Critical Heat Flux Mechanisms

Critical heat flux is the most important threshold in forced-flow boiling. Forced-flow CHF is equivalent to peak heat flux in pool boiling and represents the upper limit for the safe operation of many cooling systems that rely on boiling heat transfer. The occurrence of CHF can cause a large temperature rise at the heated surface, potentially leading to its physical burnout. Moreover, the post-CHF heat transfer regimes are inefficient. Depending on circumstances, CHF is also referred to as boiling crisis, departure from nucleate boiling, dryout heat flux, and burnout heat flux. Processes leading to forced-flow CHF are very complicated, involving the coupling of heat transfer, phase change, and two-phase flow hydrodynamics phenomena.

Consider the CHF line depicted in Fig. 13.1 which displays a portion of the boiling map previously shown in Figs. 12.4 and 12.5. Horizontal lines in this figure show qualitatively the sequence of heat transfer regimes encountered along a uniformly heated channel in steady state. Thus, moving along a horizontal line from left to right is similar to moving along a boiling channel. As noticed in the figure, depending on the heat flux, CHF can occur under subcooled or saturated boiling conditions. When CHF takes place in subcooled boiling or saturated boiling at low flow qualities, the process is called departure from nucleate boiling (see Section 12.1), a title that is descriptive of the mechanism involved.