曲轴机加工工艺与设备现状及发展趋势

曲轴在内燃机中起重要作用,因此在加工中必须确保曲轴的精度、表面硬度、表面粗糙度,笔者介绍了作为内燃机重要零件的曲轴机加工工艺和设备的现状及未来发展方向。     

高速加工中心铣削缸体刀具参数的优化

采用正交试验设计的方法,分析了在高速加工中心铣削缸体时影响表面质量的因素。通过统计分析,确定了刀具的悬伸比、前角、后角、刀尖圆弧半径对粗糙度的影响程度,并优化了一组刀具参数,将其运用于实际切削中,获得了较好的切削效果。     

Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photoelectrochemical water-splitting

Measurements of the diffuse reflectance of TiO₂ semiconductor coatings, such as are used for water splitting, are analysed using the Kubelka–Munk radiative transfer model. The widely used practice of determining the band gap of the coating directly from the diffuse reflectance is found to be inaccurate, since the diffuse reflectance depends on parameters such as the thickness, refractive index and surface roughness of the coating. However, it is shown that the absorption coefficient can be derived from the diffuse reflectance using an inversion method; the band gap can then be obtained from the absorption coefficient. Finally, the diffuse reflectance of carbon-doped TiO₂ presented by Khan et al. [Science 297 (2002) 2243-2245] is analysed; it is found that while the band-gap wavelength is extended into the visible region, it is overestimated. Moreover, light at visible wavelengths is only very weakly absorbed, and is expected to make only a minor contribution to the water-splitting efficiency.     

摇臂钻床在高精度管孔加工中的应用技术

本文通过对刀具组合及加工参数的实验研究,阐述了摇臂钻床在高尺寸精度,高表面粗糙度管孔加工中的应用技术。     

Passive cooling of outdoor urban spaces. The role of materials

This paper presents the results of a comparative study aiming to investigate the suitability of materials used in outdoor urban spaces in order to contribute to lower ambient temperatures and fight heat island effect. The study involved in total 93 commonly used pavement materials outdoors and was performed during the whole summer period of 2001. The thermal performance of the materials was measured in detail using mainly infrared thermography procedures.The collected data have been extensively analysed using statistical techniques. Comparative studies have been performed in order to identify the major advantages and disadvantages of the materials studied. Materials have been classified according to their thermal performance and physical properties into ‘cool' and ‘warm' materials. The impact of color, surface roughness and sizing has been analysed as well.The study can contribute to selection of more appropriate materials for outdoor urban applications, and thus assist to fight the heat island effect, decrease the electricity consumption of buildings and improve outdoor thermal comfort conditions.     

汽车发动机铝锡曲轴瓦的高速金刚镗削试验研究

作者针对汽车发动机铝锡曲轴瓦的内圆加工，通过在试验装置上的一系列试验研究，探索在采用高速金刚镗削工艺条件下，切削用量与内圆表面粗糙度之间的关系，为该项工艺的生产应用提供了可靠的数据与理论依据。本文结论对铝合金件及其它有色金属件的机械加工也有普遍的实用意义。     

Functional Correlation of Surface Temperature and Flow Velocity on Fouling of Cooling-Tower Water

Experimental fouling data have been analyzed on the basis of the change in overall heal transfer coefficient of the fouling test section. It is assumed that thermal hydraulic conditions in the test section remain reasonably constant for the duration of a fouling test. The model ofTaborek et al.|I| is used, and two parameters,/8,. and Rf?, that can be determined for each fouling test are derived by regression analysis. The parameter Rf? contains all the factors that influence fouling, while 1/ ?_c, contains shear stress, deposit thickness, and bonding strength of the deposit. The parameter R?is the asymptotic fouling resistance and ? is the lime constant of the fouling resistance-time curve. These parameters were determined as a function of surface temperature. Limited data were available to indicate the effect of velocity on the parameters. The parameters 1/8, and R? may be used to predict the history of fouling in a heat exchanger. Until more.data are obtained and analyzed in this fashion, the values of 1/ ?_c. and R? obtained in this paper should be applied at conditions for which the fouling data were obtained. A numerical example is presented.     

