19 research outputs found
An experimental and modeling study of the combustion of toluene.
An experimental and modeling study of the combustion of toluene
A pioneer of direct measurements to advance modern gasâphase chemical kinetics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163611/2/kin21424_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163611/1/kin21424.pd
High pressure effects on the mutual sensitization of the oxidation of NO and CH4âC2H6 blends
International audiencehe mutual sensitization of the oxidation of NO and a CH4âC2H6 (10 : 1) simulated natural gas (NG) blend was studied under fuel lean conditions (Ί = 0.5) at 50 atm and 1000â1500 K in the UIC high pressure shock tube (HPST). New experimental results were also obtained for the mutual sensitization of methane and the NG blend in the CNRS jet stirred reactor (JSR) at 10 atm. A detailed chemical kinetic model was assembled to describe the observed changes in reactivity in the CH4 and NG blends, with and without NO, in the HPST and the JSR. The data and the validated model (tested against a variety of targets) show a reduced difference of reactivity between methane and NG blends in the presence of NO at characteristic reaction times for the JSR (250â1000 ”s). However the HPST data and subsequent simulations using the validated model have revealed that at higher pressures and in the millisecond time scale regime representative of the HPST experiments (and practical combustion devices) there still persists a significant difference in reactivity between methane and NG blends in the presence of NO. The experimental data, the model development and validations and its predictions and utility as a tool to probe the NOâhydrocarbon sensitization effects under practical combustion conditions is discussed
Global Sensitivity Analysis with Small Sample Sizes: Ordinary Least Squares Approach
A new
version of global sensitivity analysis is developed in this
paper. This new version coupled with tools from statistics, machine
learning, and optimization can devise small sample sizes that allow
for the accurate ordering of sensitivity coefficients for the first
10â30 most sensitive chemical reactions in complex chemical-kinetic
mechanisms, and is particularly useful for studying the chemistry
in realistic devices. A key part of the paper is calibration of these
small samples. Because these small sample sizes are developed for
use in realistic combustion devices, the calibration is done over
the ranges of conditions in such devices, with a test case being the
operating conditions of a compression ignition engine studied earlier.
Compressionâignition engines operate under low-temperature
combustion conditions with quite complicated chemistry making this
calibration difficult, leading to the possibility of false positives
and false negatives in the ordering of the reactions. So an important
aspect of the paper is showing how to handle the trade-off between
false positives and false negatives using ideas from the multiobjective
optimization literature. The combination of the new global sensitivity
method and the calibration are sample sizes a factor of approximately
10 times smaller than were available with our previous algorithm
Resolving Some Paradoxes in the Thermal Decomposition Mechanism of Acetaldehyde
The
mechanism for the thermal decomposition of acetaldehyde has
been revisited with an analysis of literature kinetics experiments
using theoretical kinetics. The present modeling study was motivated
by recent observations, with very sensitive diagnostics, of some unexpected
products in high temperature microtubular reactor experiments on the
thermal decomposition of CH<sub>3</sub>CHO and its deuterated analogs,
CH<sub>3</sub>CDO, CD<sub>3</sub>CHO, and CD<sub>3</sub>CDO. The observations
of these products prompted the authors of these studies to suggest
that the enol tautomer, CH<sub>2</sub>CHOH (vinyl alcohol), is a primary
intermediate in the thermal decomposition of acetaldehyde. The present
modeling efforts on acetaldehyde decomposition incorporate a master
equation reanalysis of the CH<sub>3</sub>CHO potential energy surface
(PES). The lowest-energy process on this PES is an isomerization of
CH<sub>3</sub>CHO to CH<sub>2</sub>CHOH. However, the subsequent product
channels for CH<sub>2</sub>CHOH are substantially higher in energy,
and the only unimolecular process that can be thermally accessed is
a reisomerization to CH<sub>3</sub>CHO. The incorporation of these
new theoretical kinetics predictions into models for selected literature
experiments on CH<sub>3</sub>CHO thermal decomposition confirms our
earlier experiment and theory-based conclusions that the dominant
decomposition process in CH<sub>3</sub>CHO at high temperatures is
CâC bond fission with a minor contribution (âŒ10â20%)
from the roaming mechanism to form CH<sub>4</sub> and CO. The present
modeling efforts also incorporate a master-equation analysis of the
H + CH<sub>2</sub>CHOH potential energy surface. This bimolecular
reaction is the primary mechanism for removal of CH<sub>2</sub>CHOH,
which can accumulate to minor amounts at high temperatures, <i>T</i> > 1000 K, in most lab-scale experiments that use large
initial concentrations of CH<sub>3</sub>CHO. Our modeling efforts
indicate that the observation of ketene, water, and acetylene in the
recent microtubular experiments are primarily due to bimolecular reactions
of CH<sub>3</sub>CHO and CH<sub>2</sub>CHOH with H-atoms and have
no bearing on the unimolecular decomposition mechanism of CH<sub>3</sub>CHO. The present simulations also indicate that experiments using
these microtubular reactors when interpreted with the aid of high-level
