Cavitation Problem Research Articles

Rankine Vortex Suppression Employing Shaped Drain Ports: Assessment Using a New Parameter

The phenomenon of vortexing is common in engineering applications but must be prevented due to its adverse effects on draining systems. Environmental disturbances often induce rotational motions in liquid propellants, leading to the formation of vortex air cores within the propellant tank. These air cores can impede the flow of propellants through the outlet, potentially compromising engine performance and jeopardizing mission success. The formation of air-core vortices can also induce cavitation problems in the feed pumps of propellant-draining systems. To address these challenges, researchers have designed innovative drain port geometries aimed at suppressing vortex formations. However, the lack of standardized metrics for evaluating vortexing hinders the comparison of findings across different studies. To overcome this issue, the present study formulates and proposes a novel parameter that integrates various parametric characterizations used in previous studies to assess vortexing. Upon analyzing the performance of different drain port shapes using this newly developed parameter, this study identifies cruciform drain ports with an aspect ratio of 3 as the most effective ones in suppressing vortexing. The current findings not only contribute to the data on vortex suppression strategies but also highlight the need for standardized metrics to evaluate vortexing.

Multifactor response-based optimization for enhancing cavitation performance of short injection self-priming pumps

As the heart of the water supply systems, the energy conversion efficiency of the self-priming pumps with built-in injection system under overload conditions is extremely affected by cavitation problems. This study focuses on a typical short-type injection self-priming pump devoted to improve the pump performance, especially to solve the cavitation problem under overload conditions. Through a two-factor, five-level orthogonal test that combines numerical simulation and experimental validation, the influence of key structural parameters within the ejector on cavitation performance and energy characteristics are analyzed. The results demonstrate the high level accuracy of numerical simulation, as indicated by the consistency and small error between the numerical results and the experimental results for head, efficiency, and power under full flow conditions. Regression analysis based on the orthogonal test results reveals significant correlations between the hydraulic performance of the pump and the nozzle throat diameter and nozzle outlet diameter. Under overload conditions, the high-speed jet generated by the injector induces a negative pressure at the effuser and nozzle, leading to cavitation at the effuser throat and nozzle outlet. Employing appropriate structural parameters, particularly the effuser throat diameter and nozzle outlet diameter, proves effective in increasing the operating flow range. This in turn improves the energy characteristics of the entire hydraulic system. The optimal combination identified for the ejector's structural parameters in this study is a nozzle outlet diameter of 8.2 mm and a throat diameter of 18 mm. This provides theoretical insights for the optimal design of short-type jet self-priming pumps, holding practical significance in engineering applications.

Experimental and numerical comparison of multiple passive beamformers for separating intra- and extra-canal cavitation activity during transvertebral spinal cord therapy

Distinguishing intra-canal cavitation activity during transvertebral focused ultrasound sonication of the spinal cord is a challenge due to the strong prefocal cavitation emissions in the spinalis musculature overwhelming emissions originating in the canal. To achieve mapping of all cavitation sources simultaneously, two methods for enhancing detection sensitivity are investigated: (1) multiple dynamic ranges within a reconstructed volume and (2) utilizing alternative beamformers to delay-and-sum (DAS) during map reconstruction. The performance of DAS beamforming is compared to a delay-multiply and-sum beamformer (DMAS), with and without a paired multiplicative compounding method (pDMAS). Experiments and simulations were performed on a 128-element, dual-aperture transvertebral array, through stacks of ex vivo human vertebra. Numerically and experimentally obtained point spread functions were compared in 3D, producing voxel-wise cross correlation values of 0.84, 0.89, 0.97 (N = 1) for the beamformers listed above, respectively, in water. Experimental, transvertebral localizations of canal sources in isolation produced localization error of 2.8 ± 1.2, 2.9 ± 1.4, 2.7 ± 1.3 mm for a single vertebral target (N = 30 sonications), respectively. A large numerical data set investigating the prefocal cavitation problem (N > 160) is presented and compared with (N = 9) experimental data demonstrating enhancement of intracanal sensitivity in cavitation maps.

