Chemical Kinetic Properties Research Articles

SABRE hyperpolarization of nicotinamide derivatives and their molecular dynamics properties.

Signal amplification by reversible exchange hyperpolarization explores the chemical structure and kinetic properties of nicotinamide derivatives. N-Benzyl nicotinamide and nicotinic acid hydrazide compounds display relatively fast dissociation rates of approximately 7-8 s-1 and long proton T1 relaxation times of 5-20 s, respectively. Consequently, these substrates exhibit remarkable signal enhancements, reaching approximately 175 and 102 fold, respectively, underscoring the efficacy of the hyperpolarization technique in elucidating the behavior of these compounds.

Investigation of the effects of N2 and CO2 on the overpressure hazards, flame behaviors and reaction kinetics of LPG/air explosions

ABSTRACT To research the influence of inert gases on the explosion behaviors of LPG/air mixtures, LPG explosion tests suppressed by CO2 and N2 were conducted in a spherical chamber and the chemical reaction kinetic properties were analyzed. The results show that CO2 has a better inhibitory effect than N2, especially when the added amount is small. With the increasing volume fraction of inert gases, the LPG explosion flame propagation speed continuously decreases, and the phenomenon of flame uplift and wrinkling deformation occurs. The explosion pressure of LPG/CO2(N2)/air mixture and the average flame propagation speed in the initial stage follow an exponential function with an offset. The laminar flame structure of 4% LPG is similar to its mixtures with N2 and CO2. After adding inert gases, the maximum temperature of the LPG laminar flame decreases significantly, and the flame thickness increases. The key elementary reactions affecting the N2(CO2)/LPG/air explosions are given. The research results have important implications for suppression of LPG explosions.

Structure-Based Drug Discovery with Deep Learning.

Artificial intelligence (AI) in the form of deep learning has promise for drug discovery and chemical biology, for example, to predict protein structure and molecular bioactivity, plan organic synthesis, and design molecules de novo. While most of the deep learning efforts in drug discovery have focused on ligand-based approaches, structure-based drug discovery has the potential to tackle unsolved challenges, such as affinity prediction for unexplored protein targets, binding-mechanism elucidation, and the rationalization of related chemical kinetic properties. Advances in deep-learning methodologies and the availability of accurate predictions for protein tertiary structure advocate for a renaissance in structure-based approaches for drug discovery guided by AI. This review summarizes the most prominent algorithmic concepts in structure-based deep learning for drug discovery, and forecasts opportunities, applications, and challenges ahead.

Isolating gasdynamic and chemical effects on the detonation cellular structure: A combined experimental and computational study

The dependence of detonation cell regularity on mixture gasdynamic and chemical kinetic properties are investigated through hydrogen-oxygen detonations with different diluent and ozone addition. A total of seven mixtures are specifically designed such that the post-shock specific heat ratio γVN, representing the gasdynamic effect, and the effective activation energy ϵi, representing the chemistry effect, can be varied independently among these mixtures. Both experiments and two-dimensional (2D) Navier-Stokes simulations are performed. For the ranges of γVN and ϵi considered, γVN is found to moderately affect cell regularity. For mixtures of relatively large γVN (≥1.3), detonation cells become slightly irregular with decreasing γVN. A clear increase in cell irregularity is found when γVN is small (<1.3). In contrast, the impact of ϵi on cell regularity is more notable: mixtures of large ϵi produce significantly more irregular cells, corresponding to frequent appearances of transverse detonation structures that dominate the global burning behavior. Finally, ozone is qualitatively and quantitatively shown to regulate the detonation structures in both simulations and experiments tested in this study, and results suggest that trace additives that alter the ϵi of a mixture can likely be used to tune the mixture for a desired regularity.

