Study On Flashing In Natural Circulation System

The flashing occurring in the riser is one of the key phenomena for the natural circulation system in terms of driving force, especially under lower pressure condition. In this thesis, the flashing phenomenon is studied in the natural circulation system with long adiabatic riser under lower pressure. The main concern lies in the studies on the onset condition of flashing and the two phase flow characteristic across the flashing zone, which are carried out theoretically, experimentally and numerically. The main work and the results of the studies are as follows.1. On the basis of the theories of thermodynamics equilibrium, homogeneous nucleation and chemical potential, the conditions under which the flashing may occur are studied along the flowing channel. Afterwards, the thermodynamics non-equilibrium effects are introduced in the model setup. The results generated from thermodynamics equilibrium and homogeneous nucleation theory show great difference as compared with the phenomenon observed in experimental study. Furthermore, it can be concluded that the chemical potential difference is a key factor for the phase change besides the temperature difference. Moreover,the model for the critical bubble radius is also deduced on the occurrence of flashing and the meaning of the critical bubble radius is explained in this thesis.2. A natural circulation facility operating at lower pressure was used to investigate the onset of flashing and the influencing factors by both the visualization observation method and the key thermal hydraulic parameters data acquisition and analysis. The feasibility of the published model of the onset condition for the flashing is evaluated through the comparison between the predicted results and the experimental data. On the basis of the classical nucleation theory, the prediction model for the onset condition of the flashing is set up and evaluated. The relative error of the mode lies in ±16 percent as compared with experimental data.3. The mathematical model for the flow characteristics accompanied with flashing evaporation is established based on the heat transfer between phases either taken thermodynamics non-equilibrium effects into account or not. In this model,different heat transfer models are used in terms of flow pattern and the heat tranfer through both conduction between two phases and covection are considered. The thermodynamics non-equilibrium effects consideration can amend the shortcoming of the flashing model with thermodynamics equilibrium assumption. The model can also be applied to analyze the flashing process and its effects in natural circulation system with lower pressure. In addition, the predicted results show good agreement with the experimental data.4. The two-phase thermodynamic equilibrium model was also introduced to simulate the flow process with flashing evaporation. Through comparison of the results from the non-equilibrium thermodynamics model and the thermodynamics equilibrium model, it can be found that the results show the similar trends and the results of the model taking the thermodynamics non-equilibrium effects into account match better to the expermental value than the thermodynamic equilibrium model.