Studies have been made of the extent of vibrational nonequilibrium produced in supersonic expansions of undissociated N2 from known initial equilibrium conditions attained behind reflected shock waves. The N2 was shock heated and compressed to reservoir temperatures and pressures in the ranges 2800° to 4600°K, and 24 to 82 atm. It was then allowed to expand through a 15° axisymmetric nozzle coupled to the end of the shock tube. The extent of vibrational nonequilibrium was determined by measurements of local vibrational temperatures using the spectrum-line reversal method. The measurements showed the vibrational temperatures in the expansion flows to be much closer to equilibrium than the Landau—Teller relaxation theory predicts. Observations at two area ratios showed that the flow was not completely frozen in the vibrational sense. The probability of N2 vibrational de-excitation inferred from these measurements is about 15 times greater than that inferred from measurements of vibrational relaxation made behind normal shock waves. Reasons for this apparent difference are discussed in terms of the basic environmental and kinetic differences between shock-wave and nozzle-expansion flows. The validity of the application of Landau—Teller relaxation assumptions and shock-tube measured rates to predict nonequilibrium in expansion flows is thus questioned.
A study of the noise produced by turbulent premixed flames stabilized on open burners is described. It is shown that such flames may be represented acoustically as a collection of monopole sound sources in the combustion zone. The radiated sound pressure is dependent on the rate of change of the rate of increase of volume of the gas during combustion, which varies owing to the turbulence in the flow. The rate of volume increase is proportional to the rate of consumption of combustible gas mixture in the flame. To measure this quantity, an optical technique has been developed which relies on observations of changes in the intensity of emission from the free radicals C 2 or CH generated in the reaction zone. Good quantitative agreement is obtained between the radiated sound pressures calculated from these intensity measurements and the values recorded simultaneously with a microphone. A correlation observed between the mean emission intensities of C 2 and CH radicals and the total flow rate of combustible gas mixture both in the laminar and turbulent flames, and in their transition region, supports the wrinkled laminar flame concept of turbulent flame propagation.
The promotion of population inversions between electronic excited states by fluid-mechanical techniques is suggested. Three methods are proposed as illustrations, and the possible degree of inversion in each is estimated. The first makes use in part of the known inversion obtained in excited Ne levels by energy transfer from He metastables. A flow system separates the region of excitation of the He from the region of the resonant excitation of the Ne by the He metastable, and the undesirable electron excitation of the lower Ne state is avoided. The second proposal involves the supersonic expansion of a dissociated mixture of O and H atoms obtained by combustion. An inversion is anticipated between the v′ = 2 and v′ = 1 vibrational levels of the OH(2Σ+) state through the process of inverse predissociation occurring during the expansion. The third illustration involves the rapid expansion of a highly excited gas where radiative depopulation of the lower state promotes an inversion. Results of exploratory experiments involving this particular scheme are discussed. However, it was not possible to observe a population inversion within the noise level of the experiment which involved the sudden expansion of Xe heated by a shock tube.
Application of line reversal method to measurement of shock flow electron temperature radiation field existing within the gas, the intensity of which depends on the rate of loss of energy to the glass walls of the resonance tube. In general, results of this analysis are in agreement with the experimental data both qualitatively and quantitatively. The results contained in Eqs. (28) and (29) are of interest since they tell us that methane is about as effective in exciting vibration in oxygen as in itself, but that oxygen is not nearly as effective in exciting vibration in methane if all processes are compared on a per collision basis. That methane should be more effective in exciting vibration in oxygen than oxygen in methane is difficult to understand on the basis of the usual collision theory, but so then are the results obtained for methane dilute in the rare gases. Pertinent in this regard is the fact that CH4 appears to be far more effective than hydrogen, a result which is not readily explained since reduced-mass arguments favor hydrogen and the intermolecular forces between O2 and C& would not be expected to be markedly different from those dominating the 02-H 2 interaction. This is complicated still further by the findings of White,2 who has determined that the more complex hydrocarbons such as C 2 H 2 and C~ are more effective in exciting vibration in oxygen than CH4 and that C 2 H 4 is more effective than C 2 H 2 .It is evident from Eqs. (19) and (20) that the anomalous character of the experimentally determined relaxation frequencies and vibrational specific heats is due entirely to the presence of the quantity 'Y. From Eq. (12), it is apparent that the existence of 'Y depends upon {3 having a value different from zero. The existence of {3, in tum, depends on the amount of absorption occurring within the gas and also the absorption taking place in the walls of the tube. Thus {3 would differ for tubes constructed of different materials which were otherwise identical. In particular, for a metal tube the reflection coefficient would be high, the absorption low, and therefore {3 small. In this regard, it is interesting to note the measurements of Schnauss, which were carried out in a metal tube, give values of absorption per wavelength which when plotted against the ratio of frequency to pressure yield maxima consistent with specific heats calculated from the Planck-Einstein formula.Spectrum-line reversal measurements of the excitation temperature of Na atoms in expansion flows of shock-heated Ar and 1 % N2+Ar mixtures are described. The measurements were made in a conical nozzle attached to the end of a conventional shock tube. For expansion flows of pure Ar from reservoir temperatures of 3200° to 42000K and a reservoir pressure of about 37 atm, the measured Na temperatures (",,2200 0 K) were considerably in excess of the local-translational temperature (",,400 0 K). These high excitation temperatures are interpreted in terms of the excitation of Na by free electrons produced from ionization of the Na in t...
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