Experimental and Numerical Investigation of Cavitation Assessment for Runner Blades in a Francis Turbine

Cavitation in hydro turbines causes component deterioration that requires continuous and costly maintenance in the hydroelectric power generation stations. The Tarbela Dam Hydel Project (TDHP) in Pakistan is facing a cavitation problem in the Francis Turbine components, such as the runner and draft tube. Simulation work has been performed to examine and quantify the cavitation rate as a function of the suction head (SH) and flow velocity (FV) by utilizing a homogeneous cavitation model. The result shows that pressure fluctuation is maximum for overcrowded conditions and minimum for rated load conditions. Moreover, a higher cavitation rate is found for the part and overcrowded conditions compared to the rated load condition. Additionally, the cavitation rate becomes 50% higher when the SH increases from 5.54 to 12.34 m. Moreover, SEM results have verified the CFD results that higher FV and SH enhance the cavitation rate. Furthermore, the numerical work is validated by simplified hydrofoil geometry. The computational fluid dynamics results presented a good arrangement with the experimental data.

Two-phase lubrication characteristics of journal bearing in refrigerant-oil system under high-pressure environment considering gaseous cavitation

The journal-bearing system is crucial for the reliability of refrigerant compressors that operate under high-ambient-pressure environments. The pressure changes rapidly, which hereby results in the variation in solubility of refrigerant in oil, and causes gaseous cavitation of the refrigerant-oil mixture in the bearing. However, the cavitation mechanism under high-pressure environments is not well understood, and its impact on lubrication characteristics has been rarely discussed. In this paper, a numerical model is developed to simultaneously solve the gaseous cavitation and two-phase lubrication problems. The first systematic comparison of lubrication characteristics for constant-eccentricity journal bearings, in such a special environment, between the classical Reynolds boundary and gaseous cavitation boundary is presented. Results indicate the load-carrying capacity is reduced by about 3% at a lower eccentricity of 0.3, but is greatly enhanced by about 21% at a higher eccentricity of 0.9 due to the gaseous cavitation. Meanwhile, the ‘reverse’ flow rate is found to reach -0.057 at the end side of the bearing owing to sub-ambient pressure up to -1 MPa at the eccentricity of 0.8. The cavitation-induced sub-ambient pressure should be addressed for the design of the bearing operating in such pressurized refrigerant environments as ambient gas might be sucked into the bearing.

Preface of the proceeding of 11th International Symposium on Current Research in Hydropower Technologies

The international symposium on Current Research in Hydropower Technologies (CRHT) aims to bring the researchers and students from the field of hydropower around the world in a common platform to share their research experiences. Since 2018, we have also been publishing papers from this conference in IOP Journal of Physics.This year in the symposium, we received 41 papers from Kathmandu University (KU), Tribhuwan University (TU), Norwegian University of Science and Technology (NTNU), École Polytechnique Fédérale de Lausanne (EPFL), Indian Institute of Technology (IIT-Roorkee), IIT-Delhi and Gujarat Technological University, Uka Tarsadia University and CK Pithawala, Gujarat.In the symposium, the overall event was divided into three sessions. The first group of sessions contained hydraulic turbines as well as hydropower and other technologies. We had presentations related to computational and experimental techniques related to the turbines, as well as civil and environmental aspects of the power plants, including sediment traps, tunnels and finding solutions for fish friendly and sustainable power plants. In the second group, there were sessions related to other forms of renewable energies as well, such as fuel cells, hydrokinetic turbines as well as seismic assessment of structures in hydropower. In the third group, we witnessed presentations related to condition monitoring of turbines, erosion and cavitation problems and some interdisciplinary aspects of the sustainable development.After receiving comments from the presentations and a peer review process, 36 papers have been selected for publication in this proceeding. We hope that the papers will be beneficial for the researchers and students involved in the sector of hydropower.

