2D CFD Analysis and Experimental Analysis on Effect of Hub to Tip Ratios on Performance of Impulse Turbines

8/14/2019 2D CFD Analysis and Experimental Analysis on Effect of Hub to Tip Ratios on Performance of Impulse Turbines

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J. o f Thermal Science Vol. 13, No.4

2 D C F D A n a l y s is a n d E x p e r i m e n t a l A n a ly s is o n t h e E f f e c t o f H u b t o T ip R a t i o so n t h e P e r f o r m a n c e o f 0 .6 m I m p u l s e T u r b i n e

A . T H A K K E R M . E I .HEMRYW a v e E n e r g y R e s e a r c h T e a m , D e p a r t m e n t o f M e c h a n i c a l a n d A e r o n a u t i c a l E n g i n e e r in g ,U n i v e r s i t y o f L i m e r i c k , I r e l a n dE - m a i l : A j i t . T h a k k e r @ u l . i e

Th e objective o f this paper is to present the performance comparison of 2D Computational Fluid Dynam ics (CFD)analysis w ith experimental analysis o f 0.6 m impulse turbine with fixed guide van es for both 0.6 and 0.7 hub totip rat io (H/F). Also the com parison o f 2D CFD analysis of the said turbine with different values o f H/T rangingfrom 0.5 to 0.7. A 2D-cascade model was used for CFD analysis while uni-directional steady flow was used forexperimental analysis. The blade and guide vane geometries are based on 0.6 m rotor diameter, with optimumprofile, and different H/T o f 0.5, 0.6 and 0 .7. I t was concluded from 2D CFD analysis that 0.5 I-I/T rat ioperformances was higher than that of 0.6 and 0.7 H /T at peak efficiency and the operational f low ran ge for 0.5H/T w as found to be wider than that of 0.6 and 0.7 H /T ratio.

Keywords: wav e energy, impulse turbine, computational f luid dynamics (CFD ).CLC number: TK 73 0.2 Docum ent code: A Art ic le ID: 1 0 0 3 - 2 1 6 9 ( 2 0 0 4 ) 0 4 - 0 2 9 7 - 0 6

I n t r o d u c t i o nF o r t h e l a s t t w o d e c a d e s , s c i e n t i s t s h a v e b e e n

i n v e s t i g a t i n g a n d d e f i n i n g d i f f e r e n t m e t h o d s f o r p o w e re x t r a c t i o n f r o m w a v e m o t i o n . T h e s e d e v i c e s u t i l i z e t h ep r in c i p le o f a n O s c i ll a ti n g W a t e r C o l u m n ( O W C ) . O W Cb a s e d W a v e E n e r g y P o w e r P l a n t s c o n v e r t w a v e e n e r g yi n t o l o w - p r e s s u r e p n e u m a t i c p o w e r i n t h e f o r m o f b i-d i r ec t i ona l a i r f l ow . S e l f - r ec t i f y i ng a i r t u r b i nes ( whi ch a r ecapab l e o f ope r a t i ng un i - d i r ec t i ona l l y i n b i - d i r ec t i ona la i r f l o w ) a r e u s e d t o e x t r a c t m e c h a n i c a l s h a f t p o w e r ,w h i c h i s f u r t h e r c o n v e r t e d i n t o e l e c t r i c a l p o w e r b y ag e n e r a t o r . T w o d i f f e r e n t t u r b i n e s a r e c u r r e n t l y i n u s ea r o u n d t h e w o r l d f o r w a v e e n e r g y p o w e r g e n e r a t i o n ,W e l l s T u r b i n e , i n t r o d u c e d b y D r . A . A . W e l l s i n 1 9 7 6a n d I m p u l s e T u r b i n e w i th g u i d e v a n e s b y K i m e t a l. m .B o t h k i n d s o f tu r b i n e s a r e c u r r e n t l y i n o p e r a t i o n i nd i f fe r e n t p o w e r p l a n t s in E u r o p e a n d A s i a o n e x p e r i m e n t a la s w e l l a s c o m m e r c i a l b a s is . C u r r e n t l y , r e s e a r c h a r o u n dt h e w o r l d i s f o c u s e d o n i m p r o v i n g t h e p e r f o r m a n c e o ft h e s e t u r b i n e s u n d e r d i f f e r e n t o p e r a t i n g c o n d i t i o n s .

