the twin vortex draft tube surge pap-0590

8/12/2019 The Twin Vortex Draft Tube Surge PAP-0590

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The win Vortex Draft Tu be Surge

Tony L ~ a h l l ssociate Mem ber, Morris M. skinn er2 M ember,Henry T. ~a lve y3 ember

Abstract

Recent studies cond ucted at C olorado State University using a mo del of

the 700-MW turbines a t the U. S. Bureau of Reclam ation's Grand Coulee ThirdPowerplant indicate that in specific regions of the turbine hill curve a tw in vortexdraft tube surge exists. Extensive pressure fluctuation data were collectedfrom transducers mounted at the draft tube throat, and dimensionlesspressure and frequency parameters were calculated. The twin vortexproduced pressure fluctuations at 2'15-3 times the frequency typicallyassociated with draft tube surging, a nd the pressure parameter was red ucedin the twin vortex region. Evidence from the literature suggests that the twinvortex also oc curs in other similar units.

Introduction

Draft tube surging in hydraulic reaction turbines has long beenrecognized as a phenom enon caused by excess swirl in the flow leaving the

turbine runner under partial load or overload opera ting conditions. Theinvestigations of Cassidy (1969), an d Falvey and Cass idy (1970), estab lisheddimensionless pressure and frequency parameters for the pressurefluctuations due to the surge, and related these to a dimensionless swirlparameter, the length to diameter ratio of the draft tube, and the Reynoldsnumb er. The swirl parameter was com puted from gross flow variables, andthe behavior of the surge was assumed to be independent of the particularme thod use d to introduce swirl.

1 Hy r ulic n ineer U. S. Bureau of Reclamation, P.O. Box 25007, Denver,t AoracEo,?1022~.

2 As ocia e Profe s r, Dep t. of ivil Enginee ring, Co lorado State University,F o rt E o ~ ~ m s ,oPorad0, 8052

Consultant, P O ox 4 Conifer, Colorado, 80433.

1 Wahl, Skinner, Falvey



The most common flow structure associated with the vortex breakdownis a single helical vortex. However, Sarpkaya (1971), in experiments with adiverging tube, also observed a dou ble vortex at low Reynolds numbe rs (1000to 2000). The tw o spirals were intertwined together w ithin the tube, 180opposite on e another. Escudier an d Zehnder (1 982) also observed the doub levortex in straight, diverging tubes. The double vortex was unstable andintermittently reverted ba ck to a single vortex.

xperimental Procedures

Test Facility

The model turbine used for these tests is a homologous, 1:40.3 scalemo del of the 700-MW turbines (units G22, G23, a nd G24) installed at theU S.Bureau of Reclamation's (Reclamation's) Grand Coulee Third Pow erplant. Thetest facility p rovide s geom etric similarity with the proto type installation from th epenstock intake to the dow nstream tailrace. The mo del turbine was originallyinstalled in the 1970's in a Reclamation test facility at E stes Powerplant in

Colorado. The mo del was mo ved in 1988 to the Engineering Research Centerat Colorado State University (CSU).

At CSU the model is installed in a once-through flow system drawingwater directly from Horsetooth Reservoir, located immediately west of thelaboratory. The mode l was originally designed to operate in the p rototyp ehead range of 220-355 ft (67.1-108.2 m). At the cu rrent installation, themaximum available head is about 250 ft (76.2m). A butterfly valve and a 25 -ft(7.62-m) standpipe downstream of the model are used to apply backpressure. Lo ad is placed on the turbine by a water cooled, ed dy current,absorption dynamometer. The dynamom eter control console provides forbo th load and speed control of the unit.

The model draft tube is constructed of fiber glass, with a clear plastic

throat section to allow observation of the flow. The m odel is equippe d with airinjection ports in the runner crown which were used for flow visualization atsome test points. The operating status of the test facility and m ode l turbineare m onitored b y a computerized data acquisition system collecting data frompressure transducers, thermocouples, a tachometer, and the dynamometertorqu e loa d cell.

Test Procedure

The mo del was operated at test points throughout the part-load surgingregion, with the frequency a nd a mplitude of pressure fluctuations in the drafttube re cord ed for each test point. The head was maintained in the rang e110-130 ft (33.5-39.6 m) for the majority of the tests; operation at desired

points o n the turbine hill curve was achieved by varying the runner s pee d ata given gate setting. The operating status of the turbine mod el was re corde dfor each test p oint using the d ata acquisition system. The vortex was madevisible either b y cavitation occurring in the vortex core or by adm ission of smallquantities of air into the draft tube. To minimize the effec ts of cavitation all datawere co llected with tailwater levels set as high as possible; the cavitation indexa, was in the range 0.38-0.42 for m ost runs.




Pressure Fluctuation Measurements

Pressure surges in the draft tube w ere measured usin g tw o piezoresistivepressure transduc ers m ounted at the draft tube throat, below the runner exit.The transducers were placed 180 part, on the upstream side of the draft tube

(location TI ) an d o n the tailrace side (location T2), as show n in Figure 1.

