Chemical Time Scale Research Articles

CO2 absorption using diethanolamine-water solutions in a rotating spiral contactor

Results for mass transfer in a rotating spiral device are presented here for absorption of carbon dioxide from nitrogen carrier gas using mixtures of diethanolamine (DEA) and water. The ability of the device to examine the full range of flow rate ratio for the two phases while controlling the relative thicknesses of the phase layers is applied to surveying absorption performance over a wide range of DEA concentration at 312K and 1.8 bara. Comparisons are made for a fixed 86μm liquid layer thickness, which is shown to fix also the fraction of the liquid accessible by diffusion, while maintaining 90% removal of CO2 from a gas stream of 10% (mole) CO2 in nitrogen. The increasing liquid viscosity with DEA fraction is countered by reducing the liquid flow rate to maintain constant liquid layer thickness and diffusion depth. The allowed gas throughput, while meeting 90% removal, increases with DEA concentration until the increasing viscosity gives sufficient reduction in liquid flow rate to offset the increasing CO2 capacity of the liquid. The maximum gas flow rate has a broad peak centred at a DEA mole fraction of about 0.072 (31% by mass). Utilisation of the amine is increased as DEA concentration increases, apparently as a result of the longer residence time, suggesting an effect of chemical time scales on the order of seconds. For a fixed concentration, full utilisation of the amine is achieved by decreasing the liquid flow rate, which reduces layer thickness and increases diffusion time. The work highlights the use of the rotating spiral for rapid and accurate testing to determine optimum liquid composition of absorbent formulations.

Unsteady dynamics of PAH and soot particles in laminar counterflow diffusion flames

Due to their low chemical time scales, the production of soot particles in turbulent diffusion flames is highly impacted by large range of local strain rate fluctuations.In order to understand the response of soot production to strain rate fluctuations, unsteady laminar counterflow diffusion flames with an imposed oscillating strain rate are investigated both analytically and numerically. First an analytical linearized model is developed to predict the unsteady response of a flame quantity of interest from information on laminar steady flames. Three critical parameters governing flame response are identified: the Stokes number which compares the characteristic time associated to the mean imposed strain rate to the oscillation frequency, the Damköhler number associated to the quantity of interest, and a third one characterizing the response of this quantity to an imposed steady strain rate. This model is then applied to soot predictions. Parallely, the response of soot production in propane-air counterflow diffusion flames to unsteady strain harmonic oscillations is studied numerically using a detailed sectional soot model. A wide range of frequencies and amplitudes are considered. A specific trend is highlighted for soot precursors and particles production according to their respective chemical time scales: the bigger the PAH or soot particle, the higher its chemical time scale, resulting in a more damped and phase-lagged response. The particle size distribution evolves accordingly during the considered oscillations, so that the quasi-steady state behavior is not verified for high frequencies. The numerical results are compared to those obtained by the analytical approach and a very good agreement is obtained at low amplitudes. Non-linear response of soot precursors and soot particles production to strain oscillations are finally discussed in case of high oscillation amplitudes and the limits of the proposed analytical model are identified.

Simplifying ignition delay prediction for homogeneous charge compression ignition engine design and control

Advanced combustion engine concepts, such as the homogeneous charge compression ignition engine, rely on chemical kinetics for their proper operation. Accurate prediction and control of auto ignition time scales are therefore key issues. Real-time prediction and control of ignition using detailed chemical kinetic models is difficult due to the large size of such mechanisms that calls for high computational costs. Ignition models in the form of correlations can overcome these challenges, as long as they are able to predict the time scales that would be obtained using a detailed chemical kinetic model. In this work, a correlation approach based on detailed chemical models is used to simplify the determination of chemical time scales associated with kinetically controlled combustion events. Detailed combustion mechanisms for biodiesel surrogate (methyl decanoate), gasoline surrogates (isooctane/ n-heptane/toluene), bio-alcohol ( n-octanol) and ethanol/gasoline surrogate blends from the literature are used to generate ignition databases for correlation development, taking into account complex ignition behavior such as pressure-dependent limits of negative temperature coefficient kinetics. We further employ the Livengood–Wu integral method to assess the ability of the resulting correlations to predict ignition in cases where the temperature and pressure are changing prior to ignition. The integral method is compared with results of a single-zone adiabatic homogeneous charge compression ignition engine model simulation using the detailed chemical kinetic model. It is found that the simplified correlations accurately reproduce the predictions of the detailed chemical kinetic models at an insignificant computational cost relative to the detailed simulations. The correlation approach presented here can also be applied to experimental ignition data, as far as these are obtained in a manner that covers a wide range of the relevant parameters with minimal experimental uncertainties. This work enables the incorporation of realistic chemical kinetic effects in the computational analysis and control of kinetically controlled combustion systems.