T h e u l t i m a t e p u r p o s e o f t h is r e s e a r c h i s t o i m p r o v et h e p e r f o r m a n c e o f i m p u l s e t u r b i n e w i t h f i x e d g u i d ev a n e s f o r w a v e e n e r g y p o w e r c o n v e r s i o n b y m o d i f y i n g

Rece ived April 24, 2003A. T HAKKE R : Doc tor

d i f f e r e n t b l a d e a n d g u i d e v a n e g e o m e t r i c a l p a r a m e t e r s [2].T h e r e f o r e , a s t h e s t a r t i n g p o i n t , i t w a s d e c i d e d t o u s e ad i f f e r e n t h u b t o t i p r a ti o o f 0 . 6 c o m p a r e d t o t h e a l r e a d ypub l i shed and e s t ab l i shed op t i m um va l ue o f 0 .7 E31. T h es a i d t u r b i n e w a s d e s i g n e d , m a n u f a c t u r e d a n d t e s t e d a tu n i - d i r e c t i o n a l s t e a d y f l o w t e s t i n g f a c i l i t i e s o f W a v eE n e r g y R e s e a r c h T e a m a t U n i v e r s i ty o f L i m e r i c k 4j. T h i sp a p e r p r e s e n t s t h e w o r k c a r r i e d o u t t o c o m p a r ee x p e r i m e n t a l r e s u l t s w i t h 2 D C F D a n a l y s i s f o r 0 . 6 a n d0 . 7 H / T f o r 0 .6 m i m p u l s e t u rb i n e .T u r b i ne D e s i g n a n d M a n u f a c t u r e

T h e b a s i c t u rb i n e d e s i g n a n d g e o m e t r y w a s b a s e d o nt h e o p t i m u m d e s i g n p a r a m e t e r s b y S e t o g u c h i e t a l [ 3 ] . T w odi f f e r en t r o t o r s w i t h hub t o t i p r a t i o o f 0 .6 and 0 .7 w er ed e s i g n e d a n d m a n u f a c t u r e d a t t h e U n i v e r s i t y o f L i m e r i c k .T h e d e t a i l s o f d e s i g n p a r a m e t e r s f o r r o t o r s a n d g u i d ev a n e s a r e g i v e n i n T a b l e 1 . E a c h r o t o r c o n s i s t s o f 3 0b l a d e s w i t h 2 6 f i x e d a n g l e m i r r o r i m a g e g u i d e v a n e s o nb o t h s i d es o f t h e r e s p e c t i v e r o t o r w i t h i n l e t / o u tl e t a n g l e ,0 = 3 0 . B o t h t u r b i n e s h a d a s i m i l a r b l a d e p i t c h t o c h o r dof 0 .5 a s sugg es t ed i n S e t og uch i e t a l . [3 ]. A 2D ske t ch o f0 . 6 H / T r a t i o t u r b in e a t m i d r a d i u s i s s h o w n i n F i g . 1 .


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BCrCAI-I/TIrarRUR

Nomenc la ture V aBlade height, (m) zTorque coefficient ~oInpu t coefficient r/Hub-to-Tip ratio 0Chord length of rotor blade, (m) Flow rate, (m3/s) gOMean radius, m pCircumferential velocity at rn, (m/s)

Mea n axial flow velocity, (m/s)Num ber of rotor bladesPressure drop, (N/mz)Turbine efficiencySetting angle of fixed guide vane, (Deg.)Flow coefficientAngular v elocity of turbine rotor, (rad/s)Air density, (kg/m3)

Table 1 Rotor blade and guide vanes geometryParameter Symb ol 0.5 H/T 0.6 I-I/T 0.7 I-I/T

Blade profile: EllipticalNum ber of blades z 30 30 30Tip diameter (mm ) D 600.0 600.0 600.0Chord length (mm) lr 94.0 100.0 106.0Rotor blade pitch (mm ) Sr 47.0 50.0 53.0Blade inlet angle 7' 60 60 60Pitch/cho rd ratio 0.5 0.5 0.5 0.5

