Proceedings of FEDSM20082008 ASME Fluids Engineering Conference

August 10-14, 2008, Jacksonville, Florida, USA

FEDSM2008-55170

CAVITATION IN THE TIP REGION OF THE ROTOR BLADESWITHIN A WATERJET PUMP

H. WuJohns Hopkins University

Baltimore, MD, USA

F. SorannaJohns Hopkins University

Baltimore, MD, USA

T. MichaelNSWC/Carderock

MD, USA

J. KatzJohns Hopkins University

Baltimore, MD, USA

S. JessupNSWC/Carderock

MD, USA

ABSTRACTRecent upgrades to the turbomachinery facility at JHU en-

able measurements of performance, as well as flow structure, tur-bulence and cavitation within a water-jet pump. The rotor, sta-tor and pump casing in this optically index-matched facility aremade of acrylic that has the same optical index of refraction asthe working fluid, a concentrated solution of NaI in water. Theessentially “invisible” blades allow unobstructed view and accessto optical flow measurement techniques. Initial tests in water fo-cus on observations on occurrence of cavitation in the vicinity ofthe narrow tip-gap. For the present design and operating condi-tions, near the leading edge, cavitation in the tip corner of thepressure side causes accumulation of bubbles along the pressureside that extends to mid blade. As rollup of a tip vortex starts,these bubbles cross the tip gap to the suction side, and becomeprimary nuclei for cavitation inception within the tip leakage vor-tex (TLV). Bursting of this tip vortex as it migrates towards thepressure side of the neighboring blade generates a cloud of bub-bles along the aft section of the passage. As the flow in the tip gapincreases upstream of the trailing edge, cavitation also developswithin the gap, along the pressure side corner.

INTRODUCTIONDevelopment of tools for computing and predicting the com-

plex flow structure within turbomachines requires an experimen-

tal database on the flow structure and turbulence for benchmark-ing and validation. Since most the Navy water-jet turbo-pumpsare expected to operate near atmospheric pressures, occurrenceof cavitation, with its adverse effects, is also an important issue.The flow field in turbomachines is extremely complex due to si-multaneous occurrence of multiple phenomena and interactionsamong them, e.g. tip and hub vortices, highly strained turbu-lence and turbulent boundary layers as well as turbulent wakesinteracting with downstream blade rows, etc. As an introductionto our new facility, the present paper describes observation ontip leakage cavitation. In the tip region of a rotor blade, exis-tence of a tip clearance between the moving blade and the cas-ing wall, along with the pressure difference between the pres-sure and suction sides, induce a leakage flow through the finiteclearance. On the suction side, the leakage flow rolls up intoa TLV whose complex configuration depends on the blade andcasing geometry as well as flow conditions. This topic, associ-ated turbulence and cavitation phenomena have been investigatedextensively for many years, including numerous recent studiese.g. [1–10]. It is well established that TLV affects the overallperformance of a turbomachine, and plays a significant role inturbulent production and diffusion near the endwall e.g. [3,4]. Inthe case of pumps operating close to atmospheric pressure, TLVare also prone to cavitation due to the inherent low pressure inthe vortex core [11–14]. Consequently, in many cases, cavitationinception in turbomachinery is coupled with vortex structures,

1 Copyright c© 2008 by ASME

Figure 1. The new test loop.

Figure 2. The test loop corner showing the pump, settling chamber, corner with turning vanes and shaft. (unit: mm)

such as tip leakage, trailing and hub vortices as well as turbu-lent vortex structures developing in regions of flow separation.The bubbles tend to grow along the vortex core, becoming cylin-drical, and vortex models have been used in attempts to predictthe associated conditions for cavitation inception. When vorticesare stable, e.g. tip vortices, the bubbles within them persist forlong periods, sometimes until the vortex breaks down. In addi-

tion to the tip region, other forms of cavitation occur near theblades surface, such as sheet cavitation, travelling bubble cav-itation and cloud cavitation [15, 16]. Due to the complex flowstructure in pumps and physical processes involved with bub-ble dynamics, many questions still exist on the associated cav-itation phenomena. Except for empirical information, which isgeometry-dependent, we do not have reliable tools for predicting

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cavitation inception in pumps.

In the present experiments, we examine several forms ofcavitation in the tip region of a waterjet pump. We start by pro-viding a detailed description of the new test facility that has trans-parent blades and casing, providing unobstructed optical accessto the flow and associated cavitation phenomena. Some of thesephenomena are quite similar to those mentioned above, e.g. cav-itation inception within the TLV. However, the picture is consid-erably more complicated, starting with cavitation on the leadingedge side of the tip corner, which feeds bubbles across the tip gapinto the TLV. Bursting of this tip vortex within the blade passagegenerates a large bubble cloud, some of which become nuclei forcavitation within the tip gap, along the pressure side corner.

