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 DOI: 10.1243/09544050260174247

1053 2002 216:Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture

A Thakker, C Sheahan, P Frawley and H B KhaleeqInnovative manufacture of impulse turbine blades for wave energy power conversion

  

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Innovative manufacture of impulse turbine blades forwave energy power conversion

A Thakker1;2, C Sheahan1;3, P Frawley2 and H B Khaleeq1;2*1Wave Energy Research Team, University of Limerick, Republic of Ireland2Department of Mechanical and Aeronautical Engineering, University of Limerick, Republic of Ireland3Department of Manufacture and Operations Engineering, University of Limerick, Republic of Ireland

Abstract: An innovative approach to the manufacture of impulse turbine blades using rapidprototyping, fused decomposition modelling (FDM), is presented in this paper. These blades weredesigned and manufactured by the Wave Energy Research Team (WERT) at the University ofLimerick for the experimental analysis of a 0.6 m impulse turbine with ®xed guide vanes for waveenergy power conversion. The computer aided design/manufacture (CAD/CAM) package Pro-Engineer 2000i was used for three-dimensional solid modelling of the individual blades. A detailed®nite element analysis (FEA) of the blades under centrifugal loads was performed using Pro-Mechanica. Based on this analysis and FDM machine capabilities, blades were redesigned. Finally,Pro-E data were transferred to an FDM machine for the manufacture of turbine blades. Theobjective of this paper is to present the innovative method used to design, modify and manufactureblades in a time and cost eVective manner using a concurrent engineering approach.

Keywords: wave energy, impulse turbine, FDM rapid prototyping, manufacturing and concurrentengineering

1 INTRODUCTION

For the last two decades, scientists have been investigat-ing and de®ning diVerent methods for power extractionfrom wave motion. Some of these devices utilize theprinciple of an oscillating water column (OWC). OWC-based wave energy power plants convert wave energyinto low-pressure pneumatic power in the form of bi-directional air ¯ow. Self-rectifying air turbines (whichare capable of operating unidirectionally in bidirectionalair ¯ow) are used to extract mechanical shaft power,which is further converted into electrical power by agenerator [1]. Two diVerent turbines, the Wells turbineintroduced by Dr A. A. Wells in 1976 [2] and the impulseturbine with guide vanes introduced by Kim et al. in 1988[3], are currently in use around the world for this type ofwave energy power generation. Several reports haveinvestigated the performance of these two turbines.According to these reports, the Wells turbine hasinherent disadvantages: lower e�ciency and poorstarting characteristics under irregular unsteady ¯ow

conditions. On the other hand, the impulse turbinedelivers useful e�ciency over a wide range of ¯owrateand has good starting characteristics and low operatingspeeds, and thus low-noise operation [1].

The overall purpose of this research was to design,manufacture and test a 0.6 m diameter impulse turbinewith ®xed guide vanes to validate these reports. The®nal goal was to optimize the blade and the guide vanegeometry on the basis of theoretical, experimental andcomputational ¯uid dynamics (CFD) analysis [4].

This paper presents the approach used to achieve theinitial goals of the research. The main objective was todesign and manufacture turbine blades in a time andcost eVective manner. A concurrent engineeringapproach was used to achieve this target. Work wascarried out concurrently in three diVerent areas,namely design of the turbine blade, ®nite elementanalysis and rapid prototyping. The main target waseVectively to manage the manufacturing time and costof the blades, thus producing high-quality bladescomplying with the required aerodynamic and operatingcharacteristics. The design for manufacture (DFM)scheme was implemented with the goal of designingblades that were easily and economically manufacturable[5]. To achieve these targets, an innovative approach wasused for the manufacture of these blades using a fused

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SC02001 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part B: J Engineering Manufacture

The MS was received on 3 July 2001 and was accepted after revision forpublication on 2 April 2002.*Corresponding author: Wave Energy Research Team, Department ofMechanical and Aeronautical Engineering, University of Limerick,Republic of Ireland.

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decomposition modelling (FDM) rapid prototypingmachine [6].

