Cavitating Flow Regimes Research Articles

Characteristics of cavitating flows in cryogenic pump units in a combined cycle power system based on liquefied natural gas and solar energy

The cavitation of cryogenic pump units is an important cause for the low cold energy recovery efficiency and low utilization efficiency of solar energy in combined cycle power systems based on liquefied natural gas (LNG) and solar energy. Through numerical simulation of cavitation in cryogenic pump units, the research established the correlation mechanism between cavitation evolution and energy characteristics. By using the transport-based cavitation model considering the thermomechanical effect of cryogenic media, a novel numerical simulation model for two-phase flows during cryogenic cavitation was developed. In addition, the numerical model of cavitation was verified to be reliable. The cavitation performance of the two-stage LNG submerged pump in cryogenic pump units was studied under multiple conditions. In addition, the evolution of cavitating flow regimes under conditions of different flow rates and its influences on flow patterns of various flow passage components were investigated by combining analysis on temperature changes and flow fields. The entropy production diagnostic model (EPDM) was also adopted to discuss the distribution of the irreversible entropy production rate (EPR) in the evolution process of cavitation. Results show that the interfacial EPR that characterizes the phase-transition intensity after inception of cavitation exerts significant influences. Analysis of influencing mechanisms of changes in temperature, changes in flow fields, and distribution characteristics of energy consumption can provide a theoretical basis for further optimization.

Uncertainty Quantification of Turbulence Model Applied to a Cavitating Hydrofoil

This paper presents the Global Sensitivity Analysis of the coefficients of the standard k-ε turbulence model used in RANS (Reynolds Averaged Navier-Stokes) simulations aimed to predict the flow around a bi-dimensional hydrofoil operating at non-cavitating and cavitating flow regimes. The sensitivity analysis is treated by the Sobol Decomposition, where the Sobol Indices are computed through the Polynomial Chaos Expansion of the 2-nd order with a Point-Collocation Non-Intrusive approach. From the current results, it seems that the considered cavitating flow regime is less sensitive to the variability of the input parameters, at least for the prediction of lift and drag.

Dynamic mode decomposition to classify cavitating flow regimes induced by thermodynamic effects

The effect of temperature on the intensity and the dynamics of cavitation is investigated. Experiments of cavitating flows are conducted at various cavitation numbers and temperatures, leading to different cavity intensities and dynamic behaviors. The thermodynamic effects significantly influence the cavitation extent at elevated temperatures (over 58 °C) in water. Three regimes of instability, i.e. the sheet cavitation, the periodic single-cloud cavitation, and the aperiodic multi-clouds cavitation are distinguished based on their temporal-spatial evolutions. Application of DMD and POD decompositions on the velocity fields and gray level snapshots to determine the coherent structures and the various mechanics causing different shedding behaviors are discussed. The coherent structures obtained in cloud cavitation consist of the re-entrant jet and counter-rotating vortex structures, which are more aggressive dynamic behavior and are absent in the sheet cavitation. To inhibit the effects of hydrodynamic cavitation, it is recommended that a temperature range (55 °C–60 °C) of water be avoided in practical applications. Since working under this temperature range has larger potentials of transforming the steady sheet cavitation to unsteady cloud cavitation with the larger cavitation extent and the vigorous vapor cloud shedding.

An experimental/numerical assessment over the influence of the dissolved air on the instantaneous characteristics/shedding frequency of cavitating flow

The paper reports the numerical/experimental investigations of the cavitation phenomenon in the water flow in the presence of dissolved air. The dissolved air in the water was taken into account in the flow over a Clark Y 11.7% hydrofoil, one of the most common research objects of cavitating flow. The different cavitating flow regimes including the incipient, sheet/cloud and supercavitation were analyzed both numerically and experimentally with different amounts of air dissolved in the water. The dynamics of the cavitating flow, as well as; the main flow parameters, were compared and validated against the experimental data obtained from the closed-loop cavitation tunnel located at the Silesian University of Technology and the literature reports. The FFT analysis of outlet pressure fluctuation and vibration signals were applied to extract the main frequencies of the cavitating flows. The spectrum of vibrations and spectrum of pressure signals at the chamber outlet were in good agreement. The frequency of the main cavitating flow structures increased when the cavitation number increases. It was confirmed that for the tested values of dissolved oxygen in water (2.6 ppm and 5.5 ppm), it was possible to observe a detectable influence of air on cavitation frequencies and noticeable influence on cloud structures.

