Flow Visualization Using Cavitation Within Blade Passage of an Axial Waterjet Pump Rotor

2012 ◽  
Author(s):  
David Y. Tan ◽  
Rinaldo L. Miorini ◽  
Jens Keller ◽  
Joseph Katz
Keyword(s):  
Flow Rate ◽  
Rotor Blade ◽  
Incidence Angle ◽  
Leading Edge ◽  
Cavitation Number ◽  
Trailing Edge ◽  
Cloud Cavitation ◽  
Sheet Cavitation ◽  
Stator Blade ◽  
Waterjet Pump

Cavitation phenomena within an axial waterjet pump, AxWJ-2 [1,2] operating at and below the best efficiency point (BEP) are investigated using high-speed imaging. The purpose of these preliminary observations is to provide an overview of the physical appearance of several forms of cavitation under varying flow and pressure conditions. These observations provide a motivation for upcoming detailed velocity and turbulence measurements. The experiment is conducted using a transparent pump installed in an optically index-matched facility, which facilitates unobstructed visual access to the pressure and suction sides of the rotor and stator blade passages. By varying the cavitation index within the facility, the observations follow the gradual development of cavitation from inception level to conditions under which the cavitation covers the entire blade. Cavitation appears first in the tip gap, as the fluid is forced from the pressure side (PS) to the suction side (SS) of the rotor blade. Bubbly streaks start at the SS corner, and penetrate into the passage, and are subsequently entrained into the tip leakage vortex (TLV) propagating in the passage. Sheet cavitation also develops along the SS of the rotor leading edge and covers increasing fractions of the blade surface with decreasing cavitation number. At BEP conditions, the sheet is thin. Below BEP, the blade loading increases as a result of an increase in the incidence angle of the flow entering the passage relative to the blade. Consequently, the backward leakage flow also increases, further increasing the incidence angle in the tip region, and thickening the sheet cavitation there. Consistent with previous observations on swept hydrofoils, a re-entrant jet that flows radially outward develops at the trailing edge of the sheet cavitation. Only near the tip corner the trailing edge of the sheet cavitation is opened as the radial re-entrant flow is entrained into the TLV, forming an unstable and noisy spiraling pattern. Within a certain range of cavitation indices, when the sheet cavitation length at the blade tip extends to about 50–60% of the blade spacing, the sheet cavitation on every other blade begins to expand and contract rapidly, generating loud low-frequency noise. With further decrease in pressure, persistent alternating cavitation occurs, namely, the cavitating region on one blade becomes much larger than that in the neighboring one. The mechanisms involved and associated instabilities are discussed based on previous analyses performed for inducers. As the cavitation number is lowered even further, the sheet cavitation on the “heavily-cavitating” blade grows, and eventually passes the trailing edge of the rotor blade. At this condition, cavitation begins again to expand and contract rapidly on the “less-cavitating” blade, covering a significant portion of SS surface. At a lower pressure, all the blades cavitate, with the sheet cavitation covering the entire SS surface of the rotor blade. The large cavities on alternate rotor blade surfaces re-direct flow into the neighboring passages with the smaller cavities. As a result, there is a lower flow rate in the passage with the larger cavitation and higher flow rate in the neighboring passage. As the flow with the cavitating passage arrives to the leading edge of the stator flow rate, it increases the incidence angle at the entrance to the stator, causing intermittent sheet and cloud cavitation on the stator blade.

scholarly journals Investigation of Cavitation Noise in Cavitating Flows around an NACA0015 Hydrofoil

2019 ◽  
Vol 9 (18) ◽  
pp. 3736
Author(s):  
An Yu ◽  
Xincheng Wang ◽  
Zhipeng Zou ◽  
Qinghong Tang ◽  
Huixiang Chen ◽  
...  
Keyword(s):  
Spectral Density ◽  
Power Spectral Density ◽  
Leading Edge ◽  
Acoustic Power ◽  
Cavitation Number ◽  
Acoustic Analogy ◽  
Cavitation Noise ◽  
Cloud Cavitation ◽  
Sheet Cavitation ◽  
Power Spectral

To provide theoretical basis for cavitation noise control, the cavitation evolution around a hydrofoil and its induced noise were numerically investigated. A modified turbulence model and Zwart cavitation model were employed to calculate the flow field and predict the cavitation phenomenon accurately. Then, the acoustic analogy method based on the Ffowcs Williams-Hawking (FW-H) equation was applied to analyze the cavitation-induced noise. Seven cavitation numbers were selected for analysis. Acoustic power spectral density (PSD) and acoustic pressure were investigated to establish the relationship between cavitation number and their acoustic characteristics. It was indicated that as cavitation number decreases, cavitation cycle length gets shorter and the magnitude of acoustic power spectral density increases dramatically. One peak value of acoustic power spectral density induced by the extending and retracting of leading-edge cavitation can be obtained under sheet cavitation conditions, while under cloud cavitation, two peak values of acoustic power spectral density can be obtained and are induced by superposition from leading-edge cavitation and trailing vortex.