2 DESIGN

An impulse turbine is a self-rectifying turbine with a set ofmirror-image guide vanes on either side of the rotor to actas nozzle and diVuser, and vice versa, under bidirectional¯ow conditions. The major design parameters for the0.6 m diameter turbine were based on the publishedoptimum design [1]. To further the optimization ofblade geometry, a diVerent hub±tip ratio of 0.6 was inves-tigated instead of the published optimum ratio of 0.7 [4].Calculations were made to de®ne the entire blade andguide vane geometry parameters. Finally, a worksheetwas created in MS Excel to calculate all the blade andguide vane geometry parameters automatically. Theinitial design constraints required were the rotor dia-meter, the hub±tip ratio, the guide vane inlet and outletangles, the number of blades and the number of guidevanes. To validate these design data, two-dimensionaldrawings were created in AutoCAD 14. Following thisvalidation, a three-dimensional solid model was pro-duced using CAD package Pro-Engineer 2000i (Fig. 1).

The ®nal design was a solid blade with a hole at thebottom surface for M12 threading with a depth of35 mm at an angle of 7.58. This positive lean at the tipregion leads to higher blade loading and is a possiblecause of high e�ciency [1]. An aluminium hub for a 0.6hub±tip ratio rotor was also designed and generated inPro-E.

3 MANUFACTURING

A total of 30 blades were required for the rotor. The bladehad four curved surfaces, i.e. pressure side, suction sideand top and bottom surfaces with small radii at theleading and trailing edges. Looking at this complexdesign and the high number of blades to be manufactured,diVerent manufacturing techniques were evaluatedqualitatively for the manufacture of these blades. Thetechniques investigated for the said purpose were CNCmilling, investment casting, sand casting using aluminium,glass-reinforced plastics (GRPs) and an FDM rapidprototyping machine using ABS plastics (machineavailable at the University of Limerick).

Qualitative evaluation of the above-mentionedtechniques showed that rapid prototyping (RP) wouldhelp get the job done easily and economically. Thezero tool cost, reduced lead times and considerablegain in terms of freedom in product design may signi®-cantly bene®t manufacturing [7]. This will reduce theoverall cost of the blades compared with the conv-entional techniques, where tooling and labour costs aresigni®cant.

Over the last few years, RP has moved a step forwardfrom being an eVective and faster way of producingcomplex prototypes (e.g. aerospace [8] and automotiveindustries [9]) to a stage where it is considered as a timeand cost eVective tool for the production of end-userparts (e.g. automotive parts [7, 9], biomedical applica-tions such as the fabrication of custom implants andprostheses [10], mobile phone covers [11], etc.).

Based on the advantages associated with the RP tech-nique over other conventional processes, it was decidedto use the Stratasys FDM 2000 machine with ABS-P400 plastic as the raw material for the prototypeblades. After successful manufacturing and analysis ofthe ®rst prototype (Section 5.1), it was decided to useRP for the production of all the 30 blades.

4 FDM PROCESS

The process beings with the design of a geometric modelon a CAD workstation (e.g. Pro-Engineer). The designis imported into Stratasys easy-to-use software, Quick-Slice, which mathematically slices the .stl [stereo litho-graphic apparatus (SLA)] ®le into horizontal layers.Thermoplastic modelling material (ABS P400), 1.78mmin diameter, is fed into the temperature-controlled FDMextrusion head, where it is heated to a semi-liquid state.The head extrudes and deposits the material in ultrathinlayers of approximately 0.254mm onto a ®xtureless basewith precision in x±y coordinates. As each layer isextruded, it bonds to the previous layer and solidi®es.The designed object emerges as a solid three-dimensionalpart without the need for tooling. This technique givesgood surface ®nish and dimensional accuracy, and there

Fig. 1 Three-dimensional solid model of an impulse turbine

blade

1054 A THAKKER, C SHEAHAN, P FRAWLEY AND H B KHALEEQ

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is no need for jigs and ®xtures. There are therefore noadditional costs for tooling and the machine can operateunattended, thus reducing the overall cost of the parts.The largest part dimensions that can be built inside themodelling envelope of FDM 2000 are 9:4 £ 10 £ 10 in[12].

