Dr. Ruchi KhareDr. Vishnu Prasad

Manish SharmaManish Sharma

Maulana Azad National Institute of TechnologyMaulana Azad National Institute of Technology Bhopal (M.P.)

In reaction turbines significant energy comes out from In reaction turbines, significant energy comes out fromrunner in form of kinetic energy.

The draft tube in hydraulic reaction turbine is used toconverts part the kinetic energy coming out of runnerinto useful static pressure and increase the effectivehead on turbine

There are different type of draft tube but elbow drafttube is the most commonly usedtube is the most commonly used.

The amount of whirl velocity coming out from runnerl ff fl d b h d f bgreatly affects flow distribution within draft tube. It is

more at off-design operating conditions and minimumat rated conditions..

International Conference on Hydropower for Sustainable Development, Dehradun February 2015 2

The increased whirl coming out from runner ind f b k fl i d f bdraft tube makes flow very uneven in draft tubeand leads to increased vibrations.

Generally the vertical splitter in large draft tube areprovided to reduce the non-uniformity of flow

fl i diffacross flow area in diffusor.

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Numerical simulation of complete Francis turbinepassage is carried out at rated conditions withpassage is carried out at rated conditions withsplitters at different locations in draft tube.

The different draft tube parameters are computed The different draft tube parameters are computedfrom simulation results.

The effect of splitter at different locations in elbow The effect of splitter at different locations in elbowdraft tube on flow distribution and performance ofdraft tube is studied.

The pattern of head recovery along flow in drafttube is also studied.

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The complete geometry of a mixed flow hydraulict bi (F i T bi ) i i l t dturbine (Francis Turbine) is simulated.

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February 2015 5

SIMPLE DRAFT TUBE VERICAL SPLITETR IN DIFFUSER ATSIMPLE DRAFT TUBE VERICAL SPLITETR IN DIFFUSER AT DIFFUSER

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Development, Dehradun

HORIZONTAL SPLITTER IN DIFFUSERFebruary 2015 6

VERTICAL SPLITTER AT ELBOW HORIZONTAL SPLITTER AT ELBOWVERTICAL SPLITTER AT ELBOW HORIZONTAL SPLITTER AT ELBOW

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Development, DehradunCYLINDER IN CONE

February 2015 7

The flow simulation is performed at rated conditions fora constant guide vane opening of 28º and rotationalspeed of runner was 375 rpm.

Mass flow rate at inlet is specified as 22,200 Kg/sec andkept constant in all simulations.

Pressure at outlet is taken atmospheric

The density and kinematic viscosity of water is taken as The density and kinematic viscosity of water is taken as997 kg/m3 and 0.87 X 10-6 m2/sec

SST k-ω turbulence model is used.International Conference on Hydropower for

Sustainable Development, Dehradun

February 2015 8

Computation of Hydrodynamic Parameters Computation of Hydrodynamic Parameters

uavdto AQV Uniform averaged velocity at draft tube

outletdto

uavdto A

1

outlet

Mass averaged velocity at draft tube dtoA

mavdto dAvQ

V 21

Mass averaged velocity at draft tube outlet

mavdto

uavdto

VVα Uniformity coefficient

outdind

ld

TPTPH Head loss in draft tube

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Computation Loss and Flow Computation Loss and Flow ParametersParameters

ldH

Parameters...Parameters...

Relative head loss coefficient nHld

22 CCDraft tube recovery (%) 100*)

2( 43

nngHCC

Draft tube efficiency (%) 100*2)(23

24

23

CHgCC n

d

Hydraulic efficiency of turbine(%) 100*30 n

h QHnT

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Comparison of Computed Parameters

Type of Mass Uniformi Draft Eff f H d

Normal average velocity = 1.707 m/s

Type of Draft tube

Mass averaged velocity

attl t

Uniformity coeff.

