Steady-State heat transfer through micro-channels in pressurized He II

Abstract

The operation of the Large Hadron Collider superconducting magnets for current and high luminosity future applications relies on the cooling provided by helium-permeable cable insulations. These insulations take advantage of a He II micro-channels network constituting an extremely efficient path for heat extraction. In order to provide a fundamental understanding of the underlying thermal mechanisms, an experimental setup was built to investigate heat transport through single He II channels typical of the superconducting cable insulation network, where deviation from the macro-scale theory can appear. Micro-fabrication techniques were exploited to etch the channels down to a depth of ~ 16 μm. The heat transport properties were measured in static pressurized He II and analyzed in terms of the laminar and turbulent He II laws, as well as in terms of the critical heat flux between the two regions.

Heat transfer through cable insulation of Nb–Ti superconducting magnets operating in He II

“…activated adhesive film on its external part to provide the coil with mechanical stability. Breakdown voltage measurements on cables stacks showed satisfactory electrical performance of this insulation scheme [23].…”

“…However a complete study assessing whether these macro-scale laws are valid in such narrow channels is missing. The validity of the classical laminar and turbulent regimes in pressurized He II has been so far only confirmed down to 17 lm thick channels [13,22,23]. Fundamental investigations are ongoing to study possible dependencies on the geometrical size and shape for smaller dimensions [13].…”

Effective thermal conductivity of helium II: from Landau to Gorter–Mellink regimes

“…The interest on the transport properties of He II in thin or very thin channels (for instance, diameter from 1 mm to 50 µm) was in fact an advanced forerunner of the later general interest in microfluidics [4][5][6][7][8][9][10][11][12][13][14][15]. Here, we study the effective thermal conductivity of cylindrical microchannels filled with He II, a topic of interest in refrigeration of small systems, in the behavior of porous systems, and in the research on the effects of the walls on the quantized vortex lines typical of superfluid turbulence, which is a topic of fundamental interest.…”

Thermally Enhanced Cable Insulation for the Nb-Ti High Luminosity LHC Inner Triplet Model

“…Micro electro-mechanical technologies are employed to realize channels with thickness of dozens of m. The first results are reported in [10]. They confirm the independence of the turbulent heat transfer on the shape and size down to a dimension of 100 m. For smaller dimensions, the laminar regime could be identified, as well as the critical heat flux corresponding to its end that shows a peak at a bath temperature of around 1.9 K. Further investigations are ongoing to address smaller channels.…”