A numerical study on the shell-side turbulent heat transfer enhancement of shell-and-tube heat exchanger with trefoilhole

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1 Avalable onlne at ScenceDrect Energy Proceda 75 (2015 ) The 7 th Internatonal Conference on Appled Energy ICAE2015 A numercal study on the shell-sde turbulent heat transfer enhancement of shell-and-tube heat exchanger wth trefolhole baffles Guo-Yan Zhou*, Jngme Xao, Lngyun Zhu, Juntao Wang, Shan-Tung Tu Key Laboratory of Pressure Systems and Safety(MOE), East Chna Unversty of Scence and Technology,Shangha,200237,Chna Abstract Shell-and-tube heat exchangers wth trefol-hole baffles are new type heat transfer devces and wdely used n nuclear power system due to ther specal advantages, wth the flud flowng longtudnally on the shell sde. However, very few related academc lterature are avalable. In order to obtan an understandng of the underlyng mechansm of shell-sde thermal augmentaton, a CFD model ncludng nlet and outlet nozzles s proposed n the present study. Based on the RNG k- model, numercal nvestgatons on shell-sde flud flow and heat transfer are conducted by usng commercal CFD software FLUENT14.0. The results show that the flud s fully developed after the frst trefol-hole baffle. The heat transfer coeffcent and pressure drop vary perodcally along the axal drecton. Flud velocty ncreases gradually and the jet flow forms n the regon near baffles. The secondary flow s also produced on the two sdes of baffles when the flud flows through trefol-hole baffle. The jet flow and secondary flow can decrease the thckness of boundary layer and then enhance the heat transfer Publshed The Authors. by Elsever Publshed Ltd. Ths by s Elsever an open Ltd. access artcle under the CC BY-NC-ND lcense Selecton ( and/or peer-revew under responsblty of ICAE Peer-revew under responsblty of Appled Energy Innovaton Insttute Keywordstrefol-hole baffle; shell-and-tube heat exchanger; shell sde; heat transfer enhancement; numercal nvestgaton 1. Introducton Shell-and-tube heat exchangers(sthes) are wdely used n varous ndustral felds such as petroleum refnng, power generaton and chemcal process, especally nuclear power system[1]. Lots of energy could be saved by mprovng the structure of these equpment. The segmental-baffle shell-and-tube heat exchangers have been well developed and wdely used[2-3]. However, the pressure loss and the dead flow regon of them are consderably large. Moreover, the harmful vbraton and the energy consumpton * Correspondng author. Tel.: ; fax: E-mal address: zhougy@ecust.edu.cn Publshed by Elsever Ltd. Ths s an open access artcle under the CC BY-NC-ND lcense ( Peer-revew under responsblty of Appled Energy Innovaton Insttute do: /j.egypro

2 Guo-Yan Zhou et al. / Energy Proceda 75 ( 2015 ) are hgh. As a new type of longtudnal flow heat exchanger[4], shell-and-tube heat exchangers wth trefol-hole baffles s developed to solve these problems. In order to understand the underlyng mechansm of heat transfer enhancement, Dong [5] proposed the unt channel model and numercally nvestgate the flow and heat transfer characterstcs of shell-andtube heat exchangers wth trefol-hole baffles. The results showed that the heat transfer rate and pressure loss decrease wth the ncrease of the hole heght and baffle space. You[6-7] obtaned the heat transfer rate and pressure drop formulaton on the shell sde under specfed baffle space. However, these studes manly focus on the smplfcaton of the geometry and usage of the unt channel [8-9], n whch the flud flows n dfferent channels wthout nterference s assumed. So, the heat transfer performance of