A decoupled approach to enhance the Elrod algorithm

The Elrod algorithm is widely used for the study of bearing cavitation problems. However, the potential instability of the algorithm makes it difficult to use in practical applications. A simplified Elrod algorithm based on a decoupled technique was developed. The new algorithm is well-regularized because the tough switch function used for the shear flow term is removed. The most significant advantage of the algorithm is the stability, unlike the traditional algorithm where the accuracy of results irregularly depends on the choice of grids. The numerical experiments, including a journal bearing and a surface-dimpled thrust bearing, show that the simplified Elrod algorithm converges consistently for all cases studied, while the traditional Elrod algorithm fails to converge for cases with a special number of grids. The simplified Elrod algorithm is considered to be a robust alternative to the traditional Elrod algorithm.

Cavitation characteristics and energy loss in high-pressure differential control valve

PurposeThe purpose of this study is to determine the cavitation flow characteristics of the high-pressure differential control valve. The relationship between cavitation, flow coefficient and spool angle is obtained. By analyzing the relationship between different spool angles and energy loss, the energy loss at different spool angles is predicted.Design/methodology/approachA series of numerical simulations were performed to study the cavitation problem of a high-pressure differential control valve using the RNG k–e turbulence model and the Zwart cavitation model. The flow states and energy distribution at different spool angles were analyzed under specific working conditions.FindingsThe cavitation was the weakest when the spool angle was 120° or the outlet pressure was 8 MPa. The pressure and speed fluctuations of the valve in the throttle section were greater than those at other locations. By calculating the entropy production rate, the reason and location of valve energy loss are analyzed. The energy loss near the throttling section accounts for about 92.7% of the total energy loss. According to the calculated energy loss relationship between different regions of the spool angle, the relationship between any spool angle and energy loss in the [80,120] interval is proposed.Originality/valueThis study analyzes the cavitation flow characteristics of the high-pressure differential control valve and provides the law of energy loss in the valve through the analysis method of entropy. The relationship between spool angle and energy loss under cavitation is finally proposed. The research results are expected to provide a theoretical basis for the optimal design of valves.

Efficient implementation of numerical methods for solving bearing cavitation problems using symmetric system solvers

Since the successful application of the Elrod method to the Jakobsson–Floberg–Olsson (JFO) theory, many new alternative methods have been presented. In particular, over the past decade, the efficiency of cavitation algorithms has significantly improved, as demonstrated by the Fischer-Burmeister based unconstrained optimization methods and the modulus-based methods based on the generalized absolute value equation. This paper briefly reviewed the principles of these algorithms and compared their computational efficiency. Pseudocodes were included to illustrate their implementation. Taking advantage of linear direct solvers, a novel equivalent symmetric matrix equation was presented to promote the efficiency of these algorithms. A case of a surface-textured thrust bearing with multiple dimples was studied. The results showed that the method can save more than 2–5 times the computing time.

Cavitation generated by vibrating quartz tuning fork in concentrated 3He–4He mixture

The state of the problem of cavitation in superfluid helium and the content of previous works on this topic are analyzed. The transition between turbulent flow and cavitation under oscillations of a quartz tuning fork in a normal and superfluid isotopic mixture containing 15% of 3He in the temperature range of 0.5–3 K is studied experimentally. The temperature dependences of the critical velocity for the cavitation onset and the critical transition excitation force are obtained. We compare the results of our measurements with the data obtained by other groups. The results of measurements with tuning forks in restricted and unrestricted geometries suggest that the presence of acoustic emission facilitates the conditions for the onset of cavitation and indirectly confirms the role of vortex lines. A comparison of the temperature dependences of the critical velocity of cavitation onset in pure 4He and a concentrated 3He–4He mixture did not give an unambiguous answer to the reason for the sharp increase of this velocity, at the transition to the superfluid state, and the contributions of both thermal effects and the 3He impurities to the velocity increase.

