The Numerical Simulation of Enhanced Heat Transfer Tubes

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1 Aailable online at.sciencedirect.com Phsics Procedia 4 ( International Conference on Applied Phsics and Indstrial Engineering The Nmerical Simlation of Enhanced Heat Transfer Tbes Li Xiaoan, Go Renning,Li Chnang College of Mechanical Engineering Liaoning Technical Uniersit Fin, China Abstract The enhancing heat transfer mechanism of the transersall corrgated tbe,conerged-dierging tbe and corrgated tbe compared ith that of the smooth tbe as stdied in this paper. Here heat transfer models ere established ith ater as the medim of the enhanced heat transfer tbes.we sed simplec methods and the standard k- trblence model in Flent and analed the impact of different tbe strctre on heat transfer tbes, finall, achieed isaliation of flo field in enhanced heat transfer tbes. it can been conclded from the figres that: ithin the scope of this std, the order of three tbes according to the merits is transersall corrgated tbe, conergeddierging tbe and corrgated tbe Pblished b Elseier b Elseier B.V. Ltd. Selection Selection and/or and/or peer-reie peer-reie nder responsibilit nder responsibilit of ICAPIE of Organiation [name organier] Committee. Open access nder CC BY-NC-ND license. Keords:heat transfer enhancement; enhanced heat transfer tbe; nmerical simlation; trblence model 1. Introdction Dring these tent ears, chemical, petrolem and light indstr hae achieed rapid deelopment. Oing to increasing eqipment capacit can redce eqipment inestment and rnning cost, arios indstrial sectors are deeloping high-capacit and high-performance eqipment. heat transfer enhancement measres are idespread concerned b people becase it can improe thermal efficienc, the same as the problem of conection heat transfer enhancement.at present, seeral methods hae been idel sed in indstrial practice[1-], from all the methods, finned tbe has been idel applied for adantages of increasing heat transfer area and redcing thermal resistance[3]. Bt the std on heat transfer enhancement of inner finned tbe are relatiel fe, so the mechanism of it are still not er clear, it makes the design ork related to heat echanger hae no clear intitie basis to follo. Large commercial CFD softare flent can be more accrate, fast in reflecting the hole flo process of flid in the heat echanger, sch as the distribtion of elocit field, pressre field and Pblished b Elseier B.V. Selection and/or peer-reie nder responsibilit of ICAPIE Organiation Committee. Open access nder CC BY-NC-ND license. doi: /j.phpro

2 Li Xiaoan et al. / Phsics Procedia 4 ( temperatre field, from these e can directl get flo characteristics of flid in the tbe, so as to proide a good theoretical basis to actal design. In this paper, trblent flo and heat transfer characteristics in the smooth tbe and other three enhanced heat transfer tbes ere simlated in Flent ith cases of the all temperatre is constant, and the inflences of different inner finned tbes on heat transfer enhancement ere analed.. Heat Transfer Machanism in the Tbe The measres of heat transfer enhancement can be diided into actie and passie enhancement, actie enhancement measres need eternal ork, so it is rarel sed in sal, passie enhancement measres often contain etending srface, srface modification, tbe inserts and so on, from hich the tbe inserts are sall sed onl for laminar flo, hile trblent flo most se fin tbe and lo-finned tbe to strengthen[4]. The main mechanisms in enhanced heat transfer tbe are increasing the second heat transfer srface and destroing the original elocit and temperatre distribtion field. The std on heat transfer enhancement in the enhanced heat transfer tbes in this paper is mainl in the area that near the all ith lo elocit, that is, the ell-knon laminar sblaer, most of the thermal resistance is concentrated in this laer of lo elocit flo, all of enhancement technologies adopt changing the internal strctre of the tbes to distrbance flid and destro bondar laer, so as to achiee the prpose of enhancing heat transfer. The ratio of bondar laer thickness and diameter: Re [5]. d What can be aailable in heat transfer enhancement measres in tbes mainl are spirall flted tbe, transersall corrgated tbe, conerged-dierging tbe, corrgated tbe and so on. The transersall corrgated tbe actall is a kind of special spirall flted tbe, hen the heli angle increase to 90 and it is better than spirall flted tbe in strengthening effect[6]. Becase of the role of centrifgal force and torqe force, the flo of flid in enhanced tbe is mch more comple than that in smooth tbe, ith the impact of secondar flo, mainstream in high aial elocit from pipe central area ill contine to mi ith the flid that in loer aial elocit near the all, all of these can improe the trblence intensit, thereb enhancing the heat transfer. 3. Nmerical Simlation 3.1 Model There are some assmptions in the simlation process: crrent state is stable, that is, flo distribtion is onl related to the spatial location bt not the time; flid properties (densit, specific heat and flid iscosit, etc. does not ar ith temperatre; ignoring the inflence of grait. We select the optimied strctre in the phsical model of for heat transfer tbe, in order to aoid the impact of different parameters, the hdralic diameter of the tbes are set to 40mm,and the length are 600mm. The nmerical simlation se finite olme method to discrete the control eqations, and the calclation se separated implicit soler for stead state calclations. 3. Eqations Control eqations are 3D incompressible flo continit eqation and the naier-stokes eqation of momentm and energ eqation: 0

