Stanford University Research Program Shock Tube/Laser Absorption - - PowerPoint PPT Presentation

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Stanford University Research Program

Shock Tube/Laser Absorption Studies of Chemical Kinetics

Ronald K. Hanson

• Dept. of Mechanical Engineering, Stanford University
• Experimental Methods
• Butanol Kinetics
• Methyl Ester Kinetics
• Future Work

CEFRC Second Annual Conference August 17‐19, 2011 Some of the work presented here is

• unpublished. Please check with RKH/DFD

before regarding the data as final or importing it into publications. We would also value feedback from team members regarding our data and how it might be modeled. rkhanson@stanford.edu dfd@stanford.edu

Please Note

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Some of the work presented here is unpublished. Please check with RKH/DFD before regarding the data as final or importing it into publications. We would also value feedback from team members regarding our data and how it might be modeled. rkhanson@stanford.edu dfd@stanford.edu

Experiment Types

Goal: High‐quality databases to validate detailed mechanisms

• Ignition delay times provide global targets

– Shock tubes provide constant‐volume data over wide pressure range

• Species time‐histories provide strong constraints on mechanisms

– Laser absorption can provide species time‐histories for: OH, CO, CO2, CH2O, H2O, CH3, CH4, C2H4, fuel, …

• Direct determination of elementary reaction rate constants

– For reactions where estimates/theory are not sufficient

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Experimental Approach

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Kinetics Shock Tube 2 Kinetics Shock Tube 1 Aerosol Shock Tube High Pressure Shock Tube

Advances in Shock Tube Methodology

• Tailored driver gas/new driver geometry provide

extended test time for access to low T

• Driver inserts provide highly uniform reflected shock

conditions approaching constant U/V

• Aerosol shock tube provides access to low‐vapor‐pressure

fuels

• Gasdynamic modeling of shock tube flows to account for

facility effects and energy release

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Access to Low Temperatures

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• Longer driver length and tailored gas mixtures

can provide longer test times (> 40 ms)

• Shock tubes now can overlap with RCMs

2x Driver Extension

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Improvement in Temperature Uniformity

• Problem: Ignition delay times are

artificially shortened by non‐ideal facility effects!! dP/dt ≠ 0

• Solution: Driver Inserts
• Results: Near‐ideal constant‐volume

performance!! dP/dt ≈ 0

P5 T5 VRS

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Aerosol Shock Tube for Low‐Vapor‐Pressure Fuels

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Dump Tank

Diagnostics:

• Pressure
• Droplet scattering
• Fuel time history
• OH* emission

Ultrasonic Nebulizers

Driver Section Driven Section

Aerosol Tank

• Does not require heated shock tube
• Eliminates fuel cracking and partial distillation
• Provides access to low‐vapor‐pressure fuels:

large methyl esters, bio‐diesel surrogates

Evaporated Aerosol

Current Laser Capabilities for Species Detection for real‐time, in situ sensing

Infrared CO 2.3 m H2O 2.5 m CO2 2.7 m Fuel 3.4 m NO 5.2 m MeOH 9.2 m MF 9.2 m C2H4 10.5 m Ultraviolet CH3 216 nm NO 225 nm O2 227 nm HO2 230 nm CH2O 305 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

Spectra‐Physics 380 Ring Coherent MIRA Ti‐Sapphire NovaWave Mid‐IR DFG

Butanol Kinetics

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Kinetics Shock Tube 2 Kinetics Shock Tube 1

Overview of Butanol Studies

• Ignition delay times: 1.5‐45 atm, 800‐1600 K

Butanol isomers, high and low pressure

• Species time‐histories:

N‐Butanol pyrolysis: OH, H2O, CH2O, C2H4, CH4, CO N‐Butanol oxidation: OH, H2O, C2H4

• Direct determinations of elementary rxn. rate constants:

Butanol+OH=products, all isomers Butene+OH= Products, all isomers

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Survey of Ignition Delay Times: Butanol Isomers at Low Pressure

Variation in ignition delay times

• tert-butanol slowest
• 1-butanol fastest

MIT (2011) mechanism

• Fair agreement with

t- & i-butanol data

• Poorer agreement with

1- & 2-butanol data

Data of sufficient quality to refine reaction mechanisms

100 1000

4% O2/Argon 1.5 atm, =1.0

1538 K 1250 K 1429 K 1333 K

Lines - MIT (2011) 1-butanol 2-butanol i-butanol t-butanol

tign [us] 1000/T5 [1/K] 0.60 0.65 0.70 0.75 0.80 0.85

t-but i-but 2-but 1-but 12

What happens at high pressure?

