Surface energy anisotropy of tungsten

Field-ion microscopy was used to study the faceting behavior and/or surface energy anisotropy of tungsten in vacuum and in hydrogen. In vacuum below 1700 K the activation energy for {110} facet growth agreed with values previously reported for surface diffusion on tungsten. The observed anisotropy values at 0.5 T_m, where T_m is the absolute melting temperature of tungsten (～3680 K), were different from those previously reported at higher temperatures and more nearly agreed with broken bond calculations based on Mie potential using m = 5, n = 8, and a 1.5% lattice expansion. Hydrogen appeared to have a negligible effect on surface energy anisotropy, but did preferentially increase surface diffusion rates on {310} regions.     

Reductive amination agents: comparison of Na(CN)BH3 and Si-CBH

Reductive amination is a chemical reaction commonly employed by organic chemists in academics and the pharmaceutical industry. In this reaction a carbonyl group is converted to an amine via an imine intermediate, the formation of which is rate limiting. A major reagent necessary for the completion of this reaction is a hydride source, commonly sodium cyanoborohydride (Na(CN)BH₃). The objective of this research was to compare the efficacy of Na(CN)BH₃ with silica-bound cyanoborohydride (Si-CBH) as hydride sources in reductive amination reactions. Work has shown that reactions employing Si-CBH as a hydride source showed significant improvement, exhibiting an average percent conversion 25% greater than reactions using Na(CN)BH₃.     

Zum Nachweis von Chloral neben Chloroform

Ohne Zusammenfassung     

Comparative analysis of methods for heat losses in turbulent premixed flames using physically-derived reduced-order manifolds

Heat losses have the potential to substantially modify turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides. The chemistry governing these species is strongly temperature sensitive, making heat losses critical for an accurate prediction. To account for the effects of heat loss in large eddy simulation (LES) using a precomputed reduced-order manifold approach, thermochemical states must be precomputed not only for adiabatic conditions but also over a range of reduced enthalpy states. However, there are a number of methods for producing reduced enthalpy states, which invoke different implicit assumptions. In this work, a set of a priori and a posteriori LES studies have been performed for turbulent premixed flames considering heat losses within a precomputed reduced-order manifold approach to determine the sensitivity to the method by which reduced enthalpy states are generated. Two general approaches are explored for generating these reduced enthalpy states and are compared in detail to assess any effects on turbulent flame structure and emissions. In the first approach, the enthalpy is reduced at the boundary of the one-dimensional (1D) premixed flame solution, resulting in a single enthalpy deficit for a single premixed flame solution. In the second approach, a variable heat loss source term is introduced into the 1D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. The two approaches are compared in methane–air piloted turbulent premixed planar jet flames with different diluents that maintain a constant adiabatic flame temperature but experience different radiation heat losses. Both a priori and a posteriori results, as well as a chemical pathway analysis, indicate that the manner by which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism, except in situations where heat loss is unphysically fast compared to the flame time scale. A new theoretical framework to explain this insensitivity is also proposed, and its validity is briefly assessed.     

Surface energy anisotropy of iridium

Field-ion microscopy was used to study the faceting behavior and surface energy anisotropy of iridium in vacuum and in hydrogen. In vacuum below approximately 1300 K the order of faceting and the activation energy for growth of {111} facets agreed with previously published results of FIM studies. The unexpected faceting behavior of {210} planes was examined in terms of the geometry of field evaporated specimens. The observed anisotropy at temperatures above 1300 K was in qualitative agreement with Morse and Mie potential calculations and in nearly quantitative agreement with the pairwise bonding model using σ₂ = 0.4, σ₃ = 0.2. The observed maximum anisotropy of 8.8% for iridium at 1360 K fell within the range of extrapolated values for other metals at one-half the absolute melting temperature. Hydrogen appeared to lower the surface energy of each plane by only about 0.1%. An anisotropic effect of hydrogen on the faceting behavior, however, was observed and suggested that surface diffusion rates in {110} and {311} regions were preferentially increased in the presence of hydrogen.     

Dynamic mode decomposition of a direct numerical simulation of a turbulent premixed planar jet flame: convergence of the modes

Dynamic Mode Decomposition (DMD) is a technique that enables investigation of unsteady and dynamic phenomena by decomposing data into coherent modes with corresponding growth rates and oscillatory frequencies. Because the method identifies structures unbiased by energy, it is particularly well suited to exploring dynamic processes having phenomena that span disparate temporal and spatial scales. In turbulent combustion, DMD has been previously applied to the analysis of narrowband phenomena such as combustion instabilities utilising both experimental and computational data. In this work, DMD is used as a tool to analyse broadband turbulent combustion phenomena from a three-dimensional direct numerical simulation of a low Mach number spatially-evolving turbulent planar premixed hydrogen/air jet flame. The focus of this investigation is on defining the metric of convergence of the DMD modes for broadband phenomena when both the temporal resolution and number of data snapshots can be varied independently. The residual is identified as an effective, even if imperfect, metric for judging convergence of the DMD modes. Other metrics – specifically, the convergence of the mode eigenvalues and the decay of the amplitudes of the modes – fail to capture convergence of the modes independently but do complete the information needed to evaluate the quality of the DMD analysis.     

Enhancements of the G-Scheme Framework

The G-Scheme is a well established framework for multi-scale adaptive model reduction, whose effectiveness was demonstrated with reference to a number of test models, together with an identification of the critical areas that were in need of further theoretical and computational refinement. In this communication, we report on how we enhanced the solver performance. Two new features involving (i) the criteria to identify the fast and slow subspace dimensions, and (ii) a criterion to decide if and when the reuse of the CSP Basis is feasible without deteriorating the overall performance of the solver, have been proved able to increase significantly the computational efficiency of the solver without sacrificing its accuracy.     

Multilayering of a pseudo polyelectrolyte (PVPh) with a strong polyelectrolyte (PDMAC) from aqueous media

The introduction of pseudo polyelectrolytes (pPE) into the field of multilayer thin films has recently been achieved with the successful combination of poly(4-vinylphenol) (PVPh) with the weak polyelectrolyte (WPE), polyallylamine hydrochloride (PAH). This paper examines the stretching of this limit by exploring the extremes of using the pPE with the strong polyelectrolyte (SPE), poly(diallyldimethylammonium chloride) (PDMAC). UV-Vis absorbance and atomic force microscopy (AFM) topography data reveal a linear growth trend in film thickness that depends critically upon the assembly pH. At an assembly pH of 11.0 the multilayer was five times thicker compared to that assembled at pH 12.0. AFM topography images also show that the surface roughness of the films increases as the assembly pH decreases.