THE INFLUENCE OF RUNNER CONE PERFORATION ON THE … · THE INFLUENCE OF RUNNER CONE PERFORATION ON...

THE INFLUENCE OF RUNNER CONE PERFORATION ONTHE DRAFT TUBE VORTEX IN FRANCIS HYDRO-TURBINE

by

Wen-Tao SU a,b**, Xiao-Bin LI c**, You-Ning XU a, Ru-Zhi GONG b*,Maxime BINAMA b, and Alexis MUHIRWA b

a School of En ergy and Power, Shenyang In sti tute of En gi neer ing, Shenyang, Chinab School of En ergy Sci ence and En gi neer ing, Harbin In sti tute of Tech nol ogy, Harbin, China

c Sino-French In sti tute of Nu clear En gi neer ing and Tech nol ogy, Sun Yat-Sen Uni ver sity,Zhuhai, China

Orig i nal sci en tific pa perhttps://doi.org/10.2298/TSCI171011040S

In Fran cis hy dro-tur bine, the pres sure os cil la tion in the draft tube is the mainsource of flow-in duced vi bra tion and noise. The struc tural de sign of run ner coneex erts sig nif i cant in flu ence on the tan gen tial flow ve loc ity within the draft tube andthe pres sure fluc tu a tions down stream. In this work, we pro pose a sim ple and ef fec -tive method by per fo rat ing through-holes on the run ner cone to weaken or elim i nate pres sure fluc tu a tions in duced by the vor tex ropes in draft tube. An 2 and 4 holes areper fo rated on the lower half of run ner cone, and nu mer i cal sim u la tions and ex per i -ments are con ducted to un der stand the chang ing of phe nom en ol ogy and fea tureswith re gard to vor tex ropes and pres sure lev els, com par ing with the orig i nalunperforated case. The anal y ses are done for two dif fer ent guide vane open ings, a,and the re sults were val i dated by ex per i ments with high-speed pho tog ra phy. It isfound that at the two open ings, the run ner cone with per fo ra tions can re duce thevol ume of vor tex ropes, and de crease the pres sure fluc tu a tion level. The mo tion ofvor tex ropes tends to be less vi o lent with more per fo rated holes. There fore, the run -ner cone per fo ra tion has the po ten tial of re duc ing the vor tex rope strength.

Key words: fran cis hy dro-tur bine, run ner cone per fo ra tion, draft tube,pres sure fluc tu a tion, high-speed pho tog ra phy

In tro duc tion

With the in creas ing de mand on the sta bil ity and flex i bil ity of power sup ply sys tem,hy dro-tur bines (es pe cially large-scale tur bine) are fre quently op er ated un der off-de sign con di -tions, and need more ef fi cient strat egy for de sign [1]. At par tial flow load, pres sure fluc tu a tionsalong with un sta ble flow are usu ally en coun tered. They serve as in her ent source of the flow in -sta bil i ties and the in duced pres sure os cil la tions. Con se quently, some det ri men tal phe nom ena(such as struc tural vi bra tions, com po nents break age, noise, and power gen er a tion ca pac ity deg -ra da tion) arise. In Fran cis tur bine, the pres sure pul sa tions in duced by the vor tex rope in drafttube con trib ute the most to these prob lems [2, 3]. The vor tex rope de vel op ment is more as so ci -ated with the vor tex break down, which it self is an abrupt change in flow struc ture, even tu ally

Su, W.-T., et al.: The Influence of Runner Cone Perforation on the Draft Tube Vortex ...THERMAL SCIENCE: Year 2018, Vol. 22, Suppl. 2, pp. S557-S566 S557

**Corresponding author, e-mail: [email protected],**Both au thors con trib uted equally to this work

get ting highly un steady with re cir cu lat ing flows at cer tain crit i cal swirl lev els [4]. For the caseof draft tube, the flow co mes from the down stream of run ner. It is mainly driven by the tan gen -tial ve loc ity com po nent, which shifts the main flow to wards the wall, leav ing stag nated wa ter atthe cen ter of draft tube.

