Investigation On The Local Temperature Field And Two-phase Flow Distribution Characteristics In The External Vessel Cooling System

External reactor pressure vessel cooling (hereinafter referred to asERVC), as one of the severve accident mitigation measurements, has beenextensively studied and widely applicated in advanced PWRs becasure ofits simplified structures, good economy and passive safety features, andetc. The ERVC cooling capacity could be affected mainly by the local voiddistribution in the channel and heat stratification in the cooling pool.Currently, most of the existing experimental and numerical studies onERVC focus on the engineering applications. However, the researches onthe ERVC two-phase distribution and thermal stratification are very rare.This dissertation mainly focuses on advanced PWR external cooling flowchannel local void fraction and temperature distribution. The outcomingsof this dissertation are providing analysis and summaries on the featuresand mechanismes of ERVC local void fraction distribution and thermalstratification, and detailed experimental data. FLUENT simulations areperformed according to the experimental parameters, and the CFD models are evaluated with test data.During the studies on the two phase flow, the local void distributionsin the ERVC flow channel were measured with conductivity pobes. It wasfound that the local void distribution along the cross-section is mainlyaffected by the measuring point positions and air/water flow rates. Thedistribution of the void fraction was quantitativly analyed with a statisticalmethod using coefficient of kurtosis and skewness, and it was showed thatthe local void fraction distribution at the bottom of the RPV lower head iswall-peak distribution, while the upper part of RPV lower head isnear-wall peak distribution, and in the vertical pipe is basically developedinto a center peak distribution. The CFD simulation was carried out with atwo-dimensional modeling and experimental parameters; Euler-Euler andEuler-Mixture two phase models were used, respectively. Comparisonbetween the simulation results and test data shows a large difference;Though Euler model simulation result could show a similar distributionwith the test results by modifying the lift force coefficient, it is stillrequired many modifications on the FLUENT simulation to obtainaccurate results.During the investigations on the thermal stratification in the ERVCcooling pool, the spatial distribution and time histories of temperature stratification, and the effect of circulating flow rate and other factors onthermal distribution were analyzed and summarized. Experimental resultsshow that water in the tank stratified mainly in the gravity direction, and itis believd that one-dimensional model could be used to describe thetemperature distribution in the water tank. The circulation flow rate showsa critical effect on the development and distribution of thermalstratification, and the effect of initial temperature field, thermalstratification between cooling rate and other factors are very small.1:1scale,3D FLUENT simulations were performed to calculate thethermal stratification pheonema in the tank. The simulation results show agood agreement with experimental data, and it is concluded that FLUENTcode has a good capability to simulate the heat stratification and itsdevelopment process in the cooling pool.