Chemical kinetic properties of HNgF → HF + Ng (Ng = Ar, Kr, Xe, and Rn) reactions: An example of fortuitous cancelling of relevant relativistic effects

The relativistic effects on energetic quantities of the HNgF → HF + Ng (Ng = Ar, Kr, Xe, and Rn) reactions were evaluated by means of advanced four-component calculations with high-level electron correlation treatment and adequate relativistic basis sets. The results indicate that the scalar relativistic corrections can provide classical barrier height increments of as much as 4.8% (1.9 kcal mol−1) for the reaction with the heaviest noble gas, radon, while the spin-orbit coupling is almost as important in this case, lowering the value by 3.8% (−1.5 kcal mol−1). Therefore, these two relevant relativistic corrections cancel almost completely. A similar picture is seen for the energy variation from the reactant (HRnF) to the products (HF + Rn), but the contribution signs are reversed now (−4.0 and 3.4 kcal mol−1). A partial cancelling is also noticed for the HXeF → HF + Xe reaction. Such pattern is surprising since scalar relativistic effects usually predominate by magnitude orders over the spin-orbit coupling. These conclusions are reflected in the changes observed in rate constant values. Finally, an alternative decomposition route catalyzed by HF is suggested for these HNgF compounds, which can shed some light on the controversies regarding their thermal stability.

The relativistic effects on the methane activation by gold(I) cations.

The reactivity of gold has been investigated for a long time. Here, we performed an in-depth analysis of relativistic effects over the chemical kinetic properties of elementary reactions associated with methane activation by gold(I) cations, CH4 + Au+ ↔ AuCH2 + + H2. The global reaction is modeled as a two-step process, CH4 + Au+ ↔ HAuCH3 + ↔ AuCH2 + + H2. Moreover, the barrierless dissociation of the initial adduct between reactants, AuCH4 +, is discussed as well. Higher-order relativistic treatments are used to provide corrections beyond the commonly considered scalar effects of relativistic effective core potentials (RECPs). Although the scalar relativistic contributions predominate, lowering the forward barrier heights by 48.4 and 36.1kcal mol-1, the spin-orbit coupling effect can still provide additional reductions of these forward barrier heights by as much as 9% (1.0 and 2.2kcal mol-1). The global reaction proceeds rapidly at low temperatures to the intermediate attained after the first hydrogen transfer, HAuCH3 +. The relativistic corrections beyond the ones from RECPs are still able to double the rate constant of the CH4 + Au+ → HAuCH3 + process at 300K, while the reverse reaction becomes five times slower. The formation of global products from this intermediate only becomes significant at much higher temperatures (∼1500K upward). The scalar relativistic contributions decrease the dissociation energy of the initial adduct, AuCH4 +, into the global products by 105.8kcal mol-1, while the spin-orbit effect provides an extra lowering of 1.8kcal mol-1.

Modelling reaction propagation for Al/CuO nanothermite pellet combustion

A continuum-scale model for the combustion of consolidated nanothermite pellets is introduced. A simplified chemical kinetics model is used for the solid state nanothermite reaction while a two-phase porous media flow accounts for the mass and heat transfer within the consolidated pellet under an equilibrium thermodynamic assumption. The thermophysical and chemical kinetic properties of the Al/CuO nanothermite pellets are determined from the literature. A sensitivity analysis reveals the relative importance of these modelling parameters for both the advection and conduction-dominated combustion regimes. Based on the governing equations, we propose a thermodynamically consistent non-dimensionalization of the global Peclet and Damköhler numbers which are based on the characteristic Darcy velocity within the porous pellet. The non-dimensionalization results in a linear scaling between the normalized burn rate and Peclet number, which allows a collapse of the numerical results with published experimental data. We identify a transition between the conduction- and advection-dominated combustion based on the transient burn rate of the nanothermite pellet combustion.

