Parametric investigation of particle acceleration in high enthalpy conical nozzle flows for coating applications

A modified cold gas-dynamic spray technique is under development by using shock tunnel technology, which can enhance the coating quality by increasing the solid particle velocity up to 1,500 m/s. The particle diameter typically amounts to 10 μm. A theoretical model based on gas-particle flows is employed to describe the behaviour of the flow and the solid particles. This quasi-1D model is capable to consider non-equilibrium effects of the gas phase due to high reservoir temperatures, and the influence of wall friction and heat transfer averaged over the nozzle cross section. This model is used for the design and optimization of the nozzle geometry by a parametric study, which results in a conical nozzle with a half opening angle of 2.8° and a length of 325 mm. Particles for coating are injected at about 55 mm downstream of the throat. A shock tunnel facility has been set up at the Shock Wave Laboratory for performing an experimental study of this new technique. The theoretical performance of this setup is evaluated by the KASIMIR simulation software and the quasi-1D method described in this paper. The high reservoir conditions required to achieve particle velocities of 1,500 m/s can be realized by using either a very high driver pressure of about 600 bar for air as driver gas or a relatively low driver pressure of about 200 bar for helium as driver gas.