Comparative study of luminescence and chemiluminescence in hydrodynamic cavitating flows and quantitative determination of hydroxyl radicals production

Luminescence and chemiluminescence have been experimentally investigated in hydrodynamic cavitating flows. By using dedicated microdevices inserted inside a light tight box, photons counting has been made possible. Luminescence has been investigated with deionized water as the working fluid; chemiluminescence has resulted from cavitating alkaline luminol solutions, and has been correlated to hydroxyl radicals formation. For the first time, luminescent and chemiluminescent phenomena have been considered together on the same devices submitted to similar cavitating flow regimes. Degassed solutions enhance the luminescence and also the hydroxyl radical yield. Due to the small sizes of the channels, the lifetimes of the collapsing bubbles correspond to pseudo frequencies matching the range of optimal frequencies used in sonochemistry. New perspectives for the study of hydrodynamic cavitation as an advanced oxidation process are suggested.

Characterization of unsteady cavitating flow regimes around a hydrofoil, based on an extended Schnerr–Sauer model coupled with a nucleation model

The present work is aimed at the numerical spatio-temporal characterization of the unsteady cavitating flow structures on a hydrofoil by means of Proper Orthogonal Decomposition (POD) and Fast Fourier Transform (FFT) techniques. Three different cavitation regimes have been investigated: bubble cavitation, cloud cavitation and supercavitation. The homogeneous mixture approach has been used, in combination with an extended Schnerr–Sauer cavitation model. The accuracy of the numerical predictions has been improved by means of the implementation of a Density Correction Model of the turbulent viscosity, and a simplified Population Balance Modeling (PBM) which solved the spatial distribution and the temporal evolution of the nuclei. In particular, the PBM has led to a reduction of the intensity of the evaporation inside the vapor cavities and a consequent condensation enhancement at the cavity closure and in the wake downstream. This phenomenon mainly impacted on the vapor cavity dynamics during supercavition by facilitating the formation of the re-entrant jet and the vapor cavity detachment. Also, during supercavitation the nuclei density nb exhibited maximum variations of about 35.6% with respect to the inlet nuclei density. As the cavitation number increased, both the intensity and the extension of oscillations significantly reduced, and in bubble cavitation regime nb fluctuated at amplitudes of about 10% of the inlet nuclei density.The characterization of the cavitation regimes revealed that the bubble cavitation regime had a more stable and periodic dynamics highlighted by a higher hydrodynamic efficiency and a reduced root mean square of the lift force. The cloud cavitation and the supercavitation exhibited a more violent bubble detachment which caused stronger oscillations of the vapor cavity as well as the pressure upstream. This was retrieved in an increase of the average drag coefficient of about the 38% due to the presence of vapor cloud transported downstream, which promoted the surge of the flow. The vorticity analysis underlined that the formation of the re-entrant jet and the bubble detachment were promoted by the baroclinic vorticity, while the dilatation vorticity drove the dynamics of the detached clouds, governed by the phase change phenomena. The FFT analysis of the dynamics of the vapor cavity and the pressure upstream led to the detection of the most representative frequencies and Strouhal numbers of each cavitation regimes, in particular (fs=16.6Hz,St=0.355) for bubble cavitation, (fs=10.74Hz,St=0.358) for cloud cavitation, and (fs=8.79Hz,St=0.300) during supercavitation.The POD analysis allowed for the detection of the most relevant cavitating structures, in relation to the vapor cavity fluctuations and their frequency content. Furthermore, the FFT analysis of the temporal eigenfunctions demonstrated that the first POD mode of the liquid volume fraction described the overall unsteady behavior previously detected. Instead, high order POD modes revealed frequency values well above the overall ones of the main flow previously detected.