the whole heat exchangers and the mechansm of the heat transfer enhancement can not be understood and mastered. To understand the fundamental mechansms of flud flow and heat transfer on shellsde of shell-andtube heat exchanger wth trefol-hole baffles, a whole model for numercal smulaton s proposed to study the forced convecton heat transfer characterstcs and pressure drop n the present paper. Compared wth the expermental results, the feasblty and accuracy of the proposed model are verfed. Furthermore, the flow feld and heat transfer enhancement mechansm are also analysed. 2. Geometrcal model Fg. 1 depcts the sketch of a shell-and-tube heat exchanger wth trefol-hole baffles. Trefol holes are broached on the baffles for the support of tubes. The flud flows through the gaps between the tubes and baffles longtudnally. The detaled structural parameters are lsted n Table 1. Fg. 1. The geometry of shell-and-tube heat exchanger wth trefol-hole baffles: a) Structure, b) Tube layout Table 1. Geometrc parameters of heat exchanger Parameter Value Inner/Outer dameter of tube/mm 12.2/14 Central dstance of tubes/mm 19 Arrangement of tubes rotatonal regular trangle Tube number 30 Tube length /mm 1830 Trefol-hole baffle number 4 Trefol-baffle ptch /mm 400 Thckness of trefol-baffle /mm 5 Dameter of nlet and outlet nozzles /mm 45 Dameter of hexagon nscrbed crcle /mm Numercal model 3.1. The Governng equatons

3 3176 Guo-Yan Zhou et al. / Energy Proceda 75 ( 2015 ) Water s selected as the shellsde workng flud. It s treated as an ncompressble flud. The physcal propertes vary wth temperature and are calculated by usng the pecewse-lnear[10] method to mprove computng precson. The flow s assumed to be stable and turbulent. The vscous heatng and pressure power are neglected due to the lmted flud velocty and ncompressble medum assumpton. Based on the above assumptons, the governng equatons of flud flowng on the shellsde are presented as followng[11-12]: Contnuty equaton: ( ) 0 x u (1) Momentum equaton: u p ( uu k) ( ) x x x x (2) k Energy equaton: k T ( ut ) ( ) x x Cp x (3) where T and p stand are flud temperature and pressure, respectvely; u s the flud velocty; and C p are flud densty and constant pressure specfc heat, respectvely; k s heat conductvty coeffcent. Consderng the hghly swrlng on the shellsde wth non-sotropc turbulence, RNG k- turbulence model together wth standard wall functon[13] s adopted for the current computaton[10]. The conservaton equatons of turbulence knetc energy and ts dsspaton rate are gven below: Turbulence knetc energy k: k ( k) ( ku) ( k eff ) Gk t x xj x (4) j Turbulence knetc energy dsspaton rate : 2 * ( ) ( u) ( k eff ) C1 GkC2 t x x x k k (5) j j 2 k * 0 eff t t c, C1 C1 3 (1 / ) 1/2 k where, (2 Ej * Ej ), E 1 The constants[10]: C =0.085, C 1 =1.42, C 2 =1.68, =0.012, 0 =4.38, k = = The computaton doman and boundary condtons j 1 u u. 2 x j x Accordng to the symmetry of the structure, a half of the whole volume s taken as the computaton doman, whch nvolved n the shell, together wth the nlet and outlet headers and tubes. Snce the clearances between tubes and baffles are very small, the leakage effects are expected to be neglgble n the computaton. Boundary condtons are set as follows: (1) At the nlet of the shell sde, a velocty profle of fullydeveloped turbulent flow s gven, whle the flud temperature s set as unform, T n =238k. The turbulence ntensty s set as I=3%. (2) At the outlet of the shell sde, an outflow condton s mposed. (3) No slp and no penetraton are specfed for all walls. (4)The baffles and the shell wall are adabatc, the temperature of tube wall s set constant, T w =307K.

