In the design of nuclear reactors adequate cooling must be demonstrated for
all operational states as well as during and after design basis accidents. A
key aspect of this design activity is the prediction of the Critical Heat Flux
(CHF). The focus of the work in this thesis was the prediction of CHF in
non-circular channels. The Look Up Table was used to analyse several burnout studies for
non-circular channels in the literature and was found to be a poor predictive
tool for these geometries. A conventional phenomenological model developed
for round tubes was also shown to give poor predictions, with a mean error of
25% and root mean square error of 31%.
Phenomenological modelling requires correlations for the mass transfer
processes in annular flow. Deposition rates for annular flow in rectangular
channels have been determined by an analysis of upstream burnout
data. This showed good agreement with the rates in round tubes and
validated this aspect of the phenomenological approach.
The conventional one-dimensional phenomenological model was extended to
include a variation in film thickness around the periphery. This model was
fitted to experimental data from the literature for burnout in asymmetrically
heated tubes. The low mean and root mean square errors, 0.8% and 3.0%
respectively, confirmed the principle of the model.
A flow visualisation rig has been designed and successfully operated to
produce a flow-regime map for a rectangular channel of 25 mm by
2.5 mm. This map showed that the gas momentum flux required to cause
annular flow was higher than that in round tubes. A wide range of annular
flow conditions were observed and shown to be generally consistent with the
phenomenological modelling approach. However it was seen that there were
novel flow features that will need to be accounted for when predicting CHF in
these geometries.