Transient heat transfer into a closed small volume of liquid or supercritical helium

Abstract

The heat transfer from a copper surface into a closed volume of liquid or supercritical helium is investigated. The helium volume is between 0.01 and 0.17 cm 3 per cm 2 of heated surface. The surface temperature and the helium pressure are measured with a time resolution below 1 ms. In liquid helium the evaporation at the heated surface after the beginning of a heat pulse leads to a rapid pressure rise above the critical pressure. This effect reduces the duration of the high transient heat transfer in the nucleate boiling regime, t f, compared to pool boiling conditions. For small liquid helium volumes the transient heat transfer is, therefore, not much different from that of supercritical helium. Some applications in superconducting magnets imply an exponentially decreasing heat load, e.g. the plasma disruption in a tokamak. Such a heat pulse can be described by the total transferred energy and the time constant. The maximal energy, E m, which can be transferred without the onset of film boiling, was measured as a function of the time constant, the liquid volume and the initial pressure. In a third experiment the recovery time of a heated surface was determined for different cooling conditions. The results are discussed quantitatively using a simple theoretical model. Correlations for t f and E m in liquid helium and for the maximal temperature excursion after a heat pulse in supercritical helium are developed. These correlations are in good agreement with the experimental results.