3-Component Doppler Laser-Two-Focus Velocimetry Applied to a Transonic Centrifugal Compressor

The conventional Laser-Two-Focus (L2F) method, also known as Laser Transit Anemometry (LTA), measures two components of the flow vector in the plane normal to the optical axis by measuring the time-of-flight of particles crossing the two laser beams in the probe volume. Recently a new three component system was developed, named 3C, Doppler L2F, which operates with the same confocal optical set-up as the two component L2F system, thus enabling three component measurements even under the extremely difficult conditions of limited optical accessibility as they appear for example in centrifugal compressors. The new hybrid system combines the principle of the L2F technique with the basic idea of the Doppler Global Velocimetry (DGV). The two velocity components in the plane perpendicular to the optical axis are measured by the L2F time-of-flight technique, the third velocity component in the direction of the optical axis is determined by analyzing the Doppler frequency shift of the scattered light. The system was developed with respect to an application in a transonic centrifugal compressor and designed in the shape of an optical probe shown in Fig. 1. The set-up and all important components of the 3C, Doppler L2F system are described here in detail, as well as the principle of operation and calibration of the system. Tests on a free jet demonstrate the measurement accuracy of the hybrid technique. Its successful application to a transonic centrifugal compressor was the first time that three component velocity data could be collected from a high loaded, high speed centrifugal impeller. The data are presented and discussed.