Real Gas Effects in High-Pressure Ignition of n-Dodecane/Air Mixtures.

This work studies the real gas effects on the autoignition of hydrocarbon fuels under high pressures, using normal dodecane (n-dodecane) as the representative fuel and the Redlich-Kwong equation of state (EoS) as the real gas description. It is demonstrated that the real gas description yields a shorter ignition delay time (IDT) compared with the ideal gas description, especially in low-temperature regimes which could encompass the negative temperature coefficient (NTC) phenomena and has a stronger dependence on the molecular volume than the attractive potential. The study further shows that high pressure facilitates low-temperature reaction pathways, where the compressibility factors of key reactants contribute to real gas effects. Moreover, the results suggest that accounting for real gas behavior leads to an increase in the formation of polycyclic aromatic hydrocarbons (PAHs), which, in turn, promotes soot generation.

On explosive boiling of a multicomponent Leidenfrost drop.

The gasification of multicomponent fuel drops is relevant in various energy-related technologies. An interesting phenomenon associated with this process is the self-induced explosion of the drop, producing a multitude of smaller secondary droplets, which promotes overall fuel atomization and, consequently, improves the combustion efficiency and reduces emissions of liquid-fueled engines. Here, we study a unique explosive gasification process of a tricomponent droplet consisting of water, ethanol, and oil ("ouzo"), by high-speed monitoring of the entire gasification event taking place in the well-controlled, levitated Leidenfrost state over a superheated plate. It is observed that the preferential evaporation of the most volatile component, ethanol, triggers nucleation of the oil microdroplets/nanodroplets in the remaining drop, which, consequently, becomes an opaque oil-in-water microemulsion. The tiny oil droplets subsequently coalesce into a large one, which, in turn, wraps around the remnant water. Because of the encapsulating oil layer, the droplet can no longer produce enough vapor for its levitation, and, thus, falls and contacts the superheated surface. The direct thermal contact leads to vapor bubble formation inside the drop and consequently drop explosion in the final stage.

Kinetic Study of Two-Stage Low-Temperature Heat Release in iso-Octane Autoignition.

Recognizing that low-temperature ignition of alkanes is usually associated with one heat release peak, we report herein that, for iso-octane under certain ranges of initial temperatures and pressures, two separate heat release peaks were observed through computational simulations using several kinetic mechanisms. The inherent chemical reason for this phenomenon is discussed using reaction channel analysis and is identified to result from the competition between R + O2 → RO2 and the ß scission reactions. By further utilizing sensitivity and path flux analyses, an isomeric effect is identified in that the different isomeric structures produced through the H-abstraction reactions can lead to differences in the subsequent low-temperature reaction pathways.

Statistical Analysis on Rate Parameters of the H2-O2 Reaction System.

Quantitative rate determination of elementary reactions is a major task in the study of chemical kinetics. To ensure the fidelity of their determination, progressively tightened constraints need to be placed on their measurement, especially with the development of various notable experimental techniques. However, the evaluation of reaction rates and their uncertainties is frequently conducted with substantial subjectivity due to data source, thermodynamic conditions, sampling range, and sparsity. To reduce the extent of biased rate evaluation, we propose herein an approach of uncertainty-weighted statistical analysis, utilizing weighted average, and weighted least-square regression in statistical inference. Based on the backbone H2/O2 chemistry, rate data for each elementary reaction are collected from the time-history profile in shock tube experiments and high-level theoretical calculations, with their assigned weight inversely depending on uncertainty, which would overall avoid subjective assessments and provide more accurate rate evaluation. Aided by sensitivity analysis, the rates of a few key reactions are further constrained in the less investigated low- to intermediate-temperature conditions using high-fidelity flow reactor data. Good performance of the constructed mechanism is confirmed with validation against the target of the high-fidelity flow reactor data. This study demonstrates a systematic approach for reaction rate evaluation and uncertainty quantification.

Ab Initio Kinetics of Methylamine Radical Thermal Decomposition and H-Abstraction from Monomethylhydrazine by H-Atom.

