7.9. Correlation of Burnout for Rod Bundles (sub-channel analysis):

It is possible to correlate data for CHF in rod bundle geometries by using overall, mixed flow, types of correlations and these are moderately successful. However, much more precise predictions can be made by using correlations based on "sub-channel analysis." In this form of analysis, the rod bundle is sub-divided into flow zones or sub-channels and conditions are calculated for these individual sub-channels, taking account of cross-mixing between adjacent channels and of cross-flows generated by pressure differences between the sub-channels. Two types of sub-channel have them employed as illustrated in Figure 7.11:

1. "Coolant" sub-channels which are bounded by lines joining the center of individual rods as illustrated. A discussion of the application of this form of sub-channel analysis to the prediction of burnout i rod bundles is given by, for instance, Bowring (1972).
2. "Rod centered" sub-channels in which the flow zone is bounded by lines of symmetry as illustrated in Figure 7.11 (b). The application of this form of sub-channel analysis is discussed, for instance, by Gaspari et al, (1974).

In the "coolant" subchannel method, the usual practice is to devise specific CHF correlations for the sub-channel; in the application of the "rod-centered" sub-channel method, Gaspari et al., were able to employ a standard round-tube burnout correlation to the sub-channels (on an equivalent diameter basis) with reasonable success.