Design And Analysis Of Stirling Engine For Underwater Application

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Design And Analysis Of Stirling Engine For Underwater Application

Rajkumar Bhagat1, Harsh Khandelwal2, Shreyash Khandwe3, Akshay Khubchandani4, Kanhaiya Kuche5, Sohan Kulal6, Utkarsha Chinde7

1Faculty, Vishwakarma Institute of Technology, Pune, Maharashtra, India 2-7 B. Tech, Mechanical Department, Vishwakarma Institute of Technology, Pune, Maharashtra, India ***

Abstract - Selection of power systems for underwater vehicles is an important task. The power system must be compact enough to fit inside the vehicle and it should supply sufficient power. This paper presents thermodynamic analysis of the Stirling engine based on schmidt analysis for underwater application. Alpha type engine with opposed cylinder configuration is selected. After the analysis, Dimensions for each component are selected followedbystructuralanalysis of engine components.

Key Words: Stirling Engine, thermodynamic analysis, underwater application, cooler, Regenerator, Unmanned vehicle.

1. INTRODUCTION

Stirling Engine is a closed cycle heat engine in which no external air is required which makes it ideal for underwater application. The silent operation of it makes the vehicle more stealthy which makes the engine attractiveformilitaryapplications.Inthe2005wargames, USS RonaldReagan,anewlyconstructed$6.2billiondollar aircraft carrier, was sunk after being hit by multiple torpedoes. This torpedo was launched by Sweden’s submarine named “Gotland”. Gotland snuck past uss ronald reagan defenses. Americans never saw it coming and Yet despite making multiple attack runs on the Reagan, never saw it leave. Previously, diesel submarines couldonlynavigate with noisy diesel engines powered by airand stayunderwaterfor a few days. As a result, diesel submarines were most vulnerable while snorkeling and could be easily tracked. On the other hand, submarines fueled by nuclear reactors require large amounts of coolanttopreventameltdown.Hencepumpingofcoolant createsnoisesandvibrationswhichcanbeeasilydetected by SONAR. The Swedish Gotlands uses a 75 KW stirling enginewhichisanExternalcombustionengine.Therefore frequent combustion does not take place while operation and there is gradual compression and expansion of workinggaswhichmakesitmoresilent.

Stirling Engines can be used for the underwater vechicles that can operate underwater with or without a human

occupant. They can be used for surveillance and other missions that require very quiet operation. A closed-cycle heat engine has potentially better overall energy density than available battery systems. For example, 100 mile range, Submersible with 2m Diameter and 10m Length operating at 10 Knots for 1 hour requires 200 KW-Hr of energy. If we use a 35W-hr/Kg lead acid battery, We would require 6 tons of it. Above requirement can be fulfilledbythe15kW,100kgstirlingengine.Althoughthe weightofthe reactantstoragemust beaddedtothis,the total propulsion package with a Stirling could be lighter thanthepropulsionpackagewithadvanced batteries.

Among Alpha, Beta, Gamma type stirling engines, Alpha typeismoreefficientandhasmorepowerdensity.Sothat, Alpha type stirling engine with opposed cylinder configuration is designed. In this type of configuration, Insteadof displacer,Twoseparatecylindersareused.One is connected to the heater and the other is connected to thecooler.Regeneratorbetweenheaterandcooleractsas aheatreservoir.

1.1 Aim Of The Work

Thepurposeofthisarticleisto designtheStirlingengine and its various components as well as the analysis of the engine (strain, stress, deformation). Theoretical thermodynamic analysis was also performed for the engine.

2. METHODOLOGY

Work took place in three stages, first the Stirling engine wasmodeledbySchmidtanalysisusingratingstakenfrom variousreferences.Thenitwasmodeledandthenpartially importedinanalysissoftwarewherestructuralanalysisis performed.

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Fig -1:MindMap

2.1 Theoretical Analysis

Opposite cylinder configuration type is selected for the design for the optimum use of volume inside the vehicle. Schmidt's analysis is used to model the system. This analysis is still used today as the classical analysis of the Stirlingcycle.

Followingaretheassumptionsforthisanalysis:

● workingfluidobeysidealgaslaw

● Engineisperfectlysealedandthereisnoworking fluidleakage.Thustotalmassofworkingfluidisconstant

● Temperatureineachworkingspace (compression,Expansion) isknownandthereisno temperaturegradient

● Engineisrunningatconstantspeed

● Uniforminstantaneouspressureintheworking space

Followingaresomeoftheparametersselectedforthe engine:

Stroke=60mm

Crankradius(r)=30mm Clearancelength=4mm BoreDiameter(D)=52.4mm

Tc=350K

Th=923K n=1200rpm

Vregen=154cm3

Dregen=60mm

Lregen= 4Vregen/πD2 regen

Lregen=54.46mm

Now,Let’sfindoutvariationofworkingvolumeswith crankangles

Obliquityratio

Fig -2:EnginesystemConfiguration

Xp = √ √ )

Ve √

Similarly,wecanfindoutvariationofcompressionvolume withcrankangle

Vc √

Massofthegas

M=(Patm Vtotal(max)) (RTinitial)

Where, Patm=101325N/m2 Vtotal(max)=372.63cm3 Tinitial=293.15k R=287J/kg-k

M=0.4375g

Mtotal=Me+Mc +Mregen Mtotal =P/R(Ve/Te+Vr /Tr+Vc /Tc)

