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Transcript of 1 Effects of radiative emission and absorption on the propagation and extinction of premixed gas...
1
Effects of radiative Effects of radiative emission and absorption on emission and absorption on
the propagation and the propagation and extinction of premixed gas extinction of premixed gas
flamesflamesYiguang Ju and Goro Masuya
Department of Aeronautics & Space Engineering
Tohoku University, Aoba-ku, Sendai 980, Japan
Paul D. RonneyDepartment of Aerospace & Mechanical
EngineeringUniversity of Southern California
Los Angeles, CA 90089-1453
Paper No. P024, 27th Symposium (International) on Combustion, Boulder,
CO, August 5, 1998
PDR acknowledges support from NASA-Lewis
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Background Microgravity experiments show importance of radiative
loss on flammability & extinction limits when flame stretch, conductive loss, buoyant convection eliminated – experiments consistent with theoretical predictions of Burning velocity at limit (SL,lim) Flame temperature at limit Loss rates in burned gases
…but is radiation a fundamental extinction mechanism? Reabsorption expected in large, "optically thick” systems
Theory (Joulin & Deshaies, 1986) & experiment (Abbud-Madrid & Ronney, 1993) with emitting/absorbing blackbody particles Net heat losses decrease (theoretically to zero) Burning velocities (SL) increase Flammability limits widen (theoretically no limit)
… but gases, unlike solid particles, emit & absorb only in narrow spectral bands - what will happen?
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Background (continued) Objectives
Model premixed-gas flames computationally with detailed radiative emission-absorption effects
Compare results to experiments & theoretical predictions
Practical applications Combustion at high pressures and in large
furnaces• IC engines: 40 atm - Planck mean absorption
length (LP) ≈ 4 cm for combustion products ≈ cylinder size
• Atmospheric-pressure furnaces - LP ≈ 1.6 m - comparable to boiler dimensions
Exhaust-gas or flue-gas recirculation - absorbing CO2 & H2O present in unburned mixture - reduces LP of reactants & increases reabsorption effects
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Numerical model Steady planar 1D energy & species conservation
equations CHEMKIN with pseudo-arclength continuation 18-species, 58-step CH4 oxidation mechanism (Kee et al.) Boundary conditions
Upstream - T = 300K, fresh mixture composition, inflow velocity SL at x = L1 = -30 cm
Downstream - zero gradients of temperature & composition at x = L2 = 400 cm
Radiation model CO2, H2O and CO Wavenumbers () 150 - 9300 cm-1, 25 cm-1 resolution Statistical Narrow-Band model with exponential-tailed
inverse line strength distribution S6 discrete ordinates & Gaussian quadrature 300K black walls at upstream & downstream
boundaries Mixtures CH4 + {0.21O2+(0.79-)N2+ CO2} - substitute
CO2 for N2 in “air” to assess effect of absorbing ambient
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Results - flame structure Adiabatic flame (no radiation)
The usual behavior Optically-thin
Volumetric loss always positive Maximum T < adiabatic T decreases “rapidly” in burned gases “Small” preheat convection-diffusion zone - similar to
adiabatic flame With reabsorption
Volumetric loss negative in reactants - indicates net heat transfer from products to reactants via reabsorption
Maximum T > adiabatic due to radiative preheating - analogous to Weinberg’s “Swiss roll” burner with heat recirculation
T decreases “slowly” in burned gases - heat loss reduced
“Small” preheat convection-diffusion zone PLUS “Huge” convection-radiation preheat zone
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Flame structures
Flame zone detail Radiation zones (large scale)
Mixture: CH4 in “air”, 1 atm, equivalence ratio (): 0.70; = 0.30 (“air” = 0.21 O2 + .49 N2 + .30 CO2)
0
500
1000
1500
2000
2500
3000
-1 10 7
-5 10 6
0
5 10 6
-0.5 0 0.5 1
adiabaticreabsorptionoptically thin
q (reabsorption)q (optically thin)
Spatial coordinate (cm)
Reabsorptionzone (negativeloss region)
Convective-loss zone(optically thin)
400
800
1200
1600
2000
-30 -20 -10 0 10 20 30 40
Spatial coordinate (cm)
Reabsorbing flame: convective-
radiative zone
Reabsorbing flame: max. T > adiabatic flame
Optically thin: rapid downstream loss
Reabsorbing flame: slow downstream loss
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Radiation effects on burning velocity (SL) CH4-air ( = 0)
Minor differences between reabsorption & optically-thin ... but SL,lim 25% lower with reabsorption; since SL,lim ~
(radiative loss)1/2, if net loss halved, then SL,lim should be 1 - 1/√2 = 29% lower with reabsorption
SL,lim/SL,ad ≈ 0.6 for both optically-thin and reabsorption models - close to theoretical prediction (e-1/2)
Interpretation: reabsorption eliminates downstream heat loss, no effect on upstream loss (no absorbers upstream); classical quenching mechanism still applies
= 0.30 (38% of N2 replaced by CO2) Massive effect of reabsorption SL much higher with reabsorption than with no
radiation! Lean limit much leaner ( = 0.44) than with optically-
thin radiation ( = 0.68)
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Comparisons of burning velocities
= 0 (no CO2 in ambient) = 0.30
Note that without CO2 (left) SL & peak temperatures of reabsorbing flames are slightly lower than non-radiating flames, but with CO2 (right), SL & T are much higher with reabsorption. Optically thin always has lowest SL & T, with or without CO2
Note also that all experiments lie below predictions - are published chemical mechanisms accurate for very lean mixtures?
