Investigation On The Characteristics Of Two-phase Boiling Heat Transfer And Differential Pressure Fluctuation In Rod Bundle Channel

The thermal-hydraulic problems in the core is typical of gas-liquid two-phaseflow and heat transfer problems when loss of coolant accident occurs in thepressurized water reactor and boiling water reactor. Due to the impact of special rodbundles geometry, heat transfer and flow characteristics is more complex. So far, onthe one hand breadth and depth for heat transfer and flow characteristics in the rodbundle far not reaches the degree of circular tube research, and the heat transfermechanism not yet achieves satisfactory theoretical explanation. On the other hand,special rod bundles geometry lead to the significant differences of flow pattern andheat transfer characteristics through the bundle channel with common circular tube.Then the relevant theories of circular tube is not directly applied to the bundlegeometry. Based on this, the flow and heat transfer issue in the bundle channel is stillone of the hot spot of current research.A rod bundle channel flow boiling experimental test rig with self-designed andprocessed test section is established in this study. The experimental study wasconducted under the pressure ranging of0.1MPa to0.12MPa, the mass velocityranging of0~600kg/(m2s)and the inlet sub-cooling ranging of10℃to30℃. Theboiling incipience and flow boiling heat transfer was studied in the rod bundlechannel. A visualization study was also conducted to define and explore the observedflow pattern, and a flow pattern map was obtained based on the experimental data.Additionally, differential pressure fluctuation signals were studied to utilizenon-linear analysis method.By sorting and statistics the experimental data, factors affecting the onset ofboiling incipience including sub-channel position, flow rate, heat flux density isanalyzed. We make the comparison with the experimental data and the predictedresults of the common Bergles correlation. It is found that the error of the predictedresults is big. Therefore, based on the Bergles correlation, we fit the more accurate the correlation calculating the incipient boiling heat flux density. In the study of flowboiling heat transfer, the effect of heat flux, mass flux on the flow boiling heattransfer coefficients is analyzed. Heat transfer characteristics in the differentsub-channel is examined. Four correlations including Liu-Winterton、Kandlikar、Gungor-Winterton、 chen are adopted to predict the flow boiling heat transfercoefficient. The comparison of predicted results and experimental results is made andthree statistic indicator are used to assess the four correlations. The results shows thatthe Gungor-Winterton correlations are more accurate under this experimentalconditions. We also study the differential pressure fluctuation signals adoptinghierarchical entropy and multi-scale entropy algorithm. The results shows thathierarchical entropy as a nonlinear analysis method can represent the complexity oflow frequency and high frequency signals and effectively subtract noise in signal aswell as reveal its internal characteristics. It also has better robustness. However,multi-scale entropy can reveal the dynamic complexity of different flow pattern indifferent scales, and the multi-scale entropy rate effectively identify the four kinds oftypical flow patterns within the rod bundle channel. It has been demonstrated thatmulti-scale entropy has the superiority for complex time series analysis.