On the mech a nism of vor tex rope de vel op ment in draft tube, so far, there are somestud ies con ducted to un der stand the pres sure fluc tu a tions and vi bra tion. Through the ve loc ityfield mea sure ment us ing par ti cle im age velocimetry (PIV), Kirschner et al. [2, 5] stud ied the re -la tion ship be tween vor tex rope and the pres sure fluc tu a tion fre quency near the wall area in drafttube. Favrel et al. [6] ex per i men tally re vealed that the rise in the pres sure ex ci ta tion source in -ten sity was in duced by an en large ment of the vor tex tra jec tory and a si mul ta neous in crease inthe pre ces sion fre quency, as well as the vor tex cir cu la tion. Many more in ves ti ga tions have alsobeen done to an a lyze the pro files of pres sure fluc tu a tion and vor tex rope move ments [7-9].Thus, a great deal of ef fort by pro pos ing new meth ods and de signs was de voted to the tar get ofde creas ing the pres sure pul sa tions am pli tudes in duced by vor tex ropes. Ex am ples in clude wa terin jec tion, air ad mis sion, wall sur face mod i fi ca tion, etc., which tried to al le vi ate the cav i ta tion by in creas ing the lo cal pres sure level. Kirschner et al. [5] in ves ti gated the ef fi cacy of draft tube wa -ter in jec tion method un der two dif fer ent par tial load con di tions, and they showed it was ef fec -tive in terms of pres sure fluc tu a tion at ten u a tion. This wa ter in jec tion method was ini tially pre -sented by Su san-Resiga et al. [10]. The wa ter is ax i ally in jected into the cen ter of the draft tube,and the pro file of the ax ial ve loc ity at the in let of the draft tube changes, so that the de vel op mentof the vor tex rope can be avoided. Later, Tanasa et al. [11] ex tended this method to an ax ial hy -dro-tur bine. The draft tube air ad mis sion method has also been im ple mented in many cases, forin stance, Papillon and Sabourin [12] and Chirkov et al. [13], among oth ers. It has been well rec -og nized that air ad mis sion tech nique is ef fi cient to de crease the pres sure fluc tu a tions, es pe cially at low loads for Fran cis tur bine. Both the wa ter in jec tion method and the air ad mis sion methodare ac tive meth ods for pres sure fluc tu a tion al le vi a tion. How ever, these meth ods suf fer from thedraw backs in clud ing the de sign cost, com plex ity of ex tra de vices and the lim i ta tion of the op er -at ing pa ram e ters. There are also var i ous pas sive meth ods de vel oped to deal with the un sta bleflow and pres sure pul sa tions. Two ex am ples are the J-grooves [14, 15] and draft tube fins [16].Al though these two meth ods are ca pa ble of re duc ing the pres sure pul sa tions in the draft tube,their prac ti cal ap pli ca tions are lim ited due to their cost, technics and ef fi ciency.

As for the run ner cone, its con fig u ra tion and ge om e try also sig nif i cantly in flu ence thepres sure os cil la tions as it changes the dis tri bu tion pat tern of the vor tex rope in the draft tube[1, 13]. Qian et al. [17] con firmed by nu mer i cal re sults that the ex tended run ner cone, run nercone with grooves and the round-top run ner cone are able to sup press the low-fre quency pres -sure os cil la tions. Sano et al. [18] also in ves ti gated the in flu ence of spi ral grooved run ner coneon the draft pres sure pul sa tions, and they found that the am pli tude of pres sure os cil la tion was re -duced. In a very re cent study, Gogstad [19] has ex per i men tally in ves ti gated the ef fects of air in -jec tion and cone ex ten sion on the pres sure pul sa tions. It was shown that al though cone ex ten -sion de creased the peak-to-peak val ues in some flow con di tions, it also in creased these val ues atother op er a tion points.

