冲压焊接成型离心泵扬程曲线的试验研究

通过试验,主要从叶轮角度研究了低比转速冲压焊接成型离心泵扬程曲线的控制问题,得出影响H－Q曲线形状的叶轮参数变化规律。     

Experimental study of the pressure fluctuations in a pump turbine at large partial flow conditions

Frequent shifts of output and operating mode require a pump turbine with excellent stability. Current researches show that large partial flow conditions in pump mode experience positive-slope phenomena with a large head drop. The pressure fluctuation at the positive slope is crucial to the pump turbine unit safety. The operating instabilities at large partial flow conditions for a pump turbine are analyzed. The hydraulic performance of a model pump turbine is tested with the pressure fluctuations measured at unstable operating points near a positive slope in the performance curve. The hydraulic performance tests show that there are two separated positive-slope regions for the pump turbine, with the flow discharge for the first positive slope from 0.85 to 0.91 times that at the maximum efficiency point. The amplitudes of the pressure fluctuations at these unstable large partial flow conditions near the first positive slope are much larger than those at stable operating condtions. A dominant frequency is measured at 0.2 times the impeller rotational frequency in the flow passage near the impeller exit, which is believed to be induced by the rotating stall in the flow passage of the wicket gates. The test results also show hysteresis with pressure fluctuations when the pump turbine is operated near the first positive slope. The hysteresis creates different pressure fluctuations for those operation points even though their flow rates and heads are similar respectively. The pressure fluctuation characteristics at large partial flow conditions obtained by the present study will be helpful for the safe operation of pumped storage units.     

确定循环水泵站运行工况的方法比较

介绍了三种确定切削后水泵的叶轮外径和Q－H特性曲线的方法，并对其计算结果进行了比较和讨论。     

Dynamic behavior of three-dimensional composite beam under flapwise excitation

In this paper, dynamic response of a symmetrically laminated composite beam is studied under harmonic base excitation. The base is subjected to flapwise excitation tuned to the primary resonance in the presence of 2:1 internal resonance between the out-of-plane bending motion and the in-plane bending and torsional motions. In literature, modified modulation equations of composite beam have been derived and the stability of fixed points has been investigated in frequency and forced responses. However, post-critical behavior of the modulation equations is studied in this study. In bifurcation diagrams sketched near primary and internal resonances, it appears that detuning the flapwise excitation amplitude causes phenomena like jumps, period doubling, multi and quasi-periodic solutions to occur.     

ENERGETICAL APPROACH FOR SEMI-ANALYTICAL DRILLING MODELLING

Drilling modelling is a complex task and has been extensively studied from an experimental viewpoint. However, only few studies have been done on the analytical modelling of this machining operation. The geometry of a drill and the kinematics of drilling are such that tool angles as well as the cutting conditions undergo great variations along the cutting edges. The calculation of the forces in drilling thus requires a cutting model covering the whole range of these variations. Moreover, on the chisel area, the cutting speed and the angles are such that the phenomena cannot be regarded as cutting. In this case, the material is plastically deformed when passing by tool edge, just as in indentation. In this study, we propose a semi-analytical model that allows determining forces on the tool, as well as heat fluxes and temperatures along the edges of the drill. In this model, the main cutting edge and the chisel edge are considered. It also takes into account the elastic relaxation on the clearance face with edge acuity, as well as a proper modelling of the phenomena occurring at chisel edge in relation with its geometry and the kinetics of a drilling operation.     

Experimental study on trepanning drilling of the ceramic composite armor using a sintering diamond tool

Laminated ceramic composite armors made of engineering ceramics, fiber-enforced composite, and lightweight alloy are expected to be used widely in armor protection area. However, secondary machining is still a problem for the distinct properties of composing materials, and up to now, few reports can be found out about machining technology. In this work, an experimental study on drilling a typical laminated ceramic composite armor by using a special sintering diamond thinwall trepanning tool has been carried out deeply. To improve the machining efficiency, process parameters including the spindle speed and the axial force have been optimized through drilling experiments. Moreover, the three phases of the whole drilling process of the typical ceramic composite armor is proposed, and the machining mechanism and chip forms in each phase have been analyzed. Also, diamond tool skidding phenomena whether at hole exit or during drilling middle engineering ceramics have been explored, and the skidding mechanisms are presented. In addition, the surface quality of a hole wall machined has been studied, and an advisable measure is given. Results achieved in this paper can provide a technical base for diamond trepanning tool drilling similar laminated ceramic composite armors.     

