emperaure, Radiaion, Convecion, Hea ransfer enancemen, Fricion facor, Core rod, muli duc inser. 1. Inroducion Hea ransfer enancemen ecnology is e process of improving e performance of a ea ransfer sysem by increasing e convecion and radiaion ea ransfer coefficiens. Over e pas fory years, is ecnology as been exensively used in ea excangers and oer ea ransfer equipmens in ermal poer plans, cemical processing plans, air condiioning equipmen, refrigeraors and veicle radiaors. Generally e main obecive is o reduce e size and coss of ese equipmens. 1,2,3 Hea ransfer enancemen ecniques can be divided ino o caegories. Firs is e passive meod iou e use of any exernal poer source and e oer is e acive meod, ic requires exra exernal poer sources. Reverse/sirl flo devices form an imporan group of e passive augmenaion ecniques. Te reverse flo, someimes called recirculaion flo, devices or urbulaors are idely employed in ea ransfer engineering applicaions. Promvonge and Eiamsa-ard 4 repored e effec of conical-nozzle and snail enrance on ea ransfer and fricion caracerisics in a uniform ea flux ube and found a e ea ransfer rae increases considerably for using bo enancemen devices. Tey also sudied e effec of combined V- nozzle urbulaor inser and snail enry on e ea ransfer and fricion facor caracerisics under uniform all emperaure condiions. 5 Eiamsa-ard and Promvonge 6 furer repored an effec of e V- nozzle urbulaors on ea ransfer rae in a circular ube and suggesed a e nozzles ave a significan effec on ea ransfer enancemen. Tis indicaes a e crucial effec of e reverse/re-circulaion flo can promoe e ea ransfer rae in ubes. Sivasanmugam and Sures 7 examined e urbulen ea ransfer and fricion facor caracerisics of circular ube fied i full-leng elical scre elemen of differen is raio under uniform ea flux condiions. Tey also repored e ea ransfer and fricion facor caracerisics of laminar flo roug a circular ube fied i sraig elical scre-ape inser. 8 Napon 9 experimenally sudied e ea ransfer and pressure drop in orizonal double pipes i and iou ised ape inser. In mos of e noable publised ors, e experimens ad been carried ou a lo emperaures, usually less an 373 K a ic e effec of radiaion as been compleely ignored. Te aim of e presen experimenal sudy as o deermine e effec of 471 lifescience@gmail.com

2 radiaion and convecion ea ransfer as ell as e pressure drop in a ube i core rod and muli duc inser a ig emperaure. Experimens ave been performed i Reynolds numbers in e range of ,000 a four consan ube all emperaures of 373 K, 473 K, 553 K and 633 K. Experimenal Se-up and Procedure Te experimens ere performed in an open loop experimenal equipmen as son in Figure 1. Te se up consised of a bloer (4 W, 2400 Pa), orifice meer o measure e air flo rae, a 4 W inverer o adus e speed of moor in bloer and e ea ransfer es secion. Te orifice meer used o measure e flo rae as consruced and calibraed according o ASME sandard. 10 Te es secion consised of a sainless seel ube, leng, 2500 mm, inside diameer, 80 mm and ouside diameer, 88.9 mm. Teny ree bel eaers eac i maximum poer of 800 W ere beled on e ouside surface of e ube. Suiable conrol sysem as insalled in e se up o adus for e necessary poer for e eaers o provide uniform all emperaures of 373 K, 473 K, 553 K and 633 K by receiving e feed bac signal of ermocouples locaed on e surface of e ube. Te ouer surface of e es ube and eaers as ell insulaed i glass ool o minimize convecive ea loss o e surroundings, and necessary precauions ere aen o preven air leaages from e sysem. A e inle and oule of e ube, o K ype ermocouples ere placed in e cener of pipe o measure e inle and oule air emperaure of e es secion. Tireen K ype ermocouples ere