Showing papers on "Autoignition temperature published in 1997"

Low-temperature combustion and autoignition

Abstract: Combustion has played a central role in the development of our civilization which it maintains today as its predominant source of energy. The aim of this book is to provide an understanding of both fundamental and applied aspects of low-temperature combustion chemistry and autoignition. The topic is rooted in classical observational science and has grown, through an increasing understanding of the linkage of the phenomenology to coupled chemical reactions, to quite profound advances in the chemical kinetics of both complex and elementary reactions. The driving force has been both the intrinsic interest of an old and intriguing phenomenon and the centrality of its applications to our economic prosperity. The volume provides a coherent view of the subject while, at the same time, each chapter is self-contained.

Comparison of Scramjet Engine Performance in Mach 6 Vitiated and Storage-Heated Air

Abstract: To investigate the sensitivity of combustion to the test gas, an H2-fueled scramjet engine was tested at a Mach 6  ight condition with the air supplied by a combustion heater (V mode) and a storage heater (S mode). The fuel self-ignited without the assistance of igniters in the V mode. However, self-ignition was difŽ cult in the S mode. The easier ignition with vitiated air was caused by radicals supplied from the combustion heater. The combustion behavior was also affected by the test air, which suggests that the combustion was not fully mixing-controlled. As the fuel  ow rate increased, the combustion changed from a weak mode, delivering a lower thrust, to an intensive mode, with a higher thrust. Gas sampling showed that the weak combustion was caused by autoignition in the boundary layer on the engine walls. In the intensive combustion mode, the  ame was anchored near the backward-facing step on the sidewalls. However, the  ame partially detached from the step on the top wall in the combustor. The detached  ame may make the combustion kinetically controlled to produce the sensitivity to the test air.

Microwave-assisted catalytic combustion of diesel soot

Abstract: The concept of oxidizing diesel soot in a microwave-assisted catalytic trap was demonstrated. Comparisons were made to experiments using electric heating with and without catalysts to understand the influence of microwave irradiation on catalysis. The complex permittivities of the three materials (diesel soot, catalyst and support) involved in such a system were measured and the feasibility of using this combination of materials in diesel soot burn-off was evaluated, based upon these measured data. It was found that iron and copper were the most active catalysts in lowering the ignition temperature of diesel soots, while palladium was a necessary component in achieving more complete combustion. The iron-containing catalyst was also very effective and energy-efficient at low microwave input. A non-thermal or microwave enhancement effect was observed which further decreased the ignition temperature by more than 200°C. It was also found that the more vigorous burning of diesel soot by microwave heating led to an increase in carbon monoxide in the combustion products, because of the difference in the heating mechanism. However, when palladium was used, the same completeness of combustion as in electric heating could be achieved.

Method of safely initiating combustion of a gas generant composition using an autoignition composition

Abstract: The present invention relates to a low temperature autoignition composition for safely initiating combustion of a main pyrotechnic charge in a gas generator or pyrotechnic device exposed to flame or a high temperature environment. The low temperature autoignition compositions of the invention include a mixture of an oxidizer and a powdered metal, wherein the oxidizer includes a comelt or mixture comprising ammonium nitrate and at least one of an alkali metal nitrate, an alkaline earth metal nitrate, a complex salt nitrate, a dried, hydrated nitrate, an alkali metal chlorate, an alkali metal perchlorate, an alkaline earth metal chlorate, an alkaline earth metal perchlorate, ammonium perchlorate, sodium nitrite, potassium nitrite, silver nitrite, a complex salt nitrite, a solid organic nitrate, a solid organic nitrite, or a solid organic amine, and where the metal fuel and oxidizer are present in amounts sufficient to provide an autoignition composition having an autoignition temperature of no more than about 232° C. The present invention also relates to a method for initiating a gas generating composition or pyrotechnic composition in a gas generator or pyrotechnic device exposed to flame or a high temperature environment. In the method of the invention, the gas generating composition or pyrotechnic composition is placed in thermal contact with a low temperature autoignition composition of the invention.

Prediction of Autoignition Temperatures of Organic Compounds from Molecular Structure

Abstract: A quantitative structure−property relationship study is performed to develop mathematical models that relate the structures of a heterogeneous group of organic compounds to their autoignition temperature values. The molecular structures of the compounds are represented by calculated numerical descriptors which encode their topological, electronic, and geometric features. These descriptors are used to develop several multiple linear regression and computational neural network models to predict the autoignition temperatures of a data set consisting of hydrocarbons, halohydrocarbons, and compounds containing oxygen, sulfur, and nitrogen. Both genetic algorithm and simulated annealing routines are used to select subsets of descriptors based on multiple linear regression and computational neural networks. The models that are developed have predictive ability in the range of the experimental error of autoignition temperature measurements.

Comprehensive kinetic model for the low temperature oxidation of hydrocarbons

Abstract: The oxidation chemistry in the low- and intermediate-temperature regimes (600--900 K) is important and plays a significant role in the overall combustion process. Autoignition in diesel engines as well as end-gas autoignition and knock phenomena in s.i. engines are initiated at these low temperatures. The low-temperature oxidation chemistry of linear and branched alkanes is discussed with the aim of unifying their complex behavior in various experimental systems using a single detailed kinetic model. New experimental data, obtained in a pressurized flow reactor, as well as in batch- and jet-stirred reactors, are useful for a better definition of the region of cool flames and negative temperature coefficient (NTC) for pure hydrocarbons from propane up to isooctane. Thermochemical oscillations and the NTC region of the reaction rate of the low-temperature oxidation of n-heptane and isooctane in a jet-stirred flow reactor are reproduced quite well by the model, not only in a qualitative way but in terms of the experimental frequencies and intensities of cool flames. Very good agreement is also observed for fuel conversion and intermediate-species formation. Irrespective of the experimental system, the same critical reaction steps always control these phenomena. The results contribute to the definition of a limited set ofmore » fundamental kinetic parameters that should be easily extended to model heavier alkanes.« less

