|Author:||Ho, Cheuk Yei|
|Title:||A numerical study of supersonic particle-laden flow in cold gas dynamic spray coating process|
Metals -- Finishing.
Dielectrics -- Finishing.
Hong Kong Polytechnic University -- Dissertations
|Pages:||120 leaves : ill. (some col.) ; 30 cm.|
|Abstract:||The cold gas dynamic spray (CGDS) process is a promising coating technology to produce high strength, strong corrosion resistance and excellent wear-resistant coating. The CGDS has many advantages over the conventional thermal spray methods, which uses kinetic energy, instead of using thermal energy, to bond the coating materials to the target substrates at a relatively low temperature without melting the materials, so many deleterious high-temperature reactions can be avoided. Therefore this technology is able to equip the manufacturers to meet more stringent requirements of engineering components they produce. However, CGDS is not free from barriers for more advanced development. The key problem of the CGDS developers is facing is there lacks a control mechanism on the focusing of the particle beam. This problem leads to loss of costly coating powder which results in loss of deposition efficiency. Since the deposition efficiency is strongly related to the gas dynamics of the impinging jet flow, a more detailed study of the flow dynamics is needed to provide a better understanding of this process. In this project, a two-step numerical methodology is developed to simulate the CGDS process. In the first step the gas dynamics involved in CGDS process is studied It essentially consists of a development of supersonic flow within CGDS nozzle followed by a supersonic jet impinging on the substrate. To simulate the CGDS process, numerical simulations are carried out using a numerical scheme called conservation element and solution element (CE/SE) scheme. The CE/SE framework is a high resolution, multidimensional numerical framework that solves conservation laws in both space and time simultaneously, and able to resolve the conflict between numerical stability and accuracy. The CE/SE scheme has been successfully applied to many flow problems, including unsteady Euler flow, waves, travelling and interacting shock, explosion waves etc.|
The CE/SE solver used in this project is originally developed to solve low Mach number aeroacoustics problems. Therefore, it needs to prove and validate the solver is also capable of simulating supersonic impinging jet flow which may carry shock waves. The validation tasks include simulations of free supersonic jet issuing from a convergent-divergent nozzle operating at design/off-design pressure ratio, and simulations of selected supersonic impinging jet benchmark problem. The CE/SE solver is capable of reproducing some key features of these problems, such as shock cells at the nozzle exit of free supersonic jet problem, lambda shock system inside and overexpanded nozzle, stand-off shock and shear layer at the impinging zone of the impinging jet problem etc. The CE/SE numerical results show good agreement with existing experimental and numerical data. This shows that the CE/SE solver is capable of studying CGDS flow problem. In CGDS process, coating particles are injected in to the CGDS nozzle where they are accelerated by the drag force whose magnitude is determined by the velocity difference between particle and carrier gas. Therefore, as the second step of the numerical methodology, a particle model is developed to calculate the drag coefficient based on the particle Reynolds number, and applied to the flow field results to calculate the particle flight path. The particles exit the nozzle with a very high velocity and gain heat energy from the preheated carrier gas. When they impact on the substrate, they either bounce off or deposit on the substrate surface. The impact speed of particle is an important parameter that has to exceed a critical value for successful deposition. A deposition model is thus developed to account for such deposition process. The impact location, impact speed and temperature of each particle are captured during its flight and the data is analyzed in more details. The relationship between the nozzle stand-off distance, spray area and some such CGDS operating parameters as nozzle pressure ratio, temperature of carrier gas, particle inject locations are investigated and reported.
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