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We present here an experimental study of the influence of the aeroelastic coupling between the combustion chamber walls and the acoustic fluid field on the onset and development of thermoacoustic instabilities in stoichiometric propane-air premixed flames. A horizontal quasi-two-dimensional Hele–Shaw chamber formed by two parallel plates separated a small distance h is used. The flames are ignited at the open end, in contact with the atmosphere, and propagate towards the opposite closed end. The experiments reveal three distinct propagation regimes determined by the stiffness of the plates and the evolution of the pressure perturbation generated during ignition: (i) for sufficiently rigid plates, we observed secondary acoustic instabilities with large amplitude oscillations in the direction of propagation of the flame; for flexible enough walls to be compliant with ignition-related pressure changes, (ii) the propagation of the flame undergoes small-amplitude oscillations (primary acoustic instabilities) along the channel or (iii) it is smooth with no oscillations whatsoever. The flexural rigidity of the plate is modified experimentally by changing both the width W and thickness h w of the top plate of the Hele–Shaw cell. The data recorded by the pressure transducer and the accelerometer is used to plot a stability map in the W − h w parametric space to define the combination of structural parameters that triggers the onset of thermoacoustic instabilities. Our experimental measurements, supplemented with results from a theoretical analysis of the walls vibration modes, indicated that deformation-induced volume changes of around 0.1% of the volume of the Hele–Shaw cell are sufficient to suppress thermoacoustic instabilities. [ABSTRACT FROM AUTHOR]

The purpose of the present work is to analyze propagating two-dimensional flames confined in slender semi open channels, where the combustion process takes place towards the closed end. The study focuses on the calculation of the growth rate of the transition from symmetric to non-symmetric flames propagation identified by Jiménez et al. [1]. The combustion cell is initially filled with a stoichiometric mixture of fuel and air at standard conditions. Ignition is induced close to the open end of the channel under planar and gaussian profiles in temperature and species mass fractions which activate a sustained combustion process. The gases inside the chamber, initially stagnant, are accelerated due to the heat generated in the chemical reactions, leading to the development of lateral boundary layers, so that the hot gases exit the channel following a Poiseuille velocity profile. This transverse flow velocity differences are accommodated by means of a symmetric tulip shape formed after a short initial transient. Acoustic waves generated in the ignition process, keep travelling along the channel, bouncing at the walls and interacting with the flame during all the combustion process. Additionally, the flame structure, curved by Darrieus-Landau instability, interacts with the pressure waves triggering small amplitude oscillations (primary oscillation mode), which under certain conditions can transition to higher amplitude oscillations (secondary mode). This transition is observed to be highly dependent not only on the cell geometry, but also in the initial conditions generated by the ignition procedure. The aim of this work is to improve the understanding of this process, complementing the work of Jiménez et al. [1] , and to characterize the effect of the channel width in this transition. [ABSTRACT FROM AUTHOR]

In this paper, we study the effect of gravity, or buoyancy forces, on the structure of flames propagating in horizontal channels. It is shown that there are two mechanisms for the appearance of non-symmetric flames. The first, more obvious, is related to buoyancy, when a hotter gas of lower density tends to be located in the upper half of the channel. However, there is a second mechanism associated with the intrinsic flame instabilities, which also can cause the loss of flame symmetry. This mechanism can, at certain values of the parameters, act in the opposite direction, when hotter gases are enclosed in the lower part of the channel. In this case, two stable non-symmetric solutions may exist in the presence of gravitational forces and the establishment of one or another configuration depends on the initial conditions. The stability of these solutions is demonstrated by time-dependent computations. [ABSTRACT FROM AUTHOR]

In this work, a computational study on nearly two-dimensional reacting fronts under canonical configurations is proposed. Specifically, a quasi-2D description of the conservation equations with a one-step irreversible Arrhenius chemical model is used to simulate ultra-lean hydrogen-air premixed flames that slowly propagate over streams of reactants that are confined between plates and are subject to conductive heat losses. Under these conditions, recent previous experiments identified two unprecedented stable flame configurations: two-headed and circular isolated flames. This study identified buoyancy and the relative importance of conductive heat losses as the two parameters controlling the formation of these isolated flames. The numerical evaluation of the different terms ascertains the physical mechanisms that drive the formation of two-headed or circular flames. Additionally, the systematic variation of the controlling parameters depicts the maps of stable solutions that determine the capability of the two flame configurations to emerge. In particular, the existence of a range of parametric values in which both flame configurations are stable proves, also, that initial conditions are crucial to determine which of the two configurations prevails. [ABSTRACT FROM AUTHOR]

• A simplified mathematical model to describe combustion inside micro-rotary engines is proposed. • A numerical model based on high-order meshfree RBF-FD discretizations is developed. • Several benchmarks demonstrate the accuracy and effectiveness of our approach. • It aims to be instrumental in providing the needed understanding to produce more reliable micro-combustion systems. New micro-devices, such as unmanned aerial vehicles or micro-robots, have increased the demand of a new generation of small-scale combustion power system that go beyond the energy-density limitations of batteries or fuel cells. The characteristics short residence times and intense heat losses reduce the efficiency of combustion-based devices, a key factor that requires of an acute modelling effort to understand the competing physicochemical phenomena that hamper their efficient operation. With this objective in mind, this paper is devoted to the development of a high-order meshfree method to model combustion inside complex geometries using radial basis functions-generated finite differences (RBF-FD) based on polyharmonic splines (PHS) augmented with multivariate polynomials (PHS+poly). In our model, the combustion chamber of a micro-rotary engine is simulated by a system of unsteady reaction-diffusion equations coupled with a steady flow passing a bidimensional stenotic channel of great slenderness. The conversion efficiency is characterized by identifying the different combustion regimes that emerged as a function of the ignition point. We show that PHS+poly based RBF-FD is able to achieve high-order algebraic convergence on scattered node distributions, enabling for node refinement in key regions of the fluid domain. This feature makes it specially well adapted to integrate problems in irregular geometries with front-like solutions, such as reactive fronts or shock waves. Several numerical tests are carried out to demonstrate the accuracy and effectiveness of our approach. [ABSTRACT FROM AUTHOR]

