Underexpanded Free Jets Research Articles

High density windowless gas jet target

A high density gas target has been developed using a focussing effect in slightly underexpanded supersonic free-jets which are produced by axisymmetric Laval nozzles. Under constant pressure conditions the first density focus, a cylindrical flow portion of 3 mm diameter and of a length of about 5 mm and an almost homogeneous density distribution, is spatially fixed. By varying only the pressure of the gas in the entrance of the nozzle, the target thickness can be changed from less than 1 μg/cm2 to more than 100 μg/cm2. To avoid window foils in the target device, the secondary gas load is reduced by a three-stage dynamical pumping system which is built concentrically around the gas jet and which allows unrestricted 360° access to the target area.

Calculation of highly underexpanded supersonic free jets

Calculation of highly underexpanded supersonic free jets

Theoretical and experimental study of background gas penetration into underexpanded free jets

The mechanisms of penetration of single and multicomponent background gases into a free jet flowing into a stationary background are examined from theoretical and experimental points of view. The theoretical treatment is based on the asymptotic theory developed by Brook and Hamel for a spherically symmetric source flow expanding into a stationary rarefied atmosphere. Agreement between theory and experiment is reasonable; certain points of disagreement are identified for future study.

Laser-Doppler velocity measurements in an under-expanded free jet

Velocity measurements in an under-expanded supersonic free jet have been made using the Fabry-Perot laser-Doppler technique. The jet was produced by a nozzle of exit diameter 2·7 mm operating at a pressure ratio P0/Pinfinity=6·6. Measurement of the complex structure, characteristic of this type of jet, on such a small scale proved to be an excellent test of the capability of the Fabry-Perot laser-Doppler technique for making air velocity measurements.Sufficient scattering particles were present in the flow to make artificial seeding unnecessary. The particles were found to respond to the velocity change across the Mach disc (a normal shock) in a distance of similar 0·1 mm. Laser-Doppler velocity measurements were compared with velocity measurements deduced from Pitot pressure measurements. The difference in results between the two methods was < 1%. Sufficient laser-Doppler velocity measurements were obtained to construct the axial and four transverse velocity profiles which clearly showed the positions and magnitudes of the major features characteristic of an under-expanded free jet.

Aerodynamic Separation Effect on Gas and Isotope Mixtures Induced by Invasion of the Free Jet Shock Wave Structure

The shock wave system of the underexpanded free jet, in a region where the pressure is in the range of 10−2 up to 1 torr, is partly invaded by background molecules, these latter being pumped and carried by the supersonic flow. When a gas mixture is introduced into the nozzle chamber and outside the shock barrel, this aspiration acts very selectively on the light molecules, and a mixture enriched in lighter species is attainable by skimming the free jet. This new aerodynamic separation effect, which is applicable to isotopic mixtures, is very strong in comparison to the efficiency of an elementary gaseous diffusion membrane.

Diffusive separation in shock waves and freejets of nitrogen-helium mixtures.

The effects of diffusive separation were measured in shock waves and underexpanded freejets for a binary mixture of nitrogen and helium by the electron-beam technique. The measurements consisted of mole-fraction ratios in the jets, and of partial density and nitrogen rotational temperature profiles through the shock. A number of initial compositions were tested and all indicate an enrichment of the heavier particles at the axis of the jet, and an enrichment of the lighter particles in the jet boundary. The measured shock profiles indicate a qualitative agreement with the theory in that an enrichment of the lighter particles was found in the upstream portion of the shock wave. The N2 rotational temperature profiles were used to compare the heavier particle temperatures with the shock-density profiles of both of the components of the gaseous mixture. Nomenclature A — Einstein's transition probability of spontaneous emission Bv = rotational constant Ci . . . C& = constants c = speed of light d = throat diameter e = electron charge F = constant f =

Visualization Technique for Low-Density Flow Fields

A quantitative visualization technique based on an electron beam fluorescence probe has been investigated for low-density flow fields. The design considerations and operating range of the instrumentation system are described. Initial data on the visualization of the flow field of an underexpanded free jet are presented.

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.