CEA-SBT in Grenoble has a world-wide recognized expertise in cryogenic engineering. CEA/SBT had built a He refrigerator providing a large refrigeration power over a temperature range of 1.5 to 5 K, for both applied and fundamental studies, in the domain of helium cooling of superconducting magnets for fusion, and also in the domain of superfluid helium cooling of accelerators (LHC project). More fundamental studies of two phase superfluid helium flows have also been performed, and more recently two turbulence experiments have been set up and funded under the French National Agency for Research, to study some specificities of superfluid turbulence. These projects have been made possible by the unique conjunction of a multidisciplinary expertise at CEA and in the geographical area around Grenoble:
This unique environment has made possible the construction at CEA of a large facility for helium turbulence study, using the large refrigerator available at CEA, together with the expertise in the field of instrumentation of the above mentioned laboratories. CEA opens this facility to external users. Two facilities are made available by CEA: the main facility is the SHREK facility.
The SHREK facility enables to work in superfluid as well as in normal liquid helium. This is a large apparatus connected to the 400 Watt/1.8K refrigerator providing Re in the 107 range.
The 400W/1.8K Refrigerator is in operation since 2004, when it achieved its nominal performances, namely 400W at 1.8K. The refrigeration power available ranks from 120W at 1.5K to 800W at 4.5K. All the temperature levels in between have been fully explored, and temperature can be adjusted in the range of 1.5 to 4.5 K continuously. Noisy screw compressors are located in a separate building, while the cold box stays in the main hall at the vicinity of the 26 m2 control room. Compared to the refrigerators available at CERN for turbulence studies, it offers the possibility to operate below the superfluid transition, down to 1.6K.
|The 400W station|
|The Shrek facility||The HeJet experiment|
Reynolds numbers of up to Re ~ 2 107 have been obtained on the previous experiment (test line in the 1.5 m long 30 mm inner diameter pipe with a mean flow velocity of 8 m/s). Stability better than 0.05 K has been maintained over more than one hour.
A new multitest cryostat (shown on figure above) has been installed since 2009, offering large dimensions for an experiment totally independent of the refrigerator. The space available in this upgraded SHREK cryostat will roughly fit in a cylinder of 4m in height and 2 m in inner diameter. Lines providing liquid helium (normal or superfluid) and return pumping line and connections to cryogenic circulating pump are located at the bottom of the cryostat, ensuring easy access for assembly of experimental inserts.
Our new von Karman cell of unprecedented dimension (0.8m I.D. and 1.2 m in height - 0.7 m between turbines) permits to address either higher Reynolds numbers (Re up to 108) or to be able to resolve the dissipative scale for lower Re.
It is worth mentioning that our group works on other (past, future) experiments to be installed in the SHREK cryostat. These experiments, when available, could be used by external users.
Instrumentation to characterize the type of flow (mainly torques at the turbine level) as well as local measurement (microthermometer, miniature Pitot,…) can be used.
Anyway, the potential user can perform modifications to this apparatus, e.g. introducing device as heated grid inside the von Karman cell, or new sensors, etc.
Measurements available: the experiment is basically equipped with the detectors mentioned above, together with the electronics. Sensors dedicated to turbulence cascade investigation are very small, which puts a heavy constraint on the mechanical and robustness aspects: all helium facilities will certainly take a great benefit from the results obtained in the JRAs to improve the sensors under cryogenic conditions. Users wishing to install new types of sensors in the SHREK facility will need to test these sensors under similar cryogenic conditions before inserting their sensor in the SHREK facility, which requires validation of the sensor before it is inserted in the extreme flow conditions in von Karman cell. The second facility (HeJet) may help them to quickly validate these sensors.
The HeJet facility consists of a round inertial jet driven by a centrifugal pump. It is designed to perform comparative studies of the homogeneous turbulence in He I and He II at high Reynolds number (Rlambda = 2500). Comparison with classical results is easy since this kind of flow geometry is very well documented. The cylindrical test chamber dimensions are 50 cm high by 20cm in diameter, and sensors can be situated up to a distance x/Dnozzle = 30 (with currently available nozzle). The integral scale is Lint = 2 cm and the dissipative scale is a few tenth of micron.
HeJet can achieve the same pressure and temperature conditions as SHREK so that it is also a very reliable development and test bench for SHREK sensors. The self similarity of the jet flows is very interesting regarding the test of cryogenic sensors: size effects can be tracked simply by changing the distance from the nozzle to the sensor.
CEA/SBT offers two kinds of service:
(i) direct access to the facility
(ii) commissioned research.
In the first case the user attends the facility and performs himself the experiment. Technical assistance from the permanent staff at CEA/SBT is provided in the following domains:
In the case of commissioned research, everything will be installed and run by our permanent staff according to the specifications given by the users.
Maj : 06/02/2012 (771)