Hydrogen-oxygen System Research Articles

Weak and strong ignition. II. Sensitivity of the hydrogenoxygen system

This paper identifies the physical and chemical mechanisms which cause certain mixtures of hydrogen, oxygen, and argon to be very sensitive to sound wave and entropy (temperature) perturbations. Detailed simulations of the effects of sound waves in typical mixtures are used to show explicitly the effects of such fluctuations on the chemical induction time. A quantity, τ max, is defined which represents the maximum variation produced in the chemical induction time of a system, given the amplitude and frequency of a perturbation. It is observed that these perturbations may cause ignition to occur unevenly in such mixtures and this leads to ignition which appears spotty. Using detailed numerical simulations and a generalized induction parameter model derived from it, τ max is evaluated and a criterion is developed for spotty and smooth ignition behind reflected shock waves. These effects are related to weak and strong ignition observed in shock tube experiments.

Sensitivity analysis of a model for the radical recombination region of hydrocarbon-air flames

The concentration of radicals such as OH, O, and H above a flame are determined in part by three body recombination reactions. These radicals are responsible for CO oxidation and also NO production via the Zeldovich mechanism. The chemistry of the recombination region was modeled on a computer using a mechanism with nine species and nineteen reactions. A sensitivity analysis of the model was carried out to determine the effects of errors in rate constants, diffusion coefficients, and initial concentrations on the model predictions. Similar results were obtained using both the FAST (Fourier Amplitude Sensitivity Test) algorithm developed by Cukier and coworkers and a simpler brute force method. The sensitivity was large for only a few parameters. In particular, for fuel lean flames, the OH decay rate is mainly sensitive to the rate constant for H+O2+M→HO2+M. The hydroxyl concentration profiles above atmospheric, premixed, laminar methane-air flames were measured using laser absorption spectroscopy. Measurements were made for both fuel lean and stoichiometric flames. Predictions from the computer model were in reasonable agreement with the experimental profiles. The rate constant for the H+O2+H2O→HO2+H2O reaction which gives the best fit between the experiment and the model is 8±3×10−32 cm6/molecule2 at 1500 K. This is larger than that derived from earlier flame measurements, but is in reasonable agreement with shock tube results and the theoretical predictions of R. J. Blint. The model shows that the hydrogen-oxygen system stays in partial equilibrium throughout, most of the recombination zone. However, CO and CO2 do not stay in partial equilibrium.

Theory of a Hydrogen-Oxygen Diffusion Flame Part I: Profiles from a Large Damkohler Number Model

Abstract A streamwise laminar diffusion flame in a boundary layer flow is modelled for a hydrogen-oxygen system with six step reaction kinetics. The method of matched asymptotic expansions, based on the existence of a large value of the Damkohler number, provides first order inner and outer temperature and concentration solutions from which composite series are constructed for a range of fresstream reactant concentrations, some of which are chosen to coincide with experimental situations. In these cases comparisons between theoretical and experimental results are made

Scientific objectives of the Solar Mesosphere Explorer mission

The 1981–82 Solar Mesosphere Explorer (SME) mission is described. The SME experiment will provide a comprehensive study of mesospheric ozone and the processes which form and destroy it. Five instruments will be carried on the spinning spacecraft to measure the ozone density and its altitude distribution from 30 to 80 km, monitor the incoming solar ultraviolet radiation, and measure other atmospheric constituent which affect ozone. The polar-orbiting spacecraft will be placed into a 3pm-3 am Sun-synchronous orbit. The atmospheric measurements will scan the Earth's limb and measure: (1) the mesospheric and stratospheric ozone density distribution by inversion of Rayleigh-scattered ultraviolet limb radiance, and the thermal emission from ozone at 9.6 μm; (2) the water vapor density distribution by inversion of thermal emission at 6.3 μm; (3) the ozone photolysis rate by inversion of the O2(1Δg) 1.27 μm limb radiance; (4) the temperature profile by a combination of narrow-band and wide-band measurements of the 15 μm thermal emission by CO2; and, (5) theNO2 density distribution by inversion of Rayleighscattered limb radiance at 0.439 μm. The solar ultraviolet monitor will measure both the 0.2–0.31 μm spectral region and the Lyman-alpha (0.1216 μm) contribution to the solar irradiance. This combination of measurements will provide a rigorous test of the photochemical equilibrium theory of the mesospheric oxygen-hydrogen system, will determine what changes occur in the ozone distribution as a result of changes in the incoming solar radiation, and will detect changes that may occur as a result of meteorological disturbances.

