Boiling

Boiling is vaporization through the formation and growth of vapor bubbles within a liquid. It differs from surface evaporation, which can occur without bubbles in the bulk. At equilibrium, the boiling point of a pure liquid is set by equality of its saturation vapor pressure and the external pressure.

A real bubble must also overcome local hydrostatic pressure and the capillary pressure of its curved surface. Creating the interface costs energy, giving a nucleation barrier; a smooth clean vessel can therefore permit superheating. Surface cavities and boiling chips supply sites where bubbles form more readily. The observed onset depends on these sites and heating conditions, while the equilibrium boiling point remains a property specified at the chosen pressure.

At a specified external pressure, the equilibrium temperature at which a pure liquid's saturation vapor pressure equals that external pressure. Bubble nucleation, hydrostatic head and capillary pressure can affect the observed onset of boiling without redefining the equilibrium property.

Raising a solvent's boiling point at fixed external pressure by a dissolved nonvolatile solute, under conditions where the solvent vapour pressure is reduced. The simple rule does not apply unchanged to every mixture of volatile components.

A limiting heat flux associated with a transition to a less effective boiling heat-transfer regime under specified conditions. Its value depends on fluid properties, pressure, flow, heating and geometry; a single number cannot be transferred indiscriminately between different flow passages.

A transition from effective nucleate boiling toward impaired surface wetting and substantially poorer heat removal in the relevant boiling regime. It is a concern when a heated fuel surface can become much hotter at an otherwise similar imposed heat load.

The ratio of the heat flux needed to reach departure from nucleate boiling to the actual local heat flux. Greater values indicate more margin under the model and correlation used.^[ref]

The minimum local departure from nucleate boiling ratio over the specified core region or assessment. It depends on local heat loading and coolant conditions, not only on average core power.

A boiling chip is a small solid piece with cavities or a porous surface that supplies sites for heterogeneous nucleation of vapor bubbles in a heated liquid. Gas trapped in a cavity can help an incipient bubble survive the capillary pressure that would collapse a sufficiently small isolated bubble.

Many active sites encourage repeated bubble release, reducing the large, irregular boiling events associated with delayed nucleation and superheating. The chip changes the route by which boiling begins; it does not materially redefine the liquid's equilibrium boiling point. Its action is therefore understood through interface formation and bubble growth, rather than through a chemical conversion of the solvent.