Mechanism Of Stability Enhancement Research Articles

Dual stability enhancement mechanisms of axial-slot casing treatment in a high-speed mixed-flow compressor with various tip clearances

The influence of Axial-Slot Casing Treatment (ASCT) on the performance and stability enhancement mechanisms of ASCT were experimentally and numerically investigated in a high-speed mixed-flow compressor under three different tip clearances. Unsteady simulations showed the compressor stalled through end-wall stall route, i.e. the spike stall inception originating from rotor tip region, which was validated by dynamical measurements. When the ASCT was applied, greater than 20% of Stall Margin Improvement (SMI) could be achieved for the compressor under each tip clearance size. The streamwise velocity contours and flow structures in the tip region and axial slots were deeply analyzed to explore how the so called “suction and injection effects” generated by the ASCT manipulate tip clearance flow and enhance the stability of compressor under different tip clearances. It was found that the dominant stability enhancement mechanisms of ASCT varies with tip clearance size for the mixed-flow compressor. (A) For the small tip clearance, the dominant mechanism of stability enhancement is the blockage reduction generated in the blade passage by the suction effect of ASCT. (B) For the large tip clearance, the injection effect of the ASCT is the dominant mechanism of stability enhancement with ASCT, which plays the leading role in delaying the spillage of incoming/tip leakage flow interface at the rotor Leading Edge (LE) plane.

New insight into foam stability enhancement mechanism, using polyvinyl alcohol (PVA) and nanoparticles

Abstract Foam stability is critical when using foams to reduce the gas mobility and to increase the oil sweep efficiency, under harsh reservoir conditions. In this paper, we formulated foam stabilizer including a polymer, two different surfactants, and six different nanoparticles. Static foam stability tests were conducted using alpha olefin sulfonate (AOS) and cetyltrimethylammonium bromide (CTAB) as anionic and cationic surfactants, respectively; polyvinyl alcohol (PVA) polymer; and six acidic and amphoteric nanoparticles. We also tested the effect of oil on the foam stability, using a light and a heavy crude oil where NaCl brine was the aqueous phase. The effects of different stabilizer formulations, experimental variables, and their interactions were also evaluated. To assess the effectiveness of foam stabilizers, supplementary measurements were measured, including the dynamic interfacial tension, the viscosity of the aqueous solutions, the foam comprehensive index (FCI), and the diffusivity of the nanoparticles in liquids. The results indicated a significant effect of the polymer on the foam stability; when the PVA polymer was added to the foam solution containing an anionic surfactant and acidic nanoparticles, a third protective layer was formed, stabilizing the lamella. The most stable foam was obtained by using moderate concentration of Al2O3 nanoparticles in a solution with low concentration polymer and NaCl brine. This foaming solution featured over 16 times higher half-life time, t1/2, compared to the nanoparticle-surfactant system alone. It was found that the heavy oil destabilizes the foam more than the light oil; also, an increase in the salinity of the aqueous phase increased the foam stability, when other factors were fixed. This research work provides useful tips to create stable foams through using nanoparticles and polymer that can have various applications in energy and environment processes.

(Invited) PGM-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions in Reversible Alkaline Fuel Cells

The intrinsic instability of carbon largely limits its use for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as a bifunctional catalyst in reversible fuel cells or water electrolyzers. Herein, we discovered that Mn doping has a promotional role in stabilizing nanocarbon catalysts for the ORR/OER in alkaline media. Stable nanocarbon composites are derived from an inexpensive carbon/nitrogen precursor (i.e., dicyandiamide) and quaternary FeCoNiMn alloy via a template-free carbonization process. In addition to FeCoNiMn metal alloys/oxides, the carbon composites comprise substantial carbon tube forests growing on a thick and dense graphitic substrate. The dense carbon substrate with high degree of graphitization results from Mn doping, while active nitrogen-doped carbon tubes stem from FeCoNi. Catalyst structures and performance are greatly dependent on the doping content of Mn. Various accelerated stress tests (AST) and life tests verify the encouraging ORR/OER stability of the nanocarbon composite catalyst with optimal Mn doping. Extensive characterization before and after ASTs elucidates the mechanism of stability enhancement resulting from Mn doping, which is attributed to (i) hybrid carbon nanostructures with enhanced resistance to oxidation and (ii) the in situ formation of the β-MnO2 and FeCoNi-based oxides capable of preventing carbon corrosion and promoting activity. Note that the improvement in stability due to Mn doping is accompanied by a slight activity loss due to a decrease in surface area. This work provides a strategy to stabilize carbon catalysts by appropriately integrating transition metals and engineering carbon structures.

