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

TL;DR

This study investigates the dynamics of boiling and cavitation in superfluid helium-4 caused by transient heat transfer, revealing a critical heat flux leading to instant boiling driven by wave-induced cavitation.

Contribution

The paper introduces a numerical model that predicts the critical heat flux in superfluid helium-4, linking cavitation-induced boiling to wave effects and validating it with experimental data.

Findings

Existence of a critical heat flux $q^*_c$ causing instant boiling

Boiling in this regime is driven by cavitation from wave interactions

Analytical model for $q^*_c$ matches simulation across conditions

Abstract

Superfluid helium-4 (He II) has been widely utilized as a coolant in various scientific and engineering applications due to its superior heat transfer capability. An important parameter required in the design of many He II based cooling systems is the peak heat flux $q^*$, which refers to the threshold heat flux above which boiling spontaneously occurs in He II. Past experimental and numerical studies showed that $q^*$ increases when the heating time $t_h$ is reduced, which leads to an intuitive expectation that very high $q^*$ may be achievable at sufficiently small $t_h$. Knowledge on how $q^*$ actually behaves at 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 state of the He II in front of a planar heater by solving the He II two-fluid equations of motion. For an applied heat flux, we determine the heating time beyond which the He II near the heater transits to the vapor phase. As such, a curve correlating $q^*$ and $t_h$ can be obtained, which nicely reproduces some relevant experimental data. Surprisingly, we find that there exists a critical peak heat flux $q^*_c$, above which boiling occurs nearly instantaneously regardless of $t_h$. We reveal that the boiling in this regime is essentially cavitation caused by the combined effects of the first-sound and the second-sound waves in He II. Based on this physical picture, an analytical model for $q^*_c$ is developed, which reproduces the simulated $q^*_c$ values at various He II bath temperatures and hydrostatic head pressures. This work represents a major progress in our understanding of transient heat transfer in He II.