Cavitation has negative influence on pump operation. In order to detect incipient cavitation effectively, experimental investigation was conducted to through acquisition of current and vibration signals during cavitation process. In this research, a centrifugal pump was modeled for research. The data was analyzed by HHT method. The results show that Torque oscillation resulted from unsteady flow during cavitation process could result in energy variation. Variation regulation of RMS of IMF in current signal is similar to that in axial vibration signal. But RMS of IMF in current signal is more sensitive to cavitation generation. It could be regarded as the indicator of incipient cavitation. RMS variation of IMF in base, radial, longitudinal vibration signals experiences an obvious increasing when cavitation gets severe. Such single variation regulation could be selected as the indicator of cavitation stage recognition. Hilbert-Huang transform is suitable for transient and non-stationary signal process. Time-frequency characteristics could be extracted from results of HHT process to reveal pump operation condition. The contents of current work could provide valuable references for further research on centrifugal pump operation detection.

In order to study the relationship between the cavitation flow and energy conversion, based on the continuity equation, Renault N-S equation and RNG k-ε turbulence model, the whole flow field cavitation numerical simulation on unclear main pump model under design condition is carried out. Through the variation of pressure and velocity on the streamlines, combined with the basic equation of the pump, the dynamic and hydrostatic head of the nuclear main pump under different cavitation conditions are studied. The results show that the energy of fluid of nuclear main pump is provided by posterior segment of impeller, and the energy of the fluid decreases gradually from the shroud to the hub. Cavitation interferes with the flow of liquid in the impeller, which leads to the relative velocity increases and the pressure decreases in cavitation region, and the work capacity of blades is almost zero in bubble areas. At the same time, in non-cavitation region along with streamlines the dynamic head increases and the hydrostatic head decreases with the development of cavitation, and the decrease of the hydrostatic head is greater than the increase of the dynamic head, which results in the decrease of pump head and efficiency. In addition, in the cavitation region, with the development of cavitation, the sudden change of the dynamic and hydrostatic head increases, which increase the flow loss in the impeller and results in the decrease of pump head and efficiency moreover.

In this study, pressure transients are triggered by a steel ball, which is released from an upstream reservoir to hit a valve seat and shut off water flow in a horizontal straight copper pipeline. The pressure pulsations, cavitation and gas bubbles growth and collapse in the low pressure water-hydraulic pipeline are recorded by two pressure transducers and a high speed video camera, respectively. In addition, the influences of initial volume of gas bubbles in water and instant leakage in valve are investigated. The experimental results indicate that increasing initial gas bubble volume in water and the instant leakage of the valve will help to reduce magnitudes and numbers of pressure peaks during pressure transients. Then methods to reduce pressure pulsations in pipelines are put forward.

The tip leakage vortex (TLV) cavitation mechanism of axial flow pump was investigated with the results of high speed photography and pressure pulsation measurement. The tip leakage vortex cavitation morphology and the transient characteristics of the TLV-induced suction-side-perpendicular cavitating vortices (SSPCV) were analyzed under different flow rates and different cavitation numbers which were combined with the time domain spectrum of pressure fluctuation to elucidate the relationship between the tip cavitation and pressure pulsation. The results showed that cavitation inception occurs earlier with more unstable tip leakage vortex cavitation shape under part-load flow rate condition, and the cavitation is more intense with the decrease of the cavitation number. The inception of SSPCV is attributed to the tail of the shedding cavitation cloud originally attached on the suction side (SS) surface of blade, moving toward the adjacent blade perpendicular to the suction surface, resulting in a flow blockage. With further decrease of pressure, the SSPCVs grow in size and strength, accompanied with a rapid degradation in performance of the pump. The cavitation images and the corresponding circumferential pressure distribution with the same phase showed that the lowest pressure coincides with the suction surface (SS) corner, The pressure was found to decrease along with the occurrence of the cavitation structure.

