Garry L. Schott

Kinetic Studies of Hydroxyl Radicals in Shock Waves. II. Induction Times in the Hydrogen‐Oxygen Reaction

The formation of OH in the shock wave induced combustion of H2 and O2 has been measured by oscillographically recording the absorption of ultraviolet OH line radiation. The main features of the reaction course are: (1) an induction period whose length, ti, varies inversely with [O2], (2) an increase in the product [O2] ti as ti becomes short compared to the vibrational relaxation time of O2, and (3) at the end of the induction period, a sigmoid rise of [OH] to a maximum, followed by a slow decrease. ti has been studied over the ranges: 1100°≤T≤2600°K, 1.3×10—5≤[O2]≤8.0×10—4 mole/1, 0.25≤[H2]/[O2]≤5., 0.004≤[O2]/[Ar]≤0.20, and 5≤ti≤500 μsec. Agreement between incident and reflected shock experiments has been demonstrated. According to the branching chain mechanism known from explosion limit studies, ti is governed by the rate of H+O2→ lim k1OH+O according to: 2 k1[O2]ti=2.303 n, where n is the number of decades by which [OH] increases between initiation and the end of the induction period. The values of [O...

Further studies of exponential branching rates in reflected-shock heated, nonstoichiometric H2COO2 systems*

More thorough measurements are made of exponentially growing chemiluminescence, viewed end-on, during branched-chain ignition of H 2 CO O 2 Ar mixtures in reflected shock waves, to try to resolve residual questions from earlier studies by this and related techniques. Systematic variations of spectral bandpass, gas composition, and gas pressure have been made for particular H 2 :O 2 ratios and temperature spans. At H 2 :O 2 = 1/3, with CO present, exponential time constants, * , for the CO O chemiluminescence are independent of spectral bandpass over the 300–600 nm range; with CO absent, there is appreciable H 2 O 2 system ultraviolet chemiluminescence at λ > 340 nm, whose presence voids some earlier interpretations predicated on dominance of the OH A 2 Σ + → X 2 Π spectrum. At H 2 :O 2 = 0.1, * of CO O chemiluminescence exhioits a small and unaccountable variation with extent of dilution of H 2 CO O 2 mixtures in noble gas. This effect, diminished but perceptible also at H 2 :O 2 = 1/3, clouds kinetic interpretation of * in lean mixtures, but of itself does not reconcile disagreements with infrared and with incident shock CO-O exponential rates. The dilution effect is not found at H 2 :O 2 = 10, where new measurements up to T = 2400 K are made and pooled with previous rich * results to determine, for H + O 2 → k a O H + O , 1250 T 2500 K , the expression: k a = 1.22 X 10 17 T − 0.907 exp ⁡ ( − 8369 / T ) c m 3 mole − 1 sec ⁡ − 1 .

Kinetic Studies of Hydroxyl Radicals in Shock Waves. V. Recombination via the H+O2+M→HO2+M Reaction in Lean Hydrogen—Oxygen Mixtures

Recombination in highly diluted, lean hydrogen—oxygen—argon mixtures with H2/O2 ratios of 0.50 and 0.33 has been investigated with a shock tube over the temperature range 1150°—1850°K for reaction pressures of 0.85–5.41 atm. An OH ultraviolet line absorption technique was used to follow the progress of the reaction after substantial equilibration of the rapid bimolecular reactions which form OH initially. It was determined that under these conditions the recombination process is third order and its rate, Rrec, obeys the empirical relationship Rrec=keff[H][O2][Ar].keff was found to decrease slightly with increasing temperature and to depend to a small extent upon the composition of the reacting mixture. The experimental results indicate that the reaction H+O2+M→ lim kHO2+M dominates the recombination process. Further analysis of the data made it possible to obtain from keff a value of k(M = Ar) = 1.42×1015 cc2/mole2 sec at 1500°K and an estimate of k(M = H2O)/k(M = Ar) of about 30. This value of k(M = Ar) ...

Vibrational relaxation of HF in the temperature range 600–2400 °K

Hydrogen fluoride vibrational deactivation by HF, Ar, and F atoms has been studied in the temperature range 600–2400 °K using the shock tube‐laser‐induced fluorescence method. Mixtures of HF–Ar and F2–HF–Ar were heated by reflected shock waves; following establishment of thermal equilibrium, HF was vibrationally excited by pulsed radiation from an HF pin laser. Relaxation times were obtained from the time‐resolved decay of the laser‐induced vibrational fluorescence. Measurements of relaxation by F atoms were limited to temperatures > 1500 °K, where F2 dissociates completely behind the reflected shock wave. The HF self‐relaxation time, p τHF–HF, exhibits a broad maximum with temperature, peaking at ≈ 0.10 μsec · atm near 1400 °K. Fluorine atoms were found to be 2–5 times more efficient than HF molecules for HF vibrational deactivation over the range 1500–2400 °K. Argon is a very inefficient collision partner; values of p τHF–Ar over the range 800–2400 °K are reported. This is the first study in which the m...

Kinetic Studies of Hydroxyl Radicals in Shock Waves. IV. Recombination Rates in Rich Hydrogen—Oxygen Mixtures

The rate of disappearance of OH following [OH]max in shocked H2–O2–Ar mixtures has been measured by a refined ultraviolet line absorption technique. Thirty‐one experiments have been done with mixtures of 4% H2, 1% O2, 95% Ar; 2% H2, 0.5% O2, 97.5% Ar; and 8% H2, 1% O2, 91% Ar at initial pressures between 50 and 200 mm Hg. The approximate rate equation found to describe the OH disappearance is −d[OH]/dt=kapparent[OH]2[M], where [M] is total gas concentration. Assuming that the equilibrium relationship α=[OH]/[H]=K[H2O]/[H2] is maintained, and that the recombination reactions are: H+H+M→ lim k1H2+M, and H+OH+M→ lim k2H2O+M,kapparent may be interpreted as (A/α) (k1+αk2) where A is a factor near unity which incorporates the definitions of the ks as volumetric rate coefficients, the small variation of α and density during the reaction, and the participation of O and O2. (k1+αk2) increased linearly with α between α=0.01 and α=0.1, yielding k1=6±1×108 liter2 mole—2·sec—1 and k2/k1≈10±5. No dependence on tempera...

