The reference design of JET cryopump nitrogen shield consists of an outer section made of copper chevrons fastened to two cooling tubes and an inner stainless steel section and backing plate with two cooling tubes. These tubes are fed in a parallel flow arrangement. The inlet flow is divided into two parallel paths so that both tubes on either section are always at the same temperature. This arrangement was selected due to concern about conduction between warm and cold parts of the shield during cooldown transients. If the heat loads are unequal, such a parallel flow arrangement can result in flow starvation in the path with higher heat load. This will cause large temperature differences and, ultimately, structural failure. Hence, an analysis was undertaken to investigate the conduction effects in the shield for other flow arrangements. 4 refs., 8 figs

Baxi, C.B. (General Atomics, San Diego, CA (United States))

Obert, W. (JET Joint Undertaking, Abingdon (United Kingdom))

UNT Digital Library

_GA-A20587DEC2 ._#991OPTIMIZATION OF THERMAL DESIGNFOR NITROGEN SHIELDOF JET CRYOPUMPbyC.B. BAXI, W. OBERTNOVEMBER 1991__ GENERA L A TOIWICSDISCLAIMERThis report was prepared as an account of work sponsored by an agency of the UnitedStates Government. Neither the United States Government nor any agency thereof, norany of" their employees, makes any warranty, express or implied, or assumes any legalllability or responsibiU_ for the accuracy, completeness, or usefulness of. any information,apparatus, product, or proce_ disclosed, or represents that its use would not infringeprivately owned rights. Reference herein to any specific commercial product, process,or service by trade nmne, trademark, manufacturer, or otherwise, does not necessarilyconstitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or any agency thereof. The views and opinions of authors expre_ed hereindo not necessarily state or reflect those of" the United States Government or any agencythereof.GA-A--20587DE92 004295OPTIMIZATION OF THERMAL DESIGNFOR NITROGEN SHIELDOF JET CRYOPUMPbyC.B. BAXI, W. OBERT**JET Joint UndertakingThis is a preprint of a paper to be presented at the14th Symposium on Fusion Engineering, September30-October 3, 1991, San Diego, California, and to beprinted in the Proceedings.Work supported byDepartmentof EnergyContractDE-AC03-89ER51114GENERALATOMICS PROJECT 3467NOVEMBER 1991GENERAL A TOIWlCSL-_,,TP,I_Lj-FION OF THt-G DOrO(_thviE;,,T13:I.J,NLi&41;EOOPTIMIZATION OF THERMAL DESIGNFOR NITROGEN SHIELD OF JET CRYOPUMPC.B. BaxiGeneralAtomics, P.O. Box85608San Diego, California92186-9784W. ObertJET Joint UndertakingAbingdon,OxonOX14 3EA, EnglandAbstract: The reference design of JET cryopump nitrogen TUBE4,,.,O._.._ ./ SERIALMODIFIEDt ¥shield consists of an outer section made of copper chevrons faa- ,__-"_ (PARALLEL)tened to two coolingtubes and an innerstainlessteelsection _ ___,x_and backing plate with two cooling tubes. These tubes are fed ,.. ,,in a parallel flow arrangement. The inlet flow is divided into twoparallel paths so that both tubes on either section are always at TUBE/41__ uu)'T Athe same temperature. This arrangement was selected due toconcernabout conductionbetween warm and coldpartsofthe Tinshield during cooldown transients. If the heat loads are unequal, " INsuch a parallel flow arrangement can result in flow starvation in ss SIll| ,D (2)the path with higher heat load. This will cause large temper- ffo_._ature differences and, ultimately, structural failure. Hence, ananalysis was undertaken to inv(mtigate the conduction effects in PARALLELthe shield for other flow arrangements.Thermal analysis was done using a finite element computerprogram developed at GA. In this analysis, a parallel flow and CU SHIELD (1)two series flow arrangements were compared for cooldown from e300°K to about 80°K. In order to simplify the analysis, coolantwas mssumed to be a N2 gas at an inlet temperature of 80°K. COOLANTThe following conclusions were reached from this analysis:TUBE 3time for cooling down the shield from 300 ° to 80°K. Thismeans that the "heat exchanger effect" or radial conduc- ERIALtionfrom the warm partof theshieldto the cold partof-,,the shieldforseriesflowarrangementsisnot dominant.TUBE 22. Due to small conduction effects, it will be feasible to modify • cthe design to a more stable series flow arrangement. Thisflow arrangement will also have minimum cooling time.3. The inner stainless steel shield has small thermal conduc- Fig. 2. Flowarrangements.tivity and, hence, this part of the shield lags in coolingbehind the rest of the shield. This could be remedied byadding about 1 mm layer of copper in poloidal stripes tothe stainless steel fin. This will significantly reduce the Introductioncooling time of the shield, reduce the shield temperature A three-dimensional view of JET pumped divertor