Shedding is one of the most important expressions of the instability of cavitating flow. Most previous research works were focused on the shedding mechanism induced by the re-entry jet. Shock induced shedding on a wedge is identified recently by using time resolved X-ray densitometry which attracted lots of attention. In the present paper, cavitation dynamics around an axisymmetric body are investigated. Both shock propagation and re-entry jet as inducing factors of shedding are observed in different cycles in a single experiment. Relevant numerical simulations are carried out based on a fully compressible approach under the framework of the open-source code OpenFOAM. Numerical and experimental results agree well with each other. Results indicate shedding is induced by the re-entry jet in the first cycle. Re-entry jet occurs and cut the cavity off on the should which induces the shedding of cloud cavity in the first cycle. However in the second and subsequent cycles, shocks are generated by the collapse of shedding cavities and propagate to the cavity closure and induces stronger re-entry jet. Its effect on cavity instability is indirect which still needs a strong re-entry jet as the medium media.

Cavitation inside fuel injector nozzles has been linked not only to erosion of the solid surface, but also to improved spray atomization. To quantify the effects of the resulting occurrences, the prediction of cavitation through computational modeling is vital. Homogeneous mixture methods (HMM) make use of a variety of cavitation sub-models such as those developed by Kunz, Merkle, and Schnerr-Sauer, to describe the phase change from liquid to vapor and vice-versa in the fluid system. The aforementioned cavitation models all have several free-tuning parameters which have been shown to affect the resulting prediction for vapor volume fraction. The goal of the current work is to provide an assessment of the Kunz and Schnerr-Sauer cavitation models. Validation data have been obtained via experiments which employ both acoustic techniques (passive cavitation detection, or PCD) and optical techniques (optical cavitation detection, or OCD). The experiments provide quantitative information on cavitation inception and qualitative information as to overall vapor fraction as a function of flow rate, and nozzle geometry. It is shown that inception is fairly well captured but the amount of vapor predicted is far too low. A sensitivity analysis on the tuning parameters in the cavitation models leads to some explainable trends, however, several parameter sweeps results in outlier predictions. Recommendations for their usability and suggestions for improvement are presented.

Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source (SNS) uses a mercury target to generate neutrons. When the powerful 1.4 MW, 60Hz proton beam hits the target, a strong pressure wave propagates in the mercury and into the vessel wall due to the rapid temperature rise in mercury. These pressure waves induce cavitation damage on the target container and high stresses, which both limit the lifetime of the target. Since October 2017, helium gas has been injected into the mercury flow in order to mitigate the negative effects of pulse-induced pressure waves. The preliminary strain measurements suggest that gas injection is indeed efficient at mitigating the pressure wave. Tiny nozzles (8-micron diameter) at choked condition are used to generate small bubbles. The bubblers can theoretically inject a total mass flow rate of 0.75 SLPM. However, during operation the bubblers were capable of injecting only approximately 0.45 SLPM, which suggests that some of the nozzles may have become clogged. Since there is a strong desire to inject a larger quantity of gas in the target to, hopefully, mitigate even more the pressure wave, SNS has been looking at implementing swirl bubblers in the target, similar to the ones used in the Japan Proton Accelerator Research Complex (J-PARC) mercury target. In this paper, results with prototypical bubblers tested in water and mercury are presented. Bubblers were installed in prototypical targets and bubble size distributions were measured in both water and mercury. It was found that swirl bubblers can generate a large number of small bubbles, but some compromises were made to keep the pressure losses across them reasonable.

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.

Cavitation combined with degassing strongly affect the pump performance in low-pressure operation. The understanding of oil aeration and degassing mechanisms is of great interest to explain and model the pump performance. This paper presents the techniques used for the measurement of the degassing and aeration phenomena applied to turbine lubrication oil. The measurement principle is to cause oil to degas and measure the flow density, which indicates the volume of degassed air and the oil aeration. The density was experimentally measured with two devices, one based on Coriolis forces and one on X-ray transmission. The paper describes and discusses the results obtained for various operating conditions and cases, such as constant aeration, degassing transient (pressure decrease) and aeration transient (pressure increase). The transient results fit exponential laws which give the asymptotical values and the time constant of the phenomenon. The comparison of the two techniques is also discussed.

