Experimental study of a Francis turbine under variable-speed and discharge conditions

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.