Experimental investigation of two-phase instability in natural circulation with direct transient local measurement

density wave;instability;flashing;natural circulation;local measurement

Zhang, Taiyang ; Uddin, Rizwan ; Brooks ; Caleb S.

关键词: density wave;    instability;    flashing;    natural circulation;    local measurement;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/106261/ZHANG-THESIS-2019.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Nuclear reactor systems can be susceptible to thermal hydraulic instability under specific circumstances. Proper handling of the stability problem under natural circulation is critical for the reactor safety and operation under important conditions such as the start-up transient of natural circulation cooled boiling water reactors. This experimental research focuses on the two-phase instability under low system pressure (1 - 10 atm) in a natural circulation loop where an adiabatic riser is present above the heated section. Direct transient local measurement of two-phase flow is performed at multiple axial locations along the flow channel with thermocouples and conductivity probes traversed by customized high-precision mechanisms. Periodic oscillation is captured when the vapor quality at the test section exit is close to zero. A post-processing method is proposed to extract the oscillation cycles and reconstruct the instability behavior in one representative period. An averaging operation over multiple oscillation cycles is performed to reduce the random errors. Cyclic phenomena are clearly demonstrated by the post-processed data, and rapid void fraction change induced by the flashing is observed to be significant in the driving mechanism for low pressure oscillations. Parametric effects of inlet subcooling, heat flux, inlet flow restriction and system pressure are studied within the experiment range, and qualitative results are explained and compared with existing literature. The ratio between the oscillation period and the travelling time of the fluid particle is compared with the theoretical value for the density wave oscillation.The conditions captured form a new dataset containing directly-measured transient multi-dimensional two-phase data along the flow channel for natural circulation under density wave oscillations. The generated dataset is valuable for the validation of safety analysis codes and CFD codes in the simulation of natural circulation phenomena and dynamic two-phase flow.