Tribological performance of surface treated piston assembly with infiltrated layer

Journal of Mechanical Science and Technology - By applying surface treatment on piston assembly and typical samples, an infiltrated layer with the depth of about 30 μm was formed on the...     

高低压蓄能器在回转泵控液压系统中应用及仿真

为了提高挖掘机回转系统的运行稳定性,通过引入高低压蓄能器的方式提出了一种回转泵控液压系统,通过相互协同的方式来实现泵控过程,并在Simulink 平台开展了仿真分析。结果表明:单蓄能器和双蓄能器在各运动阶段和泵输出功率基本一致,形成了相同的能量释放与保持特性。相对于单蓄能器,双蓄能器回转系统缩短了约0.5 s的制动时间。高压蓄能器形成较小的油液充放范围,但能够达到较高压力,同时低压蓄能器起到弥补高压蓄能器体积偏小问题。减速制动时,高压蓄能器升高到最大压力后高低压蓄能器开始回收能量,可以更加高效回收能量,显著缩短制动时间。该系统的液压系统设计具有能量回收以及系统自动补油的功能,表现出很好的节能高效性能,对提高挖掘机液压系统运行效率具有一定的理论意义。     

A study of local deformation and damage of dual phase steel

Deformation and fracture behavior of Dual Phase (DP) high strength steel were investigated by means of a microstructure based Finite Element (FE) modeling. Representative Volume Elements (RVEs) were applied to consider effects of various microstructure constituents and characteristics. Individual stress–strain curves were provided for ferrite, martensite as well as transformation induced Geometrically Necessary Dislocations (GNDs) taking into account in the RVEs. Principally, the GNDs occurred around phase boundaries during quenching process due to the austenite–martensite transformation. Flow behaviors of individual phases were defined on the basis of dislocation theory and partitioning of local chemical composition. Then, flow curves of the examined DP steel were predicted. Furthermore, the Gurson–Tvergaard–Needleman (GTN) model was used to represent ductile damage evolution in the microstructure. Occurrences of void initiation were characterized and damage parameters for RVE simulations were hence identified. Finally, influences of the GNDs, local stress and strain distributions and interactions between phases on predicted crack initiation in the DP microstructure were discussed and correlated with experimental results.     

高速轿车车身脉动压力的数值分析

分析了高速轿车气流噪声的产生原理,通过求解Lighthill方程得出车辆外部气流噪声只与车身表面的脉动压力有关。采用RNG k-ε和LES相结合的方法求解车外流场,对三个速度下车辆外部流场和表面脉动压力进行仿真模拟,并分析了车辆外流场的特性,得到了车身关键点处的脉动压力级,并将左侧窗中部分监测点的脉动压力级的计算值和实验值进行了对比,得出两者变化趋势基本一致,误差低于5%。经计算分析表明,用大涡模拟方法来计算汽车脉动压力是可行的。     

基于发动机四参数的轮式越野车换挡加速时间计算

在只有发动机最大功率、最大扭矩及其对应转速等四大参数的情况下,引入限定车速系数,以可能的最短总加速时间为原则确定换挡车速点;以基于发动机四参数的外特性模型为基础建立轮式越野车加速度模型,进而确定等加速度车速;由加速度模型建立不含积分运算的各档加速时间计算公式;考虑每次换挡所耗时间,推导换挡加速时间计算模型。利用Matlab对某轮式越野车等加速度车速、换挡车速、加速度和换挡加速时间进行设计计算,实例结果表明,该方法易于在Matlab中用程序实现,计算精度也有所提高。     

Equalization scheme for series connected Li-ion battery pack based on drop combination strategy

为了提升飞渡电容式均衡方法的速度和效率，提出了建立电压落差的充放电组合策略，分析了落差式组合策略与常规方案的工作过程，对比了二者均衡速度的差异，将均衡电路等效为完全响应的RLC电路，通过公式推导及Matlab计算，分析了能量损失率与电路参数的关系，对比了不同充放电组合对均衡效率的影响。研究表明：落差式组合策略可显著提升飞渡电容式均衡速度，并且可以通过优化充放电组合，提升均衡效率。通过实验对所提出的方案进行了分析与验证，结果表明，该方法能有效解决电池组不均衡问题。     

