Submerged Water Jet Research Articles

Experiments and numerical analysis of rebounding behaviors of a laser-induced cloud cavitation

It is well known that a high-speed submerged water jet generates a cloud of cavitation bubbles experiencing a collective unsteady behavior that repeats growth and collapse. In particular, it is considered that a high-pressure shock wave may be emitted associated with the collapse of the cloud. However, its mechanism has not been enough clarified and hence it is required to be explored both from the experimental and numerical analyses. In this study, we fist make an observation of such a unsteady behavior of the cloud cavitation that is generated by a pulsed Ho:YAG laser-induced liquid jet using a high-speed video camera. Then, a two-dimensional numerical analysis based on the SPH method is examined to simulate the unsteady behavior of the cloud, in which the mixture model of liquids and gas bubbles is employed. Finally, the validity of our numerical analysis is illustrated by showing that the unsteady behavior of the cloud from its inception, growth, shrink, collapse to rebound is realized in qualitative agreement with the experimental data.

Evaluation of the effects of shear stress on crucian carps passing through turbines

Turbine-passed fish is inevitably exposed to one or more physical forces, such as fast pressure change, blade strikes and shear stress, which can lead to injury or fatalities. The injury mechanisms of these forces to fish and related criteria are necessary for fish-friendly-turbine design and optimization. The effects of shear stress on crucian carps are studied in a specially designed experimental system. Crucian carps are exposed to a submerged water jet at velocities ranging from 5m/s to 20m/s. High-speed, high-resolution digital videos of the exposure process was recorded for motion-tracking analysis. Fish sizes, flow directions and water jet velocities are key factors considered during the tests. Injury rate, degree of severity, site of trauma, and death rate of crucian carps after tests were studied according to repeated tests with different factors. Injuries such as shedding of fish scales, tearing of gill cover, laceration of eyeballs are observed at different water jet velocity and with different size of crucian carps. The injury mechanisms were concluded and design criteria for fish-friendly-turbine are proposed according to the study.

PIV investigation of high Reynolds number submerged water jets at high-pressure ambient conditions

High-pressure water jets bear a great technological potential to enhance geothermal deep drilling. Compared to existing water cutting technologies, significantly different operation conditions are encountered under deep-drilling conditions, such as high ambient pressures. The fundamental fluid mechanics are significantly affected by those operation conditions. In this work we examine the influence of increasing ambient pressure of up to 12.0 MPa on the water jet characteristics under submerged drilling conditions. PIV measurements of the jet flow field at changing cavitation numbers reveal two characteristic regimes, which are distinguished by a critical cavitation number. In the cavitating regime, the jet decays considerably faster with increasing distance to the nozzle than in the non-cavitating regime. In addition to that, an increasing cavitation intensity shortens the potential core length of the water jet and increases the jet spreading angle and with this has a similar effect on the jet as increasing turbulence intensity in single-phase flows. Related to the decreasing kinetic energy of the jet in the cavitating regime, the resulting impact force of the water jet on the specimen surface decreases with increasing cavitation intensity. Our investigations indicate that a technology transfer from water jet cutting to submerged jet drilling requires adjustments of both nozzle geometries and jet operation conditions.Graphic abstract

Influence of thickness of hydrophobic polytetrafluoroethylene (PTFE) coatings on cavitation erosion of hydro-machinery steel SS410

Hydrophobic materials have the potential to replace the existing surface modification materials to combat the problem of cavitation erosion (CE), owing to their various capabilities like self-cleaning, anti-corrosion, anti-fogging, anti-icing, anti-fouling, and drag-reduction. Polytetrafluoroethylene (PTFE) as a hydrophobic material exhibits specific combination of properties such as low friction coefficient, chemically neutral, good impact strength and considerable thermal firmness, that allow PTFE to be used in a wide range of applications. Therefore, in this work, an effort has been made to explore the effect of thickness of hydrophobic PTFE coatings on CE of the hydro-machinery SS410 steel. Hydrophobic PTFE coatings with three variable thicknesses have been prepared on the hydro-machinery SS410 steel using sintering process. Using high-velocity submerged water jet cavitation erosion test rig, CE behaviour of PTFE coated and bare SS410 steel was evaluated under various combinations of operating parameters: namely jet velocity, stand-off distance (SOD), and impact angle. Contact angle was reported to be increased with the decrease in the thickness of PTFE layer, which may be due to the increased surface roughness of PTFE layer with decrease in thickness. CE resistance of PTFE coated material with a maximum thickness of 121 μm, was found to be highest for all the combinations of operating parameters, which may be attributed to its lowest fracture toughness and better cushioning effect. Further, it was observed that CE of the coated and SS410 steel was found to be maximum at the chosen maximum velocity, normal impact angle, and intermediate SOD. The signatures of pits with circular and elongated morphology were observed for normal and shallow impact angles, respectively. In case of lowest PTFE layer thickness, overlapped pits with relatively higher depth were observed, which might be responsible for higher CE of the same amongst all the PTFE coatings.

