Cryogenic gaseous helium gives access to extreme turbulent experimental conditions. The very high cooling helium flow rates available at CERN have been used to reach Reynolds numbers up to Re ~ 10**7 in a round jet experiment. First results are discussed

Baudet, C

Castaing, B

Chabaud, B

Gagne, Y

Hébral, B

Ladam, Y

Lebrun, P

Pietropinto, S

Pirotte, O

Poulain, C

Roche, P E

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHLaboratory for Particle Physics(ECR)LOW TEMPERATURE GASEOUS HELIUMAND VERY HIGH TURBULENCE EXPERIMENTSS. Pietropinto1, C. Baudet2, B. Castaing3, B. Chabaud1, Y. Gagne2, B. Hébral1,5,Y. Ladam3, Ph. Lebrun4, O. Pirotte4, C. Poulain2 and P.E. Roche1,6Cryogenic gaseous helium gives access to extreme turbulent experimental conditions. Thevery high cooling helium flow rates available at CERN have been used to reach Reynoldsnumbers up to Re ~ 107 in a round jet experiment. First results are discussed.1 CRTBT CNRS-UJF-INPG, Grenoble , France2 LEGI UJF-INPG-CNRS, Grenoble , France3 Physics Department ENS Lyon-CNRS, Lyon, France4 LHC Division, CERN, Geneva, Switzerland5 CRETA CNRS, Grenoble, France6 LPMC ENS Paris-CNRS, Paris, FranceCERN LHC/2002-14Presented at the 19th International Cryogenic Engineering Conference (ICEC 19)22-26 July 2002, Grenoble, FranceGeneva, Switzerland 21 November 2002Divisional ReportAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23Low temperature gaseous helium and very high turbulence experimentsS. Pietropintoa,  C. Baudetb, B. Castaingc, B. Chabauda, Y. Gagneb, B. Hébrala,e, Y. Ladamc, P. Lebrund,O. Pirotted, C. Poulainb, P.E.  Rochea,faCRTBT CNRS-UJF-INPG, BP 166, 38042 Grenoble Cedex 9, FrancebLEGI UJF-INPG-CNRS, 38041 Grenoble Cedex 9, FrancecPhysics Department ENS Lyon-CNRS, 69364 Lyon Cedex 7, FrancedCERN, 1211 Geneva 23, SwitzerlandeCRETA CNRS, BP 166, 38042 Grenoble Cedex 9, FrancefLPMC ENS Paris-CNRS, 24, Rue Lhomond, 75005 Paris, FranceCryogenic gaseous helium gives access to extreme turbulent experimental condi-tions. The very high cooling helium flow rates available at CERN have been usedto reach Reynolds numbers up to Re ~ 107 in a round jet experiment. First resultsare discussed.Turbulence is a highly challenging problem both for fundamental understanding and industrialapplications. Present capabilities of the most powerful computer simulations remain far away fromcommon situations, and experimental data are needed to test the validity of theoretical predictions. Largeair wind tunnels ( ≥ 20 m), for example, have been used for many years; however in such largeexperiments the Re hydrodynamic parameter cannot vary on a wide range.The physical idea is to understand how the energy injected at the large scale L of the flow cascadesdown through the inertial scales to the smallest scale η where it is dissipated by viscosity. In air thisbehavior is investigated in large systems : L is in the range of several meters and η is in the millimetric orsubmillimetric range.Turbulence studies in gaseous helium at cryogenic temperatures have been achieved in the lasttwenty years and have opened a very attractive way to vary Re on several orders of magnitude in a singlegeometry and reach very high Re values. They take advantage of the very low kinematic viscosity ν  ofgaseous helium (ν = 7×10-8 m2/s at 4.2 K and 1 bar, 230 times lower than air in STP conditions). In atypical (φ = 10 cm) laboratory jet experiment, Re = VL/ν  (V is the mean velocity and L the width of thejet) may easily vary over three orders of magnitude by varying pressure and flow rate [1].A large experiment with low temperature helium flow rates is today available at CERN, in theframework of the Large Hadron Collider development. We use a large scale refrigerator producing 6 kWat 4.5 K, with flow rates from 20 g/s up to 300 g/s [2]. A specific 6 m long φ 22 cm transfer line, withconcentric tubes, feeds the experiment. The inner tube drives liquid and gaseous helium out of therefrigerator; the mixture is turned to gas at controlled temperature by a powerful heater (up to 10 kW)before entering a specific flow meter, a honeycomb and finally the φ  = 25 mm nozzle. The round jetdevelops inside the 2.5 m high experimental chamber. The investigation of the expected micrometricdissipative η scale requires an original 5 µm superconducting anemometer [3], located axially 1.25 mdown the nozzle : it gives access to the velocity measurements. The helium flows back to the refrigeratorthrough the transfer line outer tube.The grid at the nozzle entrance is a honeycomb, φ 240 mm and 20 mm thickness, with hexagons of4 mm side dimension. The nozzle diameter has been determined to maintain the flow