L08 - Combustion Instabilities

Phenomenology: cyclic loads

Unwanted pressure oscillations in the combustion chamber

Low frequency, bulk oscillations

• Thrust oscillations: control diffuculties

• Structural oscillations: accelerated fatigue

High frequency, acoustic pressure oscillations

• Pressure amplitudes > 10% of Mean chamber pressure

• Velocity amplitudes > convective, injection speeds

Phenomenology: classification

Low frequency instabilities

High frequency instabilities

Phenomenology: classification

Low frequency instabilities

• Characteristic wavelenght > CC dimensions

• Chugging/buzzing

• Pogo

(Generally preventable with modern methos)

Phenomenology: classification

High frequency instabilities

High frequency/ acoustic instabilities > 1000hz

Screeching/screaming

Least predictable, Highly destructive

Acoustics in combustors: eigenmodes of a cylinder

Assumptions of linear acoustics

Small amplitude

No viscosity

Quiescent conditions(no mean flow)

Homogeneous property distribution

Acoustics in combustors: eigenmodes of a cylinder

Eigenmodes of a cylindrical volume

Frequencies dependent on chamber geometry and gas properties

Longitudinal/transverse/combined modes

Driving mechanisms

Feedback cycle

Rayleigh Criterion

Combustion-Acoustic Coupling

Driving mechanisms: feedback cycle

Feedback coupling between energy release and pressure

Energy source is heat from combustion

Oscillating pressure defined by acoustics

Driving mechanisms: Rayleigh criterion

Periodic heat release amplifies in-phase pressure oscillations

Condition for growth (‘Rayleigh criterion’)

Driving mechanisms: combustion-acoustic coupling

Interaction of acoustic waves with spray flame

Damping elements

Baffles

Absorbers

Damping elements: baffles

Break modal symmetry

Suppress rotating modes

Dissipate acoustic energy through heat transfer and vortex shedding from edges

What are the drawbacks of damping element: baffles

Complex design

Poor predictability

Performance loss

Damping elements: absorbers

Helmholtz resonator

Quarter-wave absorber

Viscous and thermal dissipation along the walls

What are the drawbacks of damping element: absorbers

Complex design

Narrowband influence

MODELLING

low-order modelling

acoustics in combustion chambers

Modelling: low-order modelling

Introducing the Crocco (1951) n-τ model:

𝑛: interaction index

𝜏: time lag

(acoustic perturbation leads to heat release after a time delay )

• Low-order models provide fast, predictive capability for a specific configuration

• The time delay must be determined experimentally (or numerically with high-fidelity modelling)

• Difficult to scale to other systems!

Modelling: low-order modelling

Challenge

Modelling: acoustics in combustion chambers

Accounting for ‚real‘ effects

• Non-quiescent media

• Acoustic boundary conditions