Some Applications of Computational Tools for R&D of Hydraulic ...

Biraj S. Thapa et al.: Some Applications of Computational Tools for R&D of Hydraulic Turbines

Some Applications of Computational Tools for R&D of Hydraulic Turbines Biraj S. Thapa, Amod Panthee*, Hari P. Neopane Abstract— Design of hydraulic turbines involves several stages of iterative calculations. Furthermore, the selection of the turbine is unique to each site conditions. This makes the R&D of the hydraulic turbines a complicated and time consuming work. Recent advancements in computational tools and processors have added advantages to the R&D process of hydraulic turbines. These tools are able not only to compute solutions for the complex design equations but also provide the user friendly virtual environment for performance test and design optimization A new program named as “Khoj” has been developed using Matlab for design optimization of Francis runners. The optimized design from the program can be further analyzed using CFD, CAD and FSI tools, which estimate the effect of design variables on performance of the turbine. For the project on failure analysis of Khimti runner, the true size runner model has been developed using SolidWorks. Stress and fatigue analysis of the runner has been attempted using CosmosWorks. Study of the general performance behavior of the Pelton runner and identification of the possible causes of failure of the Khimti runner at its operating conditions is under progress. This paper presents the experiences of use of computational tools for research and development of hydraulic turbines at Kathmandu University. The research methodology followed and the computational tools applied, with the recent results for design optimization of Francis runner, effects of operating conditions on sediment erosion and failure analysis of Pelton runner of Khimti Hydropower will be elaborated in-depth. Effectiveness and limitations of application of computational tools for R&D of hydraulic turbines will also be discussed.9

TABLE I TOTAL HYDROPOWER POTENTIAL Major Basins Potential (at Q40) Saptakoshi

17008.3

Narayani

17800.2

Karnali

15661.16

Mahakali

2261.83 Total

52731.83

Hydraulic turbines are classified into different categories depending upon its principle and range of operation [2]. Therefore, the selection of hydraulic turbines varies depending upon the site criterions. The installation of hydropower plants despite having a huge potential becomes more complex with change in design for a slight variation in design inputs like Head (H) and Discharge (Q). Moreover, Nepal is located on the lap of Himalayas. This increases the sediment concentration in the rivers of Nepal unlike other countries with igneous mountains. The river characterized with high sediment concentration is one of the problems in operation of hydraulic turbines erected at ROR type or storage type projects [3]. The sediment eroded Francis runner of Trishuli HEP is shown in Fig. 1.

Index Terms— Computational tools, Design, Hydraulic turbines, CFD, SolidWorks,

I. INTRODUCTION

N

EPAL has huge potential of hydro power. Most of the surface water in Nepal flows through four major river basins, Saptakoshi, Narayani, Karnali and Mahakali, extending from east to the west. The total hydropower potential of Nepal at Q40 is about 53,000 MW [1] as shown in Table I. Despite having the huge potential, Nepal is still lagging behind in power generation with total harvested capacity of 650 MW.

Fig. 1: Sediment Eroded Francis Runner of Trishuli HEP

Biraj Singh Thapa is a Graduate of Mechanical Engineering from Kathmandu University. Currently, he is working as a Researcher in Turbine Testing Lab, Kathmandu University. ([email protected]). *Corresponding author Amod Panthee completed his Bacherlor’s degree in Mechanical Engineering from Kathmandu University. Currently, he is working as a Research Assistant on RenewableNepal Project at Turbine Testing Lab, Kathmandu University. ([email protected]). Hari Prasad Neopane, PhD. is Associate Professor at Department of Mechanical Engineering, School of Engineering, Kathmandu University. ([email protected]).

Rentech Symposium Compendium, Volume 1, March 2012

The design of hydraulic turbine is complex since it involves a long iterative process in design stage. And it is necessary to predict the behavior of hydraulic turbines at various conditions on real environment while designing. This makes the design of the hydraulic turbine more time consuming. But, the use of computational tools have added advantage in design of hydraulic turbines and the prediction of behavior at various operating conditions and complex iterative process can be done in short period.

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Biraj S. Thapa et al.: Some Applications of Computational Tools for R&D of Hydraulic Turbines

II. CHALLENGES IN R&D OF HYDRAULIC TURBINES There are many challenges in research and development of hydraulic turbines. The challenges are seen from the initial design stage after the selection of suitable site for erection of hydropower station. It is followed by the selection of most efficient design from a list optimized design to work on real environment. environment. The challenges continue during operation and post operation or maintenance stage of the hydraulic turbines.

A. Design of Francis Runner

requirement. The change in ooperation peration of hydraulic turbines at different operating conditions reduces the efficiency and life of the turbine. Apart from these, it also creates problem in operation and maintenance of the turbine unit [6]. Depending upon the site of operation, there is a variation in sediment concentration with different percentage composition of quartz and mica in rivers of Nepal [3].. So, if we have a list of optimized design for a specific site to be tested, it would be a problem to test with a physical model in the lab. The test becomes more costly and time consuming when tested on a physical model.

