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Calculation of HF Eigenmodes in Liquid Rocket Combustion … · 2016. 11. 18. · Rocket Engines...
Transcript of Calculation of HF Eigenmodes in Liquid Rocket Combustion … · 2016. 11. 18. · Rocket Engines...
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Calculation of HF Eigenmodes in Liquid
Rocket Combustion Chambers
COMSOL Application
R. Kaess, J. Braun, S. Koeglmeier
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Thermo-Acoustic Combustion Instability in Rocket Engines
Comsol Calulation
Summary
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1 Thermo-Acoustic Combustion Instability in Rocket Engines
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Rocket Engines
How does it work?
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http://www.nasa.gov/images/content/148709main_d4_testing_08.jpg, (accessed on 07.09.2016)
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Rocket Engines
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http://www.nasa.gov/images/content/148709main_d4_testing_08.jpg, (accessed on 07.09.2016)
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Rocket Engines
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http://www.nasa.gov/images/content/148709main_d4_testing_08.jpg, (accessed on 07.09.2016)
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Rocket Engines
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http://www.nasa.gov/images/content/148709main_d4_testing_08.jpg, (accessed on 07.09.2016)
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Rocket Engines
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http://www.nasa.gov/images/content/148709main_d4_testing_08.jpg, (accessed on 07.09.2016)
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Rocket Engines
Flowrates and Pressure Levels
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Image: Michel Bobillier
Jet d‘eau, Geneve: 500l/s, 16bar, 140m
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Rocket Engines
Thermal Power
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Image: Michel Bobillier
Neckarwestheim: 1400 MW
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Rocket Engines
Temperatures
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Image: Michel Bobillier
2 x Melting Point of Steel
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Rocket Engines
Velocities
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Approx. 4000m/s 4 x A12 maximum speed
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Rocket Engines
Need for a safe, smooth, robust mode of operation
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Image: Michel Bobillier
• Application at the physical limits
• Large quantities of energy present
• Limited margins against failures
• Small deviations can have catastrophic effects
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1 / High Frequency Combustion Instability
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Combustion chamber acoustics conserve
pressure and velocity fluctuations
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1 / High Frequency Combustion Instability
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Combustion chamber acoustics conserve
pressure and velocity fluctuations
Combustion process is sensitive to pressure and
velocity fluctuations
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1 / High Frequency Combustion Instability
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Combustion chamber acoustics conserve
pressure and velocity fluctuations
Combustion process is sensitive to pressure and
velocity fluctuations
Fluctuating combustion induces heat release fluctuations and pressure fluctuations
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1 / High Frequency Combustion Instability
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Combustion instability oscillating in the combustion chambers acoustic eigenmodes
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1 / High Frequency Combustion Instability
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Influence parameters and counter-measures:
• Combustion chamber geometry and hot gas determine the eigenfrequencies
• Propellant combination has tremendous influence on the risk of instabilities
• Proper design of Injection system reduces risk
• Stabilization devices can be used
Knowledge of the acoustic eigenfrequencies is required
Baffles:protect the flame and provide damping
Acoustic absorbers/liners: provide damping at their tuning frequency
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1 / Can COMSOL predict the combustion chamber eigenfrequencies?
Evaluation of off-the-shelf-COMSOL modules w/o modifications
Features to be considered
• Presence of mean flow (non-negligible Ma-Number)
• Presence of gradients
• (Presence of absorbers or baffles)
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Stepwise approach with increasing complexity
The results have been de-dimensionalized for confidentiality reasons
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2 Comsol Calculations
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2 / Generic Test to check set-up
Eigenfrequencies of a 2D Duct with and w/o mean flow
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Eigenmode
Analyticsolution
Mach 0 0.2
L1 114.3 109.8
L2 228.7 219.5
L3 343.0 329.3
T1L1 361.6
T1L2 412.2
1.5m
0.5
m
c=343m/s
Mean Flow
Analytic solution:
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2 / Generic Test to Check Set-Up
Eigenfrequencies of a 2D Duct with and w/o mean flow
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Eigenmode
Analyticsolution
COMSOL(Step: 0.25Hz)
Mach 0 0.2 0 0.2
L1 114.3 109.8 114.3 109.8
L2 228.7 219.5 228.8 219.5
L3 343.0 329.3 343.3 329.5
T1L1 361.6 361.8 353.5
T1L2 412.2 412.5 401.5
Physics module: Linear Euler, Frequency Domain
Acoustic source: Domain Source
Study: Frequency DomainDirect solver: MUMPS
Mesh: Quadrilateral mapped meshMax. element length: 0.025m
Set up is correct
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2 / Rocket Engine Test Case
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Physics module: Linear Euler, Frequency Domain
Boundary Conditions:Chamber wall and inlet: Rigid wallChamber outlet: Asymptotic far-field radiation + Outflow boundary
Acoustic source: Domain Source
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2 / Quiscent medium / uniform flow field
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• Zero Mach number and homogeneous medium• Simple, constant axial velocity (unphysical at nozzle)
Frequency shift and additional damping when mean flow is present
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2 / Physical mean flow of hot gas
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• COMSOL domain is truncated to keep Ma number below 1
Damping increases, double peak reduces to single peak, some spurious oscillations
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3 Summary
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3 / Titre 1
• COMSOL has been applied to calculate the acoustic properties of confined domains
• Cases with and without mean flow have been chosen
• The eigenfrequencies can be calculated using a domain source and a frequency sweep
• Mean flow increases damping and promotes spurious oscillations
• Frequency shift for mean flow can be observed
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