BTA深孔钻削中扭曲现象的探讨

本文在前人研究的基础上,通过进一步试验,着重探讨了深孔加工中的扭曲钻削现象,分析了孔的圆度畸变,钻杆的扭曲与搪动和刀具结构之间的关系。找到了减小或避免扭曲的有效措施,提高了深孔的圆度。     

Numerical simulation of water flow in an axial flow pump with adjustable guide vanes

A new adjustable guide vane (AGV) is proposed in this paper. This vane can reduce hydraulic losses and improve the performance of an axial flow pump. The formula of AGV adjustment was obtained after theoretical analysis. The fluid flow inside the axial flow pump with a fixed guide vane and adjustable guide vane was simulated. The calculated Q-H curves for the fixed guide vane agreed well with the experimental ones. The results show that the attack angle and flow separation have an important contribution to the vortices which create hydraulic losses in the guide vane channel. The AGV can decrease hydraulic losses and significantly enhance the pump head and efficiency by changing the guide vane angle.     

On-line cutting state recognition in turning Using a neural network

Tool wear, chatter vibration, chip breaking and built-up edge are the main phenomena to be monitored in modern manufacturing processes. Much work has been carried out in the analysis and detection of these phenomena. However, most work has been mainly concerned with single, isolated detection of such phenomena. The relationships between each fault have so far received very little attention. This paper presents a neural-network-based on-line fault diagnosis scheme which monitors the level of tool wear, chatter vibration and chip breaking in a turning operation. The experimental results show that the neural network has a high prediction success rate.     

A study of conformational variation of temperature-dependent PEEK molecular structures subject to stretching speeds by molecular dynamics simulations

Rapid prototyping is one of the core technologies for industry 4.0. Various studies in academia and industry have focused on producing the most qualified printing outputs by tuning operation processes, selecting appropriated printing products, etc. In addition, test & evaluation processes of the final products are essential to validate the performance of outputs. Many mechanical components in a system are under different pressure distributions depending on working environment conditions. However, mechanical characteristics of printing products subject to thermal constraints have rarely been studied based on microscopic viewpoints. This study focuses on the microscopic tensile elongation phenomena of pre-built PEEK (poly-ether-ether-ketone) structures at different working temperatures. A coarse-grained molecular model is employed to save computation time and resources. Conformational variations of PEEK structures and dependent behaviors of individual molecules are characterized by using the radial pair distribution functions, the dimensional relative variations of the radius of gyrations, the number density profiles, and averaged tetrahedral order parameters to elucidate deformation properties of microscopic structures at different uniaxial stretching speeds at three given temperatures using systematic molecular dynamics simulations.

     

微铣削力建模研究进展

微铣削加工是实现具有三维复杂结构和材料多样性特征的微型零部件制造的有效技术手段,具有日益广阔的应用前景。然而由于刀具尺寸及加工参数的急剧缩减,微铣削表现出显著不同于传统铣削的加工机理。作为理解微铣削加工机理的最重要基础之一,至今已有大量关于微铣削力建模的研究,但是它们主要针对单一现象或者某几个现象进行研究,尚少有系统完善的理论来解释微铣削加工的力学过程,因此对微铣削加工切削力的全面总结是非常必要的。结合国内外微铣削技术的最新研究进展,从微铣削与传统铣削的不同加工机理出发,对微铣削力建模进行全面的论述和总结,并重点介绍刀刃钝圆半径、刀具跳动、挠性变形和刀具磨损对微铣削力建模的影响。探讨了目前微铣削力建模方法中的热点与难点,并指出了现有微铣削力建模有待研究的内容。     

A representative ex-situ fretting wear investigation of orthodontic arch-wire/bracket contacts

Recent studies show that resistance to friction and wear between the arch-wire and the bracket are of great importance for the quality of orthodontal treatment. To optimize these phenomena specific surface treatments have been developed, but a major difficulty is the methodology for comparing and improving the given palliatives. A specific ex-situ fretting test has been developed which permits the micro-slidings generated by the occlusion movements to be superimposed on macro-slidings generated by the teeth displacement.In this study the friction force between brackets and arch-wires during combined macro-/micro-slidings has been investigated. Various stainless steel bracket/arch-wire contacts have been analyzed. The wear and friction properties of different PEPVD deposited coatings such as SiC and SiN have been studied under dry and wet artificial saliva conditions.     

