Mechanism Of Stability Enhancement Research Articles

Research on Effect of Endwall Contouring of Vaned Diffuser on Stable Operating Range of Centrifugal Compressor

Abstract The study investigates the impact of diverse endwall contouring strategies on the operational stability of a high-pressure ratio centrifugal compressor stage. Employing numerical simulation techniques, this study examines the implications of contoured wall positions, specifically convex profiles at the pressure side and/or concave profiles at the suction side, as well as asymmetric endwall contouring, on both stage performance and internal flow field within the centrifugal compressor. And the stability enhancement mechanism of endwall contouring is clarified. The results demonstrate that employing full circular contoured vaned diffusers on the hub-side wall can achieve a maximum improvement in stall margin of 15.10%. Additionally, the diffuser featuring solely convex profiles at the pressure side on the hub-side wall exhibits an augmented stall margin by 8.39%, with a reduction in performance loss at low flow rate conditions. The asymmetric endwall contouring improves stall margin by 8.39%, and mitigates the performance loss across the entire operating range. At the near-stall point, the contoured endwalls redirect fluid towards the suction side, thereby mitigating the incidence at the vane leading edge and attenuating the interaction between the impeller trailing edge vortex and the diffuser leading edge vortex. Moreover, concave profiles serve to accelerate the fluid on the suction side along the flow direction, thereby suppressing the corner separation and weakening passage blockages. Simultaneously, the advantages of endwall contouring in vane diffusers extend to the improvement in the circumferential non-uniformity of flow fields and the enhancement in diffuser stability.

Unsteady measurement in a transonic axial compressor with optimized axial slot casing treatment

Casing treatment has been widely adopted to enhance the operating range of compressors by extending the stall margin. In this article, a high-precision experimental investigation was conducted in a transonic compressor. Unsteady pressure is measured in the casing wall using a cluster of time-resolved transducers with axial and circumferential spatial resolution. The experimental data show that the optimized axial slot casing treatment improves the stall margin without peak efficiency penalty for all measured speedlines. A comprehensive flow field analysis indicates that the surge boundary of transonic compressor is sensitive to the position of shock wave and tip leakage flow. After the application of optimized axial slot casing treatment, the flow structure can be significantly modified, which specifically manifests as both the mitigation in shock wave detachment and redistribution of tip leakage flow trajectory toward the trailing edge of the blade. Furthermore, power spectral density analysis is conducted to examine the unsteady effect. The amplitude of characteristic frequency band induced by the unsteady fluctuation of tip leakage flow and shock wave is considerably suppressed, which can also be regard as the stability enhancement mechanism of casing treatment.

Integrated construction of stretchable supercapacitor with outstanding deformation stability and flame retardancy

The fast development of wearable electronics requires urgently stretchable energy storage devices, but conventional stretchable energy-storage devices suffer from poor dynamic deformation stability. The key is to create a new construction strategy of "electrode/electrolyte interface integration". Herein, we have proposed a clever strategy that combines layer-by-layer electrospinning with a self-selected vapor-phase polymerization (VPP) method to assemble the integrated stretchable supercapacitors (ISSCs). Furthermore, the interface stability enhancement mechanism is revealed through experimental and theoretical simulations, showing that the synergistic effect of molecular chain entanglement and mechanical meshing endows excellent anti-deformation stability of ISSCs. As a result, the as-assembled ISSCs deliver excellent dynamic deformation electrochemical stability with 99.5 % capacitance retention after 500 stretching cycles at strain and outstanding flame retardancy (limiting oxygen index (LOI) up to 48.1 % at a film thickness of only 0.22 mm). The innovative integrated design strategy endows a systematic construction plan for the scalable fabrication of wearable organic energy storage devices.

