Property measurement utilizing atomic/molecular filter-based diagnostics

A variety of atomic/molecular filter diagnostic techniques have been under development for qualitative and quantitative flow diagnostic tools since their introduction in the early 1990s. This class of techniques utilizes an atomic or molecular filter, which is basically a glass cell containing selected vapor-phase species (e.g., I₂, Hg, K, Rb). In filtered Rayleigh scattering (FRS), and techniques derived from it, the atomic/molecular filter is placed in front of the detector to modify the frequency spectrum of radiation scattered by flow-field constituents (i.e., molecules/atoms and/or particles) when they are illuminated by a narrow linewidth laser. The light transmitted through the filter is then focused on a detector, typically a CCD camera or photomultiplier tube. The atomic/molecular filter can be used simply to suppress background surface/particle scattering, and thereby enhance flow visualizations, or to make quantitative measurements of thermodynamic properties. FRS techniques have been developed to measure individual flow properties, such as velocity (when the scattered light is from particles) or temperature (when the scattered light is from molecules), and measure multiple flow properties simultaneously such as pressure, density, temperature, and velocity. This manuscript summarizes the background needed to understand FRS techniques, and gives example measurements that have been used to develop FRS, demonstrate its capabilities, and investigate flow fields (both non-reacting and combustion) of research interest utilizing the unique capabilities of FRS. In addition, FRS has been used in conjunction with other diagnostics to improve the technique or measure properties simultaneously such as temperature and velocity (measured with PIV), or temperature and species concentration (measured by Raman scattering or laser-induced fluorescence). Also, a brief discussion is given of similar techniques being developed which utilize atomic/molecular filters and Thomson scattering from electrons to measure the electron number density and electron temperatures in plasmas.