Increase In Particle Velocity Research Articles

Residual Stress Analysis of Cold-Sprayed Copper Coatings by Numerical Simulation

In this paper, an analysis on the residual stress evolution of cold-sprayed copper coatings on Cu and Al substrates was performed. To investigate the influences of particle velocity, temperature and material combination on the final residual stresses, an integrated frame of calculation was proposed based on the simulation results obtained from the developed thermo-mechanically coupled Eulerian model. In a single Cu splat, generally speaking, the maximum residual stress and plastic deformation are concentrated at the outside contact zone rather than at the center point of initial impact. The action of friction shear between the particle and substrate during impacting should be considered as one of the essential factors on the final residual stress. And the states of residual stresses can vary significantly depending on the material combination, particle velocity, and temperature. In a single pass Cu coating, the residual stress fluctuates across the coating and there exists both compressive stress and tensile stress within the coating. At a certain range of impacting velocities, the resultant residual stresses increase with the increase of particle velocity. The present simulated results are related to the reported experiments by others, showing that the residual stress states and stress change trend are different from some of the reported results.

Probing linear and nonlinear microrheology of viscoelastic fluids

Bulk rheological properties of viscoelastic fluids have been extensively studied in macroscopic shearing geometries. However, little is known when an active microscopic probe is used to locally perturb them far from the linear-response regime. Using a colloidal particle dragged periodically by scanning optical tweezers through a viscoelastic fluid, we investigate both, its linear and nonlinear microrheological response. With increasing particle velocity, we observe a transition from constant viscosity to a thinning regime, where the drag force on the probe becomes a nonlinear function of the particle velocity. We demonstrate that this transition is only determined by the ratio of the fluid's equilibrium relaxation time and the period of the driving.

Particle shape effects in flotation. Part 1: Microscale experimental observations

There has long been speculation as to whether some particle shapes are more ‘floatable’ than others, which might be due to differences in the induction period required to achieve attachment between the particles and the air bubbles in the pulp. To resolve this, we used the Milli-Timer apparatus to directly observe the process of particle–bubble interaction and attachment by means of a magnified, high-speed video recording, thus providing a direct measure of the induction period for attachment.To assess the influence of particle shape on induction time we used two varieties of methylated borosilicate glass particles — spheres and angular ‘frit’ — in a range of tightly-sized fractions. Other factors that could affect the induction time, such as the polar angle of sliding commencement, and approach velocity, are accounted for using multiple nonlinear regression.Our results illustrate the importance of particle shape on induction period, with angular particles exhibiting induction periods that were an order of magnitude lower than those of spheres. Furthermore, the induction period was seen to decrease with increasing particle velocity, or kinetic energy on approach, but increased as the trajectory approached the limit of just grazing the bubble. These results indicate that attention should be paid to the shape of particles obtained from the grinding operation, besides particle size.

Effect of impact angle and velocity on erosion of API X42 pipeline steel under high abrasive feed rate

The cost of erosion attack to the global oil and gas industry has drawn significant attention amongst researchers in recent years. Erosion is a complex phenomenon and in order to recommend ways to mitigate its effect it is essential to have a full understanding of the tribo-mechanisms involved. In the present study erosion tests were conducted on API X42 steel using aluminum oxide as erodent. The extent of material degradation is a function of the properties of abrasive particle, target material, erodent velocity, abrasive feed rate, attack angle and environment. In this study, erosion tests were carried out under high mean abrasive feed rate of 160gmin−1. Particle velocities and attack angles employed were as follows: 36ms−1, 47ms−1, 56ms−1 and 81ms−1 at 30°, 45°, 60° and 90°, respectively. Similar to erosion at low abrasive feed rate, it is found that erosion rate decreases with increasing impact angle and increases with increasing particle velocity. However, under the present experimental conditions, it is found that the velocity exponent (n) in the erosion rate equation (E=kVn) decreases with increasing abrasive particle angle of incidence. Contrary to common understanding for low particle feed rate tests, this finding suggests that the “n” value is mechanism dependent. Several mechanisms are found to be responsible for the behavior of API X42 steel under the experimental conditions used in this work. Heavy plastic deformation and fracture, ploughing and metal cutting are identified as operative mechanisms during erosion of API X42 pipeline steel.