We present a linear stability analysis of the Richtmyer-Meshkov instability that develops when a shock wave reaches a sinusoidally perturbed premixed flame from behind. In the hydrodynamic regime, when acoustic contributions dominate the flame growth rate, the problem is analytically addressed by the direct integration of the sound wave equations at both sides of the flame, which are bounded by the reflected and transmitted shock waves and the flame front that acts as a contact surface in the hydrodynamic limit. The resolution involves: a hyperbolic change of variables to modify the triangular spatio-temporal domain, a transformation in the Laplace variable, the resolution of the functional equations in the frequency domain, and the final inverse Laplace transform. The latter involves a novel resolution method that is proven beneficial for long-time dynamics. Asymptotic analysis is also carried out to describe the early time and late time hydrodynamic response. The nonuniform flow field resulting from distorted oscillating shocks is characterized by acoustic, rotational, and entropic disturbances, each of which exerts a substantial influence on flame dynamics. These disturbances contribute to the intricate interplay of factors shaping the behavior of the flame in response to the nonuniform flow. In particular, the sensitivity of local flame propagation to temperature disturbances is investigated. This work contributes to a deeper understanding of Richtmyer-Meshkov instability dynamics and offers insights into instability reduction through the modulation of temperature disturbances generated by the transmitted shock. • Introduction of a self-consistent analytical model for Richtmyer-Meshkov instability within shock-flame interactions. • Development of an enhanced resolution method for the functional equation in the Laplace variable. • Introduction of a new resolution method for the transient evolution via the inverse Laplace transform. • Provision of a comprehensive mechanism regarding Richtmyer-Meshkov instability reduction. [ABSTRACT FROM AUTHOR]

Gold(I) complexes containing stabilising ligands such as phosphines or N -heterocyclic carbenes (NHCs) are known to be inhibitors of the enzyme thioredoxin reductase (TrxR) and therefore act as potential apoptosis-inducing anticancer drug candidates. The conjugation of biomolecules overexpressed in cancer cells to the gold complexes makes them semi-targeted metabolites. Auranofin, an anti-arthritis agent, encompasses this property and exhibits anti-tumour activities. The synthesis, characterization and biological evaluation of four novel N -heterocyclic carbene-gold(I)-thiosugar complexes derived from glucose, lactose and galactose is reported. The reactions of 1,3-dibenzyl-4,5-diphenyl-imidazol-2-ylidene gold(I) chloride (NHC∗-Au-Cl) with pre-synthesized glycosyl thiols under mildly basic conditions gave the desired NHC-Au-thiosugar complexes in high to excellent yields (79–91%). The complexes retain the strong and redox-active Au-S bond contained in Auranofin. All complexes showed good solubility in biological media and were tested against the NCI 60 cancer cell panel for cytotoxicity. The synthesized NHC∗-Au derivatives showed good activity in the medium to low micromolar region, while complex 2 showed activity in the low micromolar to nanomolar region against the tested cell lines. To provide a theoretical structure of 4 , computational calculations were carried out based on the crystal structures of NHC-Au-SCN and NHC-Au-S-C 6 H 4 OMe. [ABSTRACT FROM AUTHOR]

We examine in this work the effect of an external electric field on the propagation velocity of a laminar, one-dimensional and lean premixed flame, with the final goal of clarifying the relative importance of each of the three different mechanisms postulated in the literature to explain the effect of electric fields on flames: ionic wind, kinetic enhancement by non-thermal electrons and ohmic heating. The one-dimensional model proposed here expands the four-reactions scheme previously presented by Sánchez-Sanz, et al. (2015) to include the effect of non-thermal electrons and activated neutral molecules on flame acceleration. Two additional reactions are included in the model to complete a minimum set of six elementary reaction capable of qualitatively reproduce the results observed in classical (Jaggers, and Von Engel, (1971).) and recent (Volkov et al., 2013; Murphy, et al., 2014,) experiments. The limit of weakly ionized plasmas is used to integrate the Boltzmann equation and to derive an explicit expression for the electron temperature proportional to the square of the electric field. The numerical integration of the conservation equations gives the flame propagation velocity for a given set of parameters. The results reveal the importance of the electric field polarity on flame acceleration, finding faster flames for positive electric fields than for equally intense negative fields. At low-intensity fields, our results indicate that the ionic wind, and the associated redistribution of the charged particles, is the main mechanism inducing flame acceleration. In more intense fields, the combined effect of the ionic wind and the heat transfer from the high-temperature electrons to the background gas induces a significant increase in the temperature field upstream and downstream of the flame front. Associated with this temperature increase, relevant changes on the flame speed are computed for positive, intense electric fields, while only modest flame accelerations are observed for equally intense negative fields, behavior that reproduces qualitatively the measurements by Murphy et al. (2014). The reduced sensitivity to an external electric field when the mixture approaches stoichiometry, observed experimentally by Jaggers, and Von Engel (1971) and Fang et al. (2015), is also reproduced by the model proposed in this work. [ABSTRACT FROM AUTHOR]

