The application of the QSSA via reaction lumping for the reduction of complex hydrocarbon oxidation mechanisms

The quasi-steady-state approximation (QSSA) has been widely applied for the purposes of chemical kinetic model reduction. Although it is essentially a low-order approximation, it can be shown to lead to significant reductions in the number of fast variables within a mechanism without significant loss of accuracy for model predictions. Due to the couplings between QSSA expressions, the species are commonly solved for using numerical inner iteration techniques. Therefore, although the stiffness of the model system can be reduced, there is a computational overhead in solving the often nonlinear QSSA equations. Greater computational savings can be made where QSS species can be removed from the chemical model via explicit analytical expressions. In many cases these expressions are equivalent to reaction lumping. Where such reaction lumping can be achieved, a reduced mechanism in standard kinetic form can be developed, which contains new lumped reaction rate coefficients, but leads to the removal of QSS species. This paper describes such an approach for mechanisms describing the oxidation of the hydrocarbon fuels n-heptane and cyclohexane, and shows that significant reductions in both species and reactions can be achieved, leading to substantial computational speed-ups. The resulting schemes clearly demonstrate the main atomic flux patterns within the oxidation process. Patterns related to the time-scales of hydrocarbon radical species within alkane oxidation mechanisms are discussed, as well as the potential significance of non-QSS radicals in determining ignition behaviour.     

Automatic mechanism generation for the combustion of advanced biofuels: A case study for diethyl ether

Advanced biofuels have the potential to supplant significant fractions of conventional liquid fossil fuels. However, the range of potential compounds could be wide depending on selected feedstocks and production processes. Not enough is known about the engine relevant behavior of many of these fuels, particularly when used within complex blends. Simulation tools may help to explore the combustion behavior of such blends but rely on robust chemical mechanisms providing accurate predictions of performance targets over large regions of thermochemical space. Tools such as automatic mechanism generation (AMG) may facilitate the generation of suitable mechanisms. Such tools have been commonly applied for the generation of mechanisms describing the oxidation of non-oxygenated, non-aromatic hydrocarbons, but the emergence of biofuels adds new challenges due to the presence of functional groups containing oxygen. This study investigates the capabilities of the AMG tool Reaction Mechanism Generator for such a task, using diethyl ether (DEE) as a case study. A methodology for the generation of advanced biofuel mechanisms is proposed and the resultant mechanism is evaluated against literature sourced experimental measurements for ignition delay times, jet-stirred reactor species concentrations, and flame speeds, over conditions covering φ = 0.5–2.0, P = 1–100 bar, and T = 298–1850 K. The results suggest that AMG tools are capable of rapidly producing accurate models for advanced biofuel components, although considerable upfront input was required. High-quality fuel specific reaction rates and thermochemistry for oxygenated species were required, as well as a seed mechanism, a thermochemistry library, and an expansion of the reaction family database to include training data for oxygenated compounds. The final DEE mechanism contains 146 species and 4392 reactions and in general, provides more accurate or comparable predictions when compared to literature sourced mechanisms across the investigated target data. The generation of combustion mechanisms for other potential advanced biofuel components could easily capitalize on these database updates reducing the need for future user interventions.     

Photoinduced Electron Transfer in Perylene‐TiO2 Nanoassemblies

The photosensitization effect of three perylene dye derivatives on titanium dioxide nanoparticles (TiO₂ NPs) has been investigated. The dyes used, 1,7‐dibromoperylene‐3,4,9,10‐tetracarboxy dianhydride (1), 1,7‐dipyrrolidinylperylene‐3,4,9,10‐tetracarboxy dianhydride (2) and 1,7‐bis(4‐tert‐butylphenyloxy)perylene‐3,4,9,10‐tetracarboxy dianhydride (3) have in common bisanhydride groups that convert into TiO₂ binding groups upon hydrolysis. The different substituents on the bay position of the dyes enable tuning of their redox properties to yield significantly different driving forces for photoinduced electron transfer (P_eT). Recently developed TiO₂ NPs having a small average size and a narrow distribution (4 ± 1 nm) are used in this work to prepare the dye‐TiO₂ systems under study. Whereas successful sensitization was obtained with 1 and 2 as evidenced by steady‐state spectral shifts and transient absorption results, no evidence for the attachment of 3 to TiO₂ was observed. The comparison of the rates of P_eT (k_PeT) for 1‐ and 2‐TiO₂ systems studied in this work with those obtained for previously reported analogous systems, having TiO₂ NPs covered by a surfactant layer (Hernandez et al. [2012] J. Phys. Chem. B., 117, 4568–4581), indicates that k_PeT for the former systems is slower than that for the later. These results are interpreted in terms of the different energy values of the conduction band edge in each system.     