lined up along e es ube all surfaces and embedded in grooved ube surfaces. Te average all emperaure as obained using e emperaure readings of all ireen ermocouple. Te ermocouples ad been calibraed o iin ±0.2 C. To pressure apes, one us before and e oer us afer e es secion ere provided and aaced o a digial on-line differenial pressure ransducer for pressure drop measuremen. Differenial pressure ransmier as calibraed iin ± 0.25 Pa deviaion before being used. Te core rod and muli duc inser used in ese experimens ere made of carbon seel i a leng of 2500 mm, and e ouer diameer for core rod is 43.5 mm, and e diameer of square a muli duc as 25 mm. Tese are son in Figure 2. Tireen K ype ermocouples ere aaced on e surfaces of core rod and muli duc o deermine is average surface emperaure. In eac experimen e flo rae of e ambien air from e bloer as adused by e inverer o e required amoun. An orifice meer i measuring range of m 3 r -1 as used o measure e air flo rae. Te required emperaure of e ube all as se on e conroller. Once reacing seady sae condiions, all emperaures and e pressure drop ic ere logged on e daa logger and noed. Using e daa obained, radiaion and convecion ea ransfer coefficiens ere calculaed as discussed laer. Experimenal uncerainy as calculaed folloing Coleman and Seele meod 11 and ANSI/ASME sandard. 12 Te calculaions soed a maximum uncerainy of ±6%, ±5%, and ±8% for Reynolds number, ea ransfer coefficien and fricion facor, respecively. Daa Reducion A e seady sae condiions, e ne ea ransfer rae Q from e inner ube surface o e ne fluid floing roug e es ube can be calculaed by subracing e ea losses Q loss from e oal elecrical poer inpu Q. Tis is also equal o e rae of e ea ransfer o e fluid passing roug e es secion, and is deermined using inle and oule emperaure difference and mass flo rae of air. Te energy balance equaions can be rien as follos: Q ne Q Q loss mc p, a T o T i (1) Q loss a As T s T (2) Te elecrical poer inpu ( Q ) o e eaer can be measured by: 2 V Q R (3) Te mass flo rae of air as calculaed by: m b u A cs (4) Te ea losses ( Q loss ) and e ea released by eaers ( Q ) ere no measured or calculaed in is or and ea absorbed by e floing air as simply deermined using equaions (1) and (4). Subracing e ea absorbed by e floing air from e ea inpu from e eaer, e ea losses ( Q loss ) ere found o be 5 o 8% of e oal elecrical poer inpu. Te ea ransfer from e es secion can be rien by: Q ne o AT ln (5) Were: T To T ln ( Ti To ) / ln T Ti (6) Here T is e consan inner surface emperaure of e ube es, ose average is being measured by e ermocouples. Te oal average ea ransfer coefficien is assumed o be: 472

3 o conv rad (7) Were ( conv ) is e average convecion ea ) is e average radiaion ransfer coefficien and ( rad ea ransfer coefficien. 8 2 ~ 2 ~ F T T T T (8) rad Were pac pac pac T ~ is e average surface emperaure of eac inser (e core rod or e muli duc), ic is measured. ~ Tpac Tpac 13 (9) And F is e sape facor from e inner surface of e ube es o e ouer surface of eac inser. Te sape facor equaions can be rien as follos: F 1 cos cos A dada A A X 2 (10) A F A F Were and (11) are e angles beeen e uni normals o e areas A and A and X is e disance beeen e o areas. Tere is conducion ea ransfer in e body mes of e muli duc inser. Terefore a seady sae condiion e emperaure of e inner surfaces of e inser are approximaely equal and uniform. I can erefore be assumed a radiaion ea ransfer in e inner surface of e muli duc inser is negligible. Te Reynolds number and average Nussel number are given by: ud Re (12) D Nu (13) In fully developed ube flo, e fricion facor (f) can be deermined by measuring e pressure drop across e es ube leng as follos: P f 2 L u