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DESIGN APPROACHThe 6FA gas turbine is a 0.69 scale of the 7FA,

just as the 9FA is a 1.2 aerodynamic scale of the7FA (Figure 2). GE has used aerodynamic scalingin gas turbine development for more than 30years. This technique is exemplified in the deriva-tive design of the 6B and 9E gas turbines, whichwere scaled from the 7E. The success of this gasturbine prod uct family in worldwide power gener-ation service illustrates the benefits of aerodynam-

ic scaling.

During all aspects of the 6FAs design, carefulattention was paid to experience gained duringthe 500,000 fired hours of operation with F tech-nology gas turbines. The F-technology fleet repre-sents the most proven advanced-technology avail-able. The fleet experience leader, located atVirginia Powers Chesterfield Station, has 35,000hours of fired hours experience.

Today, F-technology combined-cycle powerplants are operating in excess of 55% efficiencywith reliab ility in th e mid to h igh 90s. Table 2

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Table 1COMPARISON OF GAS TURBINE RATINGS (ISO, BASE, 60 Hz)

Simple-Cycle Combined-Cycle

6001B 6001FA 7001FA S106B S106FA S107FA S206FA*

Output (MW) 39.2 70.1 167.8 59.8 107.1 258.8 218.7/217.0

Heat Rate

(kJ/kWh) 11,320 10,530 9,940 7,390 6,795 6,425 6,605/6,705

EfficiencyLHV 31.8% 34.2% 36.2% 48.7% 53.0% 56.0% 54.1/53.7

Air flow

(kg/s) 138 196 432 138 196 432 196

Pressure ratio 11.8 14.9 14.8 11.8 14.9 14.8 14.8

Firing Temp.

(F/C) 2020/1104 2350/1288 2350/1288 2020/1104 2350/1288 2350/1288 2350/1288

Exhaust gas

Temp. (F/C) 1006/541 1107/597 1102/594 1000/538 1107/597 1117/603 1107/597

Gas Turbine

Speed (rpm) 5,100 5,250 3,600 5,100 5,250 3,600 5,250

* 50 Hz/60 Hz

GT24596A

Figure 2. GE F product line

GT24370

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shows a listing of materials used in primary com-ponents of th e 6FA, all of which have a proven his-tor y in G E heavy-duty gas turbines for power gen -eration.

Extensive compon ent a nd full-unit testing is an

integral part and cornerstone of the process ofnew product intro duction . During the n ine-yeardevelopment cycle of the MS7001F, componenttesting confirmed d esign assumptions. In additionto these tests, loaded instrumented tests were alsoperformed in Greenville, South Carolina, and at

Virginia Powers Chesterfield station. In addition,an instrumented load test was completed on the9F in France. Instrumented full-load tests of 7FAsat Sithe Energy, New York, and Florida Power andLight and of a 9FA at Medway, United Kingdom,

formed the baseline from which the 6FA wasdesigned.

The 6FA gas turbine configuration includes an18-stage co mpre ssor, six com bustion cha mbe rsand a three-stage turbine (Figure 3). The shaft issupported on two bearings, as it is in the 7FA, 9FA,

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Table 26FA MATERIALS

Component Material Experience

Casings

Inlet case Ductile iron All Fs

Compressor Ductile iron All Fs

Compressor discharge case 2 1/4 Cr-Mo All FsTurbine shell 2 1/4 Cr-Mo All Fs

Exhaust frame Carbon steel All Fs

Compressor

Blading C450/403+ Cb All Fs

Wheels

Compressor NiCrMoV/CrMoV All Fs

Turbine IN-706 All Fs; complete rotor since 3Q95

Combustion

Transition piece Nimonic 263 All Fs and DLN systems

Liner HASTX/HS-188 All Fs and DLN systems

Buckets

Stage 1 GTD-111DS All Fs, 6BStage 2 GTD-111 All Fs

Stage 3 GTD-111 All Fs

Nozzles

Stage 1 FSX-414 All 6Bs, Fs, EAs

Stage 2 GTD-222 All Fs, EAs

Stage 3 GTD-222 All Fs, EAs

RDC27030-1

Figure 3. 6FAgas turbine

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5P and 6B gas turbines. This design was made toenhan ce the ma intainability of these gas turbines.

