Search Results for particle tracking velocimetry

In this study, two particle-tracking velocimetry systems, the Oseir HiWatch HR2 and the Oseir HiWatch CS2 were tested. The particle-sizing velocimetry results were then compared with corresponding particle characteristics obtained using a Tecnar Cold Spray Meter and a Tecnar Accuraspray 4.0 unit. In addition, particle size distributions measured by laser diffraction were included for validation.

The advantages of the solid state deposition process Cold Spray (CS) over conventional spray technologies go hand in hand with the requirement of high and well-predictable particle velocities. The acceleration of particles primarily takes place within the CS-nozzle while measurements of their velocity are conducted downstream of its exit. Despite their essential value, these observations are limited, in that only the result of the acceleration can be evaluated, not the actual driving mechanisms themselves. Previous work has indicated that there is no conclusive understanding of these mechanisms, especially in cases of increasing particle loading. This study therefore presents a transparent rectangular CS-nozzle design (made out of quartz) for a low stagnation pressure regime. A novelty to the field of thermal spray is the first report of particle in-flight measurements within the CS-nozzle using Particle Tracking Velocimetry (PTV) at varying particle loadings and pressure levels. It is found that particle velocities in the jet decrease with increasing particulate loading as the momentum exchange of the gas is enhanced, while in the subsonic flow region, the average velocity level increases due to particle-particle interactions with shallower axial velocity profiles. This effect is aggravated for higher working pressures, as energetic collisions cause increasing losses, depending on the number density of particles. This study forms the basis for a comprehensive nozzle-internal analysis.

In this study, particle image velocimetry (PIV) is used to measure WC particle velocity during HVAF spraying. Measured velocities are compared with calculated velocities obtained using open source CFD software. Numerical simulation is also used to investigate particle temperatures. With the HVAF gun used, maximum particle velocity is reached around 18 mm from the nozzle exit with a corresponding gas temperature of 1400 K.

The image-based measurement method 2D Continuous Particle Image Velocimetry (2D Continuous PIV) is commonly used for measuring particle velocities in thermal spraying processes due to its basic measurement setup and its large measurement volume compared to methods based on point sensors. The accuracy of such image-based measurement techniques depends on the measurement algorithm, the process environment, such as distributions of particle characteristics, and the error of the imaging system. However, in the case of 2D measuring, accuracy might also depend on the fact that 2D methods measure only two of the three velocity vector components while ignoring the third component. In this paper, the impact of measuring only two of the three velocity vector components on the accuracy of a closed-source 2D Continuous PIV algorithm is investigated. The analysis is based on a virtual measuring instrument that includes optical aberrations of the imaging system and it is shown that this error contribution of measuring only in 2D is within acceptable limits for typical thermal spraying processes.

Modern thermal spray processes require wide use of diagnostics to gather an extensive process understanding. Today's diagnostic results provide the basis for future designs and advancements, particularly and increasingly on basis of computational approaches. Due to the measuring area in the square centimeter range and its quick and accurate results, the Particle Image Velocimetry (PIV) represents an enriching for thermal spray process diagnostics. Our experimental results obtained from PIV are in accordance to present theoretical and empirical derivations of some kinematic parameters of thermal spray processes. Beneath verification of well-known dominant parameters (for example powder fraction or carrier gas mass flow), this procedure enables the detection and characterization of ancillary influences on the process due to its high accuracy. By statistical analysis of our experiments, using multiple parameter variations per experiment according to the technique of "Design of Experiments" (DoE), we possibly found some hints on interactions between ancillary parameters which shall be analysed in further works carefully. In combination with detailed simulations on plasma - particle interactions and powder injection it should be possible to develop methods for thermal spray processes to minimize the particle flow expansion for an optimization of deposition rate and energy efficiency in the future.

This study investigates cold spray particle velocities achieved at different pressures and particle feed rates using particle image velocimetry (PIV). Particle dispersion and velocity evolution along the jet axis were investigated for several feedstock materials. It was found that average particle velocity decreases with increasing particulate loading. The effect is aggravated at lower pressures, but mainly depends on feedstock material, which implies more complex, volume-fraction related physics playing a role. Velocity distribution and particle dispersion were also found to be influenced by particle feed rate, depending on the material. Increased particle feed rates affect the magnitude and distribution of impact velocity and consequently the efficiency of cold spraying.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

The present work demonstrates the possibilities of atmospheric air plasma spraying. It presents various approaches to the development and optimization of spraying modes using diagnostic equipment and provides examples of industrial approbation of the technology.