Survey of Ignition Delay Times: 2‐Butanol Variation with Pressure

Very low scatter ( 5-10 %) consistent with uncertainty in T Ignition delay times scale approximately as P-0.7 MIT (2011) model P-dependence consistent with 2-butanol data Data of sufficient quality to refine reaction mechanisms

0.6 0.7 0.8 0.9 1.0 100 1000

43atm

2-Butanol 4% O2/Argon

~1.0

19atm 3 atm 1.5 atm

1053 K 1177 K 1333 K

Lines - MIT (2011)

tign [us] 1000/T5 [1/K]

1538 K

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Next step: need for species time‐histories!

N‐Butanol Pyrolysis

• First shock tube/laser absorption speciation

study of n‐butanol pyrolysis

• OH (306 nm)
• H2O (2.5 microns)
• CH2O (305 nm)
• CO (4.6 nm)
• C2H4 (10.5 microns)
• CH4 (3.4 microns)

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n‐Butanol

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N‐Butanol Pyrolysis: Species Time‐Histories OH and H2O

• OH & H2O time‐histories reveal large variation in model performance
• Clear opportunity for model refinement

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N‐Butanol Pyrolysis: Species Time‐Histories CH2O and CO

• Formaldehyde and CO uniformly underpredicted!
• Measured OH+H2O+CH2O+CO account for >90% of O‐atoms!
• Remaining O‐atoms likely in CH3CHO, CH2CO,…

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N‐Butanol Pyrolysis: Species Time‐Histories C2H4 and CH4

• All models underpredict C2H4; better agreement for CH4
• Measured C2H4+CH4+CO+CH2O account for >70% of C‐atoms!
• Remaining C‐atoms likely in C3H6, C2H6, C2H2,…

OH+Butanol→Products

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• Strong dependence on

isomer

• MIT (2011) Model:

Trends not consistent with data

• Sarathy (2011) Model:

Consistent with data

• Overall rate dependent on product channel chemistry

0.8 0.9 1.0 1.1 1.2 1E11 1E12 1E13

Rate Constant [cc/mole/s] 1000/T [1/K]

1111 K 1000 K 909 K

1-but iso-but 2-but tert-but

3E13

Sarathy (2011)

First‐order removal of OH measured using laser absorption in 30 ppm TBHP/200ppm butanol/argon mixtures

Methyl Ester Kinetics

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Kinetics Shock Tube 2 Aerosol Shock Tube

Overview of Methyl Ester Studies

• Ignition delay times: Aerosol Shock Tube Studies
• Species time‐histories during pyrolysis:

Methyl Formate: CO, OH, C2H4, CH2O, CH3, CH4, Me‐OH, MF Methyl Acetate/Propanoate: CO, CH3, C2H4 Methyl Butanoate: CO, CO2, C2H4, OH

• Reflected shock conditions:

1.5‐6 atm, 1000‐1400 K

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Methyl Oleate Methyl Decanoate

Methyl Decanoate Ignition Delay Times: Aerosol Shock Tube Studies

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Westbrook model correctly predicts ignition delay times, activation energies, and [O2] dependence Westbrook model:

• 3500 species
• 17000 reactions

Can we apply AST method to larger esters?

EA = 42.5 kcal/mol EA = 29.2 kcal/mol

Methyl Oleate Ignition Delay Times: Aerosol Shock Tube Studies

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• Data reveal weak

dependence on equivalence ratio

• Westbrook model:

‐ underestimates ignition delay times by about 50% ‐ captures reveal weak dependence on equivalence ratio

Next step: Need for species time‐histories!

