Application of differential evolution for cascaded multilevel VSI with harmonics elimination PWM switching

Harmonic elimination pulse width modulation (HEPWM) method has been widely applied to multilevel voltage source inverter (MVSI) to remove low frequency harmonics from its output voltage. However, the computation of the HEPWM switching angles for MVSI is very challenging due to several constraints, namely angle sequencing, very tight angular spacing and the numerous possibilities of angles distribution ratio. Realizing the potential of Differential Evolution (DE) to handle complex problems, this work proposes its application to solve the HEPWM problem for cascaded MVSI. Its emphasis is on improving the availability of HEPWM for higher output voltage by extending the maximum range of modulation index (M). It also removes the discontinuities in the switching angles and reduces the number of distribution ratio required to obtain the required solution. Compared to the most advanced (similar) work, i.e., 7-level MVSI with seventeen switching angles, DE covers a wider range of M; the maximum achievable M is 2.80. Furthermore, it exhibits very low second order distortion factor (DF₂): for the worst case, the value of DF₂ is 0.0014%. To verify the viability of the proposed algorithm, simulation is carried out and hardware prototype is constructed. Both results show very good agreement with the theoretical prediction.     

Surface nanodeformations caused by ultrashort laser pulse

Ultrashort laser pulse is a unique nanometallurgical tool which operates at extreme conditions: ultimate strength of material, the smallest spatiotemporal scales, and nonequlibrium crystallization. Approaches to ultimate strength and to highly nonequlibrium crystallization are tightly coupled with the smallness of spatiotemporal scales. Usage of the tool opens new opportunities to create materials with enhanced surface hardness, anticorrosion properties, and diverted optical constants. To use these advantages we have first of all to develop clear and reliable physical model. The paper presents new results concerning interactions of optical or X-ray lasers with metals. It is shown that ultrashort laser pulse melts surface layer, sends shock into bulk, and foams molten metal. Dense dislocation bilayer is created thanks to fast recrystallization (the first sublayer) and plastic transformations behind strong shock (the second sublayer). Plastic shock generated at moderate laser intensities attenuates sharply during its propagation into metal. During this attenuation, a plastic shock regenerates into a powerful elastic shock. This process defines boundary between the second dislocation sublayer and undamaged solid. Mechanical breaking of foam after its strong stretching and fly away of a part of melt together with fast freezing are responsible for appearance of chaotic frozen nanostructures at an irradiated surface.     

Improved direct inverse tracking control of a piezoelectric tube scanner for high-speed AFM imaging

For a piezoelectric tube scanner (PTS), this paper proposes an improved direct inverse tracking control algorithm and apply it to an atomic force microscope (AFM) to accomplish high-speed scanning tasks. That is, to enhance the high-speed tracking control performance of a PTS, an improved direct inverse rate-dependent Prandtl–Ishlinskii (P–I) model is firstly constructed, which includes a polynomial module to eliminate the structure nonlinearity. Based on the model, a practical feedforward control law is then designed to implement high-speed tracking control for a high-frequency trajectory with strong robustness, which presents the advantages of high-speed response, simple structure and convenient implementation. Subsequently, the designed feedforward law is combined with a feedback component, and the combined control strategy is employed in an AFM to accomplish fast imaging tasks. Numerous experimental results are then collected, which convincingly demonstrate the superior performance of the proposed practical model/control scheme.     

Investigation on phase shifting for a 4 K Stirling pulse tube cryocooler with He-3 as working fluid

He-3 is generally recognized for its ability to provide more excellent thermophysical performance than He-4, especially in the 4 K temperature range. However, this was not always the case in our preliminary experiments on a three-stage Stirling-type pulse tube cryocooler (SPTC). Our ongoing studies, as reported in this paper, demonstrate that the different working fluids also affect the performance through their phase shifting capability. This feature has been passed over in large part by researchers considering refrigerant substitution. Unlike previous theoretical analyses that focus primarily on regenerator losses, this report investigates the effects of the working fluid on the phase angle at the cold end in order to quantitatively reveal the relationship between the lowest attainable temperature and the cooling capacity. The analysis agrees well with our experimental results on a three-stage SPTC. While running with the operating parameters optimized for He-3, the lowest temperature of the SPTC decreased from 5.4 K down to 4.03 K. This is the lowest refrigeration temperature ever achieved with a three-stage SPTC.     

