Outlet tube effects on cavitation cloud dynamics and erosion in self-excited waterjets

<div class="section abstract"><div class="htmlview paragraph">Cavitation and cavitation-induced erosion have been observed in fuel injectors in regions of high acceleration and low pressure. Although these phenomena can have a large influence on the performance and lifetime of injector hardware, questions still remain on how these physics should be accurately and efficiently represented within a computational fluid dynamics model. While several studies have focused on the validation of cavitation predictions within canonical and realistic injector geometries, it is not well documented what influence the numerical and physical parameters selected to represent turbulence and phase change will have on the predictions for cavitation erosion propensity and severity.</div><div class="htmlview paragraph">In this work, a range of numerical and physical parameters are evaluated within the mixture modeling approach in CONVERGE to understand their influence on predictions of cavitation, condensation and erosion. Particular attention is paid to grid resolution, turbulence model and near-wall treatment, fuel surrogate properties, and non-condensable gas content. Assessment of cavitation predictions are conducted through comparison of measured and predicted mass flow rates and cavitation probability distributions for flow through a channel with a sharp inlet. Predictions for hydrodynamic impact loading and cavitation erosion are compared with the experimentally measured incubation period and critical site for erosion. Based on these findings, recommendations are provided for modeling turbulent cavitating flows, using the single fluid mixture modeling approach, to improve predictions for cavitation-induced erosion. In particular, to capture the fluid dynamic phenomena characterizing cavitation cloud formation, development and shedding, a Large Eddy simulation with grid resolution as fine as 2.50 μm is recommended. The assumed concentration of non-condensable gas content is observed to have a strong influence on the predicted cavitation erosion severity, which motivates the need for dissolved gas concentration measurements for future cavitation erosion experimental studies. Using the best practices established in this work, good agreement is achieved between the measured and predicted cavitation parameters, as well as the critical site for cavitation-induced erosion.</div></div>

Effects of jet impact angle on cavitation erosion intensity and cavitation cloud dynamics

Several hydro-machinery components such as impellers of submersible pump, draft tubes and turbine blades generally suffer from cavitation erosion (CE) during their operation, and due to this, service life and capability of such parts are reduced. During the design and development of these components, test rigs are usually required to evaluate their performance. In the present research work, keeping in view the economic aspects, out of different test rigs available, it is proposed to use high-velocity submerged water jet cavitation erosion test rig. The test rig was designed with flexibility in cavitation erosion parameters (velocity, angle of attack, stand-off distance, nozzle diameter) and fabricated with an aim to test the cavitation erosion of hydro-machinery steel under different cavitation erosion parameters. Calibration of the test rig was done for jet velocity, stand-off distance (SOD) and angle of attack. The CE rate of steel SS410 was evaluated using the fabricated test rig under different operating parameters consists of 3 velocities and 3 stand-off distance, keeping the other parameters like angle of attack as 90° and nozzle diameter as 3 mm. The test rig was capable of producing CE as observed from the specimen microstructure. From the microstructure analysis, the pits produced during the CE are clearly visible. The CE rate was found to be maximum for a parametric combination consist of maximum velocity (35 m/sec) and stand-off distance (10 cm). With an increase in velocity, the amount of water bubbles increases in the cavitation cloud, which contributes to maximum erosion. The cavitation erosion rate is enhanced by increasing the stand-off distance from 5 cm to 10 cm, followed by a decrement when moving from 10 to 15 cm.

It is well known that a cloud cavitation is one of the most destructive forms of cavitation. The set of equations for the motion of a spherical bubble cloud is formulated. Behavior of bubble clouds are simulated numerically when the surrounding pressure is decreased from 50 kPa to a 10 kPa and then increased to 50 kPa. To study the collapse of cloud cavitation more strictly, the internal phenomena of bubble and the compressibility of liquid are considered in the governing equations. An inward propagating shock wave is formed during the collapse of bubble cloud and the shock wave is focused in the center region of the cloud. This makes a violent bubble collapse, which causes a high emitted pressure from the bubbles, which is several hundreds times larger than the single bubble collapse. Moreover, relationship between the cloud collapse and cavitation erosion is studied.

