Some of the electronic equipment to be installed in the rack space under the future LHC dipoles and quadripoles is expected to dissipate considerable heat, putting at risk the integrity of the equipment itself and causing thermal strains on the QRL (cryogenic distribution line) and cryostat vacuum vessels. Moreover, the ventilation injection and extraction points being separated by approximately 2.5 Km (one octant), a significant increase in the air bulk temperature is also expected along the distance. Further to a simple heat balance, three alternatives have been analysed to reduce the vacuum vessels exposure to the heat dissipated by the electronic equipment

Pimenta dos Santos, M A

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHORGANISATION EUROPÉENNE POUR LA RECHERCHE NUCLÉAIRECERN - ST DivisionCERN-ST-2000-060CERN-ST/CV-2000-23019 June 2000THERMAL ANALYSIS OF ONE LHC TUNNEL OCTANTM. A. Pimenta dos SantosAbstractSome of the electronic equipment to be installed in the rack space under the futureLHC dipoles and quadripoles is expected to dissipate considerable heat, putting at risk theintegrity of the equipment itself and causing thermal strains on the QRL (cryogenicdistribution line) and cryostat vacuum vessels. Moreover, the ventilation injection andextraction points being separated by approximately 2.5 Km (one octant), a significantincrease in the air bulk temperature is also expected along the distance.Further to a simple heat balance, three alternatives have been analysed to reducethe vacuum vessels exposure to the heat dissipated by the electronic equipment.21. INTRODUCTIONEach one of the 8 LHC tunnel octants will house, in addition to the accelerator itself,a large number of electronic equipment dissipating heat at a considerable rate. Thisequipment is located under the LHC cryostat and may give rise to high temperatures inthat limited space bringing about thermal strains on the Cryogenic Distribution Line andCryostat vacuum vessels (hereafter referred to as Cryostat vv and CDL vv respectively).In addition, operating problems with the electronic equipment itself may also be an issue.The current paper compares three different alternatives for the dissipation in terms oftheir effect on the neighbouring temperature map and in particular on the cryostat andCDL vv. CFD simulations were carried out with StarCD – a commercial code based onthe finite volume method.1.1 LHC tunnel geometry and heat sources layoutOne tunnel octant comprises 23 cells of approximately 107m length, each onehousing 2 quadripoles and six dipoles. For the sake of simplicity, cells are still halved inright and left half-cells. For the LHC machine, some tunnel points will be enlarged tosmall caverns in order to house special equipment. These caverns, known as RRxx orUjxx will most probably require an independent ventilation system due to the large heatloads they will contain, hence, since no mixing of air flows will take place, they will notbe considered in this study. A possible solution would be to bypass the tunnel air througha duct as shown in Fig. 1Fig 1 – Possible solution to circumvent tunnel air through caverns RRxx and UjxxIn the current design, most of the electronic equipment is accommodated in cratesunder the cryostat as shown in Fig.2. The exact equipment distribution along one cell aswell as the power dissipated by each component is presented in appendix I.RRxx or UjxxCavernBypass Ducttunnel3Fig. 2 - Tunnel cross section (simplified)The heat dissipated in one cell by the equipment located under the cryostats(hereafter referred to as “electronics”) is 2220W for the left half and 1590W for the right,thus adding up to 23×(2220+1590) = 87630W for one octant. This heat was considereduniformly and entirely released through the back surface of the crates, i.e. to the CDLneighbouring area (Fig 2).Of particular interest is the Beam Instrumentation (hereafter also indicated as“BI”)  – an equipment dissipating 900W over a length of only 1.2m and thereby causingimportant temperature rise on the CDL and Cryostat VV. For this case, further toconsidering dissipation through the back of the crate, two other alternatives wereanalysed, namely dissipation through the front  (i.e. to the transport area) and through thetop with forced convection.In addition to the heat dissipated by the electronics, other heat sources such aselectrical cables, illumination and water pipes are also present in the tunnel. The formertwo were neglected in this study whereas the latter were considered. These 3mm thickstainless steel pipes are not insulated and can be considered at constant temperature