[Display omitted] • A flexible and deformable electrochemical sensor to detect dopamine is developed. • The sensor consists on a membrane made of alternated layers of PLA and PEDOT. • Spin-coated nanoperforated PLA layers allow to electropolymerize PEDOT layers. • The self-supported membrane acts directly as electrode, detecting dopamine. • Sensing is within the range of concentrations found for specific types of cancer. In addition of a key catecholamine neurotransmitter, dopamine is is the metabolite predominantly produced by specific types of tumors (e.g. paragangliomas and neuroblastomas), which cannot be diagnosed using conventional sensitive tests. Within this context, development of flexible electrochemical sensors to monitor dopamine levels in physiological fluids for the early diagnosis and control of diseases related to abnormal levels of such compound, is necessary. In this work, a flexible self-supported membrane, which acts directly as electrode, has been developed to detect dopamine. The membrane consists of three nanoperforated polylactic acid (PLA) layers, which provide flexibility and mechanical integrity, separated by two layers of an electroactive copolymer, which are obtained by electrochemical copolymerization of 3,4-ethylenedioxythiophene and aniline. The sensitivity and detection limit provided by the electroactive copolymer, which is accessible to dopamine molecules through the nanoperforations of the PLA outer layers, is 1.846 µA/(cm2·µM) and 1.7 µM, respectively, in a urea-rich environments that mimics urine. These values allow us to propose the self-standing flexible electrodes developed in this study for the detection of dopamine in patients affected by paragangliomas and neuroblastomas tumors, which typically present dopamine concentrations between 2 and 7 μM. [ABSTRACT FROM AUTHOR]

This paper investigates the initiation of a deflagration in a premixed boundary-layer stream by continuous heat deposition from a line energy source placed perpendicular to the flow on the wall surface, a planar flow configuration relevant for small-scale combustion applications, including portable rotary engines. Ignition is investigated in the constant density approximation with a one-step irreversible reaction with large activation energy adopted for the chemistry description. The ratio of the characteristic strain time, given by the inverse of the wall velocity gradient, to the characteristic deflagration residence time defines the relevant controlling Damköhler number D . The time-dependent evolution following the activation of the heat source is obtained by numerical integration of the energy and fuel conservation equations. For sufficiently small values of D , the solution evolves towards a steady flow in which the chemical reaction remains confined to a finite near-source reactive kernel. This becomes increasingly slender for increasing values of D , corresponding to smaller near-wall velocities, until a critical value D c 1 is reached at which the confined kernel is replaced by a steady anchored deflagration, assisted by the source heating rate, which develops indefinitely downstream. As the boundary-layer velocity gradient is further decreased, a second critical Damköhler number D c 2 > D c 1 is reached at which the energy deposition results in a flashback deflagration propagating upstream against the incoming flow along the base of the boundary layer. The computations investigate the dependence of D c 1 and D c 2 on the fuel diffusivity and the dependence of D c 1 on the source heating rate, delineating the boundaries that define the relevant regime diagram for these combustion systems. [ABSTRACT FROM AUTHOR]

A computational study of the aromatic characteristics of phenalenyl (PLY) upon replacement of the central carbon atom by Si, Ge, N, P, As, B, Al and Ga atoms comprising isoelectronic series with different charges (0, +1, −1) has been performed at the B3LYP/6-311++G(d,p) level. Two different geometries have been obtained, one planar and another one bowl-shaped. PLY derivatives exhibit a loss of the aromatic character in all cases indicated by NICS (Nucleus-Independent Chemical Shifts) values. These aromatic features are also in agreement with the pyramidalisation undergone by the central atom. The current density maps for those planar systems corroborate NICS findings. These variations in the aromaticity have been rationalised in terms of charge localisation. Harmonic oscillator model of aromaticity (HOMA) and para -delocalisation indices (PDI) have been used to evaluate the electron density delocalisation. HOMA values do not correlate with NICS but PDI indices follow the same trend than NICS. [ABSTRACT FROM AUTHOR]

There have been many experimental investigations into the ability of electric fields to enhance combustion by acting upon ion species present in flames [1] . In this work, we examine this phenomenon using a one-dimensional model of a lean premixed flame under the influence of a longitudinal electric field. We expand upon prior two-step chain-branching reaction laminar models with reactions to model the creation and consumption of both a positively-charged radical species and free electrons. Also included are the electromotive force in the conservation equation for ion species and the electrostatic form of the Maxwell equations in order to resolve ion transport by externally applied and internally induced electric fields. The numerical solution of these equations allows us to compute changes in flame speed due to electric fields. Further, the variation of key kinetic and transport parameters modifies the electrical sensitivity of the flame. From changes in flame speed and reactant profiles we are able to gain novel, valuable insight into how and why combustion can be controlled by electric fields. [ABSTRACT FROM AUTHOR]

