Numerical Analysis Of Thermal-Hydraulic Characteristics In Open-Ended Air-Cooling Decay Heat Removal System

The open-ended air-cooling decay heat removal system can be applied to small modular reactors,which is an important safety system to ensure the decay heat removal and the integrity of the pressure vessel.It has simple system and reliable long-term operate ability.In this thesis,a numerical analysis model of the open-ended air-cooling decay heat removal system is established based on the CFD method.The influence of thermal boundary conditions,radiation emissivity and thermal resistance of the insulation on the natural circulation in decay heat removal system are analyzed from the perspectives of local temperature field,velocity field,heat transfer portion and heat transfer coefficient.The local and overall heat transfer characteristics in natural,forced and mixed convection are studied,including mixed convection heat transfer impair,which reason is laminarization.The results of this thesis provide a reference for the design and optimization of the air-cooling decay heat removal system.The scope of the conditions in this thesis is:3×10³≤Re≤9×10⁴,6×10⁶≤Ra≤3×10⁹,8.89×10^-7≤Bo≤1.02×10^-4.The calculation results show that there are heat entrance effects on convective heat transfer and open-ended effects on radiative heat transfer in the heating channel.The thermal radiation does not directly transfer heat to the air,but changing the portion of convective heat transfer between the walls.The natural circulation in the system is driven by the density difference between the ascending and descending channels.With the change of the thermal resistance of the insulation,when the thermal resistance is less than a critical value,the driving force of the natural circulation drops rapidly,and the heat transfer on the pressure vessel wall deteriorates.Based on the calculation and analysis of a large number of conditions,the exponential correlations of natural convection and pure forced convection between Ra,Re and Nu are fitted respectively.The self-scaling of natural convection,the entrance effect and viscosity effect of forced convection are studied.The phenomenon of heat transfer impairment and enhancement by buoyancy in mixed convection is analyzed,and the value of heat transfer impairment section and the maximum impairment point are given,and the modified Symolon mixed convection heat transfer correlation is fitted.Finally,from the perspective of local velocity gradient and turbulent kinetic energy,this thesis indicates that the reason for heat transfer impairment is laminarization,which means the ununiform buoyancy force lead to the reduction of turbulent kinetic energy near the wall,suppress the heat transfer from the boundary layer to the center of the flow.