entropynoiseingasturbines - keele university...24of60 generaon"of"entropy"waves"...
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Entropy noise in gas turbines -‐ background and challenges
Dr Aimee S. Morgans KTH Workshop in aeroacous=cs in confined flows
of low mach number May 23rd 2014
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What is entropy noise? • Entropy noise is one (of two) components of combus=on noise • Unsteady combus=on always generates:
• Acous=c waves -‐ these propagate within the combustor.
acoustic
waves
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What is entropy noise? • Entropy noise is one (of two) components of combus=on noise • Unsteady combus=on always generates:
• Acous=c waves -‐ these propagate within the combustor.
• Entropy waves (hot/cold spots) -‐ these are “swept” downstream, advec=ng with the flow. In a non-‐accelera=ng flow they are “silent”.
entropy waves
acoustic
waves
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What is entropy noise? • Q: If entropy waves are silent, what is “entropy noise”?
• A: When the flow is accelerated, acous=c, entropy and vor=city waves all become coupled. Thus by accelera=ng entropy waves, new acous=c waves are generated.
noise
Q’ mean flow
source of directcombustion
noise
source of entropy
Regions of fluid with different densi=es undergo different volume contrac=ons
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Entropy noise in gas turbines
• Q: Are entropy waves accelerated in real combus=on systems?
• A: Yes! In a gas turbine, as the flow undergoes rapid accelera=on through the combustor exit and first turbine stage. This means that the entropy waves generate acous=c waves, called “entropy noise”, or “indirect combus=on noise”.
Stator exit shock waves from Mee et al. (1992)
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Entropy noise in gas turbines
Acousticwaves
Entropywaves
Turbine
blades
Directcombustion
noise
noiseEntropy
release rate Unsteady heat
from flame
unsteady flame
direct noiseturbine blades
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Entropy noise in gas turbines
Acousticwaves
Entropywaves
Turbine
blades
Directcombustion
noise
noiseEntropy
release rate Unsteady heat
from flame
entropy waves
direct noiseturbine blades
unsteady flame
(hot spots)
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Entropy noise in gas turbines
Acousticwaves
Entropywaves
Turbine
blades
Directcombustion
noise
noiseEntropy
release rate Unsteady heat
from flame
(indirect noise)
direct noiseturbine blades
unsteady flame
entropy noise
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Entropy noise in gas turbines
• Simplified movie of nozzle response to impulse in unsteady heat release rate.
• Note the different =me scales for returning to the flame of the “direct” and “indirect” noise.
Combustor flows are low Mach number (M < 0.2) to keep the flame adached. Thus the acous=c waves travel at M≈1, entropy waves travel at flow Mach number M << 1.
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Why is entropy noise important? • Aeroengine exhaust noise • Combus=on instability
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Why is entropy noise important? • Future aero-‐engine noise reduc=ons, as predicted by Snecma.
