Available online at ScienceDirect. International Conference on Design and Manufacturing 2013 (IConDM 2013)

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1 Avalable onlne at ScenceDrect Proceda Engneerng 64 ( 2013 ) Internatonal Conference on Desgn and Manufacturng 2013 (IConDM 2013) Numercal Analyss of Flow and Heat transfer n Sub-channel of Supercrtcal Water Reactor Aoy Debbarma a, K.M Pandey b * a M.Tech Scholar,N.I.T Slchar,Assam ,Inda b Professor, Department of Mechancal Engneerng, N.I.T Slchar,Assam ,Inda Abstract Investgaton on the thermal-hydraulc behavour n the Hgh performance Lght Water Reactor (HPLWR) fuel assembly has obtaned a sgnfcant attenton n GENERATION-IV. In ths paper, CFD analyss s carred out to study the heat transfer behavour of supercrtcal water n sub-channels of squared annular assembly. Effect of varous parameters, e.g. thermal boundary condtons and ptch-to-dameter rato on the thermal hydraulc behavour s nvestgated. Boundary condtons,.e., constant heat flux at the outer surface of claddng are appled. The effect of ptch-to-dameter rato (P/D) on the dstrbutons of surface temperature and heat transfer coeffcent (HTC) was analyzed.for typcal (gap between two rod) channel analyss, t s found that HTC ncreases wth p/d frst and then decreases sgnfcantly when p/d s <1.2. If P/D s larger than 1.2 heat transfer deteroraton (HTD) occurs as man stream enthalpy s qute small. One/eght channel of 40 pn fuel assembly show that the structure of the secondary flow manly depends on the rod bundle confguraton as well as the ptch-to-dameter rato, whereas, the ampltude of the secondary flow s affected by the thermal boundary condtons, as well. Furthermore, the HTD under wre-wrapped spacer were evaluated The The Authors. Publshed by Elsever by Elsever Ltd. Open Ltd. access Selecton under and/or CC BY-NC-ND peer-revew lcense. under responsblty of the Revew Commttee Selecton and of IConDM peer-revew 2013, under IITD&M responsblty Kancheepuram, of the organzng Chenna, and Inda. revew commttee of IConDM 2013 Keywords: Super crtcal water, Sub-channel assembly, Wre-wrapped square channel, Heat transfers deteroraton. 1. Introducton The Hgh Performance Lght Water Reactor (HPLWR), how the European Supercrtcal Water Cooled Reactor s called, s a pressure vessel type reactor operated wth supercrtcal water at 25 MPa feed water pressure and 500 O C average core outlet temperatures. It s desgned and analyzed by European consortum of 10 partners and 3 actve supporters from 8 Euratom member states n the second phase of the HPLWR proect. Most emphass has been lad on a core wth a thermal neutron spectrum, consstng of small fuel assembles n boxes wth 40 fuel pns each and a central water box to mprove the neutron moderaton despte the low coolant densty. Peak claddng temperatures of the fuel rods have been mnmzed by heatng up the coolant n three steps wth ntermedate coolant mxng. The contanment desgn wth ts safety and resdual heat removal systems are based on the latest bolng water reactor concept, but wth dfferent passve hgh pressure coolant necton systems to cause a forced convecton through the core [1]. The desgn concept of the steam cycle s ndcatng the envsaged effcency ncrease to around 44%.Moreover; t provdes the constrants to desgn the components of the balance of the plant. * Correspondng author. E-mal address: berloyme24@gmal.com, kmpandey2001@yahoo.com The Authors. Publshed by Elsever Ltd. Open access under CC BY-NC-ND lcense. Selecton and peer-revew under responsblty of the organzng and revew commttee of IConDM 2013 do: /.proeng

