Measurements of Elementary Rate Constants,Ignition Delays and Species Histories in Shock Tubes
R. K. Hanson, D. F. DavidsonStanford University
1st International Workshop on Flame ChemistryJuly 28‐29, 2012
• Shock Tube/Laser Approach• Advances in Methodology• Elementary Reaction Rate Studies• Multi‐Species Time‐Histories
Stanford Shock Tubes
2
KineticsShock Tube 2
(30 atm)
KineticsShock Tube 1(30 atm)
AerosolShock Tube(10 atm)
HighPressureShock Tube
(500 atm)
New Constrained Reaction Volume (CRV) Facility2 Configurations
3
CRV‐1Gas‐Phase
Experimentsto 100 atm
CRV‐2Aerosol
Experimentsto 100 atm
ConstrainedReaction Volume
SlidingValve
Aerosol Generation Tank
Stanford Laser Diagnostics
4
Ultra‐fast lasers used to
extend UV tuning range
(2009)
First use of tunable dye lasers in shock tubes (1982)
New lasers allow simple
access to mid‐IR (2007‐10)
UltravioletCH3 216 nmNO 225 nmO2 227 nmHO2 230 nmOH 306 nmNH 336 nm
VisibleCN 388 nmCH 431 nmNCO 440 nmNO2 472 nmNH2 597 nm HCO 614 nm
InfraredH2O 2.5 mCO2 2.7 mCH4 3.4 mCH2O 3.4 mCO 4.6 mNO 5.2 mC2H4 10.5 m
Laser Absorption Yields High SensitivityRepresentative Detection Limits: Polyatomic Molecules
• Polyatomic molecules @ 1500K: 2‐200 ppm
1000 1500 2000 2500 3000
0.01
0.1
1
10
100
1000
CH2OCO2
CH3
C2H4D
etec
tion
Lim
it [p
pm]
Temperature [K]
1atm,15cm,1MHz
H2ONH2
5
sub‐ppmsensitivity for OH, CH, CN
Laser Absorption Yields High SensitivityRepresentative Detection Limits: Diatomic Molecules
• Diatomic molecules @ 1500K:‐ sub‐ppm detectivitiy for UV absorbers‐ ppm detectivity for IR absorbers
1000 1500 2000 2500 3000
0.01
0.1
1
10
100
1000
CO
OH
Det
ectio
n Li
mit
[ppm
]
Temperature [K]
1atm,15cm,1MHz
CH
CN
6
Advances in Shock Tube Methodology
1. Improved uniformity with driver inserts
2. Longer test times with tailored gas mixtures & extended driver
3. Reactive gas modeling:problem and solutions
7
Improvement in Reflected Shock Temperature Uniformity Using Driver Inserts
• Conventional shock tube operation can provide near‐ideal uniform flows for 1‐3 ms
• But, boundary‐layers and attenuation induce dP/dtand dT/dt at longer times
8
dP/dt = 3%/msdT/dt = 1.2%/ms
Improvement in Reflected Shock Temperature Uniformity Using Driver Inserts
• Driver inserts modify flow to achieveuniform T and P at long test times
P5T5
• These effects reduce ignition delay times relative to Constant P case
• Solution: Driver Inserts
VRS
9
dP/dt = 3%/msdT/dt = 1.2%/ms
Improvement in Reflected Shock Temperature Uniformity Using Driver Inserts
• Result: dP/dt = 0 prior to ignition
Proper ign for comparisonwith Constant P simulations
P5T5VRS
10
Longer Test Times Achievable withTailored Gas Mixtures & Extended Driver Sections
• Conventional shock tube operation: ~ 1‐3 ms test time• No overlap with RCM operation~ 10‐150 ms test time
10
0.4 0.8 1.2 1.61E-3
0.01
0.1
1
10
100
1000
Test
Tim
e, I
gniti
on T
ime
[ms]
1000/T [1/K]
2500K 1000K 625K
UnmodifiedStanford ST
n-Dodecane/Air
10atm
0.4 0.8 1.2 1.61E-3
0.01
0.1
1
10
100
1000
Test
Tim
e, I
gniti
on T
ime
[ms]
1000/T [1/K]
2500K 1000K 625K
UnmodifiedStanford ST
RCM
n-Dodecane/Air
10atm
=1
11
0.4 0.8 1.2 1.61E-3
0.01
0.1
1
10
100
1000
Test
Tim
e, I
gniti
on T
ime
[ms]
1000/T [1/K]
2500K 1000K 625K
UnmodifiedStanford ST
Modified ST
RCM
n-Dodecane/Air
10atm
• Longer driver length and tailored gas mixturescan provide longer test times (> 40 ms)
2x Driver Extension
• Shock tubes now can overlap with RCMs
=1
Longer Test Times Achievable withTailored Gas Mixtures & Extended Driver Sections
12
First Use of Long Test‐Time Facility:Low Pressure n‐Heptane Ignition in the NTC regime
• Test time = 45 ms at 725K
• Enables first low‐pressure (~3 atm) n‐heptane ignition data in NTC regime
• Clear evidence of 2‐stage ignition: 1 = 8 ms, 2 = 20 ms
• Next step: add driver inserts to remove dP/dt & dT/dt
• How do the 3 atm data compare to high P data?
