Cavitation erosion prediction based on a multi-scale method

Effect of cavitation inducers' apex angle on erosion resistance of SS304 in the slurry pot test rig

Cavitation erosion would cause material loss and structural damage to the flow components of hydraulic turbines, which reduces their hydraulic efficiency and threatens the turbine's stable operation. Based on the ultrasonic cavitation test device, this paper investigates the effect of ultrasonic amplitude on the cavitation erosion of hydraulic turbine materials. The results show that the number and the volume of cavitation bubbles below the ultrasonic horn tip increased with the ultrasonic amplitude. The weight loss of S165, 1045, and S355 at ultrasonic amplitude 79 μm was 2.5, 1.67, and 1.84 times that at ultrasonic amplitude 52 μm, respectively. When the ultrasonic amplitude was 79 μm, the weight loss of S355 was 4.53 times that of S165. As the ultrasonic amplitude increased, the time for 1045 and S355 cavitation erosion to enter the maximum rate stage was shortened, while the S165 was still in the acceleration stage after 5 hours' test. The grain-boundary sliding and exfoliation of S165 were intensified, and the cavitation erosion pits in 1045 and S355 became larger and interconnected to form larger cavitation erosion pits with the ultrasonic amplitude. The number of cavitation erosion pits was more in S355 than in 1045, and the detachment of large-volume pearlite grains in 1045 and S355 at higher ultrasonic amplitude led to the large-volume cavitation erosion pits. The roughness of the three materials increased with the ultrasound amplitude. The roughness of 1045 and S355 was twice that of S165. The results were significant for the cavitation erosion protection of turbine materials.

The phenomena of cavitation and cavitation erosion affect hydraulic machines, increasing their maintenance costs. Both these phenomena and also the methods of preventing the destruction of materials are presented. The compressive stress in the surface layer created from the implosion of cavitation bubbles depends on the aggressiveness of the cavitation, which in turn depends on the test device and test conditions, and also affects the erosion rate. Comparing the erosion rates of different materials tested using different tests devices, the correlation with material hardness was confirmed. However, no one simple correlation was obtained but rather several were achieved. This indicates that in addition to hardness, cavitation erosion resistance is also affected by other properties, such as ductility, fatigue strength and fracture toughness. Various methods such as plasma nitriding, shot peening, deep rolling and coating deposition used to increase resistance to cavitation erosion by increasing the hardness of the material surface are presented. It is shown that the improvement depends on the substrate, coating material and test conditions, but even using the same materials and test conditions large differences in the improvement can be sometimes gained. Moreover, sometimes a slight change in the manufacturing conditions of the protective layer or coating component can even contribute to a deterioration in resistance compared with the untreated material. Plasma nitriding can improve resistance by even 20 times, but in most cases, the improvement was about two-fold. Shot peening or friction stir processing can improve erosion resistance up to five times. However, such treatment introduces compressive stresses into the surface layer, which reduces corrosion resistance. Testing in a 3.5% NaCl solution showed a deterioration of resistance. Other effective treatments were laser treatment (an improvement from 1.15 times to about 7 times), the deposition of PVD coatings (an improvement of up to 40 times) and HVOF coatings or HVAF coatings (an improvement of up to 6.5 times). It is shown that the ratio of the coating hardness to the hardness of the substrate is also very important, and for a value greater than the threshold value, the improvement in resistance decreases. A thick, hard and brittle coating or alloyed layer may impair the resistance compared to the untreated substrate material.

Mechanical shock of zircon based ceramic induced by cavitation erosion testing was investigated in this study. Several parameters were followed in order to determine level of material degradation during the cavitation erosion testing. Mass loss was taken as a conventional criterion for material degradation, while the level of surface degradation was evaluated by image and thermal imaging analyses. Results show high cavitation resistance of zircon ceramics and their suitability when vigorous cavitation erosion environment is expected.