Regimes describing shock boundary layer interaction and ignition in shock tubes

Regimes of shock boundary layer interaction are proposed in consideration of shock tube kinetic experiments. For this, we examine three ways that the reflected shock wave interacts with the boundary layer: incipient separation occurs when the shock is just strong enough to subject the flow to an adverse pressure gradient leading to flow reversal; shear layer instabilities manifest after a certain length of time and can cause inhomogeneities in the test gas; and shock bifurcation occurs when the back pressure of the test gas is sufficient to contain the boundary layer fluid within a stagnation bubble causing severe inhomogeneities in the test gas. Theory delineating these regimes is developed, and these delineations are compared to simulations of shock tube experiments as well as experimental data, where reasonable agreement is found. Through the theory applied to the incipient separation regime, it is determined that boundary layer separation likely occurs in most shock tube experiments; however, separation is unlikely to affect a chemical kinetic experiment except at long test times. To quantify the effect of the boundary layer, a bifurcation Damköhler number is introduced, which is found to perform sensibly well at classifying strong and weak ignition in shock tubes, implying that these combustion phenomena are determined by a competition of physical and chemical timescales. Finally, simulations suggest that tailoring the incident shock Mach number for a given experiment could provide opportunities for mitigating inhomogeneities in the test gas.

The effect of turbulent clustering on particle reactivity

The effect of turbulence on the heterogeneous (solid–fluid) reactions of solid particles is studied numerically with Direct Numerical Simulations (DNS). A simplified reaction system is used, where the solid–fluid reaction is represented by a single isothermal reaction step. It is found that, due to the clustering of particles by the isotropic turbulence, the overall reaction rate is entirely controlled by the turbulence for large Damköhler numbers. The particle clustering significantly slows down the reaction rate for increasing Damköhler numbers which reaches an asymptotic limit that can be analytically derived. This implies that the effect of turbulence on heterogeneously reacting particles should be included in models that are used in CFD simulations of e.g. char burnout in combustors or gasifiers. Such a model, based on the chemical and turbulent time scales, is here proposed for the heterogeneous reaction rate in the presence of turbulence.

Preface

Stellar mass loss is now widely recognized to have a significant impact on stellar evolution. Mass loss on the asymptotic giant branch (AGB) allows stars with initial masses under 8 solar masses to avoid the fate of going supernovae. Over 95% of stars in our Galaxy will evolve through the planetary nebulae phase to end up as white dwarfs instead of neutron stars or black holes. Massive stars undergo mass loss both in the blue and red phases of evolution and create new classes of stars such as Wolf-Rayet stars and luminous blue variables. The circumstellar matter ejected by these mass loss processes becomes new laboratories to study the physical and chemical processes of interstellar matter. The interaction between different phases of mass loss (with variable mass loss rates, ejection speeds, and directions) leads to spectacular morphological transformation of the circumstellar nebulae. The circumstellar nebulae are also sites of molecular and solid-state synthesis. Close to 100 molecular species and a variety of solids, including minerals and complex organics, have been detected in circumstellar envelopes. Since the dynamical time scale of the ejection puts an upper limit on the chemical time scale, we are witnessing a rapid synthesis of chemical species in an extremely low-density environment, creating new challenges to our understanding of chemical reactions.Effects of mass loss are not limited to single stars. Mass loss by one component of a binary system allows mass transfer to occur at separations beyond the Roche Lobe limit. Accreted wind materials on the surface of a degenerate star can lead to periodic outbursts through H-shell burning. When both components are losing mass, we have interesting dynamical systems such as symbiotic novae.The theme of this conference is “Physics and Chemistry of the Late Stages of Stellar Evolution”. We try to bring together experts in different fields to exchange ideas in the hope of solving the many unsolved problems in the late stages of stellar evolution. The size of the conference was limited to about 100 participants to allow for sufficient time for discussions and personal exchanges of opinions. Although the conference is part of the Pacific Rim series, astronomers from Asia, Europe, North and South America participated in the conference. We hope that these proceedings will serve as a record of our current understanding of the late stages of stellar evolution and be a useful resource for future generation of scientists working in the field.