Combustion reaction in multilayered nickel and aluminum foils

Abstract: Ni/Al (3/1) multilayer reaction piles were heated in a vacuum furnace and the temperature (100 readings per s for a precision of 0.2°C) of the reaction pile was measured continuously during the combustion reaction. The ignition temperature (at which the combustion starts) is almost the same independent of heating rate and the thickness of the foils. This temperature corresponds to the melting of Al which triggers the reaction. For thin Ni foils, less than 25 μm thick, the maximum temperature shows a plateau with time, close to the adiabatic reaction temperature or the eutectic temperature close to Ni3Al. For thicker Ni foils, the maximum temperature decreases with the increase of foil thickness and the combustion is incomplete. The reaction time between the ignition temperature and the maximum temperature (or the beginning of the plateau temperature) increases from 8 s for a 12.5 μm Ni foil to 125 s for a 150 μm Ni foil and the relation is almost linear. For Ni foils less than 25 μm thick, the final microstructure is homogeneous with a grain size approximately equal to the combined initial Ni and Al foil thicknesses. In the range used, 1–100°C min−1, the heating rate has very little effect on the combustion reaction. From the temperature-time profile and microstructure analysis, there are evidences of melting of Al, decomposition of Al3Ni and Al3Ni2, and the formation of NiAl and Ni3Al.

Modeling Ignition of Catalytic Reactors with Detailed Surface Kinetics and Transport: Oxidation of H2/Air Mixtures over Platinum Surfaces

Abstract: The catalytic ignition of H2/air mixtures over platinum is modeled using a stagnation-point flow model with detailed gas-phase, surface kinetics and transport using an arc-length continuation technique. Self-inhibition of the catalytic ignition of H2/air mixtures is observed in agreement with experiments. For compositions between ∼0.3 and ∼15% H2 in air at atmospheric pressure, hysteresis is created by site competition, while for mixtures with more than ∼15% H2 in air, thermal feedback is a prerequisite. It is found that the system shifts from a kinetics-limited regime on the extinguished branch to a transport-limited regime on the ignited branch. However, near ignition, the system tends toward a transport- and kinetics-limited regime. Sensitivity analysis on the reaction preexponentials shows that the competitive dissociative adsorption of H2 and O2 and the desorption of H* most affect the catalytic ignition temperature. Reaction path analysis reveals a change in dominant surface reaction paths as a func...

A Model for the Effects of Mixing on the Autoignition of Turbulent Flows

Abstract: Modelled transport equations for the conditional mean and r.m.s. temperature increments θ before autoignition in a turbulent non-premixed flow have been developed. The model is based on the recent finding that a well-defined mixture fraction, fMR, first ignites and the concept that the conditional scalar dissipation rate, χ|fMR, controls the heat losses and hence the ignition time. By introducing the fluctuations of χ|fMR, and by modelling the conditional correlation coefficients between χ and θ from results from Direct Numerical Simulations, the model is closed and calculates autoignition times in good agreement with DNS results. Extensions to complex chemistry and the relationship with the more rigorous Conditional Moment Closure are discussed. Finally, the model is incorporated in a Favre-averaged k – ϵcode(KIVA-II)to predict autoignition of a transient fuel jet in hot air with promising results.

Diesel fuel having improved qualities and method of forming

Abstract: A mixture of alkoxy-terminated poly-oxymethylenes, having a varied mixture of molecular weights, is blended with diesel fuel to form an improved fuel for autoignition engines. The mixed alkoxy-terminated poly-oxymethylenes may be produced by reacting paraformaldehyde with methylal, methanol, or other alcohol for a length of time and at a temperature and pressure sufficient to form the mixed alkoxy-terminated poly-oxymethylenes. The base diesel fuel, when blended with the mixed alkoxy-terminated poly-oxymethylenes in a volume ratio of from about 2 to about 5 parts diesel fuel to 1 part mixed alkoxy-terminated poly-oxymethylenes, provides a higher quality fuel having significantly improved lubricity and reduced smoke formation, without degradation of the cetane number or smoke formation characteristics when compared with the base diesel fuel.

Knock in spark-ignition engines: End-gas temperature measurements using rotational CARS and detailed kinetic calculations of the autoignition process

Abstract: Cycle-resolved end-gas temperatures were measured using dual-broadband rotational CARS in a single-cylinder spark-ignition engine. Simultaneous cylinder pressure measurements were used as an indicator for knock and as input data to numerical calculations. The chemical processes in the end-gas have been analysed with a detailed kinetic mechanism for mixtures of iso-octane and n-heptane at different Research Octane Numbers (RON'S). The end-gas is modelled as a homogeneous reactor that is compressed or expanded by the piston movement and the flame propagation in the cylinder. The calculated temperatures are in agreement with the temperatures evaluated from CARS measurements. It is found that calculations with different RON'S of the fuel lead to different levels of radical concentrations in the end-gas. The apperance of the first stage of the autoignition process is marginally influenced by the RON, while the ignition delay of the second stage is increased with increasing RON. © 1997 Society of Automotive Engineers, Inc. (Less)

Catalytic combustion chamber and method for igniting and controlling the catalytic combustion chamber

Abstract: The invention relates to a method of igniting and controlling a catalytic combustor (1), wherein the combustor comprises a first catalytic reactor (2) and at least a second catalytic reactor (3) arranged in series