MAE 4261: AIR-BREATHING ENGINES

Additional Topics

Mechanical and Aerospace Engineering Department

Florida Institute of Technology

D. R. Kirk

TURBINE COOLING TRENDS

• Thrust and performance increases monotonically with turbine inlet temperature, t

• Isp and thermal also increase– Because of associated

increase in c

• STRONG INCENTIVE TO INCREASE t

• Turbine efficiency decreases• Blade materials: oxidation-

resistant, high , such as Nickel and Cobalt based alloys

• Introduction of directionally-solidified and single-crystal blade materials

Increase limited bymetallurgical progress Most current advancement

due to air-cooling

WHERE DOES COOLING AIR COME FROM?

Turbine blades cooled withcompressor discharge air

Other components(burner, liners, disks, etc.)also cooled withcompressor air

OVERALL COMMENTS

• Cooling air drawn from compressor

• Must be at higher pressure than that part of flow path to be cooled

• Blades are cooled by combination of internal and external cooling

– Internal cooling: Convection driven

– External cooling: Film Cooling

COOLING SYSTEMS

COOLING STRATEGIES: INTERNAL COOLING

• Cooling air is pumped through inside of blades

– Air is pumped in at root and makes multiple passes before exiting at root

• Material is cooled by forced convection on inside surface and by conduction through blade

• Different regions of blades can have different cooling profiles

• Large surface area on inside

• Many designs employ roughened internal microfin structure

COOLING STRATEGIES: FILM COOLING

FILM COOLING BEHAVIOR

FILM COOLING OPTIONS

FILM COOLING OPTIONS

EXAMPLE OF FILM-COOLING DATA

cr

rfr

TT

TT

• Cooling Effectiveness, (often called adiabatic film effectiveness)

• Tr = Adiabatic recover temperature = the temperature wall would reach if adiabatic (no heat transfer) in the absence of film cooling

• Trf = recovery temperature in the presence of film cooling

x/D

TREMENDOUS AMOUNT OF DATA AVAILABLEB

≡ (U

) jet /

(U

) ∞ ~

0.5

B ≡

(U

) jet /

(U

) ∞ ~

1.0

COOLING JET BEHAVIOR: CFD

-100

2780

2492

2204

1916

1628

1340

1052

764

476

188

°F K

200

1800

1640

1480

1320

1160

1000

840

680

520

360

A-A: x/D = 10

A-A

x/D = 10

A-A

Attached Jet: B ≡ (U)jet / (U)∞ ~ 0.5

Lifted Jet: B ≡ (U)jet / (U)∞ ~ 2.0

COOLING STRATEGIES: TRANSPIRATION COOLING

• Wire cloth or mesh is used for exterior of blade and air is leaked uniformly through it– Consists of a plurality of

wires made of metal, ceramic or other materials, and arranged with their longitudinal axes generally and not necessarily precisely, parallel to the blade axis, either with or without a stiffener insert

• Ample porosity is provided for transportation cooling

• Cools surface and provides a protective layer

HOW TO DESIGN COOLED BLADES

One of the most difficult areas of gas turbine design

1. Estimate wall heat flux (qw) over blade surface for specified wall temperature (Tw)

2. Find thermal stresses in the skin

3. Find the reduction in qw required to limit stresses to acceptable values → find required of film cooling

4. Find the arrangement of cooling holes and cooling air flow for film cooling to provide the required effectiveness

5. Find the internal cooling airflow to absorb residual qw

ENGINE MATERIALS

• Different materials used in Rolls-Royce jet engine– Blue: titanium is ideal for strength and density, but not at high temperatures– Red: nickel-based superalloys– Orange: steel used for the static parts of the compressor– Green: Composite

PW4084: COLOR SCAN SOON!

ADDITIONAL ISSUES AND BLADE TESTING

• Other Issues

– High Cycle Fatigue

– Materials

– Manufacturing

– Containment of Blade

– Disk Rupture

– Sealing

– Tip and Hub Losses

– Turbine Cooling Bleed

– Inspection

– Replacement Parts ($) ‘Blade-Out’ Simulation

BLADE MATERIALS AND GROWTH• Although blade cooling can reduce

temperatures, heat transfer limitations still exist and t ↓

• Conventionally cast blades (inexpensive)

– Myriad of crystals which gives multidirectional mechanical properties

– Failure usually at boundary between crystals, long term creep

• Directionally solidified blades (cost ↑)

– Blade is comprised of many long or columnar crystals

– Blade made out of a Ni-base super alloy

– Directionally-solidified, resulting in a columnar grain structure which mitigates grain-boundary induced creep

• Single crystal blades (cost ↑ ↑ ↑)

– No crystal boundaries

– Multidirectional mechanical properties

– Blade is directionally-solidified to permit only one crystal to grow into blade

BLADE AND VANE MANUFACTURING (FLACK)1. Two-piece die made that is very accurate “negative” of eventual blade shape (used thousands of times).

2. If blade is hollow, ceramic core in exact shape of internal passages placed in die. Ceramic core is “negative” of passages.

3. Die is filled with hot paraffin-based wax (liquid), and wax is allowed to cool and harden. Waxes are chosen so that they do no shrink upon cooling.

4. Die is separated and wax piece is removed. Wax is now an exact replica of eventual metal blade. If eventual blade is hollow, ceramic core is still in wax replica.

5. Wax blade is coated (by dipping, spraying, or both) with a slurry and then stucco with multi-layers. A silica, alumina, or other ceramic “flour”, or a combination of these is used to create stucco shell.

6. Wax inside of stucco shell is melted and escapes through exit holes. If blade is to be hollow, ceramic core remains accurately in place in stucco shell.

