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International audience; The laminar burning rate, the explosion peak pressure, and the pressure rise coefficient have been measured for the first time for silane-nitrous oxide-argon mixtures using the spherically expanding flame technique in a constant volume combustion chamber. For these three parameters, the values obtained were higher than for hydrogen-nitrous oxide-argon and typical hydrocarbon-based mixtures. A maximum burning rate of 1800 g/m2 s was measured at 101 kPa, whereas under similar conditions, a maximum burning rate around 950 g/m2 s has been reported for hydrogen-nitrous oxide-argon mixtures. To better understand the chemical dynamics of flames propagating in SiH4–N2O–Ar mixtures, a detailed reaction model from the literature was improved using collision limit violation analysis and updated thermodynamic properties calculated with a high-level ab initio approach. The reaction model predicts the burning rate within 14% on average but demonstrates error close to 50% for the richest mixtures. The chemistry of the H–O–N system is important under all the conditions presently studied. The chemistry of the Si–H–O–N system demonstrates an increasing importance under rich conditions. In particular, the reactions (i) forming SiOx(s); (ii) describing the interaction of Si-species with N2O; and (iii) involving silicon hydrides, have an important role for the heat release dynamics. The condensed combustion products formed in the silane-nitrous oxide-argon flames were sampled and characterized using electron micrograph, electronic diffraction, energy-dispersive spectroscopy, and X-ray powder diffraction. For all equivalence ratios, silica spherical particles with a mean diameter in the range 200–300 nm were observed. In addition, for mixtures with Φ ≥ 2.2, silicon nanowires were formed. X-ray diffraction experiments showed that the silicon nanowires are composed of metal silicon characterized by a cubic structure (lattice parameter: a=5.425Å) with the Fm-3m space group.

International audience; Nitromethane has many applications, such as in racing, as a gasoline fuel additive, and as a monopropellant. Despite a large number of studies and the small size of the molecule, the combustion chemistry of nitromethane is still not well understood. To improve models, the pyrolysis of nitromethane (CH3NO2) was investigated experimentally in shock tubes and in a micro flow reactor with a controlled temperature profile (MFR), under dilute conditions. Several spectroscopic diagnostics were used in the shock tubes to follow the concentration time histories of CO, H2O (both using IR laser absorption), and CH3NO2 (UV light absorption). A quadrupole mass spectrometer was used to measure CH3NO2, NO2, CH4, C2H4, and C2H2 at various temperatures with the MFR. These unique experimental results were compared to modern, detailed kinetics models from the literature, and no mechanism was able to reproduce these data over the wide range of conditions investigated. Predictions for the CO and H2O levels were generally inaccurate, and the CH4, C2H4, and C2H2 predictions were poor in most cases for the MFR data. Importantly, all models largely differ in their predictions. A numerical analysis was performed to identify ways to improve the next generation of nitromethane models. Results indicate that nitromethane decomposition needs to be improved below 1050 K, and that hydrocarbon-NOx interactions still need to be further investigated.

Supporting or shaping a porous powder is important for industrial applications as optimized structure with high mass transfer, low pressure drop and high mechanical and chemical stability can be obtained. A new class of geopolymer-hydrotalcite composites with suitable mechanical and thermal properties were conceived as shaped materials for CO2 adsorption applications at intermediate temperature (200–400 °C). Composite monoliths were produced mixing different commercial hydrotalcite-type (HyT) powders with a metakaolin-based geopolymer matrix with a molar ratio SiO2:Al2O3 = 4.0. The compressive strength at room temperature and 500 °C ranged between 10 and 35 MPa, mainly depending on HyT powder morphology and composite total porosity. The composites were characterized and tested in term of CO2 uptake. After calcination to convert HyT into an amorphous Mg:Al mixed solid oxide able to absorb CO2, the composites were tested in the CO2 adsorption at 200 °C, with cycles of adsorption/desorption performed with intermediate regeneration at 500 °C. CO2 adsorption capacity was in the range 0.375–0.461 mmol g−1 for HyT and between 0.109 and 0.145 mmol g−1 for composites, being 0.052 mmol g−1 the value for the geopolymer matrix. A partial deactivation of the HyT phases was also detected.

The chemiluminescence originating from OH∗, the excited hydroxyl radical, is one of the most extensively used diagnostics to characterize auto-ignition delay time of gaseous mixtures behind reflected shock waves. We have carried out new experiments and modeling of this diagnostic as well as analyzed previous results for hydrogen-based mixtures, including H_2–O_2, H_2O_2–H_(2)O, H_2–N_(2)O and H_2–O_2–N_(2)O. The experiments were analyzed with a detailed chemical reaction model which included mechanisms for OH∗ creation, quenching and emission. Simulations of the reaction behind reflected shock waves were used to predict OH∗ emission profiles and compare this with measured results as well as profiles of temperature and the ground state concentrations of OH. Analysis of OH∗ rates of progress demonstrates that a quasi-steady state approximation is applicable and an algebraic model for OH∗ concentrations can be derived that relates emission to the product of concentrations of O and H for H_2–O_2 and H_(2)O_2 mixtures and an additional contribution by the product of H and N_(2)O when N_(2)O is an oxidizer.

