PDA Laser Measurements of Droplet-Laden Flows in a Four-Stage Axial Compressor

Droplet-laden flows play a significant role in the development of turbomachinery. For example, fine water droplets are injected into the flow path of axial compressors to increase the power output of gas turbines by making use of evaporation cooling, often referred to as wet compression. Another example is the ingestion of rain droplets in aircraft engines. Therefore, thermodynamic and aerodynamic measurements of air and liquid are indispensable to better predict the following behavior of the droplets in the gas flow. This article presents a new PDA measurement setup for a droplet-laden four-stage axial compressor to analyze the droplet behavior in swirled flows. To reduce optical disturbances of the laser beam path, liquid films at the casing are suppressed by a boundary layer air injection. Supplementary investigations with this treatment show a negligible influence on the compressor behavior. To minimize refraction effects of the laser beams at the casing window, the laser device with the optical probes can be translated and rotated, thus having 4 degrees-of-freedom. Furthermore, the collected droplet data are compared with a mathematical description of velocity slip in superimposed air flows. For the first time, it can be shown experimentally that droplets smaller than 10 mu m attain the velocity of air one stage after injection. For larger droplets, however, deviations from the air Mach number of up to 0.05 remain. In addition, detached liquid films behind the trailing edge of the stator blades with low velocities can be clearly detected.