One of the main causes of damage in hydraulic turbines is cavitation. While not all cavitation appearing in a turbine is of a destructive type, erosive cavitation can severely affect the structure, thus increasing maintenance costs and reducing the remaining useful life of the machine. Of all types of cavitation, the maximum erosion occurs when clouds of bubbles collapse on the runner surface (cloud cavitation). When this occurs it is associated with a substantial increase in noise, and vibrations that are propagated everywhere throughout the machine. The generation of these cavitation clouds may occur naturally or it may be the response to a periodic pressure fluctuation, like the rotor/stator interaction in a hydraulic turbine. Erosive bubble cavitation generates high-frequency vibrations that are modulated by the shedding frequency. Therefore, the methods for the detection of erosive cavitation in hydraulic turbines are based on the measurement and demodulation of high-frequency vibrations. In this paper, the feasibility of detecting erosive cavitation in hydraulic turbines is investigated experimentally in a rotating disk system, which represents a simplified hydraulic turbine structure. The test rig used consists of a rotating disk submerged in a tank of water and confined with nearby axial and radial rigid surfaces. The excitation patterns produced by cloud cavitation are reproduced with a PZT (piezoelectric patch) located on the disk. These patterns include pseudo-random excitations of different frequency bands modulated by one low carrier frequency, which model the erosive cavitation characteristics. Different types of sensors have been placed in the stationary and in the rotating parts (accelerometers, acoustic emission (AE), and a microphone) in order to detect the excitation pattern. The results obtained for all the sensors tested have been compared in detail for the different excitation patterns applied to the disk. With this information, the best location and type of sensor to detect the different excitations have been identified. This study permits improving the actual technique of detecting erosive cavitation in hydraulic turbines and, therefore, to avoid operation under these circumstances.

The experimental works presented in here contribute to the clarification of erosive effects of hydrodynamic cavitation. Comprehensive cavitation erosion test series were conducted for transient cloud cavitation in the shear layer of prismatic bodies. The erosion pattern and erosion rates were determined with a mineral based volume loss technique and with a metal based pit count system competitively. The results clarified the underlying scale effects and revealed a strong non-linear material dependency, which indicated significantly different damage processes for both material types. Furthermore, the size and dynamics of the cavitation clouds have been assessed by optical detection. The fluctuations of the cloud sizes showed a maximum value for those cavitation numbers related to maximum erosive aggressiveness. The finding suggests the suitability of a model approach which relates the erosion process to cavitation cloud dynamics. An enhanced experimental setup is projected to further clarify these issues.

Joint experiments of cavitation jet: High-speed visualization and erosion test

Factors affecting the procedure for testing cavitation erosion of GFRP composites using an ultrasonic transducer

Cavitation erosion is a concern for most hydraulic machinery. An especially damaging type of cavitation is cloud cavitation. This type of cavitation is characterized by a growth-collapse cycle in which a group of vapor bubbles first grows together in a low-pressure region and then collapses almost simultaneously when the pressure recovers. Measuring the frequency of these collapse events is possible by acoustic emission (AE), as demonstrated in this study, in which a cavitation tunnel is utilized to create cloud cavitation in the vicinity of a sample surface. These samples were equipped with AE sensors, and the initially high frequency AE signal was demodulated to detect the relatively low frequency cloud cavitation shedding. It was found that when the cavitation number is increased, AE successfully detects the changes in this frequency, confirmed by comparing the results to video analysis and to simulations from literature. Additionally, the frequency increases when cavitation erosion progresses, thus providing means to track the erosion stage. It is concluded that the presented method is suitable for both detecting the transition from cloud to sheet cavitation and the erosion evolution in the experimental cavitation tunnel. The method could probably be extended to non-intrusive hydraulic machine monitoring, as this type of cloud cavitation is common in hydrofoils.

Cavitation erosion on the wetted surface of hydraulic machinery is directly related to the cavitation behavior. In this paper, the cavitation behavior and cavitation erosion characteristics on the airfoil surface were observed experimentally, and then, image processing methods for quantifying cavitation structure and cavitation erosion were established. Laser‐CCD system was used to obtain the cavitation structure on the airfoil surface and the microtopographies of the cavitation erosion at different magnifications were obtained by SEM. The distribution and shape of cavitation pits were analyzed. An image processing method based on statistical principle was used to analyze the distribution characteristics of the cavitation structure. The mean and mean square value of the cavitation structure were obtained. The average volume and the volume change rate of cavitation cloud in each position of the flow field during a cavitation period were described. According to the characteristics of cavitation pits, an image processing method based on background correction, edge detection, and binary morphology processing was established, and then, the distribution characteristics and the area of the cavitation pits were obtained. Finally, the effectiveness of the methods is verified by the image processing of cavitation pit in different locations on the hydrofoil.