Interactions between oxygen, hydrogen and supported rhodium over a wide temperature range : II. Titration reactions in the oxygen—hydrogen system and evaluation of dispersion of rhodium—alumina catalysts

The application of ther results of titration in the oxygen—hydrogen system to the determination of metal surface seems to be influenced by water, the product of the titration process. The dependence of the titration results on the number of successive titration cycles is discussed in terms of a proposed blocking mechanism, according to which water from the support gradually blocks part of the metal surface. The results of hydrogen and oxygen chemisorption and hydrogne—oxygen titrations are compared with respect to rhodium surface area determination.

Interactions between hydrogen, oxygen and supported iridium studied by pulse gas chromatography

The interactions between oxygen, hydrogen and iridium supported on alumina, and titration reactions on the catalyst surface, have been examined by pulse gas chromatography within the temperature range 193–973° K. There are two temperature ranges for the different interactions between oxygen and iridium: 193–623° K, within which surface chemisorption dominates; and 623° K where there is considerable oxidation of the bulk catalyst. Two temperature ranges have also been distinguished for the interactions between iridium and hydrogen: 193–243° K, with increasing hydrogen adsorption; and above 243° K, with gradually decreasing hydrogen adsorption. Titration reactions of the oxygen-hydrogen system on the surface of iridium-alumina have been used to determine the iridium dispersion.

Instabilities in detonation waves near the limits of propagation

Experimental work is described on the propagation of marginal detonation waves in propane-oxygen and hydrogen-oxygen systems diluted with oxygen, nitrogen and argon. The detonation tube was of rectangular section, with internal dimensions 2.3*1.0 cm, and overall length of 30 m. A continuous monitor of the wavefront velocity was achieved using a 3 cm wavelength microwave Doppler system. A cyclic wave phenomenon which is characterized by a normal and a slow velocity regime, is observed to be present near the limits of detonability in all except the hydrogen-oxygen-argon system. The relative durations of these regimes varies from one system to another and appears to be governed by the structural regularity of the detonation front. Schlieren records show that in the low-velocity regime the shock front and reaction zone are dissociated, with a separation varying between 1 and 10 cm, and that a weak transverse wave effect appears to be present at the lead shock which is non-planar.

Some Studies on Hydrogen—Oxygen Diffusion Flame

The opposed-jet diffusion flame has been considered with four step reaction kinetics for hydrogenoxygen system. The studies have revealed that the flame broadening reduces and maximum temperature increases as pressure increases. The relative importance of different reaction steps have been brought out in different regions (unstable, near extinction and equilibrium). The present studies have also led to the deduction of the oveall reaction rate constants of an equivalent single step reaction using matching of a certain overall set of parameters for four step reaction scheme and equivalent single step reaction.

The failure of marginal detonations in expanding channels

Critical conditions for detonation failure due to tube expansion have been observed in marginal detonations propagating in a 1 4 × 3 in. (6.35 × 76.2 mm) channel. In these experiments, a well established marginal detonation propagating in the narrow channel entered a test section in which one of the narrow walls was inclined to the central axis at positive angles which ranged from 10° to 45°. Experiments were performed at pressures ranging from 60 to 200 torr (8 to 26.7 kPa) in stoichiometric hydrogen-oxygen mixtures diluted with 20, 50 and 70% argon. Smoke track records obtained on the surface which is the major dimension of the tube, were used to determine failure, incipient failure or self-sustenance of the entering wave. Because of the narrow tube used in the studies the incident waves were marginal in that their velocity was below the expected CJ (Chapman-Jouguet) value, their transverse wave spacing was larger than one would see in a large tube, and the transverse waves were of greater strength than in an ordinary detonation. All of these indicators of marginal behavior became progressively more pronounced as the pressure dropped from 200 torr (26.7 kPa) to the limit pressure of approximately 58 torr (7.73 kPa). The most interesting result of this experimental investigation is that the theoretical analyses predicted that simple one-dimensional opening of the tube should not show a pressure dependence to failure, while the experiments showed a definite decrease in the opening angle required for failure as initial pressure decreased. This behavior is related to the marginality of the incident waves, which is observed to increase smoothly with decreased pressure. It is postulated that detonation failure in the hydrogen-oxygen system occurs when the shock velocity at the end of the cell drops to about 0.60 of the CJ value due either to marginal behavior or to an expansion of the cross section of the tube.

