Nonequilibrium processes in supersonic jets of N2, H2, and N2 + H2 mixtures: (II) shock waves

We report an experimental study of the shock wave system of three continuous axisymmetric jets of pure N(2) and H(2), and of the N(2) + 2H(2) mixture, generated at p(0) = 1 bar and T(0) = 295 K through a D = 310 microm cylindrical nozzle against a background pressure p(b) = 6.1 mbar. Number density and rotational populations have been measured by Raman spectroscopy with high spatial resolution (approximately 15 microm) across the normal shock wave located at z/D approximately 8, and across the barrel shock wave at the plane z/D = 5.2. Significant differences in position, widths, and gradients of the shock waves are observed among the three jets. Such differences are qualitatively interpreted in terms of disparity in mass, inelastic cross sections, and collision numbers. Non-Boltzmann distributions of the rotational populations of N(2) are observed within the shock waves at regions where the local Knudsen number is higher than 0.05. In the N(2) + 2H(2) mixture the different behavior of the two species leads to a localized enrichment of the light component up to 20%, which might provide the basis for an efficient separation device. Measurements on the invasion of the shock wave system by the background molecules are also reported, proving such invasion to be more efficient for the lighter species H(2) than for the heavier N(2).