Abstract:
The main aim of this project is to model the effects of varied injection
parameters on the gas dynamics and droplet dynamics of the HVSFS and SP-
HVOFS processes for improving the droplet breakup and evaporation to
enhance the nanoparticles heating and deposition efficiency. Thermal spraying
processes are widely used to generate thermal-, corrosion-, and wear-resistant
layers over the machine parts, to increase the durability of the equipment under
severe environmental conditions. The liquid feedstock is used to achieve
nanostructured coatings. It is used either in the form of a suspension or a
solution precursor. The suspension is a mixture of solid nanoparticles
suspended in a liquid medium consisting, for instance, of water, ethanol, or
isopropanol. This dispersion mechanism in a liquid carrier provides adequate
flowability to the nanoparticles, which cannot be handled by conventional gas-
based feeding systems, whereas the solution precursor is mixed at the
molecular level; hence, more uniform phase composition and properties are
expected in the sprayed coatings as compared to the suspension and
conventional powder spraying.
Firstly, experiments are conducted to analyse the effects of different precursor
concentrations, solvent types and injection nozzles on the size and morphology
of synthesized nanoparticles. The results indicate that the particle size
increased with increasing precursor concentration due to the variations in the
physical properties of the mixture solution. The higher precursor concentrations
had an adverse effect on the droplet atomization and evaporation process that
led to bigger size particle formation. The use of aqueous solvent has some
limits and with higher precursor concentration the surface tension increases that
resulted in the reduction of droplets’ disintegration, and thus bigger size
precursor droplets generate larger nanoparticles. A mixture of aqueous-organic
solvents and pure organic precursors are preferred to improve the process
efficiency of the nanoparticles size and morphology. Furthermore, the
nanoparticles size can be controlled by using liquid feedstock atomization
before injecting into the HVOF torch. A new effervescent injection nozzle is
designed and compared to different types of existing injection nozzles, to see
the variations in the droplet disintegration, and its effects on the performance of
the HVOF torch processes. It is detected that the atomization would result in
smaller size particles with homogeneous morphology. In a numerical study,
different droplet injection types are analysed to see their effects on the gas and
droplet dynamics inside the HVOF torch. The group-type injection (GTI) and
effervescent-type atomization (ETI) are used effectively to overcome the heat
losses and delays in the droplet evaporation. These approaches reduce the
thermal and kinetic energy losses in the suspension-fed-HVOF torch, thereby
improving the coating formation.
The effects of using multicomponent water-ethanol mixture injection in the
HVOF torch are also modelled, and its impact on the droplet breakup and
evaporation are studied. The organic solvents have a low heat of vaporization
and surface tension, and can effectively be used in the HVOF spraying process
over the water-based solvents. Furthermore, nanoparticles are suspended in
the liquid feedstock and injected into the HVOF torch. The effect of increasing
nanoparticles’ concentration in the feedstock and its consequence on the gas
dynamics, droplet breakup and evaporation are analysed. The augmentation in
the nanoparticles loading in the suspension droplets can decrease the droplet
breakup and evaporation rate because the required heat of vaporization
increases significantly. Moreover, the size of injection droplet affects the droplet
fragmentation process; bigger sized droplets observed a delay in their
evaporation that resulted in coating porosity. The results suggest that smaller
droplet sizes are preferred in coating applications involving a higher
concentration of nanoparticles with high melting point.
Further, the gas flow rates (GFRs) are regulated to control the droplet
dispersion, atomization and evaporation inside the solution precursor fed-HVOF
torch. The size of the droplet diameter is decreased by an increment in the
GFR, as higher combustion rates increase the combustion flame enthalpy and
kinetic energy. Moreover, the increase in the oxygen/fuel flow rates dilutes the
injected precursor. It reduces ZrO2 concentration in the process and decreases the rate of particle collision; as a result, non-agglomerated nanoparticles can be
obtained.