1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
1
EFFECT OF DIFFUSER LENGTH ON PERFORMANCE
CHARACTERISTICS OF ELBOW DRAFT TUBE WITH DIVIDING PIER
Rahul Bajaj1
, Dr. Ruchi Khare2
, Dr. Vishnu prasad3
M.Tech. Student1
, Asst. Professor2
, Professor3
Department of Civil Engineering
M.A. National Institute of Technology, Bhopal (M.P.)
ABSTRACT
The hydraulic turbines extract the energy of flowing water and converts into mechanical
energy. The reaction turbine has components namely casing, stay ring, guide vane, runner and draft
tube. Each component plays some role in performance of turbine. Out of above component casing,
stay ring and distributor guide the flow while in runner and draft tube energy transfer and conversion
takes place. In reaction turbine, significant part of input energy goes out of runner unutilized in form
of kinetic energy. Draft tube are provided at exit of runner to connect turbine and tail race providing
closed conduit flow of varying cross sectional area. The development in the design of turbines leads
to different shape of draft tube to recover as much energy as possible. The elbow draft tube is mostly
used with large reaction turbine. It is found that geometry of draft tube, flow space affects its
performance to large extent but due to limitation of experimental investigation, the mesh were
limited to few shapes. The growth on computational power has made it possible to investigate the
many alternative design of draft tube. In the present paper, numerical simulation has been done for a
large elbow draft tube with pier in diffuser. The length of diffuser has been changed to see its effect
on the performance i.e. head and efficiency of draft tube.
Keywords: Draft Tube, CFD, Diffuser, Pier, Kinetic Energy, Efficiency.
INTRODUCTION
Draft tube is an important component of hydraulic reaction turbine and improves the
performance of turbine by converting the kinetic energy entering in draft tube from runner, into
pressure energy. [1] In axial flow reaction turbine, the kinetic energy of water leaving the runner is
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ISSN 0976 – 6359 (Online)
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976
ISSN 0976 – 6359(Online), Volume 5, Issue
up to 50 % of total input energy where
total energy.
In beginning of 19th
century, the hydrodynamic investigation was carried by F. Frankl and
A.V. Milovich on straight draft tube and
the shape of draft tube on the basis
investigations were carried out on straight diffuser between 1909 and
to design various shapes of draft tube
and small diameters owing to the diameter of runner D
large it is irrational to construct. To overcome these problems,
recover kinetic energy of axial and rotational
large capacity mixed flow hydraulic turbine, bell mouth
runner diameter leads increase in dimensions and weight
were overcome by elbow draft tube which is suit
when diffuser length exceeds 10 to 12 m, the dividing pier is provided
distribution and consequently improves the performance of draft tube and power characteristics of
the turbine.[2]
In hydraulic turbine, determination of optimum shape and dimension as well as prediction of
flow behavior is very difficult task. To overcome these problems the numerical simulation
become more informative tool.[3] At the best operating
minimum whirl and radial velocity
significant whirl velocity.
Since from more than a decade’s Computational Fluid Dynamics (CFD)
tool and widely used by researcher for predicting performance and flow behavior in a domain and
optimization of design of hydraulic turbine components
time consuming and to overcome this
minimized laboratory testing of models.[4]
GEOMETRY AND MESH GENERATION
The elbow draft tube has three parts namely cone, elbow and
of thickness b= 0.25 D1 are used to improve the performance of turbine.
of elbow draft tube are shown in fig. 1.
The modeling of elbow draft tube has been done in ICEM CFD for 3 length L/ D
Model of elbow draft tube for h/D1= 2.3 and L/
mm diameter at inlet and the analysis is done for L = 5 * D
length.
Fig. 1: Geometric parameter of
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976
e 5, Issue 5, May (2014), pp. 01-12 © IAEME
2
0 % of total input energy whereas in case of mixed flow reaction turbine it
century, the hydrodynamic investigation was carried by F. Frankl and
A.V. Milovich on straight draft tube and they determine the solution to the problem of determining
the shape of draft tube on the basis of theoretical investigation. Therefore large
traight diffuser between 1909 and 1929 and emphasized
draft tubes. The use of straight tubes was restricted to turbine of medium
the diameter of runner D1 increases, the length of the tube becomes so
large it is irrational to construct. To overcome these problems, bell mouth tubes were invented and
kinetic energy of axial and rotational component of flow. With the rapid deve
large capacity mixed flow hydraulic turbine, bell mouth draft tube was also absolute because large
dimensions and weight of draft tube. Later on all these problems
overcome by elbow draft tube which is suitable for large diameter hydraulic turbine.
length exceeds 10 to 12 m, the dividing pier is provided which give
consequently improves the performance of draft tube and power characteristics of
, determination of optimum shape and dimension as well as prediction of
flow behavior is very difficult task. To overcome these problems the numerical simulation
At the best operating condition the flow enters
in a draft tube but at off design condition, the flow enters with
Since from more than a decade’s Computational Fluid Dynamics (CFD)
widely used by researcher for predicting performance and flow behavior in a domain and
ion of design of hydraulic turbine components. Conventional model testing is costly and
to overcome this, CFD is an alternative tool which is a cost effective and
minimized laboratory testing of models.[4]
GEOMETRY AND MESH GENERATION
three parts namely cone, elbow and diffuser. In large draft tube piers
are used to improve the performance of turbine. The geometric dimensions
draft tube are shown in fig. 1.
The modeling of elbow draft tube has been done in ICEM CFD for 3 length L/ D
= 2.3 and L/ D1=15 is shown in fig.2. The draft tube is having 250
mm diameter at inlet and the analysis is done for L = 5 * D1, L = 10 * D1 and L = 15 * D
parameter of elbow draft tube with dividing pier [2]
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
is up to 15 % of
century, the hydrodynamic investigation was carried by F. Frankl and
they determine the solution to the problem of determining
large numbers of
and emphasized the need
tubes was restricted to turbine of medium
, the length of the tube becomes so
bell mouth tubes were invented and
flow. With the rapid development of
draft tube was also absolute because large
Later on all these problems
able for large diameter hydraulic turbine. In case
gives the better flow
consequently improves the performance of draft tube and power characteristics of
, determination of optimum shape and dimension as well as prediction of
flow behavior is very difficult task. To overcome these problems the numerical simulation has
enters mostly axially
the flow enters with
Since from more than a decade’s Computational Fluid Dynamics (CFD) is most effective
widely used by researcher for predicting performance and flow behavior in a domain and
. Conventional model testing is costly and
n alternative tool which is a cost effective and
. In large draft tube piers
The geometric dimensions
The modeling of elbow draft tube has been done in ICEM CFD for 3 length L/ D1 ratio [2].
is shown in fig.2. The draft tube is having 250
and L = 15 * D1 diffuser
[2]

