Influence of calcium on cracking of steel during the welding or casting process

The influence of calcium on the brittle–ductile transition temperature is studied for the solidification range. The test bars were cast from carbon steel with and without the addition of calcium. Mechanical properties at different temperatures around the solidus were evaluated. The fractured surfaces were examined using a scanning microscope coupled with a Kevex analyser. For the modified steel, the liquid phase appears (on heating) for higher temperature while the temperature range within which fracture can nucleate, during casting or welding, is markedly narrowed. In medium-carbon steel, calcium changes the manner of fusion of the grains. The inclusions of the liquid phase form at the triple points and the surfaces of grains remain solid above the solidus temperature.     

Heat transfer in the flow of a cold,two-dimensional draining sheet over a hot,horizontal cylinder

The paper considers heat transfer characteristics of thin film flow over a hot horizontal cylinder resulting from a cold vertical sheet of liquid falling onto the surface. The underlying physical features of the developing film thickness, velocity and temperature distributions have been illustrated by numerical solutions of high accuracy for large Reynolds numbers using the modified Keller box method. The solutions for film thickness distribution are good agreement with those obtained using the Pohlhausen integral momentum technique thus providing a basic confirmation of the validity of the results presented.     

1—甲基—3—西基咪唑六氟磷酸盐离子液的摩擦学性能

合成了1－甲基－3－西基咪唑六氟磷酸盐离子液体，采用核磁共振氢谱仪和傅立叶转换红外光谱仪表征了其结构；测定了该化合物的粘度、倾点和密度，在SRV摩擦磨损试验机上评价了其作为润滑剂对钢／钢体系的润滑作用；采用X射线光电子能谱仪和扫描电子显微镜分析了钢磨损表面形貌及化学状态，以考察其润滑机理。结果表明：1－甲基－3－丁基咪唑六氟磷酸盐离子液低温流动性能好，作为润滑剂对钢／钢摩擦副具有优异的润滑作用，摩擦系数极低，且抗磨性能优良。这是由于该离子液体在钢磨损表面形成含FePO4和FeF2等物质的边界润滑膜，从而有效地提高了摩损副的承载能力和抗磨性能。     

液晶润滑添加剂的减摩作用机理研究

液晶润滑添加剂具有优良的减摩性能，但在已有的文献报道中涉及其减摩机理的研究内容却还很少。因此，利用向列型液晶材料已氧基苯甲酸和正辛基苯甲酸作为ＨＵ－２０汽轮机油的添加剂，在给定的压力、速度和温度条件下于Ｆａｌｅｘ试验机上进行了液晶的减摩性能试验研究，并且用Ｘ射线衍射、俄歇电子能谱和扫描电子显微镜等技术，对溶解于矿物油中的液晶结构、磨损表层的元素组成及其表面形貌作了观察、分析与研究，在此基础上又对液     

A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

Influence of coolant injector configuration on film cooling effectiveness for gaseous and liquid film coolants

An experimental investigation is conducted to bring out the effects of coolant injector configuration on film cooling effectiveness, film cooled length and film uniformity associated with gaseous and liquid coolants. A series of measurements are performed using hot air as the core gas and gaseous nitrogen and water as the film coolants in a cylindrical test section simulating a thrust chamber. Straight and compound angle injection at two different configurations of 30°–10° and 45°–10° are investigated for the gaseous coolant. Tangential injection at 30° and compound angle injection at 30°–10° are examined for the liquid coolant. The analysis is based on measurements of the film-cooling effectiveness and film uniformity downstream of the injection location at different blowing ratios. Measured results showed that compound angle configuration leads to lower far-field effectiveness and shorter film length compared to tangential injection in the case of liquid film cooling. For similar injector configurations, effectiveness along the stream wise direction showed flat characteristics initially for the liquid coolant, while it was continuously dropping for the gaseous coolant. For liquid coolant, deviations in temperature around the circumference are very low near the injection point, but increases to higher values for regions away from the coolant injection locations. The study brings out the existance of an optimum gaseous film coolant injector configuration for which the effectiveness is maximum.     

