In the experimental investigation of flows the application of conventional probes is still very often necessary today although these, depending on the test conditions, more or less greatly disturb the flow field under examination or do not endure the hot ambient conditions. To avoid these disadvantages there is a great interest in non-intrusive measuring techniques. In the course of the last two decades where many approaches to velocity measurement in flows have been applied, only two techniques have really become generally accepted:
When taking measurements in turbomachinery high velocities in narrow flow channels must be determined. Due to the complicated housing geometry very often optical access is only available from one side, so that the backscatter systems with their typical, extremely low signal levels are suitable. The application of the LDA technique is more complicated in this case as a result of the greater measuring volume and the following disadvantages arise:
These restrictions are only then avoidable if the size of the measuring volume is drastically reduced.
For this purpose a principle as proposed by THOMPSON in 1968 has been further developed to the Laser-2-Focus technique at the Institute of Propulsion Technology with which flow velocities and turbulence intensities at extreme experimental conditions may be determined. The measuring principle of this method enables the 2-dimensional flow vector perpendicular to the optical axis to be determined. The small size of the measuring volume causes a high spatial resolution and facilitates the suppression of parasitic stray light from walls and windows. As a result of the high light intensity in the measuring volume particles as small as 0.1 to 0.2 mm may still be detected.
The applicability of the L2F technique especially in velocity measurements has been demonstrated in several individual cases since the first compressor tests carried out by SCHODL. L2F systems are meanwhile commercially available. Furthermore there have been some industrial and research organizations which have constructed devices for special application, e.g. for wind velocity measurements, for investigations in heat exchangers, in steam flow, in water pumps, in plasma flow, in diesel motors and in wind tunnels. Most publications on the L2F technique however, of course deal with the application in turbomachinery.
Data resulting from L2F measurements which in some cases are most detailed, have contributed greatly to the understanding of flow in turbomachinery. This holds especially for the flow within the rotating components (e.g. turbine rotors), which generally cannot be investigated at all using conventional measuring methods. Comparisons between laser anemometer data and theoretical calculations have resulted in improved mathematical models and design procedures.