Homogeneous explosion and shock initiation for a three-step chain-branching reaction model

The role of chain-branching cross-over temperatures in shock-induced ignition of reactive materials is studied by numerical simulation, using a three-step chainbranching reaction model. In order to provide insight into shock initiation, the simpler problem of a spatially homogeneous explosion is first considered. It is shown that for ratios of the cross-over temperature to the initial temperature, T-B, sufficiently less than unity, the homogeneous explosion can be quantitatively described by a widely used two-step model, while for T-B sufficiently above unity the homogeneous explosion can be effectively described by the standard one-step model. From the matchings between these homogeneous-explosion solutions, the parameters of the reduced models are identified in terms of those of the three-step model. When T-B is close to unity, all the reactions of the three-step model have a leading role, and hence in this case the model cannot be reduced further. In the case of shock initiation, for T-B (which is now the ratio of the cross-over temperature to the initial shock temperature) sufficiently below unity, the three-step solutions are qualitatively described by those of the matched two-step model, but there are quantitative differences due to the assumption in the reduced model that a purely chain-branching explosion occurs instantaneously. For T-B sufficiently above unity, the matched one-step model is found to effectively describe the way in which the heat release and fluid dynamics couple. For T-B close to unity, the competition between chain branching and chain termination is important from the outset. In these cases the speed at which the forward moving explosion wave that emerges from the piston is sensitive to T-B, and changes from supersonic to subsonic for a value of T-B just below unity

Single droplet ignition and combustion of jet-A1, hydroprocessed vegetable oil and their blends in a drop tube furnace

The environmental impact and the dependence of fossil fuels in the aeronautical sector have promoted the demand for alternative and greener fuels. This is one of the main challenges for this sector in the near future. A possible solution in the near future might be the blending of biofuels with jet fuel, which would allow the use of greener fuels, and a reduction in the greenhouse gases and emissions without significant changes in the existing fleets of the companies, which means the development of a “drop in” fuel. In this context, this work examines the ignition and the combustion characteristics of single droplets of jet-A1 (JF), hydroprocessed vegetable oil (NExBTL) and their mixtures in a drop tube furnace (DTF). The objective of this work is to evaluate the influence of the fuel mixture composition on the fuel characteristics. Droplets with diameters of 155 ± 5 µm, produced by a commercial droplet generator, were injected into the DTF, whose wall temperature and oxygen concentration were controlled. Experiments were conducted for three temperatures (900, 1000 and 1100 ºC). The ignition and combustion of the droplets were evaluated through the images obtained with a high-speed camera coupled with a high magnification lens, and an edge detection algorithm. The images allowed for the observation of the burning phenomena, and data are reported for temporal evolution of droplet sizes and burning rates. The results revealed that the fuel mixtures followed the ?? 2 law, except the mixture with 75% JF for a DTF wall temperature of 1100 ºC. This was due to the occurrence of puffing and micro explosions, which enhanced the burning rates. In addition, it was observed that the mixtures with a higher content of JF present brighter flames, and higher burning rates.O impacto ambiental e a dependência de combustíveis fósseis no setor aeronáutico promoveram a procura por combustíveis alternativos e ecológicos. Este é um dos principais desafios para este setor no futuro. Uma possível solução num futuro próximo pode ser a mistura de biocombustíveis com combustível de aviação, o que permitiria o uso de combustível mais ecológico e a redução de gases de efeito estufa e emissões sem alterações significativas nas frotas existentes das empresas, isto é, o desenvolvimento de um combustível “drop-in”. Neste contexto, este trabalho examina as características de ignição e combustão de gotas isoladas de jet-A1 (JF), óleo vegetal hidroprocessado (NExBTL) e suas misturas num forno de queda livre (DTF). O objetivo deste trabalho é avaliar a influência da composição da mistura nas características do combustível. Gotas com diâmetros de 155 ± 5 µm, produzidas por um gerador comercial de gotas, foram injetadas no DTF, cuja temperatura da parede e concentração de oxigênio eram controladas. Os testes foram conduzidos para três temperaturas (900, 1000 e 1100 ºC). A ignição e a combustão das gotículas foram avaliadas através das imagens obtidas com uma câmara de alta velocidade acoplada a uma lente de alta ampliação e um algoritmo de deteção de limites. As imagens permitiram a observação dos fenómenos de queima e avaliar a evolução temporal do tamanho das gotas e das taxas de queima. Os resultados revelaram que as misturas de combustível seguem a lei D2 , exceto a mistura com 75% de JF para uma temperatura de 1100 ºC na parede do DTF. Isso ocorreu devido à ocorrência de puffing e microexplosões, o que aumentou as taxas de queima. Observou-se ainda que as misturas com maior teor de JF apresentam chamas com maior intensidade luminosa e maiores taxas de queima

