C.-E. Paillard

Elementary reaction kinetics studies of interest in H2 supersonic combustion chemistry

Abstract Elementary reactions of interest in H2 supersonic combustion chemistry have been investigated using a shock tube technique connected to an atomic resonance absorption spectrophotometer. The rate constants for the reactions: (1) O + H 2 → OH + H (2021–3356 K ) (−2) O + O + Ar → O 2 + Ar (2740–4530 K ) (−3) O + O + N 2 → O 2 + N 2 (2740–3460 K ) (−4) H + O + Ar → OH + Ar (2950–3700 K ) (−5) H + OH + Ar → H 2 O + Ar (2790–3200 K ) (−6) H + OH + H 2 O → H 2 O + H 2 O (2790–3200 K ) have been specified in the temperature range quoted respectively. The following rate coefficients were found: k 1 =7.1×10 6 T 2,1 exp (−4140/T) cm 3 mol −1 s −1 k −2 =1.8×10 17 T −1 cm 6 mol −2 s −1 k −3 =3.6×10 17 T −1 cm 6 mol −2 s −1 k −4 =6.75×10 18 T −1 cm 6 mol −2 s −1 k −5 =3.75×10 21 T −2,1 cm 6 mol −2 s −1 k −6 =6.75×10 22 T −2,1 cm 6 mol −2 s −1 . The overall uncertainties in these expressions have been estimated by considering experimental parameters which contributed to uncertainties in the rate constants evaluation. These rate constants are compared to those reported previously in the literature. The effect of the studied reaction rate constants accuracy improvement on the H2 supersonic combustion modeling has been investigated.

Induction Delay Times and Detonation Cell Size Prediction of Hydrogen-Nitrous Oxide-Diluent Mixtures

Silane-nitrous oxide mixtures are widely used in some industries such as semiconductor manufacturing. Since the decomposition of silane is faster than that of N2O and involves the formation of H2, the H2-N2O system might be an important sub-system of the silane oxidation mechanism. The induction delay times of this system have been widely studied in the low pressure range. Aim of the present study is to investigate the high-pressure behaviour of H2-N2O-Ar. Induction delays behind reflected shock waves have been measured between 1300–1860 K, at the pressure of 910±50 kPa for mixtures with equivalence ratios of 0.5, 1, and 2. It has been shown that equivalence ratio variations have no effect on induction delays. The modeling of delays has been improved by including an excited OH* kinetic sub-mechanism. Finally, various techniques of detonation cell size prediction have been evaluated in comparison with available experimental data.

Experimental study and kinetic modeling of the thermal decomposition of gaseous monomethylhydrazine. Application to detonation sensitivity

The thermal decomposition of gaseous monomethylhydrazine has been studied in a 38.4 mm i.d. shock tube behind a reflected shock wave at 1040–1370 K, 140–455 kPa and in mixtures containing 97 to 99 mol% argon, by using MMH absorption at 220 nm. A chemical kinetic model based on MMH decomposition profiles has been developed. This model has been used, with some assumptions, to evaluate the detonation sensitivity of pure gaseous MMH. This compound is found to be much less sensitive to detonation than hydrazine.

Shock tube study of ignition delays and detonation of gaseous monomethylhydrazine/oxygen mixtures

Abstract The oxidation mechanism of gaseous monomethylhydrazine (MMH) above 1000 K consists of a simultaneous rapid decomposition and a slow oxidation followed by a rapid oxidation reaction. The ignition delays between the reflected shock arrival and the rapid oxidation were measured in a 38.4-mm-i.d. shock tube behind the reflected shock wave at 900–1440 K, 160–350 kPa, and for φ = 1–2.27, and mol.% Ar = 82–96, using MMH absorption at 220 nm. A simple relationship between ignition delay, shock conditions, and mixture composition is obtained. A chemical kinetic model based on MMH decomposition and on the oxidation of decomposition products has been developed, in order to calculate the ignition delays above 1000 K, and compare them to the experimental shock tube data. The temperature, pressure and concentration profiles, computed with a 300 reaction-model, reproduce well the pressure and absorption signals. Experimental and computed ignition delays agree in the ranges: 1070–1260 K and 190–290 kPa, within a 35% accuracy. If the ignition delay is sufficiently reduced, then a detonation wave can be initiated behind the incident shock. A study of detonation velocity and cellular structure for stoichiometric and rich mixtures, eventually diluted, has been carried out in the shock tube at low initial pressure. The critical conditions for the onset of detonation and the conditions for the propagation of a self-sustained detonation wave have been determined. The velocity deficits with regard to C-J values reach 20% at the onset of spin. Cell size versus pressure curves show that stoichiometric MMH/oxygen mixture is less detonable than both stoichiometric unsymmetrical dimethylhydrazine- and hydrazine/oxygen mixtures. No simple correlation exists between the detonation cell size and the induction distance for the explosive oxidation estimated from ignition delay data.

