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Superfluid helium-4 (He II) has been widely utilized as a coolant in variousscientific and engineering applications due to its superior heat transfercapability. An important parameter required in the design of many He II basedcooling systems is the peak heat flux $q^*$, which refers to the threshold heatflux above which boiling spontaneously occurs in He II. Past experimental andnumerical studies showed that $q^*$ increases when the heating time $t_h$ isreduced, which leads to an intuitive expectation that very high $q^*$ may beachievable at sufficiently small $t_h$. Knowledge on how $q^*$ actually behavesat small $t_h$ is important for applications such as laser ablation in He II.Here we present a numerical study on the evolution of the thermodynamic stateof the He II in front of a planar heater by solving the He II two-fluidequations of motion. For an applied heat flux, we determine the heating timebeyond which the He II near the heater transits to the vapor phase. As such, acurve correlating $q^*$ and $t_h$ can be obtained, which nicely reproduces somerelevant experimental data. Surprisingly, we find that there exists a criticalpeak heat flux $q^*_c$, above which boiling occurs nearly instantaneouslyregardless of $t_h$. We reveal that the boiling in this regime is essentiallycavitation caused by the combined effects of the first-sound and thesecond-sound waves in He II. Based on this physical picture, an analyticalmodel for $q^*_c$ is developed, which reproduces the simulated $q^*_c$ valuesat various He II bath temperatures and hydrostatic head pressures. This workrepresents a major progress in our understanding of transient heat transfer inHe II.

Boiling and cavitation caused by transient heat transfer in superfluid helium-4