Five casings form the structural shell: the inletcasing, compressor casing, compressor dischargecasing, turbine shell and exhaust frame. Figure 4shows the gas turbine section in more detail. Theaft diffuser is attached to the exhaust frame and isshipped assembled on the turbine base with thether mal insulation facto r y-installed. The inletp lenum and th e un i t p ip ing and wir ing a reshipped assembled with the unit on the base.

The basic gas turbine compressor has an evolu-tiona r y 30-year h istor y and o riginates from th eMS5001 (Figure 5). The compressor rotor usesNiCrMoV and CrMoV in its rotor construction,

alloys similar to those used on the 7FA. The com-pressor roto r h as grit-blasted flan ge surfa ces,enhancing torque transmissibility by a minimumof 70% over untrea ted f lange surfaces .Compressor extraction air, which does not req uireexternal coolers, provides the cooling for the first

two stages of buckets and all three stages of noz-zles. The cooling circuit for the buckets is internal

to the rotor and there is no loss of air in its trans-fer at stationary to rotating seals. The compressorair extraction locations are similar to the 7FA.Airfoil materials used in the compressor are thesame as those used o n th e 7FA and do not requirecoatings.

The combustion system comes in two varia-tion s, bot h of which a re capab le of multi-fuelapplications (natural gas, distillate oil, propaneand fo ssil fuels):

Dry Low NOx (DLN) standard offering

In tegra ted Gas if i ca t ion Combined Cyc le(IGCC) option for using a wide spectrum

of low heating value fuels, including gasifiedcoal or heavy oil and steel mill gases

The combustion system is comprised of sixchambers that are similar to the 9FA in designand operating conditions, and also uses commonhead end components (nozzles, swirlers, cap and

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6 Combustors 3 Stage Turbine

2 Bearing Rotor

Hand Holes andMan Holes forMaintenance

18 Stage Compressor

GT25694

Figure 4. Gas turbine configuration

GT25755

Figure 5. Growth in compressor design evolution

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end cover) with the 9FA. Commonality is affordedby the fact that the flow per can in the 6FA is with-in 2% of the 9FA. A further improvement of the6FA combustion system is the integral multiple ofstage 1 nozzle vanes with the number of combus-tors. This provides for a repeatable, cha mber-to-chamber, therma l distribution go ing into the

stage 1 nozzles. The large exit span of the transi-tion pieces, which resulted fro m a six-cham berconfiguration, has been engineered using state-of-the-art analytical techn iques. Co ld flow visualiza-t ion tests coupled with computa t iona l f lu iddynamic and finite element stress analyses havebeen used in optimizing the transition piecegeometry. Analytical predictions have been veri-fied with full temperature and pressure combus-tion tests on a fully instrumented transition piece.Figure 6 shows another transition piece used in atemperature verification test using thermal paint.

Emissions levels are at 25/15 ppm NO x/C Owith a DLN system, in the ra nge o f 40% to 100%load operating on natural gas. For operation indistillate oil with water or steam injection, the lev-els are 42/15 ppm NO x/CO. Up to 5% steammay be used for po wer augmentation.

The turbine rotor is a scale of the current 7FA.The turbine wheels, spacers and aft shaft aremade from INCO 706 with INCO 718 bolting,similar to the current 7FA. As in the compressor,the turb ine roto r also ha s grit-blasted flange sur-

faces for enhanced torque transmissibility. Theairfoil and coating materials used in the turbineare the same as tho se used in the 7FA.

The 6FA also incorporates a number of other

features to its design to enhance performanceand endurance: Static honeycomb seals and coated rotating

cutter teeth are used in locations (Figure 7)that significantly affect performance. Theseinclude the high pressure packing seal, tur-bine interstage diaphragm seals and buckettip seals. Performance is improved throughtighter clearances at these seal locations.

