The boundaries between different regions of the boiling channel are specified by experimental correlations. They are required in modeling be­cause we need to know where the heat-transfer mechanism changes from convective to bulk-boiling and where voiding occurs.

In a steam generator problem, it is necessary to integrate the heat transfer correctly and to obtain accurate time constants. The latter depend entirely on the heat-transfer coefficients (see Section 1.3.1.2). In a water-cooled core channel it is necessary to know the total core voidage, because its reactivity effect may be large (due to a removal of moderator) and fast (due to flash­ing or collapse of vapor voids).

It is possible, knowing the position of the boundary between the highly and slightly subcooled regions, to simplify the channel representation in a PWR. Here it is assumed that convective heat transfer exists below the boundary and boiling transfer exists above it. The equations for the mixed enthalpy of the fluid are used, and another experimental correlation is used to predict total subcooled voidage in the channel. The analysis for a channel that includes subcooled nucleate boiling and the experimental correlations used to predict boiling boundaries and voidage have been given by Bow­ring (9) for water systems operating in certain temperature ranges. If how­ever a channel includes bulk boiling, the small subcooled boiling range may be insignificant and the only boundary of interest is that marked by the saturation temperature or a certain superheat above this saturation temperature.