A thorough study of the structural and aromatic properties of derivatives of benzene and naphthalene where one or more –CH– groups have been substituted by a nitrogen, phosphorous or arsenic atom has been carried out at B3LYP/6-311++G(d,p) level. Relative energies between isomers range from 5.0 to 294 kJ mol −1 finding the largest relative energies in compounds with nitrogen substitutions. In general, most of the compounds show to be planar with a few exceptions, which exhibit twisted structures. Wiberg bond indexes as well as bond distances indicate that in almost all the cases the bond nature is benzene-like. Aromatic characteristics have been addressed calculating NICS values, profiles, isosurfaces and HOMA indexes. NICS(1) and (2) present values close to those of benzene showing aromatic behaviour, confirmed by NICS profiles and 3D NICS isosurfaces. HOMA indexes obtained for those compounds with reported parameterized bonds are in agreement with their corresponding aromatic nature. [ABSTRACT FROM AUTHOR]

Abstract: The effect of the equivalence ratio on the stability and dynamics of a premixed flame in a planar micro-channel with a step-wise wall temperature profile is numerically investigated using the thermo-diffusive approximation. To characterize the stability behavior of the flame, we construct the stability maps delineating the regions with different flame dynamics in the inlet mass flow rate m vs. the equivalence ratio parametric space. The flame stability is analyzed for fuels with different diffusivity by changing the Lewis numbers in the range . On the other hand, the Lewis number of the oxidizer is kept constant and equal to unity . Our results show that, for very diffusive fuels, the stability of the flame varies significantly with the equivalence ratio, transitioning from stable flames for lean mixtures to highly unstable flames when . As the fuel Lewis number approaches unity, the stability behavior of the flame for lean and rich mixtures becomes more similar to give, in the equidiffusional case , a symmetric stability map around the stoichiometric mixture . In all cases considered, the most stable flames are always found around the stoichiometric mixtures , when the flame instabilities are completely suppressed for very diffusive fuels , or are reduced to a narrow range of inflow velocities for fuel Lewis numbers equal or greater than unity. The ratio between the size of the channel and the flame thickness d turns out to be of great importance in the stability behavior of the flame. Keeping the rest of parameters constant, an increase in d for lean flames makes the flame considerably more unstable, confirming the findings of previous works. Nevertheless, as the stoichiometric ratio approaches , that trend is reversed to give flames that become more stable as the size of the channel is increased. [Copyright &y& Elsevier]

Abstract: Continuing with our interest in aromaticity, we have studied the influence that replacement of formal C–C single bonds by C–C C–C fragments, in a series of mono- (cyclobutadiene, benzene, and cyclooctatetraene) and fused-carbocycles (naphthalene and azulene), has in their properties, focusing mostly on NMR and aromaticity. We have analyzed the effect of such substitution not only in the aromaticity of the different structures, but also in the influence of low and high spin states by means of NICS values over the rings and 3D NICS isosurfaces. We have found that, in most of the cases, the substitution induces a loss of aromaticity in singlet states (both restricted and unrestricted) that can be recovered when triplet states are taken into account. [Copyright &y& Elsevier]

Abstract: Analysis of the planar premixed flames on a porous plug was performed numerically for finite activation energy within the diffusive-thermal model. The paper is focused on the influence of radiation heat loses on the flame standoff distance and its linear stability. We show that the presence of volumetric heat losses limits the range of the mass flow range as well as it can promote the flame instabilities of different kinds, both oscillatory and cellular. The oscillatory instability, which for freely propagating flames can be usually observed for the Lewis number larger than one, in the porous-plug case occurs also for flames with unity and lower than unity Lewis number. For flames with Le <1 both cellular and oscillatory instabilities can be observed simultaneously in a certain range of the mass flow rate. [Copyright &y& Elsevier]

Abstract: We have theoretically studied, using PCM–water solvation, the cation–π and π–π complexes established by the biologically relevant 5-substituted 2-pyridinylguanidinium derivatives and electron-rich and electron-depleted aromatic systems (benzene and (hexafluoro)benzene). In condensed phase using PCM–water and M06-2X/6-311++G(d,p) different cation–π and π–π complexes were found. The interactions established within these complexes have been analyzed by means of the Atoms in Molecules and Natural Bond Orbital approaches and electron density difference maps have been calculated. Finally, the effect of the cation–π and π–π complexation on the aromaticity of pyridine, benzene and hexafluorobenzene was evaluated by calculating the corresponding aromaticity indexes, NICS0, 1 and 2 as well as the NICS on the 0.001 a.u. isodensity surface. [Copyright &y& Elsevier]

Abstract: The conditions that lead to the extinction of a premixed flame propagating along a channel of height 2h 1 ∼ δ T are studied using the thermo-diffusive approximation, with δ T = D T /S L , D T and S L representing, respectively, the thermal flame thickness, the thermal conductivity and the planar flame speed. It is found that flame propagation is greatly affected by conductive heat losses to the wall , where n denotes the transverse distance to the wall, T and T 0 are, respectively, the fluid’s and wall’s temperature and is the heat transfer parameter. The sensitivity of the flame to heat losses grows as the channel narrows, leading to complete flame extinction at extremely small values of . As a way to overcome flame quenching at the walls, Lloyd and Weinberg proposed the recirculation of part of the heat stored in the exhaust gases to preheat the cold combustible mixture. In order to understand the flame dynamics in the presence of heat recirculation, we analyze the problem of two parallel channels with a combustible mixture and an inert fluid moving in opposite directions, with maximum velocities m 1 and m 2 respectively, and where heat exchange is permitted through the common wall separating both channels. It is shown that flame extinction occurs if the velocity ratio m 2/m 1 is outside the range m 2/m 1 ∈[m 21,min m 21.max ], where the limiting values m 21,min and m 21.max are obtained numerically for a given set of parameters. The amplitude of this range depends, mainly, on the Damkhöler number d =[h 1/(D T /S L )]2, quantity that represents the ratio between the channel’s height h 1 and the thermal flame thickness D T /S L . It is shown that heat recirculation is capable of extending combustion to very small h 1, but there is still a minimum value of the Damkhöler number d min below which combustion is not possible. The evolution of d min is computed numerically to give values as small as d =0.0025 for a given combination of the geometric parameters of the problem. The maximum flame temperature computed in the parallel channel configuration rises above the adiabatic flame temperature T e due to the effect of the heat recirculation. This effect becomes more significant as d is reduced, leading to maximum temperatures close to T ≃1.4T e in very narrow channels. [Copyright &y& Elsevier]

