The critical heat flux (CHF) condition forms a most important boundary when considering the performance of heat exchanger equipment in which boiling or evaporation occurs. It is characterized by a sharp reduction of the local heat transfer coefficient that results from the replacement of liquid by vapor adjacent to the heat transfer surface. For the case where the surface heat flux is the independent variable, such as in a radiantly heated furnace or a nuclear reactor fuel element, the condition manifests itself as a sharp increase in surface temperature as the critical heat flux value is reached.

The vast majority of experimental studies of the critical heat flux condition relate to water as the evaporating fluid. Fluids other than water are dealt with in a separate subsection [Section 193D(j)].

A. Nomenclature

There is considerable disparity in nomenclature for the CHF condition. The most common name is burnout, but this implies a physical destruction of the heated surface. The alternative forms DNB (departure from nucleate boiling) and dryout are equally unsatisfactory for a general description of the phenomenon, although they correctly describe specific mechanisms. The term critical heat flux condition has therefore been chosen to denote the state of the system when the characteristic reduction in heat transfer coefficient has just occurred, and the term critical heat flux (CHF) to describe the value of heat flux at which, and local to the point at which, this state of the system first occurs. The main difficulty in using the chosen nomenclature is that it implies an approach to the critical condition by increasing the heat flux, whereas, in fact, the critical condition may also be approached for a given system by varying any of the other independent variables: pressure, inlet temperature (or mass quality), and mass flux.

Considerable differences also exist in the practical methods used to define and measure the critical condition. Thus, before attempting to compare data from different sources, it is always necessary to ensure that the definition and methods used in the different works are the same or, at least, are likely to give similar results.