Raghu Sivaramakrishnan

Argonne National Laboratory | ANL · Division of Chemical Sciences and Engineering

Over the last 20 to 25 years theoretical chemistry (particularly theoretical chemical kinetics) has played an increasingly important role in developing chemical kinetics models for combustion. Theoretical methods of obtaining rate parameters are now competitive in accuracy with experiment, particularly for small molecules. Moreover, theoretical met...

Collisional energy transfer plays a pivotal role in a-priori calculations of the pressure-dependent rate constants obtained from a master equation. However, accurate determinations of collisional energy transfer parameters are rare and so most kinetic studies rely on best-fits of these parameters to experimental measurements (if available) or estim...

Quasiclassical trajectories (QCT) and newly constructed global potential energy surfaces are used to compute thermal and nonthermal rate constants for the H + HO2 reaction. The thermal QCTs rate constants are up to 50% smaller than transition state theory (TST) rate constants based on the same level of electronic structure theory. This reduction is...

The kinetics of peroxy radical (RO2) reactions have been of long-standing interest in atmospheric and combustion chemistry. Nevertheless, the lack of kinetic studies at higher temperatures for their reactions with other radicals such as OH has precluded the inclusion of this class of reactions in detailed kinetics models developed for combustion ap...

Resonantly stabilized radicals (RSR's) such as allylic radicals typically have bond energies exceeding 45 kcal/mol and are therefore reasonably long-lived up to quite high temperatures. Consequently, kinetics of the unimolecular dissociations and bimolecular reactions of these long-lived RSR's assume significance in high temperature chemically reac...

The shock-induced combustion ramjet (Shcramjet) based on oblique detonation waves (ODWs) is among the promising choices for hypersonic propulsion systems. An understanding of the ignition, propagation, and stability of ODWs is critical to harnessing their propulsive potential. In such high speed reacting flows, there is a high probability of occurr...

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...

The kinetics of prompt dissociation involves rovibrationally excited species (generally formed by exothermic reactions) which may dissociate or isomerize prior to thermalization via collisions with the bath gas. Treating such rovibrationally excited species (so-called "hot" species) with standard kinetic phenomenology may result in incorrect macros...

The binary fuel blend of H 2 /CH 4 is one of the most promising hydrogen-enriched hydrocarbon fuels in spark-ignition (SI) engines. Yet, the undesirable phenomenon of super-knock, which can severely and instantaneously damage an SI engine, limits its widespread adoption. Moreover, there is still a lack of consensus on the precise mechanism by which...

At low temperatures and high pressures, the HȮ2 + HȮ2 reaction is an important reaction in combustion. There are significant unresolved discrepancies between prior theoretical and experimental studies of this reaction. It has generally been presumed to occur as an abstraction on the triplet surface to produce H2O2 + ³O2. We employ a combination of...

A combined theoretical and experimental study showing styrene primarily decomposes to styryl radicals + H.

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here...

Recent theoretical studies have shown that termolecular chemistry can be facilitated through reactions of flame radicals (H, O, and OH) or O2 with highly-energized collision complexes (either radical or stable species) formed in exothermic reactions. In this work, radical-radical recombination reaction induced termolecular chemistry and its impact...

Insights into the structure dependence of butyl radicals in recombination, disproportionation and nonthermal reactions in the autoignition region.

Cycloalkanes and alkenes are important components of real fuels but there is little kinetic and mechanistic data on the dissociation of most large cyclic and olefinic molecules at elevated temperatures. We present here the first experimental and theoretical investigation of dissociation of cycloheptane and the initial product from ring opening, 1-h...

Recent theoretical and modeling studies have shown that reactive colliders can facilitate nonthermal reactivity (i.e. chemically termolecular reactions) in bimolecular association reactions. Highly-energized collision complexes from the incipient association reaction were shown in these studies to be intercepted by reactive radicals (H, O, and OH)...

The chemistry of low-temperature ignition in propane/air mixtures is analyzed using a recently developed pathway representation of the chemical kinetics. The “Sum Over Histories Representation” allows time-dependent kinetic observables to be computed using an expansion over global chemical pathways that follow chemical moieties as they move through...