This study evaluates laser-based diagnostic methods that can be used to control the injection of liquid feedstocks into a plasma jet and to monitor the size and velocity of particles and droplets in different zones. It demonstrates the capabilities of shadowgraphy and particle image velocimetry and investigates the spraying characteristics of different liquid feedstocks and solvents. Suspension plasma spraying examples are presented in which variations in droplet-particle velocity and diameter are measured in different areas of the jet.

In cold spray (CS) additive manufacturing process, micrometer scale particles accelerated through a supersonic nozzle are targeted on a surface with velocities in the rage of 300-1500 m/s in solid state. The impact energy of the particles leads them to deform plastically with high shear energy near the impact interface and adhere to the surface metallurgically, mechanically, and chemically. Using CS, deposition of metals, metal matrix composites, and polymers are achieved with high adhesive/cohesive strength and low porosity. Sensitivity of the CS additive manufacturing process to the variabilities in the process parameters are still being understood. Among the process parameters, particle morphology can have significant implications on drag forces, and therefore, on the particle impact velocity. This in turn affects the deposition efficiency (DE) and the quality of products. In this work, a new approach is introduced for computing DE by incorporating particle sphericity and its variation into one-dimensional numerical models. Size, sphericity, and the variability of size and sphericity of aluminum, copper, titanium, and tantalum particles are measured from static optical microscope images. The data is used for predicting impact velocity, temperature, and DE. The model results are then compared with particle velocity measurements.

In thermal spray processes, the characteristics of in-flight particles (velocity and temperature) have a significant effect on coating performance. Although many imaging systems and algorithms have been developed for identifying and tracking in-flight particles, most are limited in terms of accuracy. One key to solving the tracking problem is to get an algorithm that can distinguish different particles in each image frame. As the study showed, when noise and interference are treated, particles are more readily identified in the background, leading to more accurate size and position measurements with respect to time. This approach is demonstrated and the results discussed.

The generation of a high velocity carrier gas flow for cold metal particle applications is addressed, with specific focus on titanium cold spraying. The high hardness of this material makes cold spraying titanium difficult to achieve by industry standard nozzles. The redesign of a commercial conical convergent-divergent cold spray nozzle is achieved by the application of aerospace design codes, based on the Method of Characteristics, towards producing a more isentropic expansion by contouring the nozzle walls. Steady three-dimensional RANS SST k-ω simulations of nitrogen are coupled two-way to particle parcel tracking in the Lagrangian frame of reference. The new contoured nozzle is found to produce higher particle velocities with greater radial spread, when operated at the same conditions/cost of operation as the commercial nozzle. These numerical results have shown the potential for extending cold spray to high density and low ductility particles by relatively minor rig modifications, through an effective synergy between gas dynamics and material science.

This paper presents research highlights obtained over the past three years in the course of a DFG-funded project on new and emerging diagnostic methods for thermal coating. It describes the tools and techniques used, the particle and substrate variables monitored, the accuracy of each measurement, and various associations with coating properties. Paper text in German.

Asymmetric melting behaviour of the electrodes is a process related feature of the twin wire arc spraying (TWAS) technique since the heating of the negative connected wire is different from that of the positive connected wire. Due to these differences in melting behaviour a tracking of particle velocity and temperature for both electrodes individually is very important. Particle velocity and temperature have been recorded from anode and cathode by positioning the tracking device on each side of the spraying gun. To draw the whole picture of the spraying jet particles have been tracked also from the top side of the spray gun. The goal of this study is to have an experimental data set-up for model building and simulation of depositing process in TWAS. Corresponding measuring devices have been employed to investigate the TWAS process by spraying of massive and cored wires.

