N THESE DAYS
of intense interest
I in very fast rate processes and in
the chemical and physical behavior of gases a t very high temperatures, the shock tube has become a common laboratory device. It is, regrettably, not yet a routine tool; but i t is almost a necessary tool for attacking certain types of problems. Why is i t necessary? Because it is the best approximation that can be found to an infinitely fast-acting thermostat. If one is to study a chemical process in a condition considerably, or even drastically, removed from equilibrium, he must be able to do one of two things : either he must introduce reactants into the system and mix them in a time that is short relative to the characteristic reaction time (the halflife, for example) ; or he must premix his reactants and then change the conditions of the system (temperature, density, catalysts) to produce reaction at a rate that is measurable. The change must, of course, be accomplished a t a rate that is fast relative to the reaction rate. Otherwise, when the change has been accomplished, it is found that nieasurements are being made on a system a t chemical equilibrium, The equilibrium properties may, to be sure, be of importance; but they are not the main concern when studying rate processes. I n the early days of gaseous chemical kinetics, the usual reactor was a glass bulb in a heat bath. At best, reactions having half-lives on the order of seconds can be studied using such a system. Imagine, now, that the heat bath is modified in an ingenious way, so that its temperature can be changed from room temperature to any selected temperature, up to perhaps 25,000’ K, in 28 A
e
ANALYTICAL CHEMISTRY
about second. Suppose, further, that the heat exchange between the heat bath and the gas inside the bulb is perfect, so that the gas temperature equals the heat bath temperature, even during the rapid change. Reactions having or 10-9 half-lives as short as second can then be studied. The big problem will be to develop analytical tools that can follow such fast reactions. What has just been described is the essence of the .shock tube in its application to the study of chemical kinetics. It accomplishes the objectives of fast temperature change and avoidance of heat transfer limitations in two ways: .The temperature change is accomplished by means of a steep compression wave, traveling through the (initially unreacting) gas. As the wave overtakes the gaseous molecules, their translational energies shift abruptly-in a time on the order of 10-lo second. .The heat bath and the system t o be studied are a homogenous gas mixture. T h a t is, the heat bath molecules may be argon and the system to be studied may be, say, a diatomic molecule. The two are mixed, introduced into the shock tube, and traversed by a shock wave. Heat transfer from one to the other occurs at rates corresponding t o molecular collision rates. The virtues of the shock tube have led to numerous applications 0~7erthe past decade or two, and to \That is now a large body of literature. The applications have been possible only because of concomitant development of suitable instrumentation for the study of fast processes. A number of reviews of shock tubes, instrumentation, and
applications have appeared as articles and as books. Several of these are listed in the bibliography at the end of this article, and they can very profitably be perused by those interested in a more comprehensive understanding. Our purpose here is to present an introductory description of the basic device, together with some comments on the instrumentation, and then t o mention several examples of its applications that may give the reader a feeling for the capabilities of laboratory shock tubes. No effort is made t o be comprehensive in coverage or citation of the original literature. The Basic laboratory Shock Tube
All shock tubes, so far as we are aware, operate on the principle indicated in Figure 1, or on modifications of it. The tube is divided into two compartments. The driver gas, which is normally helium or hydrogen (which have high sound velocities), is separated from the driven gas (to be heated) by a diaphragm of plastic or metal that is just strong enough to sustain a particular pressure imbalance (Figure l a ) . The diaphragm is ruptured with a suitable device, or by introducing a slight overpressure. At the moment of rupture the pressure distribution along the tube ideally looks like Figure l b . The condition is, of course, very unstable-the driver gas pours into the driven section, sweeping the low-pressure driven gas ahead of it and piling it up on the gas boundary, or contact region ( C , in Figure I C ) ,just as though a supersonic piston were advancing down the tube. The “piled-up” gas is the shock wave. As the “piston”
REPORT F O R ANALYTICAL CHEMISTS