Recent theoretical studies have indicated that ephemeral radical collision complexes (formed by barrierless exothermic reactions of radicals with O2 or other stable species) can undergo facile termolecular reactive collisions with radicals (also another barrierless process). These termolecular reactions were found to impact global reactivity in rad...

Quasiclassical trajectories are used to compute nonthermal rate constants, k*, for abstraction reactions involving highly-excited methane CH4* and the radicals H, O, OH, and O2. Several temperatures and internal energies of methane, Evib, are considered, and significant nonthermal rate enhancements for large Evib are found. Specifically, when CH4*...

OH + molecules are an important class of reactions in combustion and atmospheric chemistry. Consequently, numerous studies have measured rate constants for these processes over an extended temperature range. A large majority of these experimental studies have utilized the decay of [OH] profiles (monitored either by absorption or laser-induced fluor...

Analysis of large-scale, realistic models incorporating detailed chemistry can be challenging because each simulation is computationally expensive, and a complete analysis may require many simulations. This paper addresses one such problem of this type, chemical-reaction selection in engine simulations. In this computationally challenging case, it...

On the cover: The cover image, by Canan Karakaya et al., is based on the Article Boundary‐layer model to predict chemically reacting flow within heated, high‐speed, micro‐tubular reactors, DOI: 10.1002/kin.21173.

Laminar natural gas flames are investigated at engine-relevant thermochemi-cal conditions where the ignition delay time τ is short due to very high ambient temperatures and pressures. At these conditions, it is not possible to measure or calculate well-defined values for the laminar flame speed s l , laminar flame thickness δ l , and laminar flame...

Chen nozzle experiments are used to study the early‐stage unimolecular decomposition of larger molecules and, sometimes, the following chemistry. The nozzle itself is typically a small‐diameter (order millimeter), short (order 20–50 mm), heated (order 1700 K) tube (nozzle) that exhausts into a vacuum chamber where a variety of diagnostics may be us...

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 10-30 most sensitive chemical reactions in complex chemical-kinetic mechanisms,...

The formation and destruction pathways of the formyl radical (HCO) occupy a pivotal role in the conversion of fuel molecules (and their intermediates) to eventual products CO and CO2, and therefore, HCO has been a prescient indicator for heat release in combustion. In this work, we have characterized the impact of including non-equilibrium effects...

The reactions of D/H with n-C4H10 and i-C4H10 have been studied with both shock-tube experiments and ab initio transition state theoretical calculations. D-atom profiles were measured behind reflected shock waves using D-atom atomic resonance absorption spectrometry (ARAS) in mixtures with C2D5I (D-atom precursor, <1 ppm) and the alkane of interest...

Although the reactions of fuel-radicals with other dominant flame radicals such as H and CH3 are important reactions in low-pressure flames, they have not been well studied. These reactions may occur through either recombination to form stabilized molecular complexes or direct abstractions and chemically activated addition-eliminations to yield bim...

Weakly bound free radicals have low-dissociation thresholds such that at high temperatures, timescales for dissociation and collisional relaxation become comparable, leading to significant dissociation during the vibrational-rotational relaxation process. Here we characterize this "prompt" dissociation of formyl (HCO), an important combustion radic...

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 micro-tubular reactor experiments on the ther...

Methylbutanoate (MB), a C4 methyl ester, represents the simplest surrogate that captures the chemical effects of the ester moiety in biodiesel and biodiesel surrogates. An updated chemical kinetic model has been developed to characterize the ignition and flame characteristics of MB. The mechanistic elements within this model that relate to the MB a...

The reactions of D/H with methyl formate (MF) and methyl acetate (MA) have been studied with both shock-tube experiments and ab initio transition state theoretical calculations. D-atom profiles were measured behind reflected shock waves using D-atom atomic resonance absorption spectrometry (ARAS) over the temperature range 1050–1270 K, at pressures...

