High Reynolds number millimeter-scale jet impingement phenomena

Abstract

Cold spray is an additive manufacturing technique in which an inert gas (often helium ornitrogen) is used to accelerate micron scale particles to supersonic velocities through a millimeter scale converging-diverging nozzle. When these particles impact a surface, they can plastically deform and form a metallurgical bond with the surface. The gas used in this process is frequently preheated to high temperatures of up to 1000°C resulting in significant heat transfer between the impinging jet and the target surface. This heat transfer between the impinging jet and surface can create complex thermal gradients and thermal histories which have been known to affect particle deposition as well as impart residual stresses into the cold sprayed deposit leading to coating delamination and other changes in the deposit's fatigue properties. A challenge to researchers is the prediction of heat transfer that will occur between the impinging jet and the substrate. Amongst subsonic impinging jets with lower Reynolds numbers than what is typical in cold spray, this phenomenon is well documented. Impinging jets in cold spray are supersonic and can feature Reynolds numbers on the order of 10^5-10^6. Within this regime there is little to no heat transfer data available. In this work, a dimensional study is developed to determine the relationship between the Nusselt number over the substrate and other nondimensional parameters, including the Reynolds number, of a normal impinging jet. Computational fluid dynamics software is used to determine local Nusselt number distributions characteristic of supersonic nitrogen and helium impinging jets with Reynolds numbers between 10^5 and 2×10^6. Gas dynamic phenomena, including bow shock positions, are also analyzed. An experimental procedure is developed to validate numerical results via the use of non-contact temperature measurements and schlieren imaging of the impinging jets. It is determined that the area averaged Nusselt number is related to a power function of the Reynolds number which varies with the radial distance from the stagnation point. The standoff distance of the nozzle is found to have little to no effect on the Nusselt number except at the stagnation region where the shock structure of the jet is relevant.--Author's abstract