Mode decomposition methods for the analysis of cavitating flows in turbomachinery

The present work is aimed at the characterization of the cavitating flow regimes by applying the coupled POD/DMD technique to the vapor volume fraction field. The proposed approach provided an improved spatio-temporal-frequency description of the flow, based on the detection of the most energetic flow structures with information about their shape and size, and their decomposition into wave patterns oscillating with specific frequency and decay rate. The novel technique was applied to numerical results concerning the bubble cavitation and the supercavitation regimes of 2D water flows around a NACA hydrofoil at ambient temperature. Numerical simulations were performed by using a homogenous mixture model equipped with an extended Schnerr-Sauer cavitation model, in combination with a Volume of Fluid (VOF) interface tracking method.The proposed approached provided a better characterization of the unsteady cavitating flow, and allowed for a deeper insight about the dynamics of the vapor cavity, especially in cases involving the more chaotic regime of supercavitation. In particular, POD results figured out the most energetic coherent vapor structures associated to each cavitation regime: the first mode highlighted the main sheet cavity which grew on the hydrofoil up to detached, the second mode pointed out the cavitating/condensating doublet structures and the third mode figured out the smaller structures owning less energy but a higher frequency content. DMD modes performed a decomposition of the coherent structures detected by means of the POD analysis, into a subset of vapor pattern periodically evolving with a single frequency and a characteristic decay rate. Furthermore, results showed that the supercavitating flow structures owned characteristic frequencies which ranged from 5 to 26 Hz, while the less intensive bubble cavitation regime was characterized by frequencies ranging from 15 to 42 Hz.

Characterization of cavitating flow regimes in an internal sharp-edged orifice by means of Proper Orthogonal Decomposition

Cavitating flow regimes of water at ambient temperature inside a sharp-edged orifice have been characterized by means of a snapshot POD combined with high-speed visualizations. The density related gray level of each experimental image was processed at each pixel point. An estimation of the energy contribution in image reconstruction of each POD eigenvalue was made. Coherent flow structures related to the most significant eigenfaces were detected. Finally an FFT analysis of the temporal eigenfunctions led to the identification of the main modal dynamic properties.It was found that the flow structure of the first POD mode was defined by a single symmetric cavity. This cavity is related to the extension of the cavitating cloud. Conversely polarities and asymmetries in flow structures of higher modes reveal the establishment of advecting vortices and swirling motions. The frequency analysis of the first four POD modes has been used for the characterization of the different cavitation regimes. As the cavitation number decreases, the jet cavitation is characterized by a rapid increase of the frequencies of modes 3 and 4, up to 1000 Hz. Instead, developed cavitation and supercavitation are characterized by a growth in frequency of the second mode up to 300 Hz, as well as they exhibit the highest FFT magnitudes of the first mode.

Numerical research on unsteady cavitating flow over a hydrofoil

Cavitation is a widely known phenomenon in pumps and water turbines installations. It can lead to significant damage of blades and walls of the rotor therefore it is crucial during pump designing and exploitation to avoid working in flow conditions, that enabled cavitation to occur. Nowadays numerical simulations of flow can provide valuable information concerning pressure and velocity distribution and can indicate if there is a risk of cavitating flow appearance. There are a few mathematical models which describe cavitating flow. In the paper Schnerr & Sauer model was chosen for simulation. Aim of the paper is to verify its utility in case of different cavitating flow regimes over Clark-Y hydrofoil. After performing the grid independence study four different cavitation regimes were investigated. The vapour areas appearance, their shapes and changes in time were observed. The assumption of isothermal, two - phase flow was made. The calculations were performed using OpenFOAM and were compared to the available measurements data. The presented results concerned sheet and cloud cavitation regimes.

