A Comparison Between Numerical and Experimental High Reynolds Number Supersonic Jets Generated by Millimeter Scale Converging-Diverging Nozzles

In thermal spray applications, such as cold spray, an inert gas jet (typically helium or nitrogen) is used to accelerate micron scaled particles to supersonic velocities. The complex gas dynamics of these supersonic jets are critical to understand via computational methods for the control of the spray. This work compares supersonic jet waveforms visualized by schlieren imaging and predicted by computational fluid dynamics simulations (CFD). A supersonic nitrogen jet is produced by a converging-diverging nozzle with inlet pressures as high as 70 bars. The nozzle being used has an inlet, throat, and exit diameters of 9.53 mm, 1.98 mm, and 6.35 mm, respectively. The length of the converging portion of the nozzle is 42.93mm, the length of the throat is 2.29mm, and the length of the diverging portion of the nozzle is 150.63mm. The Reynolds numbers of the jets being analyzed in this study range between 60,000 to 325,000 when using the nozzle exit diameter as the characteristic length. A schlieren visualization setup has been built which shows the first spatial derivative of densities within the flow field. The strong density gradients across oblique shock waves in the jets allow for clear photographs of the flow pattern of the jets to be taken using this schlieren visualization setup. These photographs are analyzed in this work to determine the location and orientation of oblique shock waves of physical jets at varying Reynolds numbers. In order to make a comparison between these physical measurements and CFD models, two dimensional axisymmetric CFD simulations are created and executed using the k-omega turbulence model. The CFD models attempt to match all parameters of the physical experiments (inlet pressure and temperature, nozzle geometry, etc.). The comparisons between the experiments and the CFD results act as a validation technique for the accuracy of the simulations in terms of the positions and orientations of the oblique shock waves. Through this study, the nozzle internal surface roughness is determined to be a critical parameter in millimeter scale nozzles for the development of the boundary layer. In this work, the CFD surface roughness parameters of the inside of the nozzle are incremented until the geometry of the oblique shock waves matches the schlieren images. This work serves to validate the simulation techniques which will be used for future jet simulations, in which shock wave locations and orientations are important, such as jet impingement on a flat plate. In the future, the use of this schlieren imaging technique will also provide a method to more accurately model nozzle surface roughness and other parameters which affect boundary layer separation such as nozzle cooling or heating.