Electron Beam Studies of the Diffusive Separation of Helium-Argon Mixtures

A narrow electron beam was used to selectively determine the component densities in rarefied helium-argon flows by spectrally analyzing the beam's luminescence. After a systematic study of the electron beam fluorescence in helium and argon had been made in a series of preliminary experiments, point-by-point measurements were made along and off the axes of underexpanded free jets and through normal shock waves produced by shock holders placed in the jets. Various argon mole fractions were used, and the jet Reynolds numbers ranged from 100 to 10 000. The argon enrichment found along the jet axis agrees well with a theory recently proposed by Sherman. Off-axis tests show an argon deficiency near the jet boundary. The experimental shock profiles agree only qualitatively with one-dimensional theories. Radial diffusion destroyed the otherwise approximate one-dimensionality of the shock waves, resulting in a significant argon enrichment inside the partially stagnated shock holder.