Turbulences of the supersonic gas flow during cold spraying and their negative effects: A DNS CFD analysis coupled with experimental observation and laser impulse high-speed shadowgraphs of the particles in-flight flow

Abstract

This paper investigates the phenomenological flow during cold spraying through DNS CFD analysis and experimental observations. The transient DNS computation shows that the gas flow begins to be instable inside the nozzle and generates self-sustained intermittent swirls across the nozzle exit due the shearing behavior of the flow. There is alternate swirling within the separated sheared layers on top and then on bottom of the jet, at sporadic time intervals. The swirls are not strictly periodic in nature, but they recur with an irregular frequency. The temperature field exhibits analogous variation and the thermal turbulence produces a heating confinement within the end zone of the nozzle, at the upper wall mostly. This phenomenon matches with the experimental erosion at the same zone due to a thermomechanical softening of the nozzle. The supersonic jet is self-oscillated along the flow direction and becomes more and more turbulent with a development of vorticity shedding at a certain distance from the nozzle exit. There is a transition from a rather stable jet towards a wake pattern that characterizes the vorticity regime. The thermal turbulence shows a development of turbulent plume which provides a clearer visualization of the straight jet and the abrupt transition into wakes. Experimental observations of particles motion ahead the nozzle exit confirm this turbulent behavior of the gas flow. High-speed shadowgraphy using nanoseconds pulsed laser spray illumination shows two regimes of particles kinematics, that is, a nearly straight jet prior to a progressive stream deviation, and then a full dispersion of the particles. Such dispersion makes possible an oblique collision of the particles on the substrate whose negative effects highlighted in the literature are high porosity within the deposit, difficulty of coating formation, and substantial decrease in deposition efficiency. These findings provide further clarifications about how both gas flow and particles flow behave during cold spraying, and how deviation effects due to turbulences of the supersonic expansion may further alter the deposition capability of LPCS that already suffers from a low DE less than 40%.