Heat Transfer Analysis of A Proposed Fuel Assembly For Supercritical Water Reactors Under Star CCM + CFD Code

ABSTRACT

Accurate knowledge and understanding of heat transfer characteristics near and above critical conditions is crucial to the successful design of the SCWR. Many studies, both numerical and experimental assessments have been conducted on bare tubes and simple geometries like annuli. However, heat transfer performance studies of the fuel assemblies for SCWR are scarce. Thus, there is still a lack of understanding of heat transfer performance in the fuel assembly designs for SCWR. Moreover, the importance of this study cannot be over-emphasized as it will broaden and better the understanding of the concept of heat transfer in the rod bundle geometry by providing more numerical data. In this study a 3D CFD code STAR CCM+ was used in assessing the performance of heat transfer in the square fuel assembly of a High Performance Light Water Reactor (HPLWR). Utilizing the computational environment and the flexibility of STAR CCM+ code, test analysis was conducted using the four turbulence models, namely AKN low-Re, Standard Lien’s low-Re, Standard Wilcox κ-ω and SST κ-ω to choose a suitable turbulence model yielding satisfying prediction (capabilities) in describing the heat transfer and flow of supercritical water in the fuel assembly “near”, at and after the pseudo-critical region. The analysis was carried out at 25 MPa system operating pressure, mass flow rate of 0.167 kg/s (601.2 kg/h), 300 °C inlet temperature with uniform heat flux of 650 kW/m2 . Turbulence sensitivity analysis was performed and SST κ-ω model was selected based on its simplicity and superiority to others especially with regard to numerical stability. Moreover, SST κ-ω model also does not involve damping functions, but allows simple Dirichlet specified boundary conditions Furthermore, using the SST κ-ω with low y+ wall treatment the selected heat transfer correlations were assessed. Overall, the Cheng et al. correlation provided the most satisfying prediction for the wall temperatures in all the sub-channels and captured closely Wataa’s Numerical data. This was followed by the McAdams correlation, but the Dyadyakin and Popov and the Petukhov correlations also yielded acceptable results. Test analysis results of the heat transfer correlation also confirmed the occurrence of heat transfer enhancement and heat transfer deterioration at the pseudo-critical point and after or beyond respectively. The maximum wall temperature was obtained in sub-channel 9, the hottest sub-channel and exceeded the design limit of 620 °C by 60 °C for the Cheng correlation while for the other correlations it was more. The difference in temperature between the hottest and coldest sub-channels was approximately 80 °C. Finally, parametric analysis was conducted in sub-channels 4 and 9 by varying mass flow rates 0.1670 kg/s (601.2 kg/h) and 0.1559 kg/s (561.2 kg/h), pressure 23 MPa and 25 MPa and with or without gravity. Results from this test analysis showed that mass flow rate, pressure and gravity have significant influence. It was observed that at low mass flow rate with varying pressure (23 MPa and 25 MPa), the temperatures significantly increased in the heat transfer deterioration region. Nevertheless, experimental investigations involving rod bundles adopted in this study should be conducted to validate the results obtained numerically and address the inconsistence of the conclusions drawn, “when compared with similar studies”. These experimental studies would also be helpful in validating similar numerical studies in future.