Predictive study of the onset of flow instability in narrow vertical rectangular channels under low pressure subcooled boiling

One of the most limiting critical phenomena, which nuclear designers must consider, is the onset flow instability (OFI) phenomenon. It is the critical phenomenon limiting the reactor power from the thermal-hydraulic point of view especially material testing reactors (MTR). In the present study, an empirical correlation is developed to predict OFI in narrow vertical rectangular channels simulating coolant channels of MTR. The developed correlation involved almost all parameters that affecting OFI phenomenon in a dimensionless form. The coefficients involved in the proposed empirical correlation are identified by experimental data of Whittle and Forgan (1967) that covers the wide range of MTR operating conditions. The correlation predictions for subcooling at OFI are compared with predictions of some previous correlations where the present correlation gives much better agreement with the experimental data of Whittle and Forgan with relative standard deviation of only 6.6%. The present correlation is then used to predict the void fraction and pressure drop in subcooled boiling where the S-curves of Whittle and Forgan for OFI are predicted with good accuracy. Based on the present correlation, the OFI locus for the IAEA 10 MW MTR generic reactor is predicted and plotted against the flow velocity, exit coolant temperature and bubble detachment parameter under both fast and slow loss-of-flow transients where a vast safety margins for OFI phenomenon are predicted in both the steady and transient states as well.