Uncertainty propagation in the derivation of phenomenological rate coefficients from theory: A case study of n-propyl radical oxidation

Global uncertainty and sensitivity analysis is used to study the propagation of uncertainties in fundamental theoretical parameters through to uncertainties in the predicted temperature and pressure dependent phenomenological rate coefficients. Predictions are obtained from ab initio transition state theory based master equation calculations. The fundamental parameters for these rate predictions include barrier heights, well depths, vibrational frequencies, collision frequency, and energy transfer parameters. A random sampling high-dimensional model representation (HDMR) approach is used to perform the global sensitivity analysis. This approach determines the predicted distributions of the phenomenological rate coefficients based on a quasi-random sample of the fundamental parameters within their uncertainty range. Sensitivity analysis then identifies the main parameters which contribute to variance in the predicted distributions. Here the approach is applied to a study of the oxidation of the propyl radical, employing the parameters derived in our recent theoretical study. We find rates at 3σ variances that typically differ from the most frequent values by factors of 4–6, with the uncertainties decreasing with increasing temperature. For the well skipping reactions there are more parameters that contribute significantly to the variance, the second-order sensitivities are greater, and the uncertainties increased with increasing pressure. For the other reactions, the uncertainties tend to decrease with increasing pressure.     

Flow phenomena in floppy tubes

Collapsible tubes, which occur all over the body, have unique properties from the point of view of both physics and physiology. A brief review is attempted of first the basic observable properties, followed by simple theory to explain the steady-flow aspects and an overview of the somewhat more complex theories for unsteady flow, in particular the flow-induced oscillations. The experimental evidence from laboratory studies is reviewed with particular emphasis on the dynamical system aspects. A final section looks at the current position and prospects.     

Holographic recording using two-photon-induced photopolymerization

Molecular excitation via the simultaneous absorption of two photons can lead to improved three-dimensional control of photochemical or photophysical processes due to the quadratic dependence of the absorption probability on the incident radiation intensity. This has lead to the development of improved three-dimensional fluorescence imaging, optical data storage, and microfabrication. The latter of these involves the fabrication of three-dimensional structures using a spatial variation in the incident intensity within a photopolymerizable resin. In the past, the translation of the focal plane of a tightly focused laser beam was used to induce localized photopolymerization and fabrication of three-dimensional structures. Here we report the first successful demonstration of large-area patterning via ultrafast holography-based two-photon polymerization of a commercially available optical resin and a large two-photon cross-section dye (AF380). This opens tremendous possibilities for the wide-spread use of two-photon absorption for the three-dimensional control of photoinduced processes. Received: 21 June 1999 / Accepted: 23 June 1999 / Published online: 8 September 1999     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.     

ISOTOPIC DESYMMETRIZATION IN THE STUDY OF HOMOGENEOUS CATALYSIS

NMR studies on isotopically desymmetrized ligands and substrates have been used to demonstrate and investigate an apparent “memory effect” in palladium-catalysed nucleophilic substitution at allylic centres. The explanation for this “memory effect” is shown to be the participation of both monomeric and oligomeric complexes, with different reaction rates.     

Elastic and inelastic electron tunnelling

This work includes a brief review of the technique of electron tunnelling in relation to other vibrational spectroscopies. A strong emphasis is given to preparation techniques. Some mathematical theory and numerical techniques used in relation to the study are also covered. The approach is an introductory one aimed at postgraduate or postdoctoral fellows entering this field of research. It is a complementary text to the Compilation of some 179 figures, displaying spectra contributed from over 20 different research groups, published by Progress in Surface Science in 1985.