Visualization of microscale cavitating flow regimes via particle shadow sizing imaging and vision based estimation of the cone angle

Recent studies show the destructive effect of the energy released from the collapse of cavitation bubbles, which are generated in micro domains, on the targeted surfaces. The cavitation phenomenon occurs at low local pressures within flow restrictive elements and strongly affects fluid flow regimes inside microchannels which results in spray formation. Extended cavitation bubbles toward the outlet of the microchannel, droplet evolution, and spray breakup are among crucial mechanisms to be considered in spray structure. In this study, various spray structures under the effect of hydrodynamic cavitation were recorded using a high speed visualization system. Acquired images were analyzed and characterized using several image processing algorithms. In this regard, the fluid flow with ascending upstream pressures from 10 to 120bar were passed through a microchannel with an inner diameter of 0.152mm. The spray at the outlet of the microchannel was analyzed for these pressures in four different segments. Particle Shadow Sizing (PSS) imaging and several image processing techniques such as contrast stretching, thresholding and morphological operations were employed to identify the flow regimes in the separated segments. In addition, a vision based estimation technique that utilizes a Kalman filter was developed to estimate cone angle of the spray. Furthermore, classification of fluid flow regimes and morphological characteristics of the spray structure were outlined based on the cavitation number.

An experimental study of water-entry cavitating flows of an end-closed cylindrical shell based on the high-speed imaging technology

The objective of this present study is to address the cavitating flow patterns and regimes in the water-entry cavity. For this purpose, an experimental study of vertical water-entry cavity of an end-closed cylindrical shell is investigated by using high-speed video cameras and visualization technique. According to the cavitating flows as observed in the experiments, two flow pattern forms of fluctuation cavitation and cloud cavitation are found around the body. A further insight into the characteristics of the cavity shape and the variation in the cavity fluctuations parameters is gained by analyzing the image data. Furthermore, the experiments at different impact velocities are conducted to analyze the effects of impact velocity on the flow patterns and parameters. Finally, the formation mechanisms of cavitation fluctuations and cavitation clouds are studied based on the basic theory of fluid mechanics. The obtained results show that the cavitation flow pattern form of fluctuation cavitation occurs under the impact velocity condition of low speed, and the cloud cavitation occurs under the velocity condition of high speed. As fluctuation cavitation, the maximal extension diameters of cavitation fluctuate periodically along the water depth, and the speeds of extension and shrinkage are both proportional to the extension diameter. The collapses are different for the two flow pattern cavitations, i.e., the fluctuation cavitation, which is of deep closure and closed at the trough of wave cavitation more than once, and the cloud cavitation, which falls off and forms the leading edge of the cylindrical shell. The frequency fluctuation is independent of the impact velocity, the corresponding pinch-off time decreases with increasing the impact velocity, and the pinch-off time decreases in a nearly linear relation with Froude number. The water poured to the cylindrical shell causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of pressure distribution and velocity field occur around the leading edge of the cylindrical shell, then the extended intensity of the cross section of the cavity shows variation in this process, which can be defined as fluctuation cavitation pattern. It appears that the re-entrant flow after the pinch-off at the trailing edge of cavity, then the laminar-turbulent transition is waken as a consequence of the re-entrant flow moving upstream, which flow pattern involved in this structure occurs as cloud cavitation.

Evaluation of the physical forces exerted on a spherical bubble inside the nozzle in a cavitating flow with an Eulerian/Lagrangian approach

An Eulerian/Lagrangian approach is used to calculate the physical forces acting on a spherical bubble. Reynolds average Navier–Stokes (RANS) equations for the Eulerian approach are solved with a finite volume scheme. The SIMPLE algorithm is utilized for pressure and velocity linkage. To model convective fluxes, an upwind scheme is used. The Reynolds stress transport model (RSTM) is used to calculate the turbulent parameters. In the Lagrangian approach, a modified form of the Reyleigh–Plesset (RP) and Maxey equations are solved with MATLAB programming software for evaluation of bubble motion and bubble dynamics. The carrying fluid in this study is diesel fuel. Continuous filter white noise (CFWN) is solved parallel to the Maxey and RP equations to calculate fluctuating terms of velocity in x and y directions. Six forces exerted on the bubble during its motion are investigated inside the cavitating flow regime. The cavitating regime can be extremely effective on bubble force and increase bubble forces up to several thousand times. Added mass force in the y direction has the highest value among all forces exerted on the bubble during its motion inside the nozzle.