The early and late portions of transient fuel injection have proven to be a rich area of research, especially since the end of injection can create a disproportionate amount of emissions in direct injection internal combustion engines. A perennial challenge in simulating the internal flow of fuel injectors is the valve opening and closure event. In a typical adaptive-mesh CFD simulation, the small gap between the needle valve and the seat must be resolved with very small cells, resulting in extremely expensive computations. Capturing complete closure usually involves a topological change in the computational domain. In this work we present a more gradual and easily-implemented model of closure that avoids spurious water-hammer effects. The algorithm is demonstrated with a simulation of a gasoline direct injector operating under cavitating conditions. The results include the the first simulation of a multiple injection event known to the authors. The results show cavitation at low valve lift. Further, they reveal post-closure dynamics that result in dribble, which is expected to contribute to unburned hydrocarbon emissions.

In engineering applications, design and manufacture of double-suction impeller are on the basis of single-suction impeller. However, there is a lack of clear view on the differences of hydraulic performance between single-suction centrifugal pump and double-suction centrifugal pump which has the same blades. In this paper, a single-suction centrifugal pump and double-suction centrifugal pump with the same blades are investigated respectively, and their hydraulic performances have been compared in terms of external characteristics, flow pattern, pressure fluctuation and cavitation characteristics. In operating range of 0.6 Q 0 ∼1.1 Q 0 , results show that the efficiency of double-suction pump is 4.14% higher than that of single-suction pump stably, and the head of single suction pump is 3.5% higher than that of double-suction pump stably. Single-suction impeller and double-suction impeller have similar jet-wake structure in impeller outlet, but the amplitude of velocity of single suction impeller changes more sharply. In the vicinity of rated condition, the amplitude of pressure fluctuation of double suction pump is about half that of the single suction pump, and the cavitation performance of double suction pump and single suction pump are basically the same. These quantitative results show that pressure fluctuation characteristics and cavitation performance of single-suction pump and double-suction pump with the same blades have little difference in the vicinity of rated condition. Compared with single-suction pump, the head of double-suction pump has declined, while the efficiency has improved remarkably. The research results have significant guidance on excavating the potential of excellent hydraulic models and guiding the design of double-inlet multistage double-suction centrifugal pump.

Cavitation instability in pump part-load condition is an ongoing subject of research due to erosion and noise. For some high specific speed (n q ) pumps, NPSH 3% -peak at part-load represents one of the cavitation instabilities. This paper describes the recent investigations and results on performance and cavitation instabilities of a high n q pump. An n q 99 impeller that shows a NPSH 3% -peak at 70% of nominal flow rate is investigated by using numerical calculations and experiments. Both experimental and numerical results show that the head curves also have instability phenomenon near 70–80% of nominal flow rate. By analyzing simulation results, part-load recirculation near the shroud is found both under non-cavitating and cavitating conditions. With the pressure decreasing, the recirculation zone become larger and cavitation bubbles influence the flow pattern near the trailing edge at the suction side.

The primary goal of this work focuses on the cavitating vortices in the tip region of an axial-flow pump with 3 and 4 blades mainly based on the high-speed imaging experiments, with special attention on the trajectory and dynamics of a large-scale cavitation structure. The hydraulic and cavitation performance between two impellers were compared, and it can be found that the model with 4 blades has a relative wide range of stable operating conditions as well as the better anti-cavitation ability. By the analysis of the cavitation curves, it confirms that the highly unsteady tip cavitation cloud near the blade trailing edge should be responsible for the severe degradation of the performance. According to the detailed study on the cavitation evolution in the two impellers, it is observed that the trajectory of tip cavitating vortices for different flow rates seems very similar determined by the operating conditions. However, the dynamics varies significantly, which is associated with the blade loading and flow passage width.

The aggressiveness of a partial cavity on a hydrofoil in a cavitation tunnel is investigated from a joint numerical and experimental analysis. The numerical approach is based on a homogeneous equilibrium model and a barotropic law for the liquid/vapor mixture [1, 2] for cold water. The unsteady behavior of the cavity appears in good agreement with experiments although the maximum cavity length is underestimated and the cloud shedding frequency overestimated. The flow aggressiveness is estimated by using the energy approach proposed by Fortes-Patella et al. [3, 4]. The technique predicts a maximum in flow aggressiveness located in the region where the cloud cavity is shed and increases as a power of the flow velocity between 2 and 3.