A study of detonation velocity and cellular structure for stoichiometric heptane/oxygen and for some stoichiometric heptane/oxygen/argon mixtures is carried out in a shock tube at low initial pressure. The critical conditions for the detonation onset and for the propagation of a self-sustained detonation wave are determined. A simplified form of the ZND model used in conjunction with a validated detailed kinetic model leads to the determination of the proportionality factor, A , between the detonation cell width, λ , and the induction distance, Δ , in the detonation wave. This A factor is of practical importance to estimate the detonation properties of n -heptane based mixtures including n -heptane/air. The prediction of detonation cell size λ for n -heptane based mixtures is discussed according to the recent semi-empirical detonation model of Gavrikov et al. The cell sizes predicted according to this detonation model are underestimated by a factor of about 8. The limitations of this model are underlined when applied to n -heptane based mixtures.

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 ...

The objective of this study is to characterize the lifetime of monomethylhydrazine mixed with O2, both in the gas phase, at 298 K. A detailed kinetic model has been built to allow a numerical study of the homogeneous MMH/O2 reactivity.

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.

International audience; The aim of the present work is to identify and characterize the type of combustion of hydrogen-air mixtures near the flammability limits for different initial temperatures (from 298 to 423 K) and pressures (100 and 250 kPa) relevant to pressurized water reactor conditions. This experimental study has been carried out using a spherical vessel equipped with a pressure transducer to monitor the pressure increase subsequent to the combustion and with two optical windows to record the flame propagation. From the schlieren images, different regimes of flame propagation have been identified depending on the temperature and pressure. The maximum pressure obtained experimentally has been compared to the theoretical maximum pressure for adiabatic combustion at constant volume. The flammability limits have been determined for different temperatures and pressures and are compared to the literature.

Because of a wide number of applications, the potential hazards of H2-CH4-air mixtures have to be characterised. For hazard evaluation, an important element is a reliable detailed kinetic scheme. In the present study, three modern kinetic models, those of Konnov, of Dagaut and the GRI-mech 03, have been evaluated with respect to a large set of experimental data, including species profiles obtained in jet-stirred reactor, laminar flame speed, ignition delay time and detonation cell size, for hydrogen-methane-air mixtures. For jet-stirred reactor data, the model of Dagaut provides significantly better results. For flame speed data modeling, the three models are as reliable. For ignition delay times, the model of Dagaut seems the most reliable. For detonation cell size predictions, the model of Konnov is the best. Important chemical reactions are underlined through sensitivity and reaction pathway analysis and are discussed in the frame of rate constant values recommended by Baulch et al.

The objectives of this investigation are to study the details of the nonignition, preignition, ignition, and combustion sequences of the NTO (nitrogen tetroxide)/MMH (monomethyl hydrazine) bipropellant combination. Several techniques have been used to visualize NTO/MMH reactivity at room temperature and recreate some failure scenarios. NTO/MMH flammability diagrams have been established experimentally.

Ignition time measurements of lean, stoichiometric, and rich n-heptane/oxygen/argon mixtures have been studied behind reflected shock waves in the temperature range 1300‐1600 K and pressure range 2‐4 atm. The experimental data were compared to previously published results and have been found to be consistent with them. The experimental data were compared to ignition delay predicted using some of the kinetic models available in the literature. Discrepancies have been interpreted kinetically, thus leading to a slightly modified detailed kinetic model able to predict accurately high-temperature ignition delay such as encountered in the heptane/oxygen detonation wave.

Experimental reaction rate data for the Al/HCl system are very scarce. Such data are needed for the comprehension and for the numerical simulation of the combustion of aluminum particles as encountered in solid segmented motors. Toward this end, we have examined the homogeneous chemistry of this system, computed rate parameters for important reactions using conventional Transition State Theory (TST) and RRKM/master equation simulations, and estimated rate parameters for reactions where rigorous computations are not presently feasible. The reaction mechanism presented in this study consists of 15 species participating in 39 reversible elementary reactions for which rate parameters have been estimated or computed.

Ignition delay times for monomethylhydrazine (MMH or CH3NHNH2)/O2/Ar gaseous mixtures have been modeled by a reaction scheme containing 70 species and 373 equilibrated elementary reactions. For many reactions, the rate constants had to be estimated or adjusted because rate constants are available for only a few reactions. The basis for a comparison with reality is measurements of ignition delay times behind reflected shock waves performed over a range of temperatures pressures, and composition. Good agreement between measured and calculated ignition delay times was obtained for lean and less dilute mixtures. A least-squares analysis of the computed ignition delays provides power dependences of the concentrations and an activation energy very similar with those obtained experimentally. The relative importance of the different reactions has been clearly shown by performing different sensitivity analyses. The reaction begins with the scission of the N-N bond in MMH, but elimination reactions from MMH also have to be considered, especially at high initial temperature. The NH2 radicals formed react with MMH to give CH3NNH2 radicals and products. These radicals react with O2 to produce methyldiazene (CH3N=NH) and HO2 radicals. These two species appear to be very important for this specific chemistry. In particular, HO2 radicals react with MMH to give CH3NNH2 radicals and H2O2, this last species playing a major role through its thermal decomposition that produces hydroxyl radicals.

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.

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.

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.