2 508 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) The concept of an LWR operatng at supercrtcal condtons has been studed n the past by dfferent vendors. A revew of the varous supercrtcal concepts that were proposed durng the last four decades was elaborated and publshed by Y. Oka. Examnaton of ths revew ndcates that, except for the Unversty of Tokyo s SCLWR-H and the CANDU-X reactor desgns, none of the other proposed concepts are lkely to be economcally compettve wth modern LWRs and therefore are less lkely to be developed nto a commercal product. Fg.1. Three-pass core concept wth target temperatures for each heat up step (left) and cross-secton of the core wth the conceptual arrangement of the fuel assembles (rght) (Schulenberg et al., 2009).[2] The desgn of HPLWR ams to utlze these advantages of supercrtcal water technologes nto the Lght Water Reactors (LWR). By operatng ts coolant at hgher temperature and pressure, t has the potental advantage of a hgher thermal effcency of 44% and a compact structure of the plant system compared wth current LWR. The reactor core wth a thermal neutron spectrum has been desgned based on the proposal by Schulenberg et al. As shown n Fg 1, the feed water flows n the RPV. The flow path of coolant through the core s desgned wth three heat-up steps wth the ntermedate mxng. The frst heat-up path s the so- Evaporator w n the nuclear core centre. Supercrtcal water enters nto evaporator at 310 ºC and then leaves t at 390 ºC. It can be found that the pseudocrtcal temperature of the coolant wll be exceeded when the coolant flows vertcally n the evaporator. Under such condtons the heat transfer coeffcent mght decrease n some regons when the mass flux s relatve low and the heat flux s relatve hgh, whch consequently results n a hgh claddng temperature, or even a severe deteroraton. 2. Mathematcal and Physcal Models Fg 2 shows the detaled fuel assembly arrangement n the thermal neutron regon. The dameter of the fuel rod (D) s 8 mm and total actve heght s 4 m. One fuel assembly has 40 pns wrapped by wres and a central water box for the moderator. The wrapped wres are capable to guarantee a cross mxng n both upward and downward flow of the coolant n the fuel assembles and also to prevent the neghbourng rods from comng close to each other due to the thermal stresses and the possble external forces. Fg. 2. HPLWR fuel assembly (a) wth wre spacers (b) ¼ top vew wthout wre.

3 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) Geometrc structure and computatonal Sub-channel Gap between two rods are consdered wth varous P/D rato, 1mm for gap wth moderator wall and fuel Assembly wall. A quarter of the thermal fuel assembly s calculated takng symmetry nto consderaton. Further geometrcally, four type of sub-channels can be consdered, see Fg 3.e. adacent to the sde of the moderator flow channels (SC-1) (moderator tube and assembly gap), central sub-channels formed by four fuel rods (SC-2), adacent to the corner of the moderator tube (SC- 3) and corner of the assembly gap (SC-4). Fg.3. Sub-channel types of the square fuel assembly by Hofmester et al. [3] 2.2 Square channel The geometres of a fuel rod wth and wthout wrapped wre nsde a square channel have been chosen as the smplfed geometres to nvestgate the mxng effect of wre. These two basc geometres are shown n Fg 4. Fg.4. Top vew of Square channel, (a) Bare rod n a Square Channel, (b) Wre-wrapped rod n a Square Channel Numercal analyss of wre-wrapped fuel pns for HPLWR s challengng for several reasons. The presence of the wre nsde the assembly results n a strong flow mxng and sweepng among sub-channels. A channel wth a length of 2000 mm (10 wre ptches) s used as a standard geometry. The wre wrappng helcally around the rod s made of ceramc materal wth a dameter of 2.5 mm (D wre ). 2.3 Selecton of turbulent models The proper choce of the turbulent models s mportant for scentfc calculaton and can lead to satsfactory smulaton results. Based on the revew of the open lterature on the turbulence models adopted under super crtcal pressure condton, Heat transfer to water at supercrtcal pressures has been numercally nvestgated usng a two-dmensonal modelng approach. The smulatons n a two-dmensonal doman have been performed usng the low-reynolds k turbulence model, and the IAPWS-IF97 formulaton to descrbe the propertes of water at dfferent condtons. The governng equatons (ANSYS14 FLUENT User Manual) are as follows: Mass conservaton equaton: x u 0 (0.1)