theoretical calculations and kinetics modeling can offer insights
into the chemistry of elusive intermediates in the high-temperature
pyrolysis of organic molecules
Elevated Pressure Thermal Experiments and Modeling Studies on the Water-Gas Shift Reaction
Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether
Dimethyl ether (DME) oxidation is
a model chemical system
with
a small number of prototypical reaction intermediates that also has
practical importance for low-carbon transportation. Although it has
been studied experimentally and theoretically, ambiguity remains in
the relative importance of competing DME oxidation pathways in the
low-temperature autoignition regime. To focus on the primary reactions
in DME autoignition, we measured the time-resolved concentration of
five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl
formate, HPMF), CH2O, and CH3OCHO (methyl formate,
MF), from photolytically initiated experiments. We performed these
studies at P = 10 bar and T = 450â575
K, using a high-pressure photolysis reactor coupled to a time-of-flight
mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization
at the Advanced Light Source. Our measurements reveal that the timescale
of ROO decay and product formation is much shorter than predicted
by current DME combustion models. The models also strongly underpredict
the observed yields of CH2O and MF and do not capture the
temperature dependence of OOQOOH and HPMF yields. Adding the ROO +
OH â RO + HO2 reaction to the chemical mechanism
(with a rate coefficient approximated from similar reactions) improves
the prediction of MF. Increasing the rate coefficients of ROO â
QOOH and QOOH + O2 â OOQOOH reactions brings the
model predictions closer to experimental observations for OOQOOH and
HPMF, while increasing the rate coefficient for the QOOH â
2CH2O + OH reaction is needed to improve the predictions
of formaldehyde. To aid future quantification of DME oxidation intermediates
by photoionization mass spectrometry, we report experimentally determined
ionization cross-sections for ROO, OOQOOH, and HPMF
Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether
Dimethyl ether (DME) oxidation is
a model chemical system
with
a small number of prototypical reaction intermediates that also has
practical importance for low-carbon transportation. Although it has
been studied experimentally and theoretically, ambiguity remains in
the relative importance of competing DME oxidation pathways in the
low-temperature autoignition regime. To focus on the primary reactions
in DME autoignition, we measured the time-resolved concentration of
five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl
formate, HPMF), CH2O, and CH3OCHO (methyl formate,
MF), from photolytically initiated experiments. We performed these
studies at P = 10 bar and T = 450â575
K, using a high-pressure photolysis reactor coupled to a time-of-flight
mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization
at the Advanced Light Source. Our measurements reveal that the timescale
of ROO decay and product formation is much shorter than predicted
by current DME combustion models. The models also strongly underpredict
the observed yields of CH2O and MF and do not capture the
temperature dependence of OOQOOH and HPMF yields. Adding the ROO +
OH â RO + HO2 reaction to the chemical mechanism
(with a rate coefficient approximated from similar reactions) improves
the prediction of MF. Increasing the rate coefficients of ROO â
QOOH and QOOH + O2 â OOQOOH reactions brings the
model predictions closer to experimental observations for OOQOOH and
HPMF, while increasing the rate coefficient for the QOOH â
2CH2O + OH reaction is needed to improve the predictions
of formaldehyde. To aid future quantification of DME oxidation intermediates
by photoionization mass spectrometry, we report experimentally determined
ionization cross-sections for ROO, OOQOOH, and HPMF
Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether
Dimethyl ether (DME) oxidation is
a model chemical system
with
a small number of prototypical reaction intermediates that also has
practical importance for low-carbon transportation. Although it has
been studied experimentally and theoretically, ambiguity remains in
the relative importance of competing DME oxidation pathways in the
low-temperature autoignition regime. To focus on the primary reactions
in DME autoignition, we measured the time-resolved concentration of
five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl
formate, HPMF), CH2O, and CH3OCHO (methyl formate,
MF), from photolytically initiated experiments. We performed these
studies at P = 10 bar and T = 450â575
K, using a high-pressure photolysis reactor coupled to a time-of-flight
mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization
at the Advanced Light Source. Our measurements reveal that the timescale
of ROO decay and product formation is much shorter than predicted
by current DME combustion models. The models also strongly underpredict
the observed yields of CH2O and MF and do not capture the
temperature dependence of OOQOOH and HPMF yields. Adding the ROO +
OH â RO + HO2 reaction to the chemical mechanism
(with a rate coefficient approximated from similar reactions) improves
the prediction of MF. Increasing the rate coefficients of ROO â
QOOH and QOOH + O2 â OOQOOH reactions brings the
model predictions closer to experimental observations for OOQOOH and
HPMF, while increasing the rate coefficient for the QOOH â
2CH2O + OH reaction is needed to improve the predictions
of formaldehyde. To aid future quantification of DME oxidation intermediates
by photoionization mass spectrometry, we report experimentally determined
ionization cross-sections for ROO, OOQOOH, and HPMF