An Experimental Research of Natural Circulation Heat Transfer for PRHR Heat Exchanger in AP1000

Transcription

1 Journal of Energy and Power Engneerng 10 (2016) do: / / D DAVID PUBLISHING An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP1000 Mnghu Duan 1, Yuzhou Chen 1, Yvfeng Lv 1, Weqng L 1, Kemng B 1, We Wang 1, Kawen Du 1 and Han Wang 2 1. Chna Insttute of Atomc Energy, P.O. Box 275(59), Bejng , Chna 2. State Nuclear Power Technology Research & Development Centre, Bejng , Chna Receved: May 20, 2016 / Accepted: June 02, 2016 / Publshed: September 30, Abstract: The heat transfer characterstcs of PRHR (passve resdual heat removal) HX (heat exchanger) are very mportant to reactor desgn and safety assessment of AP1000. The purpose of present experment was to obtan natural crculaton data n HX to research heat transfer behavor. The PRHR HX was smulated by three C-type tubes wth prototype szes mmerged n a coolng tank. Separate-effect tests of natural crculaton n HX tubes have been performed wthn wde condtons whch could cover operaton condtons n AP1000. The experment provded lots of mportant data to ndcate heat transfer phenomena of PRHR HX. The test condtons were calculated by RELAP5/MOD3.3. The calculaton results agreed well wth experment. RELAP5 could be appled wth proper correlatons to analyze heat transfer n PRHR HX under test condtons. Key words: PRHR HX, natural crculaton, separate-effect tests, AP1000. Nomenclature Nu DB Nusselt number h Convectve heat transfer coeffcent, W/m 2 K DB D k Inner-tube dameter, m Thermal conductvty, W/m K Re Reynolds number Pr Prandtl number L Tube length, m h Heat transfer coeffcent, W/m 2 K Nusselt Flm thckness, m h Shah Heat transfer coeffcent, W/m 2 K X Statc vapor qualty Pred sf Reduced bulk pressure h Superfcal heat transfer coeffcent, W/m 2 K T wo Outsde-wall temperature, C T s Saturated temperature, C q w j Local wall heat flux, W/m 2 m Inner-tube mass flow rate, kg/s H Local flud enthalpy at poston j, J/kg j L Dstance between two nner-tube temperature j measure postons j and j+1, m Correspondng author: Mnghu Duan, engneer, research feld: reactor rmo-hydraulcs. h NB Nucleate bolng heat transfer coeffcent, W/m 2 K h Convecton heat transfer coeffcent, W/m 2 K FC T The secondary temperature, C f Subscrpt o w j f Inner tube Outsde tube Wall Measure poston Flud 1. Introducton The PRHR (passve resdual heat removal) HX (heat exchanger) s an mportant emergency core coolng system of AP1000, whch can remove core decay heat by coolant natural crculaton as a result of dfferent water densty. The temperatures between two sdes of PRHR HX tubes dffer greatly, and change n a wde range. Many knds of sngle-phase and two-phase flow patterns and heat transfer modes are nvolved. Especally n natural crculaton, flow dynamc characters dffer remarkably wth forced crculaton because of