Attached cavitation is a type of common cavitation phenomenon in fluid machinery. It is important to develop methods to control its generation. From the view of cavitation inception, the generation of attached cavitation is greatly influenced by the separated boundary layer upstream of cavitation detachment. In this research, a row of microscopic delta-shaped counter-rotating vortex generators (VGs) was applied on the leading edge of the NACA0015 hydrofoil in order to suppress the boundary layer separation and then suppress the generation of attached cavitation. The application of VGs fixed the position of cavitation inception on hydrofoil thus the sheet cavitation became more stable and the cloud cavity shed from hydrofoil with trim trailing edge more regularly. It was found that cavitation inception always appeared adjacent to VGs due to the low pressure in the corner of streamwise vortices induced by VGs. Hydrofoil with VGs showed an entirely different cavitation morphology on the leading edge. A row of separate microscopic vortex cavitation was induced by the counter-rotating vortices firstly. With the lower the height of VGs, the longer the length of these vortex cavitation due to the weaker interaction between vortices and main flow. Following the vortex cavitation, the attached cavitation was developing, but without typical “finger” structure anymore.

The cavitation erosion remains an industrial issue for many applications, mainly for hydraulic machines. This paper deals with the cavitation intensity, which can be described as the fluid mechanical loading causing cavitation damage. The estimation of this intensity is a challenging problem both in terms of modeling the cavitating flow and predicting the erosion due to cavitation. For this purpose, a numerical methodology was proposed to estimate cavitation intensity from 3D unsteady cavitating flow simulations. CFD calculations were carried out using Code_Saturne, which solves U-RANS equations for a homogeneous fluid using the Merkle’s model [1], coupled to a k-ε turbulence model with the Reboud’s correction. A cavitation intensity prediction model was developped based on pressure and void fraction derivatives obtained through CFD calculations. It was previously applied to study cavitation damage [2] on a NACA65012 hydrofoil. The article briefly presents this validation case as well as the prediction of the cavitation intensity on the blades of a centrifugal pump called “SHF pump” and tested at EDF R&D laboratory [3]. The numerical predictions of cavitation damage are in good agreement with experimental results obtained by pitting.

This paper discussed about suppressions of cavitation surge in a double suction centrifugal pump. In order to suppress the cavitation surge, CFD simulation was carried out. Cavitation surge was observed near the best efficiency point, and it was difficulty to operate the pump stably. The specific speed of the tested pump was about 81 [m 3 /min, min −1 , m] or 533 [ft., USGPM, min. −1 ]. In general, the main cause of the cavitation surge is inlet reverse flows at the impeller inlet. In order to prevent the inlet reverse flow, two kinds of modification at the impeller inlet were applied. One was the reduction of impeller inlet area by using a suction ring, and the other was the reduction of impeller inlet diameter. To reduce the computational time, in CFD model, a half of the double suction centrifugal pump was modeled. CFD simulations were carried out using ANSYS CFX with the Rayleigh Preset cavitation model. It was confirmed that the head fluctuation caused by the cavitation phenomena was predicted qualitatively by use of unsteady CFD simulation in the original pump impeller. The head fluctuation was about the 16% of the time averaged head and the very low frequency was confirmed by the FFT analysis. In addition, the relationship between head characteristics and cavitation behavior was observed clearly. The objective of the suction ring was to eliminate the head fluctuations caused by the cavitation. It was concluded that the suction ring was very effective to prevent the cavitation surge. On the other hand, the decrease of impeller inlet diameter was effective to reduce the head fluctuations, which became half of that for the original pump impeller. As a result, it was suggested that the complete suppression of the cavitation surge by the reduction of impeller inlet diameter was difficult in this case. It was concluded that unsteady CFD simulations with cavitation model is very effective for clarification of the impeller inlet modification on the cavitation surge.