alongthe tunnel (42°C for the upper pipe and 27°C for the lower) as they run along the fulldistance to supply and return water from heat exchangers connected in parallel in cavernssuch as RR77, UJ82.The vacuum vessels thermally insulate their interior, i.e. there is no radial heatflux across these surfaces (the heat is conducted tangentially through the vessel). Thesevessels are made of 12mm and 6mm thick stainless steel for the Cryostat and CDLrespectively.Concerning the heat loss through the walls of the tunnel, it can be shown that theflux diminishes asymptotically with time, being negligible (compared with other sources)after 5 to 6 months of continuous operation of the accelerator (appendix II). Therefore,calculations were made assuming the most extreme case, i.e., adiabatic wall.41.2 Overall Heat balanceThe water pipes are considered at constant temperature T1 and T2 along the tunnel.The pipe surface exposed to convection is S = p.z, p being the exposed half-perimeter andz the distance along the tunnel.For a “slice” of tunnel of elementary thickness δz the following steady state heatbalance can be written:21 PIPEPIPEELECair qqqTCpm δδδδ ++=&           (1)δqPIPE1 and δqPIPE2  can be expressed as :δqPIPE1 = pδz h (T1-T)δqPIPE2 = pδz  h (T2-T)where h is the convection heat transfer coefficient. Substituting δqPIPE1 and δqPIPE2  in (1),[ ] zphTTTphTCm P δεδ 2)( 21 −++=&integrating along a tunnel segment of length z-zo∫∫ =−++ZZ PToutTin OdzCmdTphTTTph &12)(121εfinally,−−++−++=)(2exp22222121oPinOUTzzCmphTTTphTTphT&εε                 (2)a traditional expression for calculation of heat transfer in fully developed turbulent flowin smooth tubes is that recommended by  Dittus and Boelter1)4.08.0 PrRe023.0 DkhD=                                                       (3)1) Dittus, F.W., and L.M.K. Boelter: Univ. Calif. (Berkeley) Pub. Eng.,vol.2, p.443,1930.Where :δqELEC = ε δz  in which ε is the powerdissipated by the electronics per unit lengthδqPIPE1 and δqPIPE2  are the heat transferredfrom the water pipes 1 and 2m is the mass rateCpair is the specific heat for airδzT T+δTδqELECδqPIPE2 δqPIPE15Introducing the following values foreseen for the LHC tunnel ventilation:Inlet temperature Tin = 17°C (from Mr. J. Roche ST/CV)Volumetric rate = 22500m3/h = 6.25m3/s  (from Mr. J. Roche ST/CV)Thus the average velocity is u ≈ 0.8 m/sRe ≈ 170 000   Pr = 0.707 thus from (3)  h=2.63 wm-2/km=7.62Kg/s water pipes: T1=300K, T2=315K, p=pi(∅/2) ≈ 0.3mmwmwwcells /6.352461)15902220(23≈+×=εThe temperature evolution thereby calculated is shown below:Fig. 3 – Calculated bulk temperature along one octant2. THE CFD MODELAlthough forced convection has a predominant role in the heat transfer mechanismalong the tunnel, buoyancy effects cannot be neglected, this being particularly true nearbythe electronics1). On the other hand, the inclusion of buoyancy in the CFD model requiresa finer mesh and longer CPU time to calculate with an acceptable accuracy. This beingso, the tunnel was modelled one half-cell (53.5m) at a time, the field values fortemperature, velocity, density, turbulent energy and dissipation at the outlet used as inletconditions for the following half-cell.Wherever possible, the geometry was simplified in order to avoid unnecessarycomputations (see Fig. 4). Such is the case of the water pipes which are too far from theCryostat and CDL to influence the temperature map in that region (the overall influenceon the air temperature is significant though - see Fig 4). The CDL and Cryostat vv wereintroduced in the model as well as the cable trays.1 ) The importance of buoyancy with respect to forced convection is evaluated by the Grashof to Reynolds ratio (Gr/Re2) asfollows:Re=(Uδ /ν)   δ being the height of the crate dissipating surface=27 cm and23)(νδβ ∞−=TTwgGr β, volume coefficient of expansionFor the values of Tw, T∞ found in the calculations, Gr/Re2 ≈ 0.4 which means that in the vicinity of the dissipating surface,buoyancy effects are not negligible.306K2882902922942962983003023043063080 107 214 321 428 535 642 749 856 963 1070 1177 1284 1391 1498 1605 1712 1819 1926 2033 2140 2247 2354 2461z (m )T (k)63. RESULTSOne possible way to ascertain whether the method used to move from one half-cell to the following using outlet fields as inlets gives sensible results is to comparethe average temperature calculated by StarCD with the temperature obtained from asimple heat balance equation.Fig. 5 – Comparison between air bulk temperature calculated by simple heat balance equation andcalculated by StarcCDFig. 