Numerical analysis of influence of cavitation characteristics in nozzle holes of curved diesel engines

By targeting at the high-pressure common rail nozzle of diesel engines, we put forward a curved nozzle structure, established six groups of nozzle models with different curvatures. By using the CFD software STAR-CCM+, based on the incompressible fluid volume function (VOF) multiphase flow model, applying the K-Epsilon two-layer turbulence model, combined with the Schnerr-Sauer cavitation model, we investigated the degree of influence of the curved nozzle on the cavitation characteristics in the nozzle hole. Cavitation at the inlet, middle and outlet of nozzles was observed under injection pressure of 50, 100 or 150 MPa. The effects of curvature on cavitation were analyzed in detail according to cavitation volume fraction, cavitation volume content, mass flow, turbulent kinetic energy and velocity coefficient. It was found the curved nozzle can significantly reduce the cavitation degree in the nozzle holes, and the larger nozzle bending led to a smaller cavitation degree in the holes. Meanwhile, the average turbulent kinetic energy increased obviously and the average velocity decreased in both the holes and outlet, but the mass flow did not change much.     

Periodic inspection of gauge cutter wear on EPB TBMs using cone penetration testing

Cutterhead maintenance is usually required when the gauge cutters (over-cutters) wear down to the shield diameter. In soft ground pressurized shielded TBMs, inspection of gauge cutters usually involves complete stoppage of the operation. This can be a dangerous, costly, and time consuming process. In this study, a novel approach has been developed to monitor gauge cutter wear by considering the relationship between the overcut length and the length of the gauge cutters. By using the proposed testing system, frequent monitoring of gauge cutter length is possible at appropriate intervals during ring builds or maintenance without the need to enter the harsh environment of the excavation chamber. This monitoring system was used within an EPB TBM during the excavation of the University Link Light Rail Tunnel (U230) in Seattle, WA.     

Impact of TiO2 nanoparticles and nanowires on corrosion protection performance of chemically bonded phosphate ceramic coatings

The impact of the addition of TiO₂ nanoparticles and nanowires on the morphology, phase characteristics, contact angle, and electrochemical performance of chemically bonded phosphate ceramic coatings (CBPCs) was investigated. The chemical composition and surface morphology of the TiO₂ nanoparticle and nanowire modified with and without (heptadecafluoro-1,1,2,2-tetradecyl) trimethoxysilane were characterized. Results indicated that the hydrophobic –CF₂– and –CF₃ groups were successfully introduced into the TiO₂ nanoparticles and nanowires after modification. Corrosion resistance of CBPCs with TiO₂ was evidently improved compared with that without TiO₂. Such improvement was mainly due to the combined effects of low surface energy materials and micro/nano structures. In addition, CBPCs with TiO₂ nanowires exhibited higher hydrophobicity and corrosion resistance than those with TiO₂ nanoparticles because of the special columnar structure of the nanowires.     

基于新型趋近律的PMSM反演滑模控制

在永磁同步电机(PMSM)矢量控制系统中,针对传统的PI控制器控制精度不足、超调量大、易受外界扰动影响的问题,本文提出了一种反演滑模控制器替换传统的PI控制器,该反演滑模控制器是由反演法与滑模控制理论相结合产生的,且在趋近律设计方面引入双曲正切函数和系统状态变量,从而形成一种新型趋近律,代替传统的指数趋近律,提高系统的收敛速度,减小滑模抖振现象。在MATLAB/Simulink中搭建仿真模型,仿真结果表明,该方法使系统对电机的转速有着良好的控制效果,与PI控制、传统滑模控制以及变指数滑模控制相比较,具有较好的动态响应性能,转速超调量分别减少了22.15%、18%和2.75%,系统抗扰动能力提高,具有较强的稳定性。     

Bimodal fly ash size distributions and their influence on gas-particle mass transfer during electrostatic precipitation

