Lecture 12: Shock Tube Applications with Lasers

1. Laser Absorption Theory 2. Survey of Capabilities 3. Kinetics Applications: Rate Constant Measurements Multi-Species Time-Histories 4. New Species Diagnostics

1. Laser Absorption Theory • Governing Equation: Beer‐Lambert Law /0 = exp(‐Slu () Xspecies Ptotal L) Line             Line Strength     shape L

I

I0 LASER

• Quantitative absorption requires database for S,  • What species have been measured? 2

2. Survey of Capabilities: Species and Wavelengths Ultraviolet CH3 216 nm NO 225 nm O2 227 nm HO2 230 nm OH 306 nm NH 336 nm

Visible CN 388 nm CH 431 nm NCO 440 nm NO2 472 nm NH2 597 nm  HCO 614 nm 

Infrared CO  2.3 m H2O  2.5 m CO2 2.7 m Fuel      3.4 m NO  5.2 m C2H4 10.5 m

First use of tunable dye lasers in  shock tubes (1982) 

Spectra‐Physics 380

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2. Survey of Capabilities: Species and Wavelengths Ultraviolet CH3 216 nm NO 225 nm O2 227 nm HO2 230 nm OH 306 nm NH 336 nm

Visible CN 388 nm CH 431 nm NCO 440 nm NO2 472 nm NH2 597 nm  HCO 614 nm 

Infrared CO  2.3 m H2O  2.5 m CO2 2.7 m Fuel      3.4 m NO  5.2 m C2H4 10.5 m

Ultra‐fast lasers used to extend UV tuning  range  (2009) 

Coherent MIRA Ti‐Sapphire Ring Laser

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2. Survey of Capabilities: Species and Wavelengths Ultraviolet CH3 216 nm NO 225 nm O2 227 nm HO2 230 nm OH 306 nm NH 336 nm

Visible CN 388 nm CH 431 nm NCO 440 nm NO2 472 nm NH2 597 nm  HCO 614 nm 

Infrared CO  2.3 m, 4.6 m H2O  2.5 m CO2 2.7 m, 4.3 m Fuel      3.4 m NO  5.2 m C2H4 10.5 m

New lasers allow simple access to  mid‐IR (2007‐10)

NovaWave Mid‐IR Laser

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2. Survey of Capabilities: Species and Wavelengths Ultraviolet CH3 216 nm NO 225 nm O2 227 nm HO2 230 nm OH 306 nm NH 336 nm

Visible CN 388 nm CH 431 nm NCO 440 nm NO2 472 nm NH2 597 nm  HCO 614 nm 

Infrared CO  2.3 m, 4.6 m H2O  2.5 m CO2 2.7 m, 4.3 m Fuel      3.4 m NO  5.2 m C2H4 10.5 m

How sensitive are laser absorption  diagnostics in shock tubes? 6

2. Laser Absorption Yields High Sensitivity Representative Detection Limits: Large Molecules Minimum Detecitivity [ppm]

1000

CO2 100

C2H4 H2O

NH2

10

CH3

1 0.1 0.01

1atm,15cm,1MHz

1000

1500

2000

2500

3000

Temperature [K]

• Large molecules: 10‐100’s ppm 7

2. Laser Absorption Yields High Sensitivity Representative Detection Limits: Diatomic Molecules 1000

Detection Limit [ppm]

1atm,15cm,1MHz 100

CO

10

OH 1

sub‐ppm sensitivity for  OH, CH, CN

CH 0.1

CN

0.01 1000

1500

2000

2500

3000

Temperature [K]

•

Diatomic molecules @ 1500K: ‐ sub‐ppm detectivity for UV absorbers ‐ ppm detectivity for IR absorbers

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3. Kinetics Applications 1.

Foundation Fuel Kinetics OH+H2=H2O+H

2.

Methyl Ester Kinetics ME+OH=Products Methyl Formate pyrolysis

3.

