Periodic Solutions of the Duffing Differential Equation Revisited via the Averaging Theory

We use three different results of the averaging theory of first order for studying the existence of new periodic solutions in the two Duffing differential equations $\ddot y+ a \sin y= b \sin t$ and $\ddot y+a y-c y^3=b\sin t$, where $a$, $b$ and $c$ are real parameters.     

Structure and Mechanical Stability of Epoxy Modified Polyurethane Foam Under Heat and Stress

A series of phenolic epoxy resin (PEP) modified polyurethane foams (PUF) were prepared via an in-situ polymerization, one step process. It was found that the epoxy modified PUF foam exhibited a perforated network structure with larger cell size, higher open cell porosity and enhanced ovality compared with pure PUF. With increasing content of PEP, the tensile strength, elongation at break and low temperature modulus of PUF decreased. A single T_g was observed for PEP modified PUF, indicating that the two component phases of the polyurethane-epoxy were miscible. With increasing PEP content, the T_g of PUF shifted slightly to higher temperature, tan δ_max dropped to lower values, and the retention value of the storage modulus at ?20 and ?10?°C increased. For pure PUF, the cell walls degraded and the structure became disordered after aging under heat and stress, while for PUF/20wt%PEP, the degradation degree was obviously reduced, and an orientation of the cells along the stress direction and a density increase was observed. During aging at 200?°C, the retention of the mechanical properties of PUF/20wt% PEP was much higher than that of pure PUF, and it showed superior stability under heat and stress, attributed to incorporation of the thermally resistant oxazolidone rings and benzene rings in the PU backbones, the highly cross-linked networks of the polyurethane-epoxy systems and the obvious orientation of the cells under stress.     

Change of evaporation rate of single monocomponent droplet with temperature using time-resolved phase rainbow refractometry

Droplet evaporation characterization, although of great significance, is still challenging. The recently developed phase rainbow refractometry (PRR) is proposed as an approach to measuring the droplet temperature, size as well as evaporation rate simultaneously, and is applied to a single flowing n-heptane droplet produced by a droplet-on-demand generator. The changes of droplet temperature and evaporation rate after a transient spark heating are reflected in the time-resolved PRR image. Results show that droplet evaporation rate increases with temperature, from ?1.28$\times {10}^{?8}$ m²/s at atmospheric 293 K to a range of (?1.5, ?8)$\times {10}^{?8}$ m²/s when heated to (294, 315) K, agreeing well with the Maxwell and Stefan–Fuchs model predictions. Uncertainty analysis suggests that the main source is the indeterminate gradient inside droplet, resulting in an underestimation of droplet temperature and evaporation rate. With the demonstration on simultaneous measurements of droplet refractive index as well as droplet transient and local evaporation rate in this work, PRR is a promising tool to investigate single droplet evaporation in real engine conditions.     

Experimental and Numerical Investigation on Effects of the Steam Ingestion on the Aerodynamic Stability of an Axial Compressor

In order to investigate the influence of steam ingestion on the aerodynamic stability of a two-stage low-speed axial-flow compressor, multiphase flow numerical simulation and experiment were carried out. The total pressure ratio and stall margin of the compressor was decreased under steam ingestion. When the compressor worked at 40% and 53% of the nominal speed, the stall margin decreased, respectively, by 1.5% and 6.3%. The ingested steam reduced the inlet Mach number and increased the thickness of the boundary layer on the suction surface of the blade. The low-speed region around the trailing edge of the blade was increased, and the flow separation region of the boundary layer on the suction surface of the blade was expanded; thus, the compressor was more likely to enter the stall state. The higher the rotational speed, the more significant the negative influence of steam ingestion on the compressor stall margin. The entropy and temperature of air were increased by steam. The heat transfer between steam and air was continuous in compressor passages. The entropy of the air in the later stage was higher than that in the first stage; consequently, the flow loss in the second stage was more serious. Under the combined action of steam ingestion and counter-rotating bulk swirl distortion, the compressor stability margin loss was more obvious. When the rotor speed was 40% and 53% of the nominal speed, the stall margin decreased by 6.3% and 12.64%, respectively.     

