In the line of previous work done at CEA Grenoble, large size experiments were performed with the support of CERN for the validation of the LHC two phase superfluid helium cooling scheme. In order to be as close as possible to the real configuration, a straight, inclinable 22 m long line of 40 mm I.D. was built. Very accurate measurements of temperatures and pressures obtained after in situ re-calibration and verified by independent sensors allowed us to validate our two-phase flow model. Although we focus on pressure losses and heat exchange results in relation to power injected, additional measurements such as quality, void fraction, and total mass flow rate enable a complete description of the two-phase flow. Experiments were carried out to cover the whole range of the future LHC He II two-phase flow heat exchanger pipe: slope between 0 and 2.8 %, temperature between 1.8 and 2 K, total mass flow rate up to 7.5 g/s. Results confirm the validity of choice for the LHC cooling scheme

Gauthier, A

Jäger, B

Rousset, B

Van Weelderen, R

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsHE II TWO PHASE FLOW IN AN INCLINABLE 22 m LONG LINEB. Rousset1, A. Gauthier1, B. Jager1,  and R. van Weelderen2In the line of previous work done at CEA Grenoble, large size experiments were performed with thesupport of CERN for the validation of the LHC two phase superfluid helium cooling scheme. In order to beas close as possible to the real configuration, a straight, inclinable 22 m long line of 40 mm I.D. was built.Very accurate measurements of temperatures and pressures obtained after in situ re-calibration and verifiedby independent sensors allowed us to validate our two-phase flow model. Although we focus on pressurelosses and heat exchange results in relation to power injected, additional measurements such as quality,void fraction, and total mass flow rate enable a complete description of the two-phase flow. Experimentswere carried out to cover the whole range of the future LHC He II two-phase flow heat exchanger pipe:slope between 0 and 2.8 %, temperature between 1.8 and 2 K, total mass flow rate up to 7.5 g/s. Resultsconfirm the validity of choice for the LHC cooling scheme.1 CEA Grenoble / Département de Recherche Fondamentale sur la Matière Condensée/SBT, 17 rue des Martyrs, 38054 GrenobleCedex 9, France.2 LHC Division, CERN, 1211 Geneva 23, SwitzerlandPresented at the 1999 Cryogenic Engineering and International Cryogenic Materials Conference(CEC-ICMC'99), 12-16 July 1999, Montreal, CanadaGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 328Abstract1 December 19991HE II TWO PHASE FLOW IN AN INCLINABLE 22 mLONG LINEB. Rousset1, A. Gauthier1, B. Jager1, R. van Weelderen21C.E.A. Grenoble / Département de Recherche Fondamentale sur la MatièreCondensée/SBT, 17 rue des Martyrs, 38054 Grenoble Cédex 9, France.2LHC Division, CERN1211 Geneva 23, SwitzerlandABSTRACTIn the line of previous work done at CEA Grenoble, large size experiments were performedwith the support of CERN for the validation of the LHC two phase superfluid heliumcooling scheme. In order to be as close as possible to the real configuration, a straight,inclinable 22 m long line of 40 mm I.D. was built. Very accurate measurements oftemperatures and pressures obtained after in situ re-calibration and verified by independentsensors allowed us to validate our two-phase flow model. Although we focus on pressurelosses and heat exchange results in relation to power injected, additional measurementssuch as quality, void fraction, and total mass flow rate enable a complete description of thetwo-phase flow. Experiments were carried out to cover the whole range of the future LHCHe II two-phase flow heat exchanger pipe: slope between 0 and 2.8 %, temperature between1.8 and 2 K, total mass flow rate up to 7.5 g/s. Results confirm the validity of choice for theLHC cooling scheme.INTRODUCTIONLHC1 will use He II two-phase flow as the cold source. In order to maintain themagnet temperature as low as possible, two constraints have to be taken into consideration:the two-phase flow pressure losses (which means temperature difference between inlet andoutlet of He II saturated flow, i.e. longitudinal ’T) and the heat exchange between thesaturated flow and the static He II pressurized bath in which the magnets are immersed2(which means temperature difference between saturated flow and pressurized helium in amagnet cross section, i.e. transversal ’T).In the framework of LHC cooling scheme studies, previous experiments wereperformed2,3,4 using a 40 mm I.D., 1.4 % slope helical pipe and a He II two-phase flowmodel was constructed. Obtained results gave us confidence in this co-current two-phaseflow model and justified further experiments to fully validate the LHC cooling scheme. Inparticular, the influence of the slope (ranging from 0 to 1.4 % in case of LHC) had to bechecked. Furthermore, the