Current study was aimed to investigate the unsteady pressure loading in a model Francis turbine under variable-speed configurations. Focus was to investigate the time-dependent characteristic frequencies and the pressure amplitudes. Detailed analysis of both stochastic and deterministic pressure loading in the vaneless space, runner and draft tube was conducted. Total 12 pressure sensors were integrated in the turbine, including four sensors in the runner. The runner rotational speed was changed by ±30% of the rated speed, and the guide vanes were at a fixed aperture. Total four operating conditions were investigated. The measurements showed that, in the vaneless space and runner, amplitudes of unsteady pressure fluctuations increase with the runner angular speed. Pressure field at the blade trailing edge is strongly influenced by the draft tube flow at part load and low load. The variable-speed configuration allowed power generation under the stable condition, where the vortex rope effect is low. However, this led to high-amplitude stochastic frequencies in the runner and draft tube. Overall, the pressure measurements indicated that not only efficiency but also detailed study on pressure fluctuations inside the turbine is vital before designing a runner for the variable-speed configurations.
This work investigates the unsteady pressure fluctuations in a hydraulic turbine that are observed during steep ramping. Although hydraulic turbines are expected to operate seamlessly during steep ramping, the resulting pressure amplitudes are so significant that they take a toll on a machine's operating life.Objective of the present study is to investigate time-dependent pressure amplitudes in the vaneless space, runner and draft tube during power ramping-up and -down under variable-speed configuration.Novelty is to vary both discharge and rotational speed of a runner. The measurements are performed on a high-head model Francis turbine. The investigations revealed that amplitudes of characteristics frequencies, especially rotor-stator interaction, are small during steep ramping however, at the end of transient cycle, the amplitudes quickly increased 30-fold. During steep ramping, blade passing frequency was appeared in the runner, which is uncommon phenomenon in high-head Francis turbines.Strong reflection of pressure waves towards runner from vaneless space (guide vane walls) may be one of the causes for the appearance of blade passing frequency in the runner.
An analysis of the pressure in a runner channel of a low-specific speed Francis model runner during resonance is presented, which includes experiments and the development of a pressure model to estimate both the convective and acoustic pressure field from the measurements. The pressure was measured with four pressure sensors mounted in the runner hub along one runner channel. The mechanical excitation of the runner corresponded to the forced excitation from rotor-stator interaction. The rotational speed was used to control the excitation frequency. The measurements found a clear resonance peak in the pressure field excited by the second harmonic of the guide vane passing frequency. From the developed pressure model, the eigenfrequency and damping were estimated. The convective pressure field seems to diminish almost linearly from the inlet to outlet of the runner, while the acoustic pressure field had the highest amplitudes in the middle of the runner channel. At resonance, the acoustic pressure clearly dominated over the convective pressure. As the turbine geometry is available to the public, it provides an opportunity for the researchers to verify their codes at resonance conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.