Cavitation peening using pulse laser and submerged water jet to improve fatigue strength of aluminum alloy

Cavitation peening using pulse laser and submerged water jet to improve fatigue strength of aluminum alloy

Experimental study of flow characteristics of an oblique impinging jet

An experimental study has been performed by using particle image velocimetry to investigate the effect of nozzle-to-plate separation distance (L) and jet impingement angle on the flow characteristics of an obliquely inclined submerged water jet. Measurements were taken for L = 1D, 2D, 4D and 6D (where D is the diameter of the nozzle) for the jet impingement angle (θ) of 45° and 26°, and the flow characteristics in the uphill and the downhill regions are investigated at Reynolds number of 2600 (based on the nozzle diameter D and the jet velocity $$U_{{\text{o}}}$$). It is observed that surface spacing has an opposite effect in the uphill and the downhill regions in terms of wall jet flow. In the uphill region, the tendency of the wall jet to grow increases with increase in L/D. However, in the downhill region, the jet velocity and its thickness are observed to reduce as the separation distance is increased. The distance between the stagnation point and the geometric centre is observed to decrease with increase in L/D because of jet–ambient fluid interaction in the uphill region. The jet width is observed to grow for θ = 45° in the downstream of the plate due to enhanced jet–ambient fluid interaction. Flow at θ = 26° shows that after impingement the entire jet deviates towards the downhill side, which indicates the existence of a critical impingement angle below which there is no flow of the jet in the uphill region. RMS velocity fluctuations and shear stress show an increased turbulence level downstream of the plate in the downhill region for smaller impinging distance implying higher jet–ambient fluid interaction and increased jet width. They, along with the negative turbulence production term, reveal the region of flow separation and reattachment. The decrease in the peak value of Nusselt number can be related to the drop in the jet momentum at the stagnation point with increase in the surface spacing.

Experimental Study on Water‐Jet Shock Microforming Process Using Different Incident Pressures

The purpose of this paper is to demonstrate a new process technology using the cavitation phenomenon, mainly a water‐jet shock microforming, for the fabrication of a metallic foil. 304 stainless steel was exposed to a high‐speed submerged water jet with different incident pressures and certain working conditions. In this experiment, a KEYENCE VHX‐1000C digital microscope, confocal laser‐scanning microscope (Axio CSM 700), and micro‐Vickers hardness tester were utilized to observe the forming depth, surface quality, thickness distribution, and section hardness distributions under different incident pressures. The experimental results indicated that the surface morphology of the metal foils attained good geometrical features under this dynamic microforming method and there were no cracks or fracture. The forming depth and surface roughness increased with the incident pressure. In addition, the forming depth increased from 124.7 μm to 327.8 μm, while the surface roughness also increased from 0.685 μm to 1.159 μm at an incident pressure of 8 MPa to 20 MPa. Maximum thickness thinning of the formed foils occurred at the fillet region when the thickness thinning ratio was 21.27% under the incident pressure of 20 MPa, and there was no fracture at the bottom or the fillet region. The tested hardness indicated that during the cold‐rolled state of the sample, the hardness sample increased slightly along the cross section of the formed region and the hardness of the annealed 304 stainless steel foils increased significantly along the cross‐sectional region.

Numerical simulation of unsteady cloud cavitation: a comparative study of compressible mixture models

Focused on the unsteady behavior of intensively cavitating water jets numerical analysis is carried out by applying the compressible mixture flow method under assumptions of bubble cavitation and homogenous mixture. Submerged water jets issuing from a sheathed nozzle is treated when the cavitation number σ ≅ 0.1. The periodically shedding of cavitation clouds in submerged water jet is captured acceptably and the core velocity distribution evaluated by numerical simulation agrees with experiment data of PIV approximately. Concerning the effect of flow compressibility estimation, comparison of computation results reveals that it is capable to capture the unsteady behavior of cavitating flow by both the simplified bubble cavitation model and the homogeneous mixture model. However, the homogenous model shows a tendency to estimate gas volume fraction of cavitation clouds excessively. It becomes necessary to evaluate the mixture compressibility by considering the effect of bubble dynamics in modeling intensively cavitating flow. The simplified isothermal bubble cavitation model is demonstrated to be a practical method for treating bubble-liquid flow with intensive cavitation.

