Temporal and Spatial Experimental Investigationon Thin Shear-Driven Fluid Films underTurbomachinery Conditions

Thin shear stress-driven water films can be found in a wide variety of applications, such as heat exchangers,low-pressure stages of steam turbines or in compressors in the case of high fogging. In aviation, sheardriven water films are of interest in icing phenomena and heavy rain events, among others, as they pose aserious safety risk. The challenge in researching these films is the complex relationship between the turbulent boundary layer and the three-dimensional wavy film, as they influence each other. However, available literature mostly features time resolved but localized pointwise measurements of film behavior. Moreover, experimentsare often done in low-speed gas flows, which are not characteristic for turbomachinery applications. In thispaper, shear-driven water films are investigated in a new experimental test rig, which allows temporal andspatial resolution of films under turbomachinery-like conditions. The test rig can be operated at gas velocities exceeding 100 m/s and has a measuring section consisting of a 0.3 m wide rectangular duct with variableheight. Optical access to the measuring section is provided from both top and bottom side. Flow velocity profiles and turbulence intensities were obtained using 3D Laser Doppler Anemometry (LDA). The sheardriven water films were investigated with a novel light absorption measurement technique that allows films tobe recorded both temporally and spatially resolved, allowing to identify and investigate the wavy film structure.The technique is based on the light absorption of a water-ink mixture, which is illuminated by a white lightsource. The light intensity attenuation, which increases with film thickness and ink concentration, is recordedby a high-speed camera. The measurement technique was used to investigate the films behavior for variousair velocities and film Reynolds numbers.