Unsteady flow characteristics during runaway process in Francis turbine: Insights from numerical investigation

The presented paper numerically investigates the internal flow behaviors and energy dissipation during the no-load startup process toward a Francis turbine. Passive runner rotation is implemented through the angular momentum balance equation accompanied by dynamic mesh technology and user defined function. Three phases of rotational speed are identified: stationary, rapid increase, and slow increase. Head exhibits a monotonic decrease, rapid rise and fall, and eventual fluctuation. Flow rate shows quasi-linear increase. The pressure fluctuations in the vaneless region are primarily dominated by the frequencies induced by Rotor-Stator Interaction and a broad frequency range below 50 Hz, and below 30 Hz in the draft tube. Runner inlet experiences positive to negative incidence angles, causing intense flow separation and unstable structures. Draft tube exhibits large-scale recirculation and evolving vortex structures. Energy loss analysis based on the entropy production method highlights the runner and draft tube as primary contributors. The energy loss within the runner exhibits an initial increase, subsequent decrease, and then a rise again during the stationary and rapid speed increase phases. While the draft tube shows a rapid increase during the phase of rapid speed increase. Turbulent fluctuations significantly contribute to entropy production loss, with trends matching total entropy production. Maximum energy loss locations correspond to runner inlet and draft tube wall, emphasizing the importance of unstable flow and vortex generation. This study establishes foundational insights into unstable hydrodynamics and energy dissipation modes during hydraulic turbine no-load startup, paving the way for further research.

Numerical investigation of no-load startup in a high-head Francis turbine: Insights into flow instabilities and energy dissipation

Sun,

Liu,

et al. 2024

Numerical assessment of transient flow and energy dissipation in a Pelton turbine during startup

Sun,

Wang,

Zhou

et al. 2024

The Pelton turbine, known for its high application water head, wide efficient operating range, and rapid start-stop capability, is ideal for addressing intermittent and stochastic load issues. This study numerically analyzes the transient two-phase flow and energy dissipation during the startup of a Pelton turbine. Dynamic mesh technology controlled nozzle opening changes, and momentum balance equations managed runner rotation. Findings showed that the runner speed initially increased rapidly and then more slowly, and flow rate matched the nozzle opening variations. Runner torque first rose linearly, then decreased, with the fastest decline during nozzle closing. Hydraulic efficiency peaked early in nozzle reduction but then dropped sharply. Strong vortices formed due to upstream inflow and downstream backflow impact in the distributor pipe. The jet needle and guide vane improved flow in the converging section of nozzle, but flow began to diffuse with increased stroke. Initially, the jet spread fully on the bucket surface, but later only affected the bucket tips. Pressure fluctuations in the water supply mechanism were primarily due to jet needle motion, with higher amplitude during movement and lower when stationary. These fluctuations propagated upstream, weakening over distance. Reynolds stress work and turbulent kinetic energy generation, respectively, dominated energy transmission and energy dissipation, with their maximum contribution exceeding 96% and 70%. High-energy clusters corresponded to jet impact positions, highlighting jet-bucket interference as crucial for energy transport. This study established a performance evaluation method for Pelton turbine startups, supporting further investigation into characteristic parameters, flow evolution, and energy dissipation patterns.

Instability mechanism and vibration performance of a pumped storage power station under runaway conditions

Liu,

Yu,

Qiu

et al. 2024

With the large-scale access of renewable energy to the grid, the load rejection of pumped storage power stations (PSPSs) has become increasingly frequent, thus increasing the possibility of runaway accidents. This study aimed to investigate the instability mechanism and vibration performance of a PSPS by considering the coupling effect of the pressurized pipe and pump-turbine under the runaway condition. First, models of the PSPS based on the elastic water-column (elastic model) and rigid water-column (rigid model) were established. Subsequently, the vibration performances of the elastic and rigid models were compared. The comparison revealed that the runaway instability characteristics of the PSPS were mainly manifested as a high-frequency and large-amplitude vibration caused by the elastic water-column, which was not observed in the rigid model. Therefore, the elasticity of the water-column, which has normally been neglected or simplified using a rigid water-column in previous studies, has a significant effect on the runaway stability. Finally, the effects of other factors on the stability and vibration performance were clarified. The runaway stability was mainly determined by the characteristics of the runaway operating point. The discharge-head relationship coefficient (S5) for quantifying the stability performance of the runaway operating point was extracted. Moreover, increasing pipe friction loss could suppress runaway instability. Additionally, the pipe water inertia only affected the runaway vibration performance but not the runaway stability result. Overall, the study findings deepen the understanding of the physical nature of runaway instability and provide guidance for pump-turbine runner design and stable operation of PSPSs.