Analytical study of the heat transfer coefficient of the impinging air jet during cold spraying

A semi-empirical analytical model was developed to determine the convective heat transfer coefficient of an impinging air jet generated by a cold spraying nozzle. A low-pressure cold spraying unit was utilized to produce hot air jets that impinged upon a flat substrate surface. An infrared camera was used to measure the surface temperature of the substrate at different time intervals. A method involving Green's functions was employed to solve a transient two-dimensional heat conduction problem to obtain an expression for the temperature distribution within the substrate. By coupling the analytical results of temperature distribution and experimental surface temperature data, the radial variation of the non-dimensional heat transfer coefficient of the impinging air jets upon the substrate was estimated. The results showed that the maximum values of the heat transfer coefficient were present close to the stagnation point of the air jets. It was found that the heat transfer coefficient was independent of the time that the cold spray nozzle remained stationary over the substrate surface. It was found further that by increasing the stand-off distance of the nozzle, the radial variations of the heat transfer coefficient became negligible, compared to those for small stand-off distances. The close agreement between the experimental results and the predictions of the model suggested that the estimated heat transfer coefficient of the cold spray gas jet can be used to estimate the surface temperature of the substrate at any time.