Cavitation erosion is a huge problem in engineering structures working under hydrodynamic conditions. The synergistic effect of erosion and corrosion can aggravate the material removal by several orders of magnitude. Surface coatings are widely used to address material degradation by erosion–corrosion. However, non-homogenous microstructure and presence of defects leads to premature coating failure under erosion–corrosion conditions. Thus, it is imperative to identify plausible solutions to address cavitation-related material degradation. In this study, Ni-Cr-5Al2O3 coatings were deposited on stainless steel substrate using high-velocity-oxy-fuel technique. The as-sprayed coating showed highly non-homogeneous microstructure comprising splats, pores, intermetallic compounds and elemental segregation. A new thermo-mechanical processing technique, stationary friction processing (SFP), was utilized for achieving through-thickness microstructural refinement in as-sprayed coating. As-sprayed and SFP-treated coatings were tested in pure cavitation erosion, corrosion in 3.5% NaCl solution and erosion–corrosion. SFP treatment resulted in 5-times enhancement in the erosion and corrosion resistance compared to as-sprayed coating. The remarkable performance of Ni-Cr-5Al2O3 coatings after SFP treatment is attributed to significant enhancement in the mechanical properties including hardness and fracture toughness which is the consequence of complete removal of splat boundaries, pores, intermetallic compounds and uniform element distribution up to the coating–substrate interface.

In order to predict and avoid cavitation erosion, it is necessary to know the resistance of materials to cavitation impacts. It is proposed that only cavitation impacts with energy larger than a certain threshold level affect the cavitation erosion of materials. Weak cavitation impacts have small impact energy, which cannot produce elastic or plastic deformation on materials. Cavitation erosion does not take place in this situation. On the other hand, strong cavitation impacts have large impact energy, which generates plastic deformation and/or causes damage with mass loss. It is proposed that only cavitation impacts that are larger than a certain threshold level affect the cavitation erosion of materials. In this paper, the existence of such a threshold level is revealed experimentally. The fundamental threshold level of the material is a new parameter suitable for the prediction of cavitation erosion rate. The erosion rates for several materials were determined by using a cavitating jet erosion test, which forms a new ASTM standard, ASTM G 134. The cavitation impacts induced by the cavitating jet were measured by means of a specially made PVDF transducer and the energy of the cavitation impacts were then calculated. The fundamental threshold levels for several materials were obtained from the relationship between the erosion rate and the energy of the cavitation impacts. A method to predict cavitation erosion quantitatively from the fundamental threshold level of the material and the measurement of cavitation impacts is proposed.

Cavitation erosion is still one of the most important degradation modes in hydraulic turbine runners. Part of researches in this field focuses on finding new materials, coatings and surface treatments to improve the resistance properties of runners to this phenomenon. However, only few studies are focused on the deleterious effect of the environment. Actually, in some cases a synergistic effect between cavitation erosion mechanisms and corrosion kinetics can establish and increase erosion rate. In the present study, the cavitation erosion-corrosion behaviour of ASTM A27 steel in natural river water is investigated. This paper state the approach which has been used to enlighten the synergy between both phenomena. For this, a 20 kHz vibratory test according ASTM G32 standard is coupled to an electrochemical cell to be able to follow the different corrosion parameters during the tests to get evidence of the damaging mechanism. Moreover, mass losses have been followed during the exposure time. The classical degradation parameters (cumulative weight loss and erosion rate) are determined. Furthermore, a particular effort has been implemented to determine the evolution of surface damages in terms of pitting, surface cracking, material removal and surface corrosion. For this, scanning electron microscopy has been used to link the microstructure to the material removal mechanisms.

Numerical prediction of cavitation erosion is a great scientific and technological challenge. In the past, many attempts were made—many successful. One of the issues when a comparison between a simulation and erosion experiments is made, is the great difference in time scale. In this work, we do not attempt to obtain quantitatively accurate predictions of erosion process but concentrate qualitatively on cavitation mechanisms with quantitative prediction of pressure pulses which lead to erosion. This is possible, because of our recent experimental work on simultaneous observation of cavitating flow and cavitation erosion by high speed cameras. In this study, the numerical simulation was used to predict details of the cavitation process during the vapor collapse phase. The fully compressible, cavitating flow simulations were performed to resolve the formation of the pressure waves at cavitation collapse. We tried to visualize the mechanisms and dynamics of vapor structures during collapse phase at the Venturi geometry. The obtained results show that unsteady Reynolds-averaged Navier–Stokes (URANS) simulation of cavitation is capable of reproducing four out of five mechanisms of cavitation erosion, found during experimental work.