Radiative properties of materials with surface scattering or volume scattering: A review

Radiative properties of rough surfaces, particulate media and porous materials are important in thermal engineering and many other applications. These properties are often needed for calculating heat transfer between surfaces and volume elements in participating media, as well as for accurate radiometric temperature measurements. In this paper, recent research on scattering of thermal radiation by rough surfaces, fibrous insulation, soot, aerogel, biological materials, and polytetrafluoroethylene (PTFE) is reviewed. Both theoretical modeling and experimental investigation are discussed. Rigorous solutions and approximation methods for surface scattering and volume scattering are described. The approach of using measured surface roughness statistics in Monte Carlo simulations to predict radiative properties of rough surfaces is emphasized. The effects of various parameters on the radiative properties of particulate media and porous materials are summarized.     

Effect of surface roughness of top cover layer on the efficiency of dye-sensitized solar cell

In this research, the effect of surface roughness of flexible polymeric top cover layer on the efficiency of a dye-sensitized solar cell (DSSC) was investigated. In order to assess the durability of polyethylene terephthalate (PET) and polyethylene terephthalate naphthalate (PEN) the wear properties of these materials were first investigated. It was found that PEN was about 2.3 times more wear resistive than PET. PEN was used for further investigation regarding the effect of surface roughness on the light transmittance and electrical efficiency of DSSC. The surface roughness of PEN was varied by sand blasting to simulate an erosion process. It was found that PEN with a maximum average surface roughness of 1.6 μm resulted in 10% decrease in light transmittance and consequently degraded the cell efficiency of DSSC by 13%. The decrease in electrical efficiency due to increasing surface roughness was found to be proportional to the light transmittance.     

Experimental study of distribution of energy during EDM process for utilization in thermal models

Electrical discharge machining (EDM) has steadily gained importance over the years because of its ability to cut and shape a wide variety of materials and complicated shapes with high accuracy. The effectiveness of the EDM process is evaluated in terms of the material removal rate, relative wear ratio and the surface roughness of the work piece. The input discharge energy during this process is distributed to various components of the process, which further influences the material removal rate and other machining characteristics like surface roughness. Since during this process the electrical energy is converted into heat energy, hence the theoretical modeling of this process is based upon the heat transfer equations and in all existing thermal models the fraction of the energy transferred to the workpiece, is one of the important parameters. The accurate prediction of the fraction of energy effectively transferred to the workpiece will help to reduce the errors of the thermal models. In this study experiments have been performed to study the percentage fraction of energy transferred to the workpiece utilizing heat transfer equations, at different EDM parameters. This study also relates the optimum parameters with the optimum utilization of input discharge energy and hence will help to improve the technological performance of this process.     

Effect of surface roughness of the metallic interconnects on the bonding strength in solid oxide fuel cells

In this study, the joint strengths of glass-ceramic sealant placed between two metallic interconnectors (Crofer® 22 APU) are experimentally investigated depending on the surface conditions of the metallic interconnector and electrolyte/electrode materials (YSZ and NiO). The surfaces of the interconnectors are sanded with sandpaper having five different grits (60, 120, 240, 320 and 2000 grits). Thus, roughened surfaces are obtained and the adhesion is examined for each case. Profilometer is used to inspect the surface roughness of the samples. The fracture strengths of 24 samples prepared for each case are determined via tensile tests. Similarly, different electrolyte/electrode materials with modified surfaces are sandwiched between two glass ceramic layers and their mechanical performances are also measured. The results reveal that the joining strength tends to increase with the amount of surface roughness. It is also found that NiO adheres better to glass-ceramic material than YSZ. The microstructures of the adhesion interface of some cases are also investigated by a scanning electron microscopy. The images showed that good adhesion is achieved without any delamination or cracks at the interfaces. Chemical formation between the glass-ceramic sealants, interconnects and SOFC components is further investigated by XRD analyses.     