velocitysignificantly lower than the sound velocity : at 300 g/s the output velocity is half the sound velocity. Thenozzle height is 280 mm. The surface contraction factor in the nozzle is 92. Thus the flow in theexperimental chamber is independent from the incoming flow characteristics as confirmed by first testswith water: they showed that the outgoing flow from the nozzle is laminar for several cm and is alignedwith the nozzle axis before turning to the turbulent state. At the level of the detectors the flow diameter isabout 40–50 cm. The mean temperature is 4.8 K.Special care has been taken to avoid spurious heat leaks in order to limit the temperature difference∆T between the incoming and outgoing He in the experimental chamber. For heat leaks ranging around30 W, ∆T, including the temperature fluctuations of the flow induced by the turbulent regime, does notexceed 100 mK at a 50 g/s flow rate. With such a low ∆T, the outgoing He feeds directly the last stage ofthe liquefier and the He flow is in constant average temperature conditions.Figure 1  Voltage data at a sampling frequency of 312 kHz (Re = 3×106)extracted from a full 108 data recordThe anemometer operation and the data acquisition procedure will be presented elsewhere [3]. OnFigure 1, the sampling of 8000 voltage data points among a 108 data file at Re = 3×106 is shown . Astypical in turbulence rather quiet periods are interrupted by sudden bursts. The calibration procedure [3]relies on the gaussian distribution of the spatial velocity and uses an improved Taylor frozen turbulencehypothesis [1]; also special care was taken to remove the noise at “low” flow rates and high frequencyparasitic peaks.Figure 2  Third moment of the velocity fluctuations divided by r vs r at Re = 107It is expected from the Kolmogorov theory [4] that for inertial scales between L and η , 〈δVr3〉/rshould be constant and equal to –(4/5) 〈ε〉 where 〈ε〉 is the mean energy transfer rate between the variousflow scales. This behavior is roughly followed: an example is given on Figure 2 at Re = 107 from 0.5 mmup to 5 cm. The sharp decrease above 10 cm is characteristic of scales close to L. For the lowest scales thebehavior is governed, at low Re, by the viscous process and, at large Re, by the anemometer characteristicdetection size.-0.100.10.20.30 2000 4000 6000 8000Voltage (V)10- 110010110- 5 10- 4 10- 3 10- 2 10- 1<  Vr3>/r  (m2/s3)r (m)δδδδFigure 3  Kolmogorov dissipative length scale vs Re. The line is a best Re-3/4 fitover the data at CERN (full circles) and former Grenoble data (triangles, Ref. [5])From the plateau it is possible to extract a typical dissipative Kolmogorov length value: η4 = ν3/〈ε〉.For increasing Re (7.105 to 107), η decreases from 20 down to 3 µm. We present on Figure 3 the η/Ldependence with Re in the present and former [5] measurements. They nicely follow on more than threeorders of magnitude the expected Re-3/4 relation predicted by Kolmogorov; they are in excellentagreement with each other.Figure 4  Normalized histograms of the velocity fluctuations δVr for Re = 107 (r = 7, 21, 98 µm, 1, 12, 93 mm).Each histogram is shifted for clarity.Further analysis relies on the velocity increments : δVr = V(x+r) - V(x) at scale r, to investigate theintermittency process. On Figure 4, the normalized δVr histograms are presented for several values of r(Re = 107). At large r the parabolic shape is characteristic of a gaussian behavior. However when rdecreases the more and more peaked central part and the reinforced wings are typical of intermittencyprocesses.Other statistical analysis are presently under progress in order to compare the data with thetheoretical infinite Reynolds predictions.10-610-510-410-310-2104 105 106 107y = 1.825* x^(-0.749)  /LReynoldsη10-1310-1110-910-710-510-310-1101-20 -15 -10 -5 0 5 10 15 20F(  Vr)Vr/(<  Vr2> )1/2δδδδδδIncreasing rACKNOWLEDGMENTSPeople operating the refrigerator at CERN, as well as the Service Electronique at CRTBT deservespecial thanks for their support and dedication. This work was done with the support of Région Rhône-Alpes under Emergence grants n° 99809026 and 99809027.REFERENCES1. Chanal, O., Chabaud, B., Castaing, B., Hébral, B., Eur. Phys. J. B (2000) 17 3092. Erdt, W. K., Frandsen, P., Guesewell, D., Soldheim, N. O., Winkler, G., Kurtcuoglu, K., Loehlein, K., Senn, A. E.,Wagner, U., Ziegler, B., Adv. Cryog. Eng. (1994) 39A 5533. Pietropinto, S., Ladam, Y., Poulain, C., Roche, P., Baudet, C., Castaing, B., Chabaud, B., Gagne, Y., Hébral, B.,Lebrun, P., Pirotte, O., to be published4. Kolmogorov, A. N., Dokl. Acad. Nauk. SSSR (1941) 32 165. Chanal, O., PhD Thesis, Institut National Polytechnique, Grenoble (1998).