Kinetic Studies of Hydroxyl Radicals in Shock Waves. I. The Decomposition of Water between 2400° and 3200°K

The decomposition of water vapor has been studied at temperatures between 2400° and 3200°K generated in plane shock waves by following changes in the concentration of OH with time. Specific radiation absorbimetry permits these measurements to be made with microsecond resolution. The source of OH radiation used was a flash lamp containing water vapor. Measurements were made on the gas behind reflected shock waves in argon at initial pressures near 50 mm Hg and containing the reactant, H2O vapor, in amounts of the order of 1 mole %. The shock tube and associated electronic and optical equipment are described. Problems of purity, chemical analysis, and reduction of data are discussed. Absorption by OH was calibrated with the equilibrium mixtures resulting from the decomposition of H2O and from the reaction of H2 and O2. The rate of formation of OH from H2O is proportional to the H2O concentration and nearly independent of the argon pressure. The effective activation energy is about 50 kcal/mole. Addition of ...

Shocked states from initially liquid oxygen-nitrogen systems

Sets of pressures and their corresponding specific volumes and internal energies are derived from measurements on steadily propagating, planar shock waves propelled by explosively driven metal assemblies into a 1:1 atomic mixture of the elements nitrogen and oxygen in each of two liquid initial states. One of these is the equimolar solution of O2 and N2, at T≂85 K, v0≂1.06 cm3/g; the other is the pure explosive compound NO, at T≂122 K, v0≂0.79 cm3/g. Results for this system are calculated with effective spherical potentials and presented graphically for comparison with the measurements. Single‐ and reflected‐shock states are reported, as are incidental new results on pure liquid N2 at 85 K. The method of measurement is described, with reference to its previous applications to liquid O2 and Ar. First‐shock pressures from both initial forms lie between 10 and 30 GPa, and the Hugoniots intersect at a common state, near 21 GPa, where a single reflected‐shock Hugoniot is centered. Concordant measured state var...

Erratum: Shock‐Tube Study of OH Chemiluminescence in the Hydrogen–Oxygen Reaction

The exponential growth of OH*(2Σ+→2π) chemiluminescence was monitored during the induction period of the H2–O2 reaction in mixtures with [H2] / [O2] = 0.33 between 950° and 1150°K and at total gas densities between 0.50 and 1.1 × 10−2 moles/liter. An end‐on technique with reflected shock waves provided the required sensitivity. The exponential growth constant for OH* emission was found to be a factor of 2 (2.07 ± 0.17) greater than that found for O–CO emission over the same range of conditions. This result shows that OH* is formed in a reaction between two products of the chain reaction and, together with other recent evidence, indicates that the reaction is H + O + (M)→OH* + (M). The use of OH* emission to study other combustion reactions is discussed.

Quantitative line absorption spectrophotometry: Absorbance of the OH radical near 3090Å

Abstract A pulsed discharge source of OH line radiation near 3090Ais used for quantitative determination of transient OH populations in shocked gases by photoelectric absorption measurement involving a specific group of lines. The spectral intensity distribution of this line source has been determined quantitatively from photographic spectrograms, and is expressed in terms of the line shapes, their relative intensities, and the relative intensity of continuum between the lines. Computer methods have been formulated and used to synthesize the response of a thermal OH absorber population to this incident spectrum, in terms of the optical density, the absorber temperature, the band oscillator strength, individual line strengths, and a pressure broadening parameter. Finally, measurements of absorption by equilibrium OH populations in reflected shock waves at temperatures near 2800°K and pressures near 4.4 and 17.5 atm in H 2 -O 2 mixtures highly diluted with argon are reported and reconciled with computations for the source spectrum. The results demonstrate a pressure broadening effect, and corroborate the value of the band oscillator strength determined independently by others. A basis is thus established for a heretofore empirical calibration and for extending it to other regimes of temperature and pressure.

Chain-branching and initiation rates measured by spatially integrated light emission during reflected shock-wave ignition

Experimental study of ignition kinetics in H2−O2−CO−Ar mixtures by time-resolved photoelectric measurement of the growth of spatially integrated CO−O recombination radiation in reflected shock waves has been extended to cover the ranges 0.1≤H2:O2≤10.0, 1000°≤T≤2500°K, by confining the reactive gas to the downstream end of the shock tube and increasing sensitivity through a large solid angle optical view. The induction period regime of exponential growth of emission intensity, I(t)≈I0*exp(α*t), yields precise values of α* whose dependence upon [H2], [O2], and T determines, for the reactions H + O 2 → k 1 O H + O O + H 2 → k 2 O H + H O H + H 2 → k 3 H 2 O + H the functions: k1=8.6×108 exp {−(12.3±1) kcal/mole (1/T−1/1600)/R} liter/mole/sec and k2k3=1.5×1019 exp {−(20±4) kcal/mole (1/T−1/1600)/R} liter2/mole2/sec2, and indicates that k2 and k3 are, at most, about a factor of 3 from each other, in the present temperature range. The early growth of I(t) prior to the exponential regime, now observed directly, and the parameter I0*, now fairly precisely measured, bear upon the kinetics of chain initiation but require further investigation.