Blade profile: EllipticalPitch (mm ) Sg 54. 58.0 61.0Chord length (nun) lg 123.0 131.0 140.0Number of GV g 26 26 26GV inlet/outlet angle 0 30 30 30

lgFlow

No. o f bladesz = 30

0.00

No. of guide vanes " - ~ ~g = 2 6 ~

Flow

The turb ine b l ades were manufac tured us ing FDMRapid pro to typ ing machines us ing ABS P las t i c -P400.The a luminium hub was manufactured us ing convent ionalmanufac tur ing t echniques. The co pper gu ide vanes werefabricated us ing s tandard sheet metal procedures . Thedetails can be fo und in Th akk er et al . tS[.Experimen tal Facilities and Methodology

A schemat i c layout o f the expe r imenta l r ig a t W aveEnergy Resea rch Team, Unive rs i ty of L imer i ck i s shownin Fig.2. I t cons is ts of a bel l mouth entry, 0.6 m tes tsect ion, dr ive and t ransmiss ion section, a plenum c hamb erwith honeycomb sect ion, a cal ibrated nozzle and acentr i fugal fan. Air i s drawn into the bel l mouth shapedopen end; i t passes through the turbine and then entersthe p l enum chamber . In the chamber , t he f low i scondi t ioned and al l swir ls /vort ices are removed prior topass ing through a cal ibrated nozzle and f inal ly exhaust ingat the fan out le t . Using a valve a t fan exi t controls thef low ra t e . The maximum diamete r o f the t e s t s ec t ion i s0.6m with a hub-t ip ra t io of 0.6. Detai ls of the tes t r igcalibratio n etc. ca n be foun d in T hak ker e t al . [51.Experimen tal Analysis

Fig.1 0.6 m impulse turbine with fixed guide vanes Ex per ime ntal results on the run ning chara cteristic s of


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A. THA KKE R et al . Effect of Hub to Tip Ratios on the Performance of 0.6 m Impulse Turbine 317

'~ Plenum chamberFlow control valve ... -,- 7, Ro tor, - - 1 DCm otor /

Centrifugal fan .~ .~ ~]//ff/ff~ ":"?'~' H [ ,..]=~,ll].=, ,=[..].~......

; / ! \/ ,' t~ j DriveNozzle Honeyc omb section Torque transducer

Fig.2 Schem atic diagram of 0.6 m diameter test rig at University of Limerick

Mass flow inlet

-.IPeriodi~c ]

Guide vane ] Periodic ]

I Pressure outlet

[ Turbineblade ]Fig.3 Cells distribution over the domain

tu rb in e a re e x p re sse d in t e rms o f t h e t o rq u e c o e f f i c i e n tCr, i n p u t c o e f f i c i e n t CA a n d e f f i c i e n c y r / , w h ic h a rep lo t t e d a g a in s t t h e f l o w c o e f f i c i e n t 0 . Th e v a r io u sd e f in i t i o n s are:

C T =T/[p(V2a +U2)blrzrR/2] (1 )CA = t~ Q /[ p (v 2 + U2R )bl~ ZVa / 2] (2)~) : 1Ja [ U R (3 )

r = Ta~ / tYpQ = CT / CAO (4 )

Boundary conditionI t w a s n e c e ssa ry t o se t u p th re e f l u id z o n e s u s in g

s l id in g me sh t e c h n iq u e . Th e th re e z o n e s a re t h e u p s t r e a mg u id e v a n e , t h e ro to r a n d th e d o w n s t re a m g u id e v a n e .In f lo w i s se t a s ma ss f l o w in l e t , o u t f lo w i s se t a s p re ssu reo u t l e t a n d p e r io d ic w a l l s a re se t a s t r a n s i t ; ,o n a l t o a l l o wc a sc a d e e f fe c t o n b l a d e a n d g u id e v a n e to b e s imu la t e d .Th e f lu id a t ro to r i s d e f in e d a s a mo v in g r e fe re n c e f r a mew i th t h e a n g u la r sp e e d e q u iv a l e n t t o t h a t o f t h e b l a d e(3 5 0 r /m in ) . T h e f l o w i s se t a s fu l l y t u rb u le n t .