The shock tube technique has been used to study the reactions, CH3 + C2H6 → C2H4 + CH4 + H (1) CH3 + C2H4 → Products + H (2) CH3 + C2H2 → Products + H (3) Biacetyl, (CH3CO)2, was used as a clean high temperature thermal source for CH3-radicals for all the three reactions studied in this work. For reaction (1), the experiments span a T-range of 1153...

We study the role of individual reaction rates on engine performance, with an emphasis on the contribution of quantum tunneling. It is demonstrated that the effect of quantum tunneling corrections for the reaction HO2 + HO2 = H2O2 + O2 can have a noticeable impact on the performance of a high-fidelity model of a compression-ignition (e.g., diesel)...

The shock tube technique was used to study the thermal decomposition of ozone, O3, with a view to using this as a thermal precursor of O-atoms at high temperatures. The formation of O-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range, 819 K ≤ T ≤ 1166 K, at press...

The shock tube technique was used to study the high temperature thermal decomposition of dimethyl carbonate, CH3OC(O)OCH3 (DMC). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1053-1157 K at pressures ~ 0.5 atm. The H-atom profiles were s...

The shock tube technique was used to study the high temperature thermal decomposition of methylformate (MF) and methylacetate (MA). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1194-1371 K at pressures similar to 0.5 atm. The H-atom pro...

The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260-1566 K) behind reflected shock waves using high-sensitivity H-ARAS detectio...

The reactions of H/D with C2H6 and C3H8 have been studied with both shock-tube experiments and ab initio transition-state theory calculations. Rate constants for the reactions of D with C2H6 and C3H8 have been measured in reflected shock wave experiments over the temperature range, 1128–1299 K, at pressures 0.3–1 atm. D atoms are detected using ato...

The thermal decomposition of toluene-d5 (C6D5CH3) has been studied at high temperatures with the reflected shock-tube technique using H- and D-atom ARAS. The experiments were performed at high-T (1469–1859 K) at nominal pressures ≈0.25–1.50 atm.The present study utilizing the ultra-sensitive H-atom ARAS technique has provided a direct measurement o...

The thermal decompositions of benzyl (C6H5CH2) and benzyl,α,ά-d2 (C6H5CD2) have been studied at high temperatures with the reflected shock tube technique using H- and D-atom Atomic Resonance Absorption Spectroscopy (ARAS). The experiments were performed at high-T (1437–1801 K) at nominal pressures ≈ 0.3–1.3 atm.The present experiments utilizing the...

The thermal decompositions of benzyl- (C6H5CH2), benzyl α,ά-d2- (C6H5CD2), and o-xylyl-radicals (o-CH3C6H4CH2) have been studied at high temperatures with the reflected shock tube technique using H- and D-atom ARAS. The experiments were performed at high-T (1437-1801 K) at nominal pressures ≈ 0.3-1.3 atm. Using the ultra-sensitive H- and D-atom A...

A detailed chemical kinetic mechanism has been assembled for the combustion of ethanol. While there are no surprises in the mechanistic initiation and propagation routes, the uncertainties in the rate coefficients and branching ratios for many of these primary routes have inhibited an accurate description of the high temperature combustion of ethan...

The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure...

The thermal decomposition of propane has been studied using both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for propane have been measured at high temperatures behind reflected shock waves using high-sensitivity H-ARAS detection and CH(3) optical absorption. The two m...

Methylbutanoate, a C4 methyl ester, possibly represents the simplest surrogate that captures the chemical effects of the ester moiety in biodiesel and biodiesel surrogates. An updated chemical kinetic model has been used to characterize the ignition and flame characteristics of MB. The mechanistic elements within this model that relate to the MB an...

The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and high-sensitivity H-atom ARAS d...

Rate constants for H-atom abstractions by OH radicals from a series of alkanes (propane, n-butane, i-butane and neo-pentane) have been measured at high temperatures with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308nm. The experiments represent the first direct measurements of these rat...

Pyrolysis experiments on n-heptane, 1-heptene and 1,6-heptadiene have been performed using the UIC High-Pressure Shock Tube (HPST) at pressures relevant to diesel combustion systems. The experimental pressures for these experiments ranged from 25 to 50atm and temperatures varied from 1000 to 1350K with reaction times ranging from 1 to 3ms. Dilute r...