Extensive experience has been accumulatedwith honeycomb seals. They have been usedin s imi la r app l ic a t ions on GE Airc ra f tEngines since the early 1960s. They have also

been successful ly used on GE PowerGeneration heavy duty design units since1994 on 7EA, 9E and 6B units.

Tighter compressor blade and bucket t ipclearances are also maintained by equivalenttherm a l masses d ist r ibu ted a ro und theperiphery of the casings (Figure 8), whichprovide compensation for the cold flanges atthe split lines. This provides for rounder cas-ings and tighter tip clearances during opera-tion.

External casing flanges use an optimizedbolting arrangement for reduced leakage,

which has been validated by factory tests.This results in less power required for com-partment cooling and an overall improve-ment in performa nce.

Reduced use of cooling air in the hot sectionof th e turbine. Judicious use of cooling air forairfoil and shroud cooling in the stage 1 noz-zle, bucket, shroud and stage 2 nozzle haveallowed for more uniform temperature gradi-ents that improve life and performance. The

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RDC27664-11

Figure 6. Thermal paint verification test fortransition piece

GT257

High PressurePacking Seals

InterstageDiaphragm

Stage 2 & 3 BucketTip Seals

GT25770

Figure 7. Honeycomb seal locations

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stage 1 bucket serpent ine c ircui t hasenha nced lead ing edge co oling an d cast-incooling slots at the trailing edge, to improvelife in these area s.

The 6FA load gear (Figure 9) was developed inassociation with a world-class gear manufacturer,Renk AG, of Augsburg, Germany. In design, the6FA load gear is a horizontally offset gearboxdesigned to transmit 90 MW with a 1.1 ser vice fac-tor, as defined per American Petroleum Institute(API) specifications. The shaft power output fromthe 6FA gas turbine is driven through a flexiblecoupling to the high-speed pinion. The low-speedbull gear drives the generator though a rigidlycoupled quill shaft that operates at either 3,600rpm or 3,000 rpm. The 6FA gear is furnished with

case carburized and precision gro und d ouble-heli-cal gea ring. The high-speed an d low-speed shaftsare moun ted on babb itt-lined, o ffset, half-typesleeve bearings. The bearing housings are integralto th e steel-fabricated casing, and provisions areprovided for bearing metal thermocouples andeddy current vibration probes. The load gear alsoincorporates provisions for mounting a turninggear to the high-speed shaft for establishing unitbreakaway during startup.

A new level of understanding in the design ofload gears has been achieved in the design anddevelopment of the 6FA load gear. Resources

from GE Power Systems, GE Aircraft Engines andspec ia l i s t s a t GE Corpora te Resea rch andDevelopment were used in the design, develop-ment and reliability assurance studies of the 6FAload gear. Included in the process were review andapproval of:

Design parameters Stress and life analyses System lateral and torsional analyses Material specifications Forging supplier processes

Non-destructive testing (NDT) procedures First-part q ualification and testingThe generator applied with the 6FA gas turbine

is GEs model 7A6C. The 7A6C has a proven histo-ry; it is a fully packaged , base-mounted unit thathas been installed with G Es high er-rated fram e7EA gas turbines since the early 1990s. As ofAugust 1996, approximately 100 7A6C generatorshave been shipped; 90 are in operation. It is avail-able in b oth open -ventilated an d water-to-aircooled (TEWAC) configura tions and with eitherbrushless or static exciters. It can accommodatemotor start o r static start o ptions and is applicableto both 50 Hz and 60 Hz systems. An excellentreliability record has been recorded during thepast five years.

When applied at the lower 6FA rating, theincreased capability yields lower operating tem-peratures and enhanced reliability. The increasedthermal capability can accommodate demandingof f-voltage, off-freq uency conditions and can meeta wide range of req uirements.

