Temporal and spatial experimental investigation on thin shear-driven fluid films under turbomachinery conditions

Abstract

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, shear-driven water films are of interest in icing phenomena and heavy rain events, among others, as they pose a serious 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, experiments are often done in low-speed gas flows, which are not characteristic for turbomachinery applications. In this paper, shear-driven water films are investigated in a new experimental test rig, which allows temporal and spatial resolution of films under turbomachinery-like conditions. The test rig can be operated at gas velocities exceeding 100 ms-1 and has a measuring section consisting of a 0.3 m wide rectangular duct with variable height. 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 shear-driven water films were investigated with a novel light absorption measurement technique that allows films to be 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 light source. The light intensity attenuation, which increases with film thickness and ink concentration, is recorded by a high-speed camera. The measurement technique was used to investigate the films behavior for various air velocities up to 100 ms-1 and film Reynolds numbers of up to 120.