Butanol Isomer Kinetics n‐Butanol pyrolysis t‐butanol+OH using isotopic labeling

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3.1 Foundation Fuel Kinetics: •

Recent Elementary Rate Constant Measurements using Shock Tube/Laser Absorption Methods

• • • • • • • •

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

3.1 Foundation Fuel Kinetics How Do We Measure Individual Reaction Rates? - Two-Step Process 1.     Design: use sensitivity analysis to kinetically  isolate reactions 2.     Execute: use shock tubes for step heating and  laser absorption for species detection

k

Example:     H + O2  OH + O •

Accurate k values critical to combustion modeling 11

3.1 Foundation Fuel Kinetics • Example: H + O2  OH + O Rate Measurement using H2O Laser at 2.55 mm Shock Conditions: 1472 K, 1.8 atm, 0.1%O2/0.9%H2/Ar

H2O Sensitivity

H2O Time‐History

• H2O time‐history provides precise determination of k1

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3.1 Foundation Fuel Kinetics Arrhenius Plot: H + O2  OH + O

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3.1 Foundation Fuel Kinetics Arrhenius Plot: H + O2  OH + O

Modern laser  methods significantly  reduce measurement  uncertainty! 

k1=1.04x1014 exp(‐7705/T) {1=5%}

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3.1 Foundation Fuel Kinetics • Elementary Reaction Rate Determination: OH + H2  H2O + H Motivation: Large uncertainty in kOH+H2 gives large uncertainty in 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

fast upon shock heating

+ TBHP: tert‐butylhydroperoxide

+

Acetone

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3.1 Foundation Fuel Kinetics • Representative OH Absorption Data

• High SNR data, ppm sensitivity • kbest‐fit determined within 3‐5% 16

3.1 Foundation Fuel Kinetics • Arrhenius Plot: OH + H2  H2O + H

kBest‐Fit

• Very low overall uncertainty in k:  ±17% (2)  17

3.1 Foundation Fuel Kinetics • Comparison with Past Work: OH + H2  H2O + H 2500K

1667K

1250K

1000K

833K

3

-1

-1

k1 [cm mol s ]

1E13

1E12

kBest-Fit 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)

1E11 0.4

0.6

0.8

1.0

1.2

1000/T [1/K]

•

Excellent agreement with previous work, but uncertainty in k substantially reduced

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3.2 Methyl Ester Kinetics

1. Rate Constant Measurements – – – –

OH + Methyl Formate OH + Methyl Acetate OH + Methyl Propanoate OH + Methyl Butanoate

Conducted as first‐order  experiments, with TBHP  as source of OH

2. Methyl Formate Pyrolysis – Multi‐species Data

MF, CH3OH CO, CH4, CH2O

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3.2 Methyl Ester Kinetics Summary: OH+Methyl Esters  Products MB 1429K

1250K

1111K

1000K

909K

833K

Methyl Ester + OH = Products

MP k [cc/mol/s]

•

1E13

+/-25%

MA

1E12 0.7

MButanoate MPropanoate MFormate MAcetate Lines: Modified SAR (SAR x 0.75) 0.8

0.9

1.0

MF 1.1

1.2

1000/T [1/K]

• •

Low scatter data with ± 25% overall uncertainty Good agreement with modified SAR (Structure Activity Relationship)

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3.2 Methyl Ester Kinetics • Comparison with Recent Quantum Calculations: Methyl Formate +OH  Products 1429K 1E13

1250K

1111K

1000K

909K

833K

k [cc/mol/s]

+/-25%

Current Study 1E12

Tan et al. (2012) Methyl Formate + OH = Products 1E11 0.7

0.8

0.9

1.0

1.1

1.2

1000/T [1/K]

•

Shock tube/laser absorption rate 2‐4x faster than calculation

•

Confirms continuing value of high–accuracy experimental data

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3.2 Methyl Ester Kinetics 22

• Methyl Formate Pyrolysis Kinetics: Multi-Species Laser Absorption Data Can Provide Near-Complete Oxygen Balance

@ t = 300s

• •

% Oxygen balance: 5.5% in CH3OCHO 34.8% in CH3OH 44.9% in CO 5.8% in CO2 7.2% in CH2O Total: >98%

Laser data successfully tracks all major contributors to O‐atom balance Atom balance provides important new validation tool for chemical models

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3.3 Butanol Isomer Kinetics • Two Studies of Butanol Kinetics 1. n‐Butanol Pyrolysis: Challenge: Complex Pathways Solution:    Multi‐Species Time‐Histories OH, CO, CH4, H2O, C2H4

2. tert‐Butanol + OH  Products Challenge: Multiple Reaction Sites, OH Reformation Solution:  Isotopic Labeling