In this study, we pro pose a new pas sive method by per fo rat ing holes on the run nercone. The ef fects of these holes on the vor tex ropes and pres sure pul sa tions will be in ves ti gated.This kind of per fo ra tion method has been used for blades of com pres sors or fans to im prove thecom pres sor surge mar gin [20]. Re cently, we have suc cess fully ap plied this method to the run ner blades in a Fran cis hy dro-tur bine. It is found that the cav i ta tion zone was greatly de creased, andthe am pli tude of pres sure fluc tu a tions and the re lated flow-in duced noise re duced [21]. In this

Su, W.-T., et al.: The Influence of Runner Cone Perforation on the Draft Tube Vortex ...

S558 THERMAL SCIENCE: Year 2018, Vol. 22, Suppl. 2, pp. S557-S566

work, the in flu ence of run ner cone per fo ra tion on the draft tube pres sure field char ac ter is tics isstud ied for the first time. Three run ner cone model de signs, viz. unperforated, 2-hole and 4-holeper fo rated, were ex per i men tally in ves ti gated through high-speed pho tog ra phy to cap ture thevor tex rope shapes in the draft tube. Mean while, nu mer i cal sim u la tions based on ANSYS CFXwere con ducted for val i da tion.

Ex per i men tal and nu mer i cal method

Geo met ri cal model

This study in ves ti gates the in flu ence of run ner cone per fo ra tion on the flow char ac ter -is tics within a Fran cis hy dro-tur bine, and the mech a nism by which the draft tube-based vor texrope and as so ci ated pres sure fluc tu a tion am pli tudes are al tered. The stud ied tur bine model has ahy drau lic head of H = 57 m at rated con di tion. The run ner di am e ter D2 is 366.58 mm, and thenum bers of run ner blades, guide vanes and stay vanes are z = 15, zg = 24, and zs = 24, re spec -tively.

For the Fran cis tur bine, dif fer ent run ner cones with per fo ra tions were de signed. As shown infig. 1, the first two holes were drilled sym met ri cally on the bot tom half of the run ner cone, then an -other two holes were punchedon the fur ther bot tom. The di -am e ters of all holes are 7 mm,and their lo ca tions are de cidedbased on the low est pres sure lo -ca tions for the unperforatedcase (the de tails can be found in [21]). Ex cept the per fo ra tionfea ture, all other geo met ri calchar ac ter is tics are the same forthe three used run ner conemod els. The vor tex char ac ter is -tics and the cor re spond ingpres sure pul sa tions in the drafttube will be in ves ti gated.

Ex per i men tal set-up

The ex per i men tal tests were car ried out on a test rig based at Harbin Elec tric Ma chinesCom pany Ltd. (HELM) in China, where the test set-up was de signed ac cord ing to theIEC60193-1999 stan dard [22]. In or der to ob tain ad e quate data in terms of pres sure field char ac -ter is tics and its evo lu tion within the draft tube, the high-speed pho tog ra phy method wasadopted, and pres sure sen sors (Type 112A22, PCB, USA) were equipped at the wall of drafttube to mea sure the pres sure fluc tu a tions. For high-speed im ag ing, an en do scope with op ti cal fi -ber (Æ10 mm × 300 mm × DOV 50/80/90°, WOLF Inc., Ger many) and a high-speed cam era(i-SPEED 2, Olym pus, Ja pan) were used. To de crease the dis tor tion of im ages, a rect an gu larshell was de signed as the outer wall of the draft tube.

Nu mer i cal sim u la tion method

In sim u la tions, the con fig u ra tion and ge om e try is the same as that in ex per i ments, and the grids for com pu ta tions were gen er ated. Five sets of meshes were done for grid-in de pend encycheck [23], as shown in fig. 2, and fi nally the grid num ber of 7 mil lion was used.


