L. Talbot

Theory of the Stagnation‐Point Langmuir Probe

A theory is developed for a Langmuir‐type probe consisting of a collecting electrode placed at the stagnation point of a blunt body immersed in a supersonic partially ionized stream. It is shown that under certain conditions, the stagnation‐point boundary layer equations and the probe sheath equations can be solved together to yield potential vs current relationships which permit the free stream ion and electron densities and temperatures to be measured by such a probe. It is shown also that the stagnation‐point heat transfer will vary with probe potential, thus providing additional information useful in plasma jet diagnostics.

Translational nonequilibrium in free jet expansions

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 A radiation from the 31 P‐21 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 temperat...

Temperature and density in a hydrogenair flame from Rayleigh scattering

Abstract A technique has been developed for obtaining spatially resolved measurements of temperature and density under combustion conditions. The spectral structure of Rayleigh scattered laser light is determined using a Fabry-Perot interferometer. Spectra in the post-combustion zone exhibit simple Doppler broadening with a Gaussian line profile; however, spectra in the higher density, precombustion zone have appreciable Brillouin components. Radial and axial surveys of temperature and intensity in the premixed hydrogenair flame issuing from a 3.2 mm diameter welding torch tip are obtained. Measured temperatures in the post-combustion zone are in agreement with the adiabatic flame temperature.

KINETIC THEORY OF A SPHERICAL ELECTROSTATIC PROBE IN A STATIONARY PLASMA.

The spherical electrostatic probe in a weakly‐ionized gas is analyzed on the basis of kinetic theory using a Krook‐type model for the collision integral. The charged particle motions are analyzed in energy‐angular momentum phase space, and formal integral solutions for the distribution functions are obtained. Moment equations based on the Less method are derived, taking into account curvature of the particle paths, and sample solutions of these equations are presented. The results obtained agree well with the numerical analysis of Laframboise, in the collisionless limit. It is found that the effect of charged‐neutral particle collisions is to extend the probe sheath farther into the plasma and reduce the electron and ion currents, as well as the departures from neutrality within the sheath. Fair agreement is obtained with a moment method based on straight‐line trajectories for the charged particles. The continuum limit is not included in the computations.

Measurement of Shock Wave Thickness by the Electron Beam Fluorescence Method

Shock wave thicknesses and density ratios have been measured in helium, argon, and nitrogen by means of the electron beam fluorescence method, over the range 1.5 < M < 17.4, in a low density wind tunnel. The shock thicknesses in argon and helium agreed well with Mott‐Smith theory at the higher Mach numbers and were between Navier‐Stokes and Mott‐Smith theories at the lowest Mach number. In nitrogen the measured shock thicknesses were considerably greater than the predictions of Navier‐Stokes theory. Measured density ratios across the shock wave were in good agreement with theory, in the lower density flow. Poorer agreement was found at higher flow densities, leading to estimates of upper bounds for the range of linear variation of fluorescence intensity with gas density. Density ratios for shocks produced in divergent free‐jet flows were found to be in better agreement with theory after a viscous curvature correction was applied. An anomalous dip in the fluorescence intensity was found ahead of helium sho...

Vortex shedding behind rod stabilized flames

Abstract The stabilization of a premixed, turbulent V-shaped flame on a bluff body has been studied using laser Doppler anemometry for two-compenent velocity data, and Rayleigh scattering for point measurements of density. The conditions studied include isothermal flow, a lean ethylene flame at equivalence ratio φ = 0.62, and a very lean (φ = 0.54) flame that is close to blow off for the free stream velocity of 6 m/s. Examination of velocity and density fluctuation spectra and fluctuation intensity contour maps reveals the first quantitative evidence of vortex shedding in the wake of a V-shaped flame under unperturbed free stream conditions. Vortex shedding accompanied by high periodic fluctuation intensities have been clearly identified in the cases of a very lean flame stabilized on a bar (6 × 3mm) and on a rod (6 mm diameter), and in a slightly richer bar stablized case, but not as clearly for a richer rod-stabilized case. It is suggested that vortex shedding may play a role in the blowoff process, and that the assumption of steady recirculation is not always valid.

Measurements of Rotational Temperatures in a Low Density Wind Tunnel

The fluorescence of a 30‐kV electron beam has been used as a probe to measure the rotational distribution function of flowing nitrogen in a low‐density wind tunnel. The fluorescence consists predominantly of the so‐called first negative system of nitrogen, a series of bands originating from the B2Σ+ state of N2+. Measurements of the intensities of the rotational structure of the (0–0) vibrational band have been used to calculate, through a theoretical model, the original rotational distribution function of N2 before excitation. This technique was originally proposed by Muntz, who also carried out a series of experiments demonstrating the validity of the theoretical model. A series of measurements was performed in the undisturbed flows of a subsonic nozzle, a Mach 4 nozzle, and free‐jet expansions from sonic orifices at Mach numbers ranging up to 19. The rotational temperature measurements are shown to be about 3% high at 280 °K, with a progressively increasing error at lower temperatures. These measuremen...

Rotational temperatures in nonequilibrium free jet expansion of nitrogen

Rotational nonequilibrium was investigated in hypersonic free jet expansions of nitrogen using the electron beam fluorescence technique. The results confirm the conclusion of previous investigations that a dipole excitation model, with an assumed Boltzmann energy distribution, is not consistent with the measured line intensities. This discrepancy was examined quantitatively and found to be independent of density and source Knudsen number (except at number densities greater than 1016 cm−3). The effect was attributed to interactions with the ejected (ionized) electron and a new excitation model was developed and shown to be consistent with the measurements throughout the flow conditions explored. The resulting rotational temperatures were compared to a simplified relaxation model of the jet and indicate a rotational collision number of 1.9.

Characterization of the density fluctuations in turbulent V-shaped premixed flames

Abstract Rayleigh scattering has been used to measure simultaneously two-point density fluctuations in a range of turbulent premixed flames. V-shaped ethylene/air and methane/air flames stabilized on a 1 mm rod were studied at equivalence ratios of 0.6 and 0.8 with approach flow mean velocities of 5 m/s and 7 m/s and turbulent intensities of 5% and 8%. Measurements were performed at different locations downstream of the flame holder. To characterize the flames, the measured flame brush thickness and the rms of the density fluctuations are compared. Time and length scales and other statistical quantities associated with the density fluctuations are reported. The Bray-Moss-Libby model of preximed turbulent combustion has recently been extended to include such characteristics of the scalar field as time and length scales, autocorrelations, and power spectra. This model is based on the thin flame approximation and treats the time series of the density fluctuations as a “random telegraph signal.” The single-point experimental results reported here compare very well with the predictions of the model. A wrinkled laminar flame sheet model which describes the turbulent flame region in terms of the instantaneous flame-front position is used to characterize these flames. It is shown that the pdf of the instantaneous flame location in the flame brush has a Gaussian distribution. Nondimensionalizing the flame density profile by a turbulent flame thickness derived from the maximum density gradient, all the data may be collapsed onto a simple error function plot.

Density fluctuations of flames in grid-induced turbulence

Abstract Density fluctuations have been measured using Raleigh scattering in the heat release zone of a rod-stabilized C 2 H 4 /air flame propagating into a grid-induced turbulent flow field. Probability density functions show that intermediate states between burned products and cold reactants are significant in determining density statistics. A comparison of the statistics for fluctuation intensity with results predicted by the model of Bray, Moss, and Libby, in which intermediate states are neglected, further confirms this point. However, the skewness is accurately predicted by the Bray-Moss-Libby model. Since intermediate states are approximately symmetrically distributed, they do not contribute significantly to the skewness.