A note on the cavitation phenomenon in metallic plates perforated by sharp-nosed rigid projectiles

We explore the perforation process of metallic plates impacted by rigid sharp-nosed projectiles at high velocities. In particular, we are looking at the diameters of the penetration hole in the plates through a series of 2D numerical simulations, in order to check for the occurrence of cavitation in finite-thickness plates. This phenomenon has not been observed by previous workers and we were looking for its effect on the perforation process. Our simulations show that for every projectile/plate pair there is a certain impact velocity which marks the onset of cavitation. These threshold velocities depend on the normalized thickness of the plates, as well as on their effective strength. Our simulations are supported by the results from perforation tests on plates made of a low strength lead-antimony alloy. The main conclusion from our work is that analytical models for plate perforation should take into account the cavitation phenomenon, especially for high velocity impacts.

Dynamic Characterization of an Integral Squeeze Film Bearing Support Damper for a Supercritical Co2 Expander

The present work advances experimental results and analytical predictions on the dynamic performance of an integral squeeze film damper (ISFD) for application in a high-speed super-critical CO2 (sCO2) expander. The test campaign focused on conducting controlled orbital motion mechanical impedance testing aimed at extracting stiffness and damping coefficients for varying end seal clearances, excitation frequencies, and vibration amplitudes. In addition to the measurement of stiffness and damping, the testing revealed the onset of cavitation for the ISFD. Results show damping behavior that is constant with vibratory velocity for each end seal clearance case until the onset of cavitation/air ingestion, while the direct stiffness measurement was shown to be linear. Measurable added inertia coefficients were also identified. The predictive model uses an isothermal finite element method to solve for dynamic pressures for an incompressible fluid using a modified Reynolds equation accounting for fluid inertia effects. The predictions revealed good correlation for experimentally measured direct damping, but resulted in grossly overpredicted inertia coefficients when compared to experiments.

Improvement of Suction Performance Using Splitter Impeller and Inducer

Suction performance is one of the most important characteristics of the industrial pump to keep the pump capability against the cavitation under the low suction pressure condition. Inducers have been developed to improve the suction performance of pumps. They are used for rocket turbo pumps and recently for many industrial applications. It is one of the competitiveness of the industrial pumps to downsize the scale with higher rotational speed by adopting inducer. The authors had proved that pump with splitter impeller with inducer can further improve the suction performance. The splitter impeller was specially designed to be resistant to the cavitation choke. The other very important requirement for the pump against cavitation is that it does not occur cavitation surge. The cavitation surge arises at the partial flow rate of the pump. In contrast to this the suction performance become better at the partial flow rate. So the precise compromise is necessary to satisfy these requirements. This paper presents the experimental and CFD results to improve and optimize the suction performance without generating cavitation surge. The many design and combination of splitter impeller and inducer are tested to get the better performance. The test results of pumps with different inlet flow coefficient are compared not only suction performance but also the onset of cavitation surge.

Dynamic Characterization of an Integral Squeeze Film Bearing Support Damper for a Supercritical CO2 Expander

The present work advances experimental results and analytical predictions on the dynamic performance of an integral squeeze film damper (ISFD) for application in a high-speed super-critical CO2 (sCO2) expander. The test campaign focused on conducting controlled orbital motion mechanical impedance testing aimed at extracting stiffness and damping coefficients for varying end seal clearances, excitation frequencies, and vibration amplitudes. In addition to the measurement of stiffness and damping; the testing revealed the onset of cavitation for the ISFD. Results show damping behavior that is constant with vibratory velocity for each end seal clearance case until the onset of cavitation/air ingestion, while the direct stiffness measurement was shown to be linear. Measurable added inertia coefficients were also identified. The predictive model uses an isothermal finite element method to solve for dynamic pressures for an incompressible fluid using a modified Reynolds equation accounting for fluid inertia effects. The predictions revealed good correlation for experimentally measured direct damping, but resulted in grossly overpredicted inertia coefficients when compared to experiments.

Particle Image Velocimetry Measurements Near the Onset of Cavitation in a Converging-Diverging Glass Nozzle