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Why is entropy noise important? • Aeroengine exhaust noise • Combus5on instability
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Entropy noise research Five “stages” of combus=on noise relevant to research: 1. Genera5on of entropy waves by flame 2. Advec=on of entropy waves within combustor 3. Genera=on of acous=c waves as entropy waves accelerated 4. Passage of acous=c waves through turbine (including subsequent accelera=on of entropy wave) 5. Effect of reflected component of entropy noise on combus=on instability
1
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy noise research Five “stages” of combus=on noise relevant to research: 1. Genera=on of entropy waves by flame 2. Advec5on of entropy waves within combustor 3. Genera=on of acous=c waves as entropy waves accelerated 4. Passage of acous=c waves through turbine (including subsequent accelera=on of entropy wave) 5. Effect of reflected component of entropy noise on combus=on instability
2
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy noise research Five “stages” of combus=on noise relevant to research: 1. Genera=on of entropy waves by flame 2. Advec=on of entropy waves within combustor 3. Genera5on of acous5c waves as entropy waves accelerated 4. Passage of acous=c waves through turbine (including subsequent accelera=on of entropy wave) 5. Effect of reflected component of entropy noise on combus=on instability
3
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy noise research Five “stages” of combus=on noise relevant to research: 1. Genera=on of entropy waves by flame 2. Advec=on of entropy waves within combustor 3. Genera=on of acous=c waves as entropy waves accelerated 4. Passage of acous5c waves through turbine (including subsequent accelera5on of entropy wave) 5. Effect of reflected component of entropy noise on combus=on instability
4
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy noise research Five “stages” of combus=on noise relevant to research: 1. Genera=on of entropy waves by flame 2. Advec=on of entropy waves within combustor 3. Genera=on of acous=c waves as entropy waves accelerated 4. Passage of acous=c waves through turbine (including subsequent accelera=on of entropy wave) 5. Effect of reflected component of entropy noise on combus5on instability
5
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy noise: history
• Ini=al research ac=vity in 1970’s e.g. Strahle (1972), Hassan (1974), Chiu & Summerfield (1974), Marble & Candel (1977), Cumpsty & Marble (1977)
• Research resurgence over last 6 years or so.
Lean premixed low NOX combustors have increased combus=on noise and propensity to combus=on instability.
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Genera=on of entropy waves
1
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Genera=on of entropy waves
Let’s remind ourselves of what entropy “is”. It’s defined through the standard thermodynamic rela=on
from which it follows that We are only interested in entropy fluctua=ons: seong and linearising gives that:
which is equivalent to
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Genera=on of entropy waves Consider the standard flow equa=ons of mo=on in a compressible fluid: Mass Momentum Energy Assume inviscid flow, ideal gas, cp and cv constant, and zero mean vor=city. Linearise about steady, uniform mean flow, so that etc.
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Genera=on of entropy waves This linearisa=on of the Euler equa=ons gives 3 equa=ons in pressure, entropy and vor=city. See Dowling and Stow (2003) for more detailed deriva=on
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Genera=on of entropy waves This linearisa=on of the Euler equa=ons gives 3 equa=ons in pressure, entropy and vor=city.
Note 1: In the presence of unsteady heat release q’, the pressure and entropy equa=ons are coupled. The vor=city equa=on is not coupled to the other two.
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Genera=on of entropy waves This linearisa=on of the Euler equa=ons gives 3 equa=ons in pressure, entropy and vor=city.
Note 2: If q’=0, we recover the “convected wave equa=on” for pressure, and find that entropy fluctua=ons simply advect with the mean flow. All 3 equa=ons are decoupled, meaning acous=c, entropy and vor=city waves are all independent (Chu & Kovasznay 1958)
L (speed c-‐u ) R (speed c+u )
s’ (speed u) ξ ’ (speed u)
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Genera=on of entropy waves This linearisa=on of the Euler equa=ons gives 3 equa=ons in pressure, entropy and vor=city.
Note 3: If = 0, we recover which is consistent with the “direct” combus=on noise work from the 1970’s (Strahle (1972), Hassan (1974), Chui & Summerfield (1974)). Good overview in “Modern Methods in Analy=cal Acous=cs” book.
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Genera=on of entropy waves This linearisa=on of the Euler equa=ons gives 3 equa=ons in pressure, entropy and vor=city. So unsteady heat release rate, q’, acts as a “source” of both acous=c waves and entropy waves. Once away from the heat release zone, the acous=c waves propagate at speed rela=ve to the flow, while the entropy waves advect at speed
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Genera=on of entropy waves A final note on what an entropy fluctua=on “is”: Away from the heat release, we know (i) that
or equivalently
and (ii) that entropy and pressure fluctua=ons are uncoupled (independent).