NOMENCLATUREσ Cavitation number

ρ Density of working fluid

pinlet Static pressure in the inlet flow

pv Water vapor pressure

Utip Tip velocity of the rotor blade

Figure 3. Adjustable Value Mechanism of controlling flow rate

1 EXPERIMENT SETUP1.1 The upgraded turbomachine facility

In recent years, we have developed and implemented an op-tically index matched facility for studying the flow structure andturbulence within multi-stage axial turbomachines. In this setup,the blades and casing are made of transparent material (acrylic)that has the same optical index of refraction as the working fluid,a concentrated solution, 62%∼ 64% by weight, of NaI in wa-ter. Consequently, while performing PIV measurements, the al-most invisible blades do not obstruct the field of view and do notalter the direction of the illuminating laser sheet while passingthrough the blades. There is also minimal reflection from bound-aries, enabling near wall measurements. The original system isdescribed in details by [17], and results focusing on detailed anal-ysis of turbulent production and transport associated with wake-blade and wake-wake interactions are described in several paperse.g. [18–20]. To accommodate measurements within a waterjetpump, the facility has been substantially upgraded recently. Thenew test loop is illustrated in Figure 1. Except for the settlingchamber and test section, all the other, in part galvanized steeland in parts stainless steel, pipes have a 30.5 cm inner diam-eter. Three sections of double-wall pipes are used for coolingby pumping cold water, provided by the air-conditioning systemof the building, through the clearance between the double walls.The pump is connected to a 60 HP AC motor by a long shaft(see also Figure 2). The maximum motor speed is 1200 rpm, andthe associated precision speed-controller with a feedback con-trol system maintains a specified constant speed to within 1%.The 4.45 cm diameter stainless steel shaft is supported by threesets of bearings. Ball bearing is located at the point where theshaft penetrates into the loop and within the hub of the stator,and a slip brushing (journal bearing) is installed at the center ofthe honeycomb, as shown in Figure 2. Two sets of seals are in-stalled at the point of shaft penetration to minimize leakage of

Figure 4. Photos of the rotor, stator and nozzle

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Figure 5. Description of rotor and stator and definition of nomenclatures

the expensive corrosive fluid. The external seal is a dual me-chanical seal, which is lubricated independently by external wa-ter, in part due to the chemical content of the liquid, and in partto facilitate operation at low internal pressures during cavitationtests. An additional internal seal minimizes expose of the bear-ing to the NaI solution. A torque meter is mounted between theshaft and the electric motor. Two honeycombs are installed inthe settling chamber to improve the flow uniformity and reducethe scales of turbulence of the inflow to the pump. In future stud-ies, the settling chamber will be used for intentionally generat-ing nonuniform inflow. The flow rate and pressure drop in theloop are controlled by an adjustable valve shown on the top lefthand corner of Figure 1. To prevent the separated flows associ-ated with typical devices as well as cavitation at low pressures,as illustrated in Figure 3, this valve consists of a pair of perfo-rated disks, one fixed while the other one can be rotated using anexternally controlled screw post and a gear rack. The presentlyused disks are identical, providing minimal blockage when theposition of their holes matches. Varying the angle of the rotat-ing disk provides fine control of pump operating conditions. Therotating disk can be easily replaced by another one with a dif-ferent hole pattern or blockage. As shown in Figure 1, the meanpressure in the facility is controlled using a small tank, half filledwith liquid, and the rest mostly with nitrogen, which is located

Figure 6. Relative locations of all investigated regions

above the loop. The top of this tank is connected to a vacuumpump that can rapidly reduce the absolute gas pressure to below1.4KPa. We use nitrogen for pressurizing the facility instead ofair to alleviate oxidation of minute fraction of I− ion in the liquidto I3−, since the latter absorbs green light, preventing PIV mea-

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Figure 7. The casing of the test section (upper), one cross section showing the ports (lower left) and illustration of PIV measurement arrangement (lower

right)

surements [17]. The piping in the upgraded facility is designedto operate at absolute pressures up to 1000KPa. To measure per-formance and operating conditions, the facility is equipped withan acoustic time of travel flowmeter with 0.5% accuracy; a dif-ferential pressure transducer, which can be connected to variousports along the loop; a tachometer for measuring rpm, as well asthe torque meter.

