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Jet Propulsion

Lecture-11

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

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

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Materials

Temperature limitations are the most crucial limiting factors to gas turbine efficiencies.

Gas turbine components work under a variety of stress, temperature and corrosion conditions.

Compressor blades operate at relatively low temperature but are highly stressed. The turbine blades operate under extreme conditions of stress, temperature and corrosion.

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Diffuser: Low temperature, low stress

Compressor: Low temperature, but high stress.

Combustor: High temperature, low stress

Turbine: High temperature, high stress & corrosion conditions

Nozzle: High temperature, low stress

Material selection: based upon a variety of criteria.

Turbine bladesCombustor liner

Critical components of high--------------, long life GT

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StressTemperature

CorrosionInteraction complex mechanism.

Material characterization:

Turbine blade: Limited creepHigh rupture strengthResistance to corrosionGood fatigue strengthLow coefficient of expansionHigh thermal conductivity.

Failure

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Plastics (Low Temp 500-800° F)

Plastics are widely used in jet engines today. However, while they cannot withstand extreme high temperatures, they have properties that make them more versatile. They are lighter than many materials of comparable strength and also, unlike metals, do not rust. And because of their molecular stability, plastics do not break easily. Teflon, nylon, rubber, Bakelite, and a host of plastic materials are used in gas turbine engine mainly as sealing and insulation materials.

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Metals (Mid-Temp 1000-2000° F)

Many of the advanced metals that are currently in use today were specifically designed for applications in gas turbine engines. The new alloys developed can withstand over 1800° F while at the same time providing greater structural performance.

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Materials

The three main categories of super-alloys: nickel-based, iron-based, and cobalt-based.

Titanium alloys, modified to withstand high temperatures (up to 1500° F), are seeing increased use in turbine engines.

Aluminum-lithium alloys are less dense and stiffer than conventional alloys. Alloys in this group are often used in the intermediate to high temperature range (up to 650° F).

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Materials

Nickel alloys are some of the best materials used between 1200° and 1800° F. They can withstand such high temperatures because of their age hardening characteristics.

Cobalt-base alloys are used in extremely high temperature areas (upwards of 1800° F). They are commonly used in afterburners. Their use is often restricted due to high manufacturing costs.

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Common alloying elements:Al Cu TiBo Mn TC Mg VCr Ni ZrCo Si

% determines physical/chemical characteristics of the alloy & its suitability

for an application.

TemperingHardeningProcess determines

the rest

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Ceramics (High Temp 2000-3000° F)

Ceramics offer a high temperature range. However, ceramics are not very strong. To compensate for their lack of strength ceramics are usually combined with some other material to form a ceramic composite. Ceramic composites are used in combustor and nozzle components.

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Composites (All temperatures)

Composite materials offer great advantages over metals and ceramics. Not only are composites able to withstand very high temperatures, they can also be lightweight. There are three main types of composites materials: polymer-matrix, metal-matrix, ceramic-matrix.

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PMC - Polymeric materials tend to degrade when exposed to elevated temperatures. MostPMCs operate at temperatures below 570° F.

MMC - Offer not only very high temperature limits, but also increased toughness and strength against ductility. These MMCs are often used on the skin of a hypersonic aircraft.

CMC - Allow for higher temperatures inside the jet engine thus creating greater combustion efficiency (i.e. the higher the temperature, the more completely the fuel burns which leads to increased fuel efficiency and lower emissions).CMCs major downfall is their brittleness.

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Engine Components

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Fan:

Requirements:

1. High strength.2. Lightweight (Safety precaution in case it blows up).

3. Be able to handle a direct blow without breaking (bird trike).4. Temperature range: ~ -50 - 100° F

Commonly used material:Blades - Polymer Composite or Titanium alloys.

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Compressor

Requirements:

1. 200 to 300 hot hours.2. Temperature range: 800 - 1200° F

Disk1. High strength.2. Resist centrifugal stress.3. Resist fatigue.

Commonly used material:Blades - Titanium alloys (cold side). Nickel-based alloy or

Titanium alloy (hotter end).Disk - Titanium alloys (cold) and Nickel-based alloy (hot).

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CombustorRequirements:

1. 18,000 to 20,000 hours.2. 9,000 hot hours.3. Average temperature around 2,800° F.

Combustor liner1. Stresses due to thermal gradient heat.2. Transient stresses due to takeoff and cool down situations.3. Resist oxidation.

Commonly used material:Currently - Nickel-based alloy.

Future - Ceramic composite.

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TurbineRequirements:

1. Rotational strength.2. Pressure loading.3. High temperatures.4. Resist Creep.5. Resist Oxidation.6. Temperature range: 1000 - 2000° F

Commonly used material:Disk - Nickel-based alloy

Blades - Single crystal Nickel-based alloy with thermal barrier coating.

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MixerRequirements:

1. High Temperatures.2. Temperature range: 1000 - 1200° F

Commonly used material:Nickel-based alloy

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NozzleRequirements:

1. High Temperature.2. Temperature range: 1200 - 2400° F

Commonly used material:Nickel-based alloy

Titanium alloyCeramic matrix composite

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PW 4000 EngineFan rotor blades Titanium

Fan stator blades Carbon-fibre composites

LP Compressor Titanium

HP Compressor Rotor TitaniumStator Nickel steel

Turbine Nickel steel

Rotor Stator