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Study of pressure pulsations in a Francis turbine designed for frequent start-stop

Chirag Trivedi

Journal of Energy Resources Technology (2021)

The energy market aims for high flexibility that allows guaranteed power to the consumers with minimum carbon footprint. Over the last three decades, hydropower has been a reliable and an efficient option to meet the fluctuating energy demand as it allows high-ramping rate and quick start-stop. However, such critical operations bring certain challenges for hydro turbines, i.e., high-amplitude stochastic loading and the fatigue. Credible investigation of unsteady pressure pulsations and their signature is important to understand the consequences. The present work aims to study pressure pulsations in a model Francis turbine, which is designed for the frequent start–stop operations. A total of 112 numerical simulations, across the hill diagram, are conducted. Pressure pulsations in the vaneless space, blade channels, and draft tube are investigated. The results show distinct patterns of rotor-stator interaction frequencies in the turbine. In the regions of high rotational speed and low flow rate, stochastic pulsations are predominant.

Figure shows the signature of pressure pulsations in the vaneless space, point VL1. (a) Signature of pressure pulsation of blade passing frequency fb and the harmonic 2fb at the point VL1 and n*ED=1.06. (b) Comprehensive overview of signature of blade passing frequency fb and the harmonic 2fb across the hill diagram. (c) Unsteady pressure pulsations at very low load turbine operation, n*ED=1.06, Q*ED=0.13, indicate the blade passing frequency fb and the harmonic 2fb on trough position. (d) Unsteady pressure pulsations at high load turbine operation, n*ED=1.06, Q*ED=1.28, indicate the blade passing frequency fb and the harmonic 2fb on node position. (e) Illustration of guide vane opening position at 10% and the point VL1. (f) Illustration of guide vane opening position at 140% and the point VL1.

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Time-dependent inception of vortex rings in a Francis turbine during load variation: large eddy simulation and experimental validation

Chirag Trivedi

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| Journal of Hydraulic Research (2020)

The present study investigates a transient condition, i.e. load variation from the best efficiency point to the part load, of a Francis turbine. A complete turbine is considered for the study. The dynamic mesh approach is implemented to close the guide vanes and to perform load variation numerically. The primary goal is to investigate the inception, propagation and interaction of vortex rings as load on the turbine changes. The secondary goal is to determine characteristic frequencies. The continuous change in flow angle, due to the closing of the guide vanes, induces strong boundary layer separation in the runner. Flow decelerates along the blade and establishes an adverse pressure gradient due to steep curvature of the blades. Separated flow from the boundary layer creates two swirling regions attached to the blade suction side. Fully developed vortex rings exiting from the adjacent blade channels interact and induce high-amplitude stochastic fluctuations at the runner outlet

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