Unsaturated Helium Cooling Limits. (Summary of Liquid Helium-4 Boundary and Heat Transport Studies.)

The report summarizes studies of liquid He4-solid boundary conductances and liquid He4 cooling limits. Emphasis is on the final phase of the investigations concerned with cooling by means of unsaturated pressurized superfluid He II. Cooling limits arise from the destruction of superfluidity when He II locally is replaced by ordinary fluid. The limiting heat flux densities have been studied with a vacuum-insulated vertical tube zero mass flow configuration, closed and heated at its lower end. The limiting heat flux densities were found to be a function of the system pressure and temperature. For a comparison, data have been taken also for saturated He II. The pressure range of the unsaturated liquid studies extended from 1 atm to supercritical pressures of about 3 atm. The temperature had a strong effect on the limiting heat flux density which increased toward a constant value of about 3 Wsq cm below 1.8 degrees K. In contrast to saturated He II studies, no maximum in the cooling limit was detected. The unsaturated liquid data exceeded those for saturated liquid by a factor of about 2. Theoretical calculations have been carried out for both, saturated and unsaturated liquid. Author