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Department of Mechanical Engineering, Indian Institute of Technology Delhi

MEL 341: Gas Dynamics and

Propulsion

Amit Gupta

Assistant Professor

Department of Mechanical EngineeringIndian Institute of Technology Delhi

2nd Semester 2011-2012

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Essential Considerations for Burner Design

Complete combustion

Minimal total pressure loss

Combustion should be completed in burner

Easy ignition and high entrance pressure

Long life Uniform exit temperature

No internal hot spots

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Essential Considerations for Burner Design

Stable flame over range of mass flows, speeds,

other operating conditions

Flame not prone to flame out

Should have minimal volume

Overall, design of a combustor requires more

empiricism and testing compared to

compressor and turbine

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Geometry Considerations

Good mixture of fuel-to-air required along burner

axis (in primary burner zone, f~0.08)

Proper temperature must be achieved to sustain

burn Good turbulence is required

Low pressure loss is desired

Sufficient time for each particle to burn

completely

fuelreactants ignitionT T>

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Geometry Considerations

If gas velocity > flame speed, burner will flame out

Thus, large size burners required.If large size not possible, flame stabilizers used to

set up recirculation wakes

Stresses on turbine blades much higher than nozzle inlet temperature of turbine should be lower than

that of nozzle in case afterburner is used

Flame speed: velocity at which flame moves in

quiescent and uniform mixture of fuel and air

Laminar ~ 0.3 0.75 m/s; turbulent ~ 18 30 m/s

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Flame Stabilization

Source: Turns, 2nd ed. (2000)

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Flame Stabilization

Source: Turns, 2nd ed. (2000)

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Primary Combustors

Three kinds:

1) Can (or tubular)

2) Annular

3) Cannular (or can-annular)

Typically all are made of Ni-based alloys.

Ceramic materials are also popular

used on early engines

used on modernengines

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Can Burner

Primary air swirled to increase turbulence level

Secondary air bounded by outer liner (acts as heat

shield); gradually fed into combustion zone through holes

to force air away from walls

Source: Flack (2005)

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Can Burners

Self-contained

cans evenly

spacedcircumferentially

(7-14 cans)

Source: http://www.leitemlane.com/jetoperation.htm

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Annular Burner

Air enters burning

zone both from inner

and outer diameters

Source: http://www.leitemlane.com/jetoperation.htm

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Cannular Burner

Combination of can and

annular burner

Cans circumferentiallyconnected to minimize

variation in pressure

Connecting tubes allowfor flow between cans

Source: http://www.leitemlane.com/jetoperation.htm

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Advantages/Disadvantages

Can burner:

1) Fuel-to-air ratio easily controlled circumferentially

2) If particular burner malfunctions, only that burnerneed be replaced

3) Small diameter, good survivability

4) Heavier

5) High pressure drops (~7%)

6) Each can must have its own igniter

7) Flow in turbine less uniform in temperature

RARELY USED IN MODERN ENGINES

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Advantages/Disadvantages

Annular burner:

1) Simplest, most compact, lightest

2) Lowest pressure drop with good mixing and highefficiencies (~5%)

3) Controlling circumferential variation of temperature

most difficult4) Not mechanically rugged

5) Hardest to service

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Advantages/Disadvantages

Cannular burner:

1) Smaller cross-sectional area, less weight

2) Fewer ignition systems required

3) Intermediate pressure drop (~6%)

4) Replacing one component more expensive than a

can burner

5) Thermal stresses due to burner connectivity

USED IN TUBOJETS AND TURBOFANS

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Ignition and Engine Starting

Compressor=OFF high-pressure air required by

combustor is not present.

First step is to rotate the compressorDifferent techniques are:

1) High-torque DC motor to spin the shaft in small engines

2) For larger engines, stored compressed air or high-pressure air from other source used to drive air turbine

which rotates engine shaft through clutch

3) Small auxiliary gas turbines used in few larger engines4) Hydraulic motors (driven from ground source) drive the

shaft

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Afterburner

Primary combustor burns only 25% of air (well

below the stoichiometric condition)

Design fairly simple as these are rarely used(only at takeoff, climb, sudden accelerations,

max. speed bursts)

Permanently installed, and hence cause drop in

total pressure

Not installed with turbulence generators lowerburn efficiencies

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Department of Mechanical Engineering, Indian Institute of Technology Delhi

Afterburner

Source: http://www.leitemlane.com/jetoperation.htm