Combined filtered Rayleigh and Mie scattering for simultaneous planar temperature and velocity measurements

Abstract

The combination of planar Doppler velocimetry (PDV) and FRS offers a promising way to obtain simultaneously 3 component velocity in a plane and corresponding scalar temperature maps. Implementation wise the two techniques are closely related since they both make use of a molecular absorption filter. This makes it possible to combine the necessary equipment using a single laser to illuminate a plane of interest that is observed simultaneously by multiple viewing directions. While PDV determines the frequency shift utilizing an absorption measurement with the laser light stabilized on the slope of an iodine line FRS measures the residual light passing through the molecular filter with the laser frequency tuned to the absorption minimum. The main challenge arises from the fact that PDV relies on the scattered light by micron-sized particles added to the flow. The intensity of this Mie scattered light is orders of magnitude higher than the light scattered by molecules of the gas flow itself known as Rayleigh scattering. In order to be able to record both signals both simultaneously but separately two iodine cells with different operation conditions were used for each camera system. The necessary criteria for successful measurements is the sufficient suppression of the Mie scattered light by the FRS camera while maintaining a proper dynamic range for the Doppler shifted light recorded by the PDV camera system.The combined PDV – FRS system was optimized by arranging the observation directions for the velocity and temperature measurements separately. An evaluation of the combined system was performed with an analysis of measurements taken on a hot free jet. Although the PDV velocity data agree reasonably with the calculated value the performance of FRS system suffered from technical problems.