DYNAMICS OF LIQUID-HELIUM BOIL-OFF EXPERIMENTS WITH A STEP CHANGE IN PRESSURE

The results of dynamic analysis of the effect of pressure variations d uring helium boil-off experiments are presented. A general solution of the diffusion equation with a time-dependent boundary condition is em ployed to describe the dynamic response of the liquid helium system un der variable pressure conditions, and a solution is obtained for the s pecial case when the system is subjected to a step change in pressure. The calculated temperature response of the liquid indicates that most of the experiments were not likely to have reached equilibrium as a r esult of the low thermal diffusivity of liquid helium. The initial rat e of evaporation or condensation is large, and the rate decreases shar ply with time. A method is proposed to account for the transient effec t that is observed during calculation of the heat loss rate from a hel ium boil-off experiment. By assuming that there is no mixing at all, t he present analysis provides an estimate of the upper (condensation) o r lower (evaporation) bound of the heat loss rate as a result of a pre ssure increase or decrease in the system. A previously reported equili brium analysis is expected to apply to situations where complete mixin g occurred in the bulk liquid and provides the opposite limits.