Note: this has led to the concept of “excess density”
e.g.Lighthill (52), Morfey (73)
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Genera=on of entropy waves A final note on what an entropy fluctua=on “is”: Away from the heat release, we know (i) that
or equivalently
and (ii) that entropy and pressure fluctua=ons are uncoupled (independent). For an entropy wave in isola=on (no acous=c waves)
In prac=ce, even in the presence of acous=c waves, the acous=c component of temperature and density fluctua=ons is small.
Thus an entropy fluctua=on (wave) is comprised of the (non-‐acous=c component of) the temperature or density fluctua=on.
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Advec=on of entropy waves
2
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Advec=on of entropy waves
In prac=ce, the mean flow responsible for entropy wave advec=on is not uniform (Sadelmayer 2004)! Even in a simple geometry like a channel, it varies due to boundary layers.
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Advec=on of entropy waves
In prac=ce, the mean flow responsible for entropy wave advec=on is not uniform (Sadelmayer 2004)! Even in a simple geometry like a channel, it varies due to boundary layers.
? How does this affect the entropy wave strength at the combustor exit (i.e. at accelera=on)?
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Advec=on of entropy waves
The equa=on (retaining non-‐linear and viscous terms) governing entropy wave transporta=on is:
Morgans, Goh & Dahan (2013) considered entropy wave advec=on in direct numerical simula=ons (DNS) of a simple turbulent channel flow.
Define: Dissipa=on: Ds’/Dt = -‐X2 Shear dispersion: reduc=on in wave strength due spa=al varia=ons in velocity field causing smearing
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Advec=on of entropy waves
The entropy wave “impulse response” between flame and channel exit was considered for the area-‐averaged wave strength.
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Advec=on of entropy waves
• “Smearing” and shortening of wave strength seen • Entropy wave suffered negligible dissipa=on. • It suffered shear dispersion due to the mean velocity profile • Turbulent flow fluctua=ons had lidle effect.
0 1 2 3 4 5 60
0.2
0.4
0.6
0.8
1
x/h
Rel
ativ
e m
agni
tude
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Advec=on of entropy waves
• “Smearing” and shortening of wave strength seen • Entropy wave suffered negligible dissipa=on. • It suffered shear dispersion due to the mean velocity profile • Turbulent flow fluctua=ons had lidle effect.
0 1 2 3 4 5 60
0.2
0.4
0.6
0.8
1
x/h
Rel
ativ
e m
agni
tude
A Gaussian transfer func=on model was shown to give good fit.
Shear dispersion for flow condi=ons of typical combustor much less than previously thought
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Entropy wave accelera=on
3
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy wave accelera=on
• The Marble & Candel (1977) paper considered what happens to entropy waves when they are accelerated through subsonic and supersonic nozzles
• Explains direct and indirect noise mechanisms
S
acoustic waveentropy wave
M <11P
P+
P
2M >1P
S
!+1
1 !2
2
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Entropy wave accelera=on
• Consider a choked nozzle with 1-‐D flow • Assume acous=cally compact nozzle (very low frequency approxima=on) • Nozzle then acts as discon=nuity, and linearised Euler equa=ons are quasi-‐
steady giving 3 jump condi=ons across nozzle:
mass flow rate stagna=on temperature entropy Jump condi=ons wriden in terms of acous=c & entropy wave strengths
S
acoustic waveentropy wave
M <11P
P+
P
2M >1P
S
!+1
1 !2
2
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Entropy wave accelera=on
• Combining these 3 jump condi=ons gives rise to
• This equa=on acts as a boundary condi=on on both upstream and downstream flow-‐field, rather than matching condi=on across nozzle.
• Allows reflec=on and transmission coefficients to be found for incident acous=c or entropy wave.
S
acoustic waveentropy wave
M <11P
P+
P
2M >1P
S
!+1
1 !2
2
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Entropy wave accelera=on
S
acoustic waveentropy wave
M <11P
P+
P
2M >1P
S
!+1
1 !2
2
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Entropy wave accelera=on
• Recent work has focussed on extending concepts to • higher frequencies for general geometries (Stow et al. 2002, Goh & Morgans
2011) • Any frequency for specific nozzle geometries (Moase et al. 2007, Giauque et al.