C F D T e c h n i q u eComputational domainTh e d o ma in e x t e n d e d to 8 .5 c h o rd l e n g th s u p s t r e a m

a n d d o w n s t r e a m, i t i s r e s t r i c t e d to o n e b l a d e to b l a d e a n dg u id e v a n e to g u id e v a n e p a ssa g e w i th p e r io d ic b o u n da r i e s .

Mesh and solverT h e m o d e l w a s m e s h e d i n G a m b i tTM. T h e d o m a i n

w a s m e s h e d u s i n g a n u n s tr u c t u r ed m e s h . T h e m o d e l w a sme sh e d w i th 3 5 0 c e l l s o n th e b l a d e su r fa c e a n d to t a l o f1 3 0 0 0 c e l l s . A n in i t i a l t r i a l h a s b e e n c a r r i e d o u t w i th g r ids i z e 8 0 0 0 , 1 3 ,0 0 0 a n d 2 2 0 0 0 c e l l s a n d th e r e su l t s f ro m


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each grid size w ere v ery similar [61. It was foun d how ever ,that an improvement in the cell aspect ra tio a t the inle tand outle t was required to speed up convergence withmass f low inle t boundary. For this reason, the mediumdensity gr id was re-meshed, giving 13,000 cells , Fig.3.The mo del was analy zed using the Fluent 5TM . The so lve rused w as a segregated so lver as the flow is incompressible.

T u r b u l e n c e m o d e l i n gFluent provides a number of turbulence models, the

default one being the k-E model. The other models are theRN G k-E model and the Reynolds Stress Equation Model.The generic k-e model focuses on the mechanisms thataffect turbulent kinetic energy. The standard k-e modelhas two equations, one for turbulent kinetic energy k, andone for dissipation, E. The second turbulence mod el usedby Flu ent is a var ia tion of k-e model an d called the RN Gk-E model. Th e Reyno lds stress equations model analyzesthe effects of the turbulent f low rather than the f low itse lf .Turbulent f low basically consists of an instantaneouslyf luctuating f low superimposed on a steady mean f low. Inour computation, standard k-E model w as used.C F D R e s u l ts a n d D i s c u s s i o n

Two cases hav e been analy zed in order to investigatethe effects of H/T on the eff ic iency of the impulseturb ine wi th f ixed guide vanes for wave ene rgy powerconversion. For 0.6 H/T, the performance of the turbineusing CFD analysis was compared with the resultsobtained by experiments Fig.4. I t can be noted from thef igure , that the CFD results are giving higher eff ic iencywhen compared to experimental results . The differencein the eff ic iency betwee n experimental and CFD resultsare due to that the CFD results are based on 2D analysisand t ip gap losses are not taken into account. Due to thisfact , actual pressure drops across the rotor cannot bedepicted. These are the main reasons that the CFDanalysis is predicting a higher eff ic iency when com paredto experime ntal analysis.

Efficiency vs Flow Coefficient for 0.6 H/T ratio

0. 80. 60. 40, 2

-- 0-0 ,2-0 . 4-0 ,6-0 .8

Flow Coefficient

0 CFD_0.6 :

Jk Exp_0.6

Fig.4 Efficiency vs. flow coefficient

To fu r ther this discussion, coeff ic ient of torque ( Cr)verses f low coeff ic ient (~) and coeff ic ient of input (Ca )verses f low coeff ic ient (0) plots are given in Figs.5 and 6respectively. Due to highly three-dimensional nature ofthe f low, the 2-D CFD model under predicts the torquecoeff ic ient and input coeff ic ient. From Fig.5, i t can beobserved that the computed values of torque coeff ic ientarrived from 2-D model is under-predicting theexperimental values.

Torque Coef f ic ient vs Row Coef f ic ient for0.6 w r ratio

2 . . . . . . % , ,~: . . . . .

J 'iio.io;ROW Coefficient

ii CFD_ 0 .6 . :i E~0.8 i l

ii

Fig.5 Torque coe~]cient vs. flow coefficientWhere as in Fig.6, we can see that the input

coeff ic ient for experimental results is much higher thanwhat we get f rom CFD. This is due to the fact , that thetip gap losses are not considered in the CFD analysis.Therefo re the actual pressure drop meas ured duringexperiments is higher than the CFD predicted pressuredrop across the rotor . Thus we can say that 2D CFD isnot capable of picking up 3D losses in the system and forbetter and more realist ic performance predictions, 3DCFD analysis are required.