Nucleate pool boiling heat transfer characteristics of dilute Al2O3–ethyleneglycol nanofluids

Pool boiling heat transfer coefficients of dilute stabilized Al₂O₃–ethyleneglycol nanofluids as possible coolant fluid are experimentally quantified. The influence of different parameters such as heat flux, heating surface nano-roughness, concentration of nanofluids and fouling resistance on the pool boiling heat transfer coefficient of alumina nanofluids has experimentally been investigated and briefly discussed. Results demonstrated that there are two heat transfer regions with different mechanisms namely free convection and nucleate boiling heat transfer. Studies on the influence of parameter demonstrated that with increasing the heat flux, the pool boiling heat transfer coefficient of nanofluids significantly increases. In contrast, with increasing the concentration of nanofluid, due to the deposition of nanoparticles on the surface, the average roughness of the surface and the heat transfer coefficient dramatically deteriorate, while a significant increase in fouling resistance is reported. Also, studies reveal asymptotic and rectilinear behaviors of fouling resistance parameter in nucleate boiling and free convective domains.     

Surface roughness dominated wettability of carbon fiber in gas diffusion layer materials revealed by molecular dynamics simulations

High water contact angle in carbon fiber can facilitate water removal ability of gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). Water contact angle is intensively dependent on the surface hydrophobicity of carbon fiber in GDL. In this study, the hydrophobicity of commercial GDL is enhanced through the immersion and hydrothermal methods. The porosity decreases slightly while the surface roughness and surface topology diversity increase significantly in hydrothermal GDL compared with commercial reference and immersion GDL samples. The molecular dynamics simulations show that the water contact angle increases significantly with the increasing surface roughness but varies slightly with different surface topology, indicating that the water contact angle is dominated by the surface roughness. This study's findings are expected to offer an approach that can effectively enhance the water removal capacity by tailoring the surface roughness of carbon fibers in GDL materials.     

Fouling surface finish evaluation

Fouling of a surface takes place as the result of a series of complex reactions that cause deposits to form on process surfaces. For many conditions, fouling can be reduced but not necessarily eliminated. The materials considered here are: carbon steel, stainless steel, and aluminium with typical finishes.Sample plates were placed vertically in tanks and exposed to untreated lake water for various time periods. Results are presented that compare surface roughness over time, for the materials/surfaces considered. The progressive change in surface appearance with increasing immersion times is also presented.Stainless steel samples showed a relatively small change in surface appearance for most periods of immersion, with a small increase in surface deterioration for increasing immersion times. Brite aluminum, an aluminium alloy with an anodized surface film, performs similar to stainless steels. Cold rolled carbon steel has the largest variation of surface appearance over time.This review includes observations on fouling and process surface materials/finishes. Conclusions and observations regarding the materials that are commonly used in designs when fouling may be a concern are presented here. Photographs of material frontal surfaces and transient surface roughness are given for a variety of surfaces.     

Experimental study for the enhancement of heat transfer characteristics and development of thermal correlations of a roughened solar collector

The thermal performance of the flat plate solar collector is very low. The most beneficial and worthwhile method for increasing the thermal performance of a solar-powered air heater (SPAH) is to include a roughness element in the working zone of heat transfer that is located beneath the shear layer of the absorber surface. In this research work, efforts are made to enhance thermal performance and develop thermal correlations for the estimation of the Nusselt number and friction factor of a roughened SPAH. Experiments are performed for various ranges of flow, Reynolds numbers, and roughness parameters. The experimental technique of liquid crystal thermography is utilized to assess the dispersal of Nusselt number over the roughened surface for all roughness parameters. A maximum thermal performance enhancement index of 2.69 is obtained with the optimum value of the roughness parameter at a relative roughness pitch (RRP) of 9, a relative staggering distance (RSD) of 4, and a relative roughness length (RRL) of 6.15. Second, a mathematical correlation is developed using a regression model to estimate the Nusselt number and friction factor in terms of nondimensional roughness and flow parameters operated as RRP, RSD, RRL, and Re. The degree of discrepancy between the established the relationships and the findings from the experiment reveals incredibly satisfying results. Hence employing twisted V-ribs as an artificial roughness element no doubt increases the Nusselt number, and thermohydraulic performance enhancement index, but it also exerts less frictional power across the SPAH duct.     