Wear mechanisms at low-temperatures for metal-reinforced phenolic resins

Investigations have been carried out to establish the relationship between the friction force and the normal force in dry sliding with high normal pressure at low speeds when there is considerable plastic flow of material on the rubbing surfaces. The study was extended by limiting the area of contact and by providing a virgin surface. Conditions which influence seizure phenomena caused by high pressure and temperature and the extent to which it may affect the sliding characteristics are discussed. Damage due to sliding under extreme conditions has been studied to aid the better understanding of friction phenomena.     

Behaviour law for cutting process

To optimize the cutting conditions in machining, it is necessary to quantify energy parameters involved in the cutting process. These energy parameters are based on a hypothesis regarding the displacement of the material during the chip formation and the behaviour law of the material. However, a more refined model must be used to take into account complex phenomena in the cutting process. The strain gradient theory appears to be best in modelling rotation phenomena inside material. In order to solve this type of problem, the bases of the strain gradient theory are presented in this study.     

Recent progress of spray-wall interaction research

In the present article, recent progress of spray-wall interaction research has been reviewed. Studies on the spray-wall interaction phenomena can be categorized mainly into three groups: experiments on single drop impact and spray (multiple-drop) impingement, and development of comprehensive models. The criteria of wall-impingement regimes (i.e., stick, rebound, spread, splash, boiling induced breakup, breakup, and rebound with breakup) and the post-impingement characteristics (mostly for splash and rebound) are the main subjects of the single-drop impingement studies. Experimental studies on spray-wall impingement phenomena cover examination of the outline shape and internal structure of a spray after the wall impact. Various prediction models for the spray-wall impingement phenomena have been developed based on the experiments on the single drop impact and the spray impingement. In the present article, details on the wall-impingement criteria and post-impingement characteristics of single drops, external and internal structures of the spray after the wall impact, and their prediction models are reviewed.     

Wear prediction for metals

In spite of the large number of wear models found in the literature, no model can predict metal wear a priori based only on materials property data and contact information. The complexity of wear and the large number of parameters affecting the outcome are the primary reasons for this situation. This paper summarizes the current understanding of wear modelling for metals. Several recent approaches such as wear mapping and wear transition diagrams have suggested some future possible directions for improvement. Some success has been achieved in describing severe wear of steels under unlubricated conditions using thermomechanical approaches. However, modelling of mild wear remains problematic, especially under lubricated conditions. In mild wear, asperity contact events dominate the wear processes. A single asperity collision simulation apparatus has been used to study asperity-asperity contact phenomena. Shear strain and strain accumulation were found to be the dominant underlying causes for wear. It is proposed that future research in wear prediction for metals incorporate the following aspects: wear mapping, temperature, shear strain response, boundary lubricating film strength, and surface roughness.     

Analysis of non-uniform flow distribution in parallel micro-channels

Micro heat exchangers have been widely used in both of academic and industrial fields for controlling heat for decades. Especially, the printed circuit heat exchanger (PCHE) is applicable to high pressure and high temperature cases since the manufacturing process, diffusion bonding, gives high durability. However, flow does not occur uniformly in multiple channels, which causes a heat transfer difference between the channels that reduces the effectiveness of the system. Also, it makes the total pressure drop larger than when it is uniformly distributed. Flow distribution has been studied intensively for decades, but the mechanism has not been clarified yet, since flow in the header is much too complicated. As a step to reveal the mechanism, we focus on maldistribution and quantitatively investigate the phenomena. We prepared experimental devices, including a set of parallel micro-channels, inlet and outlet. We used water as a working fluid, and water soluble red ink for visualization. We measured the interface location to derive the flow speed at each channel. We focused on the dependency of channel length and flow speed. In the theoretical part, we derived the flow speed profile of the entire channels that shows good agreement with the experimental results. This study provides a theoretical basis on resolving the maldistribution problem.

     

表面微粗糙谷对热弹流润滑性能及凹陷现象的影响

分析了纯滑动工况下,静止固体带单粗糙谷对弹流润滑性能及凹陷现象的影响。就固体表面存在横向沟槽的不同工况求得了若干组数值解,并将数值结果与试验结果[1,2]进行了定性比较,两者吻合良好。指出粗糙谷对润滑性能是不利的,但不是凹陷的最根本成因。