Download - MEL341 Burners
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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