Comparison between the improvements made to the fatigue strength of stainless steel by cavitation peening, water jet peening, shot peening and laser peening

In order to make a clear comparison between the various peening methods used to improve the fatigue strength of stainless steel 316L, the fatigue strength of specimens treated by cavitation peening, water jet peening, laser peening and shot peening were examined using a plate bending fatigue test. Both cavitation peening and water jet peening were carried out using a submerged water jet system, and these are distinguished by the distance from the nozzle to the specimen. A submerged laser peening system was used for laser peening. The impact with the surface due to laser cavitation was greater than that due to laser ablation. For each peening method, the optimum coverage was examined by measuring the fatigue life at constant bending stress. Then, the fatigue strength of specimens treated with the optimum coverage was examined. The fatigue strength of the non-peened specimen was 279 MPa, whereas it was 348 MPa for cavitation peening, 325 MPa for shot peening, 303 MPa for laser peening and 296 MPa for water jet peening. The fatigue strength of each peening method was affected by the surface roughness and work hardening.

Structure optimization of submerged water jet cavitating nozzle with a hybrid algorithm

ABSTRACTIt is critical to obtain a suitable geometry for a cavitating nozzle of submerged water jet, since a suitable geometry determines its cavitation intensity and cavitation distribution. This paper puts forward a water jet cavitating nozzle optimization platform aiming to improve its axial maximum vapor volume fraction. Considering the influence of all eight geometric parameters of the nozzle itself, a hybrid optimization algorithm is developed to optimize the nozzle by combining with the CFD technique. After the optimization, the optimal geometry results to a 9.41% increment in the axial maximum vapor volume fraction at 50 m underwater depth, and improves the cavitation effect obviously especially in deeper water. From the results, it is found that the cylindrical section diameter, contraction angle and diffusion angle have a great influence on the axial maximum vapor volume fraction, while the inlet diameter have the least effect on the same. The optimization presented in this study can lay the foundation for further study on water jet cavitating nozzle.

Numerical analysis on cavitation effects in submerged water jet added with turbulent drag-reducing additives of CTAC

In this paper the cavitation phenomenon and bubble collapse in submerged water jet added with CTAC were studied. The cavitation simulation was based on an improved cavitation model established by introducing coefficients into traditional cavitation model. The coefficients were obtained by fitting the numerical solutions of Rayleigh-Plesset equation in which the viscosity term and surface tension term were not neglected. Cross model was also employed to express the shear-thinning character of the surfactant solution. Simulation results show that the whole cavitation intensity decreased but the scale of cavitation region increased in drag-reducer solution. The submerged water jet added with drag-reducer was less dispersed and the stagnation pressure near the wall was significantly improved. Simulation results of the bubble collapse process show that the bubble collapsed more slowly in drag-reducer solution in comparison with that in pure water. The two pressure peaks near the wall during the collapse process appeared later in drag-reducer solution and the maximum value of the pressure generated in drag-reducer solution was less than that in pure water.

Numerical simulation of unsteady cavitating jet by a compressible bubbly mixture flow method

Focused on the unsteady behaviour of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a compressible mixture flow method combined with a simplified bubble cavitation model. The mean flow of two-phase mixture is calculated by URANS for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the expansion and contraction of bubbles the mean flow field. High-speed submerged water jet issuing from a sheathed nozzle is treated when σ = 0.1 and Re = 4.5x105 . The distribution of cavitation clouds predicted approximately agrees with experiment data of visualization observation and the unsteady behaviour of cavitation cloud shedding is captured acceptably. Computational results reveal that cavitation occurs at the entrance of nozzle throat and bubble clouds expand and develop while flowing downstream along the boundary layer. Developed bubble clouds split into small blocks and sheds downstream periodically near the exit of sheath.