2 546 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP1000 low flow rate and great nfluence by buoyancy. The complex heat transfer characterstcs are crtcal to performance of PRHR system. Therefore, HX heat transfer behavor s very mportant to reactor desgn and safety assessment. In Amerca, several tests for PRHR HX n AP600 have been performed to study heat transfer behavor. The HX was smulated by a test secton whch conssted of three vertcal parallel tubes wth no horzontal sectons. The heat transfer has been researched between vertcal tubes and coolng tank [1]. However, a PRHR HX tube s n C shape and conssts of a mddle vertcal secton, a top horzontal secton and a bottom horzontal secton. A majorty of heat s transferred through top horzontal secton. Besdes, PRHR HX heat transfer area n AP1000 s ncreased wth more HX tubes and longer horzontal sectons of tubes. A hgher proporton of heat s transferred through horzontal secton n AP1000 than n AP600 [2]. So t s necessary to smulate natural crculaton n C-type HX tubes and research complcated rmo-hydraulcs phenomena n AP1000 PRHR HX. In our research, PRHR HX separate-effect tests were performed on a PRHR HX test faclty, whch conssted of three C-type tubes wth typcal szes n AP1000. The experment purpose was to obtan natural crculaton data, research HX heat transfer, and develop proper analyss method. The experment condtons could vary n a wde range coverng operaton condtons n AP1000. The test faclty could obtan data ncludng nner-tube water temperatures, wall surface temperatures along tubes, pool water temperatures, flow rate n tubes and so on. Several tests have been performed for natural crculaton n tubes. And expermental results were compared wth calculatons by RELAP5/MOD Test Faclty The PRHR HX test faclty was desgned to perform heat transfer tests of both natural and forced crculaton n HX tubes. The flow path of test faclty was desgned n Fg. 1. The man devces of faclty ncluded a PRHR HX test secton, a canned pump, a heater, a crculaton pump, a heat exchanger n secondary sde, a pressurzer, a hgh water tank, a low water tank, a lft pump, a make-up pump and a spray pump. There were three C-type tubes n faclty wth same sze as ones n AP1000. Accordng to AP1000, re are 689 HX tubes (Φ mm) and desgn coolant flow rate s kg/h [3]. So max desgn flow rate of three tubes should be kg/s. In our faclty, desgn flow rate was up to 1.2 kg/s whch could cover typcal flow rate of PRHR. By adjustng motor-drven valves VCO103, VAJ101 and VAJ102 n prmary loop, system resstance could be controlled, so that test flow rate could be changed. A mass flow meter was nstalled n front of heater to measure flow rate n prmary loop. In AP1000, PRHR HX s operated n 15.5 MPa, and desgn pressure s 17 MPa. The nlet temperature of HX tubes s C, whle outlet temperature s 92.8 C [3]. The test faclty preserved full temperature and pressure condtons, and vared over expected range for AP1000. The nlet water temperature could be adjusted by controllng heatng power of heater. And prmary pressure was controlled by pressurzer and make-up pump. The PRHR HX test secton was key devce of faclty. It conssted of three C-type tubes, a coolng tank and nstruments such as rmocouples. The C-type tubes mmerged n coolng tank whch smulated IRWST. When hgh temperature water flowed through tubes, HX test secton transferred heat from prmary system to coolng tank by heatng and bolng water n tank. The man techncal parameters of test secton were shown n Table 1.

3 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP Fg. 1 Flow path chart of PRHR HX test faclty. Table 1 The man techncal parameters of HX testt secton. Test condton Desgn condton Tank pressuree (MPa) Temperature n tank ( C) Tube pressuree (MPa) Inlet temperature of tubes ( C) Flow rate n each tube (kg/m Tank materal 0.1 ~100 ~15 150~ ~2,000 06Cr19N10 Tank desgn pressure (MPa) Desgntemperature n tank ( C) Tube desgn pressure (MPa) Desgntemperature n tubes ( C) Flud Tube materal Deonzed water Inconel 600 Table 2 The tube length. Tube No. A (shortest) B (center) C (longest) Horzontal secton length 2,586 mm 3,656 mm 4,724 mm Vertcal secton length 5,284 mm 5,360 mm 5,436 mm 2.1 C-Type Tubes In test secton, tube bundle conssted of three C-type tubes. The tubes were n same vertcal plane. Each tube contaned a top horzontal secton, a bottom horzontal secton and a vertcal secton, whch were connected to each or by cuttng sleeves. The outer dameter of test tube was mm and thckness was 1.65 mm as HX tubes n AP1000 [3-5]. The heat transferr for vertcal tubes has been researched, so ths experment manly focused on heat transfer n horzontal sectons. Therefore, space between horzontal conserved centerlne-to-centerlne horzontal tubes was 38 mm. The tube lengths weree shown n Table 2. The tubes respectvely smulated shortest tube, longest one and center one of PRHR HX tubes n AP1000. The materal of tubes was Inconel 600, whle that n AP1000 was Inconel 690. There s 6 percent dfference n tube materal rmal conductvty. tubes n AP1000 was n test secton. The space between adjacent

