A numerical investigation of fluid flow and heat transfer through thermal spray formed metal foam heat exchangers is presented. Experimentally obtained fluid flow and heat transfer parameters are used in the simulations. Analytically obtained values of effective thermal conductivity are used to model heat transfer. A 3D CFD model was created for a metal foam heat exchanger with a square cross-section. The external walls were deposited on the foam using a wire-arc process. The channel walls of the foam were exposed to a constant temperature of 400 K and an air flow with an inlet velocity of 2 m/s. The model was verified by comparing sample results to experiments. The effect of the foam on heat transfer was then studied by varying thermal conductivity values.

In this study, the effect of the substrate roughness and thickness on the heat transfer coefficient of the impinging air jet upon a flat substrate was investigated. A low-pressure cold spraying unit was used to generate a compressed air jet that impinged on a flat substrate. A detailed mathematical model was developed and coupled with experimental data to determine the heat transfer coefficient and surface temperature of the substrate. It was found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. It was further found that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The results of the current study were aimed to cover the influential substrate parameters on surface temperature of the substrate that eventually can affect the final quality of the cold-sprayed coatings.

The paper reports numerical simulation results of a direct current (DC) suspension plasma spray with an axial injection system. In the numerical modelling, a two-way coupled Eulerian-Lagrangian approach was employed to simulate plasma flow and suspension behavior. As effects of two-way interaction, momentum transfer and energy transfer were chosen. The plasma spray was assumed to have a rotationally symmetric, two-dimensional shape around the injection axis of suspensions (the central axis), and therefore the axisymmetric two-dimensional approximation was adopted in the numerical modelling. Working gas was pure argon and was supplied from both the anode-side and the cathode-side ports. A total volume flow rate of the working gas from anode and cathode was set to 46.5 slm. A feed rate of suspension was parametrically set to 0, 15, 25, or 35 g/min. Numerical results indicate that the temperature of a plasma hot-core region and the velocity of a plasma jet around the central axis drop more with increasing feed rate of suspension mainly because of a decrease in Joule heating with increasing it. The numerical results also suggest that the increase in feed rate of suspension leads to practically lengthening flight distance of suspensions required for completing evaporation process of disperse medium.

Time-resolved thermographic measurements were performed by a high speed infrared camera on a substrate surface during cold gas dynamic spraying without powder particles. Experiments were carried out on three commercial cold spray systems (Centerline SST, CGT Kinetiks 3000 and Inovati KM-CDS 2.2) spanning a wide range of gas pressures, gas temperatures, gun traverse speeds and stand-off distances. The substrate surface temperatures were measured directly beneath the cold spray gun nozzle as well as at predetermined distances from the gun giving a general evaluation of the heat input to the substrate in actual deposition conditions. It is shown that the substrate surface temperature can rise to a value close to the inlet gas stagnation temperature in the area located beneath the cold spray gun (bow shock region). For the same set of process conditions, significant differences were found in measured surface temperature range between the three cold spray systems, due to differences in nozzle configuration.

This paper presents a new approach for computer aided generation of optimized trajectories in thermal spray systems. Through a combination of CFD and FEM modeling, the influence of torch trajectory and speed profile on heat and mass transfer during deposition is assessed along with its effect on residual stress and coating properties. Coating experiments with WC-Co as the spray material were performed on real components in order to validate the developed programming and simulation tools.

The plasma spray deposition of a zirconia thermal barrier coating (TBC) on a gas turbine component has been examined using analytical and experimental techniques. The coating thickness was simulated by the use of commercial off-line programming software. The impinging jet was modelled by means of a finite difference elliptic code using a simplified turbulence model. Powder particle velocity, temperature history and trajectory were calculated using a stochastic discrete particle model. The heat transfer and fluid flow model were then used to calculate transient coating and substrate temperatures using the finite element method. The predicted thickness, temperature and velocity of the particles and the coating temperatures were compared with these measurements and good correlations were obtained. The coating microstructure was evaluated by optical and scanning microscopy techniques. Special attention was paid to the crack structures within the top coating. Finally, the correlation between the modelled parameters and the deposit microstructure was studied.

Experimental apparatus simulating a horizontal belt caster has been constructed for the study of thin strip casting of steels and light metal alloys. In this apparatus, the solidifying metal is deposited onto a moving substrate. The substrate was flame sprayed with various commercial coatings while its speed and the thicknesses of strip produced matched industrial values. The main objective of the present work was to determine the influence of various operational variables on local cooling rates and final microstructures. To this end, experiments were carried out to study the effects of various types of coating, roughness of the substrate, initial superheat, and strip thickness on heat fluxes. An interesting feature of this equipment is that the strip is subjected to different rates of cooling at the lower and upper surfaces, allowing two different rates of solidification to be studied simultaneously.

