Interaction of a particle-laden supersonic jet with an obstacle

Kurzfassung

At the Institute of Aerodynamics and Flow Technology of DLR the method of cold gas-dynamic spray deposition is investigated. In this method deposition of coatings is performed by the impact of high-velocity particles that are in a solid state. Powder particles are accelerated by a supersonic free jet. The temperature of the gas is sufficiently lower than the melting temperature of the materials/particles. The method enables one to produce coatings made of most metals and alloys (Al, Zn, Cu, Fe, Ti, Ni etc.) on various substrates. Because cold spray does not use high-temperature heating for melting the feed material, the method is very attractive for depositing oxygen-sensitive materials such as copper and titanium. The actual mechanism by which solid-state particles deform and bond on impact has not been well characterized. The quality of coatings depends on particle velocity, size and density, on the reservoir pressure and temperature of the gas, on the nozzle shape and length, and other process parameters. Studying supersonic two-phase flows and shock wave structures produced by jets impinging on obstacles is important for an interpretation of the process. The complexity of a mathematical description of multiphase jets requires experimental measurements covering a wide range of parameters. In particular, the importance of the character of jet-substrate interactions, the influence of particles on shock structures, particle velocities and trajectories, and particle distributions and concentrations in the jet and ahead the substrate need to be studied. We have developed short light pulse diagnostics of supersonic two-phase flows that are applied to the cold spray facility. The set-up of the facility and preliminary results were described at the STAB-Workshops 2001, 2002. Main features of our facility are: axisymmetric jets; working gas: air, flow Mach number: M=2.6-2.8; reservoir temperatures: 20-500 C, degree of underexpansion: 0.3-1.2; powder materials: Zn, Al, Cu; particle size: 10-100 mikrometer. Parameters of the constructed optical diagnostics system: - visualization method: shadowgraph and schlieren visualization; - method of particle behavior recognizing: multiexposure, particle tracking visualization; - photodetector: CCD-camera, exposure time 10 mikrosec – 10 ms; - light source: semiconductor laser, impulse length 20 ns – 10 ms; - system of synchronization allows series of light pulses, modulated arbitrarily . The behavior of particle-laden and particle-free jets impinging on substrates was studied in the cold spray facility of DLR. The dynamics of the particles in the shock layer ahead the obstacle was observed.