风机高压变频调速与液力偶合器调速运行比较

通过对风机调速的总体比较和发电厂引风机调速的实际节能比较,说明与液力偶合器相比,高压变频调速的综合经济效益是可观的。     

The benefits of planar circular mouths on suction feeding performance

Suction feeding is the most common form of prey capture across aquatic feeding vertebrates and many adaptations that enhance efficiency and performance are expected. Many suction feeders have mechanisms that allow the mouth to form a planar and near-circular opening that is believed to have beneficial hydrodynamic effects. We explore the effects of the flattened and circular mouth opening through computational fluid dynamics simulations that allow comparisons with other mouth profiles. Compared to mouths with lateral notches, we find that the planar mouth opening results in higher flow rates into the mouth and a region of highest flow that is positioned at the centre of the mouth aperture. Planar mouths provide not only for better total fluid flow rates through the mouth but also through the centre of the mouth near where suction feeders position their prey. Circular mouths are shown to provide the quickest capture times for spherical and elliptical prey because they expose the prey item to a large region of high flow. Planar and circular mouths result in higher flow velocities with peak flow located at the centre of the mouth opening and they maximize the capacity of the suction feeders to exert hydrodynamic forces on the prey.     

Absenteeism as a major cause of bottlenecks in assembly lines

This paper analyses the effects of absenteeism in assembly lines. The analysis shows that assigning temporary workers as substitutes for absentees has a major potential for generating bottlenecks. Such a bottleneck typically appears when an inexperienced worker replaces an absentee worker at their position. The inexperienced worker starts their learning curve, and for the initial period is significantly slower than other stations, and therefore is a bottleneck. The paper analyses the effect of typical absenteeism rates on the throughput, and shows it has strategic magnitude. This is verified using simulation. The effect of the absenteeism and turnover on the throughput is found to be related to the cycle time, and the amount and rate of learning. The paper discusses the simulation results, staffing requirements, strategies for overcoming absenteeism, and future research directions.     

Heat and mass transfer in a MHD non‐Darcian micropolar fluid over an unsteady stretching sheet with non‐uniform heat source/sink and thermophoresis

The effect of heat and mass transfer in a MHD non‐Darcian flow of a micropolar fluid over an unsteady stretching sheet with thermophoresis and non‐uniform heat source/sink is discussed. The fluid is electrically conducting in the presence of a uniform applied magnetic field. The arising nonlinear problem is solved by the Keller box method. The effects of various physical parameters on skin friction, local Nusselt number, and Sherwood number are presented graphically and in tabular form. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21018     

某型发动机高温转速不足分析

文章分析了某型发动机在外场点火试车试验中,发动机到达一定的转速后,发动机转速增加缓慢,伺服阀可能产生的原因.重点从系统油压波动、油液温度、液流流态等方面对伺服阀进行了分析.     

Computational modelling of shear-layer instabilities and vortex formation in DC plasma arcs

The direct-current plasma arc is the principal heating and stirring element in plasma arc furnaces. The arc is a highly dynamic system operating at very short time scales (milliseconds or less). This dynamic behaviour can be understood as the combination of several modes of instability acting together. These modes can include fluid dynamic/shear layer instabilities, near-electrode instabilities resulting from steep local temperature gradients and other electrode surface effects, or helical/twisting instabilities resulting from the magnetic field generated in one part of the arc column exerting a force on another.Computational magneto-hydrodynamic models of the arc which are able to reproduce the highly transient behaviour of the system are developed to study certain dynamic modes – in particular, the formation and evolution of axisymmetric vortices and shear layer instabilities in the arc jet. Results from this work are then compared to qualitative data from high-speed photographic imaging of large plasma arcs (up to 3 kA current).     

Viscous Potential Flow Analysis of Kelvin–Helmholtz Instability of a Cylindrical Flow with Heat and Mass Transfer

A linear analysis of Kelvin–Helmholtz instability of a cylindrical interface has been carried out when there is heat and mass transfer across the interface, using viscous potential flow theory. Both fluids are considered as incompressible, viscous, and thermally conducting with different kinematic viscosities. Both axisymmetric as well as asymmetric disturbances are considered. Stability criterion is given by a critical value of relative velocity and stability is discussed theoretically as well as numerically. Various graphs with respect to physical parameters such as wave number, viscosity ratio, heat transfer coefficients, Reynolds number, etc., have been drawn and the effect of various parameters have been described. A comparison with the linear stability analysis of inviscid fluids (Lee [10]) has been made and it is observed that viscosity has a stabilizing effect on the stability of the system. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(6): 489–503, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21092     

Pressure Drop Models for Gas/Non‐Newtonian Power‐Law Fluids Flow in Horizontal Pipes

Models commonly used in literature are evaluated versus 696 data points to predict the pressure drop of gas/non‐Newtonian power‐law fluids flow in horizontal pipes. Suitable models are recommended. A new correlation is developed by ignoring the pressure drop across the gas slug and adopting the liquid slug holdup of gas/non‐Newtonian fluid flow into the homogeneous model. The theoretical curves can capture the test data trends and the overall agreement of predicted values with experimental data is sufficient to be practically applied in industry.     

Process simulation using the expanded fluid model for viscosity calculations

The expanded fluid (EF) viscosity model was implemented and further developed for efficient integration into a commercial process simulator (VMGSim™). The model has three adjustable parameters per component and its inputs are density, pressure and low pressure gas viscosity. The model was adapted to use densities determined by the Rackett correlation (liquid phase) and the Advanced Peng–Robinson Equation of State (vapor phase). The enhanced EF model fit experimental viscosities of pure hydrocarbons, water and polar compounds important for the simulation of oil and natural gas systems with average absolute errors just above 5%. The implemented EF model was tested against experimental viscosity data that included hydrocarbon and aqueous mixtures with average absolutes errors of 0.7 and 6.2% respectively. Generalized expressions for the estimation interaction parameters of binary mixtures involving paraffins, naphthenes, aromatics, alcohols, glycols and water were obtained. The EF model was also applied to crude oil (bitumen) examples. The three key developments for the efficient implementation of the EF model in a commercial simulator were: (1) the appropriate selection of phase density models; (2) the automatic determination of model fluid specific parameters; and (3) the use of generalized mixing rules for the calculation of binary interaction parameters.     

种晶介导的二氧化铈球状粒子的形成

文章报道了一种以不同数量种晶介导合成CeO2多晶纳米颗粒的制备方法，分析以种晶介导球状的CeO2的形成机理。在生长阶段，CeO2粒子通过初级粒子在其表面的沉积来生长，从而降低了粒子的表面能，导致由许多初级粒子组成的次级粒子的形成。加入的一定量的紧密堆积的种晶聚集体优先作为初级粒子的沉积中心来降低它们的表面能，从而减小次级粒子的尺寸、增加其均匀性以及促进这些次级粒子的形成。