D 2 (14) Were P is e pressure drop across e es ube measured by e differenial pressure ransducer i a ±2% Pa accuracy, L is e es ube leng and u is e mean air velociy a e enrance of e es secion ic as calculaed from umeric flo rae divided by e cross-secion area of e ube. All of ermo pysical properies of e air are deermined a e overall bul air emperaure. T b T T / 2 o i For a consan pumping poer: (15) V P p V P (16) Te ea ransfer enancemen efficiency is defined as e raio of e ea ransfer coefficien for e ube fied i e urbulaor o a of e plain ube a similar pumping poer. I can be rien a 13 : p 3 3 f Re p f Re p pp (17) (18) Resuls and Discussion In is secion, resuls of e effec of insallaion of core rod and muli duc ube insers a various all emperaures on ea ransfer rae and flo fricion are presened. Invesigaion of e radiaion and convecion ea ransfer a uniform all emperaure of 373 K, 473 K, 553 K and 633 K are performed. Te oal ea ransfer and radiaion ea ransfer o rad are calculaed using equaions (5) and (8) respecively. Te oal average ea ransfer (i core rod or muli duc) in es secion consiss of radiaion and convecion ea ransfer. Figure 3 sos e variaion of oal ea ransfer coefficien and e radiaion ea ransfer coefficien as a funcion of Reynolds numbers and e ube all emperaure for eac inser. I can be seen a e sare of e radiaion ea ransfer coefficien is increased by increasing e surface emperaure. Te radiaion sare of ea ransfer appears o be 11.5, 13.1, 15.3 and 17.8% for core rod and 16.9, 20.0, 24.3 and 27.8% for muli duc depending on e all emperaure. I can be seen from e experimenal resuls for e core rod and muli duc inser a e ea ransfer coefficiens increase i increasing ube all emperaure. Tis increase is mosly due o e radiaion sare of e ea ransfer. Te radiaion ea ransfer is proporional o e four poer of emperaure. In is invesigaion e sape facor of e muli duc and core rod are calculaed by equaions (10) and (11) and are 0.9 and 0.54 respecively. A e same all emperaure is resuls in iger radiaion ea ransfer coefficiens for e muli duc as compared o e core rod. 473

4 Figure 4 sos e variaion of ea ransfer coefficien i Reynolds number. In is diagram e resuls for e ea ransfer coefficien for plain ube and ube equipped i core rod or muli duc insers a various all emperaures are compared. For e plain ube e resuls for various all emperaures are very similar o eac oer since e effec of radiaion ea ransfer beeen surfaces is no presen. On e oer and, e boundary layer disrupion causes a beer mixing beeen e inser and e all regions, us enancing e convecive ea ransfer process. In addiion, e use of e muli duc provides beer ea ransfer an a of e core rod a a similar all emperaure. Tis is due o e iger circulaion and iger conac surface area beeen e fluid and e eaing all surface en fluid flos inside e muli duc inser. I is or noing a for e considered all emperaures, e ea ransfer coefficien increased by 227, 299, 327 and 369% for core rod and 279, 396, 453 and 539% for muli duc inser respecively as compared o e plain ube. In Figures 5 and 6, e resuls of Nussel number and fricion facor a considered all emperaures for e plain ube are found o be in good agreemen i previous correlaion of Dius-Boeler and Blasius. 