Spectroscopic measurement of the temperature of shock-heated oxygen

SPECTROSCOPIC MEASUREMENT OF THE TEMPERATURE OF SHOCK-HEATED OXYGEN w. Cornell Aeronautical

II. WURSTER Laboratory,

Buffalo, New York

Abstract-This paper presents the preliminary findings of experiments currently in progress in which the ultra-violet absorption of the Schumann-Runge band system of oxygen has been exploited as a means of determining the vibrational temperature of heated oxygen. The apparatus is described and results obtained to date are presented. These results demonstrate the usefulness of this device in measuring temperatures between 1500’ and 35OO’K for a Current work on the extension of the measurements to limited range of optical thickness. other densities and on the applicability of this technique to the temperature of air is discussed. 1.

INTRODUCTION

One of the problems in the simulation of hypersonic flight situations, or even in basic studies of the properties of heated gases, is that of the determination of the thermodynamic state of the gas. This is reliably calculable only for the simplest of aerodynamic flows, and must in other cases be inferred from measurements. In most of these measurements it is essential that the flow remain unperturbed from an aerodynamic standpoint, making the use of optical Such is the case in the probing desirable. measurement of gas temperatures, where many methods have been reported which are based on spectroscopic techniques. They include the method of sodium line reversal(l) and the measurement of relative intensities of spectral lines of metals and of the bands of C, and CN@). These methods generally require additives to the gas and, further, the assumption of thermal equilibrium between the gas and the contaminants. In this paper is presented a discussion of the use of pure oxygen, always present in air below 5OOO”K,in the measurement of temperature by spectroscopic means. 2. ULTRA-VIOLET ABSORPTION OF OXYGEN The radiative properties of oxygen at elevated * This research was supported in whole or in part by the United States Air Force under Contract No. AF 18(603)-141, monitored by the Mechanics Division of the Air Force Office of Scientific Research of the Air Research and Development Command.

temperatures have been studied at this laboratory@,4) under the sponsorship of the Air Force Office of Scientific Research (Contract AF 49 (638)-269). One of the outgrowths of this research was the undertaking of an experimental program aimed at making use of the spectral absorption of oxygen as an optical probe to determine oxygen temperatures. The description of such a probe is given, and the first calibration measurements made with this device d are presented. An examination of the pertinent potential energy curves for the oxygen molecule (Fig. 1) reveals that the absorption of radiation by oxygen molecules at room temperature is confined to transitions with energies in excess of Such absorption produces the 60,000 cm-l. photo-dissociative continuum of oxygen in the vacuum ultra-violet at wave lengths below 2000A. It can be seen from Fig. 1 that at elevated temperatures the population of excited vibrational energy levels will result in absorptive transitions to selected vibrational levels of the upper “8; electronic state. One thereby obtains the Schumann-Runge band spectrum in absorption. A series of these spectra is shown in Fig. 2. These spectra were photographed in absorption with a large Littrow spectrograph, by passing the radiation from a high-speed argon flashlamp through a sample of pure oxygen that had been heated by an incident and reflected shock

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Fig. 2.

Ultra-violet

absorption

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spectra of oxygen at varying temperatures. cm-atmos (L=6-4 cm, p=Bp,,).

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Optical

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MEASUREMENT

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Fig. 3. Schematic diagram of the radiation probe apparatus.

and contains a set of four variable exit slits placed in the focal plane of the spectrograph. Behind each slit is a plane mirror which directs the radiation from the slit to respective lP28 ._.. photomultiplier tubes. The tube outputs are . displayed and photographed on Tektronix 502 _.. oscilloscopes. The time constant for the system was 2 psec, and the size of the pencil of radiation through the shock tube was 4 x 4 mm. The Fig. I. Potential energy curves of the oxygen pathlength through the test gas, equal to the molecule. inside dimension of the shock tube, was 2+ in. Shock wave velocity was measured at five wave in a closed shock tube. These spectra stations along the tube with the use of thin-film show the strong dependence of the oxygen resistance gauges and oscillographic display. absorption on the temperature; for example, at The measured velocity was used to calculate the 2700A the gas undergoes a change from com- temperature and density of the gas behind the plete transparency, at room temperature, to reflected shock wave. In order to monitor the almost total opacity at 2660°K. This strong duration of the equilibrium testing time, the outtemperature dependence is due to the exponenput of a fast-response pressure transducer, tial Boltzmann distribution of molecules in the mounted in the reflecting wall of the shock tube, respective excited energy levels. A method of was also recorded. The testing time may be temperature measurement based on the absorpconsidered to be the interval of time between tion of radiation in transitions from these various the passage of the reflected shock wave, and the vibrational levels will therefore yield a measure arrival of other pressure disturbances caused by of the vibrational temperature of the oxygen. subsequent wave-interface phenomena. The table included in Fig. 3 presents the wave 3. DEWRIFTION OF THE EXPERIMENTS lengths and the size of the wave length windows A schematic diagram of the apparatus is which each channel recorded, together with the shown in Fig. 3. The optical system consists vibrational quantum number of the transitions of a xenon flash-lamp, the collimating system effective in each window. The wave lengths for channels 1, 3 and 4 were chosen so that minithrough the shock tube and a Bausch and Lomb medium quartz spectrograph equipped with a mum interference was to be expected by any photomultiplier adapter. The adapter is con- NO gamma band absorption, which would be structed as a unit to fit the plate-holder slide, important in experiments with air as the test .. _..