Numerical analysis of unsteady cavitating flow in an axial inducer

This work presents the results of numerical simulation of unsteady cavitating flow through a two–bladed axial inducer. First, the analysis was carried out in a blade cascade, this two–dimensional simplified model, obtained from the studied axial inducer, was used as a test case. Later, the numerical simulations were extended to the original three-dimensional inducer. All numerical calculations were realized in cavitating flow regime. Initially, the results were obtained in steady state, and then in unsteady state.The main purpose of this study is to explore the local cavitation instabilities, such as alternate blade cavitation and rotating blade cavitation, which can appear in this type of devices when they work under certain operating conditions.The numerical results show that the fluid flow in the axial inducer is altered by the emergence of the cavitation. These vapor regions are formed, firstly near to the leading edge of each blade. The behavior of the cavitation depends on the operating conditions of the inducer, mainly by the flow rate and the suction pressure.The numerical simulation was performed using a commercial code based on a cell–centered finite–volume method. The cavitation model used for calculations assumes a thermal equilibrium between phases. It is based on the classical conservation equations of the vapor phase and a mixture phase, with mass transfer due to the cavitation appearing as a source and a sink term in the vapor mass fraction equation. The mass transfer rate is derived from a simplified Rayleigh–Plesset model for bubble dynamics.

Experimental Study of Cavitating Flow inside Enlarged Transparent Injector Nozzles and its Effect on Spray

The complicated cavitating flow inside diesel nozzles has long been concentrated on. Based on the visualization experimental system, cavitating flow characteristic inside transparent nozzles with different length-diameter ratios was investigated in this paper. Experimental results showed that the cavitation is easier to occur for diesel than for biodiesel, and the longer the length of the orifice is, the harder the cavitation and hydraulic flip phenomenon to occur. Discharge coefficient of diesel was higher than that of biodiesel in single-phase flow region, while got much smaller than that of biodiesel in the cavitating flow and hydraulic flip regions.The spray cone angle of diesel was larger than biodiesel in cavitating flow regime, and got much smaller with the increasing of the length-diameter ratios under the same level of injection pressure in diesel and biodiesel fuels.

Experimental evidence of temperature gradients in cavitating microflows seeded with thermosensitive nanoprobes

Thermosensitive fluorescent nanoparticles seeded in deionized water combined with confocal microscopy enables thermal mapping over three dimensions of the liquid phase flowing through a microchannel interrupted by a microdiaphragm. This experiment reveals the presence of a strong thermal gradient up to ~10(5) K/m only when hydrodynamic cavitation is present. Here hydrodynamic cavitation is the consequence of high shear rates downstream in the diaphragm. This temperature gradient is located in vortical structures associated with eddies in the shear layers. We attribute such overheating to the dissipation involved by the cavitating flow regime. Accordingly, we demonstrate that the microsizes of the device enhance the intensity of the thermal gap.

Mapping of cavitating flow regimes in injectors for medium-/heavy-duty diesel engines

Reducing the sac volume size of medium-/heavy-duty diesel engine injector nozzles can minimise the fuel dripping into the combustion chamber at the end of injection events, which has been linked to reduced engine-out emissions. This study demonstrates the effect of reduction in the sac volume of diesel fuel injectors utilised in medium-/heavy-duty applications on the internal nozzle flow. This is realised by comparison of two heavy-duty diesel nozzles that feature a large difference in sac volume size of almost three times. For visualisation purposes, the nozzles have been enlarged by six times, and replicas were manufactured from a transparent material. High-speed digital imaging was used to capture the instantaneous spatial and temporal characteristics of geometric as well as dynamic vortex cavitation structures. The investigation was conducted in a steady-state flow test rig for three different needle valve lifts. For all tested conditions, the flow behaviour was analysed at three distinct areas of the nozzle, these being the needle seat, the sac volume and the injection hole. Interpretation of experimental observations was supported by parallel computational fluid dynamics simulations of the exact conditions measured during the experiments. Post-processing of the captured images has revealed the ensemble – average cavitation location, its standard deviation and the cavitation structures life – time inside the sac volume. Results showed a significant dependency of the internal nozzle flow on the sac volume size and identified clear differences in the structure of the cavitation pockets inside the sac volume under certain operating conditions.

