Combination of robust point measurement techniques on the vortex tube: FRS, DGV and L2F deliver thermodynamic and flow properties

Abstract

High-end laser-optical measurements on industrial applications are often restricted to optical access via endoscopes. Since usually no single technique is able to all relevant variables of major technical flows, there is great interest in complemental combinations. This contribution presents a single-point approach to joining Filtered Rayleigh Scattering (FRS) with Doppler Global Velocimetry (DGV) and Laser-2-Focus (L2F): while L2F delivers two velocity components, temperature and pressure data are supplied by FRS. The third velocity component is added by both DGV and FRS. To put this vision into effect, measurements of all three techniques are conducted on a uniflow Ranque-Hilsch vortex tube using the same hardware: since the system has been originally designed for 3c-L2F the main focus is on the addition of FRS. While measurement results are validated literature sources and additional PIV reference data, challenges and solutions of the system setup as well as data evaluation methods are discussed: although calibrated numerical models for Rayleigh scattering spectra can be used to incorporate systematic deviations, the physically well-founded Tenti-S6-model once again proves to deliver accurate output under tough conditions. As a main result it is found that fluorescence background is emerging from signal fibers: strong Mie scattered light originating from particles needed for L2F and DGV may excite a fused silica fiber and produce broadband luminescence. Since this occurs before the FRS-typical powerful background filtering, it leads to disruptive cross-talk on the considerably lower FRS signal. Solving this issue promises simultaneous measurements of particle and fluid velocity, allowing slip analysis.