Jet Propulsion

Ujjwal K Saha, Ph. D.Department of Mechanical Engineering

Indian Institute of Technology Guwahati

Lecture-10 Prepared underQIP-CD Cell Project

Fans, Compressors & Turbines

Simple Open Cycle Gas Turbine

• Intake air is compressed by compressor turbine• Compressed air enters combustor where it mixes with fuel

and the mixture is burned, adding heat to the system• Combustion gasses enter high pressure turbine, which turns

gas generator shaft, which powers the compressor turbine• Combustion gasses then enter power turbine, which turns

output shaft, and then are exhausted

Compressor Turbine Output Shaft

Combustor

Gas Generator Shaft

Power Turbine

Exhaust

Intake

HP Turbine

Gas Turbine Shaft Types

• Single Shaft– One shaft drives both the compressor and the load– Harder to start since entire engine is mechanically

connected to the drive train

• Split Shaft– Compressor and gas-generator turbine share a

common shaft– Power turbine is decoupled and drives output shaft

independently– Gas generator section not affected by changes in

propeller loading

Gas Turbine Shaft Types

• Single Shaft • Split Shaft• Twin-Spool

– Two stage compressor, each stage driven by separate turbine

– Gas generator shaft is actually a low pressure shaft turning inside a hollow high-pressure shaft

– More complex and larger than split shaft engine

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The suitability of an aircraft engine is judged by three quantities viz.,

While it is desirable to have lower values of the first two quantities as against high values of the third, it so happens that the best of all the three aspects can not be achieved at the same time.

AERODYNAMIC DESIGN ELEMENTS

Hence, the design of an engine has always been an exercise in compromise and depends upon the intended use of the engine.

Specific weightTSFCThrust/frontal area

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• High total pressure ratio • High aerodynamic loading • High efficiency (with sufficient off-design

operating range)• Minimizing the no. of compressor stages • Minimizing the no. of blades (in rotor

and stator rows per stage)• Wider incidence range •Acceptable noise level

AERODYNAMIC DESIGN ELEMENTS

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Compressor Types

•Centrifugal–Single entry or dual-entry impellers–Air accelerates radially outward from the

hub to the diffuser–Rugged, simple in design, relatively light

weight– Large frontal area, lower efficiency, hard

to use more than one stage

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Single & Double Entry Compressors

Schematic of Centrifugal Compressor

Velocity Triangles - Centrifugal Compressor

Compressor Types

•Axial Flow–Uses several stages of rotor and stator

pairs, with decreasing diameter from front to rear

–Easy to vary compression ratio by adding or removing stages

–Stators can be fixed or variable pitch–Most commonly used type for

propulsion gas turbines

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Compressors• AXIAL

• CENTRIFUGAL

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General Electric CF6 turbofan

Pratt & Whitney PW4000 turbofan

Compressors

Axial• Higher efficiency,

(today)• Small front area• Easy stage stacking• Well known theory

Centrifugal• Robust• Not sensitive to tolerance or

disturbances• High π per stage• Large stacking loss• Same weight as axial• More real (unknown) flow

effects

Exactly the same theory! But all formulas differ!

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Complex Flow in Compressors

General Electric

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Axial Compressor V2

U

C1W1

C1

W1

U

U

W2 C2W2 C2

U

C3W3

U

C3

C4

C5

W3

W4

U

W4 C4

Ps PT

INLET GUIDE VANES

FIRST STAGE BLADE

FIRST STAGE VANE

SECOND STAGE BLADE

SECOND STAGE VANE

Velocity Triangles for One Stage

Effect of Increasing Fluid Deflection

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Important Parameters

Pressure ratioDiffusion factorFluid deflectionEfficiencyDegree of reactionLoss coefficients

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Annulus Shapes

The constant inner diameter design, mostly found in industrial units, is less expensive as compared to other designs. Compressors with constant outer diameter are used where minimum number of stages is required, and as such, they are usually found in aircraft engines. The constant mean diameter design of compressors makes the combustor to have an awkward shape, with the result that the required changes in flow direction causes additional pressure losses.

Compressor blades

• NACA 65 thickness• C4 thickness• Circular arc

– DCA– MCA

• Controlled diffusion blade– no shock– minimum BL

Note especially• Acc - dec on pressure

side• Min pressure on suction

side• Slope, BL growth on

suction side (?)