Abstract: A theoretical study of acidity and hydrogen bond acceptor properties of tetrahedric phosphine oxide derivatives have been carried out by means of MP2 computational methods. The results obtained for the mentioned complexes have been compared with the analogous ones of trimethylphosphine oxide. The strain decreases the complexation energy with metallic atoms as well as the thermodynamic and hydrogen bond acceptor (HBA) ability of the tetrahedric derivatives. [Copyright &y& Elsevier]

Abstract: This work reports the theoretical study of the aromaticity of a series of carbocycles (benzene, cyclohexane, bent and planar cyclooctatetraene) and heterocycles (pyridine, furan, thiophene, pyrrole) and their didehydro forms (arynes and hetarynes). As aromaticity probe Schleyer''s NICS were used and represented in two 3D isosurfaces of the electron density. The spatial 3D representation of the NICS is shown to be a powerful tool to visualize the aromaticity (or its absence) of different molecules. [Copyright &y& Elsevier]

Abstract: This paper investigates the conditions under which flame pinch-off occurs in forced and non-forced, buoyant laminar jet diffusion flames. The fuel jet emerges into a stagnant air atmosphere at temperature T 0, with a velocity that varies periodically in time with non-dimensional frequency ω l and amplitude A =0.5. We use the formulation developed by Liñán and Williams based on the combination of the mass fraction and energy conservation equations to eliminate the reaction terms, that are substituted by the mixture fraction Z and the excess-enthalpy H scalar conservations equations. With this formulation, valid for arbitrary Lewis numbers, the flame lies on the stoichiometric mixture fraction level surface Z = Z s and its temperature can be easily calculated as , where Z s =1/(1+ S), γ is the non-dimensional heat release parameter, S is the air needed to burn the unit mass of fuel and H s is the value of the excess enthalpy at the flame surface. Non-modulated flames ω l =0 subjected to a gravity field g are known to flicker at a non-dimensional frequency ω l,0 that depends on the Froude number Fr l . The surface of the flame is deformed by the buoyancy-induced oscillations and, for Froude numbers below a certain critical value , the flame breaks repeatedly in two different combustion regions (pinch-off). The first one remains attached to the burner and constitutes the main flame. The second region detaches from the tip of the flame, forming a pocket of hot gas surrounded by a flame that travels along the downstream coordinate z with velocity until the fuel inside the pocket is depleted. Pinch-off is affected by the modulation of the velocity of the jet, changing the critical Froude number of pinch-off Fr l,c as the excitation frequency ω l is modified. Very large ω l /ω l,0 ≫1 or very small ω l /ω l,0 ≪1 excitation frequencies do not modify Fr l,c and it remains equal to . For ω l /ω l,0 ∼ O(1), the response of the flame is determined by the ratio l/x g = γ/Fr l , where l represents the flame length and x g is the distance at which buoyancy effects become important. A strong resonance is observed at ω l ∼ ω l,0 if the flame is sufficiently long, giving Fr l,c that could be thirty times larger than . Short flames do not present that peak and Fr l,c remains almost independent of ω l . [Copyright &y& Elsevier]

Abstract: The inversion and rotation mechanisms for the isomerization of Feringa’s bithioxanthenes existing in two conformations, up/up and up/down, have been calculated at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels. The inversion mechanism that maintains the double bond nature of the central bond is a classical one but the rotation mechanisms that break the double bond to form a biradical needs to explore the singlet and triplet states. To do this we have removed the four fused phenyl rings of bithioxanthene and calculated at the CASSCF and CASPT2 levels bis(4H-thiopyran) proving that B3LYP calculations yield reasonable results for the rotation barriers. [Copyright &y& Elsevier]

Abstract: Numerical and asymptotic methods are used to investigate the structure of the hydrogen jet discharging into a quiescent air atmosphere. The analysis accounts in particular for the variation of the density and transport properties with composition. The Reynolds number of the flow Rj , based on the initial jet radius a, the density ρj and viscosity μj of the jet and the characteristic jet velocity uj , is assumed to take moderately large values, so that the jet remains slender and stable, and can be correspondingly described by numerical integration of the continuity, momentum and species conservation equations written in the boundary-layer approximation. The solution for the velocity and composition in the jet development region of planar and round jets, corresponding to streamwise distances of order Rja, is computed numerically, along with the solutions that emerge both in the near field and in the far field. The small value of the hydrogen-to-air molecular weight ratio is used to simplify the solution by considering the asymptotic limit of vanishing jet density. The development provides at leading-order explicit analytical expressions for the far-field velocity and hydrogen mass fraction that describe accurately the hydrogen jet near the axis. The information provided can be useful in particular to characterize hydrogen discharge processes from holes and cracks. [Copyright &y& Elsevier]