Fig ure 1. Three types of run ner cones; (a) with outper fo ra tion, (b) with 2 holes, (c) with 4 holes

To in ves ti gate in de tails the pres surefluc tu a tion char ac ter is tics and their evo lu -tion, two cases viz. par tial and full load con -di tions are tested, as pre sented in tab. 1.Therein, a, n11, Q11, and s are the open ing of guide vanes, the unit speed, the unit flowrate, and the cav i ta tion num ber of de vice, re -spec tively. The used SST k-w tur bu lencemodel, be ing a com bi na tion of k-w and k-e

tur bu lence mod els, has the ad van tage of ad e quately cap tur ing flow field char ac ter is tics at tar -geted flow area. As for the discretization method, high-res o lu tion scheme was used. The con ver -gence cri te rion was set to 10–5. The frozen ro tor in ter face types were adopted to con nect the ro -tat ing and sta tion ary parts. In ad di tion, the mass-flow rate con di tion, pres sure out let con di tionand no-slip wall con di tion were, re spec tively, set for the in let of spi ral cas ing, the out let of drafttube and all the walls.

Re sults and dis cus sion

Par tial load con di tion

To in ves ti gate the ef fect of run ner cone per fo ra tion on vor tex rope de vel op ment andas so ci ated pres sure fluc tu a tions, sim u la tion re sults are first pre sented and an a lyzed. Fig ure 3shows the vor tex rope struc ture within the draft tube with dif fer ent run ner cone de signs at par tial load con di tion. The vor tex ropes are pre sented in the form of iso-sur faces of pres sure val ues. Itcan be seen that un der the same flow con di tion, the vol ume of vor tex rope in the draft tube be -comes smaller as the thor ough holes per fo rated. Mean while, if look ing into the inter-blade flowvor ti ces (not shown here), they also grad u ally re duced in vol ume.

To care fully check up on the for ma tion of vor tex ropes, the de tails of vor tex ropes inthe vi cin ity of run ner cones are also pre sented in fig. 3. The vor tex rope gen er ates at the bot tomhalf of run ner cone, and grows along with the flow field in the draft tube. For the unperforatedcase, sev eral start ing points for vor tex rope gen er a tion and growth were found. For per fo ra tioncases, the num ber of start lo ca tion. In par tic u lar, there is only one vor tex rope gen er ates for the4-holes run ner cone. Mean while, with per fo ra tions the di am e ter of vor tex rope also gets smaller. The de crease of the vor tex rope vol ume would con sid er ably con trib ute to the re duc tion of vor -tex ropes' vi bra tions and the as so ci ated pres sure fluc tu a tion am pli tudes (will be dis cussed later).



Fig ure 2. (a) Grid-in de pend ency check for par tial load flow con di tion and (b) meshes for vanes and run ner

Ta ble 1. Op er at ing con di tions for the study

a[mm]

n11

[rmin–1]Q11

[m3s–1]s

Par tial loadcondition

16 80.4 0.486 0.258

Full loadcondition

28 83.2 0.768 0.277

In ex per i ments, the flow fea tures in the draft tube have been re corded with high-speedpho tog ra phy. Fig ure 4 shows the evo lu tion of vor tex rope shape within a com plete ro ta tional cy -


Figure 3. First row: Vor tex ropes with dif fer ent per fo ra tion strat e gies, in the shape of iso-sur faces ofpres sure value 44.500 Pa (a = 16 mm, s = 0.258, n11 = 80.4 rpm); (a) with out per fo ra tion, (b) with 2 holes,(c) with 4 holes; second raw: zoom-in of the shapes of vor tex ropes in the vi cin ity of run ner cones(for color image see journal web site)

Figure 4. De vel op ment of vor tex rope in draft tube with out run ner cone per fo ra tion;(for color image see journal web site)

cle for the unperforated case. At par tial load, the ro ta tional pe riod and the fre quency of vor texrope were found to be around 216 ms and 4.5 Hz, re spec tively. With out per fo ra tion, the vor texrope shows a he li cal shape, con sist ing of 2 or 3 smaller ropes, twisted down stream. Also, thestart points of these vor tex ropes are not fixed dur ing ro ta tion, but con tin u ously vary ing alongthe run ner cone. The ex is tence of sev eral ropes was found to be closely re lated to the largelow-pres sure area around the run ner cone, pos si bly giv ing rise to sev eral cav i ta tion in cep tionpoints if the low-pres sure area on the run ner out let gets larger.

