Gas/surface heat transfer in spray deposition processes

A numerical investigation of heat transfer dynamics between gas and solid surfaces during droplet spray impingement is presented. Aim of the work is to derive knowledge for control of spray deposition processes like spray painting or spray forming, analysing how the heat exchanged from the surface to the flowing gas is affected by the presence of impinging droplets. The investigation is carried on a macro- and a micro-scale, analysing velocity and temperature profiles close to a surface cooled by a spray on a scale of the whole spray and on a scale comparable to the droplet diameter, respectively. In the former case an Euler-Lagrange approach is used to reproduce the multiphase jet/spray for different nozzle geometries, gas conditions and droplets properties, as drop diameter and concentration. In the latter case, the gas flow close to the surface is studied during the collision of single and multiple droplets for different impact velocities superposed by different perpendicular gas boundary layer configurations. The "volume of fluid" (VOF) technique is utilized for the determination of the transient shape of the gas-liquid interface during droplet impact. From the data of the numerical case studies, a quantitative consideration about the global increase of surface/gas heat transfer in impinging dilute sprays as a function of the number flux of particles approaching the wall is derive

Impact of atomization and spray flow conditions on droplet μ-explosions and temporal self-similarity in the FSP process

Flame spray pyrolysis (FSP) is a technique for the synthesis of metal oxide nanoparticles by combusting precursor solutions in a spray flame. The combustion of certain precursor solutions is known to lead to severe droplet disruptions (μ-explosions) in the spray flame that are linked to the synthesis of homogeneous and phase-pure nanoparticles. In this work, a broad spectrum of suitable subsonic operating conditions for the synthesis of iron oxide nanoparticles by FSP is investigated to understand the influence of the jet Reynolds number and turbulence on the onset of μ-explosions and droplet dynamics in spray flames. In order to enable a coherent comparison between differently operated spray flames using an iron(III) nitrate nonahydrate solution, the gas-to-liquid mass ratio and, hence, the oxygen/fuel ratio have been kept constant in order to identify the influence of flow conditions on the droplet dynamics. From the analysis of the droplet sizes in the spray and in the spray flame, it is found that in all combusting sprays, the droplet sizes convert from unimodal (after atomization) to bimodal droplet size distribution (DSD) due to the presence of μ-explosions. The occurrence and evolution of the bimodal DSD reveal that high jet Reynolds numbers result in narrower DSD and in a sharper separation of both DSD probability peaks (modal values). A straightforward 1-step kinematic model is presented to describe the conversion of unimodal to bimodal DSD considering the evaporation of droplets as well as the disruption of droplets to mimic the effect of μ-explosions. The temporal evolution of droplets in FSP is investigated by spatially resolved velocity data that reveal the formation of a temporal self-similarity. The resulting iron oxide nanoparticle size decreases with increasing jet Reynolds number. The turbulent mixing and residence times in the flame, primarily set by the jet Reynolds number, are identified as key design parameters for FSP

Multiphysics modelling of the spray forming process

An integrated, multiphysics numerical model has been developed through the joint efforts of the University of Oxford (UK), University of Bremen (Germany) and Inasmet (Spain) to simulate the spray forming process. The integrated model consisted of four sub-models: (1) an atomization model simulating the fragmentation of a continuous liquid metal stream into droplet spray during gas atomization; (2) a droplet spray model simulating the droplet spray mass and enthalpy evolution in the gas flow field prior to deposition; (3) a droplet deposition model simulating droplet deposition, splashing and re-deposition behavior and the resulting preform shape and heat flow; and (4) a porosity model simulating the porosity distribution inside a spray formed ring preform. The model has been validated against experiments of the spray forming of large diameter IN718 Ni superalloy rings. The modelled preform shape, surface temperature and final porosity distribution showed good agreement with experimental measurements

A unified computer model of the spray forming process of inconel 718 rings

A unified computer model of the process of spray forming Inconel 718 rings has been developed. The target of the model is to optimize the process input parameters to achieve the correct shape, microstructure and minimum porosity. The model is divided in three main parts that correspond to the atomization phase of the material, the flight behaviour of the droplets and the solidification and growth of the ring. Different modelling techniques have been used for each part of the model. For the atomization step, stability analysis for the primary atomization and a tracking model for secondary atomization were used. Computational fluid dynamics calculations for the thermal and mechanical behaviour of the droplets in their travel to the substrate were used for the in-flight behaviour step. Shape and growth models, as well as thermal models, based on finite element calculations were used to predict the final shape and temperature history of the as sprayed ring. Various computer programs have been written to link results between different submodels and to transform the format of intermediate computer files

Laser additive manufacturing of copper–chromium–niobium alloys using gas atomized powder

Copper–chrome–niobium alloys exhibit excellent thermal and electrical properties combined with high strength at elevated temperatures. Additive manufacturing techniques such as laser metal deposition using powder as raw material offer the potential for rapid solidification as well as a high freedom of design to manufacture parts layer by layer. Powder samples of copper–chrome–niobium alloys were produced by gas atomization. Via laser metal deposition, bulk volumes without cracks and with a very low porosity can be built up. Rapid solidification leads to the formation of fine precipitates which are likely to be (Cr,Fe)2Nb. The precipitates are distributed homogeneously in the copper matrix. The copper crystals grow across the layers due to epitaxial nucleation on the preceding layer. © Carl Hanser Verlag GmbH Co. K