Gerard Claudet

submitter : Pressure drop and transient heat transport in forced flow single phase helium II at high Reynoldsnumbers

Abstract Pressure drop and transient heat transport measurements had been performed in the superfluid helium test loop at CEA/CEN-Grenoble, France. Single phase helium II was circulated through a 28 mm I.D. tube, over a length of 200 m for mass flow rates between 0.02 and 0.1 kg/s. Steady-state pressure drop measurements allow us to estimate the friction factor for Reynolds numbers in the 10 6 range. Rectangular heat pulses were applied at mid-length, and the fluid temperature was measured at several locations along the flow, upstream and downstream. A code has been developed which uses the two-fluid model to simulate time-dependent heat transport helium 11 flow. Simulation results are in good agreement with experimental measurements.

Construction and preliminary testing of perforated-plate heat exchangers for use in helium IIrefrigerators

Abstract Heat exchangers in cryogenic refrigerators must meet conflicting requirements for high thermal efficiency and low frictional pressure drops. This is particularly true in helium II refrigerators where the gas stream leaves the cold source at 15 mbar. Perforated-plate heat exchangers are likely to offer better overall performances for that application than the widely used plate-fin heat exchangers. Through their design they combine a high thermal conductance in transverse direction with a low thermal conductance in axial direction. The present paper reports on the initial stage of development of a fabrication method using photochemical milling of copper plates and stainless steel spacers, and silver brazing for bonding. Comparison of preliminary test measurements and calculations in the 80 to 300 K temperature range with flow rates between 1 and 5 g/s has confirmed that predictable results can be obtained for the thermal efficiency and the pressure drop with an accuracy of a few percent.

Closed cycle liquid helium refrigerators

Abstract We have developed closed cycle liquid helium refrigerators using a Joule Thomson circuit precooled by commercially available two staged Gifford Mac Mahon cryocoolers. The Joule Thomson counterflow heat exchangers are modular and have been thermo-hydraulically characterized. Fully automatic cool down and operation are achieved by two pneumatically driven by pass and expansion valves. Several apparatus have been built or are under assembly with cooling power ranging from 100 mW up to 5 Watt, for temperature ranging from 2.8 K up to 4.5 K. A trouble free operation with several warm up and cool down cycles has been proven over 7000 hours.

Large cooling power hybrid Gifford mac Mahon / Joule Thomson refrigerator andliquefier

Abstract In this paper, we present two refrigerators using Joule Thomson cycle providing cooling powers of 4.5 W at 4.4 K and using Gifford Mac Mahon cryocoolers for the precooling. Several smaller machines have been developped in our laboratory with refrigeration capacity ranging from 100 mW up to 1.5 W in the temperature range 3 K to 4.4 K. In the present case, to increase the cooling power, we introduced a three stage precooling scheme using a liquid nitrogen vessel plus the two stages of a Gifford Mac Mahon cryocooler. Cooldown and operation of the system are fully automatic. The first refrigerator is used to cool two Nb 3 Sn superconducting coils having a maximum field of 11.8 T. The second system is an helium liquefier, designed to produce more than 1 liter per hour.