RDC227644-8

Figure 8. Typical equivalent thermal masstocold flanges at split line

RDC27664-09

Figure 9a. Load gear in assembly at Renk AG

RDC27664-10

Figure 9b. Load gear in assembly at Renk Ag

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RELIABILITY ANDMAINTAINABILITY

The design of the 6FA and 7FA gas turbines hasfocused on o perating reliability and maintainabili-ty. Reference 2 reports the development of relia-bility features in the controls and accessories.Redundancy has been designed into the controls

and accessories areas of the gas turbine powerplant to meet these goals.

The Mark V control applied to the 6FA, similarto the 7FA has a triple-redundant, microprocessor-based computer control. During normal opera-tion, three computers share con trol of the gas tur-bine. Should one of the computers or one of thetriple-redundant sensors fail, internal voting logicswitches control of the gas turbine to the tworemaining control computers and associated sen-sors. Alarms that ind icate a fault in the other com-puter or its system of sensors are displayed. Uponrepair, the two remaining computers interrogate

the repaired system to ensure that it is functioningproperly.

Upon determining its proper function, thethree computers again share the responsibility forcontrolling the gas turbine. This type of controlsystem ha s raised r eliability from a mea n-time-between-forced-outages of 3,800 hours to 30,000hours, as demonstrated in an EPRI-sponsored teston an operating MS7001 on the Salt River Projectsystem at their Santa n site.

Redundancy has been designed into the 6FAaccessory systems in all areas, including filters,

pumps and compressors, similar to the 7FA.Redundancy of appara tus and po wer supply dupli-cation, including crossover of sources, transform-ers, switchgear for medium and low voltage andDC chargers and batteries for emergency supply,ensure starting, on-line reliability and eq uipmentsafety.

Maintainability has been considered with astep-by-step an alysis of:

Han dling means for routine or daily inspec-tions in each module. Borescope inspectionports have been provided for inspecting 5stages of the compressor and all 3 stages of the

turb ine. Four man -ho les and six ha nd -ho lesare a lso provided (Figure 4) for rout ineinspections of the tran sition pieces and a ttach-ing seals.

Major inspections of the gas turbine and themain auxiliaries

Specia l maintenance needs, such as rotorremoval, using specially-designed too ls such astrolleys for generator rotors or hoists fitted tocranes for the turbine rotor

With the 6FA being approximately the same

size as the 6B, and with fewer combustion cham-bers (six vs. eight), installation and maintenancetimes are conservatively estimated to be the sameas the 6B. Easy access to the Dry Low NO x com-bustion system was a primary focus in the designof the piping systems.

Finally, an approximate 25% parts count reduc-tion in compressor, combustor and turbine com-ponents, in comparison to the 7FA and 9FA,should manifest in faster and easier field mainte-nance operations.

PLANT ARRANGEMENTThe most frequent applications for the 6FA are

expected to be in mid-range and base load serviceas part o f combined -cycle or co-generation plan ts.Taking these requirements into accoun t, the 6FAgas turbine, like the 7FA, is designed specificallyfor combined-cycle applications with the fo llowing

features: A cold-end d rive gas turbine, which allows theexhaust to be directed axially into the heatrecovery steam generator

Factor y-assembled accessor y packages on sep-arate skids for easy installation and maintain-ability

An off-base turbine enclosure that providesmore space for maintenance and better con-trol of no ise emissions

High compressor discharge extraction capa-bility for Integrated Gasification CombinedCycle (IG CC) applications

Slab-moun ted single- (Figure 10) o r multi-shaft (Figure 1) con figurations

Air enters the unit through a standard single-stage, multiple-element filter located abo ve th egenerator and provides fouling protection for thegas turbine. Exhaust gases from the gas turbine gothrough an axial exhaust diffuser, pass throughsilencers, and either enter the heat recovery boil-er or exit to the stack.

As discussed, the shaft power output from thegas turbine is driven through a flexible couplingattached to its cold end , to the h igh-speed pinionof the load gear. The low-speed bull gear drives

the generato r though a rigidly coupled quill shaft.A turning gear for breakaway during startup isattached to the blind end of the pinion gear.