cryo-during operation and, in turn, reduce the radiation heat pump is shown in Fig. 1 [1]. The helium panel is surrounded byload on the helium shield by a factor of about 3. a N2 shield. A schematics of the N2 shield is shown in Fig. 2and geometry details are shown in Fig. 3 [2]. The outer shieldconsists of copper chevrons [3]. These can be approximated by_,_ a fin 6 mm thick and 15 cm in height [1]. The inner shield and_ CORRUGATEDH. PIPE back plate consists of 3 m thick 15 cm height stainless steel______. ROD plate.SUPPORT_i __ SUPPORTS RAP The following aasumptions were made in the analysis:_- SUPPORTCLAMP 1. The coolant is a nitrogen gas at a flow rate of 7.5 g/s and"__ DIVERT R• _,_ _,.,_ CO,t aninlettemperature°f80°K"_N 2. Specific heat of N2 is constant and density is inverselyWATER _ _ :) proportional to temperature. Effect of pressure change onCOOLED v.._:_ BACKPANEL density was neglected.BAFFLE3. Axial (in the direction of flow) conduction in the coolant,Fig. 1. A view of JET pumped divertor cryopump, tubes, and fins was neglected.18,ckPI,t. Resultsand DiscussionIm_ ~ S00cm Each of the three arrangements shown in Fig. 2 was solvedcomea_lby(_k_)tlminlem ehi fin.3 mm thick for a transient cooldown. Initial temperature of the system was15 h&jhtIScm assumed to be 300°K, the temperature of the nitrogen gas atentrance was 80°K and the flow rate 7.5 Radiationg/s.was_._ _ 3 mm heat transfer from the ambient was modeled as a heat flux onone side of the shield surfaces.$ | Frontpart---with copperchevmM In the flow Arrangement A, the flow is in series but isTubes:20mmstainlesssteeli.d. directed from shield to shield as shown in Fig. 2. Flow enters the2 mmwall lower tube of the outer (copper) shield, goes to the lower tube of__... lenath,,, SO0cm the inner shield, then to the upper tube of the outer shield andcopperr,n,: finally exits through the upper tube of the inner shield. Thus,equivalentto the temperatures of two tubes on either shield will be different6 mm 6 mm solid copperfinheight: during the cooldown transient and may degrade the performanceh ,,*15cm duringcooldowndue toconductionfrom the warmer tubeto the2mmcower colder tube. Figure 4 shows the temperature distribution for thiswo_v' flow arrangement at various times during the transient.Fig. 3. Geometric detailsof the proposedJET nitrogenshi._ld.Figure 4(a) shows the temperature distribution at 100 safter the start of the cooldown of the copper shield and tubesattached to this shield. This result shows that the temperatures4. A radiation heat load was imposed on the shield from a of tubes on the copper shield differ by up to 36°K and willsource at a temperature of 300°K with an exchange factor have conduction heat transfer from warmer to colder parts ofof 0.5. the shield. The center of the shield is hotter than either tube.Figure 4(b) shows similar result for stainless steel shield andMaterial properties were obtained from Ref. [2]. associated tubes and coolant. Comparison of Figs. 4(a) and4(b) show that, the film drop for tubes 1 and 2 is larger than forFormulation tubes 3 and 4 due to higher thermal capacity of the copper shieldThe formulation for this problem was similar to one done and larger conduction from shield to tubes. The temperaturefor analysis of helium shield of DIII-D cryopump [3]. The inner difference between tubes on a shield is reduced with time.and outer shields were divided into axial divisions. At an axial In flow Arrangement B (reference design), two tubes onlocation, each tube, each volume of the coolant, and each fin was each shield are always at the same temperature at a given axialrepresented by a node. Thus, we have ten coupled first-order location and, hence, there is no radial conduction effect. How-differential equations. The formulation is described in detail in ever, since the flow stream is divided into two paths, flow rateRef. [4]. The difference equations were solved by an explicit through each tube is only 50%. This reduces the heat transfernumerical procedure, coefficient (by about 42%) and, thus, reduces the cooldown rateslightly.............  5-r..................._'T280 _......,......_/Tr 1 "t2 '¢4A 230 .-_ 230LIJ MJ[Z: rc"P" 180- .,_ 180.,¢EZ: EZ:LIJ LIJO. O._Z _ZM.I MJP" 130- I-- 130801' 80 i i i I0 100 200 300 400 500 0 100 200 300 400 500DISTANCE(cre) DISTANCE(cre)Fig. 4(a). Arrangement A: flow = 7.5 g/s, copper Fig. 4(b). Arrangement A: flow = 7.5 g/s, stainlessshield at 100 s. Arrows indicate flow steel shield at 100 s. Arrows indicate flowdirection. See Nomenclature and Fig. 2. direction. See Nomenclature and Fig. 2.22.280Tt 2_._ 230 _ _" 230,.=, ..=,_ 130 _ 1' I_ '0 100 200 300 400 500 0 100 200 3 400 500DISTANCE(cre) DISTANCE(cre)Fig. 5(a). Arrangement (series)C: flow = 7.5 g/s, Fig. 5(b). Arrangement(series) C: flow -- 7.5 g/scoppershield at 100 s. Arrows indicate at 100 s for stainlesssteelshield. Arrowsflow direction. See Nomenclatureand indicateflowdirection. SeeNomenclatureFig. 