Mechanism study on stability enhancement of adefovir dipivoxil by cocrystallization: Degradation kinetics and structure-stability correlation

The purpose of this study is to determine the mechanism by which cocrystallization can enhance the stability of adefovir dipivoxil (AD), a diester prodrug of adefovir with known chemical stability problem. Three multi-component crystals of AD with biologically safe coformers, including gallic acid cocrystal hydrate (1:1:1), salicylate salt (1:1), and maleate salt (1:1) were prepared and characterized by thermal analysis, infrared spectroscopy, powder and single crystal X-ray diffraction. DVS measurements and stability tests were applied to evaluate the stability. The new crystalline phases exhibit improved stability compared to pure drug in the order AD gallic acid cocrystal>AD maleate>AD salicylate>AD form I. Degradation kinetics and structure-stability correlation studies demonstrate that the stability enhancement mechanism by cocrystallization involves (1) inhibition of hydrolysis of AD by replacement of drug–drug homosynthons by stronger drug–coformer heterosynthons at adenine fragments; (2) suppression of dimerization of AD by separation of adenine fragments by inserting coformers in crystal lattices; (3) further reducing rates of hydrolysis by forming hydrogen bonds with hydrate water at phosphoryl fragments. This study has important implications for use of cocrystallization approach to some easily degradable drugs in pharmaceutical.

An Improved Ti/Sb-SnO2 Electrodes with Long Lifetime and High Performance: Characterization and Mechanism of Stability Enhancement

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Experimental Investigation of Stability Enhancement Mechanism with Tip Air Injection in Axial Compressor

Experimental Investigation of Stability Enhancement Mechanism with Tip Air Injection in Axial Compressor

Thermal stability enhancement mechanism of poly(dimethylsiloxane) composite by incorporating octavinyl polyhedral oligomeric silsesquioxanes

The effects of octavinyl polyhedral oligomeric silsesquioxane (Vi-POSS), silica (SiO2), and octaphenyl POSS (Ph-POSS) on the thermal stability of poly(dimethylsiloxane) (PDMS) were studied using thermal gravimetric analysis. The decomposition mechanism of the PDMS/Vi-POSS composite was proposed by crosslinking density measurement, decomposition residues and volatiles analysis. The results showed that the Vi-POSS had significant effect on enhancing the thermal stability of PDMS. The decomposition mechanism of PDMS was changed by the chemical incorporation of Vi-POSS, but SiO2 and Ph-POSS had few effects on the polymer decomposition process, because they were mixed by simple physical blending. The cyclic depolymerization of PDMS, which produced many cyclic oligomers at relatively low temperature, was suppressed by the heavily crosslinked network in the PDMS/Vi-POSS composite. The scissions of carbon–carbon, silicon–carbon and carbon–hydrogen bonds took place at higher temperature, releasing hydrogen, methane, ethylene and ethane, left behind siliconoxycarbide and some free carbon at 800 °C.

Thermal and photocatalytic stability enhancement mechanism of poly(propylene carbonate) due to Cu(I) impurities

The thermal and photolytic decomposition of poly(propylene carbonate) (PPC) in the presence of a photoacid generator (PAG) was studied. The mechanism of previously observed shifts in the decomposition temperature of PPC was investigated. The decomposition temperature of PPC can be altered when PPC/PAG films contact copper metal. X-ray photoelectron spectroscopy (XPS) analysis showed that a small amount Cu(I) was incorporated into the PPC/PAG film causing the increased stability and the Cu(I) concentration was similar to that of the PAG. PPC/PAG temperature stability was studied for a number of iodonium and sulfonium-based PAGs. Every iodonium-based PAG showed a PPC decomposition temperature shift and no sulfonium-based PAG showed the thermal stability effect. It appears that Cu(I) interacts with iodonium in the cation of the PAG causing a change in the acid creation mechanism of the PAG.

Molecular modelling insights into DFNA15 mediated enhancement of POU4F3 stability

The POU4F3 transcription factor is expressed in the cochlear and vestibular hair cells of the inner ear and its targeted deletion results in a loss of inner ear hair cells. The DFNA15 truncation mutation has been demonstrated to result in a loss of transcriptional activity, but an increase in the stability of the protein. Molecular modelling is utilised to propose a mechanism of stability enhancement, via an interaction between the truncated POU(HD) domain and the POU(S) domain of the transcription factor.

Preparation, Tg improvement, and thermal stability enhancement mechanism of soluble poly(methyl methacrylate) nanocomposites by incorporating octavinyl polyhedral oligomeric silsesquioxanes

AbstractThe soluble poly(methyl methacrylate‐co‐octavinyl‐polyhedral oligomeric silsesquioxane) (PMMA–POSS) hybrid nanocomposites with improved Tg and high thermal stability were synthesized by common free radical polymerization and characterized using FTIR, high‐resolution 1H NMR, 29Si NMR, GPC, DSC, and TGA. The POSS contents in the nanocomposites were determined based on FTIR spectrum, revealing that it can be effectively adjusted by varying the feed ratio of POSS in the hybrid composites. On the basis of the 1H NMR analysis, the number of the reacted vinyl groups on each POSS molecules was determined to be about 6–8. The DSC and TGA measurements indicated that the hybrid nanocomposites had higher Tg and better thermal properties than the pure PMMA homopolymer. The Tg increase mechanism was investigated using FTIR, displaying that the dipole–dipole interaction between PMMA and POSS also plays very important role to the Tg improvement besides the molecular motion hindrance from the hybrid structure. The thermal stability enhances with increase of POSS content, which is mainly attributed to the incorporation of nanoscale inorganic POSS uniformly dispersed at molecular level. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5308–5317, 2007