The amplitude an d frequency of pressure fluctuations at location T I wererecorded in the form of frequency spectra using a dynamic signal analyzer.AC coupling was used to record the fluctuation of pressure, rather than theabsolute amplitude. An oscilloscope was used to monitor pha se relationshipsbetwee n the signals.

Flow

tube.Figure 1. Schematic of pressure transducer locations on the elbo w type draft

P lan levation

Results nd Analvsi ~

Vortex Breakdown

The development of the draft tube surge in the mo del followed the wellkno wn sequen ce described in the literature. As the swirl was increased, the

flow rema ined clear until a critical point at which vortex breakd own o ccurre dsuddenly, and a helical vortex developed in the draft tube in the form of aleft-handed screw. The vortex precession was in the same direction as therunner rotation. The precession frequency of the vortex was a bout one-thirdto one-fourth of the rotational frequency of the runner, comparable tofrequencies first reported by Rheingans 1937). Pressure fluctuations in thedraft tube occurring at the precession frequency were asynchronous; thesignals from locations T I an d T2 were approximately 180 out of pha se withone another.

At gate settings above 19 (full gate setting for the proto type is34 ) theamplitude of pressure fluctuations and the size of the cavitated vortex coreincreased with increasing swirl. Also, the pitch of the vortex tende d to increase

as the sw irl increased (i.e., the vortex became stretched out a long the drafttub e axis).

Twin Vortex

At gate settings less than 19 , cavitation of the vortex decrease d as theswirl increased. The vortex was no t visible at high tailwater levels, but c ouldbe seen at low tailwater levels. Injection of air into the draft tube at hightailwater pro du ced only a cloud of bubbles in the flow. Despite thedisappearance of a cavitating vortex, pressu re fluctuations were still dete cted




in the draft tube at frequencies corresponding to the precession of a single

helical vortex.

As the swirl approached 1 .2, the dominant frequency of pressurefluctuations began to shift randomly between two different frequencies. Thelower frequency corresponded to the precession of the single vortex observedpreviously. The higher frequency was generally 2V2-3 times the lowerfrequency. The shifts occurred in a random manner, at intervals ranging froma few seconds to nearly a minute. As the swirl was increased further, thefluctuation became sustained at the higher frequency. When the tailwater wasthen lowered, two helical vortices could be seen in the draft tube, as shown inFigure 2. Each vortex retained the left-handed orientation of the original singlevortex. While the twin vortex was present, the pressure fluctuations atlocations T1 and T2 became in phase with one another. A sustained twinvortex was observed at a total of twelve test points, in the range of 90_150 ateopening, and swirl values ranging from 1.2 to 3.0.

Figure 2. Photograph of twin vortex. The vortices are visible due tocavitation in the vortex cores. The exposure setting of the camera wasmatched to the strobe light frequency.

This behavior is similar to that noted by Fisher, Ulith, and Palde 1980) ina review of hydraulic model and prototype tests for the Grand Coulee Third,Marimbondo Brazil), and Cerron Grande Brazil) turbines. These installationshave similar, but not homologous, turbines and elbow type draft tubes. In testsconducted on a 1 :28 scale model of the 700-MW Grand Coulee Third turbines,a random shifting between two dominant frequencies was observed at about46 percent of the best efficiency gate setting. The frequencies observed weresimilar to those seen in these tests. This behavior was also observed in modeltests of the Marimbondo turbines at approximately the same gate setting. The

5 Wahl, Skinner J Falvey



Frequency Parameter

Figure shows the relationship of the frequency parameter to the swirlparameter. The figure clearly differentiates between the single and twin vortexcond itions. At swirl values less than about 1.2. only the single vortex modeoccurs . However as the swirl increases above 1.2 the draft tube surge mayconsist of either a single or tw in vortex.

F r e q u e n c y Parameter vs S w i r l

Twin Vortex. egion Ill

0 0 0 5 .O 1.5 2.0 2.5 3 0 3 5

raft Tube Swirl arame te r

Figure 3. Frequency param eter vs. draft tube swirl param eter.

Pressure Parameter

A contour map of the pressure parameters for the dominant pressurefluctuation at each test po int was constructed. The map was plotted on thespeed ratio , and unit power HP11 axes defined as follows:

H P , , =

bhp)

D H ~ ' =

Wahl Skinner Falvey



where: b p turbine output brake horsepower)

D throat diameter of turbine runner, ft

net hea d, ft

runner speed, rpm

Figure 4 shows tw o peaks in the pressure parameter, separated by a lowsaddle. The saddle region correspond s roughly with the reg ion in which thetwin vortex was observed.