Development of dual-crosslinked nanocomposite gel for water management in high-temperature reservoirs

Due to their controllable gelation time and high gel strength, polymer gels are widely used in the oil and natural gas industry for sealing operations. Aiming at the problems of fast gelation time and poor stability of polymer gel when the temperature exceeds 120 °C, a dual-crosslinked nanocomposite gel for high-temperature reservoir water management was developed. Polymers, crosslinking agents, nanomaterials, and retarders were studied for the effects of the concentration of each component on the initial viscosity, gelation time, stability, and viscoelastic properties of the gel. The long-term stability and water-plugging adaptability were evaluated. FTIR and viscoelastic performance analyses clarified the mechanism of action. The gel strength and thermal stability enhancement mechanism was analyzed based on SEM. At 170 °C,the gelation time is 5 ∼ 10h after adding retarder and aging in formation water for 30 days resulted in dehydration rates within 10%. The injection volume of the plugging agent is 0.3 PV, and the plugging rate of sand-filled pipes with different permeability is above 97%. The elastic modulus of the terpolymer SAV333/phenol-formaldehyde system was increased by 10 times by resorcinol and SiO2. The enhancement mechanism is mainly due to the gel’s strong dendritic dual-crosslinked network structure and the dual role of hydrogen bonding and electrostatic attraction of SiO2. Dual-crosslinked nanocomposite gel (DCNG) has shown promising performance as sealing material in high-temperature and ultra-high-temperature reservoirs.

Enhancement of emulsion pH stability by Ca2+ modulation: A novel approach for preparing egg white protein fibril cold emulsion gels

Egg white, a crucial ingredient in bulk food production with limited emulsifying properties, can be significantly improved by fibrillation modification. However, resulting emulsions may exhibit instability at critical food processing pH values. In this study, the egg white protein fibril (EWPF) emulsion gel was constructed by Ca2+, and the stability enhancement mechanism under different EWPF and Ca2+ concentrations was investigated. The results showed that higher EWPF concentrations led to improved stabilization of the oil-water interface. At 60 mg/mL EWPF concentration, water loss decreased by 13.86% and oil loss by 35.79%, while the firmness and viscosity of the emulsion gel system were enhanced. The ζ-potential was reduced by 67.98% and the ionic bond content was increased by 22 times with the addition of Ca2+, the droplets showed a uniform cross-linked structure, and the stability and plasticity of the system were improved. By adjusting EWPF concentration to 40 mg/mL and Ca2+ concentration to 30 mM, pH stability of the system was also enhanced. This study provided a reference for the high-value utilization of egg white and a possibility for the application of EWPF in food processing, such as low-fat foods.

Electron transfer route tuning caused by photo-irradiation: Stability enhancement of N-doped carbon materials in Fenton-like reactions

Improving the stability of N-doped carbon materials is still a great challenge in Fenton-like reactions. Herein, a hollow structure with TiO2 nanoparticles supported on the inner surface of N-doped carbon spheres (H-TiO2 @NC) is designed for peroxydisulfate (PDS)-based Fenton-like reactions with photo-irradiation. Impressively, the stability of H-TiO2 @NC is greatly enhanced with photo-irradiation. The degradation rate constant of bisphenol A (BPA) with photo-irradiation is ∼61 times larger than dark condition after four runs. The photovoltage enables more electrons to be transferred from BPA to PDS via active carbons. Less electrons from the carbon framework are expended, thus inhibiting the oxidation and deactivation of H-TiO2 @NC effectively. Additionally, the direct electron transfer (DET) process is improved due to conductivity enhancement, which also weakens the oxidation of carbon skeleton. This work provides a reliable method to enhance the stability of N-doped carbon materials. And giving some new insights into the mechanism of stability enhancement.

The highly stable indicator film incorporating roselle anthocyanin co-pigmented with oxalic acid: Preparation, characterization and freshness monitoring application

A novel stable PVA/HPMC/roselle anthocyanin (RAE) indicator film co-pigmented with oxalic acid (OA) was prepared, its properties, application effects and stability enhancement mechanism were investigated correspondingly. The structural characterization revealed that more stable network was formed due to the co-pigmentation facilitated generation of molecular interactions. Meanwhile, the co-pigmentation improved film mechanical and hydrophobic properties compared to both PVA/HPMC/RAE newly prepared (PHRN) or stored (PHRS) film, expressing as higher tensile strength values (12.25% and 14.44% higher than PHRN and PHRS), lower water solubility (7.22% and 10.09% lower than PHRN and PHRS) and water vapor permeability values (33.20% and 21.05% lower than PHRN and PHRS) of PVA/HPMC/RAE/OA newly prepared (PHON) or stored (PHOS) film. Compared with the PHRS film, the PHOS film still presented more distinguishable color variations when being applied to monitor shrimp freshness, owing to the stabilization behaviors of co-pigmentation in anthocyanin conformation. Hence, the co-pigmentation was an effective strategy to enhance film stability, physical and pH-responsive properties after long term storage, leading to better film monitoring effects when applied in real-time freshness monitoring.

