Assessment of criteria for Onset of Flow Instability in vertical narrow rectangular channels with downward flow

Paper in proceeding, 2018

This paper presents an assessment study of criteria for the prediction of the Onset of Flow Instability (OFI) in heated vertical narrow rectangular channels with downward flow. The onset of flow instability is a limiting safety issue in nuclear research reactors, since it may lead to flow starvation and eventually boiling crisis in some of the core channels. The experimental database consists of OFI tests at low pressure, with gap sizes between 1.37 and 3.23 mm, and with uniform and non-uniform heat flux profiles. Two typologies of criteria were tested, following a global and local strategy, respectively.
According to the global approach, the Whittle-Forgan and Stelling criteria only use global system parameters to predict OFI. Relatively good results can be obtained over the whole database. The local approach is based on the prediction of the Net Vapor Generation (NVG) along the channel. The standard Saha-Zuber correlation fails to capture OFI for Peclet numbers lower than 70000. On the other hand, the Saha-Zuber KIT correlation identifies OFI in all the experiments with uniform heat flux. In the case of a non-uniform heat flux along the width of the rectangular channel, the use of thermalhydraulic
parameters averaged over the cross section is not sufficient to predict the local onset of NVG. Nevertheless, this issue can be solved if a local estimate of the flow conditions is employed, so that the effect of the non-uniform heat flux can be accounted for. These results are consistent with the ones found
for upward flow in previous studies. No significant effect of the flow direction is therefore observed.