CHEMICAL DIAGNOSTICS OF THE MASSIVE STAR CLUSTER-FORMING CLOUD G33.92+0.11. I. 13CS, CH3OH, CH3N, OCS, H2S, SO2, and SiO

ABSTRACT Large chemical diversity was found in the gas clumps associated with the massive star cluster-forming G33.92+0.11 region with sub-arcsecond angular resolution (0.″6–0.″8) observations with ALMA. The most prominent gas clumps are associated with the dust emission peaks A1, A2, and A5. The close correlation between CH3OH and OCS in the emission distributions strongly suggests that these species share a common origin of hot core grain mantle evaporation. The latest generation of star clusters are forming in the A5 clump, as indicated by multiple SiO outflows and its rich hot core chemistry. We also found a narrow SiO emission associated with the outflows, which may trace a cooled component of the outflows. Part of the chemical complexity may have resulted from the accreting gas from the ambient clouds, especially in the northern part of A1 and the southern part of A2. The chemical diversity found in this region is believed to mainly result from the different chemical evolutionary timescales of massive star formation. In particular, the abundance ratio between CH3OH and CH3CN may be a good chemical clock for the early phase of star formation.

A parameterization of the Richtmyer–Meshkov instability on a premixed flame interface induced by the successive passages of shock waves

The interactions of a sinusoidal premixed flame interface with successive shock waves are studied by numerically solving the two-dimensional, reactive Navier–Stokes equations with high resolution schemes. The focus is the development of the perturbations on the flame interface due to the Richtmyer–Meshkov (RM) instability. A single irreversible chemical reaction for a C2H4+3O2+4N2 mixture is used to represent the combustion process during the interactions. Three cases of interactions, with different initial incident shock wave strengths (Ma=1.2, 1.7 and 2.2), are studied. The multiple interactions between the flame interface and the shock and its reflection waves lead to interesting new observations that have not been presented before. To parameterize the interactions, we introduce a time scale decoupling assumption, and propose to use two decoupled time scales, i.e., the decoupled chemical reaction time scale τC and the decoupled RM instability time scale τRM. Based on this assumption, τC is considered to be decoupled from τRM, so that both time scales can be defined and determined independently. Specifically, τRM is defined as the reciprocal of vorticity magnitudes that can be obtained by performing the computations of two-dimensional RM instability in a chemically frozen flow that removes the effect of chemical reactions. On the other hand, τC is defined as the ratio of reaction zone width of an un-stretched, laminar, premixed flame to propagating speed of the flame, during the one-dimensional laminar flame propagation without considering two-dimensional RM instability. Using these definitions, our calculations show that both τC and τRM decrease with the successive interactions of the shock waves with the flame interface in all three cases, and that τC is larger than τRM at early stages of interactions and then rapidly decreases to a level comparable with τRM at later stages of interactions. We compare the two-dimensional visualizations of the perturbation development on the interface in reactive flows with those in chemically frozen flows. The comparison shows that RM instability plays an important role in early evolution, but chemical reactions dominate later, and that the evolution is correlated with the variations of τC and τRM for all cases studied here. The quantitative correlations between the ratio of τC to τRM and chemical heat release rate in all three cases are calculated. The good correlations indicate the advantage of the approaches to determine τC and τRM proposed in present study. The observation that the ratio of τC to τRM is independent of the initial strength of incident shock wave suggests that the ratio reflects the nature of reactive RM instability, hence provides a new dimensionless parameter for understanding such flows.