HOWARD B. PALMER Professor of Fuel Science Pennsylvania State University University Park, Pa 16802 moves further down the tube, more and more gas piles up ahead of it, so that the body of shookedgas eontinuously icre88e~in length (&flusiou through the coutact region is relatively slow). Thus, if a long residence t i e for the ho$ grma 3 d e s d , a long shock t u b sbcdd be used. In an ideal shock tube, the properties of the shocked gas (datea, Figure IC)are uniform, all the way from the shock front to the contact region. In actual shock tubes, deviations from ideality =,-in general, remarkably small. Time is usually some decrease in the vehities of the front and the eontsot region a~ the wave p r o m dam the tube. The dec(“attenuation”) is assooiated with the hn~stion of an semdynamic barndmy layer along the wall of the tuba One has to be aware of She exidawe of the boundary layer, narily does not cause especially if the velo&y attolmstion is measured experimentally and if measurements on the shocked gas are made reasonably e l m to tihe front. Low p m u r e shoeke in smell tubes, Le. conditions conducive to serious boundary layer p m b h , are obviously to ba avoided. In Figure Id Is depiated the pressure distribution after sh& d e @ tion. The primary shook wave has reached the end of the tube snd the shocked gas haa piled up on the end wall. The m l t is another shock wave, the front of which moves htrther and further from the end wall a8 time passas. It moves into the once-shocked gas, BO tbe gas W i d this new front is hotter and denser than the primary shack. For 8ooe purposes, this feature is a su$aieut r e m n to prompt the IQE of refteLted
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-which is frequently at same distance behind
used for optical measurements on primary and reflected shock waves
realized that the reactions in the reflected shock may typically have
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pound whose rate of reaction is well established as a function of temperature, and measures the extents of reaction of both. The internal standard then essentially serves as a means of measuring the temperature. The method requires that there be no interaction between the reaction mechanisnis of the two compounds. “Exotic” Tubes. I n this category we include the numerous modifications of the conventional arrangement that have been devised to accomplish special objectives. As illustrations, with no attempt to be all-inclusive, the following might be mentioned: .Monster shock tubes. Examples are the very long and very powerful shock tubes that are used t o study hypersonic flow. Sometimes these tubes terminate in a nozzle, in which case they may properly be termed hypersonic wind tunnels. Another huge type of shock tube is the 61-cni diameter tube a t the A4vco-EverettResearch Laboratory. It has been used with great success in studying the density distribution and kinetic processes in shock wayes a t very low pressure (-1 torr in the shock). M a s s detection. Instead of using light absorption or emission in the shock, or instead of quenching a reflected shock, it would be a great advance to sample the hot gases and directly determine coiicentrations of various species as a function of time. This has been done by putting a leak in the end of a shock tube, feeding a sample of gas continuously to a time-offlight mass spectrometer or a quadrupole mass filter. As might be expected, there are severe difficulties; but the technique has already proved valuable and its future seems bright. Molecular beams. The notion of the pinhole or a nozzle a t the end of a shock tube suggests that one might be able t o draw from the shocked gases a molecular beam of relatively large energy and large flux (but brief duration). Efforts have been under way in this direction for a number of years, particularly by Skinner ( 2 ) of the Corne11 Aeronautical Laboratory. The problem is a particularly difficult +Circle -See
one, but the scientific rewards of success should be considerable. Modified optics, light sources, and detectors. Optical modifications take many forms: windows that transmit in the infrared, far ultraviolet, or even the aacuum ultraviolet ; special light sources that emit in suitable regions of the spectrum, from the infrared to the vacuum ultraviolet; continuum flash sources with very short duration and with controlled triggering delays; and laser sources for interferometry or schlieren studies. These various alterations in the character of the shock tube make possible measurements on otherwise rather inaccessible species. Thus H atom concentrations can be measured by atomic absorption in the vacuum ultraviolet; or infrared can be used t o look at vibrational transitions in COZ. The introduction of a laser as a light source for schlieren measurements has enabled Kiefer and Lutx ( 3 ) to make remarkably precise measurements of vibrational relaxation rates. Other detection methods. I n special arrangements, it has been possible to use microwave absorption, X-ray absorption, and electron absorption or scattering to study shock properties. Wray ( 4 ) remarks that electron scattering seems to show particular promise for the study of kinetic processes in shock waves. Alteration in initial conditions. One characteristic limitation in most shock tube arrangements has been the need to fill the tube with a gas or gas mixture that is unreactiae until its temperature is abruptly altered by a shock wave. Several ingenious ways to circunivent this limitation have been developed in the past few years. One way is to continuously run a discharge of some sort through the gas, prior to setting off the shock wave. This produces an initial condition in which atoms, radicals, ions, or excited molecular species may be present in substantial concentration before the shock wa~7e processes the gas. Their reactions can then be studied behind the shock. These methods were pioneered by Wray and Teare (6) and by Hartunian (6).