OS1807 コールドスプレー模擬単粒子衝突試験によるAl粒子のミクロ組織に及ぼす衝突速度依存性

In a cold spray technique, which used for making dense and thick metallic coatings without oxidation, a lot of fine solid particles are impinged at supersonic velocity and deposited on a substrate materials. The deposition behavior is affected by many factors, such as particle velocity, size, shape, material, temperature, surface condition, and so on. However, these influences are not completely understood, because accurate measurements of these factors are complicated. Therefore, we developed a large-scaled single particle shot system. Using this system, a pure Al particle with approx. 1 mm diameter was impinged and deposited on a pure Al substrate. The particle velocity was accurately measured and the critical velocity which deposition begins was evaluated. And the microstructure of deposited particles was investigated by SEM and EBSD. From the results, the plastic deformation in a particle increased with increasing particle velocity, and the crystal grains with large plastic deformation adjacent to the particle/substrate interface tended to contribute to adhesion between a particle and a substrate.

Warm Spray Forming of Ti-6Al-4V

Warm spray (WS) is a modification of high-velocity oxy-fuel spraying, in which the temperature of the supersonic gas flow generated by the combustion of kerosene and oxygen is controlled by diluting the combustion flame with an inert gas such as nitrogen. The inert gas is injected into the mixing chamber placed between the combustion chamber and the powder feed ports, thus the temperature of the propellant gas can be controlled from ~700 to 2,000 K. Since WS allows for higher particle temperatures in comparison to cold spray, warm sprayed particles are more softened upon impact, thus resulting in greater deformation facilitating the formation of shear instability for bonding. Recently, the combustion pressure of WS has been increased from 1 (low-pressure warm spray) to 4 MPa (high-pressure warm spray) in order to increase the velocity of sprayed particles. Effects of spray parameters on microstructure, mechanical properties, and splats formation of Ti-6Al-4V were systematically studied. Obtained coatings were examined by analyzing the coating cross-section images, microhardness as well as oxygen content. In addition, flattening ratio of splats was calculated as a function of nitrogen flow rate. It was found that the increased particle velocity caused by the increased combustion pressure had significant beneficial effects in terms of improving density and controlling the oxygen level in the sprayed Ti-6Al-4V coatings.

Impact Erosion of Quartz Crystals by Micro-Particles in Abrasive Waterjet Micro-Machining

Abrasive waterjet (AWJ) micro-machining is a precision processing technology with some distinct advantages. To understand the machining process, the erosion mechanism is presented and discussed when micro-particle impacting on a quartz crystal specimen. It is found that three types of impressions are formed which are craters, micro-dents and scratches. Small-scale craters including crashed zones and radial cracks are associated with plastic flow and subsurface micro-cracks that decrease the material strength, but cause little material removal, while large-scale craters including conchoidal fractures caused by the propagation of lateral cracks dominate the volume change of the specimen. Micro-dents are produced by the impact of particles possessing small kinetic energies, and scratches are generated by particle sliding or rolling over the target surface and make a negligible contribution to material removal. The crater volume generated by the impact of individual particle is then discussed with respect to particle impacting velocity and impact angle. It shows that an increase in particle impact angle or particle velocity increases the crater volume due to the increased conchoidal fractures during the impact process.

Research on the Coarse Particles Velocity Characteristic in Fluidized Bed

In order to understand the particle velocity distribution characteristics of the 2D bubbling fluidized bed. The paper study that several mainly factors (static bed height, particle size and gas speed) influence on velocity distribution of the outer particles by experiment. The results for particles around bubble show that: ①Under the condition of the same static bed height and particle size, the particle velocity increases with the gas speed; the particle velocity increase at the axial direction and has no rule at radial direction. ②On the basis of the same wind speed and particle size, the particle velocity reduce with the static bed height; the particle velocity decreases at the axial direction and has no regular at radial direction. ③At the same wind speed and static bed height, the particle velocity is gradual reducing with the particle size; the particle velocity also reduce at the axial direction and has no rule at radial direction.

Acoustic properties of plates with unevenly distributed macroperforations backed by woven meshes

A hybrid model describing the acoustic properties of plates with macroperforations that can be unevenly distributed on the plate surface and backed by woven or precision woven meshes with microscopic perforations is proposed. The plate perforations may be of circular or rectangular shapes. Since the perforated plate may not necessarily be considered as an equivalent fluid, its impedance is calculated by the Maa model [Noise Control Eng. J. 29, 77-84 (1987)], whereas the Johnson-Champoux-Allard model [J. Appl. Phys. 70, 1975-1979 (1991)] is used for the mesh, considered as an equivalent fluid. A simple model for the elementary cell of the mesh structure is proposed in order to calculate parameters that can be considered as the thermal characteristic length Λ' and the viscous characteristic length Λ. An effective airflow resistivity is introduced to account for the increase of particle velocity through the mesh placed behind the carrying macroperforated plate and is used in the transfer matrix approach to obtain the impedance of the whole multilayer system. The hybrid model seems to represent a good approach of this multilayer system. The theoretical predictions are compared with experimental measurements.