Experimental Investigation of the Role of Large Scale Cavitating Vortical Structures in Performance Breakdown of an Axial Waterjet Pump

2015 ◽  
Vol 137 (11) ◽  
Author(s):  
David Tan ◽  
Yuanchao Li ◽  
Ian Wilkes ◽  
Elena Vagnoni ◽  
Rinaldo L. Miorini ◽  
...  
Keyword(s):  
Rapid Decrease ◽  
Trailing Edge ◽  
Cloud Cavitation ◽  
Vortical Structures ◽  
Pump Performance ◽  
Blade Passage ◽  
Tip Leakage ◽  
Sheet Cavitation ◽  
Blade Tip ◽  
Waterjet Pump

Flow phenomena and mechanisms involved in cavitation breakdown, namely, a severe degradation of pump performance caused by cavitation, have been a longstanding puzzle. In this paper, results of high-speed imaging as well as pressure and performance measurements are used to elucidate the specific mechanism involved with cavitation breakdown within an axial waterjet pump. The experiments have been performed using geometrically identical aluminum and transparent acrylic rotors, the latter allowing uninhibited visual access to the cavitation phenomena within the blade passage. The observations demonstrate that interaction between the tip leakage vortex (TLV) and trailing edge of the attached cavitation near the rotor blade tip that covers the suction side (SS) of the blade plays a key role in processes leading to breakdown. In particular, the vortical cloud cavitation developing at the trailing edge of the sheet cavity near the blade tip is entrained and re-oriented by the TLV in a direction that is nearly perpendicular to the blade SS surface, and then convected downstream through the blade passage. Well above breakdown cavitation indices, these “perpendicular cavitating vortices” or PCVs occur in the region where blades do not overlap, and they only affect the local flow complexity with minimal impact on the global pump performance. With decreasing pressure and growing sheet cavitation coverage of the blade surface, this interaction occurs in the region where two adjacent rotor blades overlap, and the PCV extends from the SS surface of the originating blade to the pressure side (PS) of the neighboring blade. Cavitation breakdown begins when the PCV extends between blades, effectively blocking the tip region of the rotor passage. With further decrease in pressure, the PCVs grow in size and strength, and extend deeper into the passage, causing rapid degradation in performance. Accordingly, the casing pressure measurements confirm that attachment of the PCV to the PS of the blade causes rapid decrease in the pressure difference across this blade, i.e., a rapid decrease in blade loading near the tip. Similar large perpendicular vortical structures have been observed in the heavily loaded cavitating rocket inducers (Acosta, 1958, “An Experimental Study of Cavitating Inducers,” Proceedings of the Second Symposium on Naval Hydrodynamics, ONR/ACR-38, pp. 537–557 and Tsujimoto, 2007, “Tip Leakage and Backflow Vortex Cavitation,” Fluid Dynamics of Cavitation and Cavitating Turbopumps, L. d'Agostino and M. Salvetti, eds., Springer, Vienna, Austria, pp. 231–251), where they extend far upstream of the rotor and cause global flow instabilities.

Film Cooling Effectiveness on the Leading Edge Region of a Rotating Turbine Blade With Two Rows of Film Cooling Holes Using Pressure Sensitive Paint

2006 ◽  
Vol 128 (9) ◽  
pp. 879-888 ◽  
Author(s):  
Jaeyong Ahn ◽  
M. T. Schobeiri ◽  
Je-Chin Han ◽  
Hee-Koo Moon
Keyword(s):  
Rotational Speed ◽  
Film Cooling ◽  
Rotor Blade ◽  
Incidence Angle ◽  
Leading Edge ◽  
Edge Region ◽  
Pressure Sensitive Paint ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Pressure Sensitive

