Measuring flow profiles in heated miniature reactors with X-ray fluorescence spectroscopy

Density profiles of flowing gases within an opaque, heated, silicon carbide (SiC) miniature flow tube were measured, for the first time, by X-ray fluorescence spectroscopy using krypton as the fluorescent species. The experiments were performed with the flow tube at various fixed exterior wall temperatures between 300 K and 1100 K and a highly focused X-ray beam (5 μm × 7 μm; 15 keV). Spatially resolved raster scan maps of gas densities in the miniature reactor and the nearly supersonic jet eluting from the reactor were obtained. The intensity of the fluorescence signal was attenuated by the SiC, and a signal trapping model based on the tube morphology is required to convert the raw fluorescence signal into gas densities. This model was obtained, in part, from simultaneous measurements of the absorption of the incident X-rays by the SiC. Additional radiographic experiments with an unfocused beam were performed to image the SiC tube and complement the raster scans. Excellent correlation was found between the flow pattern revealed by the fluorescence measurements and the wall structure observed from the absorption measurements. Comments are made regarding refinements of the technique that will result in data of sufficient fidelity for validating computational fluid dynamic models of gas flows in heated miniature tubes.