The effect of evaporation kinetics on nanoparticle structuring within contact line deposits of volatile drops

This paper deals with the evaporation behaviour of nano-suspensions. An investigation of the relationship between reduced environmental pressure and the ring-stain formation mechanism is presented. Monodisperse, nanosuspension sessile drops formed a variety of patterns, at the macro-scale, consisting of sets of rings, concentric rings and irregularly distributed stains. “Stick-slip” motion of the triple line (TL), constantly pinned TL and very rapid evolution of the evaporation were the reasons for each of the observed patterns respectively. At the particle level (nano-scale), the promotion of close-packed, hexagonal particle structures was observed as a result of the enhanced evaporation rate and hence increased particle velocity. Beyond an optimal/critical pressure, a combination of further enhanced particle velocity with very limited space (for particles to find their most thermodynamically favourable positions) led to the formation of a disordered region at the exterior region and, as expected, to more defects. Lastly, inside the rings, particles formed random groups of particle aggregates, resembling branches, following TL motion during the “slip” phase. On the other hand, particle structures resembling ripples formed on top of a particle monolayer with increasing surface coverage as pressure was reduced. This was a direct result of fluid flow being too weak to reach the TL but strong enough to carry particles over smaller distances during the last stages of the evaporation process.

Study of the Influence of Particle Velocity on Adhesive Strength of Cold Spray Deposits

The adhesion mechanism of deposit/substrate interface prepared by the cold spray method is not fully understood at present. It seems that the adhesion strength is mainly determined by the mechanical (including the plastic deformation of particle and substrate) and thermal interaction between particle and substrate when the particles impact onto the substrate with a high velocity. In order to understand the adhesion mechanism, a novel adhesive strength test was developed to measure the higher bonding strength of cold sprayed coatings in this study. The method breaks through the limits imposed by glue strength in the conventional adhesive strength test, and it can be used to measure the coatings with a higher adhesive strength. The particle velocity was obtained with DPV-2000 measurement and CFD simulation. The relationships between the adhesion strength of deposits/substrate interface and particle velocity were discussed. The results show that stronger adhesion strength can be obtained with the increase of particle velocity. There are two available ways to improve the adhesion strength. One is to increase the temperature of working gas, and another is to employ helium gas as the working gas instead of nitrogen gas.

Electro-osmotic flows over highly polarizable dielectric surfaces

A thin-Debye-layer macroscale model is developed and analyzed for electrokinetic flows about dielectric surfaces, wherein solid polarization modifies the zeta-potential distribution. The harmonic electric potential within the solid is governed by a nonlinear boundary condition, which constitutes a generalization of the linear Robin-type condition of Yossifon et al. [Phys. Fluids 19, 068105 (2007)] to voltages comparable with the thermal scale. The resulting polarization model is demonstrated in the classical context of spherical-particle electrophoresis, where the electrophoretic mobility—now a function of applied-field magnitude and solid permittivity—is evaluated using both eigenfunction series expansions and asymptotic approximations. For strong polarization, the mobility saturates at a field-dependent value which is lower than the comparable Smoluchowski slope. At strongly applied fields, the mobility diminishes at a rate that corresponds to a logarithmic increase of particle velocity with applied-field magnitude.

Reversed Cherenkov radiation in a waveguide filled with anisotropic double-negative metamaterials

The physical properties of Cherenkov radiation (CR) are theoretically investigated for a charged particle traveling along the axis of a cylindrical waveguide filled with anisotropic double-negative metamaterials (DNMs). The reversed CR and CR conditions are obtained using analytical method. The influence of the particle velocity, the waveguide radius, and the constitutive parameters of the anisotropic DNMs are discussed. A numerical example illustrates that the total radiated energy increases with increasing particle velocity, the radiated energy spectral density has different poles at the different frequencies for different anisotropic DNMs when the loss of the anisotropic DNM is smaller, and when the radius has the same order as the operating wavelength, the influence of the waveguide radius on the total radiated energy is smaller on the whole. Since most of the metamaterials realized so far are anisotropic, our theoretical work based on anisotropic DNMs will be helpful for future experimental realizations.

A stochastic coating model to predict the microstructure of plasma sprayed zirconia coatings

A stochastic model has been developed to simulate the formation of thermal sprayed yttria-stabilized zirconia coatings. Porosity in the ceramic coating was assumed to be due to incomplete filling of cavities on the rough surface of the coating. Values of thermal spray particle size, velocity, temperature and trajectories were generated assuming they followed known distributions, with means and standard deviations measured experimentally. Predictions from the model of porosity and roughness of zirconia coatings agreed with experimental results, accurately predicting a reduction in porosity with particle size. The model also predicted that porosity, thickness and roughness would decrease with increasing particle velocity. Increasing spray-gun standoff distance had little effect on porosity (keeping particle velocity constant), while it decreased coating thickness and roughness.