Effect of substrate temperature and ambient pressure of deposition chamber on contact condition between free falling droplet and substrate surface were investigated. Cu droplet and SUS304 substrate were used for the experiment. To estimate the contact condition, temperature history of molten droplet was measured at splat-substrate interface by thermocouples embedded in the substrate and high frequency data logging system. Quantitative porosity on splat bottom surface and degree of circularity of splat shape was evaluated. Splat adhesion strength experiment was conducted and dynamic wetting behavior of substrate by molten droplet was captured by ultra-high speed video. Heating substrate and vacuuming chamber pressure enhanced the heat transfer from splat to substrate, which can be attributed to the good contact at splat bottom surface. Adhesion strength of the splat to the substrate corresponded well with degree of circularity. Consequently, substrate temperature and ambient pressure of deposition chamber have an equivalent effect to contact condition at interface between droplet and substrate surface.

Analysis of melting mode of core wire at Arc Spraying helped to explain a possibility of incomplete fusion of a charge of a core wire. Influence of physicochemical parameters of feedstock and atomization modes was established. Analysis results helped to develop a metastable austenite type Core Wire for Arc Spraying of good wear resistance. Applications results are shown.

The present work deals with numerical simulations based on a computational heat transfer model for spherical composite particles typically used under plasma conditions. Results describe heat transfer in mono and two layers steel/alumina particles immersed in an uniform infinite plasma. Time dependent behaviours interacting with phase change occurrence and taking into account the contact quality between the two layers are considered.

A numerical method and software were developed to predict non-stationary conjugated conductive heat transfer, melting and possible evaporation of materials under high energy fluxes impinging onto solid body surface (plasma jet, arc spot, laser or electron beam), and also subsequent cooling and solidification of the melted substrate layer. In the numerical procedure, the finite-element method was employed. The processes of interest can have different characteristic time and spatial scales, which in addition can suffer drastic changes at heat flux densities q ?[108 ;1014 ] W/m2. An advanced procedure was developed to enable dynamic adaptive triangulation of domains involved in the current numerical solution and characterizing the different phase states (liquid or solid) of the materials. This procedure, belonging to the class of the frontal algorithms, allows one to break a solution domain into triangles based only on the domain boundaries. The model applications of the developed simulation software are illustrated.

As the initial step of a process control design for a substrate surface temperature control solution for Vacuum Plasma Spraying (VPS), the distribution of heat transfer coefficient h between impinging plasma gases and the substrate coating surface is required. An embedded calorimeter approach was used to measure the distribution of the convective heat transfer coefficient over the surface of a substrate and the resulting distributions have been compared with empirical correlations and CFD model predictions of the plasma jet/substrate interaction, as a function of VPS process parameters.

Structured surfaces are needed for improving the heat transfer in a lot of industrial processes. The paper deals with the production and the characterization of coatings with rough and porous surface. Structured coatings of inconel or copper are deposited on copper tubes by means of radio-frequency vacuum plasma spraying. The microstructure as well as the surface roughness of the coatings are investigated. Boiling experiments are carried out on the coated tubes to measure their heat transfer coefficient. The results show that the coated tubes exhibit improved heat transfer values in comparison to smooth tubes. The enhancement ratio can reach more than 20 for inconel coatings.

The study of heat transfers in a substrate exposed to an impinging plasma jet is proposed using two different software products. Thermal exchanges between the plasma jet and the substrate were first calculated using the PHOENICS CFD software in which a two-layer extension to the Chen-Kim k-s model was implemented in order to consider both the turbulent nature of the plasma jet and heat transfer phenomena through the viscous sub-layer formed at the surface of the substrate. The model is supposed to provide accurate predictions of thermal exchanges. However this preliminary step is not described since it is part of some previous studies. In a second step, two different commercial software products are used to perform three dimensional transient calculations of the heat conduction inside the substrate. The first approach consists in the use of the finite element based SYSWELD software whereas the second one consists in the use of the finite volume based PHOENICS software. Numerical results are presented and compared for the case of an impinging plasma jet displacing linearly on the substrate. Additionally, the influence of different parameters such as the substrate sample thickness, the stand-off distance, the displacement velocity or the nature of the substrate is also discussed. The results show a good accordance between numerical predictions obtained using the two methods concerning the maximum temperature observed. These results are useful since the substrate temperature is known to have an important influence on the coating adhesion and properties.

The laser-flash method is used to determine the thermal diffusivity of HVOF sprayed WC-Co(Cr) and Cr 3 C 2 -Ni20Cr as well as APS sprayed Cr 2 O 3 and electroplated hard chromium coatings in the temperature range between RT and 600°C. Additionally bond and/or corrosion protective coatings like Ni5Al, Ni20Cr and 316L are characterized taking into account the different manufacturing methods twin wire arc spraying, HVCW and HVOF. With respect to the application example of drying rollers in paper industries the Taber-Abraser wear test is applied to evaluate the wear resistance. Finally the coatings are characterized concerning their corrosion resistance by salt fog test and by exposure to humid SO 2 environment. For WC-CoCr feedstock the effect of carbide size and micro hardness on thermal, wear and corrosion properties are studied. WC-CoCr coatings with maximum micro hardness and fine carbides show the best thermal conductivity. The use of coarse carbide feedstock permits manufacturing of coatings with the highest resistance against dry abrasive wear, but the protective function depends severely on the processing conditions.