Figure 5 show s the pressure parameter values p lotted against the swirlparameter. Points where a twin vortex was observed are m arked o n the plot.Figure 6 show s the same data divided between small gate openings less than50 percent) and large gate openings greater than 50 percent). The pressureparameter correlates well with the swirl parameter at the large gate settings.However, in the lower range of g ate settings, the pressure param eter varieswidely. The behavior of the pressure parameter in this range is betterexplained by the pressure parameter m ap Figure4 .

onclusions

Prior research has indicated that for a given draft tube shape, operatingat suitably h igh Reynolds numbers, the am plitude an d frequency of draft tub esurge pressure fluctuations depend only on the value of the draft tube swirlparameter. However, these tests have indicated that in some cases the drafttube sw irl parameter does not fully explain the behavior of the draft tube surgeover the complete operating range of a turbine, due to the existence of twopossible surging modes. The following conclusions can be drawn:

* A twin vortex draft tube surge was observed in a well defined region of themo del turbine hill curve. Evidence in the literature sugge sts that this surg ingmode may also occur in other similar models and prototype units.

* The frequency parameter is dependent not only on the draft tube swirlparameter, bu t also on the num ber of helical vortices in the draft tube. In thismo del at swirl parameter values above 1.2, the frequency parameter become sa multivalued function of the swirl, depending on whether the flow contains asingle or twin vortex.

* The transition from the single to the twin vortex is unstable. In the transitionzone, either form may exist, and the flow in the draft tube seems to alternaterandomly between the two m odes.

* The p ressure param eter is related to the sw irl parameter only at high wicketgate settings above about one-half of the full gate opening). At low gatesettings, there is little relationship between the pressure parameter and theswirl parameter; the behavior of the press ure parameter was be tter explainedby the pressure parameter map.

* The p ressure fluctuations associated with the twin vortex are well above theRheingans frequency generally associated with draft tube su rging. Theincrease in frequency due to the formation of the twin vortex is due not onlyto a dou bling of the num ber of vortices, bu t also to an increase in the vortexprecession frequency.




Twin VortexPrototype

Pressure Parameter H e a d R a n g eontour Interval 0 05

0 00 0 000 50 0 60 0 70 0 80 0 90 1 00 1 10 1 20 1 30

Speed Rat io

Figure 4. Pressure parameter map. Solid triangles indicate a sustained twinvortex. Dotte d lines are labeled with wicket gate setting in degrees . The unitpower HP is given in units of hplft3 -5. The sp eed ratio @ is dimensionless.

Wahl Skinner Falvey



P r e s s u r e P a r a m e t e r vs S w i r l

All Gate Openings

Single Vortex

Twin Vortex

Region ill

1

0 0 0 5 1 O 1 5 2 0 2 53 0 3 5

Draf t Tube Swirl Parameter

Figure 5. Pressure parameter vs. swirl for all gate settings.

P r e s s u r e P a r a m e t e r vs S w i r l

0 0 0 5 1 O 1 5 2 0 2 5 3 0 3 5

Draft Tube Swir l Parameter

Figure 6 Pressure parameter vs. swirl by gate setting.

9 Wahl Skinner Falvey



Acknowledaments

This study was funded by the Department of Civil Engineering atColorado State University, Fort Collins, Colorado.

S Uni tsValues of unit power (HP11, hplft3.5~shown on Figure 4 may be

converted to the Sl equivalent (PA , kwlrn3- ), by the following equa tion:

References

Cassidy, J. J., Experimental Study a nd Analysis of Draft Tube Surging,REC-OCE-69-5 Report No. Hyd-591 U S. Bureau of Reclamation, Oct.1969.

Cassidy, J. J., an d H. T. Falvey, Observations of Unsteady Flow Arising AfterVortex Breakdown, Journal of Fluid Mechanics Vol. 41, 1970,

pp. 727-736.

Escudier, M. P., and N. Zehnd er, Vortex Flow Regimes, Journal of FluidMechanics Vol. 115, 1982, pp . 105-121.

Falvey, H. T., an d J. J. Cassidy, Frequency and Amplitude of Pressure SurgesGenerated by Swirling Flow, Proceedings 5th Symposium IAHR/AIRHSection for Hydraulic Machinery Equipment and Cavitation, Stockholm,Sweden, 1970.

Fisher, R. K., Jr., U. J. Palde, an d P. Ulith, Com parison of Draft Tub e Surg ingof Homologous Scale Models and Prototype Francis Turbines,Proceedings 10th Symposium IAHRIAIRH Section for Hydraulic

Machinery Equipment a nd Cavitation, Tokyo, Japan, 1980.

Harvey, J. K. Some Observations of the Vortex Breakdown Phenomenon,Journal of Fluid Mechanics Vol. 14, 1962, pp. 585-592.

Palde, U. J., Influence of Draft Tub e Shape on Surgin g Characteristics ofReaction Turbines, Report No. REC-ERC-72-24 U S. Bureau ofReclam ation, July 1972.

Rheingans, W. J., Power Swings in Hydroelectric Power Plants, Transactionsof the ASME Vol. 62, No. 174, Apr. 1940, pp . 171 1 84.

Sarpkaya , T., On Stationary and Travelling Vortex Breakdow ns, Journal of

Fluid Mechanics Vol. 45, 1971, pp. 545-559.

Wahl, T. L. Draft Tube Surging Hydraulic Model Study, Thesis in partialfulfillment of the requirements for the degree of Master of Science,Colorado State University, Summer 1990.



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cavitationdraft tubehydraulic models

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Wahl Skinner Falvey

the twin vortex draft tube surge pap-0590

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