The facilities make particularly use of the phase transition experienced by Helium at 2.2 K, thus allowing experiments at very High Reynolds values with normal (above 2.2 K) and superfluid helium (below 2.2 K). It will contribute to the understanding of the fundamental differences between frictionless and normal turbulence.
Facilities and equipment
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 (Rλ = 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 = 60 (with currently available nozzles). 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. A sketch of the experiment is shown in figure 1b with its different components, in particular the heat exchanger (12) which evacuates the power dissipated in the chamber of the jet to the helium bath, the nozzle (10), the motor (1) of the centrifugal pump which ensures helium circulation, the vacuum pump (4) enabling the cooling of the helium bath down to 1.7 K.
Probes and measurements
It is possible to insert Pitot probes and hot wires. Moreover, at the time when the Euhit program starts, developments are underway in order to make acoustic measurements in the HeJet; second sound probes are also under development.
Typical measurement procedure
For HeJet, a comparable procedure to SHREK is necessary. However the time scales of the different periods is shorter: (i) assembling and cabling of new components/instrumentation takes typically two weeks, (ii) vacuum and cool-down of the experiment takes one week, and (iii) the running period last typically several days; (iv) eventually the warm-up period takes less than one week.
The working periods of external users cannot be chosen freely and they are supposed to coordinate with our own research projects.
SHREK is a large Von Karman experiment: its diameter is 0.78 m and height between propellers is 0.7 m (Figure 1). These dimensions, together with the use of cryogenic helium, allow to reach very large Reynolds numbers (typically Re ~ 108, and 2600 < Rl < 10000). It works with normal or superfluid helium in a temperature range between 1.6 K and 4.8 K, with either supercritical, gaseous, or subcooled liquid helium. Other gases could also be envisaged once their compatibility with SHREK components has been verified. The turbines are driven by two room temperature independent motors , and can be rotated each in a frequency range of 0.2 to 2 Hz.
The Von Karman cell is inserted in a cryostat immersed in a larger tank filled with liquid helium cooled by the 400 W at 1.8 K (Figure 2a and 2b) refrigerator. This refrigerator, comparable to the Tore Supra refrigerator in Cadarache, is totally dedicated to Research. It 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 500W 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 SHREK experiment (with its outer cryostat) is inserted into a so-called “Multipurpose Vessel” which is a large vessel that is connected through a cryogenic line to the refrigerator. The SHREK experiment, once inserted into this large vessel (Figure 3), can be connected to the refrigerator, and the power dissipated in SHREK is evacuated to the refrigerator.
Measurements and probes
Instrumentation to characterize the type of flow (mainly torques at the turbine level) as well as local measurement (local thermometers, miniature Pitots,…) can be used. Moreover, the potential user can perform modifications to this apparatus, or new sensors, either developed in the frame of the EUHIT, or developed independently.
Typical measurement procedure
For SHREK, allocated time can be divided in different periods: (i) assembling and cabling of new components/instrumentation, (ii) vacuum and cool-down of the experiment and (iii) running period; (iv) eventually a warm-up period is necessary before removing the experiment from the cryostat.
The time required for period (i) is typically a few days; the user who brings his own sensor should be present during this period, in order to make sure that everything is properly installed in the experiment. Period (ii) is 1 week to reach good vacuum conditions, and 2 weeks for cool down to the requested temperature. The running period (iii), dedicated to the experiment itself, is usually of the order several days, which is generally sufficient to acquire lots of data, as using liquid helium enables a drastic reduction in time scale. The working periods of external users cannot be chosen freely and they are supposed to coordinate with our own research projects. In case the user wishes to insert a new sensor in the SHREK facility, this sensor should be first tested in the HeJet facility for validation, or in another cryogenic facility of the EUHIT, or possibly in another cryostat at SBT.