4 548 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP1000 The tubes were connected to prmary system ppes by two connectors as shown n Fg. 2. A connector conssted of a vertcal cylnder tank and four nozzles as shown n Fg. 3. A top vertcal nozzle was used for ventng. Three horzontal parallel nozzles were used to connect wth HX tubes respectvely. A sngle horzontal nozzle was used to connect wth prmary ppe. The top connector was connected wth outlet lne of heater. The heated water flowed out of heater and nto test secton through top connector, n out of test secton through bottom connector. When hgh temperature water flowed through tubes, C-type tubes transferred heat from prmary system to coolng tank by heatng and bolng water n tank. The nner-tube natural crculaton was developed from densty dfference between cold water n outlet lne and hot water n nlet lne. The top connector was about 10.5 m above lowest pont of test loop, so max crculaton heght was 10.5 m. 2.2 Coolng Tank Fg. 2 Wndow The coolng tank confguraton. Overflow Connector Inlet Connector In Amerca, several tests for PRHR HX n AP600 have been performed. The results ndcated that when space between adjacent tubes was large enough, rmal effect was ndependent to each or. Typcal P/D (ptch-to-dameter) raton was 1.3 to 1.5, and n AP600 t was 2 for adjacent tubes. The tests also ndcated that when dstance between tube and tank wall was large enough, rmal effect by tank wall could be gnored [1]. Therefore, n our test faclty, IRWST was predgested to a cylnder tubular tank, as shown n Fg. 2. The coolng tank conssted of three parallel vertcal tubes connected to a top and bottom horzontal tube. Each vertcal tube of tank respectvely contaned one vertcal secton of HX tubes, whle horzontal tube of tank contaned three horzontal sectons of HX tube bundle. The sze of horzontal tubes was Φ mm, and that of Fg. 3 The connector confguraton. vertcal tubes was Φ mm. The dstance between tank wall and HX tubes was much larger than 38 mm, so tank sze would not affect heat transfer from HX tubes to coolng water, whch meant that predgested coolng tank was feasble. The secondary coolng water flowed nto coolng tank from nlet at bottom, and left t from outlet on top flange cover. There was anor outlet for coolng water overflow. In order to observe heat transfer phenomena n tank, re were two wndows on tank.

5 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP Thermocouples In test secton, many temperature data needed to be acqured, ncludng nner-tube water temperatures, wall surface temperatures along tubes and pool water temperatures. In order to measure water temperature n tubes, several rmocouple traps were mounted nto tubes. A rmocouple trap was made of a Φ mm seamless stanless steel tube wth one end closed and or open. Several shead rmocouples of 0.5 mm dameter were nserted nto trap from open end. The rmocouple traps were nserted nto and welded to HX tubes, and supported n centre of tubes. By ths way, t was assured that rmocouples measured centre temperature n HX tubes. In order to mnsh trap nfluence on flow n HX tubes, trap sze should be as small as possble. However, re were 10 rmocouples to be nstalled n A tube, 12 n B tube, 13 n C tube. In order to contan so many rmocouples, trap sze was desgned to be Φ mm. Because of rmocouple traps, flow cross secton n HX tubes was reduced by 3.6%, whch could be accepted. Several rmocouples were welded on outer wall surface of HX tubes to measure wall surface temperatures. There were 10 rmocouples on A tube, 14 on B tube, 13 on C tube. Each rmocouple on wall surface was located between two rmocouples nsde HX tubes. Besdes, 18 rmocouple traps were nserted nto and welded to tank. One rmocouple was nstalled n each trap to measure pool water temperatures. 3. Test Procedures Before natural crculaton test started, close valve VAJ101 and VAJ102 to solate canned pump from test loop. Make sure valve VCO103 open. Open valve VHO202 and VHO203, and start lft pump to pump water n low water tank to fll hgh water tank. When hgh water tank overflowed, shut down lft pump, and turn off valve VHO203. Turn on valve VHO105, VHO104 and VHO 204. Because of gravty, water n hgh water tank flled test loop ncludng pressurzer, prmary ppes and coolng tank. When test loop was full of water, turn off valve VHO105. Feed ntrogen gas nto pressurzer, and start make-up pump to ncrease prmary system pressure to test condton. Then start heater, whch was a vertcal ppe connected wth postve pole of DC power at mdpont and wth negatve pole at two ends. When prmary water flowed though heater, ts temperature rose and densty decreased, so water flowed upwards nto test secton. Adjust heatng power to make sure nlet water temperature of test secton reached test condton. Adjust motor-drven valve VCO103 to change system resstance and control flow rate n prmary loop. When hot water flowed through test secton, t was cooled by coolng tank. The water densty ncreased, so water flowed downwards and back nto heater. Ths course formed natural crculaton n prmary loop. Meanwhle, start crculaton pump to crculate water n coolng tank through secondary heat exchanger, and turn on valve VHO303 and VHO304 to cool tank water. The natural crculaton test n HX tubes was a steady test. When system was stable, computer began to record test data every second ncludng nner-tube water temperatures, wall surface temperatures along tubes, pool water temperatures, flow rate n prmary loop, heatng power and so on. 4. Test Results and Analyss The purpose of PRHR tests was to provde a database to research heat transfer behavor nsde and outsde of PRHR tubes. In our tests, parameters covered prmary pressure of MPa, qualty n tubes up to 0.2, nlet water