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3.3 Butanol Isomer Kinetics • OH+ tert-Butanol  Products Use of Isotopic Labeling • Challenges: Multiple Reaction Sites, OH Reformation  • Solution: Isotopic labeling provides strategy to identify OH  attack sites

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3.3 Butanol Isomer Kinetics • OH+ tert-Butanol  Products Conventional Experiment using t-C4H916OH •

Measurement of OH removal  by t‐C4H916OH affected by OH  reformation pathway

t‐C4H916OH + 16OH 16

16

16

16 16

•

Measurement gives 

16

k1b+ k1a (1‐BR2) = (k1b+k1a )(1‐BR1BR2)

16

16 16

16

OH reformation!  25

3.3 Butanol Isomer Kinetics • OH+ tert-Butanol  Products Conventional Experiment using t-C4H916OH •

Measurement of OH removal  by t‐C4H916OH affected by OH  reformation pathway

18H + 16OH t‐C H O 16 4 9

18 18

•

Measurement gives 

16

18

18 16

16

k1b+ k1a (1‐BR2) = (k1b+k1a )(1‐BR1BR2)

18

18 16

• Measurement of 16OH in  t‐C4H918OH gives (k1a+k1b)  with no 16OH reformation

18

No 16OH reformation!  26

3.3 Butanol Isomer Kinetics • OH Absorption Data for tert-Butanol (O16 & O18)

30 25 20 15

500 ppm t-butan16ol 17 ppm tbhp Measurement 16 k' = 2.18x10-12 1.216k' 0.816k'

10 5 0

18O‐Butanol

1020 K 1.22 atm 20

40

60

OH Mole Fraction [ppm]

OH Mole Fraction [ppm]

16O‐Butanol

80

30

500 ppm t-butan18ol 29 ppm tbhp Measurement 18 k' = 1.08x10-11 1.218k' 0.818k'

20

10

1020 K 1.22 atm 0

20

time [s]

•

Slow removal of OH during 16O butanol pyrolysis

•

Faster removal of OH during 18O butanol pyrolysis

• How do the rates compare?

40

60

80

Time [s]

27

3.3 Butanol Isomer Kinetics

(cm3/molecule/s)

• OH + tert‐Butanol (O16 & O18) Net OH Decay Rate due to Reaction with t‐Butanol 1000K

1250K

10-11

ktotal = (k1a+k1b)

Net OH decay rate

Derived from t‐C4H918OH

10-12

k1a+k1b Sarathy et al. (2012)

0.8

0.9

1.0

1.1

1000/T

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3.3 Butanol Isomer Kinetics

(cm3/molecule/s)

OH + tert-Butanol (O16 & O18) Net OH Decay Rate due to Reaction with t-Butanol 1250K

1000K

10-11

ktotal = (k1a+k1b)

Derived from t‐C4H918OH

Net OH decay rate

•

10-12

0.8

(k1a+k1b) (1‐BR1BR2)

k1a+k1b Sarathy et al. (2012) (k1a+k1b) (1-BR1BR2) Sarathy et al. (2012)

0.9

1.0

Derived from t‐C4H916OH

1.1

1000/T

•

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)

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4. New Species Diagnostics

Current Work: – Alkyl radicals, e.g., C2H5 – Alkenes, Alkynes, e.g. C3H6, C2H2 – Aldehydes, e.g., CH2O, CH3CHO

Example: Aldehydes

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4. New Species Diagnostics • Aldehyde Diagnostics Motivation:  Aldehydes provide critical information about first stages of  hydrocarbon oxidation, especially oxygenated fuels

H H

CH3 C=O

Formaldehyde

Three  Lasers

C=O

H Acetaldehyde

Challenge:  Overlapping absorption spectra Strategy: Three colors (1 and 2 in IR, and 3 in UV) IR absorption

Reflected shock wave Detectors

UV absorption

3

1,2

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4. New Species Diagnostics • Validation Experiment: Simultaneous Measurement of Known CH2O & CH3CHO Concentrations • Shock heat trioxane/acetaldehyde mixture • Observe formaldehyde formation • Recover correct CH2O/CH3CHO concentrations Measured Absorbance Time‐Histories

•

Aldehyde Concentration Time‐Histories

Recovered dCH2O/dt & plateau mole fractions:  Success!

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Summary: Laser absorption & shock tubes are a frontier for combustion kinetics

Next: Three Lectures on Laser-Induced Fluorescence (LIF)  Two level model  More complex models  Applications

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