Session: 3.4 - CFD for Nuclear Thermal Hydraulics

Paper Number: 170186

170186 - Numerical Investigation of a Control Valve for High-Temperature Gas-Cooled Reactors With Large Pressure Drop 

Abstract: 

Control valves play a crucial role in regulating fluid flow under varying operating conditions in the secondary circuit of fourth-generation high-temperature gas-cooled reactors (HTGRs). Some of these valves are designed to accommodate both liquid and gas phases, resulting in distinct throttling characteristics depending on the fluid state. To ensure feasibility across different pressure conditions, a multi-stage throttling mechanism is employed. The upper section of the valve features a perforated cylindrical sleeve optimized for low-pressure-drop conditions, while the lower section consists of stacked throttling plates designed to manage high-pressure-drop scenarios by dissipating energy and mitigating cavitation risks.
A primary concern under high-pressure-drop conditions is the phase transition of liquid water into steam. As high-pressure liquid water passes through the throttling plates, rapid depressurization induces vaporization, leading to two-phase flow within the valve. This phenomenon introduces additional complexities, including flow-induced vibrations and noise, which may affect the operational stability of the valve.
To assess the feasibility of the valve design, this study employs computational fluid dynamics (CFD) simulations to investigate the internal flow characteristics under two representative operating conditions: (1) a low-pressure-drop scenario where steam enters the valve and (2) a high-pressure-drop scenario where liquid water serves as the inlet fluid. In the latter case, a cavitation model is applied to account for phase transition effects. The study examines pressure distribution, velocity profiles, and phase change phenomena in both conditions.
The simulation results indicate that in the low-pressure-drop condition, steam experiences a moderate pressure drop due to flow resistance, leading to minor density variations. In contrast, under high-pressure-drop conditions, the throttling plates induce a substantial pressure reduction, causing localized vaporization near the plate exits. This vaporization leads to a decrease in local temperature, which suppresses further bubble growth, resulting in a liquid-gas mixture at the valve outlet rather than a fully vaporized flow. Furthermore, two-phase flow conditions introduce periodic pressure fluctuations within the valve, leading to small-amplitude flow-induced vibrations. However, these vibrations remain within acceptable limits and do not compromise the valve’s operational integrity.
These findings underscore the importance of understanding two-phase flow behavior in control valves and provide valuable insights for optimizing valve designs to enhance stability and reliability in HTGR applications.
 

Presenting Author: Shunyang Li Tsinghua University

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