Input Coeff icient vs F lowCoefficlent for 0 . 6 H / l " ratio

3.5

3, 75i 2 -; ~._06!1.5 I Jk Exp 0 6 10.5

00 0.5 1 1.5 2

Flow Coeffldent

Fig.6 Input coefficient vs. flow coefficientThe performance of the turbine using CFD was a lso

analyzed and compared with the experimental resultswith 0.7 H/F. As shown in Fig.7 the computed eff ic iencywith 2-D CFD model gives quali ta t ive agreement with


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A. THAKKER et al . Effect of Hub to Tip Ratios on the Performance of 0.6 m Impulse Turbine 319

experimental results . The difference in eff ic iency betweenCFD and experimen tal results is due to the fact that CFDresults are based on 2D analysis, where some losses arenot taken into account.

Bficiency vs Row Coefficient for 0.7 H/Tratio

++++~, 0s . . . . . .

i I . C FD 0 .7 [0 l - : EXp 0.7

-0 .5-1

Row Coefficient. +

Fig.7 E fficiency vs. flo w coefficientA coeff ic ient of torque Cr verses f low coeff ic ient

and coeff ic ien t of input CAverses f low coeff ic ient 0 plotsare given in Figs.8 and 9 respectively. From Fig.8, i t canbe observed that the computed values of torquecoeff ic ient f rom 2-D CFD mode l is under-predicting theexperimental values and experimental results are givinghigher torque coeff ic ient.

In Fig.9, we can see that the input coeff i c ient forexper imenta l r e su l t s i s much h igher than wha t we ge tfrom CFD . This is d ue to the fact , that the t ip gap losses

Tar que Coefficient v s Row Coefficient for 0 . 7 H r f ratio

21.5 /

I Q C~:D 0.7 i0.5 == Exp_0.7 [

~ o4O.5

Flow Coefficient

Fig.8 Torque coefficient vs. flow coefficient

Input Coeff icient vs Row Coeff icient for0.7 H/T ratio

32. 5

21. 5

- - 0 . 50

0 0.5 1 1 .5 2Row Coemclent

* ~Ff_O.Z= EW . _ 0 . 7

are not considered in the CFD analysis. There fore theactual pressure drop measured during experiments ishigher then the CFD predicted pressure drop across therotor . Again we can say that 3D CFD analysis isrequired for predicting the performance of turbinequali ta t ively and quanti ta t ively.Comparsion of CFD Results of 0.5, 0.6 and 0.7I-I/T Ratio Turb ines

Comparison for the CFD results among 0.5, 0.6 and0.7 hub to t ip ra tio is shown in Fig.10 with respect toeff ic iency verses f low coeff ic ient. The C FD results givesquite similar curves in trend, but the turbine with 0.5 hubto t ip ra tio is giving highe r eff ic iency when com pared to0.6 and 0.7 hub to t ip over the entire range of f lowcoefficient.

iEfficiency vs Flow coefficient for 0.5,0.6 and !0.7 H/T ratio !

i1 %+ _ +" +'~ = i

+o: . o+o+I- - " C FD 0 .6 =i CFO o.s

-0 .5 i

-' iFlow Coefficient !Fig.10 E fficiency vs. flow coefficient

Coeff ic ient of torque (Cr) verses f low coeff ic ient (0)and coeff ic ient of input (CA) verses f low co eff ic ient (0)plots are given in Figs. l l and 12 respectively. I t can beseen from Fig.11, the 2-D computed values gives quitesimilar curves in trend for a ll the tested H/T ratio,however , 0.5 H /T ratio turbine is giving higher value fortorque coeff ic ient for the full range of f low coeff ic ientwhen compared to the 0.6 and 0.7 H /T ratio.

