Design Proposal for Mechanical Components_1

DESIGN

OF CROSS FLOW TURBINE

Design of cross flow turbine...................................................................................1 ..............................................................................................................................1 1 Introduction....................... .............................................. ............................................... .................................. ................ ............ ............ ............ .......1 . 2 Design Method...................... .............................................. ............................................... ........................................... .......................... ............ ......3 2.1 Selection of turbine type........................ ............................................... ............................................... ................................ .........3 . 2.1.1 Estimated Power Power Output........................ ............................................... ..................................... .................... ............ ........3 2.1.2 Specific speed..................... ............................................ ............................................... ............................................ ...................... ..4 2.1.3 Comparison of Turgo and Crossflow turbines........................ ......................................... ................... ..4 4 2.2 Design of a Crossflow Crossflow turbine.................... ............................................ ............................... ............. ............ ............ ......... ...5 2.2.1 Method 1- SKAT generic T3 model........................................................... model........................................................... 5 2.2.2 Method 2- Self design from classical Crossflow theory...... ............ ............ ............ .......... ....5 5 2.2.3 Method 3- Hybrid design ..................... ............................................. ............................................... ............................. ......5 2.3 Materials Materials....................... ............................................... ............................................... ........................................... .......................... ............ ........ ..6 3 Appendix 1...................... .............................................. ............................................... ............................................... ................................... .............. ...8 3.1 Design from from Classical theory theory..................... ........................................... ............................ ............ ............ ............ ............ ......8 3.2 Design from SKAT .........................................................................................8 3.2.1 Design Design variables...................... .............................................. ........................................... ......................... ............ ............ ......... ...8 3.3 Number of Intermediate disks: ...................... .............................................. ............................................... ......................... ..9 4 Appendix 2- Construction Construction and Drawings..................... ...................................... ....................... ............ ............ .......... ....10 10 4.1 Construction Construction guidelines guidelines...................... .............................................. ................................ .............. ............ ............ ............ .......10 1 .0 4.2 Drawings.................... ............................................ ............................................... ............................................... ................................. ........... ..10 10 ......................................................................................................................... 10 5 References References...................... .............................................. ............................................... ............................................... .............................. ............ ......11 11

1 INTRODUCTION  The following design proposal is a requirement of a project initiated by the Malawi Industrial research and Technology Development Centre (MIRTDC). The project demands the refurbishment of a Micro-hydro power scheme in rural Malawi in

order to supply electricity to a community. The turbine is expected to provide in an annual average of 25kW of electrical power.

2 DESIGN METHOD 2.1 Selection Selection of of turbine turbine type type  The location in Matandani already has substantial civil works for the hydropower scheme. Known design data is available for average annual Flowrate and available hydraulic head, and these are presented in Table in Table 2-1

Design Factor Q Flowrate (m3/s) H Hydraulic head

0.14 29.7

(m) Table 2-1 Design factors for turbine location

Using these design factors a decision can be made on the type of turbine to be installed.

2.1.1 Estimated Power Output Output Using the design constants in Table in Table 2-2 an estimate of power available can be determined using Equation 2-1

P

ηtot g γ Q H ⋅

=

⋅

⋅

⋅

Equation 2-1

1000

ηtot g γ

Constant  Turbine efficiencyi Gravitational acceleration

0.6 9.81

(m/s2) Specific gravity of water

1000

(kgm3) Table 2-2 Design constants

 Thus the expected power P in kilowatts is:

P i

=

0.6 ⋅ 9.81 ⋅1000 ⋅ 0.14 ⋅ 29.7 1000

=

24.5

kW

Efficiency values for turbines have been calculated empirically and can vary from 60% (0.6) for a poorly constructed turbine to 75% (0.75) for a well designed and

2.1.2 Specific Specific speed specific speed (Ns) is required. The  To determine the type of turbine a value of  of specific turbine for this project should ideally match the generator with an assumed rated speed of 500rpm., i.e: N=500

Equation 2-2 deals with the calculation of specific speed.

