CFD File

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The 11th

Asian International Conference on Fluid Machinery and Paper number xxxxxxxxx

The 3rd

Fluid Power Technology Exhibition November 21-23, 2011, IIT Madras, Chennai, India

Original Paper

Evaluation of Hydro Turbine design by

Computational Fluid Dynamics

Kiran Patel1, Jaymin Desai

2, Vishal Chauhan

2 and Shahil Charnia

2,

1Manager, CFD Analysis Center, R & D Hydraulics, Jyoti Ltd., [email protected], [email protected]

2Executive, CFD Analysis Center, R & D Hydraulics, Jyoti Ltd., Nanubhai Amin Marg, Industrial Area,

P.O.Chemical Industries, Vadodara-390003, Gujarat, India.

Abstract

CFD analysis is widely used at Jyoti Ltd. for Hydro Turbines development. This paper includes CFD analysis of Francis turbine

(Ns=165) and validation of CFD result with experimental result. CFD analysis of complete system (Spiral Casing + Stay vane +

Guide Vane + Runner + Draft tube) is conducted at BEP as well as at part load and performance along with losses of various

components predicted. Predicted CFD analysis results are compared with experimental results and they show good agreement. This

case becomes guidelines for our future new development project.

Keywords: Francis Turbine, CFD Analysis of turbine, BEP (Best Efficiency Point)

1. Introduction Jyoti Ltd. is supplying small hydropower equipment since last 3 decades to Indian and overseas market. There are four basic types of turbines i.e. Francis, Kaplan, Pelton and Turgo, supplied by Jyoti ltd. Computational Fluid Dynamics (CFD) is latest state of art

technological tool which is used by designer for evaluation of hydraulic design of Turbine. This paper describes the evaluation of our

Francis turbine design by CFD. It starts with specification of design, modeling, mesh generation, problem definition and performance

evaluation using CFD. Also at the end, CFD analysis results are compared with experimental results.

2. Specification of Francis Turbine This Francis turbine design is used for head range of 50 to 142 m and discharge range of 1 to 7 m3/sec. It is installed at no. of sites and

it gives satisfactory output. It is one of our successful designs hence we decided to study and analyze using CFD. There are four basic

steps involved in CFD analysis i.e. Geometric modeling, Mesh generation, Problem definition & solution, Post processing of result.

3. Geometric Modeling Francis turbine consists of four basic components i.e. Spiral casing with stay vane, guide vane, runner and draft tube. For CFD

analysis, we need to make the model of flow passage of water passing though each component. Fig.1 shows geometric model of flow

passage of water of each components. All the components are made using commercial software. Assembly of all the components is

also shown in Fig. 1.

4. Mesh generation Mesh is generated using commercial meshing software. Grid generation is most important as CFD result quality depends on it. We

have used tetra and prism elements. Skew, aspect ratio of meshing elements are within limits. Total quality of grid is as per industry

standard, and results are independent from it. Fig. 2.A Shows mesh generated in all of the components. In addition, grid sensitivity test

has been done and CFD results are mesh independent. Detail of no of nodes in each component is given in Table 1.

Paper ID: AICFM_TM_016

Accepted for publication.Corresponding author: Kiran Patel, [email protected]

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Fig. 1 Francis Turbine Components

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Figure 2(A): Meshing of Francis Turbine components

Table 1: Element generated in each component

Sr. No. Components No. of Nodes

1 Spiral Casing 999566

2 Guide vane 963900

3 Runner 1228124

4 Draft tube 277931

In case of runner and draft tube flow separation is taking place. To capture boundary layer flow separation we have used prism

elements at wall boundary. To know whether we have captured effectively boundary layer, our y+ value in flow separation zone

should be less than 20. Using this we can predict correct head loss value for component. Fig. 2.B shows y+ plot on runner blade &

draft tube and its value is within limit.

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Figure 2(B): Y+ Plot for runner and draft tube

5. Problem Definition Steady state single-phase (only water) CFD analysis is accomplished for Francis turbine. All the components have been used in

analysis and complete system CFD analysis accomplished. At outlet atmospheric pressure and at spiral casing inlet total discharge is

specified as boundary condition as shown in Fig. 3. Solid surface of all the components is considered as a wall boundary with no slip

condition. Spiral casing, guide vane and draft tube are considered as stationary domain as they are stationary components & runner is considered as rotating domain as it is rotating component and solved with Multiple Frame of Reference (MFR) model. Domain

interface has been used for all the components to be attached with each other. Frozen rotor interface is used to attach runner with guide

vane and draft tube. Shear Stress Transport (SST) is used as turbulence model with high resolution as advection scheme. Convergence

criteria is Max 10-4 is considered with automatic time step. Computation is performed at three different guide vane openings and

result is discussed as follows.

Figure 3: Problem definition of Francis Turbine components

6. Result and Discussion Francis turbine complete system CFD analysis has been accomplished at three-different guide vane openings corresponding to three

different loading on turbine from BEP to overload. Detail discussion on results is as follows:

Fig. 4 shows pressure and velocity contour plot at central plane. Here 100% load means it is for the rated discharge value and 112%

and 123% load is for the overload discharge values. It shows gradual reduction in pressure from inlet of spiral casing to exit of runner.

Drop in static pressure into casing is almost insignificant and only friction loss is taking place. When water enters into guide vane

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passage from casing, according to opening of guide vane static pressure drop takes place. In the present case, at BEP drop in static

pressure is higher compared to overloading opening as shown in Fig.4. This causes higher velocity at exit of guide vane, which

increases friction losses as well. In addition, when opening is changing, the swirl with water is entering into runner is also getting

changed. This change in swirl at inlet of runner causes shock losses at leading edge, which also develops, and increase flow

separation.

