Search

A kind of steam impaction apparatus was developed to study the influence of microparticles on steam erosion. After 10 min exposure, no erosion pits appeared on the mild carbon steel surface under the impaction of pure water steam when the steam speed was controlled below the threshold velocity. However, at the same speed, erosion pits appeared on the surface when SiC microparticles were added into the steam. Further experimental investigations proved that the pits were not from the collision of microparticles to the metal surface, but similar to the cavitation erosion pits. It indicates that the microparticle may enhance the liquid impaction erosion besides causing the abrasion, and the generation of pit is thought to be the results of cavitation erosion.

To clarify the electron transfer behavior of the hydrogen-terminated boron-doped diamond film electrode in an electrolytic solution, its detailed electronic structures were investigated by using scanning probe microscopy and ab initio methods. The interface structure of the hydrogen-terminated diamond electrode and the electrolyte was also studied by means of cyclic voltammetry and AC impedance spectroscopy. The results showed that there exist shallow acceptors in the band gap of the hydrogenated diamond films. Neither the surface hydrogen alone nor the subsurface hydrogen could induce the shallow acceptors, though they interact strongly with the surface carbon atoms or subsurface carbon and boron atoms in the diamond film. It is the interaction of the surface adsorbates and the surface C–H bonding that gives rise to the shallow acceptors in the band gap of the hydrogenated diamond film. The surface shallow acceptors in the band gap of the hydrogenated diamond film could lower the energy barrier of the electron transfer between the diamond electrode and the electrolytic solution. Thus, electrochemical window for the hydrogenated diamond film is narrower and, its film resistance and capacitance are also smaller than those of the oxygenated one. In our work, the experimental data obtained by using scanning probe microscopy are in good agreement with the calculation results.

After a 1 min cavitation experiment performed in an ultrasonic vibration system, needle-like erosion pits appeared on a polished stainless steel surface, and a special ring area was formed around each pit. The shape of the pit and the plastic deformation of the ring area indicate that the mechanical impaction on the surface is the main reason for the cavitation erosion. On the other hand, the iridescent color, the decreased surface hardness and the appearance of the precipitated carbon ring prove that the ring area has experienced a tempering process with the temperature higher than 300 °C. Also, the lack of oxygen in the ring area proves that it is not a chemical oxygen result.

Spherical and irregular microparticles both in the size of 5 μm were put into the de-ionized water respectively to form different suspensions, and vibrating cavitation experiments were performed in the two kinds of suspensions. After the experiment, the damages on the specimen surfaces were measured and free radicals in suspensions were detected. It was found that suspensions with particles cause more severe cavitation erosion than those without particles. Compared with a spherical particle, the shape of the irregular particle has little effect on the number of the cavities, but it causes abrasion on the solid surface besides the cavitation erosion.

In cavitation erosion, stress waves will be generated and propagated in the solid when a collapse impingement is acted on it. The cavitation damages on the solid surface are considered to be under the effect of the stress waves. An ultrasonic vibration cavitation erosion experiment was performed on a polished steel specimen, and not only the plastic deformations but also the brittle fractures appeared on the surface at the incubation stage of cavitation erosion. Some characteristics such as the hemispherical shape of the crater, intergranular fractures and subsurface comminution make the damages distinguishable from the common plastics deformations, and they are thought to be the results of shear waves. Thus, stress waves are proved to take part in the cavitation erosion, and they bring some special damage styles depending on the conditions of the impaction and mechanical properties of the specimen.

To investigate the damage mechanism of the cavitation erosion at the incubation stage, a kind of polished 40Cr steel was used as the sample in the vibration cavitation erosion experiment. After 1-min experiment, three kinds of erosion pits were found on the sample surface, they are complete pits, incomplete pits and thermal pits in this paper. Numerical simulation of a bubble collapse process in Fluent environment explains the formations of the three kinds of erosion pits. The micro-jet with a strong radius flow around is the main reason for the complete pits and their step-like openings, the shock wave generated outside the jet function domain is the main reason for the incomplete pit, and the high temperature in the compressed bubble at the moment of collapse is considered as the reason for the formation of the thermal pits.

To investigate the effect of hydrophobic surface on cavitation damage, hydrogen-terminated diamond film and oxygen-terminated diamond film are prepared for the cavitation damage experiment. Because the film surfaces have the same roughness and hardness, the hydrophobic property effect is focused on. Obvious damage pits appear on the hydrogen-terminated surface after 4 h cavitation experiment, while no such pits are found on oxygen-terminated surface. Surface testing results also show that the surface roughness of hydrogen-terminated surface is higher than that of oxygen-terminated surface after the experiment. Such results indicate that the collapse of bubbles growing from the hydrophobic wall to be damaged plays important role in the generation of cavitation damage.