Methylamine radicals (CH3NH) and amino radicals (NH2) are major products in the early pyrolysis/ignition of monomethylhydrazine (CH3NHNH2). Ab initio kinetics of thermal decomposition of CH3NH radicals was analyzed by RRKM master equation simulations. It was found that ß-scission of the methyl H-atom from CH3NH radicals is predominant and fast enough to induce subsequent H-abstraction reactions in CH3NHNH2 to trigger ignition. Consequently, the kinetics of H-abstraction reactions from CH3NHNH2 by H-atoms was further investigated. It was found that the energy barriers for abstraction of the central amine H-atom, two terminal amine H-atoms, and methyl H-atoms are 4.16, 2.95, 5.98, and 8.50 kcal mol-1, respectively. In units of cm3 molecule-1 s-1, the corresponding rate coefficients were found to be k8 = 9.63 × 10-20T2.596 exp(-154.2/T), k9 = 2.04 × 10-18T2.154 exp(104.1/T), k10 = 1.13 × 10-20T2.866 exp(-416.3/T), and k11 = 2.41 × 10-23T3.650 exp(-870.5/T), respectively, in the 290-2500 K temperature range. The results reveal that abstraction of the terminal amine H-atom to form trans-CH3NHNH radicals is the dominant channel among the different abstraction channels. At 298 K, the total theoretical H-abstraction rate coefficient, calculated with no adjustable parameters, is 8.16 × 10-13 cm3 molecule-1 s-1, which is in excellent agreement with Vaghjiani's experimental observation of (7.60 ± 1.14) × 10-13 cm3 molecule-1 s-1 ( J. Phys. Chem. A 1997, 101, 4167-4171, DOI: 10.1021/jp964044z).

A cellular automata model for expanding turbulent flames.

Cellular automata models based on population dynamics, introduced by Von Neumann in the 1950s, has been successfully used to describe pattern development and front propagation in many applications, such as crystal growth, forest fires, fractal growth in biological media, etc. We, herein, explore the possibility of using a cellular automaton, based on the population dynamics of flamelets, as a low-order model to describe the dynamics of an expanding flame propagating in a turbulent environment. A turbulent flame is constituted by numerous flamelets, each of which interacts with their neighborhood composed of other flamelets, as well as unburned and burnt fluid particles. This local interaction leads to global flame dynamics. The effect of turbulence is simulated by introducing stochasticity in the local interaction and hence in the temporal evolution of the flamefront. Our results show that the model preserves various multifractal characteristics of the expanding turbulent flame and captures several characteristics of expanding turbulent flames observed in experiments. For example, at low turbulence levels, an increase in global burning rate leads to an increase in the turbulence level, while beyond a critical turbulence level, the expanding flame becomes increasingly fragmented, and consequently, the total burning rate decreases with increasing turbulence. Furthermore, at an extremely high turbulence level, the ignition kernel quenches at its nascent state and consequently loses its ability to propagate as an expanding flame.

Bouncing-to-Merging Transition in Drop Impact on Liquid Film: Role of Liquid Viscosity.

When a drop impacts on a liquid surface, it can either bounce back or merge with the surface. The outcome affects many industrial processes, in which merging is preferred in spray coating to generate a uniform layer and bouncing is desired in internal combustion engines to prevent accumulation of the fuel drop on the wall. Thus, a good understanding of how to control the impact outcome is highly demanded to optimize the performance. For a given liquid, a regime diagram of bouncing and merging outcomes can be mapped in the space of Weber number (ratio of impact inertia and surface tension) versus film thickness. In addition, recognizing that the liquid viscosity is a fundamental fluid property that critically affects the impact outcome through viscous dissipation of the impact momentum, here we investigate liquids with a wide range of viscosity from 0.7 to 100 cSt, to assess its effect on the regime diagram. Results show that while the regime diagram maintains its general structure, the merging regime becomes smaller for more viscous liquids and the retraction merging regime disappears when the viscosity is very high. The viscous effects are modeled and subsequently the mathematical relations for the transition boundaries are proposed which agree well with the experiments. The new expressions account for all the liquid properties and impact conditions, thus providing a powerful tool to predict and manipulate the outcome when a drop impacts on a liquid film.

An analysis of the explosion limits of hydrogen/oxygen mixtures with nonlinear chain reactions.