A unique variety of thermoplastic modelling materialsfor FDM systems are available, including ABS-P400. Itprovides impact resistance, toughness, heat stability,chemical resistance and rigidity to perform functionaltests on sample parts. It is ideal for the FDM processon account of its high strength and stiVness, lowshrinkage and rapid solidi®cation. Owing to theseproperties, it is feasible to machine, drill, tap, paint,glue and sand ABS models.

5 CONCURRENT ENGINEERING APPROACH

A concurrent engineering approach was applied for themanufacture of these blades. A DFM scheme [5] wasimplemented to optimize the manufacturing time andcost of the 30 blades. This was achieved by analysingthe design and material of the blades at the initialstages of the design and introducing modi®cations tocut time and cost without compromising the requiredaerodynamic and operational characteristics [13]. Keyelements involved were design validation using rapidprototyping, optimization of manufacturing time/cost,®nite element analysis to check the eVects of designmodi®cations and redesigning of the blades on thebasis of the manufacturing capabilities of the machine.The result of this parallel work was a feasible bladedesign, which could save 45 per cent of time and costcompared with the initial design.

5.1 Design validation

A prototype blade was manufactured to validate thedesign and to check the surface ®nish and dimensionalaccuracy. This blade was manufactured in 30 h using0.251 litres of material. The surface ®nish was measuredusing a Hommel tester T500, which is of accuracy class 1in accordance with DIN 4772. The surface ®nishachieved was 0.2±4.5 mm. Dimensional accuracy wasmeasured using conventional measuring tools and acoordinate measuring machine (CMM). It was foundto be within 0.01 mm.

5.2 Optimization of manufacturing time

Results obtained from the ®rst prototype suggested thatthis technique was feasible for the production of 30blades for the said purpose. The next challenge was tomanage the production of blades in such a way that itwas time and cost eVective. To achieve these objectives,

it was observed that, if the blades could be madehollow, material could be saved and thus manufacturingtime per blade would be reduced, without compromisingthe dimensional accuracy of the blades.

A new hollow blade was designed using Pro-Engineer.An enclosed hollow section was created with a wall thick-ness of 3 mm and a 10 mm thick solid base and topsection. A three-dimensional model was generated forthe ®nite element analysis (Fig. 2).

Concurrently, based on the technique used by themachine, it was observed that the machine was notcapable of manufacturing an enclosed hollow section.It was therefore conceived that, if the blades weremade in two parts and then joined together, a hollowenclosed section could be achieved. This techniquecould only be used if the stresses were found to bewithin limits in the ®nite element analysis. The vendorsof the machine endorsed this technique of makingenclosed hollow sections. A process of chemical weldingwas suggested using methyl ethyl ketone (MEK) which iscommonly known as butanone [M. Turner, Laser Lines(Industrial and Medical) Limited, 2000, personal com-munication]. Therefore, the blade was redesigned intwo parts with a base and a lid (Fig. 3). The combinedmaterial saving was calculated to be around 45 percent, and it was estimated that it would take approxi-mately 17 h to manufacture one complete blade(base and lid), as compared with 30 h required for asolid one.

Fig. 2 Wireframe drawing of a turbine blade with an enclosed

hollow section

INNOVATIVE MANUFACTURE OF IMPULSE TURBINE BLADES FOR WAVE ENERGY POWER CONVERSION 1055

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5.3 Finite element analysis (FEA)

Both solid and hollow blades were analysed using FEAsoftware Pro-Mechanica under centrifugal load [14].These loads were based on an angular velocity of36.65 rad/s, which corresponds to a turbine rotationalspeed of 350 r/min. The material speci®cations of ABS(P400) plastic are given in Table 1, and the failure criteriaadopted are given in Table 2.

The chosen design stress is based on the manufac-turing technique of the FDM machine. As explainedearlier, the model is built by laying thin layers. Althoughthe subsequent layers are fused together and there is agood bond between them, it was recommended to use afactor of 0.8 when performing stress analysis [M.Turner, Laser Lines (Industrial and Medical) Limited,2000, personal communication]. Therefore, to enhancethe safety factor, 5 MPa was taken as the design stresswhich is 80 per cent of the recommended creep ruptureof 6.3 MPa.