New Frontiers in Experimentally Observing Detonation Properties

New Frontiers in Experimentally Observing Detonation Properties

Comparative study on the laminar flame speeds of methylcyclohexane-methanol and toluene-methanol blends at elevated temperatures

Laminar flame speeds of methylcyclohexane (MCH)-methanol and toluene-methanol blends were experimentally determined with spherically expanding flame method in a constant volume bomb at atmospheric pressure, initial temperatures of 393 and 433 K, covering wide equivalence ratio range. The blending ratio of methanol in liquid volume varies as 0%, 20%, 40%, 60%, 80%, 100%. Nonlinear methodology was employed to remove the stretch effect in the data processing. Experimental results show that MCH-air flame propagates faster than toluene at the same condition. The addition of methanol into MCH and toluene results in acceleration of laminar flame speed especially at the rich mixtures. Since the published model suffers difficulty in reproducing experimental data, model refinements were carried out and the refined model yields better performance. Comprehensive analyses were developed regarding thermal and chemical kinetic properties. For MCH has lower adiabatic flame temperature than toluene and methanol addition into the two cyclic fuels decreases the adiabatic flame temperature, the thermal effect on laminar flame speed difference is negligible. Therefore, the effect of chemical kinetics was specifically discussed. MCH and toluene have different ring structures. The disintegration of aromatic ring plays as the limiting step in the high-temperature oxidation of toluene, resulting in low concentration of active radical pool and overall reactivity. However, the ring opening reaction of MCH occurs easily after the initial H-abstraction reaction, which favors the production of active intermediates and the enhancement of flame propagation. For the blending fuels, the analyses show that the laminar flame speed variation of the blends are primarily caused by the methanol substitution and the disturbance of reaction pathway through affecting the generation of important intermediates.

Experimental and kinetic modeling study of laminar flame characteristics of higher mixed alcohols

Mixtures of alcohols combining strengths of lower and higher alcohols show promising alternative properties as engine fuel. To aid the extensive application of the mixed alcohols, detailed combustion investigation is necessary. In this study, laminar flame speeds and Markstein lengths were measured for simplified higher mixed alcohols (blends of methanol and n‑hexanol/n‑heptanol/n‑octanol) with spherically propagating flame at 0.1 MPa, elevated temperature of 433 K, and different mixing ratios. A comprehensive model was developed to describe the high temperature chemistry of the mixed alcohols. The accuracy of the model was validated with present data. Results reveal that the three higher alcohols exhibited close laminar flame speeds and Markstein lengths resulting from their similar thermal, diffusion, and chemical kinetic properties. With the increasing mixing ratio of the higher alcohols, the laminar flame speed decreased, especially for the rich mixtures. The maximum laminar flame speed slightly shifted to the lean side. The Markstein length increased at extremely lean mixtures and decreased at extremely rich mixtures, with the intersection occurring at the equivalence ratio between 1.2 and 1.3. The laminar flame speed variation of the mixed alcohols was dominated by chemical kinetics. Reaction pathway analyses indicated that the cracking process of each component in the mixed alcohols remains as that in the pure alcohols. Such result demonstrated the chemical kinetic effect resulting from changes in fuel components.

On the effects of fuel properties and injection timing in partially premixed compression ignition of low octane fuels

A better understanding on the effects of fuel properties and injection timing is required to improve the performance of advanced engines based on low temperature combustion concepts. In this work, an experimental and computational study was conducted to investigate the effects of physical and chemical kinetic properties of low octane fuels and their surrogates in partially premixed compression ignition (PPCI) engines. The main objective was to identify the relative importance of physical versus chemical kinetic properties in predicting practical fuel combustion behavior across a range of injection timings. Two fuel/surrogate pairs were chosen for comparison: light naphtha (LN) versus the primary reference fuel (PRF) with research octane number of 65 (PRF 65), and FACE (fuels for advanced combustion engines) I gasoline versus PRF 70. Two sets of parametric studies were conducted: the first varied the amount of injected fuel mass at different injection timings to match a fixed combustion phasing, and the second maintained the same injected fuel mass at each injection timing to assess resulting combustion phasing changes. Full-cycle computational fluid dynamic engine simulations were conducted by accounting for differences in the physical properties of the original and surrogate fuels, while employing identical chemical kinetics. The simulations were found to capture trends observed in the experiments, while providing details on spatial mixing and chemical reactivity for different fuels and injection timings. It was found that differences in physical properties become increasingly important as injection timing was progressively delayed from premixed conditions, and this was rationalized by analysis of mixture stratification patterns resulting from injection of fuels with different physical properties. The results suggest that accurate descriptions of both physical and chemical behavior of fuels are critical in predictive simulations of PPCI engines for a wide range of injection timings.