The ignition boundary of hydrogen/oxygen mixtures is a Z-shaped curve in the pressure-temperature space, demonstrating the existence of three explosion limits. In this study, a general analysis governing all the three explosion limits in an isothermal environment is performed by considering both linear chain reactions (reactant-radical reactions) and nonlinear chain reactions (radical-radical reactions), in addition to the zeroth-order reactant-reactant reactions. For the nonlinear reactions, it is further shown that the linear-nonlinear coupling has the major influence, while the effect of nonlinear-nonlinear coupling is negligible. Phenomenologically, at low pressures, the competition between linear branching and linear termination as well as wall destruction determines the first and second explosion limits, while the nonlinear chain reactions are unimportant because of the small radical concentrations under these conditions. However, at higher pressures, both linear and nonlinear chain reactions are needed to accurately describe the third limit, which would be underpredicted by considering the linear chain reactions alone. For intermediate and high pressures, the dominant species are HO2 and H2O2, respectively. Mechanistically, the concentration of HO2 becomes higher at higher pressures due to the three-body recombination reaction, H + O2 + M → HO2 + M, such that the radical-radical reactions involving HO2 become important, while the reaction HO2 + HO2 → H2O2 + O2 renders HO2 nonessential at the third limit, with the H2O2 radical generated by the nonlinear chain reactions becoming the controlling species.

Kinetics and branching fractions of the hydrogen abstraction reaction from methyl butenoates by H atoms.

In order to explore the hydrogen abstraction reaction kinetics of unsaturated methyl esters by hydrogen atoms, we selected two molecules for study, in particular methyl 3-butenoate and methyl 2-butenoate, whose C[double bond, length as m-dash]C double bonds are at different locations. We first determined an accurate and efficient electronic structure method for the investigation by considering eight hydrogen abstraction reactions and comparing their barrier heights and reaction energies computed using several exchange-correlation density functionals to those obtained from CCSD(T)-F12a/jun-cc-pVTZ coupled cluster calculations. In this way, we found the M06-2X/ma-TZVP method to have the best performance with a mean unsigned deviation from the CCSD(T) calculations of 0.51 kcal mol-1. Based on quantum-chemical calculations by using the M06-2X/ma-TZVP method, we then computed rate constants for 298-2500 K by direct dynamics calculations using multi-structural canonical variational transition state theory including tunneling by the multi-dimensional small-curvature tunneling approximation (MS-CVT/SCT). The computed transmission coefficients were compared with those obtained using the zero-curvature tunneling (ZCT) and one-dimensional Eckart tunneling (ET) approximations. We employed the multi-structural torsional method (MS-T) to include the multiple-structure and torsional potential anharmonic effects. The results show that the variational recrossing transmission coefficients range from 0.6 to 1.0, and the multi-structural torsional anharmonicity introduces a factor of 0.5-2.5 into the rate constant, while the tunneling transmission coefficients obtained by SCT can be as large as 17.4 and differ considerably from those determined by the less accurate ZCT and ET approximations. In addition, independent of the location of the C[double bond, length as m-dash]C double bond, the dominant hydrogen abstraction reactions occur at the allylic sites.

Correction: Nonmonotonic response of drop impacting on liquid film: mechanism and scaling.

Correction for 'Nonmonotonic response of drop impacting on liquid film: mechanism and scaling' by Xiaoyu Tang et al., Soft Matter, 2016, DOI: 10.1039/c6sm00397d.

Nonmonotonic response of drop impacting on liquid film: mechanism and scaling.

Drop impacting on a liquid film with a finite thickness is omnipresent in nature and plays a critical role in numerous industrial processes. The impact can result in either bouncing or merging, which is mainly controlled by the impact inertia of the drop and film thickness. Although it is known that impact with inertia beyond a critical value on a thick film promotes merging through the breakage of the interfacial gas layer, here we demonstrate that for an impact inertia less than that critical value, increasing the film thickness leads to a nonmonotonic transition from merging to bouncing to merging and finally to bouncing again. For the first time, two different merging mechanisms are identified and the scaling laws of the nonmonotonic transitions are developed. These results provide important insights into the role of the film thickness in the impact dynamics, which is critical for optimizing operating conditions for spray or ink-jet systems among others.

Role of Carbon-Addition and Hydrogen-Migration Reactions in Soot Surface Growth.

