quemador swirl boquilla
TRANSCRIPT
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Low-Swirl Flame Stabilization Methodfor Lean Premixed Turbulent Flames
and Its Adaptation toHeating and Power Equipment
Robert K. Cheng
Senior Scientist
Leader, Combustion Technologies Group
Environmental Energy Technologies Division
Lawrence Berkeley National Laboratory
Presentation at ABMA Mid-Winter Meeting Jan. 18, 2004
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Acknowledgement
Sponsors
DOE-Basic Energy Sciences, Chemical Sciences
DOE-Basic Energy Sciences, Laboratory Technology Research
California Institute of Energy Efficiency/SoCalGas
DOE-EERE, Office of Industrial Technology
DOE-EERE, Distributed Energy Resources
Collaborators
D. Yegian, D. Littlejohn, G. Hubbard, K. Hom & I. Shepherd (LBNL)
J. Rafter & C. Taylor (Maxon), K. O. Smith (Solar Turbines)
C. Castildini (CMC Eng.), C. Benson (TIAX),M. Miyasato, V. McDonell, R. Hack & G. S. Samuelsen (UC Irvine),
H. Rieher (Industrial Combustion)
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Modes of Gaseous Combustion
Excess fuel + Air
AirAir
NOx
Partially Premixed Flame
two reaction zones
Fuel +
Excess air
NOx
Lean Premixed Flamewave-like flame front
Fuel
Air Air
NOx
Diffusion Flame
controlled by mixing
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Turbulent Combustion as a
Fundamental Research Problem
Non-premixed (dif fusion) flames
Turbulent and molecular mixing control combustion rates,efficiency and pollutant formation
Reactions occur at stoichiometric contours passive toturbulence
Combustion products diffuse into fuel and oxidizer streams
Reaction rate models expressed in terms of speciesconcentrations
Premixed flames Self propagating flame front separate reactants from products
Flame front exhibits wave behavior and generates significantfeedback to turbulent field through the pressure field
Reaction rate models expressed in terms of flame speed
No unified theory due to differences in the predominantphysical processes of non-premixed and premixed flames
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LBNLs Basic Research Focuses on
Gaseous Premixed Turbulent Flames
Theoretical Interest
Turbulence intensity and sizes of eddies
control burning rate, power density and
flame stability Technological Interest
Reduction of NOx emissions through lean
combustion Programmatic objectives
Elucidate turbulence/flame interactions
processes
Build an experimental foundation to
advance combustion theories & models
Transfer scientific knowledge to practical
use
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LBNLs Basic Research Emphasizes
Combustion Fluid Mechanics
Approach: Laboratory investigations
and theoretical development toquantify flame turbulence
interactions
clean experiments to reveal andisolate various processes
systematic variation of combustion
and turbulence parameters
Goal: Support the development ofcomputational tools suitable for the
design of advanced combustion
systems
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Addressing Problems Relevant to
Lean Premixed Combustion SystemsIncomplete knowledge on flame behavior
fast burning, compact & intense flames turbulence effects on emissions
flame interaction with combustion chambers
Flame holders dictates performance restrict operating range (5:1 turn-down vs. 10:1 for
non-premixed systems)
impact fuel flexibility, costs, & durabilityFlame generated flow dynamics
noise and vibrations
flash-back and blow-out hazards
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Conventional Flame Holders
Pilot Flame
premixture premixture
products
Fla
me
fuel
V-gutter
premixture premixture
products
Fla
me
highly swirled
premixture
Bluff Body + Swirl
recirculating
products
Flame
Based on the theoretical principle of continuous ignition source providedby a pilot flame or in the hot recirculation zone behind the stabilizer
L Bl ff d Fl I t biliti
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Lean Blow-off and Flame Instabilities
Associated With Different Flame Holders
Are Barriers to Reaching Low-Emissions
NOx
Flame Temperature, air/fuel ratio
Pollutant
concentratio
ns
CO
Onset of flame
instability
Lean
blow-off
Flammability
limit
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Low-swirl Combustion Exploits
Aerodynamics to Overcome the
Barriers to Attaining Low-Emissions
Lean
blow-offof
conventionalburne
rs
Onsetofflameinstability
inconventionalburners
NOx
Flame Temperature,
Pollutantc
oncentration
s
CO
No flame oscillations prior to lean blow-off