W. H. WURSTER

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gas.

Also, for this reason, channel 2 was purposely set in the (0, 2) gamma bandhead of NO. Typical oscillographic records for experiments in pure oxygen are shown in Fig. 4. In Fig. 4(a) the display of the flash lamp over its entire (duration is shown, both in calibration, labelled (l), and in the experiment, labelled (2). The data were read from the expanded records of Fig. 4(b) which show the intensity attenuation by the gas at the time of the reflected shock wave arrival. The lamp was synchronized with the shock wave so that its maximum output ,coincided with the arrival of the wave. The flash-lamp output is reproducible to within the ability to read the records, and allows the transmission through the gas in terms of Z/Z,, to be readily determined. Transmissions measured for as long as 100 rsec after shock wave arrival were constant, indicating the duration of the equilibrium testing time behind the reflected ,shock wave. The increased absorption in the record of Fig. 4(a) at t = O-62 msec was found to correlate exactly with a pressure increase recorded by the pressure transducer. It corresponds to subsequent heating of the gas by pressure waves resulting from the interaction of the reflected shock and the driver gas interface. 4.

R!E!WLTS

AND

CONCLUSIONS

The data from a series of experiments in pure *oxygen are presented in the graph of Fig. 5, where the transmission of the gas in each of the four wave length channels is plotted against the temperature of the oxygen for each experiment. These experiments were run at a constant molecular oxygen density of two times standard and with a pathlength of 6-4 cm. As may be expected, the transmission in channels 3 and 4, which correspond to longer wave lengths and to transitions from higherlying energy levels, does not decrease until higher temperatures are attained. Conversely, the transmission of the short-wave length ,channel 1 is attenuated at lower temperatures and rapidly reaches its minimum value. The advantage of using a series of channels is demonstrated in the broad range of temperatures (1500-3500°K) over which measurements,

SHOCK WAVE ARRIVAL +

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200

MICROSECONDS (b)

Fig. 4. Oscillosco e records of radiation intensity vs. timein channel 5 ,forthelampflash in calibration (I) and during the experiment (2). The total lamp output is seen in the monitor-oscilloscope record (a) and the enlarged portion of interest, from which the data are taken, in (b).

for these values of optical thickness, can be made. The vertical error bars on the graph reflect the ability to read the intensities from the oscilloscope records and the horizontal bars correspond to the error in temperatures introduced by an error of 1 per cent in the determination of the shock-wave velocity. A few experiments have been performed at lower values of the molecular oxygen density. To obtain readable transmission values for a given pathlength, the lower density of absorbers

Fig. 5. Curves for the measured transmission y$. temperature in each offour wavelength “windows . Optical pathlength= cm; (~/PO) 0,=2.

MEASUREMENT

OF TEMPERATURE

must be compensated for by an increase in temperature, in order to maintain an observable number of molecules in a given vibrational energy level. The results indicate that, for the pathlength of 6.4 cm, densities of oxygen molecules of O-1 standard can be conservatively measured at temperatures as low as 2500°K. For temperatures in the neighbourhood of 32OO”K, densities as low as 0.05 standard can be measured. Other wave length windows may be used to extend the range of measurement, provided the flash-lamp intensity is sufficient to provide tolerable signal-to-noise ratios. In terms of the application of this technique to other aerodynamic systems, such as tunnels and nozzles, it is of real importance to be able to predict the absorptions for densities and pathlengths other than those at which the calibrations are made. Such calculations require detailed knowledge of the number and spacings of the transitions effective in each window, the corresponding line strengths, and the dependence of the line shape on density and temperature. These parameters have been measured in the course of the study with the large Littrow spectrograph(4), and the calculations are presently being made. Preliminary calculations indicate that the shape of the transmissiontemperature curve is reasonably predictable on the assumption of a smoothed absorption coefficient over the window wave length interval. It would be most advantageous to obtain the temperature and density of the gas uniquely in a single measurement. At present, one or the

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other can be obtained if another independent measurement is available to give another relationship between temperature and density. Within the accuracy of the present system, the density dependence is too similar in the different channels, and allows only approximate values to be obtained for the density of the gas. Experiments with the radiation probe currently in progress are aimed at establishing more closely the density-temperature-pathlength limitations of the probe in pure oxygen, which will allow better comparison to be made with the theoretical transmission calculations. In addition, experiments with air are being planned which will determine the effects of possible absorption by the gamma system of NO, and which will establish the practicability of the probe for the measurement of the temperature of air in hypersonic flow situations. The author is pleased to acknowledge the contributions of Dr. C. E. Treanor and Miss Marcia Williams to this research. REFERENCES 1.

J. G. CLOUSTON,A. G. GAYDON and I. I. GLASS, Proc. Roy. Sot. A248, 429 (1958).

2.

H. C. WOLFE, Editor, Temperature-its ment and Control in Science and Reinhold, New York (1955).

3.

W. H. WURSTER,C. E. TREANORand H. S. GLICK, 1. Chem. Phys. 29, 250 (1958).

4.

C. E. TREANORand W. H. WURSTER, Measured Transition Probabilities for the Schumann-Runge System of Oxygen, to be published.

MeaszzreIndustry.