• Check off-design behaviour

Types of Blade Profiles

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COMPRESSOR STALL

• Occurs if for some reason air velocity decreases without a commensurate decrease in RPM or if RPM increases without the necessary air velocity increase.

• Similar to wing stall• Can result in blade failure

Rotating stall• One blade stall• Stall on suction side

blade• Destall on pressure

side blade• (Improving for

downstream stage)

Surge• One blade stall• Overload on upstream

blade• All compressor stall• Measured from vibration

Sub-, Trans-, Super- Sonic Machine• All subsonic• Mach 1 on blade suction surface• Relative tip inlet supersonic• All l.e. supersonic• Relative tip outflow supersonic• All rotor supersonic• Absolute flow supersonic

Shock Problems, losses and separation, influence areas

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TURBINES

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Turbines - Applications

Hydro, steam and gas turbines for electric power generationAirplane and helicopter applicationsAircraft auxiliary power unitsPump drives for gas or liquid pipelinesLand and marine applications including turbochargersExpanders in gas liquefaction and cryogenic refrigerationSpace power systems

Good Marks to Recon

Compressor– The pressure side is leading the rotation

Turbine– The suction side is leading the rotation

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TURBINES

• Develops shaft rotational energy from the kinetic energy of the hot combustion gases entering through the vanes

• Usually of axial flow design• Drives the compressor and various engine

accessories• The remaining useful energy can be used

as jet thrust or shaft mechanical work

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Turbine Construction

• STATOR– Stationary guide nozzles (vanes) discharge gas at

high velocity onto the moving blades– Attached to turbine casing

• ROTOR– Consists of a shaft and bladed wheel (disc)– Attached to the main power-transmitting shaft

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Velocity Triangles – Axial Turbines

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Parameters Affecting Turbine Blade Design

Vibration Environment

Tip Shroud

Inlet Temperature

Blade Cooling

Material

Number of Blades

Airfoil Shape

Trailing-Edge Thickness

Allowable Stress Levels (AN2)(N = Speed, RPM)

Service Life Requirements

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Important Parameters

Pressure ratioMass flow rateEfficiencyDegree of reactionLoss coefficientsBlade loading coefficientStage loading coefficient

Radial Turbines

• Radial inflow turbines are suitable for many applications in ship, aircraft, space power systems, and other systems where compact power sources are required.

• They are also used in air liquefaction plants and have been employed in power generation units providing up to 2 MW of power for a variety of industrial applications, aiming to replace heavy diesel engines.

Radial Turbines

• Radial turbines have several advantages over an axial turbine. They maintain a relatively high efficiency when reduced to very small sizes and can handle an elevated pressure ratio.

• The pressure ratio for radial turbines used in turbochargers can be up to 4:1, but in some applications, as for example in their use in power generation systems, it can be as high as 6:1.

Schematic of Radial Flow Turbine

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• Centrifugal compressors are simple, inexpensive, lightweight, and have a high pressure rise per stage

• Centrifugal compressors experience large inter-stage losses and require a large frontal area; they are typically less efficient than multistage axial compressor

• Multistage axial compressors can achieve larger compression ratios and are better suited for high-power marine applications

Summary

Summary

• Radial inflow turbines are suitable for applications where compact power sources are required.

• They maintain a relatively high efficiency when reduced to very small sizes and can handle an elevated pressure ratio.

• The pressure ratio in turbochargers can be up to 4:1, but in some applications, it can be as high as 6:1.

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References

1. Hill, P.G., and Peterson, C.R., (1992), Mechanics and Thermodynamics of Propulsion, Addison Wesley.

2. Saravanamuttoo, H.I.H, Rogers, G.F.C, and. Cohen, H,(2001), Gas Turbine Theory, Pearson Education.

3. Oates, G.C., (1988), Aerothermodynamics of Gas Turbine and Rocket Propulsion, AIAA, New York.

4. Mattingly, J.D., (1996), Elements of Gas Turbine Propulsion, McGraw Hill.

5. Cumpsty, N.A., (2000), Jet Propulsion, Cambridge University Press.

6. Bathie, W.W., (1996), Fundamentals of Gas Turbines, John Wiley.

7. Treager, I.E., (1997), Aircraft Gas Turbine Engine Technology, Tata McGraw Hill.

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