A method for computing minimum ignition energies for gaseous fuel mixtures with detailed and reduced chemistry, by numerical integration of time-dependent conservation equations in a spherically symmetrical configuration, is presented and discussed, testing its general characteristics and accuracy. The method is applied to methanol-air mixtures described by a 38-step Arrhenius chemistry description and by an 8-step chemistry description based on steady-state approximations for reaction intermediaries. Comparisons of predictions with results of available experimental measurements produced reasonable agreements and supported both the robustness of the computational method and the usefulness of the 8-step reduction in achieving accurate predictions. [ABSTRACT FROM AUTHOR]

Reported is the efficient conversion of four 3-sulfonyl prop-1-enyl azides into seven 3-sulfonyl prop-1-enyl triazoles. Results demonstrate that the stereochemical integrity of the alkene was maintained during this process. The conversion of (Z)-((3-azidoallyl)sulfonyl)benzene into the corresponding Z- enamides proved more challenging and only modest yields of (Z)- N -(3-(phenylsulfonyl)prop-1-en-1-yl)acetamide and benzamide were obtained using Williams' thioacid method. Attempts to form these compounds using Staudinger ligation-type reactions proved unsuccessful. [Display omitted] [ABSTRACT FROM AUTHOR]

Results of time-dependent, spherically symmetrical computations of the vaporization and combustion of ethanol and ethanol/water droplets are reported. Mixture-average transport was employed, along with a systematically reduced chemical-kinetic mechanism involving 15 overall steps among 17 chemical species, to speed the computations by a factor of about 100 over what would be required if full detailed chemistry had been used. Absorption of water from the gas surrounding the droplet and its diffusive transport within the liquid phase were taken into account, providing excellent agreement with previous experimental and computational results for the combustion of ethanol droplets in air. On the other hand, the assumption of rapid liquid-phase mixing produced very poor agreement when water condensation on the droplet surface or hydrous ethanol are considered. To characterize autoignition, we define the critical autoignition temperature T ∞ c as the critical ambient temperature below which autoignition is not observed. Computations for autoignition of cold ethanol/water droplets in air showed that T ∞ c decreases with increasing initial droplet diameters. In the range of parameters under consideration, ignition was found to take place always before complete vaporization of the droplet, and the ignition time was found to become longer with the increasing initial water content of the liquid ethanol droplet. On the contrary, addition of water vapor to the initial air atmosphere was found to shorten the ignition time, increasing ethanol vaporization rate as a consequence of the extra heat release associated with water absorption into the liquid. [ABSTRACT FROM AUTHOR]

We present in this short communication a modification to our previous ethanol reduced combustion chemistry (Millán-Merino, 2018) that eliminates nonphysical values of the species concentrations which we discovered in applying the mechanism to the combustion of an isolated ethanol droplet. This unsteady test is reported here to check the multipurpose character of the reduced mechanism for a problem that combines non-homogeneous autoignition, rich and lean premixed-flame propagation, and the development of a diffusion flame, as well as a the presence of a cold moving boundary at the droplet surface. During the computations, production and consumption rates of the α -hydroxyethyl (CH 3 CHOH) intermediary radical became unbalanced, invalidating its steady-state hypothesis, which was used during the derivation of the reduced scheme. This difficulty is removed here by taking CH 3 CHOH out of steady state, thereby augmenting slightly the reduced mechanism. [ABSTRACT FROM AUTHOR]

New multipurpose skeletal and reduced chemical-kinetic mechanisms for ethanol combustion are developed, along the same philosophical lines followed in our previous work on methanol. The resulting skeletal mechanism contains 66 reactions, only 19 of which are reversible, among 31 species, and the associated reduced mechanism contains 14 overall reactions among 16 species, obtained from the skeletal mechanism by placing CH 3 CHOH, CH 2 CH 2 OH, CH 3 CO, CH 2 CHO, CH 2 CO, C 2 H3, C 2 H 5 , C 2 H 6 , S − CH 2 , T − CH 2 , CH 4 , CH 2 OH, CH 3 O, HCO, and O in steady state. For the reduced mechanism, the steady-state relations and rate expressions are arranged so that computations can be made sequentially without iteration. Comparison with experimental results for autoignition, laminar burning velocities, and counterflow flame structure and extinction, including comparisons with the 268-step, 54-species detailed San Diego Mechanism and five other mechanisms in the literature, support the utility of the skeletal and reduced mechanisms, showing, for example, that, in comparison with the San Diego mechanism, they reduced the computational time by a factor of 4 (71 % faster) and 12 (93 % faster), respectively. Measures of computation times and of extents of departures from experimental values are defined and employed in evaluating results. Besides contributing to improvements in understanding of the mechanisms, the derived simplifications may prove useful in a variety of computational studies. [ABSTRACT FROM AUTHOR]