A detailed von Mises stress analysis was carried outfor both solid and hollow variants of the blade at arotational speed of 350 r/min. It was found that the

stresses in both types of blade were well distributedand were very low, as shown in Figs 4 and 5. Theselow stresses are believed to be due to the low rotationalspeed of the turbine. A maximum stress of 0.462 MPawas found for the solid blade, whereas a maximum of0.284 MPa was observed for the hollow one [6]. Basedon this analysis, it was decided that the hollow bladesare the best option in this case and would lead to a signif-icant saving in manufacturing time and cost.

5.4 Modi®ed design

To validate the calculations and chemical weldingtechnique, a second prototype was manufactured beforethe production of the 30 blades. This prototype wasmanufactured in 16 h 20 min, which was very close tothe predicted time of 17 h. It used a total of 0.137 litresof material for the two parts, as compared with the0.251 litres used for the solid blade, thus giving a savingof 45.4 per cent.

6 PRODUCTION

The satisfactory results from the stress analysis and theprototyping stage led to the production of a completeset of 30 blades. Based on the modelling envelope ofthe machine, four complete blades, i.e. four bases andlids, were manufactured per run to save machine set-uptime.

6.1 Post-production processes

All blades were then tapped for M12 £ 35 mm formounting. Finally, the bases and lids were welded

Fig. 3 Modi®ed blade in two partsÐbase and lid

Table 1 Material speci®cations P400 ABS

Tensile strength 35 MPaYoung’s modulus 2.5 GPaMass density 1050kg/m3

Maximum temperature 70 8C

Table 2 Failure criteria

Design stress for fatigue [15] 10 MPaCreep rupture (BS Code CP231) [16] 6.3 MPaChosen design stress 5 MPa

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together using MEK to produce the ®nal blade. The ®nalassembled impulse turbine rotor is shown in Fig. 6.

7 CONCLUSION

The outcome of this work was a complete set of 30 bladesfor a 0.6 m diameter impulse turbine rotor with a hub±tipratio of 0.6 for experimental analysis. A few distinct con-clusions that can be drawn from this work are as follows:

1. The FDM rapid prototype process is an excellenttechnique for the manufacture of complex three-dimensional parts.

2. The design and dimensional accuracy achieved by thistechnique is within 0.01 mm.

3. The surface ®nish obtained is 0.2±4.2 mm.4. The ABS material used to manufacture these blades

has good stress characteristics under these operatingconditions (the maximum stresses were 0.284 MPaat 350 r/min for the hollow blade).

Fig. 4 von Mises analysis of a solid turbine blade

Fig. 5 von Mises analysis of a hollow turbine blade

INNOVATIVE MANUFACTURE OF IMPULSE TURBINE BLADES FOR WAVE ENERGY POWER CONVERSION 1057

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5. MEK (butanone) was found to be an eVectivechemical for bonding two ABS parts together. Theextensive use of the rotor under varying conditionshas shown no visible eVects of cracking or other likeeVects on the blades.

6. The concurrent engineering approach (DFM) provedto be helpful in reducing the overall cost and time ofmanufacture by introducing design modi®cations inthe early design stages. A total of 45 per cent materialwas saved by making the blades hollow, whicheventually resulted in a signi®cant saving in manufac-turing time and cost.

ACKNOWLEDGEMENTS

The authors are delighted to acknowledge the input andguidance given by Professor T. Setoguchi of Saga Univer-sity, Japan, in the design phase of the research. Secondly,the authors would like to acknowledge the eVorts of theDepartment of Mechanical and Aeronautical Engineering(M&AE) and the Department of Manufacturing andOperations Engineering (M&OE) at the University ofLimerick for providing the necessary resources and ®nan-cial support to carry out this job. The authors would alsolike to acknowledge the guidance given by Dr J. Jarvis(M&AE) regarding stress analysis and the eVorts of MrV. War®eld (M&OE) in the manufacture of the bladesand of Mr F. Hourigan (M&AE) in carrying out thestress analysis. Finally, the authors would like to acknowl-edge the cooperation of all the members of the WaveEnergy Research Team at the University of Limerick.

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Fig. 6 Assembled impulse turbine rotor

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INNOVATIVE MANUFACTURE OF IMPULSE TURBINE BLADES FOR WAVE ENERGY POWER CONVERSION 1059

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