Boiling crisis / critical heat flux

Partly settled. Some of it is understood, some is still argued about.

Nukiyama's 1934 experiments first revealed that boiling heat transfer rises with surface temperature only up to a critical point, beyond which the surface is blanketed by vapor and burns out — a limit crucial for nuclear reactor and electronics cooling design. Zuber's 1959 hydrodynamic instability theory, modeling the vapor-liquid interface via Rayleigh-Taylor and Kelvin-Helmholtz instabilities, gave engineers a widely used semi-empirical formula for CHF that remains standard in textbooks. However, the underlying microscale trigger mechanism is still contested: different research groups favor near-wall dryout, macrolayer evaporation, or far-field hydrodynamic explanations, and no single first-principles model unifies all experimental regimes as of the mid-2020s. It is a textbook example of an engineering-critical number that works reliably in practice while its fundamental physical origin remains only partially resolved.

One of the most safety-critical numbers in nuclear engineering — how much heat a boiling surface can take before catastrophic burnout — still rests on a 1959 semi-empirical formula rather than a settled first-principles theory.

Explanations that circulated

• Zuber's 1959 Rayleigh-Taylor hydrodynamic-instability correlation is often presented as a first-principles derivation of critical heat flux, but it actually relies on an empirically fitted constant and does not predict CHF purely from first principles
• Treating critical heat flux as caused by a single universal mechanism, when the literature identifies at least five competing candidate mechanisms (bubble interference, hydrodynamic instability, macrolayer dryout, hot/dry-spot, interfacial lift-off) whose relative importance depends on conditions

References

• Pool boiling critical heat flux (CHF) – Part 1: Review of mechanisms, models, and correlations, Various (review article), International Journal of Heat and Mass Transfer (ScienceDirect), 2018