Electrostatic precipitators (ESPs) have previously been demonstrated to achieve substantial (up to 60–70%) removal efficiency of mercury from coal-fired power plants (CFPPs). However, a high degree of scatter exists in the pilot- and full-scale data, suggesting an incomplete understanding of the mechanism by which mercury is adsorbed within an ESP, particularly by native fly ash. The present analysis explores the influence of bimodal particle size distributions (PSDs) on the gas-particle mass transfer underlying mercury adsorption by fly ash within an ESP. The analysis is motivated by the recent discovery by other investigators of bimodal fly ash PSDs resulting from coal combustion. Results of the present analysis show that, relative to similar monomodal PSDs, bimodal PSDs exhibit greatly increased gas-particle mass transfer potential during electrostatic precipitation. For bimodal PSDs, gas-particle mass transfer potential increases with increasing particle mass in the second mode and decreasing geometric mean diameter and geometric standard deviation of the second mode. A supplemental analysis compares the mercury removal potential of native fly ash, injected fly ash, and injected powdered activated carbon (PAC) during their collection within an ESP, using representative mercury adsorption capacities and particle mass loadings for each. Results showed only marginal differences in mercury removal efficiency between the three sorbents.     

Numerical simulation on evolution process of molten pool and solidification characteristics of melt track in selective laser melting of ceramic powder

A three-dimensional high-fidelity physical model for selective laser melting (SLM) of ceramic powder was created based on computational fluid dynamics (CFD) to examine the physical mechanism of molten pool and solidified tracks at mesoscopic scale. The discrete element method (DEM) was used to generate a randomly packed powder bed, and the volume-of-fluid method (VOF) was applied to dynamically monitor the free surface of the molten pool. The formation mechanism and evolution characteristics of the molten pool were found and analyzed, and the effects of laser power on the typical characteristics of solidified ceramic tracks of SLM were investigated. The molten pool was eventually solidified into a concave geometric shape track by surface tension. The laser power played a significant impact on the shaping quality of solidified ceramic track. When the laser power was too low, the melt track suffers from severe porosity and distortion defects, which can be effectively solved with increasing laser power. The simulation results were validated via single track selective laser melting of TiC ceramic powder.     

考虑驱动耦合的绳驱动外肢体机器人运动建模及控制研究

针对绳驱动外肢体机器人由于其布线形式及驱动耦合所造成的运动建模复杂及控制精度低等问题,研究了一种考虑驱 动耦合的绳驱动外肢体机器人运动建模及控制方法。 基于 D-H 法和欧拉变换原理构建了刚柔一体外肢体机器人的运动学模 型,并根据关节耦合机理推导相邻关节间的主动解耦模型,提出了一种基于驱动解耦运动学模型的外肢体控制策略。 最后搭建 了外肢体机器人实验样机,并对其所建运动模型及控制方法进行验证。 结果表明,机器人末端多点定位误差最大为 7. 45 mm, 移动路径最大误差为 7. 24 mm,总体误差平均值为 6. 08 mm,由此验证了所提的刚柔一体外肢体机器人驱动解耦运动学模型和 控制策略的正确性,具有较好的控制精度和运动品质。     

DEM simulations of spherical particle–particle collisions

The randomness, diversity, and complexity of the high-speed particle crushing process bring great difficulties to the theoretical analysis of powder engineering. In this paper, the discrete element method is used to simulate the collision of spherical particles, which provides a reference for studying the process and mechanism of crushing between particles under impact load. The Hertz–Mindlin with bonded contact model is used as the particle–particle contact model. The central collisions of particles with different diameter ratios under different high-speed motions and the eccentric collisions with different eccentricities are discussed. The results show that the bond damage increases with the increase of relative velocity in both centre impact and eccentric impact. In centre collisions, particles of smaller objects are more fragmented than particles of larger objects. For smaller target particles, the larger the diameter ratio is, the more particle elements are detached from the target particles, and the greater the bond breakage rate. For larger target particles, the larger the diameter ratio is, the less the particle element falls off and the smaller the bond breakage rate. This provides guidance for the collision and crushing of particles with different particle size ratios and different eccentricities during high-speed motion in engineering applications in the future.