Design modification in an industrial multistage orifice to avoid cavitation using CFD simulation

BackgroundMulti-stage orifices (MSOs) are used in many industries such as oil refineries, nuclear power plants, fertilizer plants and chemical plants for pressure reduction applications. This pressure reduction often leads to an outbreak of vapors from liquid water resulting in cavitation. Cavitation imposes many problems, including loss of efficiency, erosion, and vibrations or noise. In order to mitigate the effects of cavitation on the system, it is essential to thoroughly understand the occurring phenomenon. The main objective of this study is to propose a design modification for an industrial multi-stage orifice that inhibits the onset of cavitation. MethodComputational fluid dynamics (CFD) analysis was made to simulate an actual three dimensional setup of a six-stage orifice in which cavitation occurs after the last stage of the orifice. CFD model was validated by comparing the real-time plant data with actual operating conditions. Significant FindingsVarious modifications in the geometry were tested with a focus on the orientation of the holes in orifices to eliminate the cavitation problem associated with the existing geometry. Multi-holed geometry having four holes in each orifice plate arranged at alternate positions was observed to be the best performing geometry without any cavitation effects under the same operating conditions.

Long-term decay of the water pressure in the osmotic tensiometer

Matric suction is an important state variable required for the assessment of unsaturated soil properties. Tensiometers are commonly used for direct matric suction measurement but have a limited measuring range up to 90 kPa due to the cavitation problem. Osmotic tensiometer (OT) can improve the measuring range of tensiometers by increasing the osmotic pressure of water to avoid the cavitation. However, the long-term water pressure decay that appeared in OTs caused a gradual decrease in their measuring range. In this study, crosslinked poly(acrylamide-co-acrylic acid) potassium salt (PAM-co-PAAK) was used for the preparation of OTs (five in total) to explore the mechanism of water pressure decay of OTs. The maximum water pressure in the OT versus the volume fraction of polymer filled in the OT was described based on the Flory-Huggins polymer theories and validated using WP4C dewpoint hygrometer. The long-term pressure decay of OT-1, OT-2, and OT-3 was observed for 130 d and constant pressures were found for OT-1 and OT-2, indicating that the pressure decay of OT was mainly caused by the stress relaxation of the polymer hydrogels, and standard linear solid (SLS) rheological model was appropriate to fit the decay data. For OT-1, OT-2 and OT-3, the theoretical osmotic pressure that was calculated based on the mass of retrieved polymer from OTs after 130-d pressure observation was higher than the actual osmotic pressure as observed, indicating that polymer leakage cannot explain the pressure decay of the OT. The ultraviolet–visible (UV–visible) spectrophotometry examined the change in polymer concentrations in the water containers of OT-4 and OT-5 and demonstrated that there was no increase in polymer leakage during the period of pressure decay of OT-4 and OT-5. As a result, the pressure decay of OT was not caused by polymer leakage. The results of this research suggested that the viscoelastic properties of polymers should be taken into consideration in further OT development. • Water pressure versus volume fraction of the polymer filled in OT was validated. • Pressure decay in OTs was mainly due to stress relaxation of polymer hydrogels. • No increase in polymer leakage during the period of pressure decay of OTs. • Development of new OTs should consider rheological properties of polymers.

Data-driven internet of things and cloud computing enabled hydropower plant monitoring system

Hydropower is one of the renewable energy sources that can play a crucial role to fulfil the global energy demand. However, the performance of the hydro turbine is severely affected by silt erosion and cavitation problems which causes a reduction in the overall efficiency of the plant. Various studies have been carried out and are available in the literature to investigate silt erosion and cavitation issues in hydro turbines. It has been reported that cavitation and silt erosion varies with the variation in discharge under part load and overload operating conditions of the machine. However, very few studies are available to predict the performance of the machine under variable operating conditions. Hence, there is a scope of study for monitoring the performance under these conditions in real-time, as it is difficult to predict the behavior of the machine using the existing models. In view of the above, an architecture of a data-driven IoT-based cloud computing-enabled hydropower plant monitoring system has been proposed under the present study. In order to develop this system, historical plant data has been collected and correlations are developed, which are validated with real-time data on the ThingSpeak cloud. It has been found that the developed model can accurately predict the condition of the hydro turbine with an R2-value of 0.9693 having a mean absolute percentage error (MAPE) of 0.67% at 0.89% of root mean square percentage error (RMSPE), and the power factor with an R2-value of 0.9503, having a MAPE of 0.798% at 0.91% of RMSPE.