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Mechanisms of extinction limits Why do limits exist even
when reabsorption effects are considered and the ambient mixture includes absorbers? Spectra of product H2O
different from CO2 (Mechanism I)
Spectra broader at high T than low T (Mechanism II)
Radiation reaches upstream boundary due to “gaps” in spectra - product radiation that cannot be absorbed upstream
0.1
1
10
100
1000 1300K300K
CO2
0.1
1
10
100
1000 10000
1300K300K
Wavenumber (cm -1 )
H2O
Absorption spectra of CO2 & H2O at 300K & 1300K
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Mechanisms of limits (continued) Flux at upstream boundary
shows spectral regions where radiation can escape due to Mechanisms I and II - “gaps” due to mismatch between radiation emitted at the flame front and that which can be absorbed by the reactants
Depends on “discontinuity” (as seen by radiation) in T and composition at flame front - doesn’t apply to downstream radiation because T gradient is small
Behavior cannot be predicted via simple mean absorption coefficients - critically dependent on compositional & temperature dependence of spectra
Spectrally-resolved radiative flux at upstream boundary for a reabsorbing flame
(πIb = maximum possible flux)
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Effect of upstream domain length (L1) on limit composition (o) & SL for reabsorbing flames. With-out reabsorption, o = 0.68, thus reabsorption is very important even for the smallest L1 shown
Effect of domain size Limit & SL,lim decreases
as upstream domain length (L1) increases - less net heat loss
Significant reabsorption effects seen at L1 = 1 cm even though LP ≈ 18.5 cm because of existence of spectral regions with L() ≈ 0.025 cm-atm (!)
L1 > 100 cm required for domain-independent results due to band “wings” with small L()
Downstream domain length (L2) has little effect due to small gradients & nearly complete downstream absorption
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Effect of CO2 substitution for N2 on SL
Effect of (CO2 substitution level)
= 1.0: little effect of radiation; = 0.5: dominant effect - why? (1) = 0.5: close to
radiative extinction limit - large benefit of decreased heat loss due to reabsorption by CO2
(2) = 0.5: much larger Boltzman number (defined below) (B) (≈127) than = 1.0 (≈11.3); B ~ potential for radiative preheating to increase SL
Note with reabsorption, only 1% CO2 addition nearly doubles SL due to much lower net heat loss!
B ≡ . .Blackbody radiative heat flux at ad flame temp
Convective enthalpy flux through flame front ∂ln(SL)∂ ln(Tad )
=σ Tad
4 − To4
( )
ρoS L,adCPTad
β
2; β ≡
E
RTad
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Effect of CO2 substitution on SL,lim/SL,adiabatic
Effect of (continued)
Limit mixture much leaner with reabsorption than optically thin Limit mixture decreases with CO2 addition even though CP,CO2 > CP,N2
SL,lim/SL,ad always ≈ e-1/2 for optically thin, in agreement with theory SL,lim/SL,ad up to ≈ 20 with reabsorption!
Effect of CO2 substitution on flammability limit composition
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Effect of different radiation models on SL
and comparison to theory
Comparison to analytic theory Joulin & Deshaies (1986) -
analytical theory
Comparison to computation - poor Slightly better without H2O
radiation (mechanism (I) suppressed)
Slightly better still without T broadening (mechanism (II) suppressed, nearly adiabatic flame)
Good agreement when L() = LP = constant - emission & absorption across entire spectrum rather than just certain narrow bands.
Note drastic differences between last two cases, even though both have no net heat loss and have the same Planck mean absorption lengths!
SLSL,ad
⎛
⎝ ⎜ ⎞
⎠ ⎟ln
SLSL ,ad
⎛
⎝ ⎜ ⎞
⎠ ⎟ =B
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Comparison of computed results to experiments where reabsorption effects may have been important
Comparison with experiment No directly comparable expts., BUT... Zhu, Egolfopoulos, Law (1988)
CH4 + (0.21O2 + 0.79 CO2) ( = 0.79) Counterflow twin flames, extrapolated
to zero strain L1 = L2 ≈ 0.35 cm chosen since 0.7 cm
from nozzle to stagnation plane No solutions for adiabatic flame or
optically-thin radiation (!) Moderate agreement with reabsorption
Abbud-Madrid & Ronney (1990) (CH4 + 4O2) + CO2 Expanding spherical flame at µg L1 = L2 ≈ 6 cm chosen (≈ flame radius) Optically-thin model over-predicts limit
fuel conc. & SL,lim Reabsorption model underpredicts limit
fuel conc. but SL,lim well predicted - net loss correctly calculated
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Conclusions Reabsorption increases SL & extends limits, even in
spectrally radiating gases Two loss mechanisms cause limits even with reabsorption
(I) Mismatch between spectra of reactants & products (II) Temperature broadening of spectra
Results qualitatively & sometimes quantitatively consistent with theory & experiments
Behavior cannot be predicted using mean absorption coefficients!
Can be important in practical systems Future work
“Flame balls” in H2-O2-CO2 & H2-O2-SF6 mixtures - comparison of computation & experiment indicates reabsorption important
Spherically expanding flames Elevated pressures - pressure (collisional) broadening
would lead to even greater reabsorption effects Exhaust-gas & flue-gas recirculation