6 550 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP1000 temperature of C, mass flow rate of kg/s and heatng power of up to 364 kw. The test condtons could cover operaton range for PRHR HX n AP1000. The PRHR HX had several dfferent modes of heat transfer. When power of heater was not large enough, water out of heater could not acheve saturaton temperature. So heat transfer nsde HX tubes was natural convecton. When power was large enough, prmary water out of heater was supersaturated. So heat transfer was nucleate bolng nsde tubes. Outsde tubes, heat transfer was bolng and natural convecton n pool. RELAP5/MOD3.3 has been used to model test faclty, smulate heat transfer nsde and outsde of HX tubes and assess calculaton model. There were 3 nner-tube natural crculaton test condtons calculated n Table Heat Transfer n Tube The heat transfer was natural convecton or condensaton n HX tubes dependng on heatng power. In RELAP5, heat transfer coeffcent could be calculated by Dttus-Boelter correlaton for sngle-phase lqud turbulent forced convecton [6-8], whch s gven as: hdbd NuDB 0.023Re Pr (1) k where, D means nner-tube dameter, k means flud rmal conductvty, Re means flud Reynolds number, Pr means flud Prandtl number. Dttus-Boelter correlaton could be appled wthn range of parameters: 0.7 Pr 160, Re > 6,000, L 60 D (L s tube length ). In our tests, Prandtl number was between 0.8 and 3.2, Reynolds number was above 18,000, and L 300 D. Though t was natural crculaton n tubes, Reynolds number was very large. Therefore, t was turbulence n tubes, and Dttus-Boelter correlaton was chosen n RELAP5 for heat transfer coeffcent calculaton n se test condtons. In RELAP5, heat transfer coeffcent could be calculated by maxmum of Nusselt and Shah correlatons for condensaton [6, 8]. The Nusselt expresson s gven as: k hn usselt (2) where, h Nusselt means heat transfer coeffcent, means flm thckness. The Shah correlaton s gven as: 3.8 hshah hsf (1 ) (3) 0.95 Z where, Z ( 1) P (4) red X and X means statc vapor qualty, P red means reduced bulk pressure, h sf means superfcal heat transfer coeffcent. In RELAP5, nput pressure, nlet temperature, heatng power, mass flow rate and pool temperature dstrbuton as boundary condtons, nner-tube temperature dstrbuton along tube length could be calculated. The test results and calculaton results of three tubes have been shown n Fgs From all test results, t can be ndcated that major heat transfer took place at upper horzontal secton of tubes. Along tube length, nner-tube temperature decreased because of secondary coolng water. Near nlet, flud Table 3 Test condtons. No. Pressure (MPa) Inlet temperature Heatng power (kw) ( C) Mass flow rate (kg/s) Qualty (X ) FN FN FN

7 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP (a) (a) (b) (b) (c) Fg. 4 The nner-tube temperature comparson of FN01 test. (c) Fg. 5 The nner-tube temperature comparson of FN02 test.

8 552 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP1000 (a) (b) The test results also ndcated that nner-tube temperature of upper tube was hgher than bottom one at same dstance from nlet. That was because n horzontal secton, temperature of upper tank water was hgher than bottom, so heat flux of upper tube was smaller than bottom tube. Durng two-phase flow tests, re was two-phase flow nstablty phenomenon n HX tubes. When water out of heater was supersaturated, re was two-phase flow n HX tubes. The prmary system pressure and natural crculaton flow rate fluctuated strongly. That s because n fnte space of close loop, qualty ncrease caused loop pressure rse. Wth pressure rse, water saturaton temperature rose, whch resulted n qualty and prmary pressure dropped. In order to perform stable tests, turn down valve VCO103 to ncrease system resstance. When system resstance was large enough, prmary pressure stopped fluctuatng and system became stable. The test results were shown n Fg. 6. As Fgs. 4-6, comparson between test data and calculaton results ndcated that n both sngle-phase and two-phase heat transfer condtons, calculated nner-tube temperatures of three HX tubes agreed well wth test data. Therefore, t can be concluded that Dttus-Boelter, Nusselt and Shah correlatons could be appled n RELAP5 to calculate heat transfer nsde tubes wthn test condtons. 4.2 Heat Transfer n Coolng Tank (c) Fg. 6 The nner-tube temperature comparson of FN03 test. temperature was hgh, heat transfer coeffcent and heat flux were large, so prmary temperature decreased quckly. Along tube length, heat flux decreased and nner-tube temperature dropped gradually slowly. There were two heat transfer modes outsde of HX tubes: bolng and natural convecton, whch depended on tube-wall temperature. Bolng occurred at horzontal sectons of HX tubes. Near HX tube nlet, tubes had suffcent wall superheat ( ), so local nucleate bolng occurred. Along tube length, wall temperature dropped because of secondary coolng water.

