In this paper, the phenomenological behaviour of gas flow and particles motion during cold spraying has been studied. Observations of particles behaviour show two features: a uniform jet over a short distance ahead of the nozzle exit and then, a progressive dispersion. These behaviours are explained using a computational analysis based on a direct numerical simulation of the gas flow and the kinematic interactions with the particles. The CFD computation demonstrates that the gas stream starts to be unstable inside the nozzle with more turbulence as it moves towards the exit of the nozzle. The flow is self-oscillated along the flow direction and drives the motion of the Cu particles outside the nozzle. The zone of gas flow instability does correspond to the zone of experimental particle dispersion. Outside the nozzle, the particles form a straight jet over a certain distance that corresponds to the zone of the experimental uniform particles jet. Then, they are deviated and become more and more dispersed towards a very sparse jet along the flow direction. This phenomenon is explained by a Magnus lift force that deviates the particles trajectory when the gas flow becomes highly turbulent while developing a vorticity shedding.

A model of the shock-wave induced spray process (SISP) and the criteria for bonding are used to predict whether particles traveling within the unsteady flow regime will adhere to the substrate upon impact. The results are then used to predict if a coating can be formed under specific spraying conditions. Having been validated based on particle velocity measurements, the model is used to investigate the effect of varying spray parameters, such as powder and gas initial temperature, gas heater length, and spray frequency.

This paper presents a new strategy for improving the quality of HVOF sprayed coatings as well as the deposition efficiency of the process. The highly turbulent expanding gas jet is stabilized and focused with the aid of a helical gas shroud. The effect of the design modification is demonstrated by numerical calculations and through the use of a prototype torch.

In order to find the size and shape of particle detachments in the wire-arc spraying system, it is required to find the shear force on the electrodes (wire-tips). To solve the gas flow problem, the system was divided into two regions: (1) upstream and (2) downstream of the nozzle exit. In region 1, the axisymmetric turbulent gas flow is numerically solved and compared to its approximate analytical solution. In the second numerical region, which includes the tips of the two wires, shear force on the electrode surfaces is evaluated from the gas flow velocity profile adjacent to the electrodes. The gas flow in these regions is solved using the FLUENT software. Abstract only; no full-text paper available.

This paper presents the numerical simulation of the plasma flow into a dense atmosphere. The plasma generation is performed by the simple model developed by Eichert. Two models are exposed to take in account the arc fluctuation inside the anode. They permit us to simulate plasma puffs convected into the flow. The aim of this study is to compare these two models with experiments and to determine which one is the most relevant.

Various turbulent models (Spalart-Allmaras, Standard k-ε, RNG k-ε, Realizable k-ε and Reynolds Stress Models) along with Standard, and two-zonal wall functions are used to simulate inductively coupled plasma flows. The computational results can be classified into two categories: All the turbulent models that include low Reynolds number effects, such as, Low Reynolds number k-ε model, Spalart-Allmaras one-equation model, Standard k-ε model with two-zonal wall function, RNG model with turbulent viscosity determined by a differential equation, RSM etc., give similar modelling results. These models predict almost the same temperature contours which are similar to the one predicted by laminar model. The viscosity ratios in plasma region predicted by these models are very close to zero except for in the wall-neighbouring cells, which means the plasma flow is almost laminar. The other category contains those models that do not include the low Reynolds number effects, such as Standard, RNG and so-called Realizable k-ε models with standard wall function. They predict the plasma flow to be turbulence-dominated. In comparison with the results of experimentally measured heat fluxes to a substrate, the heat fluxes predicted by these models that include low Reynolds number effect are very close to experimental measurements while these models that do not include low Reynolds number effects deviate greatly from experimental measurements. It is found that the Reynolds stress model(RSM) appears to be the best predictive model.

This paper presents a detailed numerical analysis that shows why Laval nozzles are used for vacuum plasma spraying and how to optimize their design to achieve a uniform plasma jet free of expansion waves under different spraying conditions. A Laval nozzle prototype is tested against a commercially available conical nozzle and is found to deliver a longer core jet with less turbulent scattering. Paper includes a German-language abstract.

The industrial flame spraying process has been analyzed by three-dimensional Computational Fluid Dynamics (CFD) simulation. The actual process is employed at the Volvo Aero Corporation for coating of fan and compressor housings. It involves the Metco 6P gun where the fuel, a mixture of acetylene and oxygen, flows through a ring of 16 orifices, while the coating material, a powder of nickel-covered bentonite, is sprayed through the flame with a stream of argon as a carrier gas by a central orifice. The gas flow was simulated as a multi-component chemically reacting incompressible flow. The standard, two equations, k-e turbulence model was employed for the turbulent flow field. The reaction rates appeared as source terms in the species transport equations. They were computed from the contributions of the Arrhenius rate expressions and the Magnussen and Hjertager eddy dissipation model. The particles were modeled using a Lagrangian particle spray model. In spite of the complexity of the system, the complex geometry and the numerous chemical reactions, the simulations produced fairly good agreement with experimental measurements. The powder size distribution was found to play a critical role in the amount of unmelted fraction of particles. The modeling approach seems to give a realistic description of the physical phenomena involved in flame spraying, albeit some model refinement is needed.

An analysis of a d.c. plasma jet is presented using a three-dimensional commercial fluid dynamics code, ESTET. This code solves the coupled conservation equations of mass, species, momentum and thermal energy equations for a compressible and turbulent fluid in control volume and finite difference formulation. Computations take into account fluid turbulence using a standard k-s model with the Launder and Sharma correction for the laminar zones, e.g. the plasma core. Two series of spraying conditions differing in the total gas flow rate (30 and 60 slm) and the arc current (300 and 600 A, respectively) are computed. The process parameters are independently varied about the nominal operating conditions. The effect of the variation of primary and secondary gas flow rate, effective power and powder carrier gas flow rate on flow fields characteristics, is discussed.