The unsteady cavitating flow past a three-dimensional twisted hydrofoil is numerically investigated by a large eddy simulation to obtain in-depth insight into the bubble dynamics near the cavitation erosion region. Macroscopic cavity evolution is captured by a multiphase flow computing frame, while the bubble oscillations in the cavitating flow are computed by solving the Gilmore bubble dynamic model, in which the driving force for the bubble movement is exported through the application of a discrete phase model. The cavitation erosion potential is then computed by a robust indicator developed based on the energy balance hypothesis. The relevance between the dynamics and the destructive essence of a cavitation bubble and the erosion intensity is thoroughly analyzed. The results show that the unsteadiness involved in the turbulent cloud cavitation is well reproduced, and the main cavitation erosion risk in the middle region of the hydrofoil is also accurately predicted comparing with the painting test results. A localized high-pressure region is identified near the rear part of the attached cavity where the mainstream encounters the primary reentrant jet flows. The peak bubble internal pressure can reach 487 MPa near the middle plane of the hydrofoil, during the stage when the surrounding liquid pressure is continuously increased. The bubbles with the smallest radius, ranging from 23.1 to 26.3 μm after compressing from their initial sizes (R0 = 100–700 μm) in the near wall region, are associated with the extremely high internal pressure, and they are responsible for the cavitation erosion damage on the hydrofoil surface.

This study employs a coupled multiscale method to simulate and analyze cloud cavitation flow around a twisted hydrofoil under varying water quality conditions, focusing on cavitation erosion risk. The volume of fluid method captures the vapor–liquid interface of large-scale cavitation structures, while a discrete bubble model is adopted to track microscale bubbles. A Lagrangian erosion model, accounting for asymmetric bubble collapse, is employed to predict cavitation erosion risk. The results show that the multiscale approach effectively captures both the overall evolution of cloud cavitation and the generation, growth, and collapse behavior of small-scale bubbles. The spatial distribution of microbubbles exhibits periodic variation driven by the unsteady cloud cavitation, with most bubbles originating from the main detached cavity. Two distinct power–law size distributions characterize these bubbles, reflecting multiscale bubble dynamics. The predicted cavitation erosion risk aligns closely with experimental paint tests, revealing three regions with varying erosion intensity on the hydrofoil surface, with the highest erosion risk near the sheet cavity closure line. Further analysis indicates that, under nuclei-abundant (weak water) conditions, prolonged collapse of the U-shaped cavity increases cavitation erosion near the hydrofoil's trailing edge.

The bubble size distribution (BSD) in hydrodynamic cloud cavitation is poorly understood, in spite of its importance in cavitation erosion and noise. Challenges arise owing to the heterogeneous turbulent flows and high void fraction in the cavitation regime. The use of a fiber optical probe enables us to obtain the BSD in a cavitation cloud. Two distinct power law scalings at the early stage of cloud cavitation are identified. The first generation of bubbles is produced by the fission to the shedding cavitation pocket by large-scale turbulence, whose isotropic part leads to the basic scaling −10/3, while the anisotropic part due to the effect of hydrofoil wall contributes to the deviation. The successive fragmentation of bubbles accompanied with turbulent energy cascade results in the fairly uniform scaling −4/3. The results indicate that turbulence plays a dominant role in bubble breakup at the early stage of cloud cavitation.

On the mechanisms of cavitation erosion – Coupling high speed videos to damage patterns

Recently simultaneous observation of both cavitation structures and cavitation damage, has pointed to the fact that the small scale structures and the topology of the cavitation clouds play a significant role in cavitation erosive potential. Although this opened some new insights to the physics of cavitation damage, many new questions appeared. In the present study we attached a thin aluminium foil to the surface of a transparent Venturi section using two sided transparent adhesive tape. The surface was very soft - prone to be severely damaged by cavitation in a very short period of time. Using high speed cameras, which captured the images at 30000 frames per second, we simultaneously recorded cavitation structures (from several perspectives) and the surface of the foil. Analysis of the images revealed that five distinctive damage mechanisms exist - spherical cavitation cloud collapse, horseshoe cavitation cloud collapse, the “twister” cavitation cloud collapse and in addition it was found that pits also appear at the moment of cavitation cloud separation and near the stagnation point at the closure of the attached cavity.