THERMAL COUNTERFLOW IN A DIVERGING CHANNEL - A STUDY OF RADIAL HEAT-TRANSFER IN HE-II

Heat transfer in superfluid helium is studied in a channel which imposes a radial 1/r divergence in the heat flux density, q(r). A very sensitive Au-Fe thermocouple/SQUID system is employed to measure the temperature difference between various radial positions along the channel. Both laminar and turbulent dissipation are observed. The Gorter-Mellink mutual friction model, used to describe the turbulent thermal dissipation in channels of uniform cross-section, is applied locally to this diverging flow. Comparison of the turbulent dissipation measured over two sections of the channel shows that the Gorter-Mellink model alone is inadequate to describe the thermal resistance of He II in this non-uniform flow. Effects such as a suppression of the turbulence at the channel entrance or a transition to turbulence at some position within the channel may contribute to the discrepancy. Until these effects are carefully understood, use of the Gorter-Mellink model alone to characterize radial heat transfer in He II is highly suspect, even at high heat flux densities.