Economic analysis of operation and maintenance costs of hydropower plants

The world is experiencing deep climate changes caused by increased population and rapid urbanization. Hydropower is one of the renewable energy sources that can be used to meet energy demands, but most of the hydropower plants suffer from silt erosion and cavitation problems. Therefore, it is important to decide which parts to be repair or replace, as it affects the Operation and Maintenance (O&M) costs. Presently O&M costs are speculated based on the previous years O&M costs data. Various studies are available for forecasting O&M costs, but they are plant specific. This paper deals with the assessment of the O&M costs of hydropower plants, considering the parameters and conditions involved in the O&M costs of hydropower plants. The correlations are developed to predict the O&M costs of hydropower plants and found that the developed correlation models can predict the O&M costs with an accuracy having an R2-value of 0.89, Mean Absolute Percentage Error (MAPE) of 3.53% and Root Mean Square Percentage Error (RMSPE) of 4.45% for Francis turbine-based hydropower plants, and with an R2-value of 0.97, having a MAPE of 0.17% at 1.30% RMSPE for Kaplan turbine-based hydropower plants. This study may be useful for developers, plant operators, and researchers.

Numerical investigation of vibration-induced cavitation for gears considering thermal effect

In the lubrication of high-speed gears, cavitation can be induced by the strong vibration and high temperature, resulting in the insufficient lubrication and surface damage. For further understanding the lubrication mechanism between meshing teeth, the research in cavitation problem is essential, benefiting the design and optimization of high-speed gear transmissions. In this study, regarding a pair of spur gears with oil dip lubrication as the research objective, a computational fluid dynamics method coupled with cavitation model, thermal effect and gear dynamics is proposed for the theoretical study of cavitation mechanism, where the novelties are described as: (i) the gear dynamic responses from a finite element model are introduced for simulating the vibration effect; (ii) the cavitation vapor induced by local high temperature is included; (iii) the vapor distribution along tooth width is considered. Compared with the experiments from available researches, this method is validated. The results indicate that cavitation occurs easily at high speed, moreover, the decreased load and increased speed both intensify the fluctuation of cavitation vapor on meshing face, indicating the increased erosion risk. At high speed and heavy load, the local instantaneous temperature becomes very high, aggravating cavitation most significantly, so the thermal effect should not be ignored in this situation.

Capped piston: A promising design to reduce compressibility effects, pressure ripple and cavitation for high-speed and high-pressure axial piston pumps

Raising rotational speed and operating pressure is an effective way to increase the power density of axial piston pumps. However, the axial piston pump with standard hollow pistons suffers from problems of volumetric losses, pressure ripple, and cavitation at high rotational speed and operating pressure. To reduce these fluid-related problems, the capped piston design is a promising alternative to the standard piston design by minimizing the dead volume. The capped pistons have been applied in commercial axial piston pumps but it remains unclear how the flow characteristics of axial piston pumps benefit from them. Therefore, this paper aims to clarify the mechanism of capped pistons in improving the flow characteristics of axial piston pumps. A computational fluid dynamics model is developed to compare the pump’s flow characteristics between standard and capped pistons. The simulation results show that compared with the standard hollow pistons, the capped pistons can significantly reduce the compressibility effects, pressure ripple, and cavitation over a wide range of operating conditions.

A review on erosion and erosion induced vibrations in Francis turbine

Vibrations are one of the major issues observed during the operations of Francis turbine as they are frequently operated outside their optimal operating range, with rapid load changes. In hydropower plants, vibrations occur in various parts which can be classified as: hydraulic civil parts such as reservoir, intake, dam, tail race etc. mechanical parts such as penstock, turbine, draft tube and shaft, and electrical parts such as generator and transformer. However, due to the sediments and cavitation problems, the regions around rotating parts such as turbine experiences severe vibration. In the cases where the turbines need to be operated with high sediment load, the vibration problems could be further aggravated due to sediment carrying flow and declination of the surface morphology after erosion. This paper is based on a literature review to explore the characteristic effects of erosion and cavitation which could result in losses, vibrations, fatigues on the turbine materials and oblivious turbine failure. The study aims to correlate the erosion and cavitation due to abrasion with the pressure fluctuations resulting in vibrations.