 Ns  Ns

 Ns  Ns

=

500

 N

=

P

Equation 2-2

5/4 H 5/4

24.474 5/4 29.7 5/4

=

35 .67

Referring to standard specific speed boundaries, Table boundaries, Table 2-3, 2-3 , the options for turbine type were reduced to Turgo and Crossflow.

Turbine type Pelton Turgo Crossflow Francis Propeller and

Ns 12-30 20-70 20-80 80-400 340-

Kaplan

1000

Table 2-3 Specific speed values for alternative turbine types

2.1.3 Comparison of Turgo and Crossflow turbines Analysis of design parameters in 2.1.2 determined the potential development of a  Turgo or a Crossflow turbine. A comparison is given in Table in Table 2-4.

Turgo 30-300m head Impulse turbine Good flow rate

Crossflow 2.5-100m head Impulse turbine Will operate on light

High ru running sp speed No seals to maintain Tolerant to debris

load Easy se servicing Cheap No need for flow regulation

Easy servicing Axial force on runner shaft

Table 2-4 Comparison of Turgo and Crossflow turbines

Obviously there are benefits to both but the ease of design and low cost of the Crossflow turbine meant it was the chosen type for this project.

2.2 Design of a Crossflo Crossflow w turbine turbine By reviewing the literature of theory available there were three methods that had the potential for development.

2.2.1 Method 1- SKAT SKAT generic T3 model model  The Swiss Center for Appropriate Technology has over the years produced an increasingly more efficient generic design for a Crossflow turbine. The T2 and T3 designs are free issue on the internet, but commercial licenses are available for purchase to produce model T14.  The T3 design is considered “generic” because it contains all the design drawings for manufacture, with all the variations in size and shape formed from varying head and flowrate.  This design will give a good basis for the actual design proposal.

2.2.2 Method 2- Self design from from classical Crossflow theory theory  There are numerous publications available for the theoretical design of the main components of a Crossflow turbine. The original theory work developed by Banki and Michell, and further more recent developments have been neatly condensed into a publication1 aimed specifically specifically at being designer-friendly. This step-by-step step-by-step procedure will be shown in the following section.

2.2.3 Method Method 3- Hybrid design Using the generic design available for the SKAT T3 model there was some scope for alteration. Alterations to [1 [1], explained in its follow-up document 2 suggest that the optimum number of blades differs from those proposed by SKAT. Using the procedure in [1 [1] Appendix 1 was formulated, a summary of which is given in Table in Table 2-5

SKAT design

Outside blade diameter

D1

200

Design dimens ion 421

(mm) Inside blade diameter

D2

133

295

133

(mm) Blade chord length (mm) Number of blades Outside Blade spacing

L Z t

32 19

73 16 83

15 42

(mm) Rotor axial length (mm)

B

220

67

220

Item

Hybrid

200

Table 2-5 Summary of design data  The SKAT design and construction notes can therefore be followed and altered accordingly.

2.3 2.3 Mate Materi rial als s  There is a vast selection of materials available for the construction of the hydro turbine. Historically turbines have been fabricated out of materials available e.g. wood, simple bolts, scrap sheet metal etc. The choice of material ultimately determines the following: •

Longevity of the installation (rusting, rotting, seizing, fatigue)

•

Cost of fabrication (including manufacturing techniques)

•

Maintainability (access to spare parts e.g. bearings)

•

Efficiency (smooth internal surfaces increase efficiency)

If the design is well built, to a good tolerance with maintenance in mind the long term running and upkeep by a semi-skilled individual will be made more simple and less prone to damage. This in turn results in less down-time of the turbine and a better average performance.  The manufacturing facilities available lend themselves to the use of mild steel in block and sheet form, welding, drilling and riveting, and the purchasing of  specialist parts such as the shaft bearings and drive belt.

 Table 2-6 details the construction materials of the specific components of the design.

