CAVITATION EROSION IN FRANCIS TURBINES

Abstract

The phenomenon of secondary flow is a global problem that causes cavitation erosion in hydraulic equipment. Cavitation is a phenomenon of localised corrosion of the metal surface leading to instability and highly uneven flow behaviour with a consequent excessive noise, vibration, and decreased efficiency in Francis, Kaplan, and other turbines. Both Kaplan and Francis turbines are reaction turbines. Francis turbines (FT) are used worldwide due to their relatively compact design, high efficiency, and operation underwater at heights ranging from 100 to 300 m with an efficiency ranging from 90% to 95%. The article analyses the latest relevant research conducted by various researchers on different turbine components. The analysis shows that this type of erosion depends on flow characteristics, surface, and properties of the material eroding. Tools for design optimisation, cavitation erosion, and well-conducted experiments will provide results for identifying and reducing erosion. Although some researchers conducted experimental work to study the effect of cavitation erosion, literature on computational fluid dynamics (CFD) is very scarce. Over the past two decades, experts have been applying CFD methods to detect cavitation by examining areas where the pressure is below the vapour pressure with a single-phase model. Most studies do not consider the impact of cavitation bubbles on the flow field. However, these methods cannot provide detailed information, such as the impact of cavitation on efficiency or a more accurate prediction of the cavitation bubble size. Some researchers use cavitation inducers and some of the latest visualisation methods as experimental tools to study cavitation phenomena. In the last decade, a numerical methodology has been widely used in research and experiments, yielding significant results. Studying cavitation erosion in hydroelectric turbine systems presents a complex challenge for future research. Many parameters and features still require further investigation. All the discussed studies have established that cavitation phenomena require state-of-the-art equipment for their detection and visualisation. Moreover, more work is necessary for the numerical assessment of cavitation. Keywords: hydroelectric turbine, cavitation erosion, computational fluid dynamics.