CHAPTER THREE

METHODOLOGY

2.1 Turbine Technology

Each potential site for small-scale hydropower scheme is considered unique since turbine

selection is based mostly on the water head and the available flow rate. In most cases if the head

is small the flow rate should be higher. The penstock and turbine should be increased

proportionally to support the increment [9]. Due to the uniqueness of a specific location it is

important that steps are taken to find successful approaches to provide standardized equipment,

engineering designs and implementation methods specifically for a particular location [10].

The power produced by hydropower turbine can be calculated using the following equations:

P = gHQ

H=h-hf

turbine* generator

P =7.8 HQ

Definition of terms

P = power output

= total efficiency

= density

g = gravitational constant

H = net head

Q = flow rate

h = head friction loss

f = Darcy friction factor

hf = Friction head loss

L = pipe length

V = jet velocity

D = pipe diameter

For selection of a proper turbine for a specified head (Z) and flow rate (f), turbine diameter (D)

and rotational speed of the turbine ( ω) play a significant role.

Diameter in relation to head and flow rate:

Angular velocity in relation to head and flow rate:

Choice of Turbine

Classification of water turbine according to type of head:

Turbines can be classified generally according to the table below.

Based on the charts below, the consideration of a cross flow turbine will be the most suitable

turbine for the implementation of a micro hydro power plant as the project topic implies.

Figure: Turbine Application Range. Source: James Leffel Co. USA

RESEARCH METHODS AND MATERIALS IN THE DESIGN OF A CROSS FLOW

TURBINE

Materials and Technology

As noted earlier, the purpose of this reaeaerch is to come up with a turbine design that suit the

local manufacturinng capability in terms of materials, technology and human resource. It is

therefore necessary to find out what type of technology and materials are available. Materials

survey will be done by visiting various hardware shops and suppliers in Nigeria to see what kind

of materials are easily available in the market for the completion of the fabrication of the cross

flow turbine. It is also carried out by visiting various workshops to shop available technologies

and expertise.

Design

The design work would be carried out using systematic design procedures from conceptual to

detailed design based on market survey, available technology, material survey and the reviewed

literetures. The design involves formulation of design requirementss/specifications for cross flow

turbine followed by conceptual design and detailed design of a cross flow turbine which also

includes preparation of detailed drawings, materials selection and cost estimates for

manufacturing.

Figure: Diagram of a locally made cross flow turbine

Fabrication of cross flow turbine model

Fabrication work carried out after completion of the design work. This stage also involves

purchase of selected materials. All manufacturing activities would be carried out any available

and standard workshop based south-west part of the country except for off-shelved parts such as

bearings, bolts & nuts e.t.c which were purchased from the market.

Figure: A similar cross flow turbine after fabrication.Source: Location Action Research center,

Tanzania.

Testing

Preliminary performance tests would be conducted in the Civil Engineering building and the

University water cooperation. The tests would be aimed at determining its performance

characteristics and identifying testing, procedures and techniques . However, due to limited

resources only some few parameters would be tested and the results compared with the

calculated ones.

Also, one of the tests that would be carried out is the numerical investigation of the internal flow

in the cross flow turbine using fluid flow simulation software. A 3D-CFD steady state

flow simulation would be performed using ANSYS CFX codes. The objectives of this study is to

analyze the velocity and pressure fields of the cross-flow within the runner, and to characterize

its performance for different runner speeds. Absolute flow velocity angles would be obtained at

runner entrance for simulations with and without the runner. Flow Recirculation in the runner

inter-blade passages and shocks of the internal cross-flow cause considerable hydraulic losses by

which the efficiency of the turbine decreases significantly. The CFD simulations results would

be compared with experimental data.

Figure 3: Efficiency of Various Turbines based on Discharge rate

DESCRIPTION OF CROSS FLOW TURBINE

Hydraulic Parametres and Operation Principles

The main characteristics of the Cross-flow turbine is the water jet of rectangular cross-section

which passes twice through the rotor blades arranges at periphery of the cylindrical rotor

perpendicular to the rotor shaft. The water flows from the periphery towards the center and after

crossing the open space inside the runner, from the inside outwards. Energy conversion takes

place twice; first upon impingement of water on the blades upon entry, and then when water

strikes the blades upon exit from the runner. The use of two working stages provides no

particular advantage except that it is a very effective and simple means of discharging the water

from the runner.

Figure: Schematic of Turbine. Source: BYS Nepal.

The inlet (1) consists of two curved sheets that form a logarithmical spiral, welded to two plane

side panels to form a rectangular inlet section and nozzle. The width of the inlet is denoted x in

figure above. The rotor (3) consists of n number of blades blade segments. The central shaft (3)

is also welded to the rotor discs and final machining of the rotor outside diameter, including the

blade tips as well as the shaft diameter, is done after completed welding. The drum-like rotor is

also provided with a central supporting disc for the blades, for sizes of x> 220 mm. The shaft

extends from both sides of the rotor and is usually symmetric. Depending on the application of

the turbine, either both shaft ends can be provided with pulleys to drive two machines via belt-

drive, or, if a generator is connected on one side, the other end may be used for operating a speed

governor. Bearings used are of the self-aligning spherical double-roller type, which makes

accurate machining of the bearing supports unnecessary.

Flow is controlled by the flow regulator (4). Its shaft is parallel to the rotor shaft, with two U-

channel parts connecting the regulator shaft with the rectangular tongue at the top. The latter acts

as the gate and fits neatly in- side the nozzle to keep leaks at the sides in the closed condition

within limits. The device is operated by a pushrod (5) which is either connected to a hand, wheel

(6) - requiring a thread on the pushrod and a nut in the hand wheel - or, for automatic operation,

to the hydraulic cylinder of a speed governor. The housing is completed with the base part (8)

and the rear part (91, all bolted to the foundation frame (7).

In addition, two side panels of thin sheet, stuffing boxes and rubber gaskets are required to seal

up the turbine housing.

In all cases, an adaptor is provided at the turbine inlet that connects the pen- stock with the

turbine. This part is of square shape at one end, to fit to the square inlet, and of circular cross

section at the other end to fit to the pen- stock pipe used.

Depending on the setting above tailwater in an installation, a drafttube of square shape is also

provided. For this, a flange made from sheet strips is welded to the foundation frame, so that the

draft tube can be bolted on.

Machines required

Machine tools required are standard, such as:

Turning lathe with a centre height>200 mm

Drilling machine with a capacity up to 0 25 mm and boring attachment

Milling machine or shaper

Acetylene cutting torch, plate shear (optional)

Arc welding equipment

A number of jigs and fixtures made for the purpose

general hand tools

.For higher output and depending on head and the width of the rotor (shaft bending load),

engineering know-how is required to decide whether or not parts that give greater strength must

be incorporated (such as bigger shaft diameter, supporting disc, strengthening ribs).

Sizing of main Element

Figure: General dimensions of a cross flow turbine. Source BYS NEPAL(1981)

CALCULATIONS INVOLVED