A motor torque converter that drives throughthe generator is the standard starting means.However, the generator can be a starting motorwhen supplied with a static frequency converter(SFC). The generator shaft end is kept free whenthis technique is used. The torque level can bereadily adjusted to permit fast starts and slow cool-

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down rotation of the gas turbine. A disconnectingcoupling or clutch can also be installed to allowsynchron ous condenser operation.

The mechanical accessories are motor drivenand arranged in two modules. One of these mod-u les i s used o n ly for l i qu id fue l opera t ion .Electrical devices, such as auxiliary transformers,switchgears, static frequency converters, are con-tained in the electrical/control module close tothe generator. The modules are fully assembledand factor y-tested prior to shipment. The twomechanical accessor y modules are located at fixedlocations relative to the gas turbine, which allowsfor quick field installations using prefabricatedpiping.

An array of site-specific designs for these mo d-ules provides:

Aesthetic appearance

Thermal and acoustic insulation Heating and ventilation Fire protection Redundancy of power supply Space and means available for maintenanceThese features are established for each plant

according to customer requirements and serviceconsidered (in/out doo r, new/exist ing plant ,etc.). Additionally, the skid layouts for the varioussystems have generous space to permit easy main-tenance without speciality too ls.

The typical general plant arrangement (Figure11) can be adapted to many indoor or outdoorconfigurations. However, the location of the twomain accessory modules must be retained, andthe o ff-base gas turbine enclosure must be used toachieve 85dBA maximum sound from the un it. Tominimize field installation work, the gas turbine is

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GT25758

Figure 10. Slab-mounted single-shaft arrangement

GT25759

Figure 11. Typical general plant arrangement

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mounted on a steel base structure with factory-installed piping and electrical components, simi-lar to the 6B. Side-by-side a rrangements are par tic-ularly suitable for multiple unit plants. Coolingwater needs are secured by external supply (river,sea water with intercooling, etc.) or through fin-

fan coolers. Site civil work can be kept to a mini-mum with grade-level found ation s for installationof all modules and pipeways.

STATUSFive units are scheduled to be shipped by the

end of 1996, two o f which are to be in commercialoperation by October 1996. Figures 12a through12h show ha rdware for these units in variousstages of assembly.

Orders as of August 1996 have shown the widerange of both application and customer accep-tance of the 6FA design, in both the 50 Hz and 60Hz markets. The variety of applications covered bythese pro jects, which includ e co mbin ed-cycle,cogeneration and IGCC, fully demonstrate the

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RDC27664-06

Figure 12a. Assembled stage 1 nozzle segments

RDC27664-07

Figure 12b. Stage 1 shroud (background);stage 2 shroud (foreground)

RDC27664-01

Figure 12c. Honeycomb seals on stage 2 and 3diaphragm seals

RDC27664-02

Figure 12d. Stage 3 nozzle segments withdiaphragm seal

RDC27664-04

Figure 12e. Stage 1 nozzle assembly

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capabilities of th is gas turbine. Com bined -cycleapplications include bo th single-shaft an d m ulti-shaft combined-cycle plant con figurat ions. Figure13 shows the expected fired hours accumulation

of these machines over the next three years.The two launch projec ts are DestecCogenerat ion , a natural-gas-on ly site (Figure 14)in Kingston, Ontario, Canada, and the SierraPac i f ic Power Company s P ion Pine PowerPro ject, a dual-fuel IG CC site locat ed in Reno ,Nevada (Figure 15). Both projects equipment wasshipped in the first quarter of 1996, with mechani-cal and electrical erection essentially completed inthe second quarter and first firing of the units inAugust. The Destec Cogeneration and the SierraPacific Power Projects are both scheduled to becommercial on natural gas in October 1996, with

the IGCC portion of the Sierra Pacific Projectgoing on-line in D ecember 1996.