Eight coordinate 1,2-bis(dimethylarsino) and 1,2-bis(dimethylphosphino)-benzene complexes of uranium tetrachloride, UCl4[(1,2-Me2E)2C6H4]2 where E is As or P

The eight coordinate complexes of 1,2-bis(dimethylarsino)benzene, MCl4(diars)2 where M is Th or U, and 1,2-bis(dimethylphosphino)benzene, UCl4(diphos)2, are prepared and characterized. Both coordination complexes of uranium crystallize in the tetragonal crystal system in space group I4¯2m, in which the geometry is a D2d-dodecahedron. In solution the uranium complexes do not exchange with added free ligand on the NMR time scale but they undergo bidentate ligand exchange on the chemical time scale. The 31P{1H} NMR chemical shifts of UI4(diphos)2 and UX4(dmpe)2, X=Cl, Br, I range from 1500 to 2100ppm and are strongly dependent on temperature.

Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

We study the connection of star formation to atomic (HI) and molecular hydrogen (H$_2$) in isolated, low metallicity dwarf galaxies with high-resolution ($m_{\rm gas}$ = 4 M$_\odot$, $N_{\rm ngb}$ = 100) SPH simulations. The model includes self-gravity, non-equilibrium cooling, shielding from an interstellar radiation field, the chemistry of H$_2$ formation, H$_2$-independent star formation, supernova feedback and metal enrichment. We find that the H$_2$ mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities $n <$ 1 cm$^{-3}$. Because of the long chemical timescales, the H$_2$ mass remains out of chemical equilibrium throughout the simulation. Star formation is well-correlated with cold ( T $\leqslant$ 100 K ) gas, but this dense and cold gas - the reservoir for star formation - is dominated by HI, not H$_2$. In addition, a significant fraction of H$_2$ resides in a diffuse, warm phase, which is not star-forming. The ISM is dominated by warm gas (100 K $<$ T $\leqslant 3\times 10^4$ K) both in mass and in volume. The scale height of the gaseous disc increases with radius while the cold gas is always confined to a thin layer in the mid-plane. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyrs) for total gas surface densities $\Sigma_{\rm HI+H_2} \lesssim$ 10 M$_\odot$pc$^{-2}$, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.

A Model for Predicting the Lift-Off Height of Premixed Jets in Vitiated Cross Flow

Lean premixed single-stage combustion is state of the art for low pollution combustion in heavy-duty gas turbines with gaseous fuels. The application of premixed jets in multistage combustion to lower nitric oxide emissions and enhance turn-down ratio is a novel promising approach. At the Lehrstuhl für Thermodynamik, Technische Universität München, a large-scale atmospheric combustion test rig has been set up for studying staged combustion. The understanding of lift-off (LO) behavior is crucial for determining the amount of mixing before ignition and for avoiding flames anchoring at the combustor walls. This experiment studies jet LO depending on jet equivalence ratio (0.58–0.82), jet preheat temperature (288–673 K), cross flow temperature (1634–1821 K), and jet momentum ratio (6–210). The differences to existing LO studies are the high cross flow temperature and applying a premixed jet. The LO height of the jet flame is determined by OH* chemiluminescence images, and subsequently, the data is used to analyze the influence of each parameter and to develop a model that predicts the LO height for similar staged combustion systems. A main outcome of this work is that the LO height in a high temperature cross flow cannot be described by one dimensionless number like Damköhler- or Karlovitz-number. Furthermore, the ignition delay time scale τign also misses part of the LO height mechanism. The presented model uses turbulent time scales, the ignition delay, and a chemical time scale based on the laminar flame speed. An analysis of the model reveals flame stabilization mechanisms and explains the importance of different time scale.