4 510 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) Momentum conservaton equaton: Energy conservaton equaton: RNG k u u 2 u k p uu eff eff x x x x 3 x x eff eff k k T u u u 2 u uc pt ff ff x x x u u u 3 u k p T eff eff 3 k model s adopted to close the turbulence knetc energy and dsspaton equaton. Turbulent knetc energy equaton: k ku k t x x k eff x Where Where G G k eff k b Gb s the effect of buoyancy on turbulent knetc energy, and s defne as: G b g 1 t Pr T r t x Gk s turbulent knetc energy produced by velocty gradent. G k t S 2 u u S 2, /2 x x T p g (0.2) (0.3) (0.4) (0.5) (0.6) (0.7) Turbulent knetc energy dsspaton rato: Where u C G C G C 2 R t x x x k k mol s the molecule vscosty eff 1 k 2 b eff C Where Sk /, and R mol / C mol k k 2 2 (0.8) (0.9) (1) Fg. 5. Comparson between present results and the expermental data.

5 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) Fg 5 shows the comparson between our calculaton data and the expermental data (Yamagata et al., 1972) [4]. The heat transfer coeffcent dstrbuton n the super crtcal pressure regon s calculated usng RNG k model. The RNG k model wth the enhanced near-wall treatment obtaned the most satsfactory predcton. Therefore n ths study the RNG k model s adopted to smulate the turbulent flow. 3. Results and Dscusson Effects of Ptch-to-Dameter rato (P/D) n vertcal upward flow between two rods of a Square sub-channel are smulated based on the test condtons from Yamagata [4], for Heat transfer enhancement. As the (P/D) s ncreased, the wall temperature measured n smulaton strongly ncreases at the begnnng of the heated secton. Fg 6(a) shows the wall temperature dstrbuton n between two parallel fuel rods 4m vertcal heght. Wall temperature ncreases wth the man stream enthalpy and the ncrease s qute sgnfcant after pseudo-crtcal pont. When the man stream enthalpy reaches about kj kg -1, the wall temperature ncreases very slowly. For man enthalpy value of >1900 kj kg -1, dfferent trends appear dependng on P/D value. If P/D s small, the wall temperature ncreases slowly. But f P/D s larger, the wall temperature ncreases very rapdly. (a) (b) Fg.6. P/D , q=600 kw m -2 (a) Wall temperature dstrbuton, O C (b) Heat transfer coeffcent, Kw/m 2 K Fg 6(b) shows the heat transfer coeffcent curves for dfferent P/D and man stream enthalpy between fuel rods. It can be seen that the heat transfer coeffcent ncreases wth the man stream enthalpy, reaches the maxmum value near the pseudo-crtcal pont, and then decreases. Wth the ncrease of P/D, the heat transfer coeffcent decreases and the locaton of the maxmum heat transfer coeffcent moves towards the lowere enthalpy regon. As the Reynolds Number s fxed, the flow velocty s relatvely small. Before the man stream enthalpy reaches a hgh value, HTD already cccuts. When the lecocty s small, the buoyancy force accelerates the flow velocty of the flud n the near-wall regon. Ths makes the flow lvelocty dstrbuton to be flat and generaton of turbulence energy reduced. Thus the heat transfer s deterorated. When P/D s smaller, as the Reynolds Number s fxed, the flow velocty s relatvely larger, and also the ampltude of turbulent mxng s relatvely larger. Therefore, the heat transfer condton s better and the man stream enthalpy can reach a hgh value. It can be seen that the heat transfer coeffcent decreases n towards hgher enthalpy regon. (a) (b) Fg.7: P/D 1.2, q=450,600,850 kw m -2 (a) Wall temperature dstrbuton, O C b) Heat transfer coeffcent, Kw/m 2 K Fg 7(a) shows the wall temperature dstrbuton wth dfferent Heat flux nput. The wall temperature ncreases wth the ncrease of the man stream enthalpy. The value frst ncreases rapdly, then slowly and fnally rapdly. When the heat flux