Solvent-Free Loading of Vitamin A Palmitate into β-Cyclodextrin Metal-Organic Frameworks for Stability Enhancement.

Cyclodextrin metal-organic frameworks (CD-MOFs) exhibit a high structural diversity, which contributes to their functional properties. In this study, we have successfully synthesized a novel type of β-cyclodextrin metal-organic framework (β-CD-POF(I)) that exhibits excellent drug adsorption capacity and enhances stability. Single-crystal X-ray diffraction analysis revealed that β-CD-POF(I) possessed the dicyclodextrin channel moieties and long-parallel tubular cavities. Compared with the reported β-CD-MOFs, the β-CD-POF(I) has a more promising drug encapsulation capability. Here, the stability of vitamin A palmitate (VAP) was effectively improved by the solvent-free method. Molecular modeling and other characterization techniques like synchrotron radiation Fourier transform infrared spectroscopy (SR-FTIR), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and nitrogen adsorption isotherm were applied to confirm that the VAP was successfully encapsulated into the channel formed by the dicyclodextrin pairs. Furthermore, the mechanism of stability enhancement for VAP was determined to be due to the constraint and separation effects of β-CD pairs on VAP. Therefore, β-CD-POF(I) is capable of trapping and stabilizing certain unstable drug molecules, offering benefits and application possibilities. One kind of cyclodextrin particle with characteristic shapes of dicyclodextrin channel moieties and parallel tubular cavities, which was synthesized by a facile process. Subsequently, the spatial structure and characteristics of the β-CD-POF(I) were primarily confirmed. The structure of β-CD-POF(I) was then compared to that of KOH-β-CD-MOF, and a better material for vitamin A palmitate (VAP) encapsulation was determined. VAP was successfully loaded into the particles by solvent-free method. The arrangement of spatial structure made cyclodextrin molecular cavity encapsulation in β-CD-POF(I) more stable for VAP capture than that of KOH-β-CD-MOF.

Diffusion Kinetics-Based Loading of Lipoic Acid into Cyclodextrin Metal–Organic Frameworks via a Solvent-Free Method and Mechanism of Stability Enhancement

It is difficult to accurately distinguish the amount of a drug inside versus outside of a carrier during drug loading, which limits the ability to determine the kinetic data (the drug diffusion coefficient, energy barrier, etc.). Here, we demonstrated the feasibility of determining the drug diffusion coefficient and content without distinguishing whether it is inside or outside of a carrier. Specifically, the sample of gamma-cyclodextrin metal–organic frameworks (CD-MOF) loaded with unstable and viscous alpha-lipoic acid (LA) was examined. Furthermore, the LA stability was enhanced by the CD-MOF via a straightforward and unambiguous solvent-free method, and the mechanism behind it was thoroughly explored based on the diffusion coefficient. For the first time, a combination of mathematical modeling and new experimental measurements was used to determine the diffusion coefficient of LA in CD-MOF crystals. Accordingly, the strong adhesion free energy (−8.64 ± 0.56 kcal·mol–1) between LA and the dicyclodextrin cavity in the CD-MOF was determined. Furthermore, the nature of the adhesion force was identified to be the chemisorption dominated by hydrogen binding employing a combination of molecular modeling (docking and density functional theory, DFT) and experimental methods (solid-state nuclear magnetic resonance, ssNMR). Finally, this study revealed that the stability enhancement of LA in the CD-MOF is due to the combined actions of the high binding free energy of LA with the dicyclodextrin cavity and its steric hindrance, together with the accelerating roles of the CD-MOF cage environment.

Generalized Encapsulations of ZIF-Based Fe-N-C Catalysts with Controllable Nitrogen-Doped Carbon for Significantly-Improved Stability Toward Oxygen Reduction Reaction.

