Experimental study of three-phase helium mixtures in confined channels

An experimental campaign was conducted to investigate heat and mass transfer phenomena in superfluid helium (He II) in two rectangular cross-section channels of high aspect ratios and different thickness resembling the space between steel collars in the LHC superconducting magnets. The experiments consisted of clamped heat flux tests at atmospheric pressure, in which a heater strip suddenly releases a constant heat load into the channel that is open to a helium bath on one side. The difference in thickness between the two channels allowed exploring the effect of the geometrical confinement on the propagation of both phase change fronts: i) the He II-He I 𝜆-transition front; ii) the He I-vapour first-order transition front. The observations show that, in the thinner channel, it is possible to distinguish different behaviours of the phase fronts depending on the extent of the heat flux. For increasing heat flux values, the 𝜆-front speed successively increases sharply, decreases, and increases weakly. This sequence is determined by the presence of the vapour film, which either diminishes the He II-He I transformation rate by lowering the heat transfer or pushes the 𝜆-front while expanding. In the thicker channel, the intermediate behaviour is absent as the level of confinement is lower and the He I phase never expands considerably along the highest dimension of the channel.