Pump cavitation surge is highly coupled phenomenon with unsteady cavitation inside a pump and system dynamics of the pipe flow surrounding the pump. The piping system flow dynamics can be calculated under two kinds of assumptions; lumped parameter system (LPS) and distributed parameter system (DPS). In the lumped parameter system, the equations of motion of water columns inside pipes are calculated upstream and downstream of the pump. In the distributed parameter system, wave propagations along the pipes are calculated. In this study a simple system that consists of an upstream tank, an upstream pipe, a pump with cavitation, a downstream pipe and a downstream tank is analyzed by using two methods. Cavitation inside the pump is featured in the lumped parameters of cavitation compliance and mass flow gain factor. In the lumped parameter system case, equations of motion are calculated numerically by Runge-Kutta methods. In the distributed parameter system case, wave propagations are calculated by Method of Characteristics. From the comparison of two method results, appropriate criterion for practical piping system calculation is discussed.

A vapor-compression refrigeration cycle typically uses standard refrigerants as the working fluid. Traditional refrigerants, however, have been associated with Ozone level Depletion Potential (ODP) and significant Global Warming Potential (GWP). An innovative cooling technology has been investigated using sonic multi-phase flow in a critical-flow nozzle to create a low-pressure and low-temperature region for heat absorption. The strength of the new technology is its potential to produce cooling using water and/or other working fluids with low ODP and GWP. While the full potential for using water as the working fluid may not be fully-realized, because of property limitations still under investigation, water still provides a very useful media for investigating the underlying cavitation phenomena for the development of the new technology. As part of ongoing research into the potential cooling capacity of the cavitation phenomena, cavitation in a converging-diverging nozzle is being investigated using water as the working fluid. Cavitation in a fluid is the formation of the vapor phase from the liquid phase by reduction in the pressure of the fluid below its saturated vapor pressure. Due to the constricting nature of the throat of a converging-diverging nozzle, the liquid water velocity at the throat is increased and the local absolute pressure can drop to values below the saturated vapor pressure of water at a given temperature; thus, causing the fluid to cavitate. The effect of water temperature on both the onset of the cavitation within the nozzle, and the resulting length of the cavitation region within the nozzle, are the subject of the current paper. Flow Visualization using a high speed digital camera under these different operating conditions was aimed at investigating the region of cavitation onset, which also appears to be associated with the region of boundary layer separation just downstream of the throat of the nozzle. The length of the two phase region at different operating temperatures was measured and it was observed that as the temperature of the fluid was increased, the length of two-phase region before it condensed into single-phase liquid became longer. Experimental results and analysis are presented which also show that near the onset of cavitation, the flowrate can likely go well beyond a choking condition without cavitating, and can remain in this metastable state for an extended amount of time before nucleating (cavitating) into a stable state. In particular, analysis indicates that significantly negative absolute pressures can likely be achieved within the nozzle, suggesting the presence of tension in the liquid phase just prior to cavitation.

Underwater propeller cavitation noise is composed of tonal blade rate noise and high frequency broadband noise. Cavitation usually increases overall sound pressure level in the various frequency ranges which depends on the type of cavitation. This research had been carry out to predict the radiated noise from a marine propeller in presence of cavitation with various cavitation types. The analysis is performed by coupling an acoustic code based on the Ffowcs Williams-Hawkings (FWH) equation to unsteady Reynolds-averaged Navier-Stokes (URANS) which able to simulate multiphase flows in rotational domains. A brief summary of numerical method used to model the cavitation around the underwater propeller are presented and the thrust and torque coefficients are validated in different flow conditions by experimental results. The radiated noise along the shaft direction and perpendicular to the shaft direction is studied on both cavitating and non-cavitating propellers. Then, to predict the radiated noise due to cavitation in marine propeller, the computed sound pressure level (SPL) for non-cavitating marine propeller is compared with the SPL for the same propeller in cavitation conditions at various cavitation number and advanced coefficients. The noise analysis helps to determine the dominant noise source of the underwater propeller in different conditions, which will provide a basis for proper noise control strategies.