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Lecture 12: Shock Tube Applications with Lasers
1. Laser Absorption Theory
2. Survey of Capabilities
3. Kinetics Applications:Rate Constant MeasurementsMulti-Species Time-Histories
4. New Species Diagnostics
1. Laser Absorption Theory• Governing Equation: Beer‐Lambert Law
/0 = exp(‐Slu () Xspecies Ptotal L)
• Quantitative absorption requires database for S, • What species have been measured?
Line LineStrength shape
I0
ILASER
L
2
2. Survey of Capabilities:Species and Wavelengths
UltravioletCH3 216 nmNO 225 nmO2 227 nmHO2 230 nmOH 306 nmNH 336 nm
Spectra‐Physics 380
VisibleCN 388 nmCH 431 nmNCO 440 nmNO2 472 nmNH2 597 nm HCO 614 nm
First use of tunable dye lasers in shock tubes (1982)
InfraredCO 2.3 mH2O 2.5 mCO2 2.7 mFuel 3.4 mNO 5.2 mC2H4 10.5 m
3
Coherent MIRATi‐Sapphire Ring Laser
Ultra‐fast lasers used to extend UV tuning range (2009)
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
InfraredCO 2.3 mH2O 2.5 mCO2 2.7 mFuel 3.4 mNO 5.2 mC2H4 10.5 m
4
2. Survey of Capabilities:Species and Wavelengths
NovaWave Mid‐IR Laser
InfraredCO 2.3 m, 4.6 mH2O 2.5 mCO2 2.7 m, 4.3 mFuel 3.4 mNO 5.2 mC2H4 10.5 m
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
5
2. Survey of Capabilities:Species and Wavelengths
InfraredCO 2.3 m, 4.6 mH2O 2.5 mCO2 2.7 m, 4.3 mFuel 3.4 mNO 5.2 mC2H4 10.5 m
How sensitive are laser absorption diagnostics in shock tubes?
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
6
2. Survey of Capabilities:Species and Wavelengths
2. Laser Absorption Yields High SensitivityRepresentative Detection Limits: Large Molecules
• Large molecules: 10‐100’s ppm
1000 1500 2000 2500 3000
0.01
0.1
1
10
100
1000CO2
CH3
C2H4M
inim
um D
etec
itivi
ty [p
pm]
Temperature [K]
1atm,15cm,1MHz
H2ONH2
7
sub‐ppmsensitivity for OH, CH, CN
2. Laser Absorption Yields High SensitivityRepresentative Detection Limits: Diatomic Molecules
• Diatomic molecules @ 1500K:‐ sub‐ppm detectivity 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
8
3. Kinetics Applications
1. Foundation Fuel KineticsOH+H2=H2O+H
2. Methyl Ester KineticsME+OH=ProductsMethyl Formate pyrolysis
3. Butanol Isomer Kineticsn‐Butanol pyrolysist‐butanol+OH using isotopic labeling
9
3.1 Foundation Fuel Kinetics:
• H + O2 = OH + O• H2O2 + M = OH + OH + M• OH + H2O2 = H2O + HO2
• OH + HO2 = H2O + O2
• HO2 + HO2 = H2O2 + O2
• CH3 + HO2 = Products • C2H4 + M = C2H2 + H2 + M• OH + H2 = H2O + H
10
• Recent Elementary Rate Constant Measurementsusing Shock Tube/Laser Absorption Methods
3.1 Foundation Fuel Kinetics
1. Design: use sensitivity analysis to kinetically isolate reactions
2. Execute: use shock tubes for step heating and laser absorption for species detection
Example: H + O2 OH + O
• Accurate k values critical to combustion modeling
k
How Do We Measure Individual Reaction Rates?- Two-Step Process
11
3.1 Foundation Fuel Kinetics
• H2O time‐history provides precise determination of k1
Shock Conditions: 1472 K, 1.8 atm, 0.1%O2/0.9%H2/Ar
H2O Sensitivity H2O Time‐History
12
• Example: H + O2 OH + ORate Measurement using H2O Laser at 2.55 mm
3.1 Foundation Fuel Kinetics
13
Arrhenius Plot: H + O2 OH + O
Arrhenius Plot: H + O2 OH + O
Modern laser methods significantly reduce measurement uncertainty!