Water flow through a converging-diverging glass nozzle experiences a pressure drop and its velocity increases as it flows through the converging section. For an inviscid fluid, the pressure minimum occurs at the nozzle throat, where the cross-sectional area is minimum. If the minimum pressure is below the water vapor pressure, cavitation may occur. The actual minimum pressure through a converging-diverging nozzle depends on many factors and may not occur at the nozzle throat. Additionally, fluid through the nozzle may be driven into the metastable region and subsequently cavitate at a lower pressure than the vapor pressure. All of these factors combine to create a complex and unsteady flow pattern. The precise conditions leading to the onset of cavitation in water flowing in a converging-diverging nozzle are not well understood. Utilization of a clear glass converging-diverging nozzle enabled Particle Image Velocimetry (PIV) measurements of the velocity vector field inside the nozzle without significantly promoting premature cavitation formation. Glass spheres of 10 μm diameter were selected as seed particles for use in the PIV measurements. These seed particles did not significantly affect the formation (or onset) of cavitation in the nozzle; however, larger seed particles (120 μm diameter) provided nucleation sights and promoted cavitation prematurely. The seed particles were injected into the flow significantly upstream from the nozzle to prevent disrupting the flow entering the nozzle. High seed density was needed to supply enough seed particles to interrogate small regions near the nozzle wall; however, high seed density could also cause speckling and reduce the ability to produce meaningful PIV measurements. A Nd:YAG laser provided illumination of the seed particles in the nozzle. Laser reflections off of the nozzle exterior had to be minimized to avoid saturating the PIV camera. A polarizing filter was installed on the camera to reduce reflections. An enclosure that surrounded the nozzle was also designed and utilized. The enclosure was filled with water to reduce laser reflections off of the nozzle exterior wall. The time elapsed between frames had to be adjusted for each section of the nozzle interrogated with PIV. For accurate velocity measurements, particles needed to travel at least two particles diameters but less than 25% of each interrogation cell. The large variation in velocities present in the nozzle prevented one time interval from satisfying the seed particles displacement requirements. The time interval between frames had to be tailored to each section of the nozzle, depending upon the range of velocities seen in that section. Detailed measurement of the velocity profile near the nozzle throat required precise control over all timing parameters and pushed the available hardware to its smallest possible time interval. Detailed PIV measurements near the wall in regions of recirculation and at the cavitation front required the use of a long-distance microscope. This limited the field of view and necessitated a high seed particle density, which presented problems due to the lack of control over the flow of the seed particles in the near wall region. PIV allowed for the measurement of the velocity vector field inside a converging-diverging nozzle without disrupting the flow. These measurements provided detailed velocity and flow pattern information throughout the nozzle, particularly in the regions near the cavitation front where boundary layer separation was observed along with regions of recirculating flow. These detailed velocity profiles were compiled to present a complete PIV analysis of the converging-diverging glass nozzle. Measurements of the velocity field near cavitation onset allowed for a better understanding of the conditions triggering cavitation and the degree to which the water flow was able to be driven into the metastable region.

Boundary lubrication of heterogeneous surfaces and the onset of cavitation in frictional contacts

Surfaces can be slippery or sticky depending on surface chemistry and roughness. We demonstrate in atomistic simulations that regular and random slip patterns on a surface lead to pressure excursions within a lubricated contact that increase quadratically with decreasing contact separation. This is captured well by a simple hydrodynamic model including wall slip. We predict with this model that pressure changes for larger length scales and realistic frictional conditions can easily reach cavitation thresholds and significantly change the load-bearing capacity of a contact. Cavitation may therefore be the norm, not the exception, under boundary lubrication conditions.

Cavitation Phenomena and Performance Implications in Archimedes Flow Turbines

Cavitation produces undesirable effects within turbines, such as noise, decreases in efficiency, and structural degradation of the device. Two microhydro turbines incorporating Archimedean spiral blade geometries were investigated numerically for cavitation effects using computational fluid dynamics (CFD). Separate blade geometries, one with a uniform blade pitch angle and the other with a 1.5 power pitch, were modeled using the Schnerr–Sauer cavitation model. The method used to determine where cavitation occurs along the blade and within the flow involved varying inlet flow rates and the rotation rate of the blade. Cavitation analysis was conducted locally as well as globally, using both cavitation number and Thoma number. The cavitation number was used to correlate the single-phase to the multiphase results for rotation rates of 250 and 500 rpm, allowing for the single-phase simulations to be used to determine where the onset of cavitation occurs. It was determined that cavitation occurred at the exit of the blade at a flow coefficient of approximately 0.33 for the 1.5 pitch blade geometry, while the uniform blade geometry had a value of 1.35. When the rotation rate was reduced to 250 rpm, cavitation occurred at a flow coefficient of 0.72. From the simulations at both rotation rates, it was determined that both geometry and rotation rate have a significant effect on the onset of cavitation and water vapor inception within the flow field. As the rotation rate of the turbine decreases, the onset of cavitation will be prolonged to larger flow coefficients. As the flow coefficient increased beyond the value at which the onset of cavitation occurs, the intensity of cavitation increases and efficiency drops of up to 20% were experienced by the turbines. Based on the net positive suction head required in the system and the available head in the system, the cavitation results were validated. It was determined that the inception cavitation number, Cai, where the onset of cavitation occurs is approximately −1.51.

Optical Flow Measurement of Cavitation in a Converging-Diverging Nozzle Using High-Speed Imagery

Cavitation is a phenomenon where liquids will vaporize when subjected to low pressures. Essentially, the pressure is reduced sufficiently such that the liquid boils at the given temperature. The highest pressure at which cavitation could occur is called the vapor pressure. However, the pressure associated with the onset of cavitation could be lower than the vapor pressure. This indicates the liquid exists under a meta-stable condition. The current research is investigating different aspects of cavitation and cavitating flow characteristics. Particle tracking using high-speed photography provided further insight as to what the velocity profile of cavitating flow may resemble. The research has shown that the cavitation that occurred in the current nozzle appears to have a laminar velocity profile. In the experiments that were conducted, it was also observed that as the back pressure of the downstream decreased, the volumetric flow rate would increase. However, a maximum volumetric flow rate was measured once the flow had begun cavitation regardless of the back pressure. This indicated that choked flow conditions likely exist within the nozzle. Choked flow within the nozzle indicates that near the region of the throat the fluid velocity has reached the speed of sound. Using high-speed photography, visualization of flow separation and recirculation was recorded. The information obtained from the research provides a more detailed description of the velocity profile near the onset of cavitation. The main objectives of this research were to obtain measurements of the overall flow for support of on-going research and analysis of nozzle flow cavitation. This study will provide a foundation for further and more detailed research into cavitation phenomena.