excess thermal heat exchange behaviour observed at high Vgs(Vgs represents the superficial vapour velocity, i.e. the vapour velocity if the vapour wouldoccupied the whole cross section) had to be confirmed for a long straight line to avoidentrance and exit effects. In this paper, results on an inclinable straight pipe are presented.DESCRIPTION OF TEST LOOP AND EXPERIMENTThe "Cryoloop" (Fig. 1) mainly consists of an inlet box used to create the inlet quality,the 22 m inclinable straight line, and an outlet saturated bath where excess liquid isevaporated.The main measurements consist of the total mass flow rate (taken at roomtemperature), the saturated temperature and differential pressure between the extremities ofthe line (both used to calculate pressure losses), all powers injected (in order to calculatethe quality), temperature increase in the pressurized chamber when power is on (whichgives access to the thermal heat exchange) and visualization of the flow.RESULTSHydraulic ResultsFirst, the tube friction factor was determined in Reynolds similitude using nitrogenflow at room temperature. Best fit is smooth tube correlation (Fig. 2), which was adoptedfor wall friction factor in our two-phase flow model. The interfacial friction factorintroduced in the model follows the Hanratty correlation as previously reported3,4.In situ calibration of the thermometers located along the two-phase flow enabledtemperature measurements with 10-3 K accuracy.On each figure, mass flow rate represents the sum of liquid and vapour mass flowrates.Figure 3 compares pressure loss measurements derived from various sensors. Squaresymbols represent pressure losses between inlet and outlet boxes, respectively, measuredwith a differential pressure transducer and by means of immersed thermometers (seeFig. 1). The very good agreement confirms thermodynamic equilibrium in each box andgood sensor accuracy. Line and circle illustrate comparison of measured and predictedpressure losses over the 22 m long line only (thereby excluding entrance and exit effects).The agreement is excellent and justifies the code assumptions3. Difference between squaresymbols (showing results given by sensors connected to the inlet box where liquid andvapour are at the rest) and open circle (showing results given by sensors connected to theline) are mainly due to entrance effects, i.e. acceleration of the two-phase flow at the inletof the line.3T4W2W3W4W1T5T6T8T7T12T10T11T14LHe-4.5KPumping unitDPSlope -1 to 2%Belows~ 22 mLTLTJT valveT9Thermalexchange boxVisual testsectionInletboxOutletboxFigure 1. Test facility schemeFriction factor in our 40 mmI.D. pipe, tested length: 9m  Test performed using nitrogen at room temperature   00.010.020.035.0E+04 1.0E+05 1.5E+05 2.0E+05 2.5E+05 3.0E+05 3.5E+05Reynolds numberFriction factorColebrook law pipe roughness = 50 micronsexperimental dataSmooth tubeFigure 2. Friction factor calibration0501001502002503000 1 2 3 4 5 6 7 8 9 10 11Vgs (m/s)' 'P (Pa)CodeDifferential pressure transducerThermometers (T4 and T14)Thermometers (T5 and T14)Figure 3. Various pressure losse estimations (slope 0%, total mass flow 7.2 g/s, temperature 1.77 K)40501001502002500 2 4 6 8 10 12Vgs (m/s)DP (Pa)ExperimentsHanrattyTaitel and DuklerHomogenous modelVapour Figure 4. Experiment and model comparisons (slope 0%, total mass flow 7.2 g/s, temperature 1.77 K)-500501001502002500 2 4 6 8 10 12Vgs (m/s)' 'P (Pa)ExperimentsHanrattyTaitel and DuklerHomogenous modelVapourFigure 5. Experiment and model comparisons (slope 2.8 %, total mass flow 7.3 g/s, temperature1.77 K)Figures 4 and 5 compare the experiments with various models. As flow is stratified,the homogenous model gives worst results. Pressure losses are mainly due to the friction ofvapour flow which explains the good agreement of vapour, Taitler and Dukler5 andHanratty6 models at low Vgs, especially for large slopes where liquid occupies a smallfraction of the cross section. Increasing Vgs results in a wavy interface with dropletsdragging and Hanratty correlation has to be employed to represent flow behaviour.On Figure 6, the curve obtained for 1.77 K is below the 1.91 K one. This result isdifferent if injected power replaces Vgs in the abscissa axe. Due to vapour density changeswith temperature, the same power injected results in different Vgs, e.g. 120 watt injectedcorrespond to a Vgs of 11 m/s at 1.77 K and a Vgs of 7.6 m/s at 1.91 K, and it is clear onFig. 