Pool boiling of drag-reducing polymer solutions

The effect of drag-reducing polymeric additives on the critical heat flux and minimum film boiling temperature in subcooled pool boiling of water has been experimentally examined. Three water-soluble polymers, viz. a polyethylene oxide (Aldrich No. 18946-4), a polyacrylamide (Separan AP-30), and a galactomannan polysaccharide (Galactasol-211) have been examined at concentrations of 10, 50, and 100 wppm. The experiment is performed by quenching a hot brass sphere in an isothermal pool of the fluid to be examined and obtaining the corresponding boiling curve. The experiments have been conducted at atmospheric pressure with a pool temperature of 90°C. The results show that the critical heat flux increases by more than 50% while the minimum film boiling temperature increases by more than 110°C when concentrations of 100 wppm of polyox or guar gum are used. For separan solutions, the critical heat flux and minimum film boiling temperature are lower than those for water and go through a minimum at a concentration of ～10 wppm; the maximum reductions are about 73% and 34°C respectively.     

Convective heat and mass transfer from a falling film to a laminar gas stream

A numerical study has been made of convective heat and mass transfer from a falling film to a laminar gas stream between vertical parallel plates. The effects of gas-liquid phase coupling, variable thermophysical properties, and film vaporization have been considered. Simultaneous mass, momentum and heat transfer between liquid film and gas stream is numerically studied by solving the respective governing equations for the liquid film and gas stream together. The influences of the inlet liquid temperature and liquid flowrate on the cooling of liquid film are examined for air-water and air-ethanol systems. Results show that the heat transfer from the gas-liquid interface to the gas stream is predominantly determined by the latent heat transfer connected with film evaporation. Additionally, better liquid film cooling is noticed for the system having a higher inlet liquid temperature or a lower liquid flowrate.     

载荷对氟硅共掺杂类金刚石膜摩擦学性能的影响

采用等离子体增强化学气相沉积系统成功制备氟硅共掺杂类金刚石薄膜,采用Raman,XPS对膜层结构与成分进行表征.采用划痕仪对膜层的结合力进行了测试.摩擦测试结果显示,在载荷范围为1~6 N时,膜层具有低摩擦系数(0.05~0.1);在载荷大于6 N时,摩擦系数在0.2~0.27之间.对偶磨斑的光学照片显示,在低载下(1~6 N),磨斑上存在明显的转移膜,转移膜导致膜层具有低摩擦系数;在高载下,磨合期磨斑上存在稳定的转移膜,对应于磨合期的低摩擦系数.然而在磨合期后,磨斑表面转移膜消失,结果导致高摩擦系数.     

氧化铝多孔膜振动性能的实验研究

电化学氧化法制备的氧化铝多孔膜有六角规则密排的纳米孔,其优良的结构特点使其具有良好的用途,可应用于微粒物质分离,氧化剂的载体,微电子机械系统的组件和纳米器件等。在前期研究了这种氧化铝膜的力学性能如拉伸性能和弯曲性能的基础上,本文实验研究了这种氧化铝多孔膜的振动特性即共振频率特性和振动模态,估算了该氧化铝多孔膜的相当弹性模量,与其他方法测得的相当弹性模量基本一致。     

Simultaneous heat and mass transfer inside a vertical tube in evaporating a heated falling alcohols liquid film into a stream of dry air

A numerical study of the evaporation in mixed convection of a pure alcohol liquid film: ethanol and methanol was investigated. It is a turbulent liquid film falling on the internal face of a vertical tube. A laminar flow of dry air enters the vertical tube at constant temperature in the downward direction. The wall of the tube is subjected to a constant and uniform heat flux. The model solves the coupled parabolic governing equations in both phases including turbulent liquid film together with the boundary and interfacial conditions. The systems of equations obtained by using an implicit finite difference method are solved by TDMA method. A Van Driest model is adopted to simulate the turbulent liquid film flow. The influence of the inlet liquid flow, Reynolds number in the gas flow and the wall heat flux on the intensity of heat and mass transfers are examined. A comparison between the results obtained for studied alcohols and water in the same conditions is made.     

