College Of Basic And Applied Sciences Graduate School Of Nuclear And Allied Sciences University Of Ghana

Abstract

A Gen-IV nuclear power plant cooled and moderated with supercritical water termed SCWR, is under study with the purpose to achieve a high thermal efficiency, improve safety and economic competitiveness compared to existing LWRs (light water reactors). In fact, SCWR is a logical extension of existing PWR and BWR combined with the existing technology of super-critical water cooled fossil fuel fired power plants. Thermal phenomena such as EHT (enhanced heat transfer), DHT (deteriorated heat transfer), and flow instability observed at supercritical pressures as a result of fluid property variations have the potential to affect the safety of design and operation of SCWR, and also challenge the capabilities of both heat transfer correlations and CFD physical models. These phenomena observed at supercritical pressures need to be thoroughly investigated. The main aim of this study is to investigate heat transfer and flow instability at supercritical pressure in parallel channels with water. The performance of the 3D numerical tool STAR-CCM+ CFD code in predicting dynamics characteristics such as amplitude and period of heated inlet mass flow oscillation, and flow instability boundary; and also in capturing the trends for NHT (normal heat transfer), EHT, DHT and recovery from DHT regions is examined. The system parameters such as axial power shape, pressure, mass flow rate, and gravity have significant effect on the amplitude of the heated inlet mass flow oscillation and maximum temperature of the heated outlet temperature oscillation but have little effect on the period of the mass flow oscillation. The type of axial power shape adopted in supplying heat to the fluid flowing through heat transfer system has significant effect on stability of the system. At low or high system pressures and low mass flow rates for system operated with or without gravity influence, stability of the system with HAPS (homogeneous axial power shape) or ADPS (axially decreased power shape) decreases and increases respectively below and above a certain threshold power with inlet temperature. The system with HAPS is more stable than that with ADPS. This work also investigated the effects of system pressure, mass flow rate and gravity on flow instability at lower power boundary LPB and higher power boundary HPB at supercritical pressures adopting ADPS. Only lower threshold was obtained for LPB whereas both the lower and upper thresholds were obtained for HPB. The results on flow instability in parallel channels with water at supercritical pressures have been validated with experimental data. The 3D numerical tool adopted predicted quite well the experimental results at LPB and at high mass flow rate. The numerical tool adopted also largely under-predicted experimental amplitude and quite well predicted experimental period of the inlet mass flow oscillations. This work finally investigated heat transfer in the parallel channels. The system parameters, inlet temperature, heating power, pressure, gravity and mass flow rate, have effects on WT (wall temperature) values in the NHT, EHT, DHT and recovery from DHT regions. This numerical study on heat transfer at supercritical pressures in parallel channels was not quantitatively compared with experimental data, but it was observed that the numerical tool STAR-CCM+ adopted was able to capture the trends for NHT, EHT, DHT and recovery from DHT regions. Moreover, though the heating powers used for the various simulations are below the experimentally observed threshold heating powers, heat transfer deterioration HTD was observed, confirming the previous finding by Sharabi that HTD could occur before the occurrence of unstable behavior at supercritical pressures. It is recommended that more relevant experiments at supercritical pressures should be carried out for validation of numerical tools.