Figure 4: Pressure and Velocity contour plot at center plane

Fig. 5 (a) Shows pressure contour plot on meridional plane of runner. It shows how gradually reduction in static pressure from inlet to

exit of runner. Fig. 5 also shows blade loading which is pressure distribution on two sides of runner blade. It shows, as the loading

increases pressure difference between two sides of blade also increases. Because of this rise in pressure difference, torque also

increases and hence power output increases. Fig. 5 (b) shows Blade loading on the runner blades. As there are 13 blades, more than

one line are there in the blade loading chart at hub, middle and shroud location each. Stream wise 0-1on the x-axis denotes percentage

distance between runner leading edge to the runner trailing edge. Blade loading also shows gradual reduction in pressure from inlet to

outlet, which gives sign of good design.

Figure 5 (a): Pressure contour plot on meridional plane of runner

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Figure 5 (b): Blade loading of runner

Fig.6 Shows velocity streamlines plots in blade-to-blade view on hub and shroud of runner and guide vane. It shows how water is

entering into the runner from guide vane. As flow is entering with shock at runner inlet, flow separation took place at all operating

load. This flow separation is inevitable and its intensity depends on operating load. Flow separation is one of main parameter of head

loss in runner. In addition, Fig.6 shows that flow velocity at shroud is very high and this causes higher friction losses into the runner.

Figure 6: Velocity plots at various loading condition for guide vane and runner

Fig. 7 Shows velocity vector plot at various loading condition in draft tube. It shows that the flow separation zone inside draft tube

increases as operating load increases. Water is coming out from runner at high velocity with swirl and meridional component. Swirl

component increases with operating load. This swirl component kinetic energy is not converted into pressure energy at inlet cone of

draft tube, hence head loss increases, and flow separation increases. In case of draft tube, flow separation takes place because of swirl

and change in direction of flow using draft tube bend.

100% 112% 123%

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Figure 7: Velocity vectors plots at various loading condition for draft tube

Mixing of high & low

momentum zone


momentum zone

Flow Separation

Flow Separation


momentum zone low

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Fig. 8 Shows head loss of various components of Francis turbine. It shows that maximum head loss takes place into the runner and it

increases with operating load. This head loss in runner is due to flow separation and friction loss. In case of guide vane, head loss

decreases as operating load increases. Reduction in head loss in guide vane is due to reduction in velocity because of higher opening,

which reduce skin friction losses, and due to lower drop in static pressure. When Guide vane is operated from fully close to fully open,

it is just like nozzle and as operating load increases; guide vane opening should increase. Hence, it functions like nozzle in which by

opening and closing you can change discharge as well as static pressure drop. At lower opening static pressure drop is higher hence, head loss is also high. In case of casing, as load increases discharge increases. Because of increase of discharge, velocity inside casing

also increases, which causes high skin friction losses. Hence, in casing as loading increases, head loss also increases. In case of draft

tube, as operating load increases head loss also increases. This is because of lower pressure recovery and flow separation in side draft

tube.

Figure 8: Head loss of components

Fig. 9 shows comparison of experimental and CFD result. Trend of both the result is same and shows very narrow gap between test

and CFD result. CFD performance prediction shows good agreement with experimental result.

Q11 = Q /Q BEP (Testing) (1.)

η11=η/ η BEP (Testing) (2.)

Figure 9: Comparison of Experimental and CFD result

0

1

2

3

4

5

6

0.95 1.05 1.15 1.25

He

ad

loss

(%

)

Q11 (Non-dimensional)

Casing

Guide

vaneRunner

0.9642

0.9692

0.9742

0.9792

0.9842

0.9892

0.9942

0.9992

1.0042

0.95 1 1.05 1.1 1.15 1.2 1.25

η1

1E

ffic

ien

cy (

No

n-d

ime

nsi

on

al)

Q11 (Non-dimensional)

Test CFD

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7. Conclusion The present study helps us to determine basic flow physics in various components. It also helps us to determine basic causes for losses

in various components. With this evaluation study, we determined basic two types of losses i.e. skin friction and secondary flow or

flow separation, occurred in various components. In addition, we have ascertained the trend of losses and flow pattern in various components and which becomes our guideline for our future new design assignments. Comparison of experimental and CFD results

also shows good agreement. This gives us an assurance that performance prediction by CFD is in line with experimental result.

Acknowledgement

We are grateful to Jyoti ltd. as well as all the persons involved directly or indirectly without the help of which this task of design

evaluation of Francis turbine might not be possible.

Nomenclature

BEP Best Efficiency Point Q11 Non dimensional discharge

η11 Non dimensional efficiency Q Discharge at particular point

η Efficiency at particular point QBEP(Testing) Rated Discharge measured during testing

ηBEP(Testing) Efficiency from testing results at rated point y+ Dimensionless distance of the first node from the wall

References

[1]. Nechleba, Miroslav, 1957: "Hydraulic Turbine their design and equipment", ARTIA Prague, Czechoslovakia.

[2]. Kovalev, N.N, 1965, The National Science Foundation, Washington "Hydro turbine design and constructions". [3]. Barlit, V.V., 2007,"Hydro turbine volume 1 & 2", Department of civil engineering, MANIT, Bhopal.

[4]. Čarija Z, Mrša Z., 2003, "Complete Francis turbine flow simulation for the whole range of discharges". Fourth International

Congress of Croatian Society of Mechanics, pp 18-20

[5]. Vu, T. C., and Shyy, W., 1994, “Performance Prediction by Viscous Flow Analysis for Francis Turbine Runner,” ASME J. Fluids Eng., 116, pp. 116–120.

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