We present a numerical analysis to calculate the minimum ignition energy of hydrogen–ammonia blends in air at both under and over atmospheric pressure. Unlike previous calculations, we used the full compressible and reactive Navier–Stokes equations coupled with detailed chemical kinetics (San Diego mechanism for hydrogen, complemented with the San Diego chemistry for nitrogen). The effect of the size of the energy deposition region and the deposition time are considered to determine the most efficient method to ignite the mixture. Our calculations also evaluate the impact of the gas compressibility on the minimum ignition energy after a sudden energy deposition. The results are validated first by comparing the minimum ignition energy of pure hydrogen–air mixtures as a function of the equivalence ratio ϕ with available experimental data and previous numerical results. Then, fuel blends made of mixtures of hydrogen and ammonia (NH 3) are considered to calculate the minimum ignition energy as a function of fuel composition, equivalence ratio and pressure. The full range of ammonia volumetric content in the blend is varied between the extreme cases of pure hydrogen and pure ammonia. For each fuel blend, we computed a wide range of equivalence ratios ϕ that, in the case of pure hydrogen at atmospheric conditions, ranged from ϕ = 7 to ϕ ≃ 0. 07 , near the lean flammability limit, to theoretically explain the experimental evidence of ultra-lean flames reported in the literature. • Minimum Ignition Energy (MIE) strongly depends on the deposition time. • The Minimum Ignition Energy is achieved when the deposition time is optimum. • Gas compressibility can induce energy losses above 50%. • MIE shifts towards stoichiometric mixtures with ammonia fuel content. • MIE increases exponentially with the ammonia content. [ABSTRACT FROM AUTHOR]

The ignition process produced by the instantaneous application of a linear local heat source near a cold wall is studied using irreversible Arrhenius kinetics. Special attention is devoted here to investigate the limiting case when the instantaneous source is located at a distance from the wall small in comparison with the thermal width of the flame. The ignition problem in this case is reduced to the study of flame initiation by means of a thermal dipole. Changes in the critical values of the dipole intensity for successful ignition are presented as a function of the Lewis number. The results are compared with the configuration in which the distance between the source and the cold wall is finite. This study is also relevant for safety issues in the sense of preventing accidental ignition of combustible mixtures. [ABSTRACT FROM AUTHOR]

• Modelling of a RFB that uses two immiscible electrolytes. • Fluid-fluid interface is tracked using the Arbitrary Lagrangian-Eulerian method. • Maximum power densities between 1-1.75 mW/cm 2 in discharge and 1.5-4 mW/cm 2 in charge. • The limiting current density increases with the flow as j lim ∼ R e 0.3 . • The battery performance is limited by the availability of reactant near the electrodes. We present a mathematical model to study the steady-state performance of a membrane-less reversible redox flow battery formed by two immiscible electrolytes that spontaneously form a liquid-liquid system separated by a well defined interface. The model assumes a two-dimensional battery with two coflowing electrolytes and flat electrodes at the channel walls. In this configuration, the analysis of the far downstream solution indicates that the interface remains stable in all the parameter range covered by this study. To simplify the description of the problem, we use the dilute solution theory to decouple the calculation of the velocity and species concentration fields. Once the velocity field is known, we obtain the distribution of the mobile ionic species along with the current and the electric potential field of the flowing electrolyte solution. The numerical integration of the problem provides the variation of the battery current density I app with the State of Charge (SoC) for different applied cell voltages V cell. A detailed analysis of the concentration density plots indicates that the normal operation of the battery is interrupted when reactant depletion is achieved near the negative electrode both during charge and discharge. The effect of the electrolyte flow on the performance of the system is studied by varying the Reynolds, R e , and Péclet, P e , numbers. As expected, the flow velocity only affects the polarization curve in the concentration polarization region, when V cell is well below the equilibrium potential, resulting in limiting current densities that grow with R e as j lim ∼ R e 0.3 . In addition, both the single-pass conversion efficiency ψ and the product ψ j lim decrease with R e. Concerning the later, the decay rate with R e exhibits a power law with an exponent that almost doubles previous theoretical predictions obtained by imposing a prescribed velocity profile for the electrolyte in a membrane-less laminar flow battery with a liquid oxidant and gaseous fuel. The present work constitutes the first modelling attempt that simultaneously solves the fluid dynamical system formed by the two immiscible electrolytes and the electrochemical problem that determines the response of the membrane-less battery. The proposed model could be used as a valuable tool to optimize future flow battery designs based on immiscible electrolytes. [ABSTRACT FROM AUTHOR]

The present work analyses the reliability of the Brazilian test with loading arcs. A new testing set up has allowed to determine in an effective way the real load of the failure initiation as this moment was not always or correctly detected by the universal testing machine. The instrumentation used is a simple and low-cost method that allows to know the possible pressure distribution in the contact zone as well as the final contact angle. It has been observed that the success of the test depends mainly on the surface finish of the parts involved, their geometric tolerances and the symmetry of the applied load. These boundary conditions have a direct effect in the contact pressure distribution. The possible failure modes observed experimentally have been simulated with the finite element methods. For this, the contact boundary condition has been changed and the possible stress distribution in term of Griffith equivalent stress has been obtained. The numerical analysis allows to study the influence of the initial contact condition on the success of the test and agrees with the experimental results. Furthermore, an uncertainty analysis in the expression of the tensile strength confirms that, when the test is valid, a crack appears suddenly in the central area of the disk, as observed experimentally, so there is no need to determine if the starting point is in the centre. Additionally, it has been observed that the initial crack length depends on the type of pressure distribution in the contact zone. Finally, a series of recommendations are given in order to minimize both the variability of the final contact angle and the risk of premature failure of the Brazilian disk. [ABSTRACT FROM AUTHOR]