13
0 10 20 30 40 500
1
2
3
4
5
62nd StageIgnition
OH Emission
Non-Reactive Mix w/o Inserts
Rel
ativ
e P
ress
ure
Time [ms]
n-Heptane/21%O2/Ar=1, 725K, 2.9 atm
Reactive Mix1st StageIgnition
Test time = 45 ms
Low‐Pressure, Low‐Temperature Studies:New Results for n‐Heptane in the NTC Regime
• Previous 12‐55 atm data• New 3 atm data• How do measurements compare with current models?
14
1.0 1.2 1.4 1.60.1
1
10
100
55 atm
40 atm
12 atm
1000K 625K 833K
Igni
tion
Del
ay T
imes
[m
s]
1000/T [1/K]
714K
3 atm
n-Heptane/21%O2/Ar,N2 =1
StanfordAachen
Low‐Pressure, Low‐Temperature Studies:NTC Heptane, Comparison with Model
• LLNL C7 (2000) model performs reasonably well at high P• Further tests needed at low P• Reveals value of long test time experiments
15
1.0 1.2 1.4 1.60.1
1
10
100
40 atm
12 atm
1000K 625K 833K
Igni
tion
Del
ay T
imes
[m
s]
1000/T [1/K]
714K
3 atm
n-Heptane/O2/Ar/N2 =1
Solid:LLNL C7Model
Reactive Gasdynamics Modeling: A Problem
• Most current reflected shock modeling assumes Constant‐Volume or Constant‐Pressure
But:• Exothermic energy release during oxidation or endothermic cooling during pyrolysis changes T & P behind reflected shocks not a Constant‐V or Constant‐P process!
• Example: Heptane Ignition
16
Effect of Energy Release on P Profiles:n‐Heptane Oxidation
• How does this compare with models?• Not a constant P process!
17
-100 0 100 200 300 400 500 6000
1
2
3
4
5
0.4% Heptane/4.4% O2/Ar, =11380K, 2.3 atm
Pre
ssur
e [a
tm]
Time [us]
ign
0
2
4
MeasuredSidewall P
Constant PModel
Effect of Energy Release on P Profiles:n‐Heptane Oxidation
• How does this compare with model?• Not a constant P process!• Not a Constant‐V process, even for 0.4% fuel!• So how can entire process be modeled? 18
-100 0 100 200 300 400 500 6000
1
2
3
4
5
0.4% Heptane/4.4% O2/Ar, =11380K, 2.3 atm
Pre
ssur
e [a
tm]
Time [us]
ign
0
2
4
MeasuredSidewall P
Constant PModel
Constant V Model
3 Proposed Solutions to Enable Modeling through Entire Combustion Event
1. Minimize fuel loading to reduce exothermically‐ or endothermically‐driven T and P changes‐ enabled by high‐sensitivity laser diagnostics
2. Modified gasdynamics modeling to account for P and T change during combustion‐ work in progress (but computationally intensive: 1‐D, 3‐D)
3. Use new constrained reaction volume concept to minimize pressure perturbations‐ enables constant P (or specified P) modeling
Examples: 1) Use of dilute reactive mixtures2) Use of constrained reaction volume
19
Example 1: Benefit of Dilute Mixtures3‐Pentanone Oxidation
• Pressure nearly constant throughout experiment• Good agreement between Constant H,P model and expt.• Model successfully includes temperature change
0 500 1000 15000
50
100
150
OH
Mol
e Fr
actio
n [p
pm]
Time [s]
Const. H PModel
OH Data
0 500 1000 1500
0.0
0.5
1.0
1.5
2.0
Pre
ssur
e [a
tm]
Time [s]
400 ppm 3-Pentanone/O2/Ar1486 K, 1.52 atm, =1
Low Fuel Loading Experiment
20
OH Mole Fraction
Tfinal‐TInitial =45 K
Example 2: Constrained Reaction Volume ApproachHydrogen Ignition at 950 K
• Large reaction volume gives large energy release P & T
• CRV gives reduced energy release near‐constant P
Conventional Shock Tube
21
Constrained Reaction Volume
Helium Test Mixture
Pre‐Shock
Post‐Shock
Pre‐Shock
Post‐Shock
Helium Non‐Reactive Mix TM
Large Region ofEnergy Release
Small Region ofEnergy Release
ReflectedShock Wave
• Conventional ST exhibits large pressure change!• CRV pressure nearly constant throughout experiment!• Allows kinetics modeling through ignition and combustion!
