Research On Condensation Heat Transfer In The Presence Of Noncondensable Gas In The Passive Containment Cooling System

On March 11,2011,the Great East Japan Earthquake occurred.A serious nuclear leakage accident happened at the Fukushima nuclear power plant in Japan,which shocked the world.In order to avoid such accidents,countries have rapidly carried out the researches on passive containment cooling systems(PCCS)that do not rely on external AC power and external water sources.Loop Heat Pipe(LHP)is widely used in thermal control system of space equipment.LHP is also a potential passive cooling method for nuclear reactors.Based on this,the researchers propose a new passive containment cooling technology which named LHP-PCCS.However,due to the presence of air in the containment,the noncondensable gas seriously affects the condensation heat transfer coefficient.Furthermore,air remains after the vacuuming process of the loop heat pipe,which reduces the condensation heat transfer of the steam in the loop in the condenser.The influence of noncondensable gas on the condensation heat transfer of water vapor outside the evaporator tube and in the condenser tube are studied,and the enhanced heat transfer measures are proposed,the systemic LHP-PCCS heat transfer model is established,and the heat transfer performance of LHP-PCCS by the heat transfer enhancement measures is evaluated.It has important theoretical significance for realizing the optimization and upgrading of our country’s third-generation nuclear power technology models and further improving the safety and economy of units.In this thesis,visualization experiments for condensation heat transfer with the presence of noncondensable gas outside a vertical tube are conducted.Through selecting the experimental tubes with different surface characteristics,the experiments were carried out on the heat transfer of filmwise and dropwise condensation outside a vertical tube.For the filmwise condensation heat transfer,a thermal resistance model for condensation was established to quantify the liquid film thermal resistance and the thermal resistance of the noncondensable gas layer.The gas-liquid interface temperature in the condensation process was determined by an iterative method.The heat transfer characteristics of filmwise condensation in the presence of noncondensable gas were explored.The influence mechanisms of air mole fraction,wall subcooling and steam partial pressure on condensation heat transfer were also analyzed.For the dropwise condensation heat transfer,the heat transfer characteristics of dropwise condensation in the presence of noncondensable gas were studied on the surface of the modified tube.The heat transfer performance of the filmwise condensation under the same conditions was compared.The compared results show that dropwise condensation can significantly enhance the heat transfer capacity of filmwise condensation in the presence of noncondensable gas.However,the enhancement decreases with the increase of air mole fraction.When the air mole fraction is 30%,the heat transfer enhancement by dropwise condensation was 33.7%.In this thesis,the condensation heat and mass transfer of steam-air mixture in a multichannel condenser is studied by a numerical simulation.The flow and heat transfer of the mixture are solved by the transport equations of mass,momentum,energy and components.The liquid film is independently calculated using the Eulerian Wall film model.The coupling calculation between the gas and liquid phase is performed by the phase change model based on the diffusion layer theory.The simulated results are as follows:a large amount of air is distributed in the tube bundle of the condenser,rather than in the upper or lower header.The air accumulation of in the condenser reduces the effective heat transfer area of pure steam condensation.At the same time,the heat transfer coefficient on the internal wall of the condenser is also affected by the internal flow field.Therefore,the overall heat transfer performance of the condenser decreases with the air mass linearly.The heat transfer performance can be effectively improved by setting a suitable pressure equalizing orifice plate in the upper header.Compared with the condenser without pressure equalizing orifice plate,most of the air in the condenser with the improved structure enters the lower header,and the heat transfer performance of the condenser is greatly improved,which has a maximum of 87.8%.Based on the above research on the local condensation of noncondensable gas,this paper further analyzes the coupled flow and heat transfer in the LHP-PCCS loop,and establishes a LHP-PCCS thermal-hydraulic model.Based on the objective function of minimizing the total heat transfer area of the system,a new design method of LHP-PCCS two-phase loop is proposed,and Matlab code is developed to solve the LHP-PCCS structure,saturated fluid temperature and the optimal liquid filling ratio.Based on this code,a performance evaluation model of LHPPCCS including the influence of noncondensable gases in the containment and in the loop was established to study the effect of the air mass fraction in the containment and the air partial pressure before the loop operation on the heat transfer of the LHP-PCCS two-phase loop influence mechanisms and characteristics.Combined with the strengthening measures of condensation heat transfer outside the evaporator and the improvement of the condenser structure,the heat transfer performance of the PCCS with the condensation heat transfer enhancement is analyzed by solving the thermalhydraulic model.The results show that the dropwise condensation heat transfer can increase the heat transfer capacity of the PCCS by 11.9-12.8%.As there exists the noncondensable gas in the system,the improvement of the condenser structure can improve the system power by 13.8%to 42.5%.The increment gradually increases with the increase of the air partial pressure before the loop runs.