Detailed film cooling effectiveness distributions are measured on the leading edge of a rotating gas turbine blade with two rows (pressure-side row and suction-side row from the stagnation line) of holes aligned to the radial axis using the pressure sensitive paint (PSP) technique. Film cooling effectiveness distributions are obtained by comparing the difference of the measured oxygen concentration distributions with air and nitrogen as film cooling gas respectively and by applying the mass transfer analogy. Measurements are conducted on the first-stage rotor blade of a three-stage axial turbine at 2400rpm (positive off-design), 2550rpm (design), and 3000rpm (negative off-design), respectively. The effect of three blowing ratios is also studied. The blade Reynolds number based on the axial chord length and the exit velocity is 200,000 and the total to exit pressure ratio was 1.12 for the first-stage rotor blade. The corresponding rotor blade inlet and outlet Mach numbers are 0.1 and 0.3, respectively. The film cooling effectiveness distributions are presented along with discussions on the influence of rotational speed (off design incidence angle), blowing ratio, and upstream nozzle wakes around the leading edge region. Results show that rotation has a significant impact on the leading edge film cooling distributions with the average film cooling effectiveness in the leading edge region decreasing with an increase in the rotational speed (negative incidence angle).

Investigation of Unsteady Sheet Cavitation and Cloud Cavitation Mechanisms

1999 ◽  
Vol 121 (2) ◽  
pp. 289-296 ◽  
Author(s):  
T. M. Pham ◽  
F. Larrarte ◽  
D. H. Fruman
Keyword(s):  
High Speed ◽  
Leading Edge ◽  
Pressure Measurements ◽  
Foil Surface ◽  
Mean Velocity ◽  
Cloud Cavitation ◽  
Fluctuating Pressure ◽  
Sheet Cavitation ◽  
Cavitation Instability ◽  
Unsteady Characteristics

Sheet cavitation on a foil section and, in particular, its unsteady characteristics leading to cloud cavitation, were experimentally investigated using high-speed visualizations and fluctuating pressure measurements. Two sources of sheet cavitation instability were evidenced, the re-entrant jet and small interfacial waves. The dynamics of the re-entrant jet was studied using surface electrical probes. Its mean velocity at different distances from the leading edge was determined and its role in promoting the unsteadiness of the sheet cavitation and generating large cloud shedding was demonstrated. The effect of gravity on the dynamics of the re-entrant jet and the development of interfacial perturbations were examined and interpreted. Finally, control of cloud cavitation using various means, such as positioning a tiny obstacle (barrier) on the foil surface or performing air injection through a slit situated in the vicinity of the leading edge, was investigated. It was shown that these were very effective methods for decreasing the amplitude of the instabilities and even eliminating them.

scholarly journals Experimental Study on the Relationship Between Cavitation and Lift Fluctuations of S-Shaped Hydrofoil

2021 ◽  
Vol 9 ◽  
Author(s):  
Haiyu Liu ◽  
Pengcheng Lin ◽  
Fangping Tang ◽  
Ye Chen ◽  
Wenpeng Zhang ◽  
...  
Keyword(s):  
High Speed ◽  
Developmental Process ◽  
Angle Of Attack ◽  
Cavitation Number ◽  
High Speed Photography ◽  
Cloud Cavitation ◽  
Cavitation Inception ◽  
Sheet Cavitation ◽  
Hydraulic Machinery ◽  
Stall Angle

In order to study the energy loss of bi-directional hydraulic machinery under cavitation conditions, this paper uses high-speed photography combined with six-axis force and torque sensors to collect cavitating flow images and lift signals of S-shaped hydrofoils simultaneously in a cavitation tunnel. The experimental results show that the stall angle of attack of the S-shaped hydrofoil is at ±12° and that the lift characteristics are almost symmetrical about +1°. Choosing α = +6° and α = −4° with almost equal average lift for comparison, it was found that both cavitation inception and cloud cavitation inception were earlier at α = −4° than at α = +6°, and that the cavitation length at α = −4° grew significantly faster than at α = +6°. When α = +6°, the cavity around the S-shaped hydrofoil undergoes a typical cavitation stage as the cavitation number decreases: from incipient cavitation to sheet cavitation to cloud cavitation. However, when α = −4°, as the cavitation number decreases, the cavitation phase goes through a developmental process from incipient cavitation to sheet cavitation to cloud cavitation to sheet cavitation to cloud cavitation, mainly because the shape of the S-shaped hydrofoil at the negative angle of attack affects the flow of the cavity tails, which is not sufficient to form re-entrant jets that cuts off the sheet cavitation. The formation mechanism of cloud cavitation at the two different angles of attack (α = +6°、−4°) is the same, both being due to the movement of the re-entrant jet leading to the unstable shedding of sheet cavity. The fast Fourier analysis reveals that the fluctuations of the lift signals under cloud cavitation are significantly higher than those under non-cavitation, and the main frequencies of the lift signals under cloud cavitation were all twice the frequency of the cloud cavitation shedding.

