Modelling of UnSteady Combustion in Low Emission · PDF fileModelling of UnSteady Combustion in Low Emission Systems ... combustors as heat release very sensitive to ... engine which

Aeronautics Days 2006 / 19-21 June 2006 / Vienna Austria

MUSCLES

Modelling of UnSteady Combustion in Low Emission Systems

G4RD-CT-2002-00644R&T project within the 5th Framework program of the European Union

1 June 2002 to January 2006


Motivation

Lean burn combustors inherently prone to instabilities; large potential for amplification > rich-burn combustors as heat release very sensitive to equivalence ratio close to lean extinction

Practically, this limits NOx levels attainable as customers will not accept an environmentally friendly engine which is perceived to be less safe.

Need to fundamentally understand unstable combustion in order to design combustors which avoid these phenomena


Objectives

To investigate the mechanisms by which acoustic coupling can occur in combustors

To devise methods which capture the important mechanisms in a way which can be incorporated in CFD codes

To demonstrate that unsteady combustion phenomena can be predicted in both frequency and amplitude

Where possible to recommend design practices to reduce the impact of such unsteadiness

→ more fundamental understanding of unsteady phenomena


Project StructureWP1 management and exploitationWP2 Prediction of unsteady reacting flow and validation

• ITS – near blowout (LBO) study• EBI – mixing/reaction effects at LBO• IST – Radiation/vaporisation interaction• Uni Genoa - Modelling and experimental studies of prevaporisor

WP3 Advanced diagnostics in two-phase flow field• ONERA/CORIA/LEMTA develop unsteady 2 phase measurements.• Uni Naples Measurements of fluctuations of kerosene swirled flame

WP4 Analysis of pressure/acoustic waves• Cambridge – measure transfer functions in real combustor• LU – measure injector response, and aerodynamic response, • to a pressure wave.• CORIA/ EM2C - Influence of acoustic waves on spray vaporisation and

combustion • Uni Naples – characterisation of jet in crossflow

Plus industrial implementation and integration of methods (RR/MTU/SNECMA/Turbomeca/Avio)


Main AchievementsExperimental• Characterisation of behaviour of fuel injector subject to pressure fluctuations• Improved understanding of the design of premixers• Characterisation of jet in crossflow• Effects of radiative heat transfer on droplet evaporation quantifiedMeasurement techniques• Coupling of different laser based techniques to allow simultaneous measurements

of diameter and temperature, droplet temperature and gas vapour concentration and gas temperature and species concentration

• Generation of databases for model validationNumerical• Model capable of capturing lean blow-out• Methodology for the modelling and improvement of premix ducts• DNS database of droplet evaporation for model validation• New model for mixture fraction variance source term due to droplet evaporation


Highlights - Measurements of effect of pressure oscillations on spray formation

•Particle size unaffected by fluctuations•Large variations in droplet number density•Will lead to large variations in local mixture fraction and hence heat release, thus providing a feed back mechanism


Main Achievements – Measurements of LPP injector

Liquid fuel displacement within combustion chamber

Liquid fuel release from premixing duct exit lip

Instantaneous axial velocity distribution


Measurements of flow instabilities in combustor

Frequency (Hz)P

ower

Spe

ctra

lDen

sity

(u'2 /H

z)100 101 102 103 10410-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

N=1600rpmExcited

Frequency (Hz)

Pow

erS

pect

ralD

ensi

ty(u

'2 /Hz)

100 101 102 103 10410-7

10-6

10-5

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10-2

10-1

100

101

N=800rpmExcited

Frequency (Hz)

Pow

erS

pect

ralD

ensi

ty(u

'2 /Hz)

100 101 102 103 10410-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

N=800rpmNon- Excited


Main Achievements – Characterisation of jet in cross-flow

Averaged image Intensity FluctuationszD

= 2 .27 q 0.44 W eaero−0.012

xD

⎛⎝⎜

⎞⎠⎟

0 .367

Generalized trajectory


Highlights – simultaneous measurements of droplet temperatureand gas phase vapour concentration

Finj=12 kHzC=3.5

Finj=24 kHz C=2.2

Fluorescence Fluorescence SignalsSignals VapourVapour Phase ConcentrationPhase Concentration

Data Data ProcessingProcessing

LiquidLiquid PhasePhase

VapourVapour PhasePhase


Highlights - Evaluation of temperature gradients within combusting droplets in linear stream using two colors laser-induced fluorescence

-100 -50 0 50 100

-100

-80

-60

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0

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t= 5.3 ms

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t=6.9 ms

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t= 9.6 ms

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t = 8.6 ms

Droplet temperature evolves in 2 phases• Heating phase → wet bulb temperature• Constant temperature evaporation

Internal measurements show the presence of an internal recirculation, consistent with Hill vortex


Highlights - Extension of two colors LIF to polydisperse sprays

Thermocouple

Liquid reservoir

Pressurizedair

réservoirFlow heater

Sprayer

z

T injection : 50°CT ambient : 23°C

* amb

inj amb

T TT TT −

−=


Highlights – Prediction of Lean Blow-out

measured stability limits

Stability Diagram

Flame Wall Breakup (FWB)

Lean Blow Out (LBO)• Lack of a definite stability criterion, deducible from both, experiment and CFD

open problem, actual work at EBI beyond the scope of MUSCLES

Perspectives

• More exact modeling of radiative heat losses

coupling of extended JPDF-reaction-model with a radiation model by means

of the Monte-Carlo-Method within FW6-project: INTELLECT D.M.


Highlights - Limit-cycle prediction with low-order thermoacoustic model

Experiment80mm enclosure → no instability.350mm enclosure → 350 Hz mode, velocity amplitude 0.74.

Enclosure

pressure tappings

Plenum

Air

Loudspeakers

Air

Flowstraightener

Nonlinear flame transfer function measured for 80mm enclosure byforcing with loudspeakers at a range of amplitudes and frequencies.This is used in low-order model to give linear/nonlinear predictions.

Model results80mm enclosure → stable linear mode at 353 Hz.350mm enclosure → unstable linear mode at 342 Hz,

limit cycle at 357 Hz, velocity amplitude 0.80.


Highlights - First 3D DNS database of droplet transportDNS« Exact » resolution of the fullycompressible Navier-Stokes equations

analysis of physical phenomenadevelopment of models

Dispersed phase• Lagrangian modeling

• Evolution of the properties of each droplets(position, velocity, size, temperature)

→ validation database & new model for source term in mixture fraction variance due to droplet evaporation


Highlights – measurements and predictions of unsteadygaseous and two-phase jet flames

General objective: Evaluate the capability of numerical simulations to capture spray flames/acoustic modulations


GAS EM2CModulated flames (200 Hz)


ConclusionsImproved understanding of the underlying physics of

unsteady phenomena has been achieved → potential for major improvements in combustor design for instabilities

Important advances in experimental techniques made → improved understanding of droplet evaporation and hence modelling

Improved numerical methods have been developed Transfer of technology to industry has already begun;

further benefits will become manifest as the methodologies become embedded in design processes

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Modelling of UnSteady Combustion in Low Emission · PDF fileModelling of UnSteady Combustion in Low Emission Systems ... combustors as heat release very sensitive to ... engine which