Numerical investigation of supercritical flow in a vertical tube under non-uniform heat flux

We report the study of supercritical flow of carbon dioxide in a vertical tube under non-uniform heat flux. The investigation of supercritical fluid is crucial for advanced generation IV (GIF) nuclear reactors since both supercritical carbon dioxide and supercritical water are planned to be used in those designs. The supercritical carbon dioxide is going to be used in secondary loop of liquid metal fast reactors, the supercritical water will be used as a coolant itself. Thanks to scaling parameters for fluid-to-fluid modelling at supercritical conditions based on inlet conditions approach it is possible to predict response for different supercritical fluids at different scaled conditions. Hence, the results can be used for investigation of other supercritical fluids if it is needed. Since, the heat flux in particular fuel element varies axially it is imperative to investigate the influence of non-uniform flux in this direction. Current literature provides experimental results only when uniform heat flux is applied, whereas nonuniform heat flux can show the thermal response at different stage of fuel cycle. Hence, it is possible to simulate how the flow behaves from the beginning of cycle (BOC) when the shape of the curve is a cosine distributed to the end of cycle (EOC) moment when the peaking factor is skewed to the bottom of the fuel element. The numerical investigation is performed for a 2-D axisymmetric model of a tube with use of CFD code. First, the model is validated for a case with uniform heat flux against results found in the literature. Then, non-uniform heat flux is represented with two parameter equation to describe the variation along axial direction. Sensitivity study related to influence of pressure, temperature and average heat flux in order to better understand the phenomena are conducted. Obtained results provide information about heat transfer coefficient (HTC), heat transfer deterioration (HTD) in supercritical conditions and which parameters can influence it. With better understanding of this phenomena it is possible to prevent or to avoid it, since HTD may cause overheating of the fuel elements inside reactor core and lead to an accident.