Sim i larly, figs. 5 and 6 show the de vel op ment of vor tex ropes in the draft tube with 2and 4 holes in the run ner cone in one ro ta tional cy cle, re spec tively. Af ter the per fo ra tion, thevor tex rope vol ume was found to be no tice ably re duced. With the num ber of per fo ra tion holesin creased, the vi bra tion of vor tex ropes di min ished. Es pe cially for run ner cone with 4 holes, thevol ume of vor tex rope dra mat i cally de creased around the run ner cone, where there were onlysome tiny vor ti ces in stead of a sin gle strong vor tex rope. Nev er the less, small ropes still ex isted,though the vol ume was much smaller, which in di cates that the cav i ta tion en ergy may still ex ist.In other words, the am pli tude of pres sure fluc tu a tion be comes lower but still re mark able.

To better un der stand such dif fer ences in vor tex ropes, the ve loc ity fields around therun ner cone with 2 and 4 holes were also pre sented in figs. 5 and 6. It can be seen that the thor -



Figure 5. De vel op ment of vor tex rope in draft tube with 2 holes in run ner cone (par tial load: a = 16 mm,s = 0.258, n11 = 80.4 r/min), and a snap shot of ve loc ity field (mag ni tude) around the run ner cone (right);(for color image see journal web site)

Figure 6. De vel op ment of vor tex rope in draft tube with 4 holes in run ner cone (par tial load: a = 16 mm,s = 0.258, n11 = 80.4 r/min), and a snap shot of ve loc ity field (mag ni tude) around the run ner cone (right);(for color image see journal web site)

ough holes con nected the flows of high-pres sure and low-pres sure zone. Con se quently, the low -est pres sure zone, i. e., the most prob a ble start ing points of vor tex ropes, is shifted. In these twocases, they are shifted down stream, and at the same time the vor tex zone de creases.

Full load con di tion

The full load con di tion is close to the lim i ta tion line of 95% power out put, and the hy -dro-tur bine only oc ca sion ally runs un der this con di tion. For this con di tion, the vor tex ropes arepre sented in fig. 7. They show a cy lin dri cal shape for both per fo rated case and unperforatedcase. As ex pected, the vol ume of vor tex rope de creases with the in creas ing of holes' num ber.Ex per i men tal ob ser va tions are shown in fig. 8, and they con firm that vor tex vol ume at the out letof run ner is ob vi ously re duced af ter the per fo ra tions.

For a vor tex with cy lin dri cal struc ture, the evo lu tion of its shape in time is not no tice -able. Since the vor ti ces for those three cases are sim i lar, the changes of the as so ci ated pres sure

pul sa tions should stem from the in tact area be low the run ner. Ac tu ally, by check ing the nu mer i -cal and ex per i men tal re sults, the main dif fer ence lies in the vor tex shapes be low the run ner. Un -der the full load con di tion, lumps of (cavitated) vor ti ces shed from the out let of the run ner,which ac counts for large por tion for unperforated run ner cone. As for the per fo rated cases, thenum ber of shed vor ti ces grad u ally de creases, and fi nally van ishes for 4 holes case. This fea turewould lead to com plex chang ing in the pres sure pul sa tions down stream.

Pres sure pul sa tions

The pur pose of run ner cone per fo ra tion is to re duce the pres sure fluc tu a tion lev els inthe draft tube. In this sec tion, the pres sure pul sa tions for each case are pre sented and dis cussed.Four pres sure mon i tor ing points are set at the wall of the draft tube, i. e., two points with a dis -


Figure 7. Nu mer i cal re sults of vor tex ropes un der full load con di tion with dif fer ent run ner cone de signs

Figure 8. Vor tex ropes un der large dis charge con di tion with the three run ner cone de signs

tance 0.3 D2 be low the run ner out let(left 0.3 D2 and right 0.3 D2), one pointat the in ner side of the draft tube el bow, and last point at the outer side of thedraft tube el bow. The lo ca tions of these mon i tor ing points are de cided ac cord -ing to the stan dard of IEC-60193 [22].

