Investigation on the critical heat flux in a 2 by 2 fuel assembly under low flow rate and high pressure with a CFD methodology

A high pressure critical heat flux (CHF) facility was built at the University of Wisconsin-Madison to investigate the CHF phenomena in rod bundles under lower mass flux and high pressure prototypical to small modular reactor fuel assemblies. With this facility, CHF values and locations can be obtained in a 2 by 2 fuel assembly with 2 m heated length. CFD models were used to predict the experiment data for the purpose of phenomena investigation and model validation. The CFD models can predict the dry spot at the heated surface and the temperature increase. Besides, the predicted CHF values and locations were compared with the experimental data. Good agreement can be observed for the cases with relative high mass flux. Nevertheless, the predicted CHF values were all less than experiment data and the deviations increased with decreasing the mass flux to a relative error larger than 20% for the case with mass flux around 550 kg/m2/s. The accuracies of current model for the prediction of CHF under the conditions of bubbly flow and slug flow were higher than to predict CHF under churn flow condition. The deficiency of the CHF models might be caused by the flow regime transition from bubbly flow to the churn flow with reducing the mass flux. This paper demonstrates an application of two-phase CFD models in the prediction of CHF phenomena in fuel assembly with spacers.