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
3
Fig. 2: Isometric view of elbow draft tube
In three dimensional CFD simulations, the fluid domain can be discretized by tetrahedral or
hexahedral mesh. Unstructured mesh which mainly consists of tetrahedral elements while structured
mesh consists of hexahedral elements.[5] Ansys CFX uses finite volume method (FVM) for the
discretized domain and N-s equations is solved at every node of the cell.
In the present work, meshing of draft tube is done in Ansys ICEM CFD as shown in fig. 3.
For this flow domain, unstructured three dimensional tetrahedral meshing has been adopted.
Fig.3: Mesh of elbow draft tube
BOUNDARY CONDITIONS
The numerical flow simulation is carried out for 13 whirl component varying from 3.0 m/s
to -3.0 m/s at an interval of 0.5 m/s at 3 different lengths of diffuser i.e. L = 5 * D1, L = 10 * D1
and L = 15 * D1. Cylindrical velocity component at inlet of draft tube is specified as axial
component = -15.41 m/s, radial component = 0 m/s and 13 whirl component at an interval of 0.5 m/s
as inlet boundary condition. Average static pressure is specified at outlet of draft tube as outlet
boundary condition. All the walls in the geometry is assumed as smooth and no slip. Shear Stress
Transport (SST) K-w turbulence model is used with wall function as automatic in Ansys CFX code.
Convergence criteria are set to be 10-6
as RMS value and 500 as maximum iteration.

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
4
COMPUTATION OF PARAMETARS
The following formulae are used to assess performance of draft tube using the velocity and
pressure distribution from numerical simulation.
Head loss in draft tube
03 05( )
LD
P P
H
γ
−
=
Head recovery in draft tube
2 2
03 05( )
2
RD LD
V V
H H
g
−
= −
Draft tube efficiency
2
03
2
100RD
D
gH
V
η = ×
Relative head loss
2
03
2
100LD
RLD
gH
H
V
= ×
RESULTS AND DISCUSSIONS
The 3D flow analysis has been carried out in elbow draft tube with three L / D1 ratios of 5, 10
and 15 for 13 whirl component varying from 3.0 m/s to -3.0 m/s at an interval of 0.5 m/s for constant
n1’=135.6 rpm. Fig. 4 shows the various sections at which pressure and velocity contours are taken.
Fig. 4: Pressure contour for different section (L=5D1)

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
5
Fig. 5.(a): Pressure contour at elbow section for L= 5 D1 and theta component= -1 m/s
Fig. 5.(b): Pressure contour at elbow section for L= 10 D1 and theta component= -1 m/s
Fig. 5.(c): Pressure contour at elbow section for L= 15 D1 and theta component= -1 m/s

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
6
Fig. 6.(a): Velocity contour at elbow section for L= 5 D1 and theta component= -1 m/s
Fig. 6.(b): Velocity contour at elbow section for L= 10 D1 and theta component= -1 m/s
Fig. 6.(c): Velocity contour at elbow section for L= 15 D1 and theta component= -1 m/s

