title: Multiscale and Multiphase Numerical Modelling of High Velocity Suspension Flame Spray Process for the Development of Nanostructured Thermoelectric Coatings
Creators: name: Gozali, Ebrahim
Publisher: name: University of Liverpool Repository
Year: 2016

description: The manufacture of nanostructured coatings by thermal spraying is currently a subject of increasing research efforts in order to obtain unique and often enhanced properties compared to conventional coatings. High Velocity Suspension Flame Spraying (HVSFS) has recently appeared as a potential alternative to conventional High Velocity Oxygen-Fuel (HVOF) spraying: for the processing of nanostructured spray material to achieve dense surface layers in supersonic mode with a refined structure, from which superior physical and mechanical properties are expected. The aim of this thesis is to, first, apply CFD methods to analyse the system characteristics of high speed thermal spray coating processes in order to improve the technology and advance the quality and efficiency of the HVSFS process. The second aim is to analyse heat transfer in thin films and thermoelectric thin films. The first part of this thesis aims to deepen the knowledge on such multidisciplinary process and to address current drawbacks mainly due to cooling effects and reduction of the overall performance of the spray torch. In this matter, a detailed parametric study carried out to model and analyse the premixed (propane/oxygen) and non-premixed (ethanol/oxygen) combustion reactions, the gas flow dynamics of HVSFS process, the interaction mechanism between the gas and liquid droplet including disintegration and vaporization, and finally investigation of the droplet injection point (axially, transversely, and externally), at the example of an industrial DJ2700 torch (Sulzer-Metco, Wohlen, Switzerland). The numerical results reveal that the initial mass flow rate of the liquid feedstock mainly controls the HVSFS process and the radial injection schemes are not suitable for this system. The second part of this thesis focuses on investigating the effects of solvent composition and type on the liquid droplet fragmentation and evaporation, combustion, and HVSFS gas dynamics. Here the solvent mixture is considered as a multicomponent droplet in the numerical model. The numerical results can be considered as a reference for avoiding extraneous trial and error experimentations. It can assist in adjusting spraying parameters e.g. the ratio or percentage of solvents for different powder materials, and it can give a way of visualization of the phenomena occurring during liquid spray. In the third part, effects of solid nanoparticle content on liquid feedstock trajectory in the HVSFS are investigated. Theoretical models are used to calculated thermo-physical properties of liquid feedstock. Various solid nanoparticle concentrations in suspension droplets with different diameters are selected and their effects on gas dynamics, vaporization rate and secondary break up are investigated. It is found out that small droplets with high concentrations are more stable for break up, thereby; vaporization is the dominant factor controlling the process which results in leaving some drops without fully evaporation. However, larger droplets undergo sever fragmentation inside the combustion chamber and release the nanoparticles in the middle of barrel after full evaporation. Finally a heat transfer model is developed for nanoparticles traveling inside thermal spray guns. In the absence of experimental data for Nano-scale inflight particles, the model is validated in thermoelectric thin films as candidate applications of the HVSFS process. For this purpose, one dimensional heat conduction problem in a thin film is investigated through solving three different heat conduction equations, namely: parabolic heat conduction equation (Fourier equation), hyperbolic heat conduction equation (non-Fourier heat conduction), and ballistic-diffusive heat conduction equations. A stable and convergent finite difference scheme is employed to solve the hyperbolic heat conduction (HHC) equation and the ballistic-diffusive equations (BDE). The ballistic part of the BDE is solved with the Gauss-Legendre integration scheme. Then these equations are applied across a thermoelectric thin film to investigate the steady-state and the transient cooling mechanism at the cold junction surface. The numerical results indicate that those equations predicted inaccurate results for the transient heat conduction in a thin film lead to less accurate prediction of cooling at cold side boundary, temperature, and heat flux profile in a thermoelectric film.