Polished, grinded, and milled samples made of 40Cr stainless steel were prepared for the cavitation erosion experiment. A typical phenomenon of “pits chain”, which consisted of two contact pits and a smaller pit on the ridge between them, was found on the sample surfaces after 15-minute experiment. Numerical simulation indicated that the pressure fluctuation caused by the sequentially formed pits on the solid surface was the main reason for the formation of the “pits chain”. It proves that the early-formed pits affect the formation of the subsequent erosion pits, and the whole cavitation erosion process is not a probability event. Based on the numerical analyses, the development of erosion is divided into four stages, which describe how a pit develops vertically and horizontally under the effect of the pressure perturbation. The development was validated by the characteristics of the damaged surface observed at different experimental time.

Thin fluid film is thought to be formed between the wafer surface and the pad asperity. Hydrodynamic pressure on the surface asperity is periodically generated when particles are passing through it. Fatigue fracture occurs under the effect of periodic pressure, and the fatigue begins from the top to the bottom of the asperity. The removal rate is calculated based on the energy-balance fracture theory. Particle size and its relative velocity are important parameters that affect the polishing effect. Using the multiphase model and the power–law viscosity model of the slurry, particle’s velocity and its distribution in the slurry are numerically calculated. The results indicate that the slurry film thickness needs to be in the same order of the particle size that the particle can generate effective hydrodynamic pressure to remove the asperity materials.

To analyze the effects of non-Newtonian lubricants and surface roughness in journal bearing lubrication, a modified Reynolds equation is derived. In the equation, differential viscosity and the first normal stress function are defined to specify the rheological properties of non-Newtonian medium. Flow factors are used to specify the effect of surface roughness. The modified Reynolds equation is numerically calculated using super over relaxation method. Numerical results of the lubrication show that the differential viscosity is the principal non-Newtonian property affecting the lubrication, it is determined by the material parameters of the lubricant and is affected by the shearing rate. Under its effect, the load capacity of non-Newtonian lubricants is not always higher or lower than that of Newtonian lubricants’. The effect of the first normal stress difference increases under the conditions of dynamic loading. Surface roughness shows an obvious effect on load capacity when it is greater than one-tenth of the film thickness, and the surface with longitudinal pattern affects the load capacity most. However, surface roughness has less effect on lubrication results than lubricants’ non-Newtonian property.

A drag force reduction experiment is performed on polyoxyethylene (PEO) water solution using a rotary rheometer. Longitudinal, transverse, and isotropic grooves are notched on the rotor’s surface to investigate the effect of surface roughness patterns. The experiment results show that higher drag force is generated on the surface with transverse grooves compared with the drag force on a smooth surface, while lower drag force is generated on the surface with longitudinal grooves. The drag force on the surface with isotropic grooves is between them. Forces on the side wall of the grooves play important roles in drag force reduction. They are numerically analyzed based on the Navier-Stokes equation using the finite volume method. The viscous force on the groove’s bottom plane is far less than the force on the corresponding plane of the smooth surface, but the drag force on the groove’s surface is compensated either by the pressure drag on the side walls of the transverse groove or by the viscous force on the side walls of the longitudinal groove. The pressure drag on side wall of transverse groove is always higher than the viscous force on side wall of longitudinal groove. The numerical results cope with the experiment results that only the surface with longitudinal grooves can reduce the drag force.

The purpose of this paper is to provide a lubrication equation for non-Newtonian fluid. Three nonlinear functions instead of common power law model are used to describe non-Newtonian properties more completely. They are shear dependent viscosity, first normal stress difference and stress relaxation. After the coordinate conversion which is needed for the lubricant film thickness variation, the functions are involved in the modified Reynolds equation and show their effects on the lubrication results. As the principle factor in lubrication, viscosity is expressed by a first order transfer function in frequency domain. Its variation process is described by the function’s amplitude frequency response curve, which is validated by rheological experiment. Numerical results of the modified Reynolds equation show that non-Newtonian lubricant’s load capacity is not always higher or lower than Newtonian lubricant’s, and non-Newtonian lubricant has flatter pressure profile at high working speed.

To identify the micro helicopter’s yaw dynamics, the system identification method is used and is proved to be suitable according to the validation results. In order to strengthen the information of the dynamics and reduce the effect of the noise when processing the experiment data, the conventional system identification method is modified and a weighted criterion is investigated to estimate the model parameters. In calculating the factors of the weighted criterion, a perceptron is trained to allocate the factors automatically. The model validation result shows that the model derived by this kind of method can fit the measured outputs well. The modified system identification method would be useful in identifying dynamic systems which use the multiexperiment data.