Using density functional theory and master equation modeling, we have studied the kinetics of small unsaturated aliphatic molecules reacting with polycyclic aromatic hydrocarbon (PAH) molecules having a diradical character. We have found that these reactions follow the mechanism of carbon addition and hydrogen migration (CAHM) on both spin-triplet and open-shell singlet potential energy surfaces at a rate that is about ten times those of the hydrogen-abstraction-carbon-addition (HACA) reactions at 1500 K in the fuel-rich postflame region. The results also show that the most active reaction sites are in the center of the zigzag edges of the PAHs. Furthermore, the reaction products are more likely to form straight rather than branched aliphatic side chains in the case of reacting with diacetylene. The computed rate constants are also found to be independent of pressure at conditions of interest in soot formation, and the activation barriers of the CAHM reactions are linearly correlated with the diradical characters.

Facilitated ignition in turbulence through differential diffusion.

Contrary to the belief that ignition of a combustible mixture by a high-energy kernel is more difficult in turbulence than in quiescence because of the increased dissipation rate of the deposited energy, we experimentally demonstrate that it can actually be facilitated by turbulence for mixtures whose thermal diffusivity sufficiently exceeds its mass diffusivity. In such cases, turbulence breaks the otherwise single spherical flame of positive curvature, and hence positive aerodynamics stretch, into a multitude of wrinkled flamelets subjected to either positive or negative stretch, such that the intensified burning of the latter constitutes local sources to facilitate ignition.

Dimerization of polycyclic aromatic hydrocarbons in soot nucleation.

A possible pathway of soot nucleation, in which localized π electrons play an important role in binding the polycyclic aromatic hydrocarbon (PAH) molecules having multiradical characteristics to form stable polymer molecules through covalent bonds, is studied using density functional and semiempirical methods. Results show that the number of covalent bonds formed in the dimerization of two identical PAHs is determined by the radical character, and the sites to form bonds are related to the aromaticity of individual six-membered ring structure. It is further shown that the binding energy of dimerization increases linearly with the diradical character in the range relevant to soot nucleation.

Ab initio kinetics for the decomposition of hydroxybutyl and butoxy radicals of n-butanol.

The decomposition kinetics of the hydroxybutyl and butoxy radicals (C4H9O) arising via H abstraction from n-butanol were studied theoretically with ab initio transition-state-theory-based master equation analyses. Stationary points on the C4H9O potential energy surface were calculated at either the RQCISD(T)/CBS//B3LYP/6-311++G(d,p) level or the RQCISD(T)/CBS//CASPT2/aug-cc-pVDZ level. Unimolecular pressure- and temperature-dependent rate coefficients were calculated over broad ranges of temperature (300-2500 K) and pressure (1.3 × 10(-3) to 10(2) atm) by solving the time-dependent multiple-well master equation. The "well merging" phenomenon was observed and analyzed for its influence on the branching ratios and rate coefficients. The theoretical predictions were compared with the available experimental and theoretical data and any discrepancies were analyzed. The predicted rate coefficients are represented with forms that may readily be used in combustion modeling of n-butanol.

An analysis of the explosion limits of hydrogen-oxygen mixtures.

In this study, the essential factors governing the Z-shaped explosion limits of hydrogen-oxygen mixtures are studied using eigenvalue analysis. In particular, it is demonstrated that the wall destruction of H and HO2 is essential for the occurrence of the first and third limits, while that of O, OH, and H2O2 play secondary, quantitative roles for such limits. By performing quasi-steady-state analysis, an approximate, cubic equation for the explosion limits is obtained, from which explicit expressions governing the various explosion limits including the state of the loss of non-monotonicity are derived and discussed.

Flame speed and self-similar propagation of expanding turbulent premixed flames.

In this Letter we present turbulent flame speeds and their scaling from experimental measurements on constant-pressure, unity Lewis number expanding turbulent flames, propagating in nearly homogeneous isotropic turbulence in a dual-chamber, fan-stirred vessel. It is found that the normalized turbulent flame speed as a function of the average radius scales as a turbulent Reynolds number to the one-half power, where the average radius is the length scale and the thermal diffusivity is the transport property, thus showing self-similar propagation. Utilizing this dependence it is found that the turbulent flame speeds from the present expanding flames and those from the Bunsen geometry in the literature can be unified by a turbulent Reynolds number based on flame length scales using recent theoretical results obtained by spectral closure of the transformed G equation.

Gas-phase kinetics study of reaction of OH radical with CH3NHNH2 by second-order multireference perturbation theory.

