Variable-speed operation and pressure pulsations in a Francis turbine and a pump-turbine

Abstract

The paper presents result from model measurements of the efficiency and pressure pulsation intensities for two low-specific-speed hydraulic turbines operated at variable speed, namely, one splitter-bladed Francis turbine marked with "F99" and one reversible pump-turbine marked with "RPT" and operated in a turbine mode. Both turbines have similar specific speeds, i.e. ${n}_{QH}^{(F99)}=21.88$ and ${n}_{QH}^{(RPT)}=27.26$, and for their best efficiency points, both have similar guide-vane opening angles but different operating parameters (i.e., speed factor and discharge factor). Pressure pulsation measurements were conducted for a wide operating range and at specific locations in the (1) vaneless space and (2) draft tube. Histogram method was used to obtain the peak-to-peak amplitudes of the fluctuating pressure for all operating points used to construct the performance hill-charts of the turbines. To the best of the authors' knowledge, very little or no effort has been made so far to explore the amplitudes of pressure pulsations in the turbine when operated at rotational speeds specifically optimized for maximization of the hydraulic efficiency. Results show that operation of Francis turbines at optimized rotational speeds can increase the hydraulic efficiency of the turbine, while decreasing or maintaining the same pressure pulsation amplitudes in the entire operational range. Also, it was found that the level of efficiency gain and reduction of the pressure pulsations is greatly dependent on the hydraulic design of the turbine and should be investigated individually for each case.