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, two-stage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the high-temperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.

Preheats Effects on JP8 Reforming Under Volume Distributed Reaction Conditions

Conventional noncatalytic fuel reforming provides low efficiency, large amounts of char and tar and limited control on chemical composition of the syngas produced. The distributed reaction regime can be used to assist noncatalytic reforming. In this paper, volume distributed reaction technique is used to enhance reformate quality as compared to conventional noncatalytic reforming. This work examines the intermediate regimes between volume distributed reaction and conventional flame to reform JP8 with focus on the chemical and mixing time scales. Chemical time scales were controlled with air preheat temperatures while the mixing time scales were kept constant. Progressive shift toward distributed reaction regime resulted in higher quality reformate with increased amounts of hydrogen and carbon monoxide in the syngas, but with reduced acetylene concentrations and soot formation. Visible soot formation was observed on reactor walls only under the flamelets in eddies regime. Higher hydrogen and carbon monoxide without any catalyst for JP8 reformation offers significant advantages on cost-effective plant operation, reliability, and high yields of syngas. Air preheats of 600, 630, and 660 °C showed a distributed reaction regime wherein the Damkohler number was below the Damkohler criterion, and this condition provided high H2 and CO yields and no soot. At temperature of 690 °C, laminar flame thickness approximated the integral length scale (at the interface of distributed and traditional reforming flame) showed minor soot formation. At even higher temperature of 750 °C, conventional reforming occurred with increased soot observed.

Puffing and micro-explosion of diesel–biodiesel–ethanol blends

The puffing and micro-explosion of a single burning droplet comprised of neat diesel, rapeseed methyl ester (RME); binary fuel mixtures of diesel–ethanol, diesel–RME, RME–ethanol; and ternary microemulsion of these fuel blends at various compositions have been studied using high speed backlight imaging method. Fuel droplet was suspended on the tip of a 130μm gauge thermocouple and it was ignited using a glow plug heater. Based on the temporal variation of droplet projected area, the characteristics of fuel droplets studied were classified into smooth burning, puffing and explosion. A ternary plot has been proposed to identify the mixture composition of the blends that can result in smooth burning, puffing and explosion. Micro-explosion phenomenon was observed in the ternary blends with ethanol percentages between 10% and 40%. Secondary droplets resulted from the puffing and explosion of suspended parent droplet were observed to undergo further explosion. The time scales associated with complete disintegration of secondary droplets are found to be comparable to the mixing and the chemical reaction time scales of sprays in diesel engines.

Topology and burning rates of turbulent, lean, H2/air flames

Improved understanding of turbulent flames characterized by negative consumption speed-based Markstein lengths is necessary to develop better models for turbulent lean combustion of high hydrogen content fuels. In this paper we investigate the topology and burning rates of turbulent, lean (ϕ = 0.31), H2/air flames obtained from a recently published DNS database (Aspden et al., 2011). We calculate local flame front curvatures, strain rates, thicknesses, and burning velocities and compare these values to reference quantities obtained from stretched laminar flames computed numerically in three model geometrical configurations—a counterflow twin flame, a tubular counterflow flame and an expanding cylindrical flame. We compare and contrast the DNS with these model laminar flame calculations, and show both where they closely correlate with each other, as well as where they do not. These results in the latter case are shown to result from non-flamelet behaviors, unsteady effects, and curvature-strain correlations. These insights are derived from comparisons conditioned on different topological features, such as portions of the flame front with a spherical/cylindrical shape, the leading edge of the flame, and portions of the flame front with low mean curvature. We also show that reference time scales vary appreciably over the flame, and characterizing the relative values of fluid mechanic and kinetic time scales by a single value leads to erroneous conclusions. For example, there is a two order of magnitude decrease in chemical time scales at the leading edge of the front relative to its unstretched value. For this reason, the leading edge of the front quite closely tracks quasi-steady calculations, even in the lowest Damköhler number case, DaF∼0.005.