Solution technique for equilibrium chemistry of hydrogen-oxygen systems

References Breig, W.F. and Crosbie, A.L., Two-Dimensional Equilibrium: A Semi-Infinite Medium Subjected to Cosine Varying Radiation, 1973, pp. 1395-1419. Crosbie, A.L., and Viskanta, R., The Exact Solution to a Simple Nongray Transfer Problem, Vol. 9, 1969, pp. 553-568. Crosbie, A.L., and Khalil, H.K., Radiative Transfer in a Nongray Spherical Layer: Simplified Rectangular Model, 1973, pp. 359-367. Breig, W.F. and Crosbie, A.L., Two-Dimensional Equilibrium, Journal of Mathematical Analysis and Applications, Vol.46, 1974, pp. 104-125. ' Breig, W.F. and Crosbie, A.L., Two-Dimensional Equilibrium, Journal of Quantitative Spectroscopy and Transfer, Vol. 14, 1974, pp. 1209-1237.

Dynamics of the exothermic process in combustion

The paper presents an attempt to provide a link between chemical kinetics of exothermic processes in a combustion system and the gasdynamic phenomena they are capable of generating. For this purpose experiments were performed using the reflected shock technique with a mixture of 2H 2 +O 2 +27A, while gasdynamic phenomena were observed by means of high-speed cinematographic laser schlieren photography and laser-shear interferometry. The analysis, based on the most recently available kinetic rate data for the hydrogen-oxygen system, was carried out by the use of a numerical technique especially developed for handling properly the “stiff” differential equations describing the rates of chemical reactions, combined with an appropriate numerical technique for the integration of the partial differential equations of non-steady, inviscid gasdynamics in Lagrangian form. The agreement between experimental and analytical results is quite satisfactory, providing thus, on one hand, a rationalization for the observed gasdynamic effects of exothermic reactions, and, on the other, an experimental check of the validity of currently available chemical kinetic rate data for the hydrogen-oxygen system with respect to the macroscopic effects of its reaction mechanism.

Electron spin resonance studies of gas-phase oxidation reactions. The hydrogen-oxygen system at atmospheric pressure

An apparatus is described in which radicals can be identified by ESR and molecular products by polarography or gas chromatography. The radicals are removed from the reaction mixture through a micro-leak and condensed in a matrix on a cold finger. From a study of the system, 1 H 2 (1 D 2 ) - 1 O 2 - 4 N 2 at 546°C and a pressure of 1 atm, in reactors with and without a B 2 O 3 coating, evidence has have been produced for the presence of HO − 2 and DO − 2 radicals. Following the earlier suggestion that the formation of H 2 O 2 occurs by bimolecular reaction of HO − 2 , a mechanism is studied and compared with the experimental results.

Studies on Hydrogen–Oxygen Systems in the Electrical Discharge. VI. Detection by Laser Raman Spectroscopy of O2 and O3 Trapped at 80 °K

