Translational nonequilibrium in free jet expansions

Abstract

The velocity distribution function for helium in translational nonequilibrium in hypersonic free jet expansions has been investigated with an electron beam fluorescence technique. A unique computer controlled Fabry‐Perot interferometer and data acquisition system has been used to observe the 5015.67 Å radiation from the 3 1 P‐2 1 S transition in helium. From the spectral structure, the Doppler broadening and shift of the line due to the velocity distribution function can be determined. With this technique parallel and perpendicular gas kinetic temperatures, mean flow velocity, and relative density measurements were made along the centerline of the free jet, from near equilibrium conditions through the midpoint of transition (T ‖ /T ⊥ = 2.0) . A comparison with kinetic theory predictions of the temperatures and velocity distribution functions indicate that the adoption of the ellipsoidal model, although sufficient for fitting the measured distribution function, is inadequate for determining the parallel temperature. The experimental data have been compared with a Navier‐Stokes theory, with moment solutions of the Boltzmann equation, and a rarefication parameter suggested from previous Monte Carlo calculations. Determination of an effective hardsphere diameter from the experimental data is also presented.