Explicit analytic prediction for hydrogen–oxygen ignition times at temperatures below crossover

This paper addresses homogeneous ignition of hydrogen-oxygen mixtures when the initial conditions of temperature and pressure place the system below the crossover temperature associated with the second explosion limit. A three-step reduced mechanism involving H2, O2, H2O, H2O2 and HO2, derived previously from a skeletal mechanism of eight elementary steps by assuming O, OH and H to follow steady state, is seen to describe accurately the associated thermal explosion. At sufficiently low temperatures, HO2 consumption through HO2 + HO2 → H2O2 + O2 is fast enough to place this intermediate in steady state after a short build-up period, thereby reducing further the chemistry description to the two global steps 2H2 + O2 → 2H2O and 2H2O → H2O2 + H2. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be used in describing the resulting thermal runaway, yielding an explicit expression that predicts with excellent accuracy the ignition time for different conditions of initial temperature, composition, and pressure.This work was supported by the Comunidad de Madrid through project # P2009/ENE-1597. The first two authors also acknowledge support from the EU through the Marie Curie ITN MYPLANET and from the Spanish MCINN through projects # ENE2008-06515 and CSD2010-00011.European Community's Seventh Framework ProgramPublicad

Asymptotic analysis of n-heptane ignition and cool flames with a temperature-explicit model

An empirical four-step mechanism has previously been proposed for describing ignition of heptane-air mixtures. This mechanism captures the low-temperature and high-temperature ignition behavior as well as the intermediate-temperature behavior, between roughly 800 K and 1100 K, where a negative temperature dependence of the overall rate is observed. The present paper derives simplified overall rate formulas for ignition times from this four-step mechanism and uses those formulas to derive a temperature-explicit model whose simplicity facilitates analysis of more complex ignition phenomena. Methods of activation-energy asymptotics are employed for the temperature-explicit model to investigate ignition in homogeneous, adiabatic systems, ignition by compressional heating in homogeneous systems, and structures and quasisteady propagation velocities of cool flames in weakly strained mixing layers. It is shown that, in the range of negative temperature dependence, there is a plateau in the ignition time when the criterion of thermal runaway is employed. Near this plateau region, cool flames with three-zone structures can propagate at velocities that increase with increasing initial temperature. Besides providing qualitative descriptions of ignition processes for hydrocarbon-air mixtures, the results lead to quantitative predictions that can be compared with experiment

Carbon monoxide oxidation rates computed for automobile exhaust manifold reactor conditions

Carbon monoxide oxidation rates for automobile exhaust manifold reactor condition

Theory of explosions and detonations for a three-step chain-branching chemistry model