Table 2-6 Materials selection Componen t

Material

Detail

Roto Rotorr sha shaft ft Rotor blades

2” Mild Mild stee steell rod rod Steel tube cut into strips or Steel plate rolled to correct curvature

Spacers

Mild steel rod, various diameters Brass rod, various diameters

Bushes

Inside of steel tube is often used for low cost productions- but has a high surface roughness reducing efficiencies considerably. Achieving an accurate roll with sheet metal is difficult.

3 APPENDIX 1 3.1 Design from Classic Classical al theory theory  To produce the geometry of the rotor the following relationships are used:  Jet inlet angle is usually set at 16°. Hydraulic efficiency of the nozzle (ηh ) is considered to be 0.95. Work coefficient of the turbine (Ψ) is set at 2.  Therefore: nozzle velocity: Inlet velocity : Vu

V1 =

=

ηh

V1cos16

 Tangential flow velocity: U

2gh

=

0.95

2 ×9.8

=

22 2

=

60 U

60 ×11

D1 =

Diameter ratio =

D2/D1 = 0.7

Inner diameter

D2 = 0.7 x D1

a=

Blade spacing

t=

Number of blades Rotor length B = Q ×

Z 4

 N π

D1 − D2

=

2

a 0.764

=

=

t

×

=

0.21

500 × π

=

22.9 m/s

−

=

0.147

0.041

π × D1 × V1sin16

0.42m

=

= 0.295m

0.063m

0.083m

π × 0.21

1

=

= 0.7 x 0.21

2

0.032 0.764

π × D1

Z=

29.7

11 m/s

Rotor Outer diameter

Annulus width

×

22 m/s

Vu ψ

=

=

=

=

16 blades

0.067m

3.2 Design Design from from SKAT SKAT 3.2.1 Design Design variables variables We already know from the calculations in 2.1.1 and 2.1.2 that the expected power output and specific speed are 24.5kW and 71.3rpm respectively. The SKAT design therefore allows the calculation of the rotor length bo from the following (where Qs is the specific speed of the design, Qs= 0.15, and η=0.6)

 b o

=

102

×

η × H ×Qs

P ×

H

=

102

×

24 .7

0.6 × 29 .7 × 0.15 × 29 .7

=

172 .95 mm

 Taking the nearest design value bo becomes 220mm Substituting this value into the values for bo in the SKAT design, the new hybrid design can be produced for construction and testing

3.3 Number Number of Intermed Intermediate iate disks: disks: From a stress consideration, when the rotor length (B, bo) reaches certain values the inlet force on the blade would produce bending. This bending over time would produce fatigue and therefore permanent damage. This can be avoided by the inclusion of intermediate disks along the rotor length. Table length. Table 3-7 has been formulated from design data in Scheurer et. al3 in order to determine the appropriate number of intermediate disks. Design flow for this l ocation is 140 l/s.

Flow

No. Intermediate

rate > 85 l/s > 125

disks 1 2

l/s > 155

3

l/s > 180

4

l/s Table 3-7 Calculation of number of intermediate disks on runner shaft  The SKAT design allows for 2 intermediate disks.

4 APPENDIX 2- CONSTRUCTION

AND

DRAWINGS

4.1 Constructi Construction on guidelines guidelines  The SKAT publication contains comprehensive design guidelines. In addition to this a useful document to consult is the Compendium In Small Hydro (2002)4 which contains many of the referenced documents and further approaches to Crossflow fabrication techniques.

4.2 4.2 Draw Drawin ings gs  To be completed.

5 REFERENCES

1

Crossflow Turbine Design, Soft Technology Number 35, ATA Melbourne Australia, Ian Scales, p33-

39 2

Crossflow Turbine Design, Soft Technology Number 37, ATA Melbourne Australia, Ian Scales, p16-

17 3

Scheurer, H., Metzler, R., and Yoder, B., 1980. Small water turbine; instruction manual for the

construction of a Crossflow turbine. German Appropriate Technology Exchange (GATE), Eschbom,

Germany. 4

Compendium in Small Hydro, Furze. J., University of Aarhus, 2002