Draft tube headloss ( )

Head recovery

(m)

Eff. of draft tube( %)

Hydra--ulic

eff. of turbine

Relative head

loss %outlet(m/s)

(m) ( %) tu b e(%)

Simple-draft 1.980 0.862 0.6528 3.643 81.163 91.69 14.54tubeVerticalplate indiffuser

1.956 0.873 0.6800 3.598 80.496 91.08 15.21diffuserHorizontalplate in

2.030 0.841 0.9652 3.363 74.509 91.30 21.38

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diffuser

February 2015 11

Comparison of Computed Parameters...Comparison of Computed Parameters...

Type of Draft tube

Mass averaged velocity at

outlet

Unifor-mity

coeff.

Draft tube headloss

Head reco-very

Eff. of draft tube

( %)

Hydra--ulic

eff. of t bi

Relative headoutlet

(m/s)loss (m)

very(m) turbine

(%)

headloss %

Verticalplate in 2.271 0.752 1.0474 3.308 73.362 91.23 23.22plate inelbow

0.752

Cylinder in 1.971 0.866 1.3056 3.036 64.873 91.16 27.9cone

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60

45505560

)

25303540

ecov

ery

(%)

Simple draft tubeCylinder in coneVertical splitter in diffuserHorizontal splitter in diffuserH i l diff i lb

510152025

Hea

d re Horizontal diffuser in elbow

Vertical splitter in elbow

-505

1 2 3 4 5 6 7 8 9 10 11 12 13

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Sections along stream flow

February 2015 13

The total stream flow length of draft tube fromi l l i di id d i 12 i d hinlet to outlet is divided into 12 sections and thehead recovery between each section is calculatedfor all variants of draft tube.

Most of the head recovery occurs in the draft tubecone. The head recovery has rising trend up to endof cone and then gradually decreases and becomesof cone and then gradually decreases and becomesnearly zero in last 3 sections towards outlet.

In case of horizontal splitter in diffuser, the headrecovery in 5th section is negative indicating thatloss in more than change in kinetic energy and alsothere is large fluctuation in head recovery along thethere is large fluctuation in head recovery along theflow.

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WITHOUT SPLITTER VERTICAL SPLITTER AT DIFFUSERWITHOUT SPLITTER VERTICAL SPLITTER AT DIFFUSER

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HORIZONTAL SPLITTER AT DIFFUSER HORIZONTAL SPLITTER AT ELBOW

February 2015 15

VERTICAL SPLITER AT ELBOW

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CYLINDER AT CONE

February 2015 16

SIMPLE DRAFT TUBESIMPLE DRAFT TUBE

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HORIZONTAL PLATE AT MIDDLE OF DIFFUSER

February 2015 17

The mass average computed velocity from simulationresults at draft tube outlet is higher than normalresults at draft tube outlet is higher than normalaverage velocity and this indicates the concentration offlow to smaller area.

The value of uniformity coefficient has maximum value(0.916) with horizontal splitter in elbow and minimum( ) pvalue (0.752) with vertical splitter in elbow.

It is observed that provision of splitter in draft tube It is observed that provision of splitter in draft tubeincreases the loss in draft tube due to increasedexposed surface area to flow.

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The maximum loss is seen with cylinder in cone The energy recovery is also affected by splitters

and found to be decreasing with minimum recoveryfor cylinder in cone.o cy de co e

The draft tube efficiency is found to be maximum81.163% for simple draft tube and minimum with

li d icylinder in cone. The hydraulic efficiency of turbine is maximum for

simple draft tube and minimum with vertical platep pin diffuser. This is because the velocity head is4.50m at exit of runner which is only 4.73% of thenet head 95m of turbinenet head 95m of turbine.

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It is observed that Flow behavior and performance of drafttube is significantly affected due to splitters at variousg y plocations.

Efficiencies of both draft tube and turbine are found todecrease due splitters and hence there is no advantage ofdecrease due splitters and hence there is no advantage ofsplitter at rated condition.

The uniformity of flow at draft tube outlet is found to bei d i d f b i h h i l li i lbimproved in draft tube with horizontal splitter in elbow.

The most of head recovery occurs in draft tube cone.

The splitter may improve the flow behaviour and performanceof draft tube at part load operation of turbine where largeamount of whirl comes out of runner but location of splitter isto be found again by simulationto be found again by simulation.

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