Phenomena, mechanism, and prediction of flashing instability investigated by experiments, analytical modeling, and numerical simulation

Understanding and predicting flow instability are challenging topics paramount to the reliable operation of nuclear reactors. In start-up transients, some reactors may experience low-pressure low-flow-rate conditions susceptible to flow instability. Flashing instability is one of the most widely reported two-phase flow instabilities in low-pressure natural circulation. The knowledge of flashing instability is therefore of practical interest to natural-circulation reactors. This dissertation is correspondingly devoted to enhancing the current understanding of flashing instability by investigations through experiments, analytical modeling, and numerical simulations. On a 5-meter-tall low-pressure natural circulation loop, stability tests capture the occurrence of flashing instability and record its consequences. Utilizing a novel technique based on movable sensors and ensemble averaging, periodic oscillations are measured. This generates high-quality two-phase data beyond steady states, with statistically quantified uncertainties and resolved lateral two-phase structures. A new benchmark dataset of flashing instability is therefore collected capturing both stability boundaries and the long-term asymptotic flow behaviors. Linear stability models are then developed with tractable formulations analytically derived from physical simplifications. Validation confirms acceptable predictions of steady-state flow rate, stability boundaries, and oscillation periods. Three dominant pressure responses to flow perturbations are extracted. The triggering mechanism of flashing instability is also identified. Some qualitative features, such as the trend of stability, magnitude of a time-scale ratio, and preferred oscillation modes, are explained analytically and physically. These analytical models contribute new theoretical bases to physically understanding flashing instability. Validations of a system analysis code, ASYST, are conducted in simulating flashing instability by numerically solving 1-D Two-Fluid Model. Underprediction of void fraction is observed in single-channel simulations with prescribed periodic boundary conditions. Its cause from potential modeling defects is revealed by case studies. Full-loop simulations are found underpredicting the unstable range in high-subcooling operational regions but reasonably capturing the stability boundary with low subcooling. In addition, for periodic conditions with predicted instability, ASYST exhibits generally acceptable performance even in predicting oscillation waveforms. In general, this dissertation contributes to the knowledge of flashing instability with a new experimental dataset, tractable theoretical models, and the validation of a system analysis code.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Zhang, Taiyang

Contributors dc:contributor

• Brooks, Caleb
• Uddin, Rizwan
• Kozlowski, Tomasz
• Chamorro, Leonardo

• Flashing Instability
• Natural Circulation
• Stability Test
• Transient Two-phase Measurement
• Linear Stability Analysis
• Analytical Solution
• Instability Mechanism
• Thermal Hydraulic System Analysis
• Numerical Simulation
• Validation

Statement dc:rights

• Copyright 2023 Taiyang Zhang

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/121421

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F121421