A new approach on anti-vortex devices at water intakes including a submerged water jet

A new approach on anti-vortex methods as hydraulic-based anti-vortex was investigated experimentally in the present study. In the investigated method, a submerged water jet is used as the anti-vortex mechanism. The added jet acts as a source of external momentum. This leads to change the intake-induced hydrodynamic pattern in the near-field of the intake structure, which can prevent formation of undesirable intake vortices. The experiments were carried out on a horizontal pipe intake. By performing 570 test cases in two different categories, including the inclined jet with respect to the axis of the intake, and the inclined jet with respect to the water surface, the effects of the jet inclination angle on the anti-vortex performance were investigated. It was found that the inclined jet with respect to the water surface is the best alternative to consider as the water jet injection pattern. Results showed that using the inclined jet with respect to the water surface can simply reduce the amounts of the expected water jet momentum more than 50% compared to that of the similar condition of the horizontal injection pattern. Moreover, it was concluded that the intake critical submergence can easily be minimized using the inclined jet with respect to the water surface.

Effect of pulsation on the near flow field of a submerged water jet

The current study investigates the effect of pulsation frequency on the near field characteristics of a submerged water jet using the technique of dye visualization. Flow visualization was performed in water over the range: Reynolds number 540–1540, Strouhal number 0.16–1.75, and at constant amplitude of pulsation of 18%. The results show that the mixing and entrainment process at lower Reynolds number occurs due to diffusion process owing to relatively stable shear layer for the case of a steady jet, whereas the external pulsation promotes an early instability in the shear layer where irregular structures promotes mixing between the jet and surrounding fluids. Images of streaklines show that initial mixing and entrainment processes in the potential core of the jet is due to the development of large vortical structures. While beyond the potential core, mixing and entrainment are governed by the small-scale structures. Further results show that the initiation and growth of vortices in the shear layer depends on the pulse frequency. For a given Reynolds number and amplitude, the number of vortical structures and their size changes with frequency. With an increase in the pulsation frequency, there is an increase in the spreading of the jet along with stretching of the vortical structures. An optimum pulsating frequency at which the effect of pulsation on the flow is maximum occurs at St = 0.44, independent of Reynolds number. These results should eventually lead to a better understanding of the physical phenomena responsible for enhanced mixing and entrainment processes in the presence of pulsating jets.

Periodic Behavior of Cavitation Cloud Shedding in Submerged Water Jets Issuing from a Sheathed Pipe Nozzle

The behavior of cavitation cloud shedding in submerged water jets issuing from a sheathed pipe nozzle is investigated experimentally by high-speed camera visualization observation. Experiments are carried out under different cavitation numbers decreased to 0.01 with increase of the injection pressure, and the frequency spectrum of cavitation cloud shedding is evaluated by statistical analysis of a sequence of high-speed camera images. Experiments demonstrate that cavitation clouds appear when the cavitation number σ decreases to the level of 0.5-0.7 and developed cavitation clouds shed downstream periodically at multiple frequencies. The low frequency components of cavitation cloud shedding is basically dependent upon the pressure pulsation of plunger pump, which is often employed in various industry application of water jets. However, the high frequency components are closely related to the shedding of vortexes and the collapsing of cavitation clouds, which are dependent on the flow structure of submerged jets and the property of cavitation clouds consisted of numerous bubbles.

Vortex Dissipation Using a Hydraulic-Based Anti-Vortex Device at Intakes

In the present research, dissipating free surface vortices over the intakes by designing a hydraulic anti-vortex method is studied experimentally. In this method, an up-stream submerged water jet is used as a hydraulic-based anti-vortex mechanism. The imposed jet acts as a point source of external momentum to the formed free surface area. The added momentum changes the hydrodynamic balance of the vortex zone, and leads to instability and disability of the vortex to form and develop. Experiments were conducted on a projected horizontal pipe intake to investigate the efficiency of the proposed anti-vortex hydraulic device on dissipating and preventing formation of free surface vortices. Results show that the efficiency of the imposed jet is dependent on the jet linear momentum and vertical distance between the nozzle and intake positions. Moreover, the critical submergence of the intake can potentially reduce up to the nozzle submergence in which less than 10% of the jet momentum is required by decreasing nozzle submergence about 50%. Finally, an empirical equation is suggested to predict the critical submergence of the intake with respect to the jet linear momentum and the nozzle submergence.