The thermal dissociation of acetaldehyde has been studied with the reflected shock tube technique using H(D)-atom atomic resonance absorption spectrometry detection. The use of an unreversed light source yields extraordinarily sensitive H atom detection. As a result, we are able to measure both the total decomposition rate and the branching to radi...

The dissociation of a molecule by simple bond cleavage generally leads to two radical fragments. However, as the radicals separate they can roam, sampling large volumes of orientation space. This roaming occurs at large radical-radical separations (> 3 ). During this roaming, the radicals may sample orientations where there is a barrierless path le...

Primary and secondary reactions involved in the thermal decomposition of NH2OH are studied with a combination of shock tube experiments and transition state theory based theoretical kinetics. This coupled theory and experiment study demonstrates the utility of NH2OH as a high temperature source of OH radicals. The reflected shock technique is emplo...

High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801–1347 K, and represent the first direct measurements of the title rate constants at T>500 K for cyclopentane and cyclohexane and the only hi...

High-temperature rate constant experiments on OH with the five large (C5-C8) saturated hydrocarbons n-heptane, 2,2,3,3-tetramethylbutane (2,2,3,3-TMB), n-pentane, n-hexane, and 2,3-dimethylbutane (2,3-DMB) were performed with the reflected-shock-tube technique using multipass absorption spectrometric detection of OH radicals at 308 nm. Single-point...

The water-gas shift reaction influences the chemistry between the postcombustion gases of a rocket and the rocket's graphite nozzle. The rocket's operating pressures (70-600 atm) exceed those for existing water-gas shift reaction data, and further study of the chemistry undersimilar conditions is essential for optimum rocket design. To investigate...

The mutual sensitization of the oxidation of NO and a CH(4)-C(2)H(6) (10 : 1) simulated natural gas (NG) blend was studied under fuel lean conditions (Phi = 0.5) at 50 atm and 1000-1500 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 s...

This paper reports measurements of the thermal dissociation of 1,1-difluoroethane in the shock tube. The experiments employ laser-schlieren measurements of rate for the dominant HF elimination using 10% 1,1-difluoroethane in Kr over 1500-2000 K and 43 < P < 424 torr. The vinyl fluoride product of this process then dissociates affecting the late obs...

The high pressure oxidation of dilute CO mixtures doped with 150–200 ppm of H2 has been studied behind reflected shock waves in the UIC high pressure single pulse shock tube. The experiments were performed over the temperature range from 1000 to 1500 K and pressures spanning 21–500 bars for stoichiometric (Φ = 1) and fuel lean (Φ = 0.5) oxidation....

The thermal decomposition of vinyl fluoride was investigated at pressures 75-1370 Torr and high temperatures 1500-2300 K in shock tubes using two different independent detection techniques: time of flight mass spectrometry and laser schlieren densitometry. The measured rate coefficients show a strong fall off from the high pressure limiting rate co...

It is clear that since the development and validation of one of the most comprehensive and widely used models for Methane and Natural Gas Combustion, GRIMECH 3.0, there have been a number of fundamental studies that have led to the revision of thermochemical parameters for key species (OH, HO2) and rate parameters for key reactions involved in H2/C...

The heterogeneous reaction C(s)+CO2 has been studied under high pressure and high temperature conditions. The high-pressure shock tube (HPST) facility at the University of Illinois at Chicago was utilized to perform experiments with post-shock pressures ranging from 35-412 atm and a temperature range between 1475-2658 K with reaction times lasting...

Pyrolysis experiments on n-heptane, 1-heptene and 1,6-heptadiene have been performed using the UIC High Pressure Shock tube (HPST) at pressures relevant to diesel combustion systems. The experimental pressures for these experiments ranged from 25-50 atm and temperatures varied from 1000-1350K with reaction times ranging from 1-2 ms. Dilute reagent...

