Proceedings of the 6th International and 43rdNational Conference on Fluid Mechanics and Fluid Power

December 15-17, 2016, MNNITA, Allahabad, U.P., India

FMFP2016–PAPER NO. 295

Design and Flow Simulation of Straflo Turbine for Micro Hydro Power Station

Ajay Singh Vishnu Prasad

Assistant Professor, Professor,
Mechanical & Automation Engg. Deptt, Civil Engineering Department,
DTC, Greater Noida - 201306 MANIT, Bhopal - 462003
Email: ajaysinghredhu@gmail.com Email: vpp7@yahoo.com

Abstract highly twisted [2] due to which there occurs change in angular
The micro hydro power plants are low head and Straflo momentum which forces the rotor to rotate along with
turbine is the best choice for the hydro power generation generator shaft which in turn generates electricity.
where water is conveyed through pipe line at slope. The
efficient design of straflo requires detailed flow behaviour Using CFD, the detailed flow behaviour in turbine space
inside the turbine space which largely depends on can be obtained. Variations in both local and global flow
geometry of runner and distributor blades. In present parameters due to changing operating conditions in turbine can
paper, straflo turbine has been designed and flow be studied in detail. CFD had been used by many investigators
simulation has been carried out using Ansys CFX to assess to carry out flow simulations for axial flow turbines to analyse
its performance. The predicted performance results are their performance, pressure pulsation prediction, swirling flow
found to be in accordance to an axial turbine investigation etc. [2, 3 and 4] and validated. In this paper,
characteristics. distributor, runner and a conical draft tube are designed for a
Keywords: straflo turbine; specific speed; flow simulation; runner low head horizontal axial flow straflo turbine. Steady state
numerical simulations have been carried out for 3D turbulent
I. INTRODUCTION flow using SST turbulent model in the whole turbine space by
As more and more need for sustainable development is felt, considering distributor and draft tube as stationary domain and
there is need to increase the use of renewable sources of energy runner as rotating domain in Ansys CFX. The velocity and
over other sources like fossil and nuclear energy sources. Out pressure distribution from simulation results are used to
of available renewable sources of energy, hydro energy is compute axial flow turbine characteristics parameters and
having least operating and maintenance cost after its installation presented in graphical and tabular form.
[1]. Around one-fifth of the world power requirement is
fulfilled by hydropower. Micro, mini and small hydro plants II. DESIGN AND FLOW SIMULATION
play a key role of rural electrification in many countries and The straflo turbine is designed for the discharge of 550 kg/s,
they have greater capacity than all other renewable energy head 7.5m and 35 kW power. The axi-symmetric axial
sources to make instant impact on the replacement of fossil distributor with fixed 7 vanes, propeller runner with 4 fixed
fuels. The water is conveyed mostly in pipes on large sloping blades and conical draft tube are designed. The computed
ground. The water gains additional energy due to falling slope values of inlet and outlet blade angles are used in Ansys
and this energy can be utilised for power generation. Bladegen to generate blades. A suitable stagger angle variation
is taken from hub to shroud so as to get blades with smooth
The axial flow turbines are high specific speed machines curvature. The inlet flow angle for runner is taken as blade
and used for low head and high discharge. The turbine is an outlet angle for guide vanes. Axial guide wheel with twisted
important part in a hydro power plant because hydro energy is blades is used ahead of the runner. Then three components are
extracted from water and is converted into mechanical energy. modelled separately taking only one blade for both the runner
Straflo turbines comprises turbine and generator into single unit and distributor and full draft tube as shown in Fig.1. The 3D
are the best choice for micro hydro power development from turbulent flow simulation has been carried out using periodicity
water flowing in pipes. The generator is mounted around the in propeller type runner and distributor with fixed blades.
periphery of runner. The blades of axial flow turbines are Tetra/mixed mesh using robust (octree) method is used for
1
 meshing in Ansys ICEM CFD and the components are increased discharge giving a linear relationship between
assembled by proper interfaces. The mesh dependency is done discharge factor and speed factor.
to arrive at the best suited mesh and to get desired Y+ value.
85

80

Hydraulic Efficiency (%)

900 rpm
75

70

65

60

55

50
20 40 60 80 100
Speed Factor
Fig. 2 Variation of hydraulic efficiency

0.70
900 rpm
0.65
Fig. 1 3D geometry of axial flow pipe turbine model
0.60
III. RESULTS AND DISCUSSIONS Discharge Factor 0.55
The grid dependency is checked by performing numerical 0.50
simulations for a number of tetrahedral elements ranging from
0.45
570637 to 1326245 at the design operating conditions. It was
found that after 800000 tetrahedral elements there is a minor 0.40
change of 0.08% in hydraulic efficiency. Thus all the numerical 0.35
simulations are carried out for 795223 number of elements in 0.30
the present paper. 20 40 60 80
Speed Factor
Fig. 3 Variation of discharge factor
The flow is simulated for discharge values varying from 450
kg/s to 650 kg/s at intervals of 50 kg/s and for constant
rotational speed 900 rpm. Accordingly, computed speed and
discharge factors vary from 31.28-84.50 and 0.35-0.66 shroud
0.9
respectively. The variations of hydraulic efficiency with speed hub
factor for different discharge at constant runner speed are mid span
shown in Fig.2. Hydraulic efficiency shows a parabolic 0.4
variation with both the speed factor and discharge factor which
CP