We have developed a numerical simulation code that can predict the cavitation erosion and the residual stress of a material, which are both closely related to plastic deformation. The shock wave generated at the cavitation bubble collapse hits the material surface and the impact energy causes plastic deformation which changes the residual stress. When the impact energy is high, mass loss (i.e., erosion) occurs after the plastic deformation stage. We numerically simulated impulsive bubble pressures that varied on the order of a microsecond in the cavitating flow with the ‘bubble flow model’. The bubble flow model simulates the abrupt time-variations in the radius and inner pressure of bubbles based on the Rayleigh-Plesset equation. Although the shock wave propagation was not simulated, the impact energy was estimated based on the bubble pressure. The predicted impact energy was compared with the distribution of plastic deformation pits, which were observed on the impeller blade surface of a centrifugal pump. The predicted impact energy was also compared with the distribution of residual stress measured on a stainless steel plate after a cavitating jet impinged on the plate. The distribution of the impact energy corresponded qualitatively to that of the residual stress improvement caused by the plastic deformation. High correlation between the predicted impact energy and the plastic deformation of material was confirmed, and we found that our numerical method is relevant for the prediction of cavitation erosion and residual stress on a material surface.

Hydro-abrasive erosion and cavitation-silt erosion characteristics of HVOF sprayed WC-Ni cermet coatings under different flow velocities and sand concentrations

The prediction of cavitation erosion by using numerical flow simulation is necessary for designing small high-speed pumps. This erosion is closely related to the bubble dynamics near the solid surfaces such as impeller blades and casing walls. We developed a new numerical simulation code for investigating bubble behavior in a centrifugal pump. We simulated distributions of the bubble number density, the bubble size and the bubble pressure, which were close to the impeller blade. The numerical results were consistent and revealed a potential of this code to predict flow and bubble behavior under the cavitation erosion conditions in a pump.

Effective hydraulic turbine design prevents sediment and cavitation erosion from impacting the performance and reliability of the machine. Using computational fluid dynamics (CFD) techniques, this study investigated the performance characteristics of sediment and cavitation erosion on a hydraulic Francis turbine by ANSYS-CFX software. For the erosion rate calculation, the particle trajectory Tabakoff–Grant erosion model was used. To predict the cavitation characteristics, the study’s source term for interphase mass transfer was the Rayleigh–Plesset cavitation model. The experimental data acquired by this study were used to validate the existing evaluations of the Francis turbine. Hydraulic results revealed that the maximum difference was only 0.958% compared with the CFD data, and 0.547% compared with the experiment (Korea Institute of Machinery and Materials (KIMM)). The turbine blade region was affected by the erosion rate at the trailing edge because of their high velocity. Furthermore, in the cavitation–erosion simulation, it was observed that abrasion propagation began from the pressure side of the leading edge and continued along to the trailing edge of the runner. Additionally, as sediment flow rates grew within the area of the attached cavitation, they increased from the trailing edge at the suction side, and efficiency was reduced. Cavitation–sand erosion results then revealed a higher erosion rate than of those of the sand erosion condition.

Numerical prediction of vibration-induced cavitation erosion in high-speed gears using erosion risk indicators

Cavitation-induced degradation of metallic materials presents a significant challenge for engineers and users of equipment operating with high-velocity fluids. For any metallic material, the mechanical strength and ductility characteristics are controlled by the mobility of dislocations and their interaction with other defects in the crystal lattice (such as dissolved foreign atoms, grain boundaries, phase separation surfaces, etc.). The increase in mechanical properties, and consequently the resistance to cavitation erosion, is possible through the application of heat treatments and cold plastic deformation processes. These factors induce a series of hardening mechanisms that create structural barriers limiting the mobility of dislocations. Cavitation tests involve exposing a specimen to repeated short-duration erosion cycles, followed by mass loss measurements and surface morphology examinations using optical microscopy and scanning electron microscopy (SEM). The results obtained allow for a detailed study of the actual wear processes affecting the tested material and provide a solid foundation for understanding the degradation mechanism. The tested material is the Ni-based alloy INCONEL 625, subjected to stress-relief annealing heat treatment. Experiments were conducted using an ultrasonic vibratory device operating at a frequency of 20 kHz and an amplitude of 50 µm. Microstructural analyses showed that slip bands formed due to shock wave impacts serve as preferential sites for fatigue failure of the material. Material removal occurs along these slip bands, and microjets result in pits with sizes of several micrometers.

Polyurethane coating with heterogeneity structure induced by microphase separation: A new combination of antifouling and cavitation erosion resistance