Extension of the Eddy Dissipation Concept for turbulence/chemistry interactions to MILD combustion

Over the past 30years, the Eddy Dissipation Concept (EDC) has been widely applied in the industry for the numerical simulations of turbulent combustion problems. The success of the EDC is mainly due to its ability to incorporate detailed chemical mechanisms at an affordable computational cost compared to some other models. Detailed kinetic schemes are necessary in order to capture turbulent flames where there is strong coupling between the turbulence and chemical kinetics. Such flames are found in Moderate and Intense Low-oxygen Dilution (MILD) combustion, where chemical time scales are increased compared with conventional combustion, mainly because of slower reactions (due to the dilution of reactants). Recent modelling studies have highlighted limitations of the standard EDC model when applied to the simulation of MILD systems, noticeably a significant overestimation of temperature levels. Modifications of the model coefficients were proposed to account for the specific features of MILD combustion, i.e. an extension of the reaction region and the reduction of maximum temperatures. The purpose of the present paper is to provide functional expressions showing the dependency of the EDC coefficients on dimensionless flow parameters such as the Reynolds and Damköhler numbers, taking into account the specific features of the MILD combustion regime, where the presence of hot diluent and its influence on the flow and mixing fields impacts on the reaction rate and thermal field. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet in Hot Co-flow (JHC) burner at different co-flow compositions (3%, 6% and 9% O2 mass fraction) and fuel-jet Reynolds numbers (5000, 10,000 and 20,000). Results show promising improvement with respect to the standard EDC formulation, especially at diluted conditions and medium to low Reynolds numbers.

DNS of swirling hydrogen–air premixed flames

Direct numerical simulation is employed to investigate the turbulent flow characteristics and their effect on local flames for mean reaction rate modelling in turbulent swirling premixed flames. Two swirl numbers having significant effects on the formation of a central recirculation zone in the combustor are considered. The large velocity gradients in the higher swirl number case produce high turbulence intensity in a relatively upstream region compared to the lower swirl number case. The conditional Probability Density Functions (PDFs) of the reaction rate and dissipation rates of turbulent kinetic energy and scalar fluctuations are also examined. The PDFs show correlations between the turbulence energy dissipation and reaction rates and between the scalar dissipation and reaction rates, suggesting that the heat and radicals from the hot products trapped in the recirculation zones are mixed with the reactants, not only through scalar dissipation rate (i.e. scalar gradient) but also by small-scale processes of turbulence relevant to turbulent kinetic energy dissipation rate. Therefore, both scalar and velocity gradients have a strong influence on the chemical reactions through mixing of cold reactant and hot products. A conventional flamelet and EDC models are used to estimate the mean reaction rate, and to study the balance between these two mixing mechanisms. Although both models show a qualitative agreement with the DNS results, these models compensate their limitations each other, depending on the local turbulence and thermochemical conditions. A simple approach is proposed to exploit the advantages of these two models by considering the balance of two mixing mechanisms based on the chemical and turbulence time scales. The estimated mean reaction rate using the proposed model is significantly improved for the higher swirl number case, although the estimated value slightly shifts away from the DNS results for the lower swirl number case. The improved modelling estimate and the balance of turbulence and chemical time scales suggest that the locations of intense reaction zones are strongly related to the dissipation rates of both scalar and turbulent kinetic energy.

Effect of fuel composition and differential diffusion on flame stabilization in reacting syngas jets in turbulent cross-flow