A novel interphase mass transfer model toward the VOF simulation of subcooled flow boiling

In this study, a novel interphase mass transfer model has been developed to predict mass and energy exchange in the vicinity of the phase interface. The proposed model involves the new time relaxation parameter which takes into account the effect of the subcooling, the wall superheat, and Reynolds number, and is obtained from simulation results by the method of trial and error. Transient simulations of subcooled flow boiling in Hua’s experimental horizontal rectangular channel are performed. The Volume of Fluid (VOF) method is used to track the vapor–liquid interface using OpenFOAM software package. The constant coefficients of new time relaxation parameter are determined using fitted method. The predicted results show that the deviation between the predicted and experimental wall superheats using the proposed model, which is within 20%, is smaller than that of Lee model. As a further step, the novel interphase mass transfer model has been validated in other experimental data under similar condition. These results indicate that the novel interphase mass transfer model is quite accurate and robust.     

Virtual access hydraulic experiment for system dynamics and controls education

A Virtual Access Laboratory (VAL) for undergraduates is currently being built at Georgia Tech. This laboratory will house a three-degree of freedom, hydraulically actuated device resembling a fork lift truck. Experiments with the hydraulic equipment can be run remotely via the Internet. Such an experimental apparatus will give students exposure to hydraulics, technology that is used expansively in industry, yet lightly treated in most US undergraduate engineering curricula. The software responsible for providing the user interface and controlling the manipulator is comprised of both client and server programs that communicate via the Internet. The client code is written in Java and features a Graphical User Interface to provide a user-friendly environment for students. Code for the server is written in C++ to run on Windows NT with Hyperkernel for deterministic real-time control. This code will offer the students the ability to run experiments such as system identification, PID control, and trajectory planning. A camera monitors the system and provides students with visual data.     

Integrated planning and scheduling under production uncertainties: Bi-level model formulation and hybrid solution method

We propose a novel method for integrating planning and scheduling problems under production uncertainties. The integrated problem is formulated into a bi-level program. The planning problem is solved in the upper level, while the scheduling problems in the planning periods are solved under uncertainties in the lower level. The planning and scheduling problems are linked via service level constraints. To solve the integrated problem, a hybrid method is developed, which iterates between a mixed-integer linear programming solver for the planning problem and an agent-based reactive scheduling method. If the service level constraints are not met, a cutting plane constraint is generated by the agent-based scheduling method and appended to the planning problem which is solved to determine new production quantities. The hybrid method returns an optimality gap for validating the solution quality. The proposed method is demonstrated by two complicated problems which are solved efficiently with small gaps less than 1%.     

Effect of hydrophilic pipe structure of proton exchange membrane fuel cell on water removal from the gas diffusion layer surface

In a proton exchange membrane fuel cell (PEMFC), effective GDL surface water elimination is significant to water management. This paper used the volume-of-fluid method (VOF) method to carry out simulation research on transferring liquid water in the flow channel with a hydrophilic pipe. The findings indicated that compared with a straight channel, a hydrophilic pipe structure could effectively remove water from the gas diffusion surface (GDL) and reduce the surface water coverage of the GDL. With the increase in the diameter and height of the pipe structure, the GDL surface's water coverage first increased and then decreased, and it was less with the pipe structure than with the direct flow channel. The removal rate of water on the GDL surface was accelerated. The spacing of hydrophilic pipes has a significant impact on the transportation of water. As the spacing increases, the removal rate of water on the GDL surface slowed. A hydrophilic pipe structure with a diameter of 75 μm, a height of 400 μm, and spacing of 300 μm has good water removal performance on the GDL surface. This research work proposes a new internal structure design of the flow channel, which has specific implications for removing water on the GDL surface.