The gas-phase kinetics of H-abstraction reactions of monomethylhydrazine (MMH) by OH radical was investigated by second-order multireference perturbation theory and two-transition-state kinetic model. It was found that the abstractions of the central and terminal amine H atoms by the OH radical proceed through the formation of two hydrogen bonded preactivated complexes with energies of 6.16 and 5.90 kcal mol(-1) lower than that of the reactants, whereas the abstraction of methyl H atom is direct. Due to the multireference characters of the transition states, the geometries and ro-vibrational frequencies of the reactant, transition states, reactant complexes, and product complexes were optimized by the multireference CASPT2/aug-cc-pVTZ method, and the energies of the stationary points of the potential energy surface were refined at the QCISD(T)/CBS level via extrapolation of the QCISD(T)/cc-pVTZ and QCISD(T)/cc-pVQZ energies. It was found that the abstraction reactions of the central and two terminal amine H atoms of MMH have the submerged energy barriers with energies of 2.95, 2.12, and 1.24 kcal mol(-1) lower than that that of the reactants respectively, and the abstraction of methyl H atom has a real energy barrier of 3.09 kcal mol(-1). Furthermore, four MMH radical-H(2)O complexes were found to connect with product channels and the corresponding transition states. Consequently, the rate coefficients of MMH + OH for the H-abstraction of the amine H atoms were determined on the basis of a two-transition-state model, with the total energy E and angular momentum J conserved between the two transition-state regions. In units of cm(3) molecule(-1) s(-1), the rate coefficient was found to be k(1) = 3.37 × 10(-16)T(1.295) exp(1126.17/T) for the abstraction of the central amine H to form the CH(3)N(•)NH(2) radical, k(2) = 2.34 × 10(-17)T(1.907) exp(1052.26/T) for the abstraction of the terminal amine H to form the trans-CH(3)NHN(•)H radical, k(3) = 7.41 × 10(-20)T(2.428) exp(1343.20/T) for the abstraction of the terminal amine H to form the cis-CH(3)NHN(•)H radical, and k(4) = 9.13 × 10(-21)T(2.964) exp(-114.09/T) for the abstraction of the methyl H atom to form the C(•)H(2)NHNH(2) radical, respectively. Assuming that the rate coefficients are additive, the total rate coefficient of these theoretical predictions quantitatively agrees with the measured rate constant at temperatures of 200-650 K, with no adjustable parameters.

Ab initio kinetics for thermal decomposition of CH3N•NH2, cis-CH3NHN•H, trans-CH3NHN•H, and C•H2NNH2 radicals.

The thermal decomposition of the CH(3)N(•)NH(2), cis-CH(3)NHN(•)H, trans-CH(3)NHN(•)H, and C(•)H(2)NNH(2) radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the ß-scission of NH(2) to form methyleneimine is the predominant channel for the decomposition of the C(•)H(2)NNH(2) radical due to its small energy barrier of 13.8 kcal mol(-1). The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10(19)T(-1.672) exp(-9665.13/T) s(-1). In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH(3)NHN(•)H and trans-CH(3)NHN(•)H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol(-1) for their isomerization. The ß-scission of CH(3) from the cis-CH(3)NHN(•)H radical to form trans-diazene has an energy barrier of 35.2 kcal mol(-1), and the ß-scission of CH(3) from the trans-CH(3)NHN(•)H radical to form cis-diazene has an energy barrier of 39.8 kcal mol(-1). The CH(3)N(•)NH(2) radical undergoes the ß-scission of methyl hydrogen and amine hydrogen to form CH(2)═NNH(2), trans-CH(3)N═NH, and cis-CH(3)N═NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol(-1), respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01-100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.

Detonative propagation and accelerative expansion of the Crab Nebula shock front.

The accelerative expansion of the Crab Nebula's outer envelope is a mystery in dynamics, as a conventional expanding blast wave decelerates when bumping into the surrounding interstellar medium. Here we show that the strong relativistic pulsar wind bumping into its surrounding nebula induces energy-generating processes and initiates a detonation wave that propagates outward to form the current outer edge, namely, the shock front, of the nebula. The resulting detonation wave, with a reactive downstream, then provides the needed power to maintain propagation of the shock front. Furthermore, relaxation of the curvature-induced reduction of the propagation velocity from the initial state of formation to the asymptotic, planar state of Chapman-Jouguet propagation explains the observed accelerative expansion. Potential richness in incorporating reactive fronts in the description of various astronomical phenomena is expected.