The vigorous development of efficient platinum group metal-free catalysts is considerably important to facilitate the universal application of proton exchange membrane fuel cells. Although nitrogen-coordinated atomic iron intercalated in carbon matrix (Fe-N-C) catalysts exhibit promising catalytic activity, the performance in fuel cells, especially the short lifetime, remains an obstacle. Herein, a highly-active Fe-N-C catalyst with a power density of >1 w cm-2 and prolonged discharge stability with a current density of 357 mA cm-2 after 40 h of constant voltage discharge at 0.7 V in H2 -O2 fuel cells using a controllable and efficient N-C coating strategy is developed. It is clarified that a thicker N-C coating may be more favorable to enhance the stability of Fe-N-C catalysts at the expense of their catalytic activity. The stability enhancement mechanism of the N-C coating strategy is proven to be the synergistic effect of reduced carbon corrosion and iron loss. It is believed that these findings can contribute to the development of Fe-N-C catalysts with high activity and long lifetimes.

Stability enhancement of molten pool and keyhole for 2195 Al[sbnd]Li alloy using fiber-diode laser hybrid welding

Molten pool and keyhole stability are vital concerns in laser welding of 2195 AlLi alloy, potentially affecting the weld quality and porosity defect. Fiber-diode laser hybrid welding, which coaxially superimposes dual laser beams with different wavelengths and spot diameters, is superior in improving welding stability and quality. In this paper, the molten pool dynamic characteristics associated with the keyhole during fiber-diode laser hybrid welding were innovatively investigated to explore the impact of different laser energy on welding stability. The inner keyhole was observed by utilizing the “sandwich” structure with 2195 AlLi alloy and refractory glass. The experimental results indicate that during fiber-diode laser hybrid welding, a larger molten pool and plasma plume, deeper keyhole depth, and smoother keyhole rear wall are generated compared to single fiber laser welding. It is shown that the closing of the keyhole entrance and the swelling of the molten pool are effectively inhibited due to the superposition of the diode laser. The average keyhole depth during fiber-diode laser hybrid welding at the PD of 2500 W is substantially increased compared to that during single fiber laser welding. Based on the keyhole and molten pool characteristics, the mechanism of stability enhancement with the assistance of a diode laser is understood. In addition, the fiber-diode hybrid welding method can achieve a weld seam (WS) with high-quality surface morphology and a low porosity ratio of 0.7 %. This work provides the necessary theoretical basis and technical guidance for the fiber-diode laser hybrid welding technology of aluminum alloy.

Charge transport characteristic and charge stability enhancement mechanism of polyimide aerogel as an ultralight weight electret

In this study, polyimide aerogel (PIA) is applied as an ultralight-weight and thermally stable electret material, meanwhile its charge transport characteristic and charge stability enhancement mechanism are investigated. It is found that PIA can exhibit excellent electret properties at 120 °C after corona charging at room temperature, and its charge stability acquires further promotion after charging at elevated temperatures or heat treatment, which far exceeds conventional polyimide-based electrets. Further research indicates that the outstanding charge retention characteristics of PIA originate from the enhanced local defect density, fast retrapping effect, weakened internal electric field by dipole polarization, and prolonged conduction path, which are inextricably linked to its porous structure. In addition, the high gas breakdown strength in the pores of PIA effectively limits the breakdown induced conductivity, thereby ensuring high space charge density and large piezoelectric response of PIA electrets. Considering the extremely low density and inherently excellent properties of polyimide substances, PIA may have promising prospects as an electret in the lightweight sensing and energy conversion devices at elevated temperatures.

Effect of self-recirculating casing treatment on the unsteady flow and stability of counter-rotating axial-flow compressor

Abstract Counter-rotating axial-flow compressor (CRAC) is a promising technology to enhance the thrust-to-weight ratio of aero-engines. Self-recirculating casing treatment (SRCT) is an efficient flow control technique for increasing stall margin in conventional compressors. With the purpose of investigating the applicability and mechanism of SRCT in the CRACs, a two-stage CRAC is selected to investigate the stability enhancement mechanism of SRCT and its effect on the unsteady flow near the rotor tip, and the effect of injector location on the stability improvement capacity of SRCT is also studied. Results show that about 7.73% stall margin improvement can be achieved by configuring the SRCT on the near rotor top, and the injector location also has a significant influence on the stability expansion potential of SRCT. The SRCT delays the stall occurrence by weakening the intensity of tip leakage flow (TLF) and restraining the leading-edge spillages of TLF. The SRCT reduces the unsteady interference between the adjacent rotors by receding the disturbance of the upstream wake and inhibiting the potential flow effect of the downstream. Furthermore, the SRCT reduces the self-excited oscillation frequency of TLF and damps its fluctuation amplitude.