6 512 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) value s hgher, the ncreases of wall temperature take place very slowly n mddle enthalpy regon. Wth the decrease of heat flux, as the P/D s fxed, the change of temperature n the mddle enthalpy regon ncreases rapdly. Fg 7(b) shows heat transfer coeffcent dstrbuton. As the rod dameter and the gap sze between the rod and wall s fxed, wth the ncrease of heat flux the heat transfer coeffcent ncreases and reversely occur when heat flux decreases. Furthermore, the locaton of the maxmum value of the heat transfer coeffcent moves towards the low enthalpy regon wth the P/D decreasng. (a) (b) Fg.8. 1/4 secton fuel assembly,p/d 1.2, q=600 kw m -2 (a) velocty Magntude, m/s(b) Statc Wall temperature dstrbuton, O C Fg 8 shows the Contours of velocty Magntude and Statc Temperature of a quarter sectons at outlet and throughout the length respectvely. It can be seen that the maxmum velocty occurs n the typcal quadrlateral sub-channels and the velocty dfference between each typcal quadrlateral sub-channel s small. The velocty near the wall s much smaller. As a result, the temperature near the wall s hgher, and n the typcal quadrlateral sub-channels s lower. The hghest temperature occurs n the corner regon. Fg. 9 shows the velocty dstrbuton and temperature dstrbuton near the pseudocrtcal pont. Fg 10 shows the temperature dstrbuton at outlet secton of each ndvdual channels. It s seen that towards the rods wall, temperature s hgher than towards moderator wall. Fg 11(a,b,c,d) descrbes the temperature dstrbuton throughout the axal drecton of the channels. Inlet temperature ncreases rapdly n the entry regon, t can be seen n the mddle regon were the temperature ncreases slowly and last regon ncreases rapdly agan. Fg 11(e,f,g,h) shows n the flatten regon heat transfer coeffcent ncreases. As a result, the claddng temperature ncreases from the nlet as the heat transfer coeffcent

7 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) ncreases. At the pont where the heat transfer has a maxmum, the claddng temperature decreases. Fg.9. 1/4 secton fuel assemblyat the pseudo-crtcal pont P/D 1.2, q=600 kw m -2 (a) velocty Magntude, m/s (b) Statc Temperature, o C Beyond the pseudo-crtcal lne, where the heat transfer coeffcent decreases, the claddng temperature ncreases. The maxmum claddng temperatures n the dfferent sub-channels wth P/D 1.2 are wthn the allowable lmts of 620 o C for the HPLWR. Fg 11 s explaned wth color code, black colors ndcate the bulk flud temperature. Fg.10. Statc Temperature dstrbuton, O C (sub-channel assembly, P/D 1.2, q=600 kw m -2 )

8 514 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) (a) (e) (b) (f) (c) (g) (d) (h) Bulk flud temperature Claddng surface-4 Claddng surface-1 Claddng surface-6 Claddng surface-2 Claddng surface- 7 Claddng surface-3 Claddng surface-12 SC-1(a,e), SC-2(b,f), SC-3(c,g), SC-4(d,h) Fg.11. Temperature dstrbuton, O C (a,b,c,d), Heat transfer coeffcent, Kw/m 2 K (e,f,g,h), The purpose of square annular channel s to the performance n the non-crcular channels, whch s closer to the realstc flow channel n the fuel assembly. In HPLWR the desgn wth wre spacers rase several challengng ssues wth regard to the transfer behavour of the water under a supercrtcal pressure, wth the spacers lead to a good mxng and a low pressure drop n the assembly. The smulatons are performed at a mass flux densty of 1000 kg/m2s and at heat fluxes of 600kw/m2. The results of the non-crcular channel wth and wthout wre-wrapped are shown Fg 12 and Fg 13 respectvely, ts shows the dfference of