The Computational Singular Perturbation/Perfectly Stirred Reactor Approach in Reduced Chemistry of Premixed Ethanol Combustion

Ethanol is a bio-fuel widely used in engines as a fuel or fuel additive. It is, in particular, attractive because it can be easily produced in high quality from renewable resources. Its properties are of interest in many fields, such as gas turbines applications as well as fuel cells. In the past decades, research in chemistry and engineering has put a lot of effort into a better understanding of its gas-phase chemical kinetic properties during combustion processes. This work describes a methodology to define an optimal expression of the reaction progress variable in the context of tabulated chemistry in laminar premixed combustion. The choice of the reaction progress variable is based on the investigation of the wide range of consumption rates of the species involved in the reaction. Two methods are used: the computational singular perturbation method and a sensitivity analysis of the time scales evaluated with a perfectly stirred reactor. The thermochemical databases computed with these techniques are compared in the cases of a freely propagating flame and a Bunsen flame, in the laminar premixed regime and under stoichiometric conditions. The influence of the chemical kinetics on the laminar flame speed is estimated from the results of the freely propagating flame. The case where the differences in the performance between the databases become more pronounced is the Bunsen flame, where some databases lead to a premature ignition prediction of the flame.

Dual Fuel Combustion of N-heptane/methanol-air-EGR Mixtures

Numerical simulations are performed to study the combustion processes of n-heptane and methanol/air/EGR under high-temperature and high-pressure conditions relevant to a dual-fuel compression-ignition engine. Detailed chemical kinetic mechanism and transport properties are considered in the simulation. The simulations are carried out by performing three-dimensional (3D) direct numerical simulation (DNS) in a cuboid constant volume enclosure and two-dimensional (2D) DNS in a constant volume domain corresponding to the 3D domain. The results reveal three combustion modes involved in dual fuel engines, the ignition of the n-heptane jet, the propagation of thin reaction fronts in the methanol/air/EGR mixture, and finally the onset of ignition of the entire mixture. In dual-fuel combustion under high initial temperature conditions (1000K or higher) the three different combustion modes occur rather quickly with two distinct peaks of heat release corresponding to the two ignition modes. At low temperatures (800K or lower) the ignition delay time of the mixture is longer and more complete mixing is achieved before the onset of a single ignition of the n-heptane/methanol mixture.

Liquid Fuel Composition Effects on Forced, Nonpremixed Ignition

The effects of jet fuel composition on ignition probability have been studied in a flowfield that is relevant to turbine engine combustors, but also fundamental and conducive to modeling. In the experiments, a spark kernel is ejected from a wall and propagates transversely into a crossflow. The kernel first encounters an air-only stream before transiting into a second, flammable (premixed) stream. The two streams have matched velocities, as verified by hot-wire measurements. The liquid fuels span a range of physical and chemical kinetic properties. To focus on their chemical differences, the fuels are prevaporized in a carrier air flow before being injected into the experimental facility. Ignition probabilities at atmospheric pressure and elevated crossflow temperature were determined from optical measurements of a large number of spark events, and high-speed imaging was used to characterize the kernel evolution. Eight fuel blends were tested experimentally; all exhibited increasing ignition probability as equivalence ratio increased, at least up to the maximum value studied (∼0.8). Statistically significant differences between fuels were measured that have some correlation with fuel properties. To elucidate these trends, the forced ignition process was also studied with a reduced-order numerical model of an entraining kernel. The simulations suggest ignition is successful if sufficient heat release occurs before entrainment of colder crossflow fluid quenches the exothermic oxidation reactions. As the kernel is initialized in air, it remains extremely lean during the initial entrainment of the fuel–air mixture; thus, richer crossflows lead to quicker and higher exothermicity.