A High Temperature Chemical Kinetics Study of the Reaction: OH+Ar = H+O+Ar by Atomic Resonance Absorption Spectrophotometry

The OH radical dissociation has been studied behind shock waves in the temperature range of 2950-3700 K at total pressures of about 220-310 kPa by Atomic Resonance Absorption Spectroscopy using mixtures of H2 and O2 highly diluted in Ar. The OH decomposition was followed by monitoring the time dependent O concentration in the post shock reaction zone. In our experimental conditions this reaction is very sensitive to O-atom profiles. The rate constant k−2 of the termolecular recombination reaction between H, O and Ar as collision partner was calculated from OH dissociation rate constant k2 measurements and the equilibrium constant. The results expressed in the simple Arrhenius form are: with an uncertainty of ± 30 %, which are in good agreement with the estimate of Tsang and Hampson in 1986. No experimental results were reported by earlier investigators.

Shock tube study of the effect of nitrogen or hydrogen on ignition delays in mixtures of monomethylhydrazine + oxygen + argon

Abstract Ignition delays have been investigated for mixtures of monomethylhydrazine (MMH) + O 2 + Ar at high temperatures (850–1440 K) in the pressure range 160–400 kPa, for equivalence ratios of 1–4.3 and in the Ar dilution range 82–96 mol%. The replacement of argon by nitrogen leads to a twofold increase of the delay. The addition of 0.88–2.4 mol% hydrogen to the MMH + O 2 + Ar mixture shows that hydrogen has, globally, a promoting effect on the ignition in MMH + O 2 + (Ar). Moreover, it is shown, with the help of a kinetic model that the addition of MMH by forming radicals during its oxidative decomposition promotes hydrogen oxidation in the temperature range 900–1000 K.

The Onset of Detonation Behind Shock Waves of Moderate Intensity in Gas Phase

The shock-to-detonation transition (SDT) in gaseous n-heptane/oxygen/argon mixtures has been experimentally studied, using a shock tube, at low initial pressure (2–4 kPa) for a better understanding of the deflagration-to-detonation transition process. The detonation is generated by a precursory shock wave (PSW), with a Mach number smaller than that of the self-sustained detonation. Pressure (P2) and temperature (T2) behind incident shock waves have been accurately determined from the PSW velocity. The transition occurs in the measurement zone located between 3.20 m and 3.65 m from the shock tube diaphragm. The auto-ignition of mixture behind PSW is immediately followed by the onset of a combustion wave, which propagates at near Chapman–Jouguet (CJ) detonation velocity in the mixture carried at P2, T2 conditions. Consequently, the pressure peak can reach 350 times the initial pressure during the transition. The combustion wave merges with the PSW to form an overdriven detonation propagating in the initial mixture at velocity that progressively decreases to a value close to CJ value, of the order of 2 000 m.s−1. The experimental particles heating times are compared with the computed ignition delay times, τI,2, by using a detailed kinetic model of n-heptane oxidation. For stoichiometric n-heptane/oxygen mixtures diluted by 50% Ar, the experimental delay times are compatible with those computed. In this case, τi,2 is governed by the so-called “high temperature mechanism” (T2 > 1000 K) and varies exponentially with temperature. For undiluted mixtures, the particle heating times is much shorter than τi, 2. Then, the chemistry is governed by the “low temperature mechanism” (T2 < 950 K). Between 750 K and 950 K, τi, 2 decreases or small changes with temperature decrease but is sensitive to pressure. Turbulence in boundary layer could also promote the SDT.

Domains of Existence of the Bifurcation of a Reflected Shock Wave in Cylindrical Channels

The bifurcation of a reflected shock wave at the end of a cylindrical shock tube has been analyzed under conditions similar to those neccessary for studying chemical reactions. The experiments have been carried out using shocks in argon, nitrogen and CH4/O2/N2 mixtures. The triple configuration, deduced from pressure signals, has been compared to configurations provided by different models. The polar method has been used to calculate the parameters of the configuration and a method for the estimation of the domain of existence of the reflected shock bifurcation has been proposed.