Conventional Shock Tube
22
Constrained Reaction Volume
0 1 2 3 4 50
2
4
6
8
Pressure
4%H2-2%O2-ArNormal Filling979K, 3.44 atm
Pres
sure
[at
m]
Time [ms]
Emission
0 1 2 3 4 50
2
4
6
8
Pressure
4% H2-2%O2-ArCRV = 4 cm956K, 3.397 atm
Pre
ssur
e [a
tm]
Time [ms]
Emission
Ignition
Example 2: Constrained Reaction Volume ApproachHydrogen Ignition at 950 K
Elementary Reaction RateDeterminations
GoalNear‐direct determination of rate constants for specific reactions
ExamplesOH + ketones:
acetone, 2‐butanone, 2‐ & 3‐pentanoneOH + alkanesOH + butanol isomersOH + methyl esters
23
Experimental Strategy
fast upon shock heating
tert‐butylhydroperoxide
• TBHP used as a prompt OH precursor• Useful T range (850 to 1350 K) • Pioneered by Bott and Cohen (1984)• Also used at Argonne
• Fuel in excess, pseudo‐first order experiment
24
Acetone
+ +
Representative 3‐Pentanone+OH Data: 1188 K and 1.94 atm
0 50 100
0
5
10
15
20
OH
Mol
e Fr
actio
n [p
pm]
Time [s]
Current Study 1.23x1013 cm3 mol-1 s-1
211 ppm 3-Pentanone / Ar17 ppm TBHP1188 K, 1.94 atm
0 50 100-4
-3
-2
-1
0
1
OH
Sen
sitiv
ityTime [s]
3-Pent+OH=Products CH3+OH=CH2*+H2O C2H5=C2H4+H
• High‐quality data allows high‐precision comparison with model• Sensitivity analysis confirms pseudo‐first order behavior• Near‐direct determination of reaction rate constant!
25
Results for 3‐Pentanone + OH Products
• No other high temperature data available
• NUI model (Serinyel et al. ) in excellent agreement
0.7 0.8 0.9 1.0 1.1 1.21E12
1E13
1E14
833K1429K 909K1000K1111K
Current Study Serinyel et al. - NUI Galway (2010)
k [c
m3 m
ol-1 s
-1]
1000/T [1/K]
3-Pentanone + OH = Products
1250K
26
0.7 0.8 0.9 1.0 1.1 1.21E12
1E13
1E14
833K1429K 909K1000K1111K
Current Study
k [c
m3 m
ol-1 s
-1]
1000/T [1/K]
3-Pentanone + OH = Products
1250K
+/‐ 20%
Summary: OH+KetonesProducts
• How do data compare with Structural Activity Relationship (SAR) model?• Data agree within 25% with SAR‐estimated rate constants• Similar measurements performed with methyl esters
27
0.7 0.8 0.9 1.0 1.1 1.21E12
1E13
833K909K1000K1111K1250K
Acetone 2-Butanone 3-Pentanone 2-Pentanone
k Ket
one
+ O
H [c
m3 m
ol-1 s
-1]
1000/T [1/K]
Ketone + OH = Products
1429K
Lines: Modified SAR (SAR x 0.75)
Summary: OH+Methyl Esters Products
• Data agree within 25% with SAR‐estimated rate constants• Current work: OH + aldehydes, alcohols
28
0.7 0.8 0.9 1.0 1.1 1.2
1E12
1E13
833K909K1000K1111K1250K
MButanoate MPropanoate MFormate MAcetate
Lines: Modified SAR (SAR x 0.75)
k Met
hyl E
ster
+ O
H [c
m3 m
ol-1 s
-1]
1000/T [1/K]
Methyl Ester + OH = Products
1429K
Multi‐Species Time‐Histories
MotivationMulti‐species provide greater constraint on mechanism refinement/evaluation
Recent WorkKetones: Acetone, Butanone, 3‐Pentanone
Alcohols: 1‐, 2‐, tert‐,& iso‐Butanol
Alkanes: n‐Hexadecane
Esters: Methyl Formate, MA, MP, MB, EP
29
Multi‐Species Approach: 3‐Pentanone Pyrolysis
3‐Pent = 3.39 m
CH3 = 216 nm
C2H4 = 10.5 m
CO = 4.6 m
0 100 2000
100
200
3000.1% 3-Pentanone / Ar
.
