Endoscopic filtered Rayleigh scattering for the analysis of ducted gas flows

Laser-induced Rayleigh scattering makes use of the fact that elastically scattered laser light from atoms or molecules hold information on pressure, temperature and velocity inside the observed region of interest. Being several orders of magnitude weaker than other elastic light scattering effects such as Mie scattering from larger particles or laser flare from surfaces, the accuracy of Rayleigh scattering measurements strongly depends on the attenuation of these unwanted but omnipresent noise effects. The image-based technique of filtered Rayleigh scattering makes use of the absorption bands of atomic or molecular gases to remove strong elastic scattering effects from the measured signal. The concept is extended by frequency tuning a narrow line-width continuous wave laser light source along the absorption filter’s transmission profile. In acquiring images at several known frequencies, this results in spectra for each camera pixel from which time-averaged temperature, pressure and velocity fields can be deduced simultaneously. In order to qualify the frequency scanning filtered Rayleigh scattering technique for applications with strongly limited accessibility, it is applied to characterize the flow inside a bell-mouthed circular duct. The image data are acquired endoscopically via a fiberscope that is placed downstream from the measurement plane.