Abstract
An investigation is conducted on the Francis turbine’s cavitation characteristics and its influence on system hydraulic stability using two experimental methods, namely the flow visualization and acoustic emission methods. The investigated turbine is of Francis type with a 15-blade runner and has a specific speed of 202 rpm and a rated head of 30 m. Having tested the machine under a wide range of cavitation conditions, the gap cavitation is the earliest to develop as the cavitation coefficient gradually decreases and has no obvious effect on the machine’s external performance characteristics. The airfoil cavitation follows and causes the increase and decrease in machine flow rate and head, respectively, showing its drag reduction effect, where, at the same time, the pressure pulsation amplitude gets to its peak value. There is also the formation of constant cavitation zones and the involvement of an unsteady surge close to the wall of the draft tube’s cone. Pushing the cavitation coefficient to even lower values, there is the formation of an annular cavitation zone, accompanied by a sharp drop in cone pressure pulsation amplitudes while the former drag reduction effect disappears. The trend of noise is basically the same as that of pressure fluctuation, which confirms its trustworthiness when it comes to cavitation occurrence detection within Francis turbines.
Highlights
The and development of cavitation in the tested turbine model is studied, the influence of cavitationofoncavitation hydraulic in stability is analyzed invesInwhere this paper, the development the tested turbinethrough model the is studied, of eventual changes on in the machine’s external performance characteristics and wheretigation the influence of cavitation hydraulic stability is analyzed through the investiga◦
Tests were conducted at a constant guide vane opening of 35.5°
A large cavitation zone formed in the channel and blocked the fluid movement, resulting in the increased resistance to water flow, which in turn led to the decrease in the flow rate and subsequent water head increase
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Owing to its ability to quickly change its operations from high to low power generation or vice versa, hydropower ranks among the most efficient means to deal with RES-inflicted grid instabilities [13,14] In these plants, Francis turbines constitute roughly 60% of the worldwide installed capacity and are either used for base load generation or, as mentioned above, stabilization of electrical power grids [15]. With a continuous decrease in cavitation number, the cavitation volume increased both at the runner blade’s trailing edges and within the core of a draft tube axisymmetric cavitating vortex rope The latter exhibited self-excited collapses and re-emergence behaviors for very low cavitation numbers, which generally leads to extremely high pressure pulsation amplitudes [53]. The experimental findings presented in this article provide support for the cavitation performance optimization for turbine runners
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