Three-step approach for prediction of limit cycle pressure oscillations in combustion chambers of gas turbines

Currently, gas turbine manufacturers frequently face the problem of strong acoustic combustion driven oscillations inside combustion chambers. These combustion instabilities can cause extensive wear and sometimes even catastrophic damages to combustion hardware. This requires prevention of combustion instabilities, which, in turn, requires reliable and fast predictive tools. This work presents a three-step method to find stability margins within which gas turbines can be operated without going into self-excited pressure oscillations. As a first step, a set of unsteady Reynolds-averaged Navier–Stokes simulations with the Flame Speed Closure (FSC) model implemented in the OpenFOAM® environment are performed to obtain the flame describing function of the combustor set-up. The standard FSC model is extended in this work to take into account the combined effect of strain and heat losses on the flame. As a second step, a linear three-time-lag-distributed model for a perfectly premixed swirl-stabilized flame is extended to the nonlinear regime. The factors causing changes in the model parameters when applying high-amplitude velocity perturbations are analysed. As a third step, time-domain simulations employing a low-order network model implemented in Simulink® are performed. In this work, the proposed method is applied to a laboratory test rig. The proposed method permits not only the unsteady frequencies of acoustic oscillations to be computed, but the amplitudes of such oscillations as well. Knowing the amplitudes of unstable pressure oscillations, it is possible to determine how these oscillations are harmful to the combustor equipment. The proposed method has a low cost because it does not require any license for computational fluid dynamics software.     

Optimal cell approach to osmotic properties of finite stiff-chain polyelectrolytes

We propose a self-consistent geometry optimized cell model approach to study osmotic properties of stiff-chain polyelectrolyte solutions. In contrast with the usual monotonic Poisson-Boltzmann prediction, the cell model predicts the correct nonmonotonic dependence of the osmotic coefficient on concentration. A lower degree of polymerization is found to reduce significantly the counterion condensation in a typical dilute strong polyelectrolyte. The results agree quantitatively with simulations of a corresponding many-body bulk system up to a dense semidilute regime.     

Photoluminescence properties of Ba3B′B″2O9 (B′=Mg, Co or Zn and B″=Nb or Ta) ceramics with perovskite structure

Optical properties of near-stoichiometric compositions of Ba₃B′B″₂O₉ (B′=Mg, Co or Zn and B″=Nb or Ta) perovskite type materials were studied by means of photoluminescence (PL) spectroscopy. Ba₃B′B″₂O₉ perovskites belong to the family of perovskite materials that exhibit phase transition from a completely disordered system having space group Pm-3m to a 1:2 ordered one with space group P-3m1. As revealed by X-ray diffraction, deviation from stoichiometry results in the formation of Ba- and Nb-rich phases. Photoluminescence measurements demonstrated the presence of two broad peaks, the position of which depends on secondary phase formation: the first one, attributed to the disordered Pm-3m phase, is positioned at approximately 430 nm and the second band, positioned at lower energies (i.e., longer wavelengths, around 900 nm), was ascribed to the formation of the 1:2 ordered P-3m1 phase. The position of the first peak is influenced by the bond network through distortions in the local environment caused by the specific electronic structure of the B′ cation.     

Evidence for Non-Innocence of a β-Diketonate Ligand

β-Diketonates, such as acetylacetonate, are amongst the most common bidentate ligands towards elements across the entire periodic table and are considered wholly redox-inactive in their complexes. Herein we show that complexation of 1,1,1,5,5,5-hexafluoroacetylacetonate (hfac⁻) to Cr^II spontaneously affords Cr^III and a reduced β-diketonate radical ligand scaffold, as evidenced by crystallographic analysis, magnetic measurements, optical spectroscopy, reactivity studies, and DFT calculations. The possibility of harnessing β-diketonates as electron reservoirs opens up possibilities for new metal–ligand concerted reactivity in the ubiquitous β-diketonate coordination chemistry.     

4-(1-Haloalkyl)-3-nitrotetrahydrofurans as versatile scaffolds for the synthesis of diversely functionalized tetrahydrofurans

4-(1-Haloalkyl)-3-nitrotetrahydrofurans, which are accessible by tandem oxidative oxa-Michael addition/radical cyclization/ligand transfer reactions, can be processed to diversely substituted tetrahydrofuran derivatives. Selective epimerization at the nitro function provides tetrahydrofuran diastereomers, which cannot be prepared by the tandem process. Intramolecular alkylations furnish interesting bridgehead nitro oxabicyclo[3.1.0]hexane derivatives in high yields. Intermolecular substitution reactions of the halide functions succeed only with nucleophiles, which are not basic enough to trigger intramolecular alkylations. The aryl substituent in 2-aryl-3-nitrotetrahydrofurans can be selectively oxidatively transformed to carboxylic acid derivatives using catalytic Ru(III) and NaIO₄ without affecting the nitro group. Reduction and hydrogenation reactions provide differently substituted 3-aminotetrahydrofuran derivatives depending on the conditions with moderate to good chemoselectivity.     

Oxidative tandem alkoxide conjugate addition to nitroalkenes/radical 5-exo cyclizations—a versatile synthesis of functionalized 3-nitrotetrahydrofurans

Structurally diverse functionalized 4-(1-haloalkyl)-3-nitrotetrahydrofurans were conveniently obtained in moderate to good yield and moderate to very good diastereoselectivity by an oxidative tandem process consisting of conjugate addition reaction of lithium allyloxides to nitroalkenes followed by SET oxidation of the resulting nitronates. This triggers a radical cyclization; ligand transfer from the oxidant provides the products. The influence of the counter ion of the initial alkoxide and intermediate nitronate, the solvent and additives on the outcome of the tandem process was investigated. Optimal conditions for the tandem reactions consist of using butyllithium as the base for deprotonation in DME as the solvent. Cupric halides proved to be the SET oxidants of choice in the tandem reactions. A stereochemical model for the radical cyclization and ligand transfer steps is proposed.     

Synthesis,structural characterization and anticancer properties of p-cymene Ru(II) complexes with 2-(N-methyl-1H-1,2,4-triazol-3-yl)pyridines

Transition Metal Chemistry - The structures of new p-cymene Ru(II) complexes with 2-(1-methyl-1H-1,2,4-triazol-3-yl)pyridine and 2-(1-methyl-1H-1,2,4-triazol-5-yl)pyridine were established based on...