Co3O4/LaCoO3 nanocomposites derived from MOFs as anodes for high-performance lithium-ion batteries

• The lanthanide perovskite LaCoO 3 is synthesized from pyrolysis of MOF. • The introduction of LaCoO 3 improves the lithium storage performance and structural stability of Co 3 O 4 . • The capacitance contribution rate of the composite material reached an incredible 94.7% at 1.0 mV/s. Co 3 O 4 has been extensively studied due to its high theoretical capacity in lithium ion batteries (LIBs) anode materials. Unfortunately, the large volume change of Co 3 O 4 during charging and discharging causes severe capacity degradation, which limits its practical application. In this paper, the metal organic framework (MOF) synthesized by the microwave method was used as the precursor, and the Co 3 O 4 /LaCoO 3 nanocomposite with unique heterostructure was synthesized by calcination. The introduction of LaCoO 3 improves the lithium storage performance and structural stability of Co 3 O 4 . Under the best doping ratio, the reversible charging capacity of Co 3 O 4 /LaCoO 3 reaches 1024 mAh/g (after 100 cycles), breaking the theoretical capacity limit of Co 3 O 4 . Impressively, even at a high current density of 1 A/g, the composite electrode still exhibits an ultra-high reversible capacity of 799 mAh/g. The excellent electrochemical performance of Co 3 O 4 /LaCoO 3 is attributed to the introduction of LaCoO 3 which provides a buffer space for electrode volume changes and shortens the diffusion path of Li + . Finally, kinetic analysis shows that the capacity and stability enhancement mechanism is related to the ultra-high capacitance ratio of the composite material (94.7% at 1 mV/s). This study demonstrates a MOF in-situ synthesis project: a way to overcome the capacity and cycle limitations of Co 3 O 4 in LIBs anodes by compounding lanthanide perovskites.

Numerical study on the stability enhancement mechanism of self-recirculating casing treatment in a counter-rotating axial-flow compressor

Counter-rotating axial-flow compressor (CRAC) is a promising technology to increase the thrust-to-weight ratio of aero-engines. Self-recirculating casing treatment (SRCT) is selected to explore its stability enhancement capacity in the CRAC. In the present work, an effective SRCT is designed by the design of experiment method, and the stability improvement potential of the SRCT and its mechanism of stall margin enhancement are studied. Results show that the optimal SRCT scheme increases the stall margin by about 7.73% and without remarkable peak efficiency loss, and the overall performance of the compressor is enhanced at the near-stall condition. The SRCT improves the quality of the tip flow field by sucking out low-energy fluid and restrains the spillage of the tip leakage flow by the jet effect from the injection port. In addition, the SRCT also reduces the unsteady interference between the adjacent rotors. The effect of speed ratio on the effectiveness of SRCT is further investigated, and results indicate that the SRCT increases the stall margin of the CRAC by about 4.13–5.80% at some off-design speed ratio conditions. The speed ratio affects the first stall stage of the CRAC, thus affecting the effectiveness of the SRCT.

Research on the stability enhancement mechanism of multi-parameter interaction of casing treatment in an axial compressor rotor

The research on geometric parameters of casing treatment (CT) has always been a hot topic, yet the multi-parameter interaction is rarely studied. In order to gain better knowledge of the interaction mechanism of geometric parameters of CT, the experimental and numerical study based on the response surface method has been carried out in an axial compressor rotor. First of all, the statistical analysis based on the database of experimental and numerical results is presented to summarize the influence law of varied parameters on the stability enhancement. It was found that axial overlap, open area ratio, and their interaction had the most significant influence on the stability enhancement of CT. Subsequently, the interaction between axial overlap and open area ratio was analyzed by visualization flow field in details, which provided a deeper insight into stability enhancement mechanism of CT. It indicated that the mass flow and momentum dominated by injection and the suction effect played a key role for extending stability. With smaller open area ratio, it was difficult for the slots to manipulate and control the tip leakage flow or secondary tip leakage flow, resulting in the weak effect of CT on compressor performance. Finally, the underlying flow physics in the tip region and dominant region of CT has also been discussed to penetrate the essential reason of multi-parameter interaction.