k1=1.04x1014 exp(‐7705/T) {1=5%}
14
3.1 Foundation Fuel Kinetics
3.1 Foundation Fuel Kinetics
fast upon shock heating
TBHP: tert‐butylhydroperoxide
Motivation: Large uncertainty in kOH+H2 gives large uncertaintyin modeled H2/air flame speeds
Experimental Strategy: Direct determination of rate constant• Pseudo‐first order experiment• Fuel in excess• TBHP used as a prompt OH precursor
‐ Useful T range (850 to 1350 K) ‐ Pioneered by Bott and Cohen (1984), also used at Argonne
Acetone
+ +
15
• Elementary Reaction Rate Determination:OH + H2 H2O + H
3.1 Foundation Fuel Kinetics
• High SNR data, ppm sensitivity• kbest‐fit determined within 3‐5%
16
• Representative OH Absorption Data
3.1 Foundation Fuel Kinetics
• Very low overall uncertainty in k: ±17% (2)
kBest‐Fit
17
• Arrhenius Plot: OH + H2 H2O + H
3.1 Foundation Fuel Kinetics
• Excellent agreement with previous work,but uncertainty in k substantially reduced
0.4 0.6 0.8 1.0 1.21E11
1E12
1E13
2500K
kBest-Fit
833K1000K1250K
k 1 [cm
3 mol
-1 s
-1]
1000/T [1/K]
Current Study Krasnoperov and Michael (2004) Michael and Sutherland (1988) (revised Kc) Davidson et al. (1988) (revised Kc) Frank and Just (1985) Oldenborg et al. (1992)
1667K
18
• Comparison with Past Work: OH + H2 H2O + H
3.2 Methyl Ester Kinetics
1. Rate Constant Measurements– OH + Methyl Formate– OH + Methyl Acetate– OH + Methyl Propanoate– OH + Methyl Butanoate
2. Methyl Formate Pyrolysis– Multi‐species Data
Conducted as first‐order experiments, with TBHP as source of OH
MF, CH3OHCO, CH4, CH2O
19
3.2 Methyl Ester Kinetics
• Low scatter data with ± 25% overall uncertainty• Good agreement with modified SAR
(Structure Activity Relationship)
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 [c
c/m
ol/s
]
1000/T [1/K]
Methyl Ester + OH = Products
1429K
+/-25%
MB
MP
MA
MF
20
• Summary: OH+Methyl Esters Products
3.2 Methyl Ester Kinetics
• Shock tube/laser absorption rate 2‐4x faster than calculation
• Confirms continuing value of high–accuracy experimental data
0.7 0.8 0.9 1.0 1.1 1.21E11
1E12
1E13
+/-25%
Tan et al. (2012)
833K909K1000K1111K1250K
k
[cc
/mol
/s]
1000/T [1/K]
Methyl Formate + OH = Products
1429K
Current Study
21
• Comparison with Recent Quantum Calculations:Methyl Formate +OH Products
3.2 Methyl Ester Kinetics
• Laser data successfully tracks all major contributors to O‐atom balance• Atom balance provides important new validation tool for chemical models
@ t = 300s
% Oxygen balance:5.5% in CH3OCHO34.8% in CH3OH44.9% in CO5.8% in CO2
7.2% in CH2OTotal: >98%
22
22
• Methyl Formate Pyrolysis Kinetics:Multi-Species Laser Absorption Data Can ProvideNear-Complete Oxygen Balance
3.3 Butanol Isomer Kinetics
1. n‐Butanol Pyrolysis:Challenge: Complex PathwaysSolution: Multi‐Species Time‐Histories
OH, CO, CH4, H2O, C2H4
2. tert‐Butanol + OH ProductsChallenge: Multiple Reaction Sites,
OH ReformationSolution: Isotopic Labeling
23
• Two Studies of Butanol Kinetics
3.3 Butanol Isomer Kinetics
• Challenges: Multiple Reaction Sites, OH Reformation
• Solution: Isotopic labeling provides strategy to identify OH attack sites
24
• OH+ tert-Butanol ProductsUse of Isotopic Labeling
3.3 Butanol Isomer Kinetics
16
16
16
16
16 16 16
16
16
16
• Measurement of OH removal by t‐C4H9
16OH affected by OH reformation pathway
• Measurement gives
k1b+ k1a (1‐BR2)
= (k1b+k1a )(1‐BR1BR2)
t‐C4H916OH + 16OH
OH reformation! 25
• OH+ tert-Butanol ProductsConventional Experiment using t-C4H9
16OH
3.3 Butanol Isomer Kinetics
16
16
16
18
18
18
18
18 18
16
18
16
• Measurement of OH removal by t‐C4H9
16OH affected by OH reformation pathway
• Measurement gives
k1b+ k1a (1‐BR2)
= (k1b+k1a )(1‐BR1BR2)
• Measurement of 16OH in t‐C4H9
18OH gives (k1a+k1b) with no 16OH reformation
t‐C4H9O18H + 16OH
No 16OH reformation! 26
• OH+ tert-Butanol ProductsConventional Experiment using t-C4H9
16OH
3.3 Butanol Isomer Kinetics
0 20 40 60 80
10
20
30
500 ppm t-butan18ol29 ppm tbhp
Measurement 18k' = 1.08x10-11
1.218k' 0.818k'
OH
Mol
e Fr
actio
n [p
pm]
Time [s]
• Slow removal of OH during 16O butanol pyrolysis• Faster removal of OH during 18O butanol pyrolysis
• How do the rates compare?
