Particle-based approach to model fuel droplet impact and thermal spray coating processes
Date
2022-08
Authors
Subedi, Kshitiz Kumar
Major Professor
Advisor
Kong, Song-Charng
Michael, James B
Lee, Jonghyun
Hsu, Ming-Chen
Ward, Thomas
Committee Member
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Abstract
This dissertation primarily focuses on a particle-based approach to modeling drop-wall interactions encompassing two disciplines: fuel spray dynamics and thermal spray coating. The first part is about the consecutive drop-wall interactions at different impingement frequencies at a wide range of wall temperatures and ambient pressure conditions. The second part is centered around the solidification of a molten droplet at the realistic thermal spray conditions and subsequent splat and substrate behaviors.
A theoretical splashing criterion is presented based on the K parameter incorporating the impingement frequency. A Lagrangian-based method, Smoothed Particle Hydrodynamics (SPH), is used to simulate the impact of a droplet train on a heated wall below and above the Leidenfrost temperature at different impingement frequencies and ambient pressure conditions. The developed SPH method is validated against the experimental results on the propagation of the crown rim’s diameter as a function of the non-dimensional time. Impact regimes are identified for various impact conditions based on the study of the time evolution of the post-impingement process. Visualizations of the impact outcomes are provided to explain the interaction between the succeeding droplets and the liquid film created by the preceding droplets. Simulations were also conducted for n-heptane droplets at different impingement frequencies and droplet diameters. It was found that the K parameter provides a better prediction for the impingement outcomes, and the effect of impingement frequency is seen as the kinematic discontinuity among the spreading lamellae, film formation, and rapidness of droplet impact. To further characterize the shift in the Leidenfrost behaviors and the corresponding impact outcomes caused by the change in ambient pressure, simulations were also conducted at 5 bar and 20 bar ambient pressures. The inhibition by high ambient pressure on the crown rim propagation was also quantified. Results show that the increase in ambient pressure impedes splashing, and the film is concentrated inwards near the impingement point. The results of this study can be used to further improve spray/wall interaction models at realistic engine conditions.
In the second part of this dissertation, the existing SPH model is enhanced to investigate the outcomes of droplet impacts on substrates under thermal spray conditions. Numerical results were validated by experimental data for impact outcomes and splat thickness. The threshold of deposition splat of yttria-stabilized zirconia (YSZ) droplet impact on two different substrates, including YSZ and stainless steel, was investigated. The results show that the kinematic parameters of the droplet and initial substrate temperature collectively determine the formation of the deposition splat. A regime diagram is proposed to characterize the outcomes of thermal spray impact in terms of the Sommerfeld parameter (K parameter) and substrate temperature. As the substrate temperature increases, the splashing threshold decreases slightly, resulting in an irregular splat morphology. The heat transfer effect is more pronounced at lower substrate temperatures, causing the droplet to solidify quickly. Furthermore, a comprehensive solidification model was developed to simulate the phase change of the droplet and substrate. The developed model was validated against the experimental results using tin droplets on stainless-steel and aluminum substrate and copper droplet on copper substrate. The histories of the spread factor, the substrate temperature, and the splat height at the impingement point are validated. A parametric study on the impact of the high melting point molybdenum droplets on different substrate materials (including tin, stainless steel, zinc, yttrium stabilized zirconia, and aluminum) was performed. The temporal evolution of the solidification interface and the height/thickness of the solidified splat are reported. Temperature distributions across the splat, substrate, and the corresponding melting/re-solidification are investigated. It was found that the cooling rates for the same impingement velocity were nearly the same for the same substrate regardless of the initial substrate temperatures but were higher for higher impingement velocities. A modified Biot number and thermal diffusivity were used to describe the heat transfer characteristics of the splat substrate combination. Results show that a higher modified Biot number for the splat-substrate combination indicates a faster cooling of the splat and a higher maximum substrate temperature. Furthermore, a substrate with high thermal diffusivity also causes heat at the impingement location to be dissipated quickly, resulting in a fast-cooling rate and accelerated solidification.
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dissertation