4 – Simplified geometry and mesh used in the modelAverage air temperature290290.5291291.5292292.5293293.5294294.50 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500z (m)T (K)starCDheat balance (consideringpipes)CDL vv7Fig.5 shows a good agreement between these two calculations for the first 4 cells(427.6m). Thus, to avoid unnecessary computations, calculations were resumed onlyfor the very last cell of the octant, using as inlet temperature the prediction of Fig.4for that point. Consequently, upon comparing results for the first and last cell, it wasnoticed that, apart from the difference in the ambient temperature, the temperatureand flow patterns were very similar. Furthermore, the BI being by far the mostimportant heat source, attention was focused on the tunnel segment containing the 2ndcryodipole (see appendix I) where this equipment is located. Hence, for the sake ofbrevity, only the results for this segment will be presented hereafter.3.1 Alternative 1: All electronics dissipating to the CDL side of thecrateIn the first case to be analysed the heat is dissipated through the back surface ofthe crate for all the electronics, as shown below:Fig. 6 – Alternative 1: dissipation to the CDL side of the crateTemperature maps for the tunnel segment containing the 2nd cryodipole are shownin fig.8 for the tunnel sections indicated in fig.7.Fig. 7 – 2nd Cryodipole of each cell and power released by the equipment beneath.CrateCryostatCDLHeatH = 270mm180 W 60W BI900 W200WCross-Section: A B D EC8Fig. 8 – Temperature maps for cross-section A to E (tunnel segment containing 2nd cryodipole of last cell)Fig. 8 makes clear that the influence on the temperature map is limited to thevolume delimited by the crates, the CDL and the Cryostat. In Fig.9 the temperature mapon the dissipating surface is shown for all the electronics housed under the 2nd cryodipoleof the last cell.Cross-Section ACross-Section ECross-Section DCross-Section CCross-Section B°C9Temperature maps on the vacuum vessels are shown next in figs 10 and 11.CrateCryostCDLh =270mmThisViewFlowBeam Instrumentationdissipating surface2777 W/m2Fig. 9 – Temperature Map for the Crates Dissipating Surface(along 2nd dipole of last cell)VA:Reserve QRL Linde617 W/m21.2 m100W × 2 crates1.2 m450W × 2 crates0.6  m60W × 1 crate2.4  m45W × 4 cratesCR: Tunnel Cryogenics370.4 W/m2MP: Heater PowerSupply277.8 W/m2Flow10FlowCut A-ACut A-ABeam Instrumentation’sdissipating surfaceFig. 10 – Surface Temperature for the 2ndcryodipole of last cellBeam Instrumentation’sdissipating surfaceFlow11FlowCut A-AFig. 11 – Cryogenic Distribution Line (CDL) Surface Temperature(Segment parallel to the second dipole of the last cell of the octant)Beam Instrumentation’sdissipating surfaceFlow123.2  Alternative 2: “Beam Electronics” dissipating to the transportarea side of the crateWith the aim of reducing the temperature rise on the CDL shell, in the secondalternative to be analysed, the BI dissipating surface is turned to the transport areaside of the tunnel, as shown below (note that the other electronics stays unchanged):Fig. 12 – Alternative 2: BI dissipation to the transport area side of the crateThe temperature map of the BI crate dissipating surface is shown in Fig.13The fact that the BI crate dissipating surface is now more exposed to convectionand out of reach from the other upstream heat sources accounts for the difference withthe temperature map of Fig. 9.The CDL and LHC Cryostat surface temperature for this case are shown next infig. 14 and 15.BICrateCryost.CDLHeatFig. 13 – Temperature Map for the BI Crate Dissipating Surface(as seen from the transport area)TransportAreaBeam Instrumentationdissipating surface2777 W/m2Flow13FlowCut A-ABeam Instrumentationdissipating surfaceFig. 14 – Surface Temperature for the 2nd cryodipole of last cellBeam Instrumentation dissipating to the transport area side14FlowDissipating surface for BeamInstrumentationCut A-ABeamInstrumentationFig. 15 – Surface Temperature for the Cryogenic Distribution Line (CDL)Segment parallel to the second dipole of the last cell of the octantBeam Instrumentation dissipating to the transport area side153.3 Alternative 3: Beam Electronics Ventilated to the Transport AreaAgain with the purpose of reducing the temperature rise on the CDL and Cryostatvacuum vessels, another possibility is to ventilate the crate containing the beaminstrumentation. The crate height has therefore to be increased to 350 mm to allocatefan units as shown in Fig. 16.Heat dissipation was assumed uniform through an imaginary surface immediatelyunderneath the fan units. The fans simulated in this study3) are very similar to thosecommonly found in overhead projectors and other equipment alike4). In this simplifiedmodel the pressure drop across the crate was approximately 8 Pa and the outlet velocity3.2m/s as shown in Fig.17. Since a continuous row of fans was considered along the(1.2m long) crate the volumetric rate adds up to 0.3m3/s. This high volumetric rateaccounts for the small increase in temperature through the crate and reduced effect on thetunnel temperature map as shown in Fig.18.3) fans are simulated in CFD by adding a momentum source to fluid cells4) In fact, the velocity at the outlet of an overhead projector fan is about 3.0 m/sCryostatCDL350 mmHeatFan UnitsOutletInletFig. 