2012)
• Paper last year (Duran & Moreau 2013) extends to any frequency and any
geometry!
S
acoustic waveentropy wave
M <11P
P+
P
2M >1P
S
!+1
1 !2
2
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Entropy wave accelera=on
Duran & Moreau (2013) start with same quasi 1-‐D linearised Euler equa=ons as Marble & Candel (1977)
Aver a convenient change of variable and considering harmonic varia=ons, these can be recast as
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Entropy wave accelera=on This is solved using a Magnus expansion (Magnus 1954)
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Entropy wave accelera=on Comparing Duran & Moreau’s Magnus expansion method with experiments
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Entropy noise passage through turbine
4
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Entropy noise passage through turbine • Each blade row modelled as discon=nuity -‐ compact assump=on
(Cumpsty & Marble 1977) • Jump condi=ons applied across this e.g for stator row
• Similar for rotor row but rota=ng frame used (e.g. rothalpy instead of
enthalpy)
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Entropy noise passage through turbine Interac=ons of successive blade rows then follows from straighworward matrix mul=plica=on (Cumpsty & Marble 1977) Simple wave propaga=on between rows Cumpsty & Marble (1977) compare mesaured & predicted rear arc acous=c power for 3 real gas turbines Good match at low jet veloci=es (when jet noise low)
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Entropy noise passage through turbine
• Recent results from CERFACS (Toulouse) for 1 stator row followed by 1 rotor row
Reflected wave Transmided wave
___ analy=cal . , + numerical, simple entropy dispersion model (Leyko 2010)
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Effect of entropy noise on combus=on instability
5
direct
noiseentropy
noise
combustorturbine stages
flameacoustic
waves
entropywaves
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Effect of entropy noise on combustor stability
Consider simple model combustor with entropy waves present downstream of flame (Goh & Morgans 2013). Choked upstream and downstream. Assume plane acous=c waves, compact flame.
etc in region 1.
Similar in region 2, except density fluctua=on has entropic component:
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Effect of entropy noise on combustor stability
(Linearised) conserva=on of mass, momentum, energy applied across flame. Flame model required for closure
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Effect of entropy noise on combustor stability
Simple =me delay spread model for entropy wave dispersion
Area = 1 Area = k (<1)
flame outlet
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Effect of entropy noise on combustor stability
Find the combustor modes (frequency and growth rate) by numerically finding poles of relevant transfer func=on
Poles in white
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Effect of entropy noise on combustor stability
Entropy noise can (Goh & Morgans 2013): • Destabilise otherwise stable modes • Stabilise otherwise unstable modes • Cause mode switching • Cause an acous=c-‐entropic instability in which heat release rate amplitudes
stay small
Mode destabilisa=on as entropy waves increasingly accounted for (k increasing from 0 to 1)
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Effect of entropy noise on combustor stability
Entropy noise can (Goh & Morgans 2013): • Destabilise otherwise stable modes • Stabilise otherwise unstable modes • Cause mode switching • Cause an acous=c-‐entropic instability in which heat release rate amplitudes stay
small
Mode switching as entropy wave dispersion varies (k=1)
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Final note: entropy noise experiments
• Experiments on entropy noise complicated by difficul=es in measuring entropy waves – measuring temperature fluctua=ons at high speed a challenge!
Entropy wave generator DLR Berlin (Bake et. al 2009)
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Conclusions
• Entropy noise important for aero-‐engine noise and combus=on instability
• Generated by accelera=on of “hot-‐spots” / “cold-‐spots” resul=ng from unsteady heat release
• Entropy wave (i) genera=on, (ii) advec=on, (iii) accelera=on, (iv) passage through turbine and (v) effect on combus=on instability all ac=ve research areas!
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Thank you!