Kinetic Modeling for the Insight into Combustionakrmys.com/research/symp161123.pdfAkira Miyoshi...
Transcript of Kinetic Modeling for the Insight into Combustionakrmys.com/research/symp161123.pdfAkira Miyoshi...
Akira Miyoshi
Kinetic Modeling for the Insight into Combustion
The 54th Symposium (Japanese) on Combustion (Sendai, Japan) Nov. 23, 2016
1
Department of Chemical Systems Engineering, University of Tokyo
燃焼をわかるための化学反応モデリング
2 Selecting Kinetic Models
hierarchy of combustion kinetic mechanisms
• Fuel ............................
• High-T & Low-T oxidation
• Sub-mechs (NOx, PM/PAH, etc. …)
Flame propagation- need High-T mech only (usually!)- small fuel dependency
Autoignition- require both High-T & Low-T- strong fuel dependency
• Reduction
Nearly automatic down to 1/5~1/10- ANSYS Reaction Workbench(license for which is now included in academic labo license)
Kinetics of Autoignition
3
4 Branched Chain Reactions
H + O2 OH + OO + H2 OH + H
OH + H2 H2O + H—————————
net: 2 H2 + O2 OH + H + H2O
H2-O2 - Typical branched chain reactions (Chain Explosion)
λmax > 0 ... diverging term
zyx
RRRRR
RRR
zyx
321
21
321
0
i
iii
ta esx
Chain carriers cannot be cancelled out
H OOH
branch
exponential growth exp(λmaxt )
• Selfmultiplication of chain carriers Autoacceleration Explosion
x = [H], y = [O], z = [OH],R1 = k1[O2], R2 = k2[H2], R3 = k3[H2]
Jacobian Matrixremains unchanged during the induction period where p, T and x are nearly constants
5 Autoignition Limit : λmax = 0
H + O2 OH + OH + O2 HO2
O + H2 OH + HOH + H2 H2O + H
zyx
RRRRR
RRRR
zyx
321
21
3241
0
λmax < 0slow rxn
λmax > 0explosion
i
iii
ta esx
Autoignition Limit of H2:O2 = 2:1 mixture
Jacobian Matrix
The 1st and 2nd explosion limits can be explained by nearly constant Jacobian matrix
mol
e fra
ctio
ns
mol
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ns6 Autoignition of Alkanes
Before Cool Flame: Chain-Reaction Controlled (chain explosion)
After Cool Flame: Thermally Controlled (thermal explosion)
Ignition mechanism depends on the condition !
λmax
mol
e fra
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ns
0 1 2t / ms
20 atm, 714.3 K(OH addition)
x0(OH)= 1 10 8 total ra
dicalRO 2
QOOHO 2QOOH
OH
R
HO 2
H 2O 2
total XOOH
KetOOH
Active Species Addition7
• x(OH) ≈ x(total radical) → independent of species added• x(OH) promptly decays to steady state → robust steady state• Additive effect diminishes exponentially with time
Low‐Temperature Oxidation Steady State8
OH R RO2 QOOH O2QOOH
OO
OOH
OOH
OO
OOOH
KetOOH
OO
O
+ OH
+ OH+ OH
+ HO2
+fuel +O2 +O2
HO2
subetantial‘branching’
‘propagation’only
• Reactions up to KetOOH formation are Straight Chain Reactions(No increase in active species)
• Only the decomposition of KetOOH is Chain Branching Reaction
substantial'branching'
Straight Chain ReactionsBranched
Chain Reactions
Livengood‐Wu (L‐W) Integral9
Assumptions: Autoignition occurs when the concentration of "pertinent reaction product
x" reaches the critical value (x)c. The rate of increase of (x)/(x)c is a function of T and p and equal to τ –1(T, p), reciprocal of the ignition delay time, τ(T, p), at this condition. This relation holds for all T and p during the piston compression and
expansion
J. C. Livengood and P. C. Wu, Proc. Combust. Inst., 5, 347–356 (1955).
ign
0 ,d1
t
pTt
2
1
1
(x)(x)c
ignition
2
1
1
(x)(x)c
ignition
2
(x)(x)c
1 ignition
condition-2
(x)(x)c
1
ignition1
condition-1
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RO 2
QOOHO 2Q
OOH
OHR
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ns
Integrand of Livengood‐Wu10
• Integrand is log(XOOH)or log(radical) for t < τc .
c> c <(a) τc dominant (b) Δτ dominant
c c
H. Ando, Y. Ohta, K. Kuwahara, and Y. Sakai, Rev. Automotive Eng., 30, 363–370 (2009).
• Integrand is heat for t > τc .
Autoignition of Gasoline/air (p‐dependence)11
• Shifts upper-right with decrease of p• Larger τ2 – τ1 separation at lower p
Difference between const‐p & const‐V12
• Essentially NO difference for τ1• τ2 is slightly shorter for const-V than const-p
Problem in Given‐V Simulation for RCM13
• Cool flame expansion cannot be reproduced in core-gas model• Deviation of from experiments where τ2 – τ1 separation is visible
1.1 1.2 1.3 1.4
10
100
1000 / TEOC(cgm)
exp.sim.
5.51 MPaCH4-C3H8(25%)
= 0.7
adiabatic core
EOCcompression cooling
timepres
sure
end ofcompression
Effect of the Change of Equivalence Ratio14
• Shifts upper-right with decrease of ϕ• Changes mostly τ2 but not so much of τ1
Flame Propagation
15
Laminar Flame Propagation16
0.5 1.0 1.50
10
20
30
40
50
• Laminar flame speed (S L0) decreases with pressure
and increases with temperature
Flame Propagation at High p and T17
puS
u02
1
100 cm s–1 under in-cylinder condition
18 Laminar Flame Structure
• In-cylinder flame thickness: 10~20 μm (require submicron resolution)
— Structure & flame thickness
Flame Propagation vs. Autoignition19
• Autoignition kinetics affects the calculation of laminar flame speedat high temperatures (artefact - NOT true laminar burning velocity)
Flame Propagation vs. Autoignition20
• Small difference between RON~100 S5H (high-octane) & RON~90 S5R (regular)
1.5 mm
xu ignxu =3.5 mm
Summary21
— Flame Propagation
• Autoignition competes under in-cylinder condition
— Kinetics of Autoignition
[before cool flame]• Chain carrier conc. increases exponentially with time, or
log x(radical) increases LINEARLY with time
[after cool flame]• Thermal ignition dominates