9 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) the velocty and turbulence knetc energy between the bare rod and wre-wrapped channel. The result of the comparson shows that velocty ncreases wth wre-wrapped, also turbulence occurs more. At hgh mass flow rate buoyancy effect can be neglected unless wth low flow rate, even though the buoyancy should not be relevant, heat transfer s stll deterorated (as measured by Ornatsk et.al. [5]. The nfluence of buoyancy s small and there are clearly other mechansms whch cause the decrease n heat transfer. Accordng to Koshzuka et.al may lead to the decrease n heat transfers coeffcent [6]. Fg 14 shows the temperature dstrbuton, here the red colors ndcate the surface contact wth wre spacer, and Fg 15 shows the heat transfer coeffcent Fg.12. Bare rod n a Square Channel at outlet; (a) Velocty dstrbuton, m/s (b) Turbulent knetc energy, (k) (m 2 /s 2 ) Fg.13. Wre-wrapped rod n a Square Channel at outlet; (a) Velocty dstrbuton, m/s(b) Turbulent knetc energy, (k) (m 2 /s 2 ) Black Claddng Surface, Red Claddng surface contact wth wre wrapped Fg.14. Temperature dstrbuton n a square channel, o C (a) bare rod, (b) Wre-wrapped rod

10 516 Aoy Debbarma and K.M. Pandey / Proceda Engneerng 64 ( 2013 ) Conclusons Black Claddng Surface, Red Claddng surface contact wth wre wrapped Fg.15. Heat Transfer Coeffcent n a square channel, Kw/m 2 K (a) Bare rod, (b) Wre-wrapped rod In present study, numercal study was performed on the rod gap, rod bundle sub-channels based on one of the HPLWR proposed model. The RNG k model wth the enhanced near-wall treatment obtaned the most satsfactory predcton and evaluated for the flow and heat transfer characterstcs n typcal sub-channels were nvestgated. In the 2D, gap between two rods sub-channel, the effect of P/D on HTC was studed. When P/D s <1.4, HTC ncrease wth P/D frst and then decreases sgnfcantly. When P/D s lower TD occurs when the man stream enthalpy s stll very small and more than one peak occurs. In the wall channel the maxmum temperature occurs between the rod and wall, where the temperature gradent s very large. Wth the ncrease of P/D, the temperature dfference between the wall and man stream ncreases, and HTC decreases. Furthermore, the locatons of the maxmum value of the HTC move towards the low enthalpy regon wth the P/D decreasng. In the one/eght channel near the pseudo-crtcal pont, the temperature dfference s small, whle n the sub-crtcal and super crtcal regons t s large. The non unformty at the outlet s largest. In the mddle heght of the channel, the flud reaches the pseudo-crtcal pont n the near-wall regon, whch causes the heat transfer coeffcent to decrease and the wall temperature ncreases rapdly. It s also found, n the corner regon (SC-4) the temperature rasng faster comparson to other channels (SC-1, SC-2, SC-3).On the study of wre-wrapped, results proved wth the spacers- lead to a good mxng and a low pressure drop n the assembly. Wth-wre wrapped spacer the sustanablty of claddng surface are mprove hgher as the temperature are under the lmts of ts materals. There are stll a lot to do to mprove the accuracy and relablty of CFD method and better models are needed to descrbe the flow and heat transfer characterstcs for the flud n super crtcal condtons n the future. Acknowledgements The author wshes to acknowledge Mechancal Engneerng Department for provdng the ANSYS 14 CFD software and support for ths study. The author would also lke to acknowledge the help of Dr. K.M Pandey, Professor of Mechancal Engneerng Department. Ths research was workouts durng post graduate perod, authors are grateful to Natonal Insttute of Technology, Slchar, Assam, Inda. References [1] Sarah Mokry, Amad Farah, Krysten Kng, Sahl Gupta, Igor Poro, Development of Supercrtcal Water Heat-Transfer Correlaton for Vertcal Internatonal Conference Nuclear Energy for New Europe Bled / Slovena / September [2] Fscher, K,. Schulenberg, T,. Lauren, E, cooled reactor wth a three- Nuclear Engneerng and Desgn 239, p [3] Hofmester, J., Schulenberg, T., Starflnger, J Optmzaton of a Fuel assembly for a HPLWR, ICAPP 2005, Seoul, Korea, paper [4] Yamagata, K,. Nshkawa, K,. Hasegawa, S,. Fu, T,. Yoshda, S, conve Internatonal Journal of Heat and Mass Transfer 15, p [5] Ornatsk, A.P,. Glushchenko, L.F,. Kalachev, S.I, Heat Transfer wth Rsng and Fallng Flows of Water n Tubes of Small Dameter at Thermal Engneerng, 18, p

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