Ignition of solid propellants

At present, the study of solid-propellant ignition is of particular interest owing to the adoption of hybrid motors [1–3]. The status of experimental and theoretical research in this field can be evaluated on the basis of the rather extensive survey of American papers in [2]. It is noteworthy that a common deficiency in available references is the absence of exact ignition criteria; in most cases the propellant is assumed to have ignited when its surface temperature reaches a prescribed level (gasification temperature), or when the rate at which the temperature increases with time at the propellant surface is sufficiently high. Exact criteria for this rate, however, are not given. In this article, we present ignition criteria for solid propellants and these are based on a diffusion-burning model. It is shown that for a diffusion flame to exist above the propellant surface, two conditions must be satisfied simultaneously: 1) the propellant surface temperature must equal the gasification temperature for that propellant and 2) the temperature gradient at the surface must be smaller than some value which depends on the kinetics of the chemical reaction in the diffusion flame and on the rate of oxidizer input to the propellant surface during burning.Two ignition techniques are examined as examples: ignition by hot gases or radiant heat flow and ignition by means of an active film which reacts with a cold oxidizer; the film is applied to the propellant surface prior to ignition.     

Two-phase structure above hot surfaces in jet impingement boiling

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found.     

2024铝合金在干摩擦往复运动条件下的磨损图研究

研究了 2 0 2 4铝合金 / 4 5 #钢摩擦副在干摩擦往复运动条件下的摩擦学特性 ,系统考察了载荷 98～ 4 90 N和速度0 .0 83～ 1.16 7m/ s范围内摩擦副材料的磨损行为 ;通过对磨损表面、断面以及铝在摩擦偶件的转移膜及磨屑的显微分析 ,建立了 2 0 2 4铝合金的磨损机制转变图 .结果表明 :2 0 2 4铝合金在低速和轻载条件下的磨损机制主要是磨粒磨损和剥层磨损 ;而在高速重载条件下的磨损机制主要是严重熔融磨损 ,并伴随向偶件材料表面的大量转移     

Lithium Diffusion and Stress in a Polycrystalline Film Electrode

In the present work, the two-dimensional analytical solution for Li diffusion in grains and grain boundaries of a polycrystalline film electrode is established with consideration of Li-segregation at the grain boundary. The stress field induced by the inhomogeneity of Li concentration, called chemical stress here for brevity, is analyzed via the finite element simulation.The effects of the grain boundary including its size, its diffusion coefficient as well as the segregation phenomenon on the solute concentration and the chemical stress are examined. It shows that grain boundaries can assist fast diffusion and significantly affect the stress profile in the whole film. It proves that tailoring the grain boundary size or other grain boundary-related parameters may provide an alternative strategy for improving the overall diffusivity and mechanical stability of battery electrodes.     

Rheology of crosslinking poly vinyl alcohol systems during film formation and gelation

Gelation of crosslinking polymers, with ionic groups, is of interest in membrane processing of these materials. The gelation of polyvinyl alcohol in the presence of an ionic crosslinker, sulfosuccinic acid, was monitored through rheological measurements. The evolution of rheological material functions during film formation, which involves solvent evaporation and crosslinking, was observed at different polymer and crosslinker concentrations. Relative effect of water evaporation leading to physical gelation and the chemical crosslinking was examined. To understand the effect of crosslinker type, the rheology of nonionic crosslinker, glutaraldehyde was also examined. The gel points as well as network parameters for these covalent networking systems were different and depended on crosslinker type and polymer/crosslinker concentrations. However, qualitative evolution of rheological behavior during film formation was largely similar, demonstrating the dominating effect of solvent evaporation. Crosslinking in the absence of solvent evaporation was examined at different temperatures. It can be concluded that similar mechanism was involved, independent of temperature, in the temperature range of interest. The effect of crosslinker concentration on evolution of rheological properties near gelation was insignificant for ionic crosslinker though marginally significant for nonionic crosslinker.     