9 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP When wall superheat was not large enough, bolng dd not occur. Consderng large volume of tank, flow rate n secondary crculaton was very small, so heat transfer mode could be consdered to be natural convecton n large room. The local wall heat flux could be calculated by steady-state energy balance equaton as below: m qw j ( H j H j 1) (5) D Lj where, q wj means local wall heat flux, m means nner-tube mass flow rate, H j means local flud enthalpy at poston j whch was determnedd by nner-tube temperature, Lj means dstance between two nner-tube temperature measure postons j and j+1. In RELAP5, bolng heat transfer coeffcent could be calculated by Chen correlaton as below. Fg. 7 The bolng heat transfer curve n coolng tank. htp h h NB FC (6) h NB where, means nucleate bolng heat transfer coeffcent, whch s Forster-Zuber pool bolng heat transfer coeffcent modfed by a suppresson factor. h FC means convecton heat transfer coeffcent whch could be calculated by Dttus-Boelter correlaton tmes a Reynolds number factor. The bolng heat transfer curve could be correlated by local wall heat flux and wall superheat as Fg. 7. The calculaton results by RELAP5 were also shown n t. Generally speakng, calculaton trend agreed well wth test data. At low superheat regon, calculated heat flux was close to test data. At hgh superheat regon, calculated heat flux was hgher than test data. When local wall heat flux ncreased largely, wall superheat ncreasee slowed down. The wall superheat was less than 30 C. Compared wth large temperature dfference between nner-tub temperature dfference n tank water was relatvely small. Therefore, Chen correlaton was enough to calculate bolng heat transfer n test condtons. water and coolng tank water, Fg. 8 The natural convectonn heat transferr curve n coolng tank. The natural convecton heat transferr coeffcent could be calculated by Churchll-Chu and McAdams correlatons n RELAP5 [6]. The natural convectonn heat transfer curve could be correlated by locall wall heat flux and dfference between walll temperature and secondary temperature ( ) as Fg. 8. The calculaton results were also shown n t. The calculated heat flux was lower than test data, and calculaton trend agreed well wth testt data. Generally speakng, dfference between nner-tube water and outsde-wall temperature was much larger than that between tank water and outsde-wall temperature. The man rmal resstancee n PRHR HX exsted n nner-tubtube wall conductve heat transfer. Therefore, heat transfer of prmary natural convecton heat transfer and

10 554 An Expermental Research of Natural Crculaton Heat Transfer for PRHR Heat Exchanger n AP1000 sde was more mportant than secondary sde. For heat transfer n coolng tank, RELAP5 could calculate heat flux wth enough accuracy by usng Chen, Churchll-Chu and McAdams correlatons. 5. Conclusons Accordng to nner-tube natural crculaton experment research of PRHR HX for AP1000, t can be concluded that: (1) A PRHR HX test faclty wth 3 typcal C-type HX tubes of AP1000 has been bult. It can smulate natural crculaton n HX tubes under pressure of MPa, qualty up to 0.2, nlet water temperature of C and flow rate of kg/s, whch could cover operaton range n AP1000. (2) The experment provded lots of mportant data to research heat transfer behavor nsde and outsde of C-type tubes. (3) RELAP5 could be appled wth proper correlatons to analyze heat transfer n PRHR HX under test condtons. Acknowledgments Ths research was supported by Large Advanced PWR Plant Natonal Scence and Technology Major Project (2011ZX ). References [1] Hochreter, L. E., Peters, F. E., and Paulsen, D. L AP600 Passve Resdual Heat Removal Heat Exchanger Test Fnal Report. WCAP-12980, Revson 3. [2] Nuclear Regulatory Commsson Fnal Safety Evaluaton Report Related to Certfcaton of AP1000 Standard Desgn. NUREG-1793, Intal Report. [3] Ln, C. G., and Yu, Z. S An Advanced Passve Plant AP1000. Bejng: Atomc Energy Press. [4] Rryes, J., and Woods, B Testng of Passve Safety System Performance for Hgher Power Advanced Reactors. US: Oregon State Unversty Press. [5] Schulz, T. L Westnghouse AP1000 Advanced Passve Plant. Nuclear Engneerng and Desgn 236 (5): [6] Informaton Systems Laboratores, Inc RELAP5/MOD3.3 Code Manual. Volume IV: Models and Correlaton. [7] Corlett, M. M., Hochreter, L. E., and Stewart, W. A AP600 Passve Resdual Heat Removal Heat Exchanger Test. Fnal Report, WCAP Rev.0. [8] Hao, L. M Bolng Heat Transfer and Gas (Vapor)-Lqud Two-Phase Flow. Bejng: Chna Natonal Nuclear Corporaton.