Validated Stability-Indicating GC-MS Method for Characterization of Forced Degradation Products of Trans-Caffeic Acid and Trans-Ferulic Acid

When dealing with simple phenols such as caffeic acid (CA) and ferulic acid (FA), found in a variety of plants, it is very important to have control over the most important factors that accelerate their degradation reactions. This is the first report in which the stabilities of these two compounds have been systematically tested by exposure to various different factors. Forced degradation studies were performed on pure standards (trans-CA and trans-FA), dissolved in different solvents and exposed to different oxidative, photolytic and thermal stress conditions. Additionally, a rapid, sensitive, and selective stability-indicating gas chromatographic-mass spectrometric method was developed and validated for determination of trans-CA and trans-FA in the presence of their degradation products. Cis-CA and cis-FA were confirmed as the only degradation products in all the experiments performed. All the compounds were perfectly separated by gas chromatography (GC) and identified using mass spectrometry (MS), a method that additionally elucidated their structures. In general, more protic solvents, higher temperatures, UV radiation and longer storage times led to more significant degradation (isomerization) of both trans-isomers. The most progressive isomerization of both compounds (up to 43%) was observed when the polar solutions were exposed to daylight at room temperature for 1 month. The method was validated for linearity, precision as repeatability, limit of detection (LOD) and limit of quantitation (LOQ). The method was confirmed as linear over tested concentration ranges from 1−100 mg L⁻¹ (r²s were above 0.999). The LOD and LOQ for trans-FA were 0.15 mg L⁻¹ and 0.50 mg L⁻¹, respectively. The LOD and LOQ for trans-CA were 0.23 mg L⁻¹ and 0.77 mg L⁻¹, respectively.     

Quinones for redox flow batteries

Quinones are electroactive species that have shown great promise for redox flow batteries due to the ability to tune their properties and to act as both negative and positive electrolytes. The following review outlines highlights of work in the last couple of years working to provide materials with higher stability, solubility, and performance. Developments toward stable negolytes have provided opportunities for potential commercial opportunities when paired with alternate chemistries. However, the stability of quinones in high potential electrolytes is still not sufficient and the number of potential quinones limited.     

Ni、Mo共掺杂对SnO2材料性能影响的仿真研究

针对AgSnO₂触头材料存在的不足,采用基于密度泛函理论的第一性原理对SnO₂、Ni单掺杂、Mo单掺杂以及Ni-Mo共掺杂SnO₂材料进行了电性能与力学性质的研究,计算了各体系的形成能、能带结构、态密度、弹性常数等各项参数。结果表明,掺杂后的材料可以稳定存在,且仍为直接带隙半导体材料。与未掺杂相比,掺杂后体系的能带结构带隙值减少,其中Ni-Mo共掺杂时的带隙值最小,载流子跃迁所需能量减少,极大地改善了SnO₂的电性能;由弹性常数计算了剪切模量、体积模量、硬度等参数,其中Ni-Mo共掺杂时的硬度大幅降低,韧性增强,有利于AgSnO₂触头材料后续加工成型,且其普适弹性各向异性指数最小,不易形成裂纹。综合各项因素,Ni-Mo共掺杂能够很好地改善SnO₂的性能,为触头材料的发展提供了理论指导。     

Stabilization of a viscoelastic rotating Euler‐Bernoulli beam

In this paper, we consider a rotating Euler‐Bernoulli beam. The beam is made of a viscoelastic material, and it is subject to undesirable vibrations. Under a suitable control torque applied at the motor, we prove the arbitrary stabilization of the system for a large class of relaxation functions by using the multiplier method and some ideas introduced by Tatar (J. Math. Phys. 52:013502, 2011).