1.2 Jet pump test section

The axial waterjet pump consists of a seven blades rotor andan eleven blades stator, as shown in Figures 4 - 6. The hub ofthe rotor and stator has an elliptical cross section. The rotor tipdiameter is 30.43 cm, and the tip gap is 0.7 mm. Starting fromthe entrance to the stator passage and further downstream, theouter diameter gradually decreases from 30.5 cm at its inlet to a16.2 cm nozzle shown in Figure 7. At the exit from the nozzle,the channel suddenly expends back to 30.5 cm. The measured

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Figure 8. Performance Comparison with CFD prediction. Marks are

measured points, dash lines are projections of CFD prediction from

300rpm (left) to 1000rpm (right). Pink solid line and blue squares are

the CFD prediction at 1190rpm

performance curve of the pump is compared to the design predic-tions generated by a RANS code, in Figure 8. As is evident, re-sults agree quite well. The selected flow condition for subsequenttests at 900 RPM corresponds to a flow rate of 0.14 m3/s and totalhead rise in fresh water of 6 m. The entire rotor, stator and cas-ing of the test section are made of acrylic, enabling us to performobservations almost everywhere. As noted before, the refractiveindex of the acrylic is equal to that of the fluid used during PIVmeasurements, a 62%∼ 64% by weight solution of NaI in wa-ter. This fluid has specific gravity of 1.8 and kinematic viscosityof 1.1×10−6m2/s, i.e. very close to that of water. Within thisfluid, the blades become almost invisible, enabling unobstructedvelocity measurements essentially everywhere. The casing hasseveral ports distributing throughout the test chamber, enablingus to insert kiel probes, pitot tubes, hydrophones or inject air in-tentionally (shown in Figures 4 and 7).

As we assembled the new setup and tested it, we proceededwith observations on the occurrence of cavitation in the tip re-gion of the rotor blade, and results are described in the followingsection. During these tests, we recorded images using a Lavi-sion 2K×2K high resolution camera for still pictures and a highspeed CMOS camera that can record full 1K×1K images at 2000frames/s, and partial frames up to 120,000 frames/s for serial pic-tures. For now, the camera is normally to the outer surface of thecasing, and we use a strobe light or high power continuous lightfor illumination at high frame rates. Optical setup that will beused for PIV measurement is illustrated in Figure 7. We will alsouse stereo PIV for planar measurements of all three velocity com-ponents, obtaining phase-averaged 3-D data by patching multipleplanes. Tomographic PIV, a relatively new technique [21], willalso be used to obtain the instantaneous 3- dimensional distribu-

tions of 3 velocity components, especially in the tip region.

2 PRELIMINARY RESULT - CAVITATION IN THE TIPREGION

Figure 9. Inception of cavitation along the pressure side of the tip (Re-

gion1)

In this section we provide a series of images on the occur-rence of cavitation in several sections of the tip region. For clar-ity, Figure 6 illustrates the location of images. Region 1 coversthe leading edge; region 2 is the mid chord area, covering boththe suction and pressure sides; region 3 focuses on the area ofTLV detachment, propagation and bursting in the mid chord area;and region 4 focuses on the tip clearance slightly upstream of thetrailing edge, but downstream of the point of TLV detachment.For defining the cavitation index, we use the mean pressure andvelocity of the blade tip

σ =pinlet− pv

12ρU2

tip

(1)

Near the leading edge, region 1, Figure 9 demonstrates thatcavitation inception occurs on the pressure side of the tip corner.The flow pattern visualized by the bubbles suggests that interac-tion of the blade with the wall casing generates a corner vortex,

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Figure 10. Bubble trapped at the pressure side tip corner (Region2 left

part) σ=0.26

similar to a hub vortex that develops along wing-body junctions.Cavitation appears to occur within this corner structure. Whenwe apply grease marks on the leading edge, as scales, they be-come cavitation “inducers” along the pressure side of the blade.Note that since the cavities are located at different depth, onlyone of them is in focus. The pressure side tip corner cavitationgenerates a train of bubbles that extends to the mid chord regionof the pressure side, as shown in images of Region 2 in Figure6, this time at a lower pressure (σ=0.26). Such persistence indi-cates that in this region, there is very little flow across the tip tothe suction side. Furthermore, it is likely that there is a vortexstructure that traps the bubbles along the pressure side tip corner,as suggested by the shape of the bubbles shown in Figure 10. Asthe TLV start rolling up at mid chord, Figure 11 shows that thetrapped bubbles occasionally start crossing from the pressure tothe suction side with the tip leakage flow, and are subsequentlyentrained into the developing vortex, forming cylindrical cavita-tion along the vortex core. The location of bubble crossing andnumber of bubbles being entrained vary, and with it, the initialappearance of TLV cavitation, as illustrated by two samples inFigure 11. A sample sequence of three images recorded at 8000frames/s showing entrainment of bubbles into the TLV and re-sulting inception of tip leakage cavitation is presented in Figure12. As the tip leakage flow increases past the mid chord, allthe bubbles trapped along the pressure side are forced throughthe tip gap to the suction side, and are entrained into the TLV.Subsequent to TLV rollup, it detaches from the blade, and prop-agates towards pressure side of the neighboring blade. Uponreaching the vicinity of the neighboring blade, at about mid-passage, there is an abrupt change to the shape of this vortex.At relatively high cavitation indices (σ=0.33), Figure 13 shows