Reduction in Temperature Difference of Prime Mover Stack in Loop-Tube-Type Thermoacoustic Cooling System by Applying Phase Adjuster

The use of a phase adjuster (PA) is proposed to reduce the temperature difference between the upper and lower ends of the prime mover stack in a loop-tube-type thermoacoustic cooling system. PA is a device for increasing particle velocity. By placing PA in the loop tube, the resonance is forcibly one wavelength. It is confirmed that the temperature difference of the prime mover stack with PA is smaller than that without PA. Because PA prevents the resonance wavelength shift from one-wavelength resonance to two-wavelength resonance, the loop tube suppresses the increase in temperature at the prime mover stack, so that the viscous boundary layer thickness is smaller than the stack channel radius. This induces an effective energy conversion. Consequently, the temperature difference is reduced.

Direct numerical simulation of particle collisions in two-phase flows with a meshless method

The element-free Galerkin (EFG) method was used to simulate particle motion in two-phase flows with a new node distribution arithmetic suitable for meshless methods. The control equations were discretized with the standard Galerkin method in space and a fractional step finite element scheme in time. Regular background cells were used for the quadrature. The forces of the fluid on the particles were obtained by integrating the stress and shear forces on the particle surfaces. Simulation of the movement of a particle in a channel showed the Karman vortex street forming behind the particle with increasing particle velocity. Simulation of the movement of two particles in a channel showed the well-known drafting, kissing and tumbling, which has been obtained experimentally. Multi-particle flows were simulated with the results showing that meshless methods are capable of dealing with real particle collisions, which makes meshless methods superior to all mesh-based methods. Moreover, the theoretical relation of interparticle collision rate derived using kinetic theory was compared with the present numerical results and it is found that the collision rate is much lower than the theoretical estimation based on kinetic theory.

Size Shifts in Measurements of Droplets with the Aerodynamic Particle Sizer and the Aerosizer

Observations of the size of liquid droplets using the Aerodynamic Particle Sizer (APS) and the Aerosizer indicated that the measured size was significantly different from the aerodynamic diameter as calculated by measuring droplet settling velocity. The size shifts appeared to be caused by droplet distortion in the detector flow field for the Aerosizer. However, for the APS, droplet sizing was further affected by droplet impaction on the upstream side of the focusing nozzle. It is suggested that liquid accumulated in and constricted the nozzle, resulting in a particle velocity increase at the sensor. The size shift can occur with the deposition of < 0.5 μ L liquid onto the nozzle; the size shift can occur in 1–10 minutes even at concentrations of 1000 particles/L; and the size shift can disappear after cessation of liquid aerosol sampling. CFD calculations provided information about the amount of velocity increase at the APS sensor for a selected constriction. Both solid and liquid particles are affected ...

Reactive nanojets: Nanostructure-enhanced chemical reactions in a defected energetic crystal

Nanofluidics of chemically reactive species has enormous technological potential and computational challenge arising from coupling quantum-mechanical accuracy with largescale fluid phenomena. Here, we report a million-atom reactive force field molecular dynamics simulation of shock initiation of an energetic crystal with a nanometer-scale void. The simulation reveals the formation of a nanojet which focuses into a narrow beam at the void. This, combined with the excitation of vibrational modes through enhanced intermolecular collisions by the free volume of the void, catalyzes chemical reactions that do not occur otherwise. We also observe a pinning-depinning transition of the shock wave front at the void at increased particle velocity and the resulting localization-delocalization transition of the vibrational energy.

On high velocity impact of micro-sized metallic particles in cold spraying

In this study, a systematic examination of particle deformation behaviour in cold spraying was conducted for Cu particle using both the Lagrangian and Arbitrary Lagrangian Eulerian (ALE) methods. It is found that the meshing size in modelling by Largrangian method influences significantly the localized shear instability at interface areas. With refining the meshing size the onset velocity for interface shear instability decreases. The extrapolation of these results yields a reasonable critical velocity comparable to the actual one in cold spray practice. The results indicate that both the flattening ratio and compression ratio of the deformed particles increase with the increase in particle velocity, which are in good agreement with the experiment results. The ALE method provides a suitable way to examine the particle deformation in cold spraying. Moreover, the numerical results also show that there exists the similarity for the deformation of particles of different diameters.

FeAl and Mo–Si–B intermetallic coatings prepared by thermal spraying

FeAl and Mo–Si–B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe–24Al (wt%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 to 700 m/s. Mo–13.4Si–2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo–Si–B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity. For Mo–Si–B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and α-Mo. The T1 phase was retained after spraying at 350 m/s.