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

Abstract : 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 instabilityis a limiting safety issue in nuclear research reactors, since it may lead to flow starvation and eventuallyboiling crisis in some of the core channels.The experimental database consists of OFI tests at low pressure, with gap sizes between 1.37 and 3.23mm, 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 systemparameters 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 theother 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 thermalhydraulicparameters 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 theeffect of the non-uniform heat flux can be accounted for. These results are consistent with the ones foundfor upward flow in previous studies. No significant effect of the flow direction is therefore observed.
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Submitted on : Friday, December 13, 2019 - 2:28:50 PM
Last modification on : Tuesday, April 28, 2020 - 11:28:14 AM

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A. Ghione, B. Noel, P. Vinai, C. Demazière. Assessment of criteria for Onset of Flow Instability in vertical narrow rectangular channels with downward flow. 12th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety (NUTHOS-12), Oct 2018, Qingdao, China. ⟨cea-02339113⟩

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