14 Using e experimenal daa, e folloing empirical correlaions for Nussel number and fricion facor are derived for e plain ube and are presened in equaions (19) and (20) respecively. Tey are found o represen e experimenal daa o iin ±1 o 5 % error limis Nu 0.2 Re Pr (19) f Re (20) Figure 7 sos e variaion of fricion facor i e Reynolds number a e various all emperaures. For plain ube and ube equipped i insers, i is clear a fricion facor for e core rod and muli duc configuraion are iger an a for plain ube. Also as expeced for ese cases fricion facor decreases i increasing Reynolds number. Te prediced Nussel number and fricion facor for ube i core rod and muli duc inser ere correlaed and are son in equaions (21) o (24) respecively. Te prediced Nussel number and fricion facor ere compared i e experimenal daa in Figures 8 o 11. Te maoriy of e daa falls iin ±12%, ±8% of e proposed correlaions for ea ransfer coefficien and fricion facor, respecively T Nu Re Pr ~ (21) cor Trod T f Re ~ (22) cor Trod T Re Pr Nu ~ (23) mul T mul T f Re ~ (24) mul Tmul Using equaions (17), (20), (22) and (24), e Reynolds number for e plain ube Re p can be rien as a funcion of e Reynolds number for e core rod and muli duc inser urbulaor Re : (Re p ) cor Re T ~ T rod (25) T (Re ) Re ~ (26) p mul Tmul Employing equaions (18), (19), (21) and (23), e enancemen efficiency a consan all emperaure for e core rod and muli duc inser urbulaor can be rien as: T Re ~ (27) cor p Trod pp T Re ~ (28) mul p Tmul pp Figure 12 sos Variaion of enancemen efficiency i Reynolds number for bo core rod and muli duc insers. I indicaes a e enancemen efficiency of insers increases i increasing ~ 1 emperaure, since e raio T T is increased a ig emperaure. In e muli duc e fluid ad iger urbulency, iger conac surface area and bigger radiaion sape facor an e core rod. Terefore e oal ea ransfer and e enancemen efficiency for is inser are iger. Te average enancemen efficiency for e considered uniform all emperaures of 373, 473, 553 and 633 K ere 1.63, 1.83, 1.97 and 2.28 for e core rod and 2.32, 2.83, 3.14 and 3.36 for e muli duc inser respecively. Te average raio of enancemen efficiency of muli duc inser o a of e core rod for ese emperaures and a various Reynolds numbers ere 1.42, 1.55, 1.59 and 1.47, respecively. pac 474

5 Figure 1: Scemaic diagram of e experimenal apparaus Figure 4: Verificaion of ea ransfer coefficien a various emperaure beeen plain ube, muli duc and core rode inser Figure 2: Te ube fied i core rod and muli duc neor inser Figure 5: Verificaion of ea ransfer coefficien a various emperaure in plain ube Figure 3: Verificaion of radiaion and oal ea ransfer coefficien a various emperaure of core rode and muli duc inser Figure 6: Verificaion of fricion facor a various emperaure in plain ube 475

6 Figure 7: Verificaion of fricion facor a various emperaure beeen plain ube, core rod and muli duc inser Figure 10: Nussel numbers obained from e presen correlaion and experimenal daa for core rod inser Figure 11: Fricion facors obained from e presen correlaion and experimenal daa for core rod inser Figure 8: Nussel numbers obained from e presen correlaion and experimenal daa for muli duc inser Figure 9: Fricion facors obained from e presen correlaion and experimenal daa for muli duc inser Figure 12: Variaion of enancemen efficiency i Reynolds number for core rod and muli duc inser 476

7 Conclusions Experimenal invesigaions ave been conduced o sudy e ea ransfer in a circular ube equiped i core rod and muli duc insers. For Reynolds numbers in e range of 6000 o for any given uniform all emperaure, e sare of radiaion ea ransfer o e oal ea ransfer are 11.5, 13.1, 15.3 and 17.8% for core rod and 16.9, 20.0, 24.3 and 27.8% for muli duc inser respecively. For all cases sudied e oal ea ransfer coefficien for bo e core rod and muli duc inser increased due o increased urbulency, enanced surface area and increased radiaion effecs. As menioned e raio of radiaion ea ransfer as separaed from e oal average ea ransfer, and by increasing e sape facor in muli duc o core rod, I as seen a is parameer for muli duc as iger an a for e core rod. I as noed a e ea ransfer coefficiens