On the Interactions of a Rotor Blade Tip Flow With Axial Casing Grooves in an Axial Compressor Near the Best Efficiency Point

2018 ◽  
Author(s):  
Huang Chen ◽  
Yuanchao Li ◽  
Joseph Katz
Keyword(s):  
Flow Rate ◽  
Rotor Blade ◽  
Leading Edge ◽  
Secondary Flows ◽  
Leakage Flow ◽  
Flow Rates ◽  
Tip Leakage ◽  
Flow Features ◽  
Blade Tip ◽  
Corner Vortex

Previous studies have shown that axial casing grooves (ACGs) are effective in delaying the onset of stall, but degrade the performance of axial turbomachines around the best efficiency point (BEP). Our recent experimental study [1] in the JHU refractive index-matched liquid facility have examined the effects of ACGs on delaying stall of a one and half stage compressor. The semicircular ACGs based on Müller et al. [2] reduce the stall flow rate by 40% with a slight decrease in pressure rise at higher flow rates. Stereo-PIV (SPIV) measurements at a flow rate corresponding to the pre-stall condition of the untreated machine have identified three flow features that contribute to the delay in stall. Efficiency measurements conducted as part of the present study show that the ACGs cause a 2.4% peak efficiency loss. They are followed by detailed characterizations of the impact of the ACGs on the flow structure and turbulence in the tip region at high flow rates away from stall. Comparisons with the flow structure without casing grooves and at low flow rate are aimed at exploring relevant flow features that might be associated with the reduced efficiency. The SPIV measurements in several meridional and radial planes show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the blade suction side (SS). The inflow velocity magnitude is substantially lower than that occurring at a flow rate corresponding to the pre-stall conditions of the untreated machine. Yet, entrainment of the PS boundary layer and its vorticity during the inflow phase generates counter-rotating radial vortices at the entrance to the groove, and a “discontinuity” in the appearance of the tip leakage vortex (TLV). While being exposed to the blade SS, the backward tip leakage flow causes flow separation and formation of a counter-rotating vortex at the downstream corner of the groove, which migrates towards the passage with increasing flow rate. Interactions of this corner vortex with the TLV cause fragmentation of the latter, creating a broad area with secondary flows and elevated turbulence level. Consequently, the vorticity shed from the blade tip remains scattered from the groove corner to the blade tip long after the blade clears this groove. The turbulence peaks around the corner vortex, the TLV, and the shear layer connecting it to the SS corner. During periods of inflow, there is a weak outflow from the upstream end of the groove. At other phases, most of the high secondary flows are confined to the downstream corner, leaving only weak internal circulation in the rest of the groove, but with a growing shear layer with elevated (but weak) turbulence originating from the upstream corner. Compared to a smooth endwall, the groove also increases the flow angle near the blade tip leading edge (LE) and varies it periodically. Accordingly, the magnitude of circulation shed from the blade tip and leakage flow increase near the leading edge. The insight from these observations might guide the development of ACGs that take advantage of the effective stall suppression by the ACGs but alleviate the adverse effects at high flowrates.

Effect of Hydrofoil Planform on Tip Vortex Roll-Up and Cavitation

1995 ◽  
Vol 117 (1) ◽  
pp. 162-169 ◽  
Author(s):  
D. H. Fruman ◽  
P. Cerrutti ◽  
T. Pichon ◽  
P. Dupont
Keyword(s):  
Reynolds Number ◽  
Incidence Angle ◽  
Pressure Coefficient ◽  
Tangential Velocity ◽  
Leading Edge ◽  
Velocity Profiles ◽  
The Other ◽  
Tip Vortex ◽  
Trailing Edge ◽  
Cavitation Inception

