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7.8. CHF With a Nonuniform Heat Flux

In the introduction to the CHF concept correlation we illustrated the effect of uniform heating of a channel versus nonuniform heating. In general there are two ways to handle nonuniform heating:

(1) Develop a correction for a particular type of nonuniform heating pattern.
(2) Develop specific CHF correlations for nonuniform heating.

Let us consider each approach below.

The differences in tex2html_wrap_inline5440 for nonuniform heating led Tong, et al., (1966) to propose the so-called "F-factor" method for the prediction of critical heat flux in nonuniform axially heated channels. Their method is illustrated schematically in Figure 7.10, the three curves shown in this figure are respectively:

  1. The flux profile q"(Z).
  2. The critical heat flux tex2html_wrap_inline5444 predicted for the local quality condition using a standard correlation for uniform heating.
  3. The critical heat flux, tex2html_wrap_inline5446 for the nonuniform heating situation calculated by multiplying tex2html_wrap_inline5444 by the F-Factor defined as:

    equation3092

where tex2html_wrap_inline5402 is the distance from the channel inlet to the point at which the burnout flux is being predicted and tex2html_wrap_inline5452 is given by the empirical expression:

equation3104

where G is in tex2html_wrap_inline5456 .

Nonuniform heating experiments have been investigated in the past, primarily for axial variation of the heat flux. Each of these studies have resulted in an empirical correlation for CHF with nonuniform heating. The basic assumption in directly correlating the CHF data from such tests is that one correlates the CHF values for the actual local conditions (e.g., G, tex2html_wrap_inline5130 , P). Kirby (1966) used some of the past data and more recent data from Harwell to develop such a correlation (Table 7.2). This is suggested for the conditions listed for nonuniform axial heating situations as an alternative for the F-factor.

Table 7.2. Kirby Correlation for Non-Uniformly Heated Round Tubes

CorrelationEquation
equation3328


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Last Modified: Tue Sep 2 15:06:55 CDT 1997