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7.1. Introduction and Objectives

The critical heat flux condition was originally discussed in the context of pool boiling. In this situation it represents a condition in which the flow boiling regime dramatically changes near the heater wall. This is the most important flow regime transition when considering the performance of heat exchange equipment with evaporation and boiling. The critical heat flux condition is characterized by a large decrease in the heat transfer coefficient from the heater wall to the bulk fluid. The reason for the reduction is due to the flow regime transition when continuous liquid is replaced by continuous vapor at the heater wall, whether locally or over a larger area.

Let us consider the critical heat flux for two different boundary conditions. For the purpose of this section, CHF is defined as follows:

  1. For a surface with a controlled heat flux (e.g., with electrical heating, radiant heating or nuclear heating), CHF is defined as that condition under which a small increase in the surface heat flux leads to a large increase in the wall temperature.
  2. For a surface whose wall temperature is controlled (e.g., one heated by a condensing vapor), CHF is defined as that condition in which a small increase in wall temperature leads to a large decrease in heat flux. The term "burnout' is sometimes taken as being synonymous with the terms "critical heat flux" (CHF), "departure from nucleate boiling" (DNB), "boiling crisis" and "dryout." In this discussion the implication of "burnout" is that the rise in surface temperature is sufficient to cause physical damage to the heat surface.


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