The role of diacetylene as a soot precursor and the importance of its chemistry in the chemistry of larger aromatic hydrocarbons have been postulated but not thoroughly investigated experimentally. The absence of experimental data for the pyrolysis of diacetylene at high pressures has led to incomplete validation of existing diacetylene models at p...

The water gas shift reaction (WGSR) influences the chemistry between the post-combustion gases of a rocket and the rocket's graphite nozzle. The rocket's operating pressures (70-600 atm) exceed those for existing WGSR data and further study of the chemistry under similar conditions is essential for optimum rocket design. In order to investigate the...

The mutual sensitization of the oxidation of NO and a CH4-C2H6 (10:1) blend (NG) was studied under fuel lean conditions (Φ=0.5) at 50 atm and 1000-1500 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...

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Part 1 in this two-part series focused on the high-pressure experimental results and the high-pressure limiting rate coefficients for the primary steps in toluene decomposition. The present work...

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Experiments were performed at nominal high pressures of 27 and 45 bar and spanning a wide temperature range from 1200 to 1900 K. A variety of stable species, ranging from small hydrocarbons to s...

The high-temperature, high-pressure pyrolysis of the prototype aromatic, benzene, has been studied behind reflected shock waves in the UIC High Pressure Single Pulse Shock Tube. Three sets of experiments were performed at nominal pressures of 30 and 50 bars in the high temperature regime from 1200–1800 K. Stable species sampled from the shock tube...

An extensive experimental study of ethane oxidation and pyrolysis has been conducted in the high pressure shock tube at UIC covering reflected shock pressures from 5–1000 bar, reaction temperatures up to 1550 K and stoichiometric (Φ = 1), fuel rich (Φ = 5), and pyrolytic mixtures. The experimental data has been used to develop a single model that c...

Density functional theory (DFT) has been used along with isodesmic reaction schemes to estimate heats of formation for aromatics and polynuclear aromatic hydrocarbons (PAHs). Calculations have been performed for 42 molecules, 12 of which have uncertain or unknown experimental values, using the B3-LYP functional with the small 6-31G(d) basis set. He...

An extensive experimental study of ethane oxidation and pyrolysis has been conducted in the high pressure shock tube at UIC covering reflected shock pressures from 5-1000 bar, reaction temperatures up to 1550 K and stoichiometric (Φ = 1 ), fuel rich (Φ = 5), and pyrolytic mixtures. The experimental data has been used to develop a single model that...

A detailed chemical kinetic model has been developed to predict the oxidation of toluene in the high temperature regime (1200–1500K) over wide pressure ranges (25–610bar) based on experimental data obtained in the high pressure single pulse shock tube at the University of Illinois at Chicago. The experimental data were obtained at pressures of 25,...

The high-pressure single pulse shock tube (HPST) at the University of Illinois at Chicago has been used to study the oxidation of toluene at reflected shock pressures from 22 to 550 bar and temperatures from 1210 to 1480 K. Experiments were performed for dilute stoichiometric, Φ=1Φ=1, and rich, Φ=5Φ=5, reagent mixtures. Stable species were analyzed...

A unique high-pressure single pulse shock tube has been used to obtain the first experimental data for ethane oxidation and pyrolysis at very high pressures and temperatures. Experiments were performed at two nominal reaction pressures of 340 bar and 613 bar in the temperature range 1050 K to 1450 K. The major stable species were identified and the...

Two chemical thermometers, 1,1,1-trifluoroethane and cyclohexene, have been used to calibrate the temperatures behind reflected shock waves, T5real, in a unique high-pressure, single-pulse shock tube. Experiments with 1,1,1-trifluoroethane were performed at nominal postshock pressures of 5000 psi and 9000 psi, and T5real was calculated from the ext...

Pyrolysis experiments on n-heptane, 1-heptene and 1,6-heptadiene have been performed using the UIC High Pressure Shock tube (HPST) at pressures relevant to diesel combustion systems in order to examine the correlation between an increase in the degree of unsaturation in biologically derived diesel fuels and an increase in NO formation through acety...

Thesis (M.S. in Chemical Engineering)--University of Illinois at Chicago, 2002. Includes bibliographical references (leaves 134-135).