almost at the theoretical flammabili ty limit
Flammability
limit
L i l C b i E l i
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Low-swirl Combustion Exploits
Aerodynamics to Overcome the
Barriers to Attaining Low-Emissions
NOx
Flame Temperature,
Pollutantc
oncentration
s
CO
No flame oscillations prior to lean blow-off
almost at the theoretical flammabili ty limit
Flammability
limit
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Premixed Flames Stabilized
by Low SwirlNovel concept discovered in 1991 at LBNL
Defies recirculation theory on f lame stabilization
Scientific Interest
Scientific background lacking for low-swirl flows
Challenging modeling problem
Excellent laboratory research tool
Technological Interest
Capability to support ultra-lean flames
Simple design
Patent awarded 1998
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Flame Holders Were Considered
Essential to Anchor
Lean Premixed Flames
Premixed flames requires a physical stabilizer so that it can anchor. This
flame is stabilized by a bluff body of about 1 cm diameter
Reactants
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Low-Swirl Eliminates the Need for a
Flame Holder
By introducing a very small amount of swirl air (swirl number S 0.6), this
video shows that the flame can self propagate without the bluff body
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Low-Swirl Flame Stabilization Exploits
Propagating Nature of Premixed Flames
Fuel/Air
mixture
Propagating against the divergent
flow, the flame settles where the
local velocity equals the flame speed
Small air jets swirl the perimeter of
the fuel/air mixture but leave the
center core flow undisturbed
Flow divergence (generated by low-swirl) above the burner tube is the
key element for flame stabilization
L Di ti Ch t i d
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Laser Diagnostics Characterized
Flame Stabilization Mechanism
Flow divergence provides a
much more stable mechanismfor lean flames than high swirl
flows or flame holders
Flame brush propagates atturbulent flame speed that
increases linearly with
turbulence intensity
flashback conditions predictable
Swirl intensity controls flame lift
off position r / d
x/d
-1 -0.5 0 0.5 1
0
0.5
1
1.5
2
2.5
1.0
0.8
0.6
0.4
0.2
0.1
0.0
-0.1
U / Uexit
_ _
c
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Scientific Studies
Using Jet-LSBs Exploiting LSBs capability to
supports premixed turbulent
flames under a wide range ofturbulence and mixture
conditions has helped to resolve
key scientific problems Investigate evolving turbulent
flame structures from low to
intense turbulence
Verify new theory on
classification of premixed
turbulent flames
Relate turbulent f lame speed to
combustion intensity
SWF29
SWF28
SWF26
Increas
eturbulence
These images of OH fluorescence
obtained by laser diagnostics are
the data for studying combustion
intensities and flame structures
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First Technology
Transfer ProjectSupported by
DOE-LTR (1994-1997)
CRADA with Teledyne-Laars
Adaptation to pool heaters
Design and test LSBs that meetsthe operational requirements of15 KW to 100 KW units
Sizes similar to laboratory LSBs
Non-modulated systems, noturndown requirement
Issues
need simpler design requiringonly one flow supply (no jets)
firing sideways or downwards toattain > 85% efficient
stable inside chamber
cannot compromise on energyefficiency
cost must be lower than Alzetaburner ($100/per unit)
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Vane-Swirler Developed for LSB
New design fundamentally different than that of conventional vane-swirler Open center channel allows a portion of flow to bypass swirl vanes
Angled guide vanes induce swirling motion in annulus
Screen balances pressure drops between swirl and center channel
Patent awarded in 1999
Vane-swirler
Rh
R
Screen
Premixture
Exit tube
Top view of patentedvane-swirler
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Development of Vane-Swirler
Relied on Laser Diagnostics
-40 -30 -20 -10 0 10 20 30 40-3
-2
-1
0
1
2
3
4
5
r (mm, Rh = 20.5)
Velocity
(m/s)
Jet Swirler
Vane SwirlerU (m/s) W (m/s)
Varied screens
blockage, vane
angle, and
inner tube
diameter
Measured
mean velocity
profiles and
compare withprofiles of jet-
swirler
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Vane-Swirler for LSB Can Be Made
From Simple and Low-Cost MaterialsVane-LSB produces
the same shape as Jet-
LSB
Lifted flame does not
transfer heat to burner
throat
Estimated fabricationcost for pool heaters