Thermal quenching of mixed eddies in non-premixed flames

Extinction in a turbulent non-premixed flame can lead to a local state or eddy where the fuel, oxidizer and combustion products are mixed together. The process of quenching and re-ignition of these eddies in turbulent non-premixed flames is analyzed. The corresponding critical conditions are first related to the eddy Damköhler number, and then, through the spectrum of the mixed eddy sizes, to the turbulent Karlovitz number of the flow and the properties of the stoichiometric pre-mixture in the eddy. It is shown that the mixed eddies at the reaction length scale are the most reactive, and there is a continuous range of reaction completeness between the overall quenching and re-ignition. Based on the spectrum of eddy sizes in the flow, a local volume fraction of reactive eddies is defined that can be used as a factor in the expression of the reaction rate in turbulent non-premixed combustion models. Representative chemical kinetic properties necessary to implement this approach in fire modeling are introduced. The model predictions are compared with test data on quenching by dilution, blow-off extinction limit, and extinction strain rate as a function of diluent type and concentration.

Surrogate Definition and Chemical Kinetic Modeling for Two Different Jet Aviation Fuels

For emulation of the chemical kinetic combustion phenomena and physical properties of S-8 POSF 4734 and Jet-A POSF 4658, two surrogate fuels were formulated by directly matching their molecular structure and functional groups. The same functional groups, CH3, CH2, CH, C, and phenyl, were chosen to formulate the S-8 and Jet-A surrogates with n-dodecane/2,5-dimethylhexane (0.581/0.419 mole fraction) and n-dodecane/2,5-dimethylhexane/toluene (0.509/0.219/0.272 mole fraction), respectively. The numerical results using the surrogate fuels were compared with the experimental data and the results predicted by other surrogate fuel formulation methods. The results show that the present method can formulate surrogate mixtures of both jet fuels and Fischer–Tropsch real fuels and reproduce the combustion characteristics in homogeneous ignition and the flow reactor oxidation process. The idea presented here could be extended to other real fuels with the appropriate choice of surrogate fuel components.

Heat transfer and flame stabilization of laminar premixed flames anchored to a heat-flux burner

Measurement of the burning velocity of unstretched laminar hydrogen/air premixed flames suffers from large uncertainties owing to the highly diffusive nature of hydrogen that can give rise to flame instability. This paper reports on a numerical study of the structures and stability of laminar premixed CH4/O2/CO2 flames and H2/O2/N2 flames anchored to a heat-flux burner using a high-order numerical method with detailed chemical kinetic mechanisms and detailed transport properties. The aim is to elucidate the effect of the flow and temperature inhomogeneity generated by the burner plate holes on flame structures and burning velocity. Heat transfer flux between the burner plate and the surrounding gaseous mixture is investigated under various standoff distances and burner plate temperatures. The burning velocity and the detailed flow, temperature and species distributions in flames with a zero net heat flux between the flames and the burner plate are analyzed. It is found that for the methane flames, when the standoff distance is sufficiently small, the burner can essentially suppress the intrinsic flame instability, but the plate holes can give rise to flame wrinkles of the size of the holes. At high standoff distances, the non-uniformity of the flow from the burner plate holes has a minor effect on the flame surface wrinkling; however, large-scale cellular structures can appear on the flame surface due to intrinsic flame instability. For the studied methane flames the effect of non-uniformity of the flow from the burner plate holes on the burning velocity is fairly small. For the studied hydrogen flames the burner plate could not totally suppress the intrinsic flame instability. The intrinsic flame instability can give rise to a significant increase in the flame surface area and mean burning velocity, with more than 25% increase in the burning velocity.

