Gas-particle Two Phase Flow In Supersonic Cold Gas Dynamic Spray

Cold gas dynamic spraying(CGDS) was a newly-developed coating techniques proposed in 1980 's, in which the carrier gas was accelerated to very high speed (usually supersonic) by de Laval nozzle, then the particles carried by gas impinged on the substrate and form coating. Compared with traditional thermal spraying, CGDS eliminates the negative effect of high temperature on the coating and the substrate. It also has many other merits such as avoiding oxidation, small residual stress, high efficiency of particles depositing, low porosity of the coating, coating material reclaimable and so on. As a well-prospective coating techniques, CGDS attract more and more attentions.Since the process of CGDS is a supersonic jet, the velocity profile of carrier gas cannot be measured easily, which limits experimental study. By employing the numerical simulation, the distribution and variation of thermal parameters such as pressure, temperature, velocity and so on can be conducted in detail. We can also optimize the spraying distance, choose the best carrier gas and particle size and etc.For the simulation of gas-particle two-phase flow, two primary methods are generally used at present, including two fluid model (Euler- Euler model) and particle track model (Euler-Lagrange model). In the process of CGDS, the volume content of particles is less than 10%, so the Euler-Lagrange formulation is appropriate for the modeling of the acceleration of the particles. In the Euler-Lagrange model, particles were treated as dilute phase, and their effect on the gas phase was neglected.A structured non-uniform grid system had been used to discretize the computational domain. RNG k- ?model was used to solve the turbulent parameters, and SIMPLE method to solve pressure and velocity. Particles are supposed to be sphere-shaped with the same size, and their interaction was neglected.In this thesis, the effect of different carrier gas, spraying distance, particles' size and material on the spraying has been investigated. The result indicates thata. The ability of Helium to accelerate particles is better than N2;b. Particles with smaller mass can obtain higher velocity, but when particle mass is lighter than a certain value, particles cannot reach the substrate, flowing away with carrier gas, or whirling near the substrate because of their small inertia.c. The particles reach the highest velocity, among the three calculated spraying distances, when the spraying distance is 18cm.