Mechanism of Stability Enhancement with Shallow ‎Reversed Slot-Type Casing Treatment in a Transonic ‎Compressor

In this paper, the influence of a shallow reversed slot-type casing treatment on the performance of a tip-critical transonic compressor has been numerically investigated. Firstly, the complex flow fields in the rotor tip region are studied in details. It shows the severe blockage induced by suction surface boundary separation triggers compressor stall at 100% design speed, while the blockage due to tip leakage vortex dominates at 80% design speed. Secondly, the mechanism of stability extension is presented at different rotating speeds. The casing treatment alleviates greatly the tip blockage by manipulating the tip leakage flow, accompanied by the redistribution of aerodynamic loading and mass flux. As a result, the casing treatment is more efficient for the blockage induced by tip leakage vortex (at 80% design speed). Further analysis of the pressure field and passage shock distribution demonstrates that the passage shock intensity and its location will affect the effectiveness of casing treatment. Finally, the instability characteristics of compressor with casing treatment are revealed. The numerical results reflect when the mass flow approaching the instability boundary, the stator passage blockage presumably is dominant for triggering the compressor stall.

Thermal stability enhancement mechanism of engine oil using hybrid MoS2/h-BN nano-additives with ionic liquid modification

One of the primary causes of lubricating oil failure in automobile engines is thermal degradation caused by high temperatures, which prejudices the lube oil capacity between friction pairs. This research aims to comprehensively investigate the effect of the h-BN/MoS2 hybrid nanoparticles and ionic liquid (IL) on the thermal performance of engine oil (5 W-40). The rheological results showed that nanolubricants (NL) behave like non-Newtonian fluids and IL promoted the viscosity value of lubricant due to the extension of alkyl chains. The thermal peculiarities of lube oil were analyzed by thermogravimetric (TGA/DTG/DSC) analysis to explore the thermal stability of h-BN/MoS2 hybrid nano-lubricants modified by IL and further revealed how IL improved the thermal characteristics. The results strongly showed the synergistic effect between the IL and h-BN/MoS2 hybrid nanoparticles produces a complicated interaction at high temperatures, forming a layer of film which can delay the erosion of high temperature on the surface of the nanoparticles. Ultimately, this study revealed a delay of burnout temperature in thermal degradation by approximately 8.1% under the use of hybrid nano-lube oils in a wide temperature range.

Enhanced Stability and Epitaxial Growth Mechanism of the Honeycomb Borophene Monolayer on a Two-Dimensional Ti2C Substrate

The stability enhancement mechanism of honeycomb borophene on a typical MXene material, known as Ti2C, was investigated systematically by theoretical energetic and kinetic studies. On this basis, t...

Mechanism of Stability Enhancement for Adiponitrile High Voltage Electrolyte System Referring to Addition of Fluoroethylene Carbonate.

In order to improve the stability of high voltage electrolyte for 5 V-level LiNi0.5Mn1.5O4 cathode material, adiponitrile (ADN) with high oxidation stability was selected as the main solvent, meanwhile, 2% fluoroethylene carbonate (FEC) as the additive with good film forming effect was also used. And then, the effect of 2 mol L−1 LiBF4-GBL/ADN+2% FEC on the electrochemical performance of LiNi0.5Mn1.5O4 was explored at room temperature. The electrolyte system containing FEC can improve the cycle stability of the battery. At 1 C rate, the cycle capacity retention rate can reach 83% after 100 cycles, while the capacity retention rate of the electrolyte system without FEC and the ordinary commercial electrolyte system is only 77 and 68%, respectively. Besides, the rate performance of the battery with the addition of FEC also shows excellent performance, however, this kind of advantage is not obvious under the conditon of large rate. In addition, under the conditon of the synergistic effect between adiponitrile and fluoroethylene carbonate, the high-voltage electrolyte exhibits the good compatibility and lithium reversibility in the full cell with Li4Ti5O12 as the negative electrode.