The effect of the planform of hydrofoils on tip vortex roll-up and cavitation has been investigated by testing three foils having the same NACA 16020 cross section but different shapes. One foil has an elliptical shape while the other two are shaped like quarters of ellipses; one with a straight leading edge and the other with a straight trailing edge. Experiments were conducted in the ENSTA, Ecole Navale and IMHEF cavitation tunnels with homologous foils of different sizes to investigate Reynolds number effects. Hydrodynamic forces as well as cavitation inception and desinence performance were measured as a function of Reynolds number and foil incidence angle. Laser Doppler measurements of the tangential and axial velocity profiles in the region immediately downstream of the tip were also performed. At equal incidence angle and Reynolds number, the three foils show different critical cavitation conditions and the maximum tangential velocity near the tip increases as the hydrofoil tip is moved from a forward to a rear position. However, the velocity profiles become more similar with increasing downstream distance, and at downstream distances greater than one chord aft of the tip, the differences between the foils disappear. The rate of tip vortex roll-up is much faster for the straight leading edge than for the straight trailing edge foil and, in the latter case, a significant portion of the roll-up occurs along the foil curved leading edge. The minimum of the pressure coefficient on the axis of the vortex was estimated from the velocity measurements and correlated with the desinent cavitation number for the largest free stream velocities. The correlation of data is very satisfactory. At the highest Reynolds number tested and at equal lift coefficients, the straight leading edge foil displays the most favorable cavitation desinent numbers.

Scale Effects on the Cavitation Development in Fluid Machinery

2011 ◽  
Author(s):  
Weiping Yu ◽  
Xianwu Luo ◽  
Yao Zhang ◽  
Bin Ji ◽  
Hongyuan Xu
Keyword(s):  
Turbulence Model ◽  
High Speed ◽  
Scale Effects ◽  
Design Procedure ◽  
Leading Edge ◽  
Characteristic Size ◽  
Cavitation Model ◽  
Fluid Machinery ◽  
Cloud Cavitation ◽  
Sheet Cavitation

The prediction of cavitation in a design procedure is very important for fluid machinery. However, the behaviors of cavitation development in the flow passage are believed to be much different due to scale effects, when the characteristic size varies greatly for fluid machines such as pumps, turbines and propellers. In order to understand the differences in cavitation development, the evolution of cavity pattern in two hydro foils were recorded by high-speed video apparatus. Both foils have the same section profile, and their chord lengths are 70mm and 14mm respectively. For comparison, the cavitating flows around two foils were numerically simulated using a cavitation model based on Rayleigh-Plesset equation and SST k-ω turbulence model. The experiments depicted that for both hydro foils, there was attached sheet cavitation near the leading edge, which separated from the rear part of the cavity and collapsed near the foil trailing edge. There was clear cloud cavitation in the case of the mini foil. The results also indicated that the numerical simulation captured the cavitation evolution for the ordinary foil quite well compared with the experiments, but could hardly predict the cloud cavitation for the mini foil. Thus, it is believed that both the cavitation model and the turbulence model should be carefully treated for the scale effect on cavitation development in fluid machinery.

Effects of Solid Particle Ingestion Through an HP Turbine

2012 ◽  
Author(s):  
Adel Ghenaiet
Keyword(s):  
Gas Turbines ◽  
Rotor Blade ◽  
Leading Edge ◽  
Suction Side ◽  
Operating Conditions ◽  
Pressure Side ◽  
Trailing Edge ◽  
Premature Failure ◽  
Narrow Strip ◽  
Tracking Model

Modern gas turbines operate in severe dusty environments, and because of such harsh operating conditions, their blades experience significant degradation in service. This paper presents a numerical study of particle dynamics and erosion in an hp axial turbine stage. The flow field is solved separately from the solid phase and constitutes the necessary data in the particle trajectories simulations using a Lagrangian tracking model based on the finite element method. Several parameters consider a statistical description such as particle size, shape and rebound, in addition to the turbulence effect. A semi empirical erosion correlation is used to estimate erosion contours and blades deteriorations, knowing the locations and conditions of impacts. The trajectory and erosion results show high erosion rates over the pressure side of NGV near trailing edge, in addition to extreme erosion observed toward the root corner, due to high number of particles impacting with high velocities. On the suction side, erosion is mainly over a narrow strip from leading edge. Erosion in the rotor blade is shown along the leading edge and spreading over the fore of the blade suction side, owing to a flux of particles entering at high velocities and incidence. On the pressure side, regions of dense erosion are observed near the leading edge and trailing edge as well as the tip corner. Critical erosion spots seen over NGV and rotor blade are signs of a premature failure.

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