16 – Beam Electronics ventilated to thetransport area by a set of fan units installed at thetop of the crate.Crate containing the Beam InstrumentationFans16Fig. 17 – ventilated tunnel cross-section velocity fieldFig. 18 – ventilated tunnel cross-section temperature field17Beam InstrumentationFlowFig. 19 – Surface temperature for the 2nd Cryodipole of Last Cell(Beam Instrumentation crate ventilated to the transport area side)18FlowFig. 20 – Surface Temperature for the Cryogenic Distribution Line (CDL)(Beam Instrumentation crate ventilated to the transport area side)BeamInstrumentationCut C-C193.4 Summary of resultsA possible criterion to compare the three alternatives herein discussed is themaximum temperature difference in one single shell cross-section as presented in thetable below:Table 1Maximum cross-sectional temperaturedifference on the Cryostat vacuumvesselMaximum cross-sectional temperaturedifference on the CDL vacuum vesselAll electronics (BI included)dissipating to the CDL side 7 K 14 KBI  dissipating to theTransport Area side 4.5 K 3 KBI Crate Ventilationto the Transport Area side 0.5 K 2.3 KTable 1 shows clearly that ventilating the BI crate reduces significantly thetemperature disturbance on both vacuum vessels. It can also be noticed that BI ventilationreduced slightly the effect of the other electronics on the CDL vv  as it removes heated airfrom that volume. The effect of the other electronics on the cryostat (0.5 K) is negligiblebecause they dissipate considerably less power per crate than the BI. (see appendix I).Table 2 summarises the maximum temperatures attained on the dissipating surfacesfor the equipment located under the 2nd cryostat (which is well representative of theequipment under the other cryostats – see appendix I).Table 2 (see Fig.9)Name of theelectronicsNr. of cratesoccupiedTotal powerdissipatedT max on the dissipating surfaceCR: tunnelCryogenics1 60W 39.5 °CMP: HeaterPower Supply4 180W 41.5 °CVA: ReserveQRL Linde2 200W 42.5 °CDissip to CDL side :    64.5 °CBeamInstrumentation 2 900WDissip to Transp. Area side:  59.5°CNote that the ambient temperature in the last cell is 34 °C – the highest in the octant-so the values presented in table 2 are also the maximum temperatures to expect for all theelectronics in the tunnel.204. CONCLUSIONSThe steady state temperature rise along one octant due to the heat dissipated bythe electronics and the cooling water pipes, is expected to be approximately 17°C, i.e.Tair ≈34°C at the end of the octant. This value was calculated assuming adiabatictunnel walls and would be reached after a transient evolution of 5 to 6 months in whichthe heat flux through the tunnel walls decreases as the surrounding concrete and rockheats up.Among all the electronic equipment installed in the rack space under thecryostat, one in particular – the Beam Instrumentation – releases a significant power ina short volume (two crates). For this equipment, the use of active ventilation wasshown to be necessary to minimise the temperature rise on the LHC cryostat andCryogenic Distribution Line vacuum vessels. Ventilation should be done towards thetransport area of the tunnel to protect the Cryogenic Distribution Line and can beachieved by ordinary fan units, similar to those found in computers, overheadprojectors, etc. With the volumetric rate commonly delivered by these fans, thetemperature of the air expelled from the Beam Instrumentation crate would be lessthan 3 °C above the ambient temperature in that part of the tunnel.The rest of the equipment causes little temperature rise on both vacuum vesselsand therefore can be let to dissipate passively towards the Cryogenic Distribution Line.In this case, temperatures in the immediate vicinity of the dissipating surfaces are inthe order of 39 to 43°C for the equipment located in the last cell of the octant, i.e. 5 to9°C above the ambient temperature.

Thermal analysis of one LHC tunnel octant

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