is a characteristic of an axial turbine. The turbine is found to -0.1

have a maximum hydraulic efficiency of 83.05% at a discharge
of 550 kg/s and at a rotational speed of 900 rpm, which are its -0.6
designed operating parameters. The hydraulic efficiency
decreases with a change in either discharge or rotational speed
-1.1
due to increase in shock losses in turbine.
0.0 0.2 0.4 0.6 0.8 1.0
Leading edge to trailing ege(0-1)
The variation of discharge factor with speed factor at Fig.4 Runner blade loading at design operating condition
constant speed with different discharges is shown in Fig. 3. At
constant rotational speed, as discharge increases, net head also Variations of pressure from leading edge to trailing edge on
increases and due to combined effect of discharge and net head, runner blade surfaces at hub, shroud and mid span for design
discharge factor decreases. Also, at rotational speed being operating conditions are shown in Fig. 4. There is sharp change
constant, speed factor decreases due to increase in head with in pressure at both pressure and suction surfaces due to impact
and then it decreases smoothly towards the trailing edge. The

2
 flow to blade is smoother near hub. The blade loading is nearly
uniform from runner hub to its shroud which is required in a
hydraulic turbine for its stability otherwise there would be
vibrations in the turbine and also the blades will not be long
lasting. Span wise velocity variations on three sections from
hub to shroud on runner blade are shown in Fig. 5. It shows
higher values of relative velocities at shroud and lower values
at hub.

1.4
hub
1.2 shroud
mid span
1.0

0.8
CV

0.6

0.4

0.2 Fig.6 Blade to blade view showing (a) pressure contours and (b)
streamline pattern, at mid span for Q = 550 kg/s and N = 900 rpm
0.0
0.0 0.2 0.4 0.6 0.8 1.0 IV. CONCLUSIONS
Leading edge to trailing edge (0-1)
It is found from the present study that hydraulic efficiency
Fig.5 Velocity variation on runner blade at design operating
of turbine is found to have parabolic variation with speed factor
conditions
with maximum hydraulic efficiency 83.05% at design speed
Pressure contours for blade to blade at mid span of both and discharge. The blade loading of turbine is uniform from
distributor and runner blade rows at design speed and discharge hub to shroud with slight shock at inlet toward tip.
are shown in Fig. 6 (a). As seen from Fig.6 (a) that there is The variation pattern of efficiency and computed velocity
slight change of pressure in distributor but in runner, pressure components are very similar to those obtained from the axial
decreases from leading to trailing edge of blade and also there flow turbine model test and are in close resemblance with
is difference in pressure on two sides of runner blade due to its performance characteristic curves of an axial flow turbine. The
rotation. Streamline pattern at mid span shown in Fig.6 (b) experimental testing is costly and hence CFD may be used as a
depicts that velocity is nearly same on two sides of guide vanes. tool for assessment of flow behaviour in hydraulic turbines
There is smooth flow at runner inlet and velocity difference on after proper validation with an experimental tested model.
two sides of runner and increases from inlet to outlet due to
pressure drop. The pressure and velocity pattern obtained in REFERENCES
1. D.K. Okot, Review of small hydropower technology, Renewable and
blade to blade is very similar to that obtained in experiments
Sustainable Energy Reviews, 26(2013), 515-520
conducted on axial turbine cascade flow. The computed 2. V. Prasad, Numerical simulation for flow characteristics of axial flow
velocity components in normalised form at runner inlet and hydraulic turbine runner, Energy Procedia, 14(2012), 2060-2065.
outlet from flow simulation at design operating conditions are 3. S. Liu, J. Mai, J. Shao, Y. Wu, Pressure pulsation prediction by 3D turbulent
unsteady flow simulation through whole flow passage of Kaplan turbine,
shown in Table 1. The absolute, whirl velocities decreases
Engineering Computations, 26(2009), 1006-1025.
while relative velocity increases from inlet to outlet of runner. 4. Y. Wu, S. Liu, H. S. Dou, S. Wu, T. Chen, Numerical prediction and
There is slight change in meridional flow velocity. similarity study of pressure fluctuation in a prototype Kaplan turbine and the
model turbine, Computers & Fluids, 56(2012), 128-142.
Table 1 Flow parameters at design operating conditions 5. V.V. Barlit, P.Krishnamachar, M.M.Deshmukh, A.Swaroop, V.K.Gehlot,
Hydraulic Turbines Volume 1 & 2, MANIT, Bhopal(2007).
c1* c2* cu1* cu2* cm1* cm2* w1* w2* ηh(%)
0.58 0.52 0.23 -0.11 0.53 0.50 1.16 1.45 83.05