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Circle No. 70 on Readers’ Service Card VOL. 39, NO. 14, DECEMBER 1967
35 A
REPORT FOR ANALYTICAL CHEMISTS
The CHROMALOG is a portable integration system designed for use with any gas chromatograph. It produces an automatic digital print-out of the area of each peak, and the incorporation of a dip detector enables a reading to be obtained for partially resolved peaks. Write for Bulletin
I n another approach, flash photolysis is combined with a shock wave. The flash creates unstable species which then react a t the shock conditions. The method is difficult but it has strong potential. It was developed by Burns and Hornig ( 7 ) and by Bradley (8). Finally, mention may be made of the over-dissociation technique used by Wray ( 9 ) in a study of the recombination rate of 0 atoms. H e introduced ozone, shocked it, and watched the 0 atoms recombine to 02. Ozone, being very unstable, decomposed almost instantaneously upon encountering the oncoming shock front. The principle has now been employed by Fishburne and Edse (10) in studying other reactions of 0 atoms. It may be possible to extend the technique t o other species. Shock Tube Chemistry
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36A
D
ANALYTICAL CHEMISTRY
We have seen t h a t the shock tube is basically a straightforward derice, and that i t is amenable to a wide variety of modifications that permit the study of many different systems by suitable techniques. The kinds of studies that have been carried out are, for the most part, discussed very thoroughly in one or another of the references in the Bibliography. Let us therefore limit ourselves here t o a very brief statement of the objectives of the types of chemical measurements carried out using shock tubes. Relaxation rates. In a sense, relaxation rates include all rate processes. However, for present purposes vie exclude changes in chemical composition and define relaxation as the reattainment of a Boltzmann distribution of internal energies after an abrupt change in the translational energy of a system. Therefore we include: ( a ) Rotational relaxation. This is generally extremely fast and is measured essentially by measuring the thickness of the shock front. It is such a fast process t h a t a chemist can safely assume rotational energies to be completely Boltzmann in character throughout the course of all reactions t h a t he is apt t o encounter. (b) Vibrational relaxation.
This process is usually much slower than rotational relaxation, and may be slow enough t o limit the rate of chemical reaction in some instances -e.g., the dissociation of diatomic molecules a t very high temperature. Shock tube studies have provided all the information t h a t exists on this signficant matter, and have provided a large body of knowledge on vibrational relaxation rates in chemically unreacting systems. (c) Electronic relaxation. The equilibrium population of excited states of atoms and molecules increases when they traverse a shock front. The rate a t which the new equilibrium state is attained is a subject of considerable ignorance, though some useful knowledge exists. V o r k has been done on the problem, using shock waves [e.g., by Levitt (11)] ; but it merits greatly increased emphasis. Ionization rates. The phenomenon of thermal ionization is very closely connected to electronic excitation. As a rate phenomenon, it has been studied with some success using shock tubes, but it cannot be considered to be \vel1 defined as yet. Impurities seem t o be particular hazards in attaining reliable information. Chemical reaction rates. The types of reactions studied in shock tubes include: ( a ) Dissociation of diatomic molecules. This has been an important contribution of the shock tube method, but unsolved problems still exist. (b) Recombination of atoms. The work of Wray on 0 atoms has been mentioned. Reconibination studies in shock waves are rare. (e) Decomposition of polyatomic molecules. Many important studies have been reported in the shock tube literature, and have contributed strongly to current basic knowledge of reaction kinetics. Triatomics have been especially well examined ( 1 2 ) . (d) Isomerixation and exchange reactions. Cndoubtedly the most work on these processes using shock tube methods has been performed by Bauer (13) and his group a t Cornel1 University. (e) Other complex reactions. 4 s illustration we may cite com-
REPORT
bustion processes, which, because they usually are very fast, have been particularly difficult to unravel by most available techniques. They have been much studied in recent years by shock tube (and other) methods. Perhap the most outstanding accomplishment in shock tube studies of combustion has been the gradual elucidation of the H A 2 reaction through the efforts of Schott (141, Gardiner ( M ) , and their colleagues, together with information gathered by other means. Epilibrivm pvpmtiea. Earlier, it was pointed out that chemical equilibrium is frequently attained in shmk waves before the contact region is reached. I n q m e favorable cmes, it hae been poasible toilse this feature to determine, or at least shed light upon, some significant thermochemical quantities such as the heata of dissociation of 4, Nz,and CO. Another way of using the equilihrium region is to make spectroscopic measurements on it. In this way one may be able to mzaaure transition probabilities of high-temperature atomie or molecular species. Measuremen@ of the light emitted from the equilibrium region have been used to determine the rate a t which atoms combine to form molecules by two-body processes, as opposed to the more frequent three-body prooeases.
