Study On The Global Characteristics Of Natural Convection And Thermal Stratification Of IRWST

The In-containment Refueling Water Storage Tank(IRWST)and Passive Residual Heat Removal Heat Exchanger(PRHR HX)play an important role in removing the core decay heat under non-Loss of Coolant Accident(LOCA)accident.In this paper,Particle Image Velocimetry(PIV)and Laser Induced Fluorescence(LIF)techniques were used to investigate the natural convection and thermal stratification inside the IRWST experimentally so as to optimize the design of the IRWST,and to improve the reliability of the reactor safety system.According to the H2 TS methodology,combined with the theoretical analysis of natural convection phenomena,scale-down ratio was proposed which needed to be satisfied in the experimental facility.PC board and stainless steel were used to construct the main part of the facility with five electric heated C-shape tubes.And the optical diagnostic platform and thermocouple temperature measurement were built accordingly.Then PIV measurement was used to investigate the evolution of the flow field of total twelve surfaces in three axial directions under three steady-and two variable-heating conditions.Attempts to use the LIF temperature measurement technology to study the evolution of the thermal stratification at the X-3 plane,and proposed to use the point-to-point calibration method to post-process the original image.The experimental results show that there is a violent upflow around the C-shape heating tube bundle due to the large thermal driving force,and the maximum height that can be achieved is continuously reduced as the heating time increases.And then a diffusion zone is present.The flow field indicates that there are a large number of small eddies around the diffusion flow.And there is a large cycle inside the water tank.The existence of circulation and convection makes the difference in fluid temperature at the same height position measured less than 1 °C.Both the LIF measurement and the thermocouples data indicate that there is a significant thermal stratification along the axial height.The layer phenomenon,and with the extension of the heating time,the thermal stratification phenomenon tends to be stable,and the temperature tends to be uniform along the radial direction of the water tank.According to the thermal stratification and natural convection phenomenon in the axial direction,the water tank was divided into three regions along the axial height,which are hightemperature zone,convection zone and low temperature zone.Natural convection is caused by temperature imbalance between fluids,which in turn is caused by weak convection between fluids.At the initial stage of PRHR HX activiation,the whole fluid temperature in the IRWST is uniform.As heated fluid accumulates in the upper region of the IRWST,the difference in fluid density decreases and the convection weakens.The weakening of the convection will increase the thermal stratification of the fluid in the axial direction.It also exacerbates the convective heat transfer between fluids.In summary,PIV and LIF technique can be used to measure hydraulic characteristics of the IRWST and the flow field and temperature field can be quantitatively analyzed.Analyze the internal flow heat transfer,and the obtained full field experimental data can provide direct quantitative and qualitative comparison for the verification of numerical calculation,optimize and improve the design of the IRWST and PRHR HX.