Hydropower is one of the most reliable renewable sources of electricity generation. With high efficiency and good regulating capacity, hydropower has the ability to meet rapid changes in power demand. Large investments in intermittent renewable energy resources have increased the demand for balancing power. This demand has pushed hydraulic turbines to generate electricity over the operating range from part load to full load. High-amplitude pressure pulsations are developed at off-design conditions, which cause moderate damage to the turbine components. The pressure pulsations may be either synchronous-(axial)-type, asynchronous-(rotating)-type or both. In this study, pressure measurements on low specific-speed prototype Francis turbines were performed; one of them was vertical axis and another was horizontal axis type. Four pressure sensors were mounted on the surface of the draft tube cone. Pressure measurements were performed at five operating points. The investigations showed that, in the vertical axis turbine, amplitudes of asynchronous pressure pulsations were 20 times larger than those of the synchronous component; whereas, in the horizontal axis turbine, amplitudes of asynchronous pressure pulsations were two times smaller than those of the synchronous component.

speed. For the start-up cycle, the guide vanes are initially opened by a few percentages from a completely closed position. As the discharge to the runner increases, the runner starts spinning at a synchronous speed to that of the corresponding turbine. The generator then is coupled to the grid network at a minimum load. After synchronization, the power output is further increased to the set value by increasing the discharge to the runner. Unsteady pressure measurements observed during the transient cycles of a Francis turbine model indicated that a turbine could experience more than twice the pressure amplitudes observed at the best efficiency point (BEP). 13-15 Pressure measurements in a draft tube indicated that high amplitude pressure oscillations occur during the load variation and start-stop cycles. The oscillations were nearly three times higher than that of the normal operating conditions. The literature indicated that the repeated transients affect the operating life of the turbine components. 16-19 During a transient condition, the turbine passes through a rapid pressure variation and experiences low-cycle fatigue. 20,21 The repeated fatigue loading to the runner blades initiate cracks where the blades are welded. 22 Pressure pulsations developed in a draft tube are primarily related to the vortex breakdown. The pressure pulsations are composed of two different phenomena that occur simultaneously at the same frequency, which may be synchronous (axial) and asynchronous (radial) types. 23,24 The synchronous component may have equal phase and amplitude in the runner and the draft tube. The pressure may be considered as a plane wave propagating to the hydraulic system through the draft tube. The asynchronous component is a pressure pattern that develops at the runner downstream and that rotates about the circumference of the draft tube. The rotation period is dependent on the circumference and the runner angular speed. The synchronous component may not be present at high load conditions. In a straight/conical draft tube, only asynchronous type pulsations exist. 25 Both synchronous and asynchronous types of pressure pulsations cause different impacts on turbine operation. It is important to analyze and distinguish such pulsations properly. During the steady-state operating condition of a turbine, the pulsations are periodic and may repeat over certain periods of time. 26-29 However, during load variation and start-stop, the pressure pulsations are non-repeating and largely dependent on the runner's instantaneous speed as well as guide vanes' apertures at that time. Recently, velocity measurement conducted by Goyal et al. 30,31 indicated that the vortex structure in the draft tube evolves over a certain time as the runner accelerates/decelerates. The study showed that, unlike a steady-state load, 32-35 amplitudes of both types of pressure pulsations vary with time as the load changes from one operating point to another. The frequency of synchronous pulsations appeared before the asynchronous pulsations during...

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