Modelling of convection heat transfer in the SNU Riser Heat transfer Experimental Facility (RHEF)
Du Toit, C.G.
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The Reactor Cavity Cooling System (RCCS) of a High Temperature Gas-cooled Reactor (HGTR) plays an important role in ensuring the integrity of the HTGR during normal operating and accident conditions. The performance of the RCCS depends on the convection heat transfer rate in the riser ducts. Under various conditions forced, free or mixed convection may occur. In order to determine the heat transfer characteristics of the rectangular riser ducts of the PMR200 the Riser Heat transfer Experimental Facility (RHEF) had been constructed at the Seoul National University (SNU) to investigate heat transfer inside a riser duct for various heat flux and flow rate conditions. The experimental results showed that mixed convection occurred for certain conditions which leads to heat transfer deterioration. The heat transfer coefficients obtained from the experimental results were not consistent with those predicted by the mixed convective heat transfer correlations published in the literature. Therefore, a modified correlation was derived to fit the experimental data for the RCCS riser duct. In the current study a 1D systems model of the riser duct was constructed using Flownex and the proposed correlation was implemented to obtain the required heat transfer coefficients. A selected set of the SNU experiments were simulated and the predicted heat transfer coefficients and the wall temperatures extracted from the results. The experimental data was also processed using the proposed correlation to predict the heat transfer coefficients and the wall temperature. These two sets of results were compared with the measured wall temperatures and the heat transfer coefficients obtained directly from the measured data. It was found that when forced or free convection conditions dominate the agreement between the three sets of results are very good. However, when mixed conditions prevail -- neither forced and free convection dominate -- the Flownex and processed experimental results were in good agreement, but differed markedly from the results obtained directly from the measurements. This needs to be investigated in depth to develop a fundamental understanding of the interaction between the relevant physical phenomena