Frequency scanning filtered Rayleigh scattering in combustion experiments

Abstract

To increase the overall efficiency in aero engines as well as stationary gas turbines detailed information on flow field properties under realistic operating conditions are required. While conventional probe based technology is readily available and thoroughly tested, most of these techniques are point-wise. To gain insight into complex flow structures spatially and temporally resolved data is needed. Planar optical measurement techniques are capable of providing this type of data in a cost effective manner. The paper describes an image based filtered Rayleigh scattering (FRS) technique that, on frequency scanning a narrow linewidth light source, provides time averaged temperature (or density) fields. FRS uses molecular absorption to attenuate elastically scattered light from surfaces or particles in the flow. The filtered light is collected by a camera sensor at a multitude of light source frequencies allowing the temperature to be inferred from the pixel intensities. The measurement technique is validated on a McKenna type premixed flame and shows a better than 2% agreement to corresponding single point CARS measurements. The technique is further applied on a pressurized single sector combustor in an effort to visualize and quantify the cooling film on an effusion cooled combustor liner wall. In this case the light sheet was directed straight onto the wall and aligned with a single row of effusion holes. Due to the molecular filtering the laser flare on the wall was completely attenuated in the acquired images allowing temperature measurements to within a tenth of a millimeter from the liner wall.