Numerical and experimental investigation on the transient heat transfer characteristics of C-shape rod bundles used in Passive Residual Heat Removal Heat Exchangers

The heat transfer effect of Passive Residual Heat Removal Heat Exchanger (PRHR HX) and buoyancy-induced flow in the In-containment Refueling Water Storage Tank (IRWST) are of great importance for the efficient and safe removal of the residual heat in the AP1000 reactor. Although some numerical studies have been conducted, only the standard k-epsilon model has been applied. Experimental validation of the simulation results was also not sufficient because of the lack of appropriate experimental data. In the present work, the applicability of different Reynolds Average Navier-Stokes (RANS) turbulence models and Large Eddy Simulation (LES) were examined, utilizing the commercial CFD software CFX 14.5. Further, two types of grids were built for the high/low-Reynolds turbulence models, and the y+ values as well as grids sensitivity were carefully analyzed. Meanwhile, overall scaled IRWST and PRHR HX models were built to simulate the thermal-hydraulic process in the residual heat removal accident, which was a new overall scaled separate effect IRWST&PRHR HX experiment. More than 150 thermocouples were utilized to measure the temperature in the key regions, and Particle Image Velocimetry (PIV) was utilized for the measurement of the flow velocity. Based on the validation of turbulence models in simulating the overall variations of temperature and velocity field in the IRWST model, the transient heat transfer capacity of PRHR HX was then analyzed. The results indicated that the low-Reynolds Shear Stress Transport (SST) model with multi-sublayer grid was appropriate for the simulation of buoyancy-induced flow. Nusselt numbers obtained from numerical simulations, experimental data, and empirical correlations were further compared to analyze the heat transfer mechanism. Combination factors including the special C-shape, flow resistance, and turbulent mixing effects imposed important influences on the heat transfer effects of the PRHR HX model. It was confirmed by numerical results, experimental data, as well as empirical correlations that the heat transfer capability of the vertical section was better than the horizontal section. (C) 2015 Elsevier Ltd. All rights reserved.