The Application of Multipole Expansions to Unsteady Combustion Problems*
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Transcript of The Application of Multipole Expansions to Unsteady Combustion Problems*
The Application of Multipole Expansions to Unsteady Combustion Problems*
T. Lieuwen and B.T. Zinn
Schools of Aerospace and Mechanical Engineering
Georgia Institute of Technology
Atlanta, GA
*Research Supported by AGTSR and AFOSR; Dr. Dan Fant and Dr. Mitat Birkan, Contract Monitors
Background
• Behavior of unsteady combustion systems controlled by complex interactions that occur between the combustion process and acoustic waves– Combustion instabilities
– Pulse Combustors
– Combustion Noise
• Predicting or controlling the behavior of these systems requires capabilities for understanding and modeling these interactions
Approximate Models of Unsteady Combustion Systems
• Approximate techniques to analyze acoustics of combustion chambers are well developed– e.g., Galerkin based techniques
– Unified approaches allow consistent treatment of nonlinearities, mean flow effects, etc.
• Approximate techniques to analyze combustion process are not well developed– primarily ad-hoc approaches
Modeling Approaches
• Wave Equation with distributed heat source
LCombustion Region Interfaces
'q)1(i'pk'p 22
q1’….. qn’
Modeling Approaches
• “Concentrated Combustion” Approximation
Plane Acoustic Disturbances
p’2(t)-p’1(t)=0
v’2(t)-v’1(t)=Q’(t)
V
dV)t,x('q)t('Q
1 2
Modeling Approaches
• “Concentrated Combustion” Approximation– Peracchio, A.A., Proscia, W.M., ASME paper # 98-GT-269
(1998).
– Lieuwen, T., Zinn B.T., AIAA Paper # 98-0641 (1998).
– Dowling, A.P., J. Fluid Mech, 346:271-290 (1997).
– Fleifil, M. et al., Comb. and Flame, 106:487:510 (1996)
– Culick, F., Burnley, V., Swenson, G., J. Prop. Power, (1995)
– Many others
• Combustion Process treated as a single lumped parameter
How accurate is the Concentrated Combustion Approximation?
0
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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
L/
Err
or
(%
)
L
“Exact” Rate of Energy addition
Approximate Rate of Energy addition
“Exact” Rate of Energy addition
Error=
Concentrated Combustion Approximation
• Source of error - in general, information needed to describe combustion process - acoustic coupling is not contained in one lumped quantity, Q’
Can Approach be Generalized to Reduce the Error?
Distributed Heat Release
(i.e., infinite parameter model)
Single Lumped Parameter Model
Multiple Lumped Parameter Model
Increasing Model Complexity
Multipole Expansions
• Classical Acoustics - Sound Radiation often described in terms of “fundamental sources”
– “Monopole”
– “Dipole”
Multipole Expansions
• Farfield radiation from an arbitrary compact body can be decomposed into the radiation from these fundamental sources
Can decompose sound field of this body:
p’(farfield) = monopole component + (L/)* dipole component + higher order poles
L
Multipole Expansions
• Application to Ducted Problems:– Develop expression of form:
p’2-p’1=(L/)Q’1+(L/ )2Q’2 +...
v’2-v’1=Q’+(L/ )Q’1+(L/ ) 2Q’2 + ...
Multipole Expansions
• Result to O(L/):
p’2-p’1=(L/)Q’1
v’2-v’1=Q’
• Can generalize to arbitrary order in L/
V
1 dV)L
x('q'Q
Example
)(1
' ikxn
ikxn
tin eBeAe
cv
)(' ikxn
ikxn
tin eBeAep
• Equations describing plane waves:
• Boundary Conditions:
– x=0: p’=poe-it
– x=Lcomb: p’ =0
• Heat Release distribution: q’=sin(x/L)e-it
Plane Acoustic Disturbances
LCombustion Region Interfaces
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0.02
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0.06
0.08
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0.12
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0 0.2 0.4 0.6 0.8 1 1.2
x/Lcomb
|p'|
ExactValueO(1)
O(KL)
O(KL^2)
Combustion Region
Result - L/=0.02
O(L/)
O(L/)2
Result - L/=0.08
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0 0.2 0.4 0.6 0.8 1 1.2
x/Lcomb
|p'|
Exact Value
O(1)
O(KL)
O(KL^2)
Combustion Region
O(L/)
O(L/)2
Predicted Combustion Driving
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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
L/
Err
or
(%
)
O(1)
O(kL)
O(kL2)O(L/)
O(L/)2
O(1)
“Exact” Rate of Energy addition
Approximate Rate of Energy addition
“Exact” Rate of Energy addition
Error=
Summary and Conclusions
• Developed method that improves current capabilities for modeling combustion - acoustic interactions
• Method generalizes “Concentrated Combustion” approximation– Describe combustion process as a series of lumped
elements
Supporting Slides
Multipole Expansions
• where:
dS)x('vi4
S
)L
(OR
eD)
L(
R
eS)x('p
S
sn
2
ikRikR
Monopole term Dipole term Higher order poles
Multipole Expansions
Lumped Heat Source Model (i.e., concentrated
combustion)
Example of two parameter Heat Source Model
Q’(t)
Qa’(t)
Qb’(t)
Illustration of Method
• Energy Equation:
• Integrate Energy Equation over combustion region volume:
• Obtain: v’2-v’1=Q’+O(L/ )
VS
iidVp
p
iQdS ''
nv'
'q)1('vpt
'p
Illustration of Method
• Evaluating Volumetric Pressure term: – Take moment of momentum equation, integrate over combustor
volume, and integrate by parts
• Obtain more accurate energy balance:
• v’2-v’1- (L/)(p’2-p’1) =Q’+O(L/ )2
S
ii
V
ii
V
dSpdVxiN
dVp )''(1
' nxv
Illustration of Method
• In same way, improved momentum balance:
• p’2-p’1- (L/)(v’2-v’1) = (L/)Q’1+O(L/ )2
• where
V
1 dV)L
x('q'Q