The boiling curve is not monotonic. As the wall superheat rises, the heat flux first climbs steeply, reaches a peak, and then falls. That peak is the critical heat flux, and it is the governing quantity for safety.
The regimes
Natural convection boiling. At small superheat there are no bubbles; heat leaves by single-phase convection.
Nucleate boiling. Bubbles form at nucleation sites on the surface, depart, and generate strong mixing. Heat transfer is most efficient here; Rohsenow's correlation models this regime by reducing it to the single-phase convection induced by bubble motion [1].
Critical heat flux. Bubble production becomes so dense that liquid can no longer reach the surface.
Film boiling. The surface is blanketed with a continuous vapour layer. Because its conductivity is low, heat transfer collapses and the wall temperature jumps — in a power-controlled system that means the surface burns out.
Why designs are based on critical heat flux
In power-controlled systems (electric heaters, fuel elements, power electronics) the transition is irreversible: the moment the flux exceeds the peak, the system jumps to film boiling and the temperature runs away. The acceptance criterion is therefore not the mean heat transfer but the margin to critical flux.
Flow boiling is different
With flow inside a tube, the regime also depends on the flow pattern: bubbly, slug, annular. In the annular regime, if the liquid film on the wall is exhausted (dryout) the same temperature jump can occur without the critical flux being reached.
A modelling warning
Boiling models depend strongly on empirical coefficients and on surface properties — roughness, wettability, nucleation site density. The same fluid on a differently coated surface gives a different curve. The result should be used comparatively rather than as an absolute, and the source of the coefficients stated. See the significant digits note.
References
- Rohsenow, W. M. (1952). A method of correlating heat transfer data for surface boiling of liquids. Transactions of the ASME, 74, 969–976.
- Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605