Figure 11. Bubble crossing the tip clearance and entrained into the TLV

(Region2) σ=0.45

that the bubbly core becomes initially helical, and then breaks upinto a cloud of bubbles. With further reduction in pressure, e.g. atσ=0.26, the breakup process of the cavitating core becomes moreviolent, and fills the entire aft part of the blade passage with bub-bles (Figure 14). This flow pattern suggests that vortex bursting(breakdown) occurs in the blade passage, presumably due to theadverse pressure gradients along the suction side of the blade.This well-known phenomenon frequently occurs in aerodynamic

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Figure 12. A time sequence of the evolution of the bubbles entrained

into the TLV (Region2 The blade is moving to the left. ∆t=1ms σ=0.45)

Figure 13. Migration and bursting of TLV near pressure side of following

blade (Region 3) σ=0.33

flows and combustion chambers [22–25]. Fraction of the bubblesgenerated by the vortex breakup is entrained into the tip leakageflow, and subsequently by the TLV on the other side of the blade,as shown in Figure 15.

As the flow across the tip gap becomes stronger in theaft/rear part of the rotor passage, attached sheet cavitation in-ception occurs within the tip gap, starting from the pressure sidecorner of the blade. At intermediate cavitation indices, the pro-

Figure 14. Burst of TLV providing bubble nuclei for the TLV of following

blade at σ=0.26

Figure 15. Sheet cavitation inside the tip clearance (Region 4) σ=0.26

cess is intermittent, but as Figure 15 demonstrates, atσ=0.26sheet cavitation covers substantial fractions of the tip gap. Theresulting bubbles are fed into the suction side of the blade.

3 SUMMARY AND CONCLUSIONThis paper describes recent upgrades to the JHU optically in-

dex matched turbomachine facility. The flow complexity is thendemonstrated by the simultaneous occurrence of several types of

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cavitation in the tip region of a rotor blade of waterjet pump witha narrow, 0.7 mm, tip gap. Near the leading edge, cavitation de-velops in the tip corner of the pressure side. At low pressures,bubble and attached cavitation also appear. Accumulation ofbubbles along the pressure side corner that extends to mid chordindicates the tip leakage flow does not start until mid blade. Theshape of some of the cavitation suggests that it occurs within avortex located in the pressure side corner, similar to a hub vortex,and that the bubbles remain trapped within it until mid passage.As the tip leakage flow starts developing at mid chord, these bub-bles cross the tip gap to the suction side, and become the nucle-ation sites for cavitation inception within the tip leakage vortex.In the forward part of the blade, lack of bubble flux across thetip gap, and formation of a vortex along the pressure side cornersuggest that the pressure gradients across the tip gap are weakor even reversed. In support of the latter, occurrence of bubblecavitation in the pressure side corner prior to any cavitation inthe suction side indicates that near the leading edge, the pressurein the pressure side corner is lower than that in the suction side.In the present flow conditions, pressure gradients across the gap,which force a leakage flow towards the suction side and rollupof a TLV occur only starting from the middle of the blade. Thevalidity of this and other related statements require validationby the upcoming velocity measurements. As the TLV develops,it detaches from the suction surface and propagates towards thepressure side of the neighboring blade. There, the initially lineartrain of bubbles within the vortex core becomes helical, and thenbreaks down into a large cloud of small bubbles. With decreas-ing pressure, this process becomes more violent. Some of thesebubbles are forced to the suction side by the tip leakage flow ofthe neighboring blade, and are subsequently entrained by its tipvortex. These observations suggest that TLV breakdown occurswithin the rotor passage, presumably due to exposure to adversepressure gradients along the suction side. As the flow across thetip gap increases in the aft/rear part of the blade, attached cavi-tation develops within the tip gap, along the pressure side cornerof the tip. With decreasing pressure, this cavitation seems to fillsubstantial fractions of the gap.

ACKNOWLEDGMENT

This project is sponsored by the Office of Naval Researchunder grant number N00014-06-1-0160. The program officer isKi-Han Kim. Funding for the upgrades to test facility was pro-vide by ONR DURIP grant No. N00014-06-1-0556. We wouldlike to thank Yury Ronzhes and Stephen King for their majorcontributions to the construction and maintenance of the facilityand thank Alan Becnel for RANS calculations using Ansys CFX.

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