Thermal Hydraulic Characteristics Of Loop Heat Pipe For Passive Containment Cooling System

On 11 March 2011,the Fukushima nuclear power plant in Japan was affected by the tsunami and all cooling equipment using electric power could not operate without power in the entire plant,which resulted in a nuclear accident and continuing damage to the environment till today.In response to such extreme accidents,countries are seeking and designing passive containment cooling system(PCCS)that do not rely on external power and water sources.Among them,PCCSs based on loop heat pipes(LHPs)have received much attention and research because they can effectively suppress the pressure and temperature rise inside the concrete containment and prevent the leakage of radioactive materials due to the overpressure failure of the containment.However,LHP-PCCS is a large scale and space engineering system with high reliability requirements.Operating parameters that affect the thermal hydraulic characteristics of the LHP,such as the containment pressure and filling ratio,directly determine whether the system can be properly started and maintained in stable operation.Furthermore,when the evaporator,which is an important part of the LHP,operates at low heat flow densities,the gas-liquid distribution and heat transfer coefficient inside the tube determine how much heat can be transferred from the loop.Therefore,it is important to study the influence of the filling ratio and containment pressure variation on loop thermodynamics,explore the gas-liquid distribution in the evaporator,establish a PCCS thermodynamic calculation model,and propose a new PCCS design and performance evaluation method,which is of great application value to improve the stable operation and safety of third-generation nuclear power plants.An experimental platform with an LHP using water as the working medium is designed and built in this paper to investigate the stable working condition of PCCS.The influence of the containment pressure and filling ratio on the heat transfer,operating temperature,heat transfer coefficient and pressure distribution at each part of the loop is separately examined,the operating parameters of the evaporator in the loop are analyzed and the heat transfer ability at each part of the loop is investigated with thermal resistance.Experimental results show that:when the containment pressure changes,a)it will change the effect of the noncondensable gas partial pressure inside the containment on the steam condensation on the outer wall surface of the evaporator;b)it will change the increase in steam saturation temperature inside the containment and the loop operating temperature,which will affect the driving temperature difference of the loop.When the filling ratio changes,the area of effective heat exchange of saturated boiling in the loop will be affected.When the containment pressure is 0.32-0.36 MPa,a lower filling ratio corresponds to a lower starting position of nucleate boiling,and more benefit from enhancing the heat exchange capacity of the evaporator.When the containment pressure is 0.36-0.46 MPa,a higher filling ratio corresponds to a more effective heat exchange area of saturated boiling,and more benefit from enhancing the heat exchange capacity of the evaporator;however,all filling ratios correspond to a unique containment pressure,which allows the LHP to have the highest heat transfer.In addition,the evaporator heat flow density and subcooling are low and the evaporator tube is not a fully developed nucleate boiling,but rather a mechanism where nucleate boiling heat exchange acts in conjunction with convective heat transfer.This paper establishes a numerical model for heat transfer by flow boiling in an evaporation tube,and preform numerical simulations of flow boiling at different filling ratios and tube diameters to investigate the influence of the filling ratio and tube diameter on the flow pattern and heat transfer characteristics in an evaporation tube and reveal the impact mechanism from the perspective of bubble behaviors,such as bubble detachment,aggregation,merging and rupturing.Based on this impact mechanism,a numerical simulation study is performed to analyze the evolution of the flow pattern,heat transfer characteristics,bubble behavior and flow stability in the evaporation tube for practical engineering applications.The simulation results show that a lower filling ratio,corresponds to a lower position of the two-phase flow level,a larger ineffective heating surface and a smaller heat transfer capacity of the system;a larger tube diameter,corresponds to a lower the height of the two-phase flow level,a larger ineffective heat transfer area and a lower equivalent heat transfer coefficient.In small diameter tubes,bubble,cap and slug flow will occur,but continuous gas slugging will cause significant pulsation of the liquid surface.In larger tubes,although slug flow will occur,the slug bubble does not stably exist and will rapidly change to a churn flow when the slug bubble grows to a certain size.Based on the experimental and simulation results,a thermal hydraulic calculation model for LHP-PCCS is established by analyzing the thermal resistance network and flow process in the LHP by coupling the boiling heat transfer,flow resistance and void fraction distribution in the evaporator with one another using the drift flow model.The new global optimization theory is used to obtain the optimal structure of the PCCS and its corresponding unique operating temperature.A new method to evaluate the reliability of the PCCS is proposed,which considers of the filling ratio and noncondensable gas.A sensitivity analysis is completed for three key parameters:the containment pressure,filling ratio and cooling tank temperature,and an effective prediction of the PCCS start-up characteristics is obtained.The calculation results show that an increase in cooling tank temperature decreases the temperature difference between the PCCS heat source and the cold source,which significantly increases the boiling heat transfer thermal resistance of the outer pool of the condenser tube,increases the total system thermal resistance,and consequently reduces the heat transfer capacity of the system.A step in the system heat transfer capacity may occur during PCCS start-up,which is due to the change in heat transfer mode of the outer surface of the condenser at different cooling tank temperatures.If the step occurs,PCCS can be successfully started.