Study on Scaling Method for Natural Circulation Instability in Narrow Channel

Abstract Natural circulation is an important phenomenon in the passive safety system of advanced nuclear reactors. Two-phase flow instability occurring during natural circulation may cause severe mechanical vibrations, interfering with the control system and causing thermal fatigue failures. Therefore, understanding the characteristics of two-phase flow instability in natural circulation and accurately predicting the flow instability onset is of great significance for improving the safety of the reactor system. In order to improve and validate the prediction of flow instability onset, and to balance with the feasibility, timeliness, and economical efficiency factors of the test facilities, practitioners usually choose to study the prototype based on test facilities with different scales. The scaling method plays a key role in the design of these test facilities, as it provides a theoretical basis for mapping test facilities results to the prototype efficiently, which share certain physics in common. The prototype model can be validated with the test facilities using the scaling methods, as test facilities can explain part of the prototype phenomena. The results on scale facilities can guide the design improvement of prototype. This work focuses on understanding the characteristics of two-phase flow instability in natural circulation using the scaling methods. The objective of this work is to increase the accuracy of two-phase flow instability onset prediction model. This two-phase natural circulation system works with a narrow rectangular channel and uses water as flow medium under high pressure. The flow instability onset occurs during the transient process, which is caused by a step-type power change. The natural circulation system is consist of one preheating section, one condensation section, and one pressurizer. Scaling analysis was performed employing various methods, including Hierarchical, two-Tiered Scaling (H2TS) method and Dynamical System Scaling (DSS) method, to predict the flow instability onset of this narrow channel two-phase natural circulation system. To evaluate the instability similarity, the differences and variation laws of flow instability onset mass flow rate and heat flux between prototype and scale model were calculated and analyzed. And the important similarity criterion numbers of prototype and scale model, such as the subcooling number (Nsub) and phase change number (Npch), were calculated and analyzed, as well as the flow instability boundary maps under different scale sizes. The ratios of important similarity criterion numbers between prototype and scale model were calculated and analyzed. Scaling analysis results among different scaling methods were compared and discussed in terms of their performance. Results show that the prototype model validated by test facilities provides reasonable predictions. The ratios of simulation results of key thermal-hydraulics parameters of prototype and scale model instability can basically reflect the prototype changes. It lays the corresponding theoretical foundation for further analyzing the new scaling method to represent the flow instability onset.