text: The understanding of CHF phenomenon and an accurate prediction of the CHF condition are important for safe and economic design of many heat transfer units including nuclear reactors, fossil fuel boilers, fusion reactors, electronic chips, etc. Therefore, the phenomenon has been investigated extensively over the world since Nukiyama first characterized it.  In 1950 Kutateladze suggested the hydrodynamical theory of the burnout crisis. Much of significant work has been done during the last  decades with the development of water-cooled nuclear reactors.  Now many aspects of the phenomenon are well understood and several reliable prediction models are available for conditions of common interests.
  The use of the term critical heat flux (CHF) is inconsistent among authors. The United States Nuclear Regulatory Commission has suggested using the term “critical boiling transition” (CBT) to indicate the phenomenon associated with a significant reduction in two-phase heat transfer.  For a single species, the liquid phase generally has considerably better heat transfer properties than the vapor phase, namely thermal conductivity. So in general CBT is  the result of some degree of liquid deficiency to a local position along a heated surface. The two mechanisms that result in reaching CBT are: departure from nucleate boiling (DNB) and liquid film dryout. 
  Departure from nucleate boiling (DNB) occurs in sub-cooled flows and bubbly flow regimes. DNB happens when many bubbles near the heated surface coalesce  and  impede the ability of local liquid to reach the surface. The mass of vapor between the heated surface and local liquid may be referred to as a vapor blanket. 
  Dryout means the disappearance of liquid on the heat transfer surface which results in the CBT.  Dryout of liquid film occurs in annular flow. Annular flow is characterized by a vapor core, liquid film on the wall, and liquid droplets entrained within the core. Shear at the liquid-vapor interface drives the flow of the liquid film along the heated surface. In general, the two-phase HTC increases as the liquid-film thickness decreases. The process has been shown to occur over many instances of dryout events, which span a finite duration and are local to a position. The CBT occurs when the fraction of time a local position is subjected to dryout becomes significant. A single dryout event, or even several dryout events, may be followed by periods of sustained contact between the liquid film and the previously dry region [https://mediaspace.wisc.edu/media/Dryout-rewet/1_7sn93wd2] . Many dryout events (hundreds or thousands) occurring in sequence are the mechanism for significant reduction in heat transfer-associated dryout CBT.      
  Post-CHF is used to denote the general heat transfer deterioration in flow boiling process, and liquid could be in the form of dispersed spray of droplets, continuous liquid core, or transition between the former two cases. Post-dryout can be specifically used to denote the heat transfer deterioration in the condition when liquid is only in the form of dispersed droplets, and denote the other cases by the term Post-DNB.