7 Li Xiaoan et al. / Phsics Procedia 4 (01 70 79 F p t ( ( F p t ( ( F p t ( ( t K i k i D k k G D t 1 t k i i D C G C Dt k k C = 0.09, 1 C =1.44, C =1.9, k =1.0,. 3.

3 7 Li Xiaoan et al. / Phsics Procedia 4 ( F p t ( ( F p t ( ( F p t ( ( t K i k i D k k G D t 1 t k i i D C G C Dt k k C = 0.09, 1 C =1.44, C =1.9, k =1.0,. 3.3 Bondar conditions and flo parameters Model ses heahedral mesh and mesh all encrption; Inlet temperatre of orking flid T = 300k, stress is a standard atmospheric pressre, temperatre of the all T = 450k. Renolds nmber Re = 6680, bondar conditions are inlet elocit, temperatre of the all is constant and theotlet conditions are free flo; Conted ithin the entire region, ignoring the differences of all temperatre. In order to find the natre of trblence better, the model se standard k- model, pressre - elocit copling ith Simplec algorithm. 4. Analsis of the simlation reslts Three-dimensional trblent flo nmerical simlation of smooth tbe, transersall corrgated tbe, conerged-dierging tbe and corrgated tbe based on Flent are shoed in this part, in the process, e can clearl obsere the distribtion of temperatre, elocit, trblent kinetic energ and skin friction coefficient from seeral different sections, the analsis and comparison of the inflence of the different enhanced tbe on heat transfer and flo characteristics are as follos.( the code of smooth tbe, transersall corrgated tbe, conerged-dierging tbe and corrgated tbe in folloing diagrams are No.1No.No.3No Working process of the enhanced heat transfer tbes The hole process of heat transfer inside tbes can be clearl seen throgh the folloing diagrams: Figre 1. Speed contor distribtion cross-section of No.1

4 Li Xiaoan et al. / Phsics Procedia 4 ( Figre. Speed contor distribtion cross-section of No. Figre 3.Speed contor distribtion cross-section of No.3 Figre 4.Speed contor distribtion cross-section of No.4 Figre 5.Aial trblence kinetic energ distribtion of No.1