7. Stucco shell, which is heat resistant, is filled with blade material and blade material is allowed to cool.

8. Stucco shell is removed from blade by air or sand blasting.

9. If hollow core is present, ceramic core is removed by immersion in a caustic solution that dissolves internal ceramic core.

10. Finishing or trimming accomplished by removing any metal used for holing blade in place.

11. Blades are inspected by X-ray and fluorescing surface die for internal and surface defects. If minor defects are found, they are repaired. If major imperfections are found, blade is discarded.

12. Some blades are coated with a very thin film of a poor heat conductor (usually ceramic)

13. Any surface holes for film cooling are “drilled” using precise, electrochemical, electrodischarge, waterjet, or laser machining

14. Some machining may be needed on root (fir tree) for placement on wheel or drum. After this expensive and long multi-step process, blades are ready for installation.

HIGH FUEL TO AIR PROBLEM / CHALLENGE

Flow Direction

Compressor Combustor Turbine

• To increase specific thrust, future engines will increase overall fuel-air ratios

• JSF affected

JSF

F119

PHENOMENOLOGICAL OVERVIEW

PW229

EMISSIONS INTO TURBINE

EXHAUSTMIGRATION

SURFACE HEAT FLUX IMPACT

F119-100 1st ROTOR

F119-100 1st ROTOR

BOAS: BLADE OUTER AIR SEAL

BLADE OUTER AIR SEAL POST EVENT

DETAIL

TURBINE ROTOR BLADE FAILURE (ROLLS-ROYCE)

RESEARCH QUESTIONS• What is impact to turbine surfaces due to secondary reactions?

• What is change in surface heat flux due to a local reaction over a range of operating conditions

– What is influence of blowing ratio, B?

– What is influence of the total fuel content, E?

– What is influence of flow and chemical time scales, Da = flow/chem?

– Etc…

• What if you knew answers?

– How do you use this information?

– How to incorporate into a design system framework?

EXPERIMENTAL INVESTIGATION

Air-Side Injection

Nitrogen-Side Injection

Heat Flux Gauges

Fuel rich air flow

EFFECT OF LOCAL REACTIONB = 1.0, Da = 13, CO = 65,000 ppm (Moderate Energy Content)

4.E+05

6.E+05

8.E+05

1.E+06

1.E+06

-20 -10 0 10 20 30 40Position, x/D

He

at F

lux,

W/m

2

Air SideNitrogen SideCorrelationPredicted Cooling

25%

Coolant Injection at x/D = 04.E+05

6.E+05

8.E+05

1.E+06

1.E+06

-20 -10 0 10 20 30 40Position, x/D

He

at F

lux,

W/m

2

Air SideNitrogen SideCorrelationPredicted Cooling

25%

Coolant Injection at x/D = 0

25% augmentation over inert sideCooled side injection agrees to within 10% of literature values and correlation

Upstream

Downstream

-100

2780

2492

2204

1916

1628

1340

1052

764

476

188

°F K

200

1800

1640

1480

1320

1160

1000

840

680

520

360

x/D = 10

Da < 1Maximum Temperature = 1200 K, 0 % of potential (cold flow)

Da > 1Maximum Temperature = 1715 K, 80 % of potential

A-A: x/D = 10A-A

Note maximum wall heat release at z/D = +/- 0.5

CFD STUDY: B = 0.5 (ATTACHED JET) TOTAL TEMPERATURE CONTOURS — Tflame = 1840 K

-100

2780

2492

2204

1916

1628

1340

1052

764

476

188

°F K

200

1800

1640

1480

1320

1160

1000

840

680

520

360

Da < 1Maximum Temperature = 1200 K, 0 % of potential (cold flow)

Da > 1Maximum Temperature = 1683 K, 75 % of potential

x/D = 10

x/D = 10

Note maximum wall heat release at z/D = 0.0

CFD STUDY: B = 2.0 (LIFTED JET) TOTAL TEMPERATURE CONTOURS — Tflame = 1840 K

IN-LINE AND STAGGERED HOLE GEOMETRIES

900 K

2500

2100

1700

1300

B = 1.0, Da = 0.3, H* = 0.54, Qs ~ 0%

B = 1.0, Da = 0.3, H* = 0.54, Qs ~ 70%

Numerical studies extended to engine conditions

Staggered hole (z/D~3) at low B (0.5-1.0) provides ‘good’ surface protection: burning is kept off-surface, > 0.15

ENGINE BEARINGS AND ARRANGEMENTS

• Engines are unusual among engineering structures in that such a large fraction of their total mass is rotating at high speed

• This large rotating mass must be supported on bearings to maintain close clearances between blade tips and stationary casing

– On order of 1 mm on a rotor of 1 m diameter, or 1 part in 103

• All existing engines use ball and roller bearings to support the rotating assemblies

BEARING TYPES

• Each rotating spool is supported by one ball bearing

– Positions spool axially

– Absorbs radial loads

• Each rotating spool also supported by one or more roller bearings

– Accepts radial loads

– Allows axial movements to accommodate thermal expansion and structural deformations

• Also provides location for squeeze films for damping

– Increases speed at which rotor can operate

– Decreases vibrations

ADDITIONAL EXAMPLES: THERMAL STRESSES

• Beam constrained by two unmovable walls

• Heat added at some location

• When material gets hotter, volume expands (thermal expansion)

– But no room for body to move since constrained by both walls

• Compressive stress is induced in material to produce a strain that cancels thermal expansion

• If body is locally cooled → thermal contraction → induced tensile stress

EXAMPLE OF THERMAL STRESSES

ZOOM IN TO SEE DETAILS

Bearing locations

Mechanism to varycompressor stator angles

ROTATING SHAFT DYNAMICS: SIMPLE MODELS

Simplified spring mass damper model

Very large amplitudes possibleif near critical frequencies

DISK DESIGN