The magnitude for the input coeff ic ient, Fig.12, for

Torque Coeff icient vs Row Coeff icient for0.5,0.6 and 0.7 Hrr ratio

"+- 0.60.5 ,i* +++0. 40. 30. 2

~ 0.1I-

..01Row Coefficient

i I CF D_0.7 C FD 0 .6

C FD _ 0 .5

Fig.9 Input coefficient vs. flow coefficient F ig .l l To rqu e coe ffic ien t vs. flo w coe ffic ien t


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320 Journal o f Thermal Science, Vo1.13, No.4, 2004

Input Coefficient vs Row Coefficient for0.5,0.6 and 0.7 H/T ratio

0.8 , -0 .6 O0.4 "

" 0.20

0 0.5 1 1.5 2ROW Coefficient

Fig.1 2 Inpu t coefficient vs. flow coefficient

C A C F D 0 . 7C A C F D 0 . 6s _ p _ i n l e t C F D _ 0 . 5-- i

0.5 H /T ra t io is higher than fo r 0.6 and 0.7 I-I /T ra t io, butdue to the higher torque generat ing character is t ics of 0.5H/T ra t io tu rb ine , t he ove ra l l pe r formance was found tobe bet ter than 0.6 and 0.7 H/T ra t io turbine. Thusperformance of 0.5 H/T ra t io turbine proves that i t i s abe t t e r hub to t i p ra t io when compared to prev ious lyes tabl ished opt imu m v alue o f 0.7 Se toguchi e t a l . I31.C o n c l u s i o n s

The e f fec t s o f hub to t i p ra t io on the pe r forman ce ofthe impul se tu rb ine for wave ene rgy convers ion havebeen inves t igated by 2-D CFD analys is andexperimental ly by model tes t ing under s teady f lowcondi t ions . The 2-D CFD resul ts for 0.6 and 0.7 H/Tra t io gave h ighe r e f f i c i ency when compared wi thexperim ental resul ts. This w as due to the fact that t ip gaplosses were not taken into 2-D CFD analys is .

The resul ts from 2D CFD analys is sugges ted that 0.5H/T ra t io tu rb ine pe r fo rmance was h ighe r than tha t o f 0 .6and 0.7 I-I/T ratios. A f urthe r study is requ ired to clar ifythe e f fec t o f hub- to- t ip ra t io on the pe r formance of theimpulse turbine.

A c k n o w l e d g m e n t sThe authors would l ike to acknowledge the f inancial

support given by Wave Energy R esearch Team, Departmentof Mechanica l & Aeronaut i ca l Enginee r ing , Unive rs i tyof Limerick, Ire land.R e f e r e n c e s[1] Kim, T W, Kaneko, K, Setoguchi, T, et al. Aerodynamic

Performance of an Impulse Turbine with Self-Pitch-Controlled Guide Vanes for Wave Power Generator. In:Proceedings of the 1 t KSMY-JSME Thermal and FluidsEngineering Conference, Korea., 1988

[2] Thakker, A, Frawley, P, Kalleeq, H B. An Investigation o fthe Effects of Reynolds Number on the Performance of0.6 m Impu lse Turbines for Different Hu b to T ip Ratios.In: Proceedings of the 12~ International Offshore andPolar Engineering Conference, Kitakyushu, Japan, 2002.682--686

[3] Setoguchi, T, Santhakumar, S, Maeda, H, et al. A Revie wof Impulse Turbines for Wave Energy Conversion.Renewable Energy, 2001, 23:261--292

[4] Thakker, A, Khaleeq, H B, Setoguchi, T. PerformanceComparison of 0.3 m and 0.6 m Impulse Turbine withFixed Guide Vanes - Part I. In: Proceedings of the 4 mEuropean Wave Energy Conference, Aalborg, Denmark,2000a

[5] Thakker, A, Sheahan, C, Frawley, P, et al. The ConcurrentEngineering Approach to the Manufacture of ImpulseTurbine Blades using Rapid Prototyping. The RapidPrototyping Journal, 2001, 7(3): 1 -- 5

[6] Thakker, A, Frawley, P, Kalleeq, H B, et al. Experimentaland CFD Analysis of 0.6 Impulse Turbine with FixedGuide Vane. In: Proceedings of 11 h ISOP E Conference,Stavanger, Norway, 200 l, 1: 625-- 629

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2D CFD Analysis and Experimental Analysis on Effect of Hub to Tip Ratios on Performance of Impulse Turbines