Startup Process Optimization of Hydropower Unit Based on Stress Measurement of Francis Turbine Runner

The energy efficiency characteristics of the turbine, cavitation performance of the runner, and operation stability of the unit are the three major concerns of a hydropower field, the latter two factors leading to the units’ cavitation and stability problems under some operation conditions and fatigue failure of the runner blades, which are the main causes of structural and mechanical failure for the hydropower units. In this article, three conventional unit startup processes are introduced, and combined with the optimal impulse control strategy, an optimized unit startup method is proposed. As the optimal guide vane opening adjustment setting value is obtained by solving the planned startup control strategy, 3D modeling and CFD simulation are carried out, then the equivalent stress characteristics are obtained by finite element calculation when the unit is running from startup to no load at a rated speed on three different startup modes. At last, the dynamic stress tests of the runner blade are performed at the hydropower station by using wireless acquisition and transmission technology, and the peak value and characteristics of the dynamic and static stresses of the blade under different startup processes are obtained. In this, the validity of the model analysis is proved by the comparison between the simulation calculation and the measured results, and the effectiveness of the startup control strategy is proved by combining the runner blade dynamic stress simulation, calculation, and the field dynamic stress test results. The optimization method of the startup process proposed in the article can alleviate the abnormal vibration of the unit and provide a basis for long-term stable operation of the hydropower unit.

Numerical prediction of cavitation phenomena on marine vessel: Effect of the water environment profile on the propulsion performance

Abstract Energy-saving and emission reduction are crucial since shipping activity due to the global maritime trade has increased exponentially. Several agreements have been engaged to optimize ship energy efficiency composed of ship design and shipping operation planning. However, most up-to-date studies focused on speed and route optimization. The interaction analysis between speed and route efficiency below varied environmental conditions is limited. To attain energy and cost efficiency, a study of cavitation on the propeller that considers the ocean environmental condition will be discussed in this work. Although researchers have previously observed cavitation phenomena, the predictability of simulations is not yet such that problems can be eliminated. Since the multiphase flow of water and vapor is sensitive to environmental conditions, it leads to varying observation accuracy. Thus, the current paper proposes a new performance indicator of the ship propeller under cavitation predicted by computational fluid dynamics (CFD). CFD-based simulation to observe the propeller cavitation was used to model the Zwart cavitation and Kunz cavitation models under two turbulence models of K−ε at different flow conditions and operating environments. Initial validation tests between experimental and numerical simulation show good agreement with a mean error of 4.7% in the Zwart model and 3.7% in the Kunz model, where the k−ε turbulence model provides an almost higher relative error. It is revealed from the result that the increase in temperature causes the rise in the cavitation problem. It is revealed from the result that the increase of temperature causes the increase in cavitation problem.

A semi implicit compressible solver for two-phase flows of real fluids

The development of numerical solvers able to simulate compressible two-phase flows is still a great challenge in computational fluid dynamics. The interaction between acoustic waves and interfaces is of major concern for several engineering and biomedical applications, among which atomization in combustion chambers, cavitation problems, underwater explosions and bubble shock interactions. For instance, there are experimental evidences that acoustic waves can have an important effect on the atomization process, and this could have a great impact on combustion. However, usual approaches for DNS of primary atomization are based on incompressible solvers and therefore are not able to capture the propagation of acoustic waves and therefore cannot be used to simulate such phenomena. The numerical problem associated with the simulation of compressible two-phase flows is challenging, mostly because of the huge spatial variations of the speed of sound and the corresponding low Mach number in the liquid phase. In the present work, a numerical solver able to study subsonic compressible two-phase flows is presented. The solver is based on a complete formulation of the Navier-Stokes equations with real fluid equations of state, which are solved with a semi-implicit projection method. It is shown that the solver can handle a large range of compressible subsonic flows, both for a single phase or for two phases, as the flow induced by free convection, a bubble expansion in isothermal or isentropic conditions, and interaction between acoustic waves and liquid-gas interfaces. Eventually, attention will be given to the simulation of a water droplet in air, under the excitation of a stationary acoustic wave. It is also shown that the solver exhibits equivalent performances as an incompressible solver in configurations where compressible effects have no effects.