Further study by Raman spectroscopy of the condensed products from electrically dissociated water vapor and other related systems has revealed the presence, not only of ozone in appreciable amounts, but also of molecular oxygen in still greater amounts trapped at 80 °K. The concentration of ozone relative to the hydrogen oxides varied somewhat with experimental conditions, and also locally due to some segregation effect in the glassy condensate. The concentration of trapped oxygen showed still wider fluctuations depending particularly on surface effects in the discharge–flow system. On warming up under vacuum the intensity of the O2 and O3 bands began to decrease even before the crystallization temperature was reached (about 150 °K), thereby confirming that the gas evolution at that stage is mainly a desorption process. Therefore, the often quoted ratio of total evolved O2 to residual H2O2 could not be a reliable index of the formation of hydrogen polyoxides, H2O3 and H2O4, in these systems. A mechanism is proposed for the formation in situ of the trapped gases and in particular, for a source of oxygen atoms in dissociated water vapor.The fundamental vibrations of the ozone molecule, ν1 = 1106, ν2 = 703, and ν3 = 1036 cm−1, were confirmed by polarization and isotope shift measurements.

The Hydrogen-Oxygen Reaction in Potassium Chloride-Coated Vessels in the Region of the Third Explosion Limit

Abstract New measurements are reported in the region of the third explosion limit in the hydrogen-oxygen system. Detailed studies have been made of the effects of temperture, mixture composition and vessel diameter on the third limit pressure. These results have been interpreted in terms of the chemical processes occurring and of the Frank-Kamenetskii theory of thermal explosions. The Frank-Kamenetskii theory is capable of giving a good quantitative account of the principal experimental observations.

Auto-ignition of hydrocarbons behind reflected shock waves

The paper reports on the study of auto-ignition of hydrocarbon-oxygen mixtures behind reflected shock waves. Because of their bearing on the problem of knock in internal combustion engines, n-heptane and iso-oc??ne were chosen as the combustible species. Their stoichiometric mixtures with oxygen had to be diluted with 70% argon to reduce the influence of the boundary layer. Photographic records demonstrated the existence of two different modes of ignition, as has been previously established for the hydrogen-oxygen system. One, called strong ignition, manifested itself by a shock wave generated in the immediate proximity of the back wall of the shock tube. The other, refered to as mild ignition, had chemical reactions starting at distinct points in the gaseous medium producing essentially constant pressure flames. The pressure-temperature limits between the regions of mild and strong ignition (strong ignition limit) were determined. From the same experimental tests, induction time data was obtained over the pressure range of 1–4 atm and the temperature interval of 1200–1700°K. The strong ignition limits were found to correlate best with the lines of constant partial derivatives of the logarithm of induction time with respect to temperature at constant pressure.

Studies on Hydrogen–Oxygen Systems in the Electrical Discharge. V. Raman Spectra of the Trapped Products

The condensed products (at – 180 °C) of electrically dissociated water vapor and other related systems were examined by means of laser Raman spectroscopy. A new type of total reflection cell had to be developed for the study of metastable species trapped in an amorphous matrix. Besides the characteristic O—O stretching band of hydrogen peroxide at 878 cm−1, always the strongest, another fairly strong band at 500 cm−1, and a third one at about 760 cm−1 were present in all cases. A couple of weaker bands around 820–830 cm−1 and 430–440 cm−1 occurred only in oxygen-rich system (H2O2 vapor or H2O–O2 mixtures). In the latter, the presence of condensed ozone was confirmed by a band at 1025 cm−1, and occasionally another faint one around 1120 cm−1.The relative intensities of the bands varied locally in a given sample, indicating uneven composition.Isotopic shifts with 18O confirmed that these Raman bands arise from vibrations of oxygen atoms. Essentially the same results were obtained in hydrogen as in deuterium systems, except that in the latter case the new bands were appreciably stronger and sharper. The present results support the assignment of previous infrared spectra to hydrogen polyoxides, H2O3 and H2O4.