Low Temperature Gaseous Helium and very High Turbulence Experiments

Cryogenic gaseous helium gives access to extreme turbulent experimental conditions. The very high cooling helium flow rates available at CERN have been used to reach Reynolds numbers up to Re ~ 10 7 in a round jet experiment. First results are discussed. Turbulence is a highly challenging problem both for fundamental understanding and industrial applications. Present capabilities of the most powerful computer simulations remain far away from common situations, and experimental data are needed to test the validity of theoretical predictions. Large air wind tunnels (≥ 20 m), for example, have been used for many years; however in such large experiments the Re hydrodynamic parameter cannot vary on a wide range. The physical idea is to understand how the energy injected at the large scale L of the flow cascades down through the inertial scales to the smallest scale η where it is dissipated by viscosity. In air this behavior is investigated in large systems : L is in the range of several meters and η is in the millimetric or submillimetric range. Turbulence studies in gaseous helium at cryogenic temperatures have been achieved in the last twenty years and have opened a very attractive way to vary Re on several orders of magnitude in a single geometry and reach very high Re values. They take advantage of the very low kinematic viscosity ν of gaseous helium (ν = 7×10-8 m 2 /s at 4.2 K and 1 bar, 230 times lower than air in STP conditions). In a typical (φ = 10 cm) laboratory jet experiment, Re = VL/ν (V is the mean velocity and L the width of the jet) may easily vary over three orders of magnitude by varying pressure and flow rate [1]. A large experiment with low temperature helium flow rates is today available at CERN, in the framework of the Large Hadron Collider development. We use a large scale refrigerator producing 6 kW at 4.5 K, with flow rates from 20 g/s up to 300 g/s [2]. A specific 6 m long φ 22 cm transfer line, with concentric tubes, feeds the experiment. The inner tube drives liquid and gaseous helium out of the refrigerator; the mixture is turned to gas at controlled temperature by a powerful heater (up to 10 kW) before entering a specific flow meter, a honeycomb and finally the φ = 25 mm nozzle. The round jet develops inside the 2.5 m high experimental chamber. The investigation of the expected micrometric dissipative η scale requires an original 5 µm superconducting anemometer [3], located axially 1.25 m down the nozzle : it gives access to the velocity measurements. The helium flows back to the refrigerator through the transfer line outer tube. The grid at the nozzle entrance is a honeycomb, φ 240 mm and 20 mm thickness, with hexagons of 4 mm side dimension. The nozzle diameter has been determined to maintain the flow velocity significantly lower than the sound velocity : at 300 g/s the output velocity is half the sound velocity. The nozzle height is 280 mm. The surface contraction factor in the nozzle is 92. Thus the flow in the experimental chamber is independent from the incoming flow characteristics as confirmed by first tests with water: they showed that the outgoing flow from the nozzle is laminar for several cm and is aligned with the nozzle axis before turning to the turbulent state. At the level of the detectors the flow diameter is about 40-50 cm. The mean temperature is 4.8 K. Special care has been taken to avoid spurious heat leaks in order to limit the temperature difference ∆T between the incoming and outgoing He in the experimental chamber. For heat leaks ranging around 30 W, ∆T, including the temperature fluctuations of the flow induced by the turbulent regime, does not exceed 100 mK at a 50 g/s flow rate. With such a low ∆T, the outgoing He feeds directly the last stage of the liquefier and the He flow is in constant average temperature conditions

Low Temperature Gaseous Helium and very High Turbulence Experiments

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