Reversed Heat Flux Study of Impinging-Jet Water Cooling for Helium-Cooled Finger-Type Divertors

Experimental evaluation of the thermal-hydraulic characteristics of helium-cooled divertor concepts is important in developing commercial magnetic fusion energy (MFE). Although experimental studies of a variety of concepts have been performed at the Georgia Institute of Technology (GT) over the last decade, achieving prototypical steady-state incident heat fluxes of 10 MW/m2 remains a major challenge. As an alternative to heating the test section, this work presents an initial assessment of a “reversed heat flux approach” that cools the test modules (instead of heating them) with water to determine the heat transfer coefficients (HTC). This approach was pioneered by the Karlsruhe Institute of Technology (KIT) in their initial studies of the helium-cooled modular divertor with multiple jets (HEMJ).The objectives of this design study are to: 1) determine whether such a reversed heat flux approach can be used to experimentally study the thermal-hydraulic performance of helium-cooled divertor concepts, while minimizing safety and operational issues associated with the extremely high temperatures (>1200°C) reached when testing at prototypical conditions (inlet conditions of 700°C and 10 MPa with an incident heat flux of 10 MW/m2), and 2) determine the design and operational parameters for a small-scale submerged water jet impingement cooling facility suitable for validating these numerical predictions. Numerical simulations were performed to determine the impinging-jet (water) mass flow rates required to remove heat fluxes up to 10 MW/m2 from a single HEMJ module at prototypical conditions (i.e., 700°C and 10 MPa). Initial axisymmetric simulations at water pressures up to 3 MPa suggest that a submerged single-phase impinging water jet at (300 K, 1 MPa) and = 3.5 kg/s can remove heat fluxes as great as 7.5 MW/m2 over a 2 cm diameter area.

Theoretical and experimental study of scour depth by submerged water jet

The characteristics and working principles of the impinging by submerged water jet are analyzed, and the relevant mathematical model is optimized based on Rajaratnam’s theoretical and experimental study. A new mathematical model is constructed by adding an important parameter called impinging angle. A new experiment is designed according to the working conditions of various impinging distances and angles. In combination with the experiment data and with the use of the curve fitting method, the functional relationship between the impinging distance and angle as well as the coefficient C4is obtained. The experiment results show that the scour depth decreases as impinging distance increases, followed by a trend from decline to rise before falling again; those two turning points occur within the range of 20d–25d. The scour depth constantly increases with rising impinging angle, and the maximum and minimum increasing ranges can reach 180% and 50%, respectively, with the impinging angle increasing from 40° to 90°.

Numerical analysis of cavitation cloud shedding in a submerged water jet

Focused on the unsteady behavior of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a practical compressible mixture flow bubble cavitation model with a simplified estimation of bubble radius. The mean flow of two-phase mixture is calculated by unsteady Reynolds averaged Navier-Stokes (URANS) for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas bubbles whose radius is estimated with a simplified Rayleigh-Plesset equation according to pressure variation of the mean flow field. High-speed submerged water jet issuing from a sheathed sharp-edge orifice nozzle is treated. The periodically shedding of cavitation clouds is captured in a certain reliability compared to experiment data of visualization observation and the capability to capture the unsteadily shedding of cavitation clouds is demonstrated. The results demonstrate that cavitation takes place near the entrance of nozzle throat and cavitation cloud expands consequentially while flowing downstream. Developed bubble clouds break up near the nozzle exit and shed downstream periodically along the shear layer. Under the effect of cavitation bubbles the decay of core velocity is delayed compared to the case of no-cavitation jet.

Mixing and entrainment characteristics of a pulse jet

Pulsating jets can be more useful for enhancing mixing and heat transfer than steady jets; however, their flow physics is still not clear. In this work, particle image velocimetry based measurements have been undertaken to investigate the flow structure and mixing characteristics of axisymmetric pulsating jets. This experimental study deals with flow characterization of 20mm diameter of axisymmetric submerged water jet over a wide range of test parameters. The purpose of this study is to understand the influence of frequency and amplitude on the flow characteristics, and the characteristics of turbulent/non turbulent (T/NT) interface of a pulsating jet. The flow characteristics across the T/NT interface has been compared for potential core, transition zone and fully developed zone. The introduction of pulsation results in widening of the jet and shortening of the potential core length as compared to steady jets. It is also observed that shear layer in pulsating jet oscillates with lower frequency as compared to the jet frequency, while the amplitude depends on the location in the flow field. Further, entrainment and mixing of surrounding fluid increases up to an optimal frequency. These results should eventually lead to a better understanding of the physical phenomena responsible for enhanced mixing and heat transfer in pulsating jets.