Experiments on the Breakdown of Heat Transfer in Liquid Helium II

Abstract

Transient heat transport in counterflowing Helium II is investigated experimentally in a rectangular channel of 2.6 cm x 2.6 cm cross section. The mechanism of heat transfer, based on internal convection, is affected strongly by mutual friction due to the presence of superfluid vortex lines. Simultanous measurements of temperature, vortex line density (VLD) and counterflow velocity have been performed, using especially designed high sensitivity probes with a short response time. It is shown that, with increasing input heat flux, the counterflow velocity and hence the internal heat flux reaches a maximum depending on the flow parameters, which leads to a strong increase of temperature in the region close to the heater (and may cause there even evaporation or a phase change to He I). As shown by corresponding measurements and supported by theoretical calculations, this is correlated with the presence of a large amount of superfluid vortex lines, that have been produced during the passage of the heat pulse and decay afterwards. The experimental findings compare favourably with a proposed simple phenomenological model.