5 74 Li Xiaoan et al. / Phsics Procedia 4 ( Figre 6.Aial trblence kinetic energ distribtion of No. Figre 7. Aial trblence kinetic energ distribtion of No.3 Figre 8. Aial trblence kinetic energ distribtion of No.4 The srface heat transfer coefficient is mainl determined b the changes of the radial elocit and trblence intensit ithin transition one, that is to sa this three is consistent ith each other. It is clear from Figre 1 - Figre 4 that hen the flid flos throgh cone rib ring of the transersall corrgated tbe, the speed direction near the all srface ill change, then the radial distrbance appears, and thereb preents the formation of the bondar laer, that enhanced heat transfer to some etent. Besides, hen the flid flos throgh the grooe ring, the elocit near the all erodes bondar laer in aial and then aial orte formed, that can increase the distrbance of bondar laer and promote the pdate of srface laer, and is condctie to the heat transfer throgh the bondar laer; In the conerged-dierging tbe, the decrease of circlation area in contraction segment accelerates the elocit of the flid, high-speed flid erode and distrb the bondar laer, becase there are changes in the flid particle elocit, dramatic spiral and secondar flo are formed in the transition section, it

6 Li Xiaoan et al. / Phsics Procedia 4 ( promotes miing of flid from the core and bondar laer area, makes the bondar laer thinning, so it cold hae a better heat transfer effect; In corrgated tbe, as it shon in Figre 4,not onl the elocit of the flid in the trogh section is increasing, ashing bondar laer ith high-speed, bt also in pstream flo of each crest hae emerged a closed recirclation region, and accompanied ith circlar flo in a certain speed in the recirclation one. These ill keep the cold flid from central region and the hot flid near the all echanging. Finall make the heat pass off. Figre 5 - Figre 8 shon, in the three enhanced heat transfer tbes, flid temperatre and trblent kinetic energ hae been greatl improed compared ith smooth tbe, particlarl in transersall corrgated tbe and conerged-dierging tbe, their trblent kinetic energ reached to a larger ale at the part of the all. The periodic changes in epansion and contraction section make the flid trblence enhanced. The scope of trblent kinetic energ changing is mch larger in corrgated tbe than that in other tbes, its trblent kinetic energ increases along the flo direction, ith the changes of crosssections, trblent kinetic energ gradall transferred to the central region, shoing the trend of increasing, and get to the maimm ale in the eport section. 4. Comparison of heat transfer effect Figre 9.Aial temperatre distribtion section of No.1 Figre 10.Aial temperatre distribtion section of No. Figre 11. Aial temperatre distribtion section of No.3

7 76 Li Xiaoan et al. / Phsics Procedia 4 ( Figre1. Aial temperatre distribtion section of No.4 Figre 13. Otlet temperatre section of No.1 Figre 14. Otlet temperatre section of No. Figre 15. Otlet temperatre section of No.3

8 Li Xiaoan et al. / Phsics Procedia 4 ( Figre 16. Otlet temperatre section of No.4 As can be seen in Figre 9-Figre 16, the heat transfer effect of transersall corrgated tbe is most obios and Its maimm temperatre reached , bt the temperatre in ais area is loer. It indicates that its distrbance is ineffectie; the heat transfer effect of conerged-dierging tbe is orse than the former, its otlet temperatre is mainl in , bt the distrbance effects in the pipeline region and niformit of temperatre distribtion are better than that of the former; The otlet temperatre of corrgated tbe reached , its performance of the flid distrbance miing effect and the niformit of otlet temperatre is best. 4.3 Analsis of integrated performance The folloing is heat transfer resistance coefficient inside the tbe changes ith the tbe length: Figre 17.Wall srface friction coefficient of No.1 Figre 18. Wall srface coefficient of No.