2. and Fig. 2.In the flow Arrangement C, flow is in series. Conduction Cooldown rates of stainless steel fin for the three arrange-effects will be slightly less than series parallel flow Arrange- ments axe compared in Fig. 7. These plots show that ali threemerit A, but will be more than parallel flow Arrangement B. arrangements are similar from cooldown consideration. Arrange-The temperature distribution for the copper shield and associ- ment C, the series arrangement, is slightly superior than theated coolant and tubes at 100 s from the start of cooldown is other two. Purely parallel arrangement (reference design) hasshown in Fig. 5(a). Similar results for stainless steel shield are the longest cooldown time.shown in Fig. 5(b). The temperature differences along the heightof the shields axe slightly smaller than for Arrangement A, re- A significant observation from this analysis is that thesuiting in smaller conduction heat transfer. The temperature stainless steel shield (inner shield) lags behind the rest of thedistributions at 3600 s are shown in Figs. 6(a) and 6(b). The shield during cooldown for ali three arrangements. This occurscooldown of the stainless shield lags behind the copper shield, due to very small thermal conductivity of stainless steel and28O 280-A 230 230-IJJ Iu .__.__.Tf 2CC._ 180 ._ 180"IZ: OC:I.U uJ /Tc 4o. Q,, Tt 4130 _ 130-Tt380 _'_ 80, _ , . , _0 100 200 300 400 500 0 100 200 300 400 500DISTANCE(cre) DISTANCE(cre)Fig. 6(a). Arrangement (series) C: flow = 7.5 g/s, Fig. 6(b). Arrangement (series)C: flow = 7.5 g/scoppershieldat 3600 s. Arrowsindicate at 3600 s for stainless teelshield.Arrowsflow direction. See Nomenclature and indicateflowdirection.SeeNomenclatureFig. 2. and Fig. 2.3280 280liel (B) i<I,230 o_, 230 _ \_: \ _'x SerleeParallel (A) LU ! '::_ \_.._. \_,_..._.. ..__ _rv" \ _ Flow = 12.5 g/sI_ 180 "_" I"" 180 "., _I.U I0 g/s .13= /,_, 13= _._ ,, ",, 7 g/s=i ...._.... Series(C) =E _,-__U.l 130 uJ 130I-- Series(C) with 1 mm Cu I'-Added to Stainless Steel Panel 7".-_ ......................25 g/s80 .... 80 .......o 2oo 8ooo o 2ooo4oo0 8oooTIME(sec) TIME(sec)Fig. 8. Effect of flow rate on cooldownofFig. 7. Cooldownof stainlesssteel fin for flow stainlesssteel fin with 1 mm copperrate of 7.5 gis. for ArrangementC (series).relatively long conduction path between the coolant channels Acknowledgment(15 cre)and the center of the shield. This work was sponsored, by the U.S. Department ofThis deficiency (high temperature of the stainless steel Energy under Contract No. DE-AC03-89ER51114. The authorsshield) could be remedied by adding a layer of copper to the also wish to thank T. Simonen, P.M. Anderson, A. Langhorn,stainless steel shield. The layer need not be continuous axially and J.V. Del Bene for useful discussions on this paper.(alongthelength),ithas tobe continuousfrom tube tothe cen-teroftheshield.Such a layercouldbe added by flamespraying, Referencesa processthatisplannedtobeusedfornitrogenshieldofDIII-Dcryopump, ltisimportant to pointout that thisisessential [1]Obert, W., etal.,"JET Pumped DivertorCryopump,"Syrup.on FusionTechnology(SOFT) Conference,Londonnot onlyforthe cooldown,but alsotoinsurethat the nitrogen (1990).shieldtemperatureisbelow 100°K duringsteadystateoperation.The stainlessteelshieldat a highertemperaturewillimpose [2]"SelectedCryogenic Data Book," BNL-10200-R, Vol. Iunnecessarilyhighradiationheatloadon thehelium shield. (August 1980).The lowestcurve in Fig.7 shows the effectof a 1 mm [3]Baxi, C.B., A. Langhorn, K. Schaubel,and J. Smith,copper layer on stainless steel shield on the cooldown of the "Thermal Analysis of a Coaxial Helium Panel of a Cryo-shield. Addition of the copper layer has reduced the temperature genie Vacuum Pump for Advanced Divertor of DIII-Dof the shield from over 160°K to under 130°K. Tokamak," presented at Cryogenic Eng. Conf., Huntsville,Finally, effect of coolant flow rate on cooldown rate was Alabama (1991); submitted for publication in Advancea inanalyzed. Figure 8 shows the effect of flow rate on cooldown of Cryogenic Engineering.the stainless steel fm with a 1 mm copper layer for the series [4] Baxi, C.B., "Thermal Analysis of JET cryopump NitrogenArrangement C. Increasing the flow can significantly reduce the Shield," General Atomics Report GA.-A20371 (Februarycooldown time but may not be feasible due to system limitations. 1991).NomenclatureT - temperatureSubscriptscl = coolant 1, etc.fl = copper finf2 = stainless steel fintl = tube i, etc.4

Optimization of thermal design for nitrogen shield of JET cryopump

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Optimization of thermal design for nitrogen shield of JET cryopump

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