A computational methodology for formulating gasoline surrogate fuels with accurate physical and chemical kinetic properties

Gasoline is the most widely used fuel for light duty automobile transportation, but its molecular complexity makes it intractable to experimentally and computationally study the fundamental combustion properties. Therefore, surrogate fuels with a simpler molecular composition that represent real fuel behavior in one or more aspects are needed to enable repeatable experimental and computational combustion investigations. This study presents a novel computational methodology for formulating surrogates for FACE (fuels for advanced combustion engines) gasolines A and C by combining regression modeling with physical and chemical kinetics simulations. The computational methodology integrates simulation tools executed across different software platforms. Initially, the palette of surrogate species and carbon types for the target fuels were determined from a detailed hydrocarbon analysis (DHA). A regression algorithm implemented in MATLAB was linked to REFPROP for simulation of distillation curves and calculation of physical properties of surrogate compositions. The MATLAB code generates a surrogate composition at each iteration, which is then used to automatically generate CHEMKIN input files that are submitted to homogeneous batch reactor simulations for prediction of research octane number (RON). The regression algorithm determines the optimal surrogate composition to match the fuel properties of FACE A and C gasoline, specifically hydrogen/carbon (H/C) ratio, density, distillation characteristics, carbon types, and RON. The optimal surrogate fuel compositions obtained using the present computational approach was compared to the real fuel properties, as well as with surrogate compositions available in the literature. Experiments were conducted within a Cooperative Fuels Research (CFR) engine operating under controlled autoignition (CAI) mode to compare the formulated surrogates against the real fuels. Carbon monoxide measurements indicated that the proposed surrogates accurately reproduced the global reactivity of the real fuels across various combustion regimes.

Chemical kinetic and combustion characteristics of transportation fuels

Internal combustion engines running on liquid fuels will remain the dominant prime movers for road and air transportation for decades, probably for most of this century. The world’s appetite for liquid transportation fuels derived from petroleum and other fossil resources is already immense, will grow, will at some future time become economically unsustainable, and will become infeasible only in the very long term. The ongoing process of augmenting and eventually replacing petroleum-derived fuels with liquid alternative fuels must necessarily involve approaches that result in comparatively much lower net carbon cycle emissions from the transportation sector, most likely through a combination of carbon sequestration and renewable fuel production. The successful growth and establishment of a sustainable, profitable alternative fuels industry will be best facilitated by approaches that integrate alternative products into petroleum-derived fuel streams (i.e., gasolines, diesel, and jet fuels) and consider synergistic evolution of and integration with prevailing refining and liquid fuel distribution infrastructures.The emergence of low temperature combustion strategies, particularly those implementing dual fuel methods to achieve Reaction Controlled Compression Ignition (RCCI), offers the potential to significantly improve operating efficiency and reduce emissions with minimal aftertreatment. For all advanced combustion engine technologies, but especially for RCCI, a clear understanding of fuel property influences on combustion behaviors will be important to achieving projected engine performance and emissions.To achieve the benefits projected by emerging engine technologies, the properties of petroleum-derived fuels themselves must be modified over time, but the effects of blending candidate alternative fuels with these conventional fuels must also be understood. Predicting the coupled physical and chemical property effects of real fuels on energy conversion system performance and emissions is a daunting problem, even for petroleum-derived real fuels, since each is composed of several hundred to thousands of individual chemical species typically belonging to one of a few organic classes (e.g., n-paraffins, iso-paraffins, cyclo-paraffins, olefins, aromatics). For specific combustion applications, it is often the global combustion response to variations in the composition of fuel mixtures – inclusive of those occurring by blending petroleum-derived fuel with alternative fuel candidates – that is of interest for fuel property optimization. This paper presents an overview of tools used for evaluating and emulating combustion-relevant properties of real fuels and alternative fuel candidates. New analytical and statistical methods can provide important insights as to how the ensembles of distinct molecular structures found in a given fuel mixture contribute to the physical and chemical kinetic properties that govern its combustion in energy conversion processes. Such tools can in turn assist in screening candidate alternative fuels for more detailed study.

The possibility of membrane-based solvent-extraction (MBSX) as a separation method for Zr and Hf in aqueous solutions

The possibility of membrane-based solvent-extraction (MBSX) as a separation method for Zr and Hf in aqueous solutions. The focus of this study was to determine the suitability of this MBSX technology to purify and simultaneously concentrate Zr. It is known that Zr and Hf possess inherently slow chemical kinetic properties. However, by optimising the design and the operating conditions, we succeeded in removing Hf from the Zr solutes, while almost completely recovering the desired Zr.