G. Dupré

Doppler interferometry study of unstable detonations

Near-limit detonations are highly unstable and characterized by very large longitudinal velocity fluctuations that can range from 0.4 to 1.8 times the normal Chapman-Jouguet value. The period of the fluctuations also varies over a wide range from a few to a hundred tube diameters. In an attempt to establish a criterion for detonation limits, the velocity fluctuations of near-limit detonations are studied. A novel microwave Doppler technique based on a single coaxial mode has been developed for this purpose to give an unambiguous quasi-continuous velocity measurement of the detonation wave over the entire length of its travel. The near-limit unstable behavior in the detonable stoichiometric mixtures of hydrocarbons (C2H2, C2H4, C2H6, C3H8) with O2, air or N2O, tested in this work, have been characterized by four distinct modes of unstable behavior. This classification allows a qualitative description of the wide range of velocity fluctuations occurring near the detonation limit, including galloping waves.

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.

Unstable detonations in the near-limit regime in tubes

Unstable detonations in near-limit gaseous mixtures are investigated in five tubes of 152, 97, 74, 49, and 38 mm i.d. respectively and 11 m long, interconnected by 1980 o bends to form a continuous loop. The detonation is initiated in the largest tube by a solid explosive charge and transmits to the smaller tubes through the bends. A short section in which the wall is lined by an acoustic absorbing material is placed at the beginning of each tube immediately after the bend to damp out any transverse perturbations associated with the cellular detonation itself or induced by the propagation through the bend. In this manner, a standard “clean” initial conditionis obtained. A short section of defined wall roughness is also placed immediately after the damping section to introduce some finite transverse perturbations to assist the transition to detonation. Most of the experiments were carried out at 1 atm, with lean mixtures of H 2 , C 2 H 4 and C 3 H 8 in air, some of them with stoichiometric H 2 /O 2 mixtures diluted with argon. The results show that immediately after the damping and rough wall sections of earch tube, a dissociated “shock-reaction zone complex” propagating at approximately half the Chapman-Jouguet velocity of the mixture is obtained. It is found that in general, for a cell size over diameter ratio λ/d≤1, re-transition to detonation occurs rapidly and the detonation formed is stable. However for λ/d>1, the re-transition occurs far downstream and the phenomenon is unstable with large velocity fluctuations resembling those of galloping detonations. Even with standardized initial conditions, a λ/d criterion for detonation limits in a given mixture cannot be obtained. Highly unstable detonations are observed for concentrations up to λ/d=13. Mixtures wit different fuels or different diluents also exhibit different behaviors for the same λ/d ratio indicating that, in the near-limit regime, the detonation phenomenon cannot be characterized by a single parameter. The present results also raise the fundamental question of the existence of detonation limits since it requires a unique definition for a detonation wave itself.

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.

Ignition of a Combustible Mixture by a Hot Unsteady Gas Jet

ABSTRACT The study of the ignition of a combustible mixture induced by means of an unsteady gas jet, at an initial temperature ranging from 700 to 3000 K, requires the construction of an original test facility which consists of a shock tube connected to a combustion chamber via an injector. With this new experimental setup, the ignition conditions of hydrogen-air ( + carbondioxide) mixtures, induced by a hot hydrogen-argon mixture, have been extensively studied, resulting in the determination of the ignition limits of these combustible mixtures, at an initial pressure and temperature of 100 kPa and 403 K respectively.

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.

High-Temperature Reaction of O(3P)+H2S

The reaction of atomic oxygen (3P) with hydrogen sulfide was investigated by the shock tube - laser photolysis method at high temperatures (1100–2000 K), where time dependences of O and H atoms were monitored with atomic resonance absorption spectrometry (ARAS). O atoms were produced by the photolysis of SO2 by an KrF excimer laser behind reflected shock waves. The overall rate constant for the reaction

Hydrogen–nitrous oxide delay times: Shock tube experimental study and kinetic modelling

O + {H_2}S \to products