0 20 40 60 80
5
10
15202530
OH
Mol
e Fr
actio
n [p
pm]
500 ppm t-butan16ol17 ppm tbhp
Measurement 16k' = 2.18x10-12 1.216k' 0.816k'
time [s]
16O‐Butanol 18O‐Butanol
1020 K1.22 atm
1020 K1.22 atm
27
• OH Absorption Data for tert-Butanol (O16 & O18)
ktotal = (k1a+k1b)Derived from t‐C4H9
18OH
0.8 0.9 1.0 1.1
10-12
10-11
1000K
k1a+k1b
Sarathy et al. (2012)
Net
OH
dec
ay ra
te
(cm
3 /mol
ecul
e/s)
1000/T
1250K
28
• OH + tert‐Butanol (O16 & O18)Net OH Decay Rate due to Reaction with t‐Butanol
3.3 Butanol Isomer Kinetics
(k1a+k1b) (1‐BR1BR2)Derived from t‐C4H9
16OH
• first direct determination of overall rate (k1a+k1b)• Ratio (18O/16O expts.) gives (1‐BR1BR2) = 0.2• Since BR1 = 0.95, then BR2 = 0.8 (±0.05)
0.8 0.9 1.0 1.1
10-12
10-11
1000K
k1a+k1b
Sarathy et al. (2012) (k1a+k1b) (1-BR1BR2)
Sarathy et al. (2012)
Net
OH
dec
ay ra
te
(cm
3 /mol
ecul
e/s)
1000/T
1250K
ktotal = (k1a+k1b)Derived from t‐C4H9
18OH
29
• OH + tert-Butanol (O16 & O18)Net OH Decay Rate due to Reaction with t-Butanol
3.3 Butanol Isomer Kinetics
4. New Species Diagnostics
Current Work:– Alkyl radicals, e.g., C2H5
– Alkenes, Alkynes, e.g. C3H6, C2H2
– Aldehydes, e.g., CH2O, CH3CHO
Example: Aldehydes
30
4. New Species Diagnostics
31
Motivation: Aldehydes provide critical information about first stages of hydrocarbon oxidation, especially oxygenated fuels
H
HC=O C=O
H
CH3
Challenge: Overlapping absorption spectraStrategy: Three colors (1 and 2 in IR, and 3 in UV)
1,2
IR absorption
3
Formaldehyde Acetaldehyde Reflectedshock wave
Detectors
Three Lasers
UV absorption
31
• Aldehyde Diagnostics
4. New Species Diagnostics
Measured Absorbance Time‐Histories Aldehyde Concentration Time‐Histories
• Recovered dCH2O/dt & plateau mole fractions: Success!
• Shock heat trioxane/acetaldehyde mixture• Observe formaldehyde formation• Recover correct CH2O/CH3CHO concentrations
32
• Validation Experiment: Simultaneous Measurement ofKnown CH2O & CH3CHO Concentrations
Next: Three Lectures onLaser-Induced Fluorescence (LIF)
Two level model More complex models Applications
33
Summary:Laser absorption & shock tubes are a frontier for combustion kinetics
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