An intensity-based LIF measurement technique to quantify film thicknesses in the air gap of an electric motor with direct liquid cooling

The air gap of an optically accessible model of a directly cooled radial flux electric motor is investigated using laser-induced fluorescence (LIF). The cooling oil enters the air gap and a film forms on the stator surface, which resembles a thin film in a shear flow. The flow phenomena at different rotational speeds (2000 rpm to 10,000 rpm) are described. An intensity-based LIF measurement technique is developed and used to measure the film thickness on the stator in a realistic air gap environment. The rotational speed influences the flow phenomena and the film thickness of the stator film. With increasing rotational speed, i.e., increasing gas Reynolds number, the film thickness probability density functions (PDFs) shift to lower film thicknesses and become narrower, which is in agreement with the characteristic behavior of films in shear flows as reported in the literature. Additionally, the velocities of the wave crests which move across the film surface are evaluated and used to calculate the film Reynolds number Re_F, which characterizes the investigated operating points.