The propagation of low-Lewis-number premixed flames is analyzed in a partially confined Hele-Shaw chamber formed by two parallel plates separated a distance h apart. An asymptotic-numerical study can be performed for small gaps compared to the flame thickness δ T. In this narrow-channel limit, the problem formulation simplifies to a quasi-2D description in which the velocity field is controlled by dominant viscous effects. After accounting for conductive heat losses through the plates in our formulation, we found that the reaction front breaks into one or several isolated flame cells where the temperature is large enough to sustain the reaction, both in absence and in presence of buoyancy effects. Under these near-limit conditions, the isolated flame cells either travel steadily or undergo a slow random walk over the chamber in which the reacting front splits successively to form a tree-like pathway, burning only a small fraction of the fuel before reaching the end of the chamber. The production of quasi-2D circular or comet-like flames under specific favorable conditions is demonstrated in this paper, with convection, conductive heat losses and differential diffusion playing an essential role in the formation of the isolated one and two-headed flame cells. [ABSTRACT FROM AUTHOR]

Abstract An experimental study of methane, propane and dimethyl ether (DME) premixed flames propagating in a quasi-two-dimensional Hele-Shaw cell placed horizontally is presented in this paper. The flames are ignited at the open end of the combustion chamber and propagate towards the closed end. Our experiments revealed two distinct propagation regimes depending on the equivalence ratio of the mixture as a consequence of the coupling between the heat-release rate and the acoustic waves. The primary acoustic instability induces a small-amplitude, of around 8 mm, oscillatory motion across the chamber that is observed for lean propane, lean DME, and rich methane flames. Eventually, a secondary acoustic instability emerges for sufficiently rich (lean) propane and DME (methane) flames, inducing large-amplitude oscillations in the direction of propagation of the flame. The amplitude of these oscillations can be as large as 30 mm and drastically changes the outline of the flame. The front then forms pulsating finger-shaped structures that characterize the flame propagation under the secondary acoustic instability. The experimental setup allows the recording of the flame propagation from two different points of view. The top view is used to obtain accurate quantitative information about the flame propagation, while the lateral view offered a novel three dimensional perspective of the flame that gives relevant information on the transition between the two oscillatory regimes. The influence of the geometry of the Hele-Shaw cell and of the equivalence ratio on the transition between the two acoustic-instability regimes is analyzed. In particular, we find that the transition to the secondary instability occurs for values of the equivalence ratio ϕ above (below) a critical value ϕ c for propane and DME (methane) flames. In all the tested fuels, the transition to the secondary instability emerges for values of the Markstein number M below a critical value M c. The critical Markstein number varies with the gap size h formed by the two horizontal plates that bound the Hele-Shaw cell. As h is reduced, the critical Markstein number is shifted towards larger values. [ABSTRACT FROM AUTHOR]

In their 1 H NMR spectra in CDCl 3 3-oxo- N -acyl homoserine lactones (OHLs) show significant downfield chemical shifts of the amide N H proton when compared to the parent N -acyl homoserine lactones (AHLs). NMR spectroscopic and DFT calculation studies have shown that this is most likely due to the presence of a stabilising intramolecular H-bond from the N H to the 3-oxo group. The 1 H NMR spectra also show evidence for the enol tautomers and that the amount of enol present for a range of OHLs is 4.1–4.5% in CDCl 3 and 6.5–7.2% in CD 3 CN. In contrast, DFT calculations show that the lowest energy enol tautomer and the keto tautomer are of equal energy in the gas phase, but that the keto tautomer is more stable in chloroform, acetonitrile and water solution. The calculations also show that there is no evidence for any n → π ∗ or C5H-bonding interactions being present in either the lowest energy keto or enol tautomer of the OHLs in solution or the gas phase, which is in contrast to the reported solid-state structure. [ABSTRACT FROM AUTHOR]

Thermoacoustic instabilities arise for lean hydrogen-air flames propagating in narrow channels. We provide here a detailed experimental analysis of such phenomena in a semi-confined vessel, analyzing the effect of the mixture composition, geometry and gravity on the onset of acoustic-driven flame vibrations. Downward-propagating flames leaner than a critical value vibrate smoothly and transit to the secondary oscillating instability, which develops strong variations of pressure that couple with the propagation dynamics. The transition threshold changes during the propagation along very narrow channels, where heat losses are no longer negligible. The parametric region of equivalence ratio for the secondary thermoacoustic instability diminishes, showing an additional transition for very lean flames. There, the front breaks into several structures and the flame-wave feedback becomes weaker. The influence of gravity is studied by comparing upward and downward propagating flames, where the Rayleigh–Taylor instability arises for sufficiently small values of the Froude number in slow-propagating lean flames. For a constant mixture, buoyancy-driven upward-propagating flames develop less wrinkled fronts than those propagating downwards, and remain unresponsive to acoustic-front interaction. We show here a direct relation between front shape and thermoacoustics. In agreement with previous studies [1–3] , curvature and strain effects on conduction and diffusion characterize the response of the flame to pressure perturbations, with the Markstein number controlling the aforementioned transition. Nevertheless, the theoretical analyses found in the literature can only be used on nearly equidiffusional mixtures, and are not accurate enough to describe the highly diffusive fuel mixtures (i.e. lean hydrogen-air flames) considered in our experiments. [ABSTRACT FROM AUTHOR]