Methyl Formate Pyrolysis: Time‐Histories and Rate Data

• Species time‐histories: CO, OH, C2H4, CH2O, CH3, CH4, MeOH, MF
• Rate constant determination for all three major

decomposition channels (Princeton/NUI 2010)

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1: MF → CO+MeOH using CO (4.6 m) using MeOH (9.23 m) 2: MF → CH4+CO2 using CH4 (3.4 m) using CO2 (2.7 m) 3: MF → 2CH2O using CH2O (306 nm)

Methyl Formate Decomposition: CH3OH+CO channel

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Direct measurement of CH3OH+CO channel possible with CO laser

200 400 600 800 0.00 0.05 0.10 0.15 0.20

1202K 1285K 1376K 1488K

CO Mole Fraction [%] Measurement Dryer et al. 2010 Time [s]

1607K

0.1% MF/Ar 1.5 atm

CO Time‐Histories MF → CH3OH+CO Excellent agreement at with Dooley et al. (2010) particularly at lower T

0.60 0.65 0.70 0.75 0.80 0.85 10

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10

10

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1333 1176 1429 1250 1538 Current study Best fit Dryer et al. 2010 (1.6 atm) Curran et al. 2008 (1.6 atm)

k1 [cm

• 3mol
• 1s
• 1]

1000/T [K]

1667K

P = 1.48-1.72 atm

Methyl Formate Decomposition: CH4+CO2 channel

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Direct measurement of CH4+CO2 channel possible with CH4 or CO2 laser CH4 Time‐Histories

0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.1 0.2 0.3 0.4

3% MF/Ar 1.5 atm

1408K

CH4 Mole Fraction [%] Time [ms] Current study Dryer et al. 2010 Updated k1

1289K

MF → CH4+CO2 Excellent agreement at with Dooley et al. (2010)

0.60 0.64 0.68 0.72 0.76 0.80 10

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Current study Best fit Dryer et al. 2010 (1.45 atm) 1250 1316 1389 1470 1562 k2 [cm

3mol

• 1s
• 1]

1000/T [K] 1667 K P = 1.36-1.54 atm

Methyl Formate Decomposition: CH2O+CH2O channel

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Direct measurement of CH2O+CH2O channel possible with CH2O laser CH2O Time‐Histories

200 400 600 800 1000 0.0 0.2 0.4 0.6 0.8

Current study Dryer et al. 2010 Updated k1, k2

1347K 1462K

CH2O Mole Fraction [%] Time [s]

1257K

3% MF/Ar 1.4 atm

MF → CH2O+CH2O Excellent agreement at with Dooley et al. (2010)

0.60 0.65 0.70 0.75 0.80 0.85 10

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10

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10

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1176 1538 1250 1333 1429 1538 Current study Best fit Dryer et al. 2010 (1.4 atm)

k3 [cm

3mol

• 1s
• 1]

1000/T [K]

P = 1.37-1.56 atm 1667 K

Oxygen Balance in Methyl Formate Pyrolysis

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• Laser data successfully tracks all major

contributors to O‐atom balance

• Significant opportunity for mechanism validation

% Oxygen balance: 5.5% in MF 34.8% in MeOH 44.9% in CO 5.8% in CO2 7.2% in CH2O Total: 98.2% @ t = 300s

100 200 300 0.0 0.2 0.4 0.6 0.8 1.0 CH4 (CO2) CH2O CH3OH CO

Oxygen Atom Balance Time [s]

CH3OCHO T = 1420K, P = 1.5atm (0.2-3% MF/Ar)

Future Work

• Butanol Kinetics

– Extend species time‐history database to all isomers – Extend speciation database to oxidation

• Methyl Ester Kinetics

– Extend ignition delay time database to large bio‐diesel components – Extend speciation studies to other methyl esters (small and large)

• Other Opportunities

– Collaborate with modelers – Apply multi‐species methods to other fuels/surrogates

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