6 that resulting pressure loss is higher at low temperature.Finally, the influence of liquid mass flow rate can be deduced from Figure 7. Thisparameter has negligible effect, which confirms that pressure losses are mainly due tovapour flow.50501001502002500 1 2 3 4 5 6 7 8 9 10 11Vgs (m/s)' 'P (Pa)Experiments     1.77 KHanratty Experiments     1.91 KHanratty Figure 6. Temperature influence (slope 1.4 %, total mass flow 7.2 g/s)02550751001251500 1 2 3 4 5 6 7 8Vgs (m/s)' 'P (Pa)Experiments   2.6 g/sHanratty   Experiments   7.2 g/sHanratty           Figure 7. Total mass flow influence (slope 0 %, temperature 1.77 K)Thermal ResultsOne other goal for this new test circuit was the confirmation of excess heat exchangewith respect to stratified flow observed on the helicoidal line (i.e. heat exchange increasesat high Vgs while liquid mass flow decreases) and the potential influence of using a straightline. As shown on figure 1, two heat exchange boxes instrument the Cryoloop. One islocated 10 m downstream of the inlet, and the other 20 m downstream of the inlet. Resultsobtained are identical for the two boxes, and only data acquired on the second box arepresented here.Figure 8 compares the helicoidal line with the present Cryoloop. In both cases, heatflux to be transferred from pressurized bath to saturated flow is equal to 22 W/m2 (valuecalculated using the whole pipe perimeter). Excess wetting is always present but appears athigher Vgs in the Cryoloop. This excess wetting consists probably of a tiny liquid layer dueto interception of droplets by the pipe wall.To verify this assumption, heat flux to be transferred was increased in order to dry outthe wall liquid layer. The result is that this excess wetting can be partially evaporated whenheat flux is increased (figure 9). Furthermore, the wetting dependence with heat fluxexhibited a critical vapour velocity which is consistent with the apparition of excesswetting.6Low Vgs. On Figures 10, 11 and 12, numerical and experimental curves are parallel atlow Vgs. The under-estimation of the wetting prediction could be due to wavy flow.Gravity plays a major role and wetting varies considerably with slope. As Vgs decreases,two-phase flow tends to an open channel liquid flow. Experimental slope uncertainties canalso explain difference between measured and calculated values, especially for thehorizontal case.202530354045500 1 2 3 4 5 6 7 8 9 10Vgs (m/s)Wetted perimeter (%) Cryoloop            7.2 g/s, 1.77 KHelicoidal line  6.3 g/s, 1.81 KFigure 8. Comparison of thermal heat exchange for helicoidal line and cryoloop (slope 1.4%, heat flux22 W/m2)202530354045500 2 4 6 8 10Vgs (m/s)Wetted perimeter (%) W=10 W/m²W=22 W/m²W=42 W/m²W=70 W/m²Figure 9. Influence of heat flux on apparent wetted perimeter  (slope 1.4%, total mass flow 7.2 g/s,temperature 1.77 K)High Vgs. Gravity has no influence, even on over-wetting. Unfortunately, it was notpossible to access the mass flow or temperature influence, because in our experimentscritical Vgs was not obtained for all curves. Over wetting is probably due to liquid dropletdeposition on wall. The model, which use separate vapour and liquid flow can't take intoaccount such phenomena.71020304050600 2 4 6 8 10Vgs (m/s)Wetted perimeter (%)Experiments   slope 0%Code  Experiments   slope 1.4%Code Experiments   slope 2.8%CodeFigure 10. Slope influence (total mass flow 7.2 g/s, temperature between 1.77 and 1.8 K, heat flux 22 W/m2)01020304050600 2 4 6 8 10Vgs (m/s)Wetted perimeter (%)Experiments   2.6 g/sCodeExperiments   6.1 g/sCodeExperiments   7.2 g/sCodeFigure 11. Total mass flow influence (slope 0%, temperature between 1.77 and 1.8 K, heat flux 22 W/m2)1015202530354045500 1 2 3 4 5 6 7 8 9 10Vgs (m/s)Wetted perimeter (%)Experiments   T=1.77 KCodeExperiments   T=1.91 KCodeFigure 12. Temperature influence (slope 1.4%, total mass flow 7.2 g/s, heat flux 22 W/m2)8CONCLUSIONWe have investigated the thermohydraulic behaviour of He II two-phase flow. Thelarge amount of data acquired in this experiments (only the most representative arepresented here) confirms the excellent pressure loss prediction given by the Hanrattymodel.Concerning heat transfer, apparent excess wetting appearing at high Vgs mightpossibly be due to an onset of change in flow pattern. Further measurements are needed toverify the presence of liquid droplets in the vapour flow. The code gives a conservative(under estimated) value for heat transfer.REFERENCES1. Lebrun P., "Superfluid Helium Cryogenics for the Large Hadron Collider Project at CERN"Cryogenics 34 (1994)2. Rousset B., Gauthier A. & Grimaud L., "Stratified Two-phase Superfluid Helium Flow:  I" Cryogenics 37(1997)3. Grimaud L., Gauthier A., Rousset B. & Delhaye J.M. "Stratified Two-phase Superfluid Helium Flow-Part: II" Cryogenics 37 (1997)4. Rousset B., Gauthier A. , Grimaud L. & van Weelderen R., "Latest Developments on HeII co-currentTwo-phase Flow Studies" Advances in Cryogenics Engineering 43 (1998)5. Taitel Y. & Dukler A.E., "A Model for Predicting Flow Regime Transitions in Horizontal and NearHorizontal Gas-liquid Flow", AIChE Journal (1976), Vol. 22, 47-55.6. Andritsos, N. & Hanratty, T.J., "Influence of Interfacial Waves in Stratified Gaz-liquid Flows", AIChEJournal (1987), Vol. 33, 444-454.

HE II Two Phase Flow in an Inclinable 22 m Long Line

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