1346 K, 1.67 atm
CH
3 Mol
e Fr
actio
n [p
pm]
Time [s]
0 500 1000 15000.0
0.5
1.0
1.5
2.0
1343 K, 1.60 atm
C2H
4 Mol
e Fr
actio
n [%
]
Time [s]
1% 3-Pentanone / Ar
0 500 10000.0
0.4
0.8
1.2 1% 3-Pentanone / Ar
3-
Pen
tano
ne M
ole
Frac
tion
[%]
Time [s]
1323 K, 1.32 atm
0 500 10000
1000
2000
3000
1325 K, 1.60 atm
CO
Mol
e Fr
actio
n [p
pm]
Time [s]
0.25% 3-Pentanone / Ar
30Excellent SNR, high sensitivity data
Comparison with Serinyel et al. (2010) Galway NUI
3‐Pent CH3
C2H4CO
0 100 2000
100
200
3000.1% 3-Pentanone / Ar
Serinyel et al.
1346 K, 1.67 atm
CH
3 Mol
e Fr
actio
n [p
pm]
Time [s]
0 500 1000 15000.0
0.5
1.0
1.5
2.0
1343 K, 1.60 atm
C2H
4 Mol
e Fr
actio
n [%
]
Time [s]
1% 3-Pentanone / Ar
0 500 10000.0
0.4
0.8
1.2 1% 3-Pentanone / Ar Serinyel et al.
3-
Pent
anon
e M
ole
Frac
tion
[%]
Time [s]
1323 K, 1.32 atm
0 500 10000
1000
2000
3000
1325 K, 1.60 atm
CO
Mol
e Fr
actio
n [p
pm]
Time [s]
0.25% 3-Pentanone / Ar Serinyel et al.
Two Differences: 1) 3‐P decomposition rate; 2) CO/C2H4 yields31
3‐P Data Enables Revision of Decomposition Rate
3‐P data show strong sensitivitiesto k1 + k2
3‐Pentanone Sensitivity
Revised ktotal 3.5x Serinyel et al. rate
CO yields still not correct!
Arrhenius Plot: 3‐Pent → Products
0.7 0.8 0.91
10
100
1000
10000
100000
1111K1250K
k tota
l [1/s
]1000/T [1/K]
3-Pentanone = Products1.6 atm
Serinyel et al. (2010)
1429K
32
2 Channels
CO Yield Resolved through Use of O‐Atom Balance
• Model underpredicts CO and overpredicts methyl ketene
• Why? Methyl ketene decomposition pathway missing in mechanism
• Introduce CH3CHCO C2H4 + CO (assume k the same as ketene decomp.)33
3‐P and CO data yield total O‐atoms O‐atom concentration at 1.5 ms
Laser Absorption3‐Pentanone:CO: Sum:
23%69%
Simulation (with new k1+k2)3‐Pentanone:CO: CH3CHCO:
Sum: 93%
23%43%27%
92%
0 500 1000 15000.0
0.2
0.4
0.6
0.8
1.0
Mol
e Fr
actio
n [%
]
Time [s]
1% 3-Pentanone / Ar1248 K, 1.6 atm
3-PentanoneCO
Revised Model Improves 3‐P Simulations
CH3
CO
0 500 10000
1000
2000
3000
1325 K, 1.60 atm
CO
Mol
e Fr
actio
n [p
pm]
Time [s]
0.25% 3-Pentanone / Ar.
1215 K, 1.68 atm
1403 K, 1.57 atm
1272 K, 1.64 atm
Final Modifications to Serinyel et al.3‐Pentanone Mechanism• Revised decomposition rate:
3‐pentanone products• Additional reaction:
methylketene C2H4 + CO
34
C2H4
• Good agreement with 3‐P, CH3, CO, and Low‐T C2H4 time‐histories
Ongoing Work
• Diagnostics development & spectroscopy: – aldehydes (CH2O, CH3CHO)– alkyl radicals (C2H5)– methyl esters (methyl formate)– Alkenes (C3H6, C4H8)
• Continued improvement of shock tube methods:– constrained reaction volume
• Direct measurement of elementary reactions:– OH + oxygenates (aldehydes, ethers, alcohols)– decomposition of oxygenates– CH3, HO2 reactions with HC
35
Acknowledgements
• ARO, AFOSR, DOE, NSF
• Students:• Genny Pang, Matt Campbell, Wei Ren, Brian Lam, Sijie Li,
Sreyashi Chakraborty
36