The en ergy dis tri bu tions of pres -sure pul sa tions un der the par tial loadare shown in fig. 9. With out per fo ra -tions, the pres sure pul sa tion at theouter side of el bow is the larg est, and italso de creases the most af ter per fo ra -

tions. Other points show the same trend of pres sure pul sa tion mit i ga tion. How ever, for the fullload con di tion, the trend of pres sure pul sa tion for var i ous run ner cones is not con sis tent. As in -creas ing the holes num ber, the mag ni tude of pres sure pul sa tions de creases at the points of left0.3 D2, right 0.3 D2, and the outer side of el bow at full load. At the same time, the main fre -quency also de creases. Whilst for the point at the in ner side of el bow, the en ergy of pres surefluc tu a tions de creases not too much, but the main fre quency ob vi ously in creases, tab. 2. Thisshould be due to the com plex chang ing in the flow field, and it re quires a fur ther in ves ti ga tion.



Figure 9. En ergy dis tri bu tion of pres sure pul sa tions at par tial load con di tion (a = 16 mm, s = 0.258,n11 = 80.4 r/min)

Ta ble 2. Pres sure fluc tu a tions for var i ous run ner cones and flow con di tions

Con di tionsConede sign

n11

[r/min]DH/H[%]

Mainfre quency

[Hz]

0 hole 6.13 4.52

a = 16 mm 2 holes 80.4 7.14 4.52

4 holes 6.17 4.52

0 hole 1.67 3.05

a = 28 mm 2 holes 83.2 1.36 2.93

4 holes 1.47 8.06

Con clu sion

This work pro posed a new method of run ner cone per fo ra tion to mit i gate the pres surepul sa tion in the draft tube in a Fran cis hy dro-tur bine. Nu mer i cal and ex per i men tal stud ies withtwo de signs (2 hole per fo ra tions and 4 hole per fo ra tions in the run ner cone) are per formed to in -ves ti gate the their in flu ence on the evo lu tion of vor tex ropes and the as so ci ated pres sure pul sa tionmag ni tudes. It is found that the run ner cone per fo ra tion pos i tively im pacts the flow char ac ter is ticsin the draft tube by re duc ing the vor tex rope vol ume. At par tial load, af ter per fo ra tions, the vor texrope pat terns changes from sev eral twisted ropes to a sin gle thin rope, and there fore the pres surepul sa tion mag ni tude greatly de creases for all the four mon i tor ing points. How ever, for the fullload, al though the vol ume of cy lin dri cal vor tex rope de creases af ter per fo ra tion, its ef fect on thepres sure fluc tu a tions is not the same for dif fer ent mon i tor ing points. There fore, the run ner coneper fo ra tion method could serve as an ef fi cient way to deal with the pres sure pul sa tion in ducedprob lems in the tur bine. How ever, more re search are still re quired to deepen the un der stand ingabout the hy dro dy nam ics be hind the pre sented method and to ex plore its ap pli ca tions.

Ac knowl edg ment

This work is sup ported by na tional Nat u ral Sci ence Foun da tion of China (51606050,51506037), Chi nese Post doc toral Sci ence Foun da tion (2016M591527), Heilongjiang Post doc -toral Fund (LBH-Z16057), and Nat u ral Sci ence Foun da tion of Heilongjiang Prov ince(E2017038).

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[16] Kawamoto, K., Satoh, J., Re straint of Whirl Vi bra tion in the Draft Tube by Air Ad mis sion and by Fins,Trans ac tions of the Ja pan So ci ety of Me chan i cal En gi neers Se ries B, 59 (1993), 558, pp. 487-491

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Paper submitted: October 11, 2017 © 2018 Society of Thermal Engineers of Serbia.Pa per re vised: November 25, 2017 Pub lished by the Vin~a Institute of Nuclear Sciences, Bel grade, Ser bia.Pa per ac cepted: November 25, 2017 This is an open ac cess ar ti cle dis trib uted un der the CC BY-NC-ND 4.0 terms and con di tions.



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