Experimental investigation of two phases evaporative heat transfer coefficient of carbon dioxide as a pure refrigerant and oil contaminated under forced flow conditions in small and large tube

The evaporative two-phase heat transfer coefficient of CO₂/oil contaminated as a refrigerant under forced flow conditions through a smooth horizontal tube was experimentally investigated. The experiments were carried out for two test sections of evaporators. The test sections were made of seamless precision steel tubes with a length of 1.12 m and two inner diameters of 4 and 10 mm to fulfill the influence of the evaporator geometry. Experimental parameters include mass fluxes varied from 90 to 750 (kg m⁻² s), heat flux ranged from 5 to 40 (kW m⁻²), evaporation temperatures changed from −10 to −35 °C, and the oil concentration is varied from 0.2 to 7 %. The results from the experiment are compared with those calculated from correlations reported in the literature. The results of this study are of technological importance for the efficient design of evaporators when systems are assigned to utilize CO₂ as a refrigerant.     

Design and experimental investigation of the high efficiency 1.5 W/4.2 K pneumatic-drive G-M cryocooler

The development of a high cooling power and high efficiency 4.2 K two stage G-M cryocooler is critically important given its broad applications in low temperature superconductors, MRI, infrared detector and cryogenic electronics. A high efficiency 1.5 W/4.2 K pneumatic-drive G-M cryocooler has recently been designed and developed by ARS. The effect of expansion volume rate and operation conditions on the cooling performance has been experimentally investigated. A typical cooling performance of 1.5 W/4.2 K has been achieved, and the minimum temperature of the second stage is 2.46 K. The steady input power of the compressor at 60 Hz is 6.8 kW, while the operation speed of the rotary valve is 30 rpm. A maximum cooling power of 1.75 W/4.2 K has been obtained in test runs.     

Φ177mm PQF无缝管连轧机前横移系统改进

对鞍钢股份无缝钢管厂连轧机前毛管坯横移装置存在的设计缺陷进行了分析,并提出改进方案,将原横移装置由高架轨道横移机构及上下升降链传动装置,改进为地面轨道横移系统及同步旋转臂实现取管、放管过程。实施后消除了生产故障,缩短了轧制周期,降低生产成本。     

The dynamic creep rupture of a secondary superheater tube in a 43 MW coal-fired boiler by the decarburization and multilayer oxide scale buildup on both sides

A dynamic creep mechanism has been proposed and verified through a case study. A secondary superheater tube burst occurred in a 43 MW coal-fired boiler. Microstructural examination indicates that the overheating temperatures reached 900 °C (above Ac3). The overheating duration was estimated to be 3 h by calculating with LMP formula. The 710 μm steam-side scale and 960 μm fireside scale built-up in the short time of overheating. The burst scenario was a short-term severe overheating on the basis of the long-term creep. The multilayer oxide scales on both sides have been studied with ESEM/EDS, indicating FeO. At 900 °C, full decarburization had gone throughout the tube. As the strength reduced due to the decarburization, the creep mechanism transformed from long-term intergranular creep to short-term transgranular rupture. The two types of dimples on the fractograph and two types of cracks in the microstructures confirmed the mechanism transformation. The overheating, the scale buildup and the decarburization constituted the full picture of the dynamic creep rupture.     

Experimental study on the heat transfer and flow characteristics of nanorefrigerants inside a corrugated tube

The heat transfer and flow characteristics of MWCNT-R141b nanorefrigerant with different mass fractions have been studied through experiments. Experimental results were compared with existing correlations. A two-step method was used to prepare the nanorefrigerants. Span-80 was used as surfactant with an average particle diameter of 20 nm. Transmittance method was used to evaluate the stability of nanorefrigerants. Results showed that the stability of MWCNT-R141b nanorefrigerant, which is the added dispersant, was good during the experiments. The 0.3 wt% MWCNT-R141b nanorefrigerants had optimal heat transfer enhancement effects compared with pure refrigerants. The maximum Nusselt number increased by 40%. The specific pressure drop of nanorefrigerant increased as the Reynolds number (Re) increased, and the specific pressure drop of the pure refrigerant was minimum, which is similar to R141b.     

氨基羟基脲脉冲辐解研究

研究了脉冲辐解过程中氨基羟基脲与水辐解活性粒子（e^-_aq、·OH和·H）及单电子氧化剂·CO₃^-的反应动力学过程。反应近似为准一级反应，反应速率常数分别为k（e^-aq）=1.41×10⁸ L/(mol·s)、k（·OH）=1.05×10¹⁰ L/(mol·s)、k（·H）=2.68×10⁵ L/(mol·s)、k（·CO^-₃）=4.25×10⁸ L/(mol·s)。其中氨基羟基脲与·OH的反应速率常数最大，故在辐解过程中其为主要反应。