Micro spherical particles were found in a vibrating cavitation erosion experiment. Examination of the spherical particles reveals the dendritic pattern on the surface and the hollow structure of the interior. The surface and interior structures of the particle are also related to the particle’s size. For smaller particle, the dendritic structure is replaced by the fine cell and the interior becomes solid. Such special structures are considered to be the result of particle’s solidification from molten state at a rapid cooling rate, which happens in a special transient environment with transient high temperature and high pressure provided by cavity collapsing. The specific area and surface tension force are the main reasons for the different structures of the particles in different size.

An ultrasonic vibration cavitation erosion experiment was performed to study the thermal effect during the erosion process. The ring affected zone was observed on the sample surface around the erosion pit at the incipient stage of the cavitation erosion. The results of the surface testing on roughness, hardness, and chemical composition proved that the zone was caused by thermal effect, and that the zone surface experienced a tempering process with the temperature higher than 300°C. Numerical simulation results show that the high temperature domain in the bubble directly contacting the solid wall is a necessary condition for the occurrence of the tempering process on the zone surface, or the heat in the bubble can hardly be transferred to the solid wall under the effects of the great temperature gradient in the bubble and the quick cooling process in the water.

The cavitation-erosion is induced by the collapse of bubbles in the water. In the cavitation-erosion of hydro-machinery, it is generally accepted that the collapse of bubbles in water is attributed to the sharp rise of the pressure induced by the fluctuation of the fluid. However, it is proposed in this research that the collapse of bubbles is attributed to the interaction between micro-particles and bubbles in fluid. The cavitation -erosion experiments conducted in a rotating-disc flow shows that the most of micro-particles suspended in the water were damaged by the collapse of bubbles after cavitation-erosion, and played indispensable roles to induce the collapse of bubbles in the fluid. In the experiments, damage of the micro particles was evaluated by a transimission electron microscope (TEM) and the collapse of bubbles was estimated by the erosion pits on the solid surface.

The effect of transverse grooves on friction force was considered, differing from the longitude riblets. A discontinuous traveling wave distribution was developed based on the traditional continuous one. The friction force was measured directly in a high-speed water tunnel with flow velocity varying from 17 to 22m/s. The force increased quadratic with the increasing of velocity. Compared with smooth surface, the friction drag reduction was about 12.5%. The computational fluid dynamics simulations, using RNG k-e turbulent model, showed that the vortex was formed in the groove. On the upside of the vortex, the revolving direction of the fluid was in consistent with the main flow, which decreases the flow shear stress rate. The vortex increases the height of the turbulent boundary layer, and decreases the turbulent intensity of main fluid field, thus decreases the slope of velocity and the friction force. Correspondingly, the transverse grooves induce pressure drag. When the increase of pressure drag is less than the decrease of the friction force, the total drag is decreased. The function of the vortex in the groove was like a rolling bearing. In this wok, the computational simulation explained the experimental results well.

The pits formed at the incipient stage of cavitation erosion of 45 steel have been investigated by using a standardized vibration cavitation apparatus with de-ionized water. Scanning electron microscopy (SEM) was used to characterize the morphology and composition of the pits. The results show that there are more than 20 percent of the erosion pits accompanied with MnS inclusions. It is considered that these pits are induced by MnS inclusions and formed by a series of actions: first, the MnS inclusion is attacked and activated, then it dissolves and accelerates the local corrosion process, finally the dissolution of both the inclusion and the matrix around it will lead to the formation of the pit.

Cavitation includes the occurrence, growth and collapse of vapor bubbles in a liquid. Cavitation noise is the sound uttered during the cavitation progress, and it is mainly induced by the collapse of the bubbles. Under certain presuppositions, the micro-jet and micro-shockwave can be produced when the bubbles collapse. The impact of the microjet and micro-shockwave on the surface of material is strong enough to damage the structure of the material and finally erosion takes place. Accordingly, compared with cavitation, the sound emitted during the cavitation erosion progress has a more source that is induced by the vibration of the solid surface when it is attacked by the micro-jet. In this work, the effect of the surface material on the cavitation erosion noise has been studied. Spectrum characteristics obtained for different metal materials: stainless steel, aluminum bronze, bronze, brass (in the same experimental conditions) has also been presented in this paper. The purpose is to validate the contributions of the impact noise to the cavitation noise.