Concluding Remarks
It is hoped that the reader of this survey will appreciate the power of the shock tube technique, but a t the same time see its essential simplicity. We have ~ s c i o u s l y avoided dwelling on the many factors that one has ta worry about when doing shock tube research. There am errors and uncertainties (in fact, shock tube research ia rather famous for them!) but .the shock tube is by far the best tool that we have for studying most of the significant chemist@ and physics of high temperature procesees in gases, and particularly for studying the rate8 of tho= processes. Precision of experiments and confidenoe in the shock tube method are increasing steadily, while a t the same time we see that there are con-
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3
REPORT
stantly appearing new and imaginative variants on the method that will continue to widen its applicability. Specific References
W.Tsang, J . C h e m . Phys., 40, 1171 (1964) ; 44,4283 (1966) ; 46,2817 (1967). (2) G. T. Skinner, Phys. Fluids, 4, 1172 (1961). (3) J. Kiefer and R. Lutz, J . Chem. Phys., 44, 658, 668 (1966). (4) K. L. Ft‘ray, item 5 in the Bibliog-
(1)
raphy.
( 5 ) K . L. Wrap, J . Chem. Phys., 44, 623 (lQA6’). \__..,
(6) R. Hartunian, 11:. Thompson, and E. Hevitt, J . Chem. Phys., 44, 1765 (1966). (7) G. Burns and D. F. HorniE. Can. J . Chem., 38, 1702 (1960). ( 8 ) J. N.Bradley and R. Tuffnell, Proc. Roy. SOC.( L o n d o n ) , A280, 198 (1964). ( 9 ) IC. L. Wray, J . C h e m . Phys., 38, 1518 (1963). I
(IO) E. Fishburne and R. Edse. J . Chem. Phus.. 44. 515 (1966). (11) B . ’ P.‘Levitt, Trans. Faraday Soc., 59, 59 (1963). (12) H. A . Olschewski, J. Troe, and H.
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Gg. Wagner, Eleventh Symposium ( I n ternational) on Combustion, The Combustion Institute, Pittsburgh (1967), p. 1
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IDD.
I
(13) R.S. Watt, P. Borrell, D . Lewis, and S. H. Bauer, J . C h e m . Phys., 45, 444 (1966). (14) G. L. Schott, J . Chem. Phys., 32, 710 (1960) ; C. W. Hamilton and G. IT. Schott, Eleventh S y m p o s m n (International) o n Combustion, The Combustion Institute, Pittsburgh (1967), p.,635. (15) D. L. Ripley and W.C. Gardiner, J . Chem. Phys., 44, 2285 (1966).
Bibliography
Papers or book: chapters (1) H. B. Palmer, “The Shock Tube as a
Tool in Fuel and Conibustion Research,” J . Inst. Fuel, 34, 359 (1961). (2) S. H. Bauer, “Chemical Kinetics in Shock Tubes,” Science, 141, 867 (1963). (3) S.H . Bauer, “Shock TT’aves,” in Annual Review of Physical Chemistry, 16, Annual Reviews, Inc., Palo Alto, Calif. (1965). (4) H. B. Palmer, “Chemical Kinetics
and Hypersonic Flow,” in Fundamental Phenomena in Hypersonic Flow, J. Gordon Hall, Ed., Cornel1 University Press, Ithaca, N.Y. (1966). (5) K. L. Wray, “New Experimental Techniques $r Kinetic Studies in Shock Tubes. Reot. AMP 232. Avco Everett Reeearch f,aboratory, Eiwett, Maas., July (1967). (6) R. A . Strehlow. “Detonation and the Hydrodynamics of Reactive Shock Waves,” Preprints, Div. of Fuel Chemistry. Am. Chem. Soc., 11, No. 4, September (1967) 1. ( 7 ) S.H. Bauer,’ ’Shock Front Structure --A Chemical Kinetics View,” Ibid., p. 59.
Books (1) A. Ferri, Ed., “Fundamental Data
Obtained from Shock-Tube Exoeriments,” Pergamon Press, Ling Iiland CitJ-, N. Y. (1961).
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ANALYTICAL CHEMISTRY
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