A three-step kinetics scheme is used to model chain-branching reactions but also thermal reactions that are traditionally modelled using a one-step scheme. The chain-branching crossover temperature $T_B$ is adjusted to accommodate for the different reaction types. Two main scenarios are considered: a homogeneous reactive gas in a closed vessel, and the initiation of detonation waves in a tube induced by a shock that is driven either by a piston or a contact. In the homogeneous scenario, the reaction behaves more and more like a chain-branching reaction the smaller $T_B$ is below the initial temperature (unity in our non-dimensionalized scales). For $T_B > 1$ however, heat is released from the outset, and the reaction proceeds in a thermal manner similar to what occurs with one-step schemes. It is shown how the three-step scheme can be matched to widely used two-step and one-step models in the cases $T_B 1$ respectively. With piston-driven shock-induced detonations and for $T_B — 1$ sufficiently large, we reproduce situations similar to those which occur with one-step thermal schemes. For $T_B ~ 1$, both thermal and chain-branching effects are witnessed. With $T_B$ sufficiently below unity, chain-branching is very prominent from the outset, and since no secondary shock is formed, the situation is unlike what usually occurs with one-step thermal schemes, but is in good agreement with predictions of a simplified two-step chain-branching model. The major qualitative difference when using an acoustically permeable contact discontinuity to drive the shock instead of a piston occurs when $T_B — 1$ is sufficiently large, where we witness the temperature maximum and the fuel minimum move away from the driving surface

Solvent nature effect in preparation of perovskites by flame pyrolysis: 1: carboxylic acids

The effect of a series of carboxylic acids (C(2)-C(8)), as solvents for the preparation by flame spray pyrolysis of LaCoO(3) catalyst for the flameless combustion of methane, has been investigated. Acetic acid showed to be unsatisfactory from several points of view: low phase purity of the catalyst, higher amount of unburnt carbonaceous residua, lower catalytic activity and low thermal stability. By increasing the carbon chain length of the solvent, the consequent increase of flame temperature led to an increase of crystal phase purity and of particle size and to a decrease of specific surface area of the catalyst. Catalytic activity showed only marginally affected by the last parameter, phase purity seeming more important. Thermal resistance showed directly related to flame temperature, i.e. to the combustion enthalpy of the solvent, but a relatively high amount of residual organic matter can negatively affect this property

Navigating Reactor Safety in Catalytic Microchannel Reactors

High temperature catalytic reactions are being intensely studied since many decades due to their large industrial potential, such as in pyrolysis, total oxidation (i.e. combustion) and partial oxidation of hydrocarbons. The reactions are characterized by extreme reaction temperatures (T> 1000°C) where homogeneous (i.e. non-catalytic gas phase) reactions can occur in parallel to catalytic reactions. This occurrence of homogeneous reactions is typically an undesired feature, since it complicates the understanding of reaction mechanisms, leads to selectivity losses, and often poses a safety hazard due to potentially explosive behavior. Since free surfaces tend to bind radical species, eventually lead to a quenching of gas-phase reactions. Microreactors, i.e. chemical reactors with characteristic dimensions in the sub-millimeter range, hold great promise for fundamental studies of existing processes offering small thermal inertia, high heat and mass transport rates, compactness etc. Due to their large surface-to-volume ratio, microreactors can be expected to suppress undesirable gas phase reactions and thus form safe reactor configurations for highly explosive processes. In the present study, we numerically investigate the reactive flow of H₂/air mixtures in a microchannel to gain insights into the reason for the absence of explosion observed in previous experiments. The H₂ oxidation reaction is chosen as model reaction due to its high exothermicity and wide flammability range. It also constitutes an important sub-set of reactions in hydrocarbon oxidation. In a two-dimensional boundary layer numerical model, we used coupled mechanisms with detailed elementary-step kinetics for gas-phase and catalytic surface reactions. The influence of different wall materials, reactor dimension, feed conditions and reaction pressure on the coupling of heterogeneous and homogeneous reaction pathways in the microreactor was studied. The results demonstrate that the attainability of 'intrinsic safety' in microchannel reactors is strongly dependent on a complex interplay between homogeneous and heterogeneous reaction pathways in the individual reaction system. In particular, it is found that intrinsic reactor safety breaks down at sufficiently high reactor pressure. Generalized equations for the current reaction systems are derived. As an outlook, other industrially relevant reaction systems, i.e. CO oxidation and NOx formation, are preliminary investigated with respect to the effect of heterogeneous-homogeneous interactions and radical quenching in particular, on the behaviour of these reaction systems