Abstract
Cavitation erosion is the most important erosion mechanism in Francis turbine runner blades. For this reason, knowledge of a material’s ability to resist cavitation is important in defining how suitable it is for use in a Francis turbine. In this study, three Francis turbine materials were subjected to cavitation erosion in a high-speed cavitation tunnel. One of the materials was a low-alloy steel, and the other two were stainless steels. The cavitation tunnel produced an annular cavitation field on one face of a cylindrical specimen. The test specimens underwent cavitation erosion until the erosion had reached a maximum penetration depth of about 0.5 mm. The material surface profiles were measured at regular intervals to calculate volume and mass loss. These losses were compared to those of several other materials that had undergone the same tests with the same setup and operational parameters. The materials were compared according to their steady-state erosion rates. The steady-state erosion rate represents a material’s ability to resist cavitation erosion once cavitation damage has already started to develop. The low-alloy steel eroded four times faster than the two stainless steels. One of the stainless steels tested here (Stainless steel 1) had the lowest erosion rate, along with another previously tested stainless steel. The other stainless steel (Stainless steel 2) had a slightly greater erosion rate than the first, falling into the same class as other lower-grade stainless steels and a nickel aluminum bronze alloy. The results show that in choosing a turbine blade material, stainless steels outperform nonstainless ones. The choice of which type of stainless steel to use is significant in turbines with cavitation problems. The eroded surfaces were analyzed with scanning electron microscopy in order to study the erosion mechanisms, and these studies showed that most of the damage is probably due to low-cycle fatigue.
Highlights
The use of renewable energy sources, such as wind or solar power, is increasingly common in electricity production, which means that a large amount of power with modifiable output is required
Our results show that the low-alloy steel has the poorest resistance to cavitation, falling into the same category as the 7075 aluminum alloy in terms of volume loss rate
The stainless steels were specimens from turbines that are of a type that is still in use, so they are relevant in the field of turbine design
Summary
The use of renewable energy sources, such as wind or solar power, is increasingly common in electricity production, which means that a large amount of power with modifiable output is required. The reasons for studying cavitation erosion resistance in a hydrodynamic testing rig, rather than in a static liquid, have been highlighted by Karimi and Avellan in Ref. The hydrodynamic methods produce both different-size bubbles and flow vorticity effects, as well as other interactions with the liquid flow For these reasons, it is often regarded that a hydrodynamic method, such as was used in this study, provides a better comparison of materials than the static liquid method. Deplancke et al [25] have studied the erosion of ultrahigh molecular weight polyethylene coatings, using the same operating parameters as Franc et al [24], and they found that such coatings are highly resistant to cavitation This type of coating could protect against cavitation in hydro machines, if it were economically feasible. The SEM images allowed the resulting cracks and removed particles to be studied in detail
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