4261 Combustors

8/12/2019 4261 Combustors

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MAE 4261: AIR-BREATHING ENGINES

Gas Turbine Engine Combustors

Mechanical and Aerospace Engineering DepartmentFlorida Institute of Technology

D. R. Kirk


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COMBUSTOR LOCATION

Military

F119-100

Commercial

PW4000

Combustor

Afterburner


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MAJOR COMBUSTOR COMPONENTS

Compre

ssor

Tu

rbine


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MAJOR COMBUSTOR COMPONENTS

Key Questions:

Why is combustor configured this way?

What sets overall length, volume and geometry of device?

Compre

ssor

Tu

rbine

Fuel


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COMBUSTOR EXAMPLE (F101)

Henderson and Blazowski

Fuel

Compressor

Turb

ine

NG

V


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VORBIX COMBUSTOR (P&W)


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COMBUSTOR REQUIREMENTS

Complete combustion (hb 1)

Low pressure loss (pb 1)

Reliable and stable ignition Wide stability limits

Flame stays lit over wide range of p, u, f/a ratio)

Freedom from combustion instabilities

Tailored temperature distribution into turbine with no hot spots Low emissions

Smoke (soot), unburnt hydrocarbons, NOx, SOx, CO

Effective cooling of surfaces

Low stressed structures, durability

Small size and weight

Design for minimum cost and maintenance

Futuremultiple fuel capability (?)


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CHEMISTRY REVIEW

OHmnCOOmnHC mn 22224

478.4

1

m

n

s

22222

478.3

278.3

4N

mnOH

mnCONO

mnHC mn

Stoichiometric Molar fuel/air ratio Stoichiometric Mass fuel/air ratio

General hydrocarbon, CnHm(Jet fuel H/C~2)

Complete oxidation, hydrocarbon goes to CO2and water

For air-breathing applications, hydrocarbon is burned in air

Air modeled as 20.9 % O2and 79.1 % N2(neglect trace species)

Complete combustion for hydrocarbons means all C CO2and all H H2O

2878.3324

12

mn

mns

Stoichiometric = exactly correct ratio for complete combustion


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COMMENTS ON CHALLENGES

Based on material limits of turbine (Tt4), combustors must operate below

stoichiometric values

For most relevant hydrocarbon fuels, s~ 0.06 (based on mass)

Comparison of actual fuel-to-air and stoichiometric ratio is called equivalence ratio

Equivalence ratio = f = /stoich

For most modern aircraft f~ 0.3

Summary

If f= 1: Stoichiometric

If f> 1: Fuel Rich

If f< 1: Fuel Lean


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VARIATION OF FLAME TEMPERATURE WITH

FlameTem

perature

Flammability LimitsStill too hot

for turbine


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WHY IS THIS RELEVANT?

Most mixtures will NOTburn so far away fromstoichiometric

Often called Flammability Limit

Highly pressure dependent

Increased pressure, increasedflammability limit

Requirements for combustion, roughly f> 0.8

Gas turbine can NOToperate at (or even near)stoichiometric levels

Temperatures (adiabatic flame temperatures)associated with stoichiometric combustion areway too hot for turbine

Fixed Tt4implies roughly f< 0.5

What do we do?

Burn (keep combustion going) near f=1 withsome of compressor exit air

Then mix very hot gases with remaining air tolower temperature for turbine


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SOLUTION: BURNING REGIONS

Air

Compressor

Turbine

f ~ 1.0

T>2000 K

f~0.3

Primary

Zone


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COMBUSTOR ZONES: MORE DETAILS

1. Primary Zone

Anchors Flame

Provides sufficient time, mixing, temperature for complete oxidation of fuel Equivalence ratio near f=1

2. Intermediate (Secondary Zone)

Low altitudeoperation (higher pressures in combustor)

Recover dissociation losses (primarily CO CO2) and Soot Oxidation

Complete burning of anything left over from primary due to poor mixing High altitudeoperation (lower pressures in combustor)

Low pressure implies slower rate of reaction in primary zone

Serves basically as an extension of primary zone (increased tres)

L/D ~ 0.7

3. Dilution Zone (critical to durability of turbine) Mix in air to lower temperature to acceptable value for turbine

Tailor temperature profile (low at root and tip, high in middle)

Uses about 20-40% of total ingested core mass flow

L/D ~ 1.5-1.8


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COMBUSTOR DESIGN

Combustion efficiency, hb= Actual Enthalpy Rise / Ideal Enthalpy Rise

h=heat of reaction (sometimes designated as QR) = 43,400 KJ/Kg

34 tt

Rb

P TTQ

cf

h

General Observations:1. hb as p and T (because of dependency of reaction rate)

2. hb as Mach number (decrease in residence time)

3. hb as fuel/air ratio

Assuming that the fuel-to-air ratio is small

hm

TmTmmc

f

tatfaP

b

34h


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COMBUSTOR TYPES (Lefebvre)

Single Can

Tubular

or Multi-Can

Tuboannular

Can-Annular

Annular


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COMBUSTOR TYPES (Lefebvre)


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EXAMPLES

CAN-TYPERolls-Royce Dart

ANNULAR-TYPEGeneral Electric T58


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EXAMPLES

CAN-ANNULAR-TYPE

Rolls-Royce Tyne


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CHEMICAL EMISSIONS

Aircraft deposit combustion products at high altitudes, into upper troposphere and

lower stratosphere (25,000 to 50,000 feet)

Combustion products deposited there have long residence times, enhancing impact NOx suspected to contribute to toxic ozone production

Goal: NOx emission level to no-ozone-impact levels during cruise


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AFTERBURNER (AUGMENTER)

Spray in more fuel to use up more oxygen

Main combustion can not use all air

Exit Mach number stays same (choked Mexit= 1) Temp

Speed of sound

Velocity = M*a

Therefore Thrust

Penalty:

Pressure is lower so thermodynamic efficiency is poor

Propulsive efficiency is reduced (but dont really care in this application)

As turbine inlet temperature keeps increasing less oxygen downstream for AB and

usefulness decreases Requirements

VERY lightweight

Stable and startable

Durable and efficient


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RELATIVE LENGTH OF AFTERBURNER

Why is AB so much longer than primary combustor?

Pressure is so low in AB that they need to be very long (and heavy)

Reaction rate ~ pn(n~2 for mixed gas collision rate)

J79 (F4, F104, B58)

Combustor Afterburner


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INTRA-TURBINE BURNING


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BURNER-TURBINE-BURNER (ITB) CONCEPTS

Improve gas turbine engine performance using an interstage turbine burner (ITB)

With a higher specific thrust engine will be smaller and lighter

Increasing payload

Reduce CO2emissions

Reduce NOxemissions by reducing peak flame temperature

Initially locate ITB in transition duct between high pressure turbine (HTP) and low

pressure turbine (LPT)

Conventional

Intra Turbine Burner (schematic only)


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SIEMENS WESTINGHOUSE ITB CONCEPT

Tt4


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UNDERSTANDING BENEFIT FROM CYCLE ANALYSISFrom Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by

Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001

Conventional Intra Turbine Burner


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2 additional burners 5 additional burners




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Continuous burning in turbine



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4261 Combustors