This study evaluates the effect of hammer peening on the wear behavior of cermet coatings. WC-FeCMnSi coatings were produced by twin wire arc spraying and post-treated on a five-axis machining center equipped with pneumatic peen. The surface topography of the peened coatings was examined and compared to as-sprayed and polished samples. Dry sliding friction and abrasive wear tests showed that the treated coatings had lower friction coefficients, but were less wear resistant than non-treated samples. Likewise, strain hardening effects revealed by nanoindentation testing were offset by process-induced cracking of embedded carbides, which contributes to break-outs and third-body wear.

This study assesses the effect of machine hammer peening (MHP) and carbide grain size fraction on the friction and wear behavior of arc-sprayed WC-W 2 C FeCMnSi coatings. SEM examination shows that post-treatment by MHP compresses the coating, reducing both thickness and porosity, particularly in coatings with ultrafine carbides. The treatments also cause cracking, however, especially in carbide phases. Ball-on-disk tests were carried out on as-sprayed and treated samples to determine sliding wear and friction properties, and dry sand rubber wheel tests were used to evaluate abrasion resistance. SEM and EDX analyses before and after wear testing show how coating microstructure and grain size correlate with the friction and wear test results obtained and the given surface treatments.

In cold spray, optimum process conditions to accelerate particles vary with different densities and melting temperatures of the materials. Therefore, material-specific nozzle designs are required. In the present study, a nozzle geometry optimization concept based on 3D-CFD simulations was developed to provide a specific nozzle design for a given material. Al6061 and pure copper with mean particle diameters of 40 μm were taken as examples. Together with a design of experiments (DoE) approach, the model seeks for the optimal nozzle geometry. In order to reach the highest particle velocity prior to impact upon the substrate, different geometry parameters were varied, such as the nozzle throat cross section, the aspect ratio, and the nozzle divergent section length. The process gas was nitrogen with set stagnation pressure and temperature of 50 bars and 500 °C. For both materials, the simulation identified nozzle divergent section length as the most influential parameter, followed by the throat cross-section. The aspect ratio must be tuned to avoid over expansion of the gas in the nozzle.

The two-phase flow properties of copper particle laden nitrogen are measured and compared to computational fluid dynamic calculations, determining the achievable degree of consistency between model and reality. Two common, commercial nozzles are studied. A two-way coupled Lagrangian scheme along with the RSM turbulence model is used to track the particles and to model the interactions between the gas and the particulate phase. Significant agreement is found for the geometrical gas flow structure, the resulting particle velocities, and the dependence of the two-phase flow on the particulate phase mass loading. The particle velocities decrease with increasing mass loading, even for modest powder feed rates of less than 3 g/s. The velocity drop occurs even when the gas flow rate is kept constant. Adiabatic gas flow models neglecting the energy consumption by the particles are thus inaccurate, except for very dilute suspensions with low technical relevance. For the cases modeled, the experiments evidence the high predictive power of the chosen approach.

In cold spray, 5-150 μm particles (of metal, ceramic, composite, and other materials) are accelerated to supersonic velocities through a deLaval nozzle with an inert gas (generally He or N 2 ) that can reach 1000 °C. In the process, the gas jet impingement on the target and the extreme plastic deformation of impacting particles cause heat generation in the coating layers and the substrate. The heat generation has been argued to cause residual stress, which may cause coating-substrate delamination. In this study, heat generation due to gas impingement and particle plastic deformation has been predicted from CFD and FEA simulations, respectively. Furthermore, a finite volume method has been presented for transiently simulating the coating buildup and bulk heat generation in the coating and the substrate. The model is intended to assist researchers understand thermal affects in the coating process and help design more informed coating patterns to reduce negative thermal effects.

Although the HVOF process has shown to be a technological alternative to the many conventional thermal spray processes, it would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow coating of ceramic powders. Based on a numerical analysis, a new nozzle was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the new nozzle was investigated by spraying several coating materials including metallic (Nickel, MCrAlY, Stainless Steel), carbide (WC-Co), and ceramic (Al 2 O 3 ) powders. Particle spatial distribution, velocity, and temperature corresponding to the new nozzle and the standard HVOF gun were compared. The new nozzle provides a superior particle spatial distribution, as well as higher and more uniform particle velocity and temperature.