9 78 Li Xiaoan et al. / Phsics Procedia 4 ( Figre 19. Wall srface friction coefficient of No.3 Figre 0. Wall srface friction coefficient of No.4 In Figre17-Figre0, de to the all strctres, the srface friction coefficient of three enhanced heat transfer tbes increased in different leels compared ith smooth tbe. Ignoring the changes of srface friction coefficient in entrance section, in the fll deeloped section, the srface friction coefficient of smooth tbe is abot 0.001,and for transersall corrgated tbe it is abot 0.004, srface friction coefficient of conerged-dierging tbe is mainl in , bt that of corrgated tbe is increasing ith the length of the tbe for the effect of distrbance, the initial ale is abot 0.00 and ith the length its groth is 5% 40%,so the length design ofcorrgated tbe plas a er important role in heat echanger, in actal, the srface friction coefficient can be controlled in a certain range throgh the tbe length. The srface friction coefficient of transersall corrgated tbe is the largest, the srface friction coefficient of conerged-dierging tbe is relatiel small, ith smooth transition srface, conerged-dierging tbe is difficlt to scale, so it is applied to heat transfer ith dst particles flid. 5. Conclsion This paper sed the comptational flid dnamics (CFD softare Flent to anale the flo in three sorts of enhanced heat transfer tbes and heat transfer process, ith comparison of smooth tbe. The flo of heat transfer enhancement and heat transfer process are described in detail, inclding seeral comparison reslts of heat transfer enhancement processes, heat transfer theor, the heat transfer effect and the resistance coefficient. Analsis of heat transfer mechanism of the three enhanced heat transfer tbes are as follos: transersall Corrgated tbe constantl changes the srface shape of tbes to make flid continosl prodce radial distrbance and aial orte, ths to strengthen the role of the erosion and the distrbance on bondar

10 Li Xiaoan et al. / Phsics Procedia 4 ( laer, making heat transfer capacit is greatl enhanced; Conerged-dierging tbe adopt the periodic changes of epansion and transition section to strengthen the flid trblence, it also prodce orte flo and secondar flo, so as to redce the laminar sblaer, strengthening heat transfer effect; For corrgated tbe, the eistence of recirclation region in each crest is the ke to the heat transfer enhancement. Within the scope of this std, considering the trblent kinetic and heat transfer effect, the order of the priorit is: transersall corrgated tbe, conerged-dierging tbe, corrgated pipe; Bt from the resistance to consider :flo resistance of transersall corrgated tbe is maimm, flo resistance of Conerged-dierging tbe is minimm, so this tbe is applicable to the heat transfer flid containing dst particles, flo resistance of corrgated tbe can be different according to the length, its good miing effect of flid flo is also one of adantages. According to the specific needs, the design of heat echanger shold nder the consideration of their adantages and disadantages. References [1] Gao Xiang, Lo ZhongangZho Jinsongetal Std on heat transfer enhancement characteristics ith a decaing sirl flo[j]proceedings of the CSEE0033( (in Chinese [] Ci HaitingYan XiganYao Zhongpengetal Eperimental inestigation on the heat transfer and flo resis-tance characteristics of a special-shaped spirall flted tbes[j]proceedings of the CSEE0033(617-0(in Chinese [3] Y BoWang QiangTao WenqanEperimental std on thepresser drop and heat transfer characteristics of tbes ith internal ae-like longitdinal fins[j]jornal of Engineering Thermophsics0001( (in Chinese [4] Y Bo Tao W Q.Pressre drop and heat transfer characteristics of trblent flo in annlar tbes ith internal ae-like longitdinal fins[j]heat and Mass Transfer [5] Qian Songen, Zh Dongsheng, etal. Heat transfer technolog in tbe heat echanger[j]. Chemical indstr,003. Article in a conference proceedings [6] Li Jip. Eperimental inestigation on the mechanical properties of transersall corrgated tbe[j]. Chemical Machiner, 1998, 5 (3 : 70-7

SIMULATION OF TURBULENT FLOW AND HEAT TRANSFER OVER A BACKWARD-FACING STEP WITH RIBS TURBULATORS

THERMAL SCIENCE, Year 011, Vol. 15, No. 1, pp. 45-55 45 SIMULATION OF TURBULENT FLOW AND HEAT TRANSFER OVER A BACKWARD-FACING STEP WITH RIBS TURBULATORS b Khdheer S. MUSHATET Mechanical Engineering Department,