Three-dimensional direct numerical simulation results of a transverse syngas fuel jet in turbulent cross-flow of air are analyzed to study the influence of varying volume fractions of CO relative to H2 in the fuel composition on the near field flame stabilization. The mean flame stabilizes at a similar location for CO-lean and CO-rich cases despite the trend suggested by their laminar flame speed, which is higher for the CO-lean condition. To identify local mixtures having favorable mixture conditions for flame stabilization, explosive zones are defined using a chemical explosive mode timescale. The explosive zones related to flame stabilization are located in relatively low velocity regions. The explosive zones are characterized by excess hydrogen transported solely by differential diffusion, in the absence of intense turbulent mixing or scalar dissipation rate. The conditional averages show that differential diffusion is negatively correlated with turbulent mixing. Moreover, the local turbulent Reynolds number is insufficient to estimate the magnitude of the differential diffusion effect. Alternatively, the Karlovitz number provides a better indicator of the importance of differential diffusion. A comparison of the variations of differential diffusion, turbulent mixing, heat release rate and probability of encountering explosive zones demonstrates that differential diffusion predominantly plays an important role for mixture preparation and initiation of chemical reactions, closely followed by intense chemical reactions sustained by sufficient downstream turbulent mixing. The mechanism by which differential diffusion contributes to mixture preparation is investigated using the Takeno Flame Index. The mean Flame Index, based on the combined fuel species, shows that the overall extent of premixing is not intense in the upstream regions. However, the Flame Index computed based on individual contribution of H2 or CO species reveals that hydrogen contributes significantly to premixing, particularly in explosive zones in the upstream leeward region, i.e. at the preferred flame stabilization location. Therefore, a small amount of H2 diffuses much faster than CO, creating relatively homogeneous mixture pockets depending on the competition with turbulent mixing. These pockets, together with high H2 reactivity, contribute to stabilizing the flame at a consistent location regardless of the CO concentration in the fuel for the present range of DNS conditions.

Investigation of chloromethane complexes of cryptophane-A analogue with butoxy groups using ¹³C NMR in the solid state and solution along with single crystal X-ray diffraction.

Host‐guest complexes between cryptophane‐A analogue with butoxy groups (cryptophane‐But) and chloromethanes (chloroform, dichloromethane) were investigated in the solid state by means of magic‐angle spinning 13C NMR spectroscopy. The separated local fields method with 13C‐1H dipolar recoupling was used to determine the residual dipolar coupling for the guest molecules encaged in the host cavity. In the case of chloroform guest, the residual dipolar interaction was estimated to be about 19 kHz, consistent with a strongly restricted mobility of the guest in the cavity, while no residual interaction was observed for encaged dichloromethane. In order to rationalize this unexpected result, we performed single crystal X‐ray diffraction studies, which confirmed that both guest molecules indeed were present inside the cryptophane cavity, with a certain level of disorder. To improve the insight in the dynamics, we performed a 13C NMR spin‐lattice relaxation study for the dichloromethane guest in solution. The system was characterized by chemical exchange, which was slow on the chemical shift time scale but fast with respect to the relaxation rates. Despite these disadvantageous conditions, we demonstrated that the data could be analyzed and that the results were consistent with an isotropic reorientation of dichloromethane within the cryptophane cavity. Copyright © 2015 The Authors. Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.

Conformational Plasticity in Glycomimetics: Fluorocarbamethyl-L-idopyranosides Mimic the Intrinsic Dynamic Behaviour of Natural Idose Rings.

Sugar function, structure and dynamics are intricately correlated. Ring flexibility is intrinsically related to biological activity; actually plasticity in L-iduronic rings modulates their interactions with biological receptors. However, the access to the experimental values of the energy barriers and free-energy difference for conformer interconversion in water solution has been elusive. Here, a new generation of fluorine-containing glycomimetics is presented. We have applied a combination of organic synthesis, NMR spectroscopy and computational methods to investigate the conformational behaviour of idose- and glucose-like rings. We have used low-temperature NMR spectroscopic experiments to slow down the conformational exchange of the idose-like rings. Under these conditions, the exchange rate becomes slow in the (19) F NMR spectroscopic chemical shift timescale and allows shedding light on the thermodynamic and kinetic features of the equilibrium. Despite the minimal structural differences between these compounds, a remarkable difference in their dynamic behaviour indeed occurs. The importance of introducing fluorine atoms in these sugars mimics is also highlighted. Only the use of (19) F NMR spectroscopic experiments has permitted the unveiling of key features of the conformational equilibrium that would have otherwise remained unobserved.