Intrinsic coupling of near-tip matrix crack formation to mode III delamination advance in laminated polymeric matrix composites

Laminated carbon/epoxy specimens are loaded in anti-plane shear to investigate the relationship between near-tip matrix crack formation and the apparent mode III delamination toughness. Specimens are tested with different insert lengths to various load levels and examined fractographically. Near-tip matrix cracks are found to initiate and propagate intralaminarly before the onset of planar growth. These cracks are inclined at approximately 45° to the delamination plane and are perpendicular to the direction of maximum near-tip tensile stress. It is found that this represents an intrinsically coupled sequence of events for anti-plane shear loading of continuous fiber laminated polymeric composites when a preexisting delamination is bounded by plies that have their fiber direction aligned with the direction of macroscopic advance. This sequence of events violates the assumptions used in the reduction of data from common mode III tests. It therefore invalidates the associated toughness measurements, and may account for or strongly contribute to the common observation that laminated polymeric composites exhibit an apparent mode III delamination toughness that is dependent on test geometry.     

Viscous liquid film flow down an inclined corrugated surface. Calculation of the flow stability to arbitrary perturbations using an integral method

Viscous liquid film flow along an inclined corrugated (sinusoidal) surface has been studied. Calculations were performed using an integral model. The stability of nonlinear steady-state flows to arbitrary perturbations was examined using the Floquet theory. It has been shown that for each type of corrugation there is a critical Reynolds number for which unstable perturbations occur. It has been found that this value greatly depends on the physical properties of the liquid and geometric parameters of the flow. In particular, in the case of film flow down a smooth wall, the critical waveformation parameter depends only on the angle of inclination of the flow surface. The values of the corrugation parameters (amplitude and period) were obtained for which the film flow down a wavy wall is stable to arbitrary perturbations up to moderate Reynolds numbers. Such parameter values exist for all investigated angles of inclination of the flow surface.     

Local heat transfer from a hot plate to a water jet

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Because of its importance to many cooling procedures, investigations on basic mechanisms of jet impingement boiling heat transfer are needed. Until now, most of the experimental studies, carried out under steady-state conditions, used a heat flux controlled system and were limited by the critical heat flux (CHF). The present study focuses on steady-state experiments along the entire boiling curve for hot plate temperatures of up to 700°C. A test section has been built up simulating a hot plate. It is divided into 8 independently heated modules of 10 mm length to enable local heat transfer measurements. By means of temperature controlled systems for each module local steady-state experiments in the whole range between single phase heat transfer and film boiling are possible. By solving the two dimensional inverse heat conduction problem, the local heat flux and the corresponding wall temperature on the surface of each module can be computed. The measurements show important differences between boiling curves measured at the stagnation line and those obtained in the parallel flow region. At the stagnation line, the transition boiling regime is characterised by very high heat fluxes, extended to large wall superheats. Inversely, boiling curves in the parallel flow region are very near to classical ones obtained for forced convection boiling. The analysis of temperature fluctuations measured at a depth of 0.8 mm from the boiling surface enables some conclusions on the boiling mechanism in the different boiling regimes.     

钢材的微观结构对某些添加剂抗磨特性的影响

用四球试验机和HQ-1试验机研究了钢的化学组成、硬度和微观结构对含硫添加剂、含磷添加剂和含磷、氮添加剂抗磨作用的影响。结果表明,在某些情况下,钢材的微观结构对添加剂抗磨性能的优劣几乎起着决定作用。此外,作者还根据XPS、AES和Mossbauer谱的分析结果,对影响钢材与添加剂配伍性的作用机制进行了讨论。