An artificial neural network approach to investigate cavitating flow regime at different temperatures

Identification of cavitating regime is an important issue in a wide range of fluid dynamic systems. The cavitation behavior is affected by several parameters, as the operating pressure and the fluid temperature. In the present study the cavitating behavior of water inside an orifice was analyzed by images analysis and by pressure signals. Four cavitation regimes were characterized: no-cavitation, developing cavitation, super cavitation and jet cavitation. A three-layer Elman neural network was designed to predict the cavitation regime, from the frequency content of the pressure fluctuations, recorded upstream and downstream the internal orifice. Cavitation regimes were successfully predicted. The designed neural networks were useful also to underline the influence of each operating parameter on the phenomena under investigation; in particular it was possible to identify the frequency ranges that characterize the different cavitation regimes and the influence of the fluid temperature.

Draft tube discharge fluctuation during self-sustained pressure surge: fluorescent particle image velocimetry in two-phase flow

Hydraulic machines play an increasingly important role in providing a secondary energy reserve for the integration of renewable energy sources in the existing power grid. This requires a significant extension of their usual operating range, involving the presence of cavitating flow regimes in the draft tube. At overload conditions, the self-sustained oscillation of a large cavity at the runner outlet, called vortex rope, generates violent periodic pressure pulsations. In an effort to better understand the nature of this unstable behavior and its interaction with the surrounding hydraulic and mechanical system, the flow leaving the runner is investigated by means of particle image velocimetry. The measurements are performed in the draft tube cone of a reduced scale model of a Francis turbine. A cost-effective method for the in-house production of fluorescent seeding material is developed and described, based on off-the-shelf polyamide particles and Rhodamine B dye. Velocity profiles are obtained at three streamwise positions in the draft tube cone, and the corresponding discharge variation in presence of the vortex rope is calculated. The results suggest that 5–10 % of the discharge in the draft tube cone is passing inside the vortex rope.

Hydrodynamic cavitation in microsystems. II. Simulations and optical observations

Numerical calculations in the single liquid phase and optical observations in the two-phase cavitating flow regime have been performed on microdiaphragms and microventuris fed with deionized water. Simulations have confirmed the influence of the shape of the shrinkage upon the contraction of the jet, and so on the localisation of possible cavitating area downstream. Observations of cavitating flow patterns through hybrid silicon–pyrex microdevices have been performed either via a laser excitation with a pulse duration of 6 ns, or with the help of a high-speed camera. Recorded snapshots and movies are presented. Concerning microdiaphragms, it is confirmed that very high shear rates downstream the diaphragms are the cause of bubbly flows. Concerning microventuris, a gaseous cavity forms on a boundary downstream the throat. As a consequence of a microsystem instability, the cavity displays a high frequency pulsation. Low values Strouhal numbers are associated to such a sheet cavitation. Moreover, when the intensity of the cavitating flow is reduced, there is a mismatch between the frequency of the pulsation of the cavity and the frequency of shedded clouds downstream the channel. That may be the consequence of viscous effects limiting the impingement of a re-entrant liquid jet on the attached cavity.

Cavitation in flow through a micro-orifice inside a silicon microchannel

Hydrodynamic cavitation in flows through a micro-orifice entrenched in a microchannel has been detected and experimentally investigated. Microfabrication techniques have been employed to design and develop a microfluidic device containing an 11.5μm wide micro-orifice inside a 100.2μm wide and 101.3μm deep microchannel. The flow of de-ionized water through the micro-orifice reveals the presence of multifarious cavitating flow regimes. This investigation divulges both similarities and differences between cavitation in micro-orifices and cavitation in their macroscale counterparts. The low incipient cavitation number obtained from the current experiments suggests a dominant size scale effect. Choking cavitation is observed to be independent of any pressure or velocity scale effects. However, choking is significantly influenced by the small stream nuclei residence time at such scales. Flow rate choking leads to the establishment of a stationary cavity. Large flow and cavitation hysteresis have been detected at the microscale leading to very high desinent cavitation numbers. The rapid transition from incipient bubbles to choking cavitation and subsequent supercavitation suggests the presence of radically different flow patterns at the microscale. Supercavitation results in a thick cavity, which extends throughout the microchannel, and is encompassed by the liquid. Cavitation at the microscale is expected to considerably influence the design of innovative high-speed microfluidic systems.