On the shock-induced ignition of explosive gases

The paper elucidates the distinction between two regimes of ignition, one strong and the other weak, that characterize the onset of combustion in homogeneous gaseous mixtures whose reaction mechanism is controlled by a chain-branching step, such as that associated with the H+O2 collision in the hydrogen-oxygen system. On the basis of experimental observations, made primarily by the use of a stroboscopic laser-schlieren system yielding a sequence of photographic records of the entire flow field behind a reflected shock near the closed end of the tube at a repetition rate of 2 microseconds between frames, it has been revealed that strong ignition is manifested by a practically instantaneous appearance of a relatively plane pressure front associated with the generation of a flow field across the whole cross section of the tube, while mild ignition starts in the form of distinct flame kernels whose growth is comparatively slow and essentially devoid of any gasdynamic effects. The demarcation line between the two regimes of ignition is then shown to be associated with a critical value of the gradient of the induction time with respect to temperature at constant initial pressure. Specifically, for a stoichiometric hydrogen-oxygen mixture, this turns out to be −2 microseconds/°K. Although basically controlled by the same elementary processes as those that establish the second explosion limit, this so-called strong ignition limit differs from the locus of states obtained by direct extrapolation of the second limit. Moreover, it does not intersect with any of the explosion limits, encompassing a region that is fully contained within the classical explosion regime.

Studies on hydrogen–oxygen systems in the electrical discharge. IV. Spectroscopic identification of the matrix-stabilized intermediates, H2O3 and H2O4

The infrared absorption of the products from electrically dissociated H2O or D2O vapor and other hydrogen–oxygen systems trapped at liquid nitrogen temperature was measured between 4000 and 300 cm−1. Four new absorption bands were found in the deuterated systems at 857, 820, 760, and about 440 cm−1. By isotopic substitution of 18O the frequencies are shifted to 806, 775, 717, and ~420 cm−1 as expected for O—O vibrations. In the hydrogen systems this region is obscured by the strong libration bands of H2O and H2O2 molecules. Temperature and composition effects show that more than one new species is involved. Accordingly the new spectra are assigned to the often postulated polyoxides, H2O3 and H2O4, stabilized in the water–peroxide matrix. The more abundant, and also more stable, H2O3 has a half-life of some 5 h at −65 °C.The observed frequencies are consistent with zigzag chain structures linked by single covalent bonds as in hydrogen polysulfides. Relative concentrations of the polyoxides are estimated at 5 to 10 mole% depending on the composition of the starting material. Possible mechanisms of formation and decomposition are discussed.

Studies on hydrogen–oxygen systems in the electrical discharge. III. Surface effects

Surface effects in the reactions of dissociated hydrogen–oxygen systems and the products condensed therefrom have been investigated. Water vapor at about 0.1 Torr was streamed at high velocity through an electrodeless discharge confined in tubes of different materials or with various surface coatings. In all cases the products trapped in liquid nitrogen evolved oxygen gas on warming, but the relative amounts varied considerably from one type of surface to another. In some cases there was clear evidence that the walls of discharge tube were attacked by hydrogen atom bombardment. The decomposition, both thermal and electrical, of pure hydrogen peroxide vapor was studied likewise. The pyrolysis products gave off very little oxygen on warming. By contrast the products from electrical decomposition, even at low power level, evolved much oxygen, most of it above the melting point.It is concluded that there is always some decomposition of hydrogen peroxide in the trapped products. However, this does not seem sufficient to account for all the evolved oxygen; at least not in the case of dissociated water vapor.

On the influence of non-steadiness on the thickness of the detonation wave

In order to investigate the influence of the non-steadiness of wave motion on the thickness of the detonation front, a 'quasi-steady’ kinetic model of the induction process is considered and the flow field treated as that of a decaying blast wave. The ‘quasi-steady' model is based on the assumption that the induction time is governed by the same law as that corresponding to steady flow conditions, while the thermodynamic parameters of state vary because of the non-steady nature of the blast wave. For a point, line or plane symmetrical motion, the induction period and the corresponding wave thickness, identified as the distance between the shock and the combustion front, have been expressed, under such circumstances, in terms of the Mach number at the moment when a given particle was overtaken by the front. The results demonstrate that in the non-steady case the wave thickness can become significantly larger than that corresponding to the same Mach number in steady flow, and that, in fact, as the initially overdriven wave decays, the induction time rapidly approaches infinity, indicating a complete extinction of the detonation process. The theory has been shown to be in satisfactory agreement with experimental observations of marginal detonation in a hydrogen-oxygen system, and the point of extinction was found to occur there when the front velocity became roughly equal to the Chapman-Jouguet value.