There is a variety of components, which are subject to high wear and/or corrosion stress on the one hand and are used to transfer heat on the other hand. Two examples are drying cylinders in paper production and condensing boilers. Up to now there are no data available for the thermal design of thermal spray coated components except for some MCrAlY and thermal barrier coatings for turbine applications. Also guidelines for the optimization of thermally sprayed coatings concerning heat transfer including the effect on the wear resistance are missing. HVOF sprayed cermet coatings are widely used for combined wear and corrosion protection these days. In addition to WC-CoCr 86 - 10 4 and 75 Cr 3 C 2 - 25 Ni20Cr conventional Ni5Al and Ni20Cr bond coats are evaluated concerning their thermal conductivity in the range between room temperature and 600 °C. Also the thermal contact resistance is determined depending on the substrate material: mild steel S355J2G3 (1.0570), grey cast iron GG25 (0.6025) and austenitic stainless steel X5CrNi18-10 (1.4301, AISI 304). The applied Laser- Flash method requires knowledge of the heat capacity, thermal expansion and density, which are determined before. HVOF spraying has only negligible influence on the heat capacity of WC-CoCr feedstock, as the temperature depending functions are almost identical. The use of spraying feedstock with average WC particle sizes of 800 nm, 3 µm and 5 µm permits to investigate the influence of the specific surface area of the hard phases both on the thermal conductivity and wear resistance. Furthermore the influence of the coating porosity is determined. In accordance to the drying cylinder application the wear resistance is determined by Taber-Abraser wear tests. Bond coats are produced by HVOF, HVCW and arc spraying and compared concerning microstructure and thermal conductivity. A comparison to the properties of electroplated hard chromium coatings is drawn.

A mathematical model is developed to analyze thermo-physical phenomenon within an individual solution precursor droplet travelling in the high temperature plasma jet. The model involves the motion and evaporation of the droplet and the heat and mass transfer within the droplet. The influence of Stefan flow and the variable physical properties of the environment gas around the droplet are considered. The internal circulation due to the relative velocity between the droplet and the plasma jet, which can be described by the Hill’s spherical vortex, is considered as well. The trajectory, temporal droplet surface temperature and radius variation are predicted. The temperature and concentration distribution inside the droplet as well as the difference between the droplets with different injection parameters are presented.

This paper discusses the challenges of constructing mathematical models of physicochemical and heat-mass transfer processes associated with reactive heterogeneous materials used in laser additive manufacturing. The results of calculations of thermocapillary convection induced by laser heating in an aluminum melt with an admixture of nickel particles are presented. Models of interphase and chemical interactions with the formation of intermediate phases and intermetallic compounds on nickel particles added to the melt during laser alloying or cladding are proposed, which make it possible to calculate the composition of intermetallic phases in the trace of the beam after crystallization and cooling.

Recently the research about technical ceramics knows a renewed interest. The synthesis process of nanopowders plays a significant role in the production of such materials. This work, concerns particularly the synthesis of the nanopowders by laser pyrolysis. A numerical model is developed in order to understand the influence of the heat transfers occurring in the reactor and around the reaction zone, for a better control of the process. The study highlights the importance of the thermal transfer in the cell. A simple one dimensional analysis enabled us to define a stability condition of the flame function of the precursor flow rate and the cell pressure. The extended 3D calculations underline the complex fluid flow and heat transfer developing in the synthesis reactor.

PTA (Plasma Transferred Arc) reclamation of aluminum alloys by hard materials with a much higher melting temperature is very difficult. This is due to the high thermal diffusivity of these al1oys. Below a critical heat flux φc nothing happens and over φc the substrate melts very rapidly contrarily to what is observed with steel substrates. That explains probably why PTA is mainly used for steel reclamation. Thus the knowledge of heat flux transferred to the anode is a critical point to develop PTA reclamation on aluminum alloys and this is the aim of this paper. An experimental set-up was built to study the heat transferred to three substrates made of different materials : cast iron for reference, aluminum alloy and copper for its high thermal conductivity. The plasma torch was a Castolin Eutectic gun and allowed to inject a sheath gas around the plasma column. The copper, aluminum alloy and cast iron substrates, easily interchangeable, were the top of a water-cooled calorimeter allowing to determine the variation of the received heat flux with the working parameters : arc current, stand off distance, plasma forming gas momentum, sheath gas composition and momentum. The determination of the arc electric field allowed to calculate the arc diameter which was compared first with pictures taken with a video camera and second, with wear traces left on the anode material. Several correlations have been established to characterize the arc voltage and the anode heat flux.