Subsequent projects scheduled to go commer-cial in late 1996 or 1997 include a cogenerationfacility in Finland , a single-shaft ba se loa d com -

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RDC27664-03

Figure 12f. Turbine shell

RDC27664-12

Figure 12g. Rotor being installed into unit atGreenville, South Carolina, plant

RDC27644-05

Figure 12h. Unit being assembled atGreenville, South Carolina, plant

0

20

40

60

80

100

120

140

3Q96 1Q97 3Q97 1Q98 3Q98 1Q99

End of Year

60 Hz Applications

50 Hz Applications

- 6FA Fleet (Total)

Hours

(Thousands)

GT25766

Figure 13. 6FA operational experience

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bined -cycle facility in Ita ly and a multi-shaft com-bined -cycle facility in the U nited States.

The no-load tests on these first three units havesuccessfully demonstrated that the design toolsused a ccurately predicted the operating character-istics of the unit (Table 3). The units exhibitedflawless starting an d acceleration to full-speedconditions. Rotor vibration levels for these unitswere well below design criteria and indicated satis-factor y st if fness characterist ics of th e scaled

design. All bolted flanges and shell/casing jointsexhibited no leakage.

CONCLUSIONGEs design philosophy, based on a firm ana lyti-cal foundation and years of experience of gas tur-bine operat ion, has resulted in reliable, heavy-dutygas turbines. On this basis, successful designs arecarefully scaled to larger or smaller sizes. Scalinghas been used to produce similar designs thatrange from 25 MW to 200 MW. Improved materi-als and components that have been prudently andcarefully applied to increase power and thermalefficiency have resulted in the evolution of provendesigns. Finally, designs are carefully tested anddemonstrated in extensive development facilities

and by fully instrumented unit tests in order toprovide full confirmation of the design underactual operating cond itions.

Using this methodology, the 6FA has beenscaled from the proven 7FA and successfullylaunched into production. Five units are sched-uled to be shipped by the end of 1996, two ofwhich are scheduled to be operational in thesame period. The full-speed no -load tests and ini-tial site startup operations of these first units weresuccessful.

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GT25781

Figure 14. Destec CogenerationKingston, Ontario, Canada

RDC27640-6

Figure 15a. Sierra Pacific Power CompanysPion Pine Power Project

GT27640-2

Figure 15b. Sierra Pacific Power CompanysPion Pine Power Project

Table 3

6FA TEST RESULTS AT FULL SPEED NO LOAD

ISO Performance ISO Performance

Expected (nominal) as Tested

Airflow (lb/s / kg/s) 437 438

Compressor pressure ratio 10.68 10.74

Compressor efficiency 86.4% 86.2%

Turbine efficiency 84.6% 84.9%

Turbine inlet temperature (F/C) 1074/579 1083/584

Turbine exhaust temperature (F/C) 490/254 495/257

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REFERENCES1. Rowen, W.I., O pera ting Ch ara cteristics of

Heavy-Duty Gas Turbines in Utility Service,ASME paper No. 88-G T-150, presented at theGas Turb ine and Aeroengine Congress ,Amsterd am, Netherland s, June 6-9, 1988.

2. D esign of H igh-Reliabi l i ty Ga s Turbin e

Controls and Accessories, EPRI Final Report,AP-5823, Jun e 1988.

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1996 G E Compan y

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Michael C. Conway

Michael Conway has 16 years of power generation experience and iscurrently Prod uct Line Manager, F Technology. He graduated fro mClarkson Un iversity with a BS in engineering.

Jay Ramachandran

Jay Ramachandran is currently Manager, 6FA Engineering Programs.He has 18 years of design and project management experience at GEsPower Generation and Aircraft Engine divisions. His engineering experi-ence is primarily in th e design of turbine h igh-temperature com po-nents. He also has significant experience gas turbine system design fromhis contribution to G Es advanced H -generation ma chines.

Jay graduated from the University of Cincinnati with an MS in engi-neering. He is also a grad uate of G Es ABC gas turbine engineering pro-gram. He holds two patents on his work in gas turbine engineering atGE.

A l ist of figures and tables appears at the end of thi s paper