Impinging jets for convective cooling are used in several technical applications. Piston cooling with impinging oil jets is one key application. To improve the heat transfer between the surface of the piston and the oil film it is necessary to understand the underlying mechanisms of heat transfer at the boundary face. For this reason it is important to analyze the oil flow and to identify and evaluate the influence of the parameters governing film formation. Also the oil jet is investigated, because the film formation can be influenced by the jet. In the experiments the oil temperature is set to 30 °C or 60 °C and the pressure at the nozzle inlet is varied between 1.6 bar and 4.2 bar. The minimal Reynolds number is 125 and the maximum is 1924. The liquid Weber number varies between 2.2 × 10−2 and 52.9 × 10−2. The results of the visualization measurements reveal the influence of the exit velocity, oil temperature and the related material properties on the film formation process. On the one hand the results show the macroscopic relation between Reynolds number and the level of instability. On the other hand the relation between Weber number and the break-up at the surface of the jet and accordingly of the film surface can be demonstrated.

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