Introduction of a length correction factor for the calculation of laminar flow through microchannels with high surface roughness

The main objective of this work is to introduce a length correction factor, based on CFD simulation results, in order to extend the method for taking into account the surface roughness effects in the calculation of laminar flow through rough microchannels described in [J.R. Valdés, M.J. Miana, T. Pütz, Numerical investigation of the influence of roughness on the laminar incompressible fluid flow through annular microchannels, Int. J. Heat Mass Transfer 50 (2007) 1865–1878], to surface roughness values much larger than the ones considered in the previous study. The method proposed in Valdés et al. (2007) consisted in building an equivalent smooth channel with the same flow resistance as the rough one. As in this case, the new method is validated with results of CFD simulations of microchannels with different roughness values.     

先进表面工程技术的发展前沿

叙述了表面工程的分类及各种表面工程技术的应用现状和发展趋势,指出,双层辉光等离子表面冶金技术、激光表面冶金技术、离子注入、加弧辉光离子渗镀技术和闭合场非平衡磁控溅射等先进技术是表面工程技术研究应用的前沿。     

An active coolant cooling system for applications in surface grinding

In many precision machining processes such as surface grinding, coolant is typically used to provide functions such as lubrication and cooling. In order to reduce surface grinding temperatures effectively, an active coolant cooling system is proposed. The system is based on the use of forced convection of the heat generated during the machining process. The coolant cooling system utilizes a commonly used air conditioner for ease of use and to reduce costs. In the proposed design, the evaporator of the heat pump is connected to the coolant tank of a surface grinding machine to reduce grinding temperatures for improved stability of accuracy and surface quality. This can be done without compromising production efficiency. System structure is explained and a coolant temperature model presented. Experimental testing on a prototype active cooling system is presented. The coolant temperature can be reduced to approximately −2 °C under no load condition, and to approximately 3 °C under loaded condition. The time constants of the cooling system were estimated. The results of the experimental tests demonstrated the effectiveness of the proposed system for applications in surface grinding for active coolant cooling in comparison with passive cooling.     

汽油喷雾撞壁雾化的数值模拟

撞壁雾化是现代直喷式中小缸径发动机中普遍存在的现象,研究撞壁雾化的特性对改善发动机的燃烧具有重要的意义。根据Mundo等人提出的飞溅喷雾撞壁模型,对定容室中的汽油喷雾撞壁现象进行了模拟计算与分析。通过改变壁面温度、壁面粗糙度、喷雾速度及网格数,对比分析了不同条件下喷雾撞壁过程发生的变化,为进一步进行喷雾撞壁试验、完善喷雾撞壁模型提供了参考。     

Effect of Micropillar Spacing and Temperature of Substrate on Contact Angle Dynamics

ABSTRACT

Contact angle dynamics of droplets deposited on a structured surface were studied in this work and the effects of substrate microstructure and temperature were investigated. Microstructures consisting of uniformly-sized, cubic micropillars with varying pillar spacings were constructed by microfabrication. Droplets (of the order of tens of microlitres in volume) were deposited on these surfaces and dynamic contact angles were observed using various techniques. Advancing and receding contact angles were measured using tilting of the surfaces or by injection and aspiration of fluid from a horizontal droplet by syringe. Droplets on these surfaces appeared to be mainly in the Wenzel state. Contact angle hysteresis was obtained as a function of pillar spacing or, equivalently, surface roughness. Depinning force was deduced and a linear dependence on maximal three phase contact line was found. The techniques of tilting the surface on which the droplet was deposited and uniformly increasing and reducing the volume of the droplet via the syringe both gave the same contact angle hysteresis for a given micropillar spacing. The effect of temperature was then assessed using a heated tilting plate. Contact angle hysteresis was found to increase with temperature. Further work to elucidate mechanisms governing this dependence will be undertaken.