7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
7
It is seen in figure 5.(a),(b) and (c) that maximum pressure variation is obtained at outer side
of elbow and in fig. 6. (a),(b) and (c) that at the periphery, velocity is zero and velocity is increases
as moves from outer side to inner side of elbow.
Fig. 7.(a): Pressure contour at inlet of diffuser for L= 5 D1 and theta component= -1 m/s
Fig. 7.(b): Pressure contour at inlet of diffuser for L= 10 D1 and theta component= -1 m/s
Fig. 7.(c): Pressure contour at inlet of diffuser for L= 15 D1 and theta component= -1 m/s
Fig. 8.(a): Velocity contour at inlet of diffuser for L= 5 D1 and theta component= -1 m/s
Fig. 8.(b): Velocity contour at inlet of diffuser for L= 10 D1 and theta component= -1 m/s

8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
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Fig. 8.(c): Velocity contour at inlet of diffuser for L= 15 D1 and theta component= -1 m/s
From fig 7.(a),(b) and (c), it is seen that the maximum pressure zone occurs at divider and in fig
8.(a),(b) and (c) minimum velocity zone is occuer at divider and periphery of diffuser.
Fig. 9.(a): Pressure contour at diffuser for L= 5 D1 and theta component= -1 m/s
Fig. 9.(b): Pressure contour at diffuser outlet for L= 10 D1 and theta component= -1 m/s
Fig. 9.(c): Pressure contour at diffuser outlet for L= 15 D1 and theta component= -1 m/s
Fig. 10.(a): Velocity contour at diffuser outlet for L= 5 D1 and theta component= -1 m/s

9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
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Fig. 10.(b): Velocity contour at diffuser outlet for L= 10 D1 and theta component= -1 m/s
Fig. 10.(c): Velocity contour at diffuser outlet for L= 15 D1 and theta component= -1 m/s
It is observed from fig 9.(a),(b) and (c) that pressure variation is nearly constant and in fig
10.(a),(b) and (c), velocity is minimum at upper side of diffuser due to sharp curvature at divider.
Fig. 11.(a): Pressure Contour at L= 5 D1 and theta component= -1 m/s
Fig. 11.(b): Pressure Contour at L= 10 D1 and theta component= -1 m/s

10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
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Fig. 11.(c): Pressure Contour at L= 15 D1 and theta component= -1 m/s
The pressure contour for a draft tube is shown in Fig.11. (a), (b) and (c) at L/ D1 = 5, 10 and
15 respectively. From these contours it can be seen that after achieving a optimum pressure at the
inlet of dividing pier, pressure is nearly constant throuhout the length of draft tube, there is sudden
drop in pressure due to curvature at elbow section, where as .
Fig. 12.(a): Velocity stream line at L= 5 D1 and theta component= -1 m/s
Fig. 12.(b): Velocity stream line at L= 10 D1 and theta component= -1 m/s

11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
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Fig. 12.(c): Velocity stream line at L=1 5 D1 and theta component= -1 m/s
The streamline patterns for a draft tube is shown in Fig.12. (a), (b) and (c) at L/ D1 = 5, 10
and 15 respectively. From the above figure it can be seen that the amount of whirl incresase with
increase in diffuser length. The whirl is higher in the left of the dividing pier as compared to whirl in
right of dividing pier , it may be because of eccentricity of the dividing pier with the centre line of
draft tube inlet. The velocity reduces from inlet to outlet which confirms the cherecterstic of draft
tube.
The draft tube efficiency and relative loss in each numerical simulation are computed and
graphical representation is shown in fig. 13 and fig. 14. As shown in fig.13 the maximum draft tube
efficiency obtained at 0 m/s whirl component in case of L/D1 = 10. All these numerical simulation
shows that when diffuser length is L > 10 D1, the highest efficiency is achieved. From fig. 14, it is
seen that, minimum relative loss is for L/D1 = 5 and 10.
Fig. 13: Efficiency of Draft Tube at Fig. 14: Relative Loss in Draft Tube at
different L / D1 ratios different L / D1 ratios
10
20
30
40
50
60
70
80
90
-4 -3 -2 -1 0 1 2 3 4
DraftTubeEfficiency(%)
Theta Component (m/s)
L= 5 D1
L= 10 D1
L= 15 D1
0
10
20
30
40
50
60
70
80
-4 -3 -2 -1 0 1 2 3 4
RelativeLoss(%)
Theta Component (m/s)
L= 5 D1
L= 10 D1
L= 15 D1
Poly. (L= 15
D1)

12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME
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CONCLUSION
From numerical simulation of elbow draft tube with dividing pier it may be observed that that
maximum efficiency is achieved at L= 10 * D1 length of the draft tube. The comparative study of the
pressure variations and velocity contours at the inlet section of draft tube and just after the elbow
section shows that location of dividing pier effects the velocity distribution significantly. Efficiency
of draft tube is affected due to whirl component.
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