The design of hydraulic turbines involves many assumptions. Initial assumptions during design of Francis turbine are infinite number of thin blades, streamlines are symmetric about the rotational axis and no frictional losses are present [4]. But it is difficult to achieve these conditions in real working conditions. The other assumptions based on the empirical al relation to achieve the desired result with assumptions stated above are given in Table II [4] TABLE II INITIAL DESIGN ASSUMPTION OF FRANCIS TURBINE

Outlet Peripheral Velocity (U2) 35 m/s – 43 m/s

Outlet Angle (β ( 2) 18o – 22o

The first step in design of a hydraulic turbine is to assume a streamline. The shape of a streamline is given by Equation (11). ). The shape of the streamline changes with change in values of constants ‘a’ and ‘b’ assumed in the equation from a circle to an ellipse [5].
   1   

(1)

A A_inlet  1.1  A_outlet

(2)

The remaining streamlines on a runner blade of the runner is obtained using the continuity equation (2) ( ) and the principles of geometry. The geometry of the streamlines is shown in Fig. 2 where the constants have their usual meaning. All these assumptions and iterative processes make the design process a challenging task.

Fig. 3: Root Crack in Pelton Runner

Despite having a sound hydraulic design, a hydraulic turbine fails during operation. A typical case of such failure of Pelton runner of Khimti HEP is shown in Fig. Fig 3.. The reasons for failure of turbines during operation may be manufacturing defects, modes of operation which are steady state and transient state operation and maintenance procedure applied in the hydraulic turbines.

Fig. 4: Pelton Bucket

Fig. 2: Distribution of Points along the Streamline Streamline

B. Operation of a Hydraulic Turbine Hydraulic Turbines are operated at various conditions. It is a usual trend to design a hydraulic turbine at Best Efficiency Point (BEP). But, a hydraulic turbine is operated from part load to full load depending upon the energy Rentech Symposium Compendium, Volume 1, March 2012

During the operation of hydraulic turbines, the mode of operation changes from steady state to transient state upon variation in load. This produces vibrations and stress on power production unit which might cause a failure of the hydraulic turbines. When the load is transferred from point A to point B, shown in Fig. Fig. 44,, the stress developed in the runner varies which might cause the untimely failure of the runner when operated on a cycle [7].

C. Maintenance of Hydraulic Turbine The turbine components are passed through a series of 33

Biraj S. Thapa et al.: Some Applications of Computational Tools for R&D of Hydraulic Turbines heat treatment procedure during maintenance. The residual stress is induced when a large temperature difference has been created at different sections within the runner components [8]. Also, the improper cooling of the hydraulic turbines forms weld decay which is a combination of Carbon and Chromium [9], the ingredients of material used in manufacturing hydraulic turbines. This induces residual stress in turbine component and the immediate site of failure[7].

III. METHODOLOGY The Francis runner blade has been developed using general principles mentioned in Section II and literature [5] for “Design Optimization of Francis Runner” and “Effects of Operating Conditions on Sediment Erosion” using the Matlab program “Khoj” and CAD software. These files are exported to AnSys CFX for further analysis.

constants and depend upon the particle and wall material combination. The blade profile is meshed with hexahedral element using Turbo-Grid. The analysis is done on two modes of operation, i.e. at best efficiency point with guide vane opening of 160 and at full load point with guide vane opening of 220. The sediment particles chosen for analysis are of spherical and non-spherical types. The boundary condition is set as uniform injection of sand particles of uniform diameter for both modes of operation [6].

C. Failure Analysis of Pelton Runner

A. Design Optimization of Francis Runner A range of optimization criterion has been set on outlet diameter, number of pole pairs in generator, reduced peripheral velocity at inlet acceleration of flow through the runner, height of the runner and blade angle distribution [10]. Several optimized design is then developed using the optimization criterion. The comparison of the optimized design is done with the reference design by setting the erosion factor and erosion tendency as reference indicators. Erosion Tendency (3) is quantification of tendency of a specific design of the runner to be eroded in similar sediment conditions. Erosion Factor (4) is the ratio of erosion tendency of each new design with respect to reference design.

∑  ∗    ∑   =

( )#$% &$'( ( ))$ *$+$ &$'(

m3/s3

(3)

-

(4)

The erosion factor for the reference design is set 1 and compared to the results obtained for optimized design.

B. Effects of Operating Conditions on Sediment Erosion Factors responsible sediment erosion are particle mass, particle velocity, grain shape and size, concentration of particles and angle of attack at which the collision occurs between the particle and the surface of the runner blade. The empirical erosion model, Tabakoff erosion model (5), is used in analysis which incorporates the factors responsible for erosion to determine the erosion rate [6]. 

 = , ∙ .(/) ∙ 01  ∙ 234  / ∙ 51 − 71 − ,8 ∙ 01 ∙ sin /: ; + , ∙ 701 ∙ sin /:

8

The 3D model of the Pelton Runner, shown in Fig. 5, has been developed using the orthographic projection of runner sections provided by Himal Power Limited using SolidWorks. The other model runner was developed for stress and fatigue analysis of the Pelton runner because of software limitation. The information on analysis conditions is tabulated in Table III. TABLE III ANALYSIS CONDITIONS ON COSMOSWORKS

Load 85630 N normal to the reference plane Uniform Distribution and Sequential Loading Restraint On 1 Face (at center) Description Study Property Solid Mesh Mesh Type 4 Points Jacobian Check 55709 No. of Elements

Force Distribution

IV. RESULTS A. Design Optimization of Francis Runner

(5)

Where, .(/) = 51 + , ∙ , ∙ sin