P=Mtotal R/(Ve/Te+Vr /Tr+Vc /Tc)

Tr =(Th -Tc)/ln(Th /Tc)

Tr=590.9K

2.2 Flywheel Design

Torquedevelopedbyexpansionpistononcrankwillbe givenby-

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Te=rFp [sin +sin(2 )/2√[(n2-sin2 )] Where, Fp=π/4D2 P

Torquedevelopedbycompressionpistononcrankwillbe givenbyTc=rFp [cos +sin(2 )/2√(n2-cos2 )]

Totaltorque(T)=Te +Tc

Chart -1:Turningmomentdiagram

Workdevelopedbytheengine=workrequireforan application

Areaofturningmomentdiagram=Tmean 2pi

Tmean =Areaofturningmomentdiagram/2pi Tmean =22.88/2pi=3.64Nm

LetenergyatpointabeE

A:E b:E+A1 c:E+A1-A2 d:E+A1-A2+A3=E

MaximumfluctuationofenergyΔE=maximumenergy-Minimumenergy ΔE=energyatb-energyatc ΔE=(E+A1)- (E+A1-A2) ΔE=A2=46.85Nm

Coefficientoffluctuationofspeed(Cs)=0.03 Soliddisktype flywheelisselected Maximumfluctuationofenergyinflywheel=½Iw2Cs ΔE=½Iw2 Cs

Where, W=2pin/60=125.66rad/s

I=0.098Kg-m2

I=M(Ro 2-Ri2)/2 M=ρ ㅠ (Ro 2-Ri2)t

I= ρ ㅠ (Ro 2-Ri2)2 t/2

ρ=7800kg/m3 (carbonsteel) Ri=shaftradius=10mm t=10mm

Ro=31.53mm

2.3 Cooler

For cooler, Equivalent model was designed using ansys and the analysis is carried out. Taking the regenerator temperature as the inlet, We must achieve the temperatureof 350K.Thesurfaceareaiscalculatedwhich can produce that much temperature difference. Following parametersaretakenfortheanalysis:

Table -1: Specifications

Parameters Specifications

Coolertemperature 20-300C,Average:250C

Chart -2:TurningMomentwithArea

A1=45Nm

A2=46.85Nm A3=2.5Nm

Heatertemperature 923K=6500C

Heatingmethod liquidoxygenanddiesel tocreatetheheatingof theengineinthe

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Flywheel

combustionchamber.

Coolingmethod coldseawater material copper tubediameter 300mm shelldiameter 1000mm length 4000mm Itwasobservedthatsurfaceareaof0.2827m2 issufficient toproducethetemperaturedifference

2.4 CAD Modelling

Piston

The pistons of the Stirling engine are hermetically sealed and are driven to move up and down as the gas inside expands.Weusegraycastironmaterialforpiston.

Thedesignoftheflywheelisdoneintheprevioussection. Itisusedtominimizethefluctuationsintheoutputpower. Carbonsteelisusedasamaterial

Fig -2:PistonCADModel

Connecting Rod

A device used to connect two moving parts, where it is usedbetweenthecrankshaftandthepiston.Herewehave usedcarbonsteelmaterialforit.

Gudgeon pin

Fig -4:FlywheelCADModel

TheGudgeon/pistonpinisusedtoconnectthepistonto theconnectingrod.Italsoprovidesabearingonwhichthe connectingrodrotatesasthepistonmoves.

Fig -5:GudgeonPinCADModel

Regenerator

In a Stirling engine, the regenerative is an internal heat exchanger and a heat reservoir temporarily placed betweenthehotandcoldspacessothattheworkingfluid passes through it first in one direction and then in the other. Its function is to retain in the system heat that would otherwise be exchanged with the intermediate temperaturemediumatthemaximumandminimumcycle temperatures.Copperisusedasamaterial.

Fig -3:ConnectingRodCADModel

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Crankshaft

The shaft through which the mechanical work is transferredfromthepistontotheflywheel.

Fig -6:RegeneratorCADModel

Engine housing

Anenginehouseisastructurethatholdsthemovingparts.

Fig -9: CrankshaftCADModel

Bearing

Abearingisapartofmachinerythatlimitsmotionrelative to only the desired motion and reduces friction between movingparts.

Fig -7:EnginehousingCAD

Model

Spur Gear

Spur/Cylindricalgearsareusedinmechanicalapplications toincreaseordecreasethespeedofequipmentormultiply torquebytransmittingmotionandpowerviaabeltdrive.

Fig -10:BearingCADModel

Pipe

Fig -11:PipeCADModel

Fig -8:SpurgearCADModel

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Chart -8:VariationofPressurewithExpansionVolume

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3.3 CFD Analysis

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4. CONCLUSION

Bydoingthethermodynamicanalysis,Itisconcludedthat workdoneforcompressionis smallerthanworkobtained intheexpansionstroke.Sothatnetworkof10.735Jouleis obtained.considering1200rpmspeed,cyclecompletedin 1second will be20hencepowerobtainedis214.7Watts. Structural analysis is done in ansys and the deformation, Vonmisesstress,strainiscalculated.Stressinducedinthe piston was 41 Mpa and 21 Mpa for the connecting rod whicharewellwithinthepermissiblelimits.

5. FUTURE SCOPE

Effect of pressure drop due to friction can be taken into considerationtocorrectlymodelthermodynamicanalysis Working fluid like helium can be used for the efficient operation due to its high conduction coefficient and low dragcoefficient.

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