increase i increasing ube all emperaure. Tis increase is mosly due o e radiaion sare of e ea ransfer and i can be seen a e radiaion ea ransfer is proporional o e four poer of emperaure. Moreover e increased enancemen efficiency for e muli duc as compared o a of e core rod a any given uniform all emperaure is aribued o e increasing of e sape facor, e circulaion, and e increased surface area for e muli duc. Nomenclaure A : Hea ransfer surface area, m 2 Cp,a : Specific ea capaciy of air, J g -1 K -1 D : Hydraulic diameer, m f : Fricion facor F : Sape facor of surface i respec o surface : Average ea ransfer coefficien, W m -2 K -1 : Termal conduciviy, W m -1 K -1 L : Leng of es secion, m m : Mass flo rae, g s -1 Nu : Average Nussel number Pr : Prandl number Q : Hea ransfer rae, W R : Elecrical resisance of e eaer elemen, Ω Re : Reynolds number T :Temperaure, K T ~ : Average emperaure, K u : Mean axial velociy, m s -1 V : Volage oupu from e Auo-ransformer,V V : Volumeric flo rae, m 3 s -1 X : Disance beeen e o areas Gree symbols : Enancemen efficiency : Kinemaic viscosiy, m 2 s -1 : Densiy of e fluid, g m -3 : Wall emissiviy : Angle beeen e uni normals and e area T ln : Logarimic mean emperaure difference, K ΔP : Pressure drop, Pa Subscrips a : Air b : Bul cor : Core rod Conv : Convecion cs : Cross secion area mul : Muli duc i : Inle : Inner : Ouer loss : losses o : Oule p : Plain ube pac : Pacing pp : Pumping poer rad : Radiaion rod : Rod s : Side long area of eaer : Turbulaor o : Toal : Volage : Wall : Amosperic air References 1. Marner W.J., Bergles A.E. and Cenoe J.M., On e presenaion of performance daa for enanced ubes used in sell and ube ea excangers, Transacion ASME Journal Hea Transfer, 105, (1983) 2. Bergles A.E. and Webb R.L., Guide o e lieraure on convecion ea ransfer augmenaion, Advanced in Enanced Hea Transfer, 43, (1985) 3. Bergles A.E., Tecniques o augmen ea ransfer, In: Roseno W.M., Harne J.P. and Ganie E., (Eds) Handboo of Hea Transfer Applicaion, McGra-Hill, Ne Yor, (1985) 4. Promvonge P. and Eiamsa-ard S., Hea ransfer enancemen in a ube i combined conicalnozzle insers and sirl generaor, Energy Conversion and Managemen 47, (2006) 477

8 5. Promvonge P. and Eiamsa-ard S., Hea ransfer augmenaion in a circular ube using V-nozzle urbulaor insers and snail enry, Experimenal Termal and Fluid Science 32, (2007) 6. Eiamsa-ard S. and Promvonge P., Experimenal invesigaion of ea ransfer and fricion caracerisics in a circular ube fied i V- nozzle urbulaors, Inernaional Communicaion Hea and Mass Transfer 33, (2006) 7. Sivasanmugam P. and Sures S., Experimenal sudies on ea ransfer and fricion facor caracerisics of urbulen flo roug a circular ube i elical scre-ape insers, Cem. Eng. Process 46, (2007) 8. Sivasanmugam P. and Sures S., Experimenal sudies on ea ransfer and fricion facor caracerisics in laminar flo roug a circular ube fied i elical scre-ape insers, Journal of Applied Termal Engineering 26, (2006) 9. Napon P., Hea ransfer and pressure drop in e orizonal double pipes i and iou ised ape inser, Inernaional Communicaions in Hea and Mass Transfer 33, (2006) 10. ASME Sandard., Measuremen of fluid flo in pipes using orifice, nozzle and venure, ASME MFC-3M (1984) Unied Engineering Cener 345 Eas 47 Sree, Ne Yor (1984) 11. Coleman H.W. and Seele W.G., Experimenal and Uncerainy Analysis for Engineers, Wiley, Ne Yor (1989) 12. ANSI/ASME, Measuremen uncerainy, PTC 19,1 1985, Par I (1986) 13. Yau K., Sain B. and Canbazoglu S., Performance and flo-induced vibraion caracerisics for conical-ring urbulaors, Appl Energy 79(1), (2004) 14. Incropera F. and Dei P.D., Inroducion o ea ransfer. ird ed, Jon Wiley and Sons, Inc (1996). 1/15/