4 Guo-Yan Zhou et al. / Energy Proceda 75 ( 2015 ) The mesh and the computaton scheme The 3D grd system s generated by usng the commercal software ANSYS. The computaton doman, except near-baffles regon, s dscretzed wth structured tetrahedral elements. The mesh of the regons adjacent to the tubes and supported plate are refned to meet the requrement, as shown n Fg. 2. The fnte volume method s adopt for the dscretzaton, and all the varables are treated wth the second-order upwnd scheme[14], except the pressure term wth standard scheme[13]. Numercal computatons are conducted wth the pressure-based solver, and pressure and velocty are coupled wth the SIMPLE algorthm. The convergent crterons are set as: relatve resdual of 1E-8 for energy and 1E- 5 for other varables. In order to ensure the accuracy of the calculaton, grd ndependency of the results has been checked. It s found that the relatve dscrepances of both Nusselt number Nu and the pressure loss for the latter two are wthn 1%, whch demonstrates that the CFD smulaton s of reasonable precson. Thus the grd system of s adopted for the fnal computaton. 4. Model verfcaton Fg. 2. Meshes of the studed model: a) Grds of cross secton, b) Grds near trefol-hole baffle In order to verfy the accuracy of the proposed CFD model, an experment for shell-and-tube heat exchanger wth trefol-hole baffle s carred out. Nusselt number Nu Expermental results Numercal results Pressure drop p /kpa Expermental results Numercal results Reynolds number Re Reynolds number Re Fg. 3. Comparson of the expermental and numercal results The expermental results are shown n Fg. 3. It shows that the numercal Nusselt number and the pressure loss are n good agreement wth expermental results. The maxmum error of Nusselt number and the pressure loss are 17.4% and 20.3%, respectvely. It ndcated that the proposed physcal model and the numercal calculaton method s feasble and accurate. 5. Results and dscussons 5.1. Heat transfer and pressure drop performance The mean heat transfer coeffcent of the cross secton h av s defned as: hav q ( Tw Tav ). where q s the average heat flux, T av s the average flud temperature, T w s the temperature of tubes.

5 3178 Guo-Yan Zhou et al. / Energy Proceda 75 ( 2015 ) Mean heat transfer coeffcent h av /kwm trefol-hole baffle Axal length z /mm statc pressure p av /kpa trefol-hole baffle Axal length z /mm Fg. 4. The varaton of a) mean heat transfer coeffcent and b) statc pressure along wth axal drecton z The varaton of the average heat transfer coeffcent h av, statc pressure p av of cross sectons along wth axal length s shown n Fg. 4. It can be seen that the flud s fully developed after the frst trefol-hole baffle. The flow and heat transfer characterstcs of the central shellsde are mportant and can be used as desgn rules. In addton, the pressure loss comes from flow resstances and local loss flowng through baffles The flow dstrbuton Fg. 5 shows the velocty dstrbuton on the longtudnal secton. It can be seen that the most of flud flows longtudnally and unformly. The acceleraton and expanson of the workng flud are clear when t flows cross baffles, and the flow drecton vares, whch generates multdrectonal jet and results n the turbulence enhancement. Ths s because that the flow area decreases and the velocty ncreases sharply. Snce the block of the baffle, the flud n front of the baffles flows from the center to the edge of the shell, and the flud has passed through the baffles flows back to the center form the edge, whch produce the secondary flow and enhance the convecton heat transfer on shell sde effectvely. Due to the effect of shell wall and baffles, the velocty near the nner shell wall s low, and the velocty near the center s hgh. Along the radal drecton, the velocty decreases gradually, whch leadng to the decrease of the average convectve heat transfer coeffcent. Average temperature T av /K Axal length z /mm Fg. 5. Flow dstrbuton of the longtudnal secton(x=8mm) Fg. 6. The varaton of average temperature along the axal length The temperature dstrbuton T av along the axal length s shown n Fg. 6. It can be seen that the temperature near baffles ncreases obvously due to the generaton of the jet and secondary flow. Thus, the local heat transfer s enhanced relatvely. 6. Conclusons The numercal analyss of heat transfer and flud flows n the shell sdes of the shell-and-tube heat exchanger wth trefol-hole baffles s carred out, wth the am to mprove the overall themo-hydraulc performance n longtudnal flow heat exchangers. The followng conclusons be drawn from the study:

6 Guo-Yan Zhou et al. / Energy Proceda 75 ( 2015 ) ) It has been shown that the numercal Nusselt number and the pressure loss are n good agreement wth those from experments. The relatve devaton s lower than 17.4% and 20.3%, respectvely. It ndcates that t s reasonable to use the proposed CFD model and computaton method n the analyss of shell-and-tube heat exchanger wth trefol-hole baffles. 2) Due to the structure characterstc, the flud on the shellsde of shell-and-tube heat exchanger wth trefol-hole baffles flows perodcally. The gradent temperature and pressure also recur perodcally. The flud flow s fully developed after the frst trefol-hole baffle. 3) For flud flowng through the baffles, the acceleraton and expanson of the workng flud are clear and the jet and swrl flows are generated due to the decrease of the flow area. Along the radal drecton, the velocty decreases gradually, whch leadng to the decrease of the average convectve heat transfer coeffcent. Acknowledgements Ths study s sponsored by Shangha Pujang Program. References [1] Bell K. Heat exchanger desgn for the process ndustres[j]. Journal of Heat Transfer, 2004, 126(6): [2] Sh Jngu. Research on flow-feld and temperature numercal smulaton to shell and tube heat exchanger[d]. Nanjng: Nanjng Unversty of Technology, [3] Muhammad Mahmood Aslam Bhutta, Muhammad Hassan Bashr, Ahmer Ras Khan, Kanwar Naveed Ahmad, Sarfaraz Khan. CFD applcatons n varous heat exchangers desgn: A rewew[j]. Appled Thermal Engneerng, 2012, (32): [4] Zhang Janfe, He Yalng, Tao Wenquan. 3D numercal smulaton on shell-and-tube heat exchangers wth mddleoverlapped helcal baffles and contnuous baffles-part I: Numercal model and results of whole heat exchanger wth mddleoverlapped helcal baffles[j]. Internatonal Journal of Heat and Mass Transfer, 2009, 52: [5] Dong Qwu, Ba Capeng, Lu Mnshan, Wang Dan. Study on flow and heat transfer of shell sde of heat exchangers wth trefol baffle. Heat and mass transfer at the Chnese academy of engneerng thermal physcs learnng academc conference proceedngs[c]. X an: Chnese Socety of Engneerng Thermophyscs, 2011: 1-5. [6] You Yonghua, Fan Awu, La Xuejang, Huang Suy, Lu We. Expermental and numercal nvestgatons of shell-sde thermo-hydraulc performances for shell-and-tube heat exchanger wth trefol-hole baffles[j]. Appled Thermal Engneerng, 2013,50: [7] You Yonghua, Fan Awu, Chen Chen, Fang Shul, Jn Shpng, Huang Suy. Numercal study of shellsde performance of heat transfer and flow ressance for heat exchanger wth trefol-hole baffles[j]. Advanced Materals Research, 2012, 557: [8] Dong Q W, Wang Y Q, Lu M S. Numercal and expermental nvestgaton of shellsde characterstcs for RODbaffle heat exchanger[j]. Appled Thermal Engneerng, 2008,28 (7): [9] Dong Q W, Lu M S, Zhao Z D. Research on the characterstc of shellsde support structures of heat exchanger wth longtudnal flow of shellsde flud[j]. ASME Trans., 2005, 8(2): [10] FLUENT14.0 User s Gude. [11] Xong Q, L B, Xu J. GPU-accelerated adaptve partcle splttng and mergng n SPH. Comput. Phys. Commun. 2013, (184): [12] Xong Q, Deng L, Wang W, et al., SPH method for two-flud modelng of partcle-flud fludzaton. Chem. Eng. Sc. 2011, 66: [13] Tao Wenquan. Numercal Heat Transfer [M].2nd ed. X an: X an Jao Tong Unversty Press, [14] Barth T J, Jespersen D C. The desgn and applcaton of upwnd schemes on unstructured meshes. AIAA Paper, 1989, 89(366): Bography Dr. Guo-yan Zhou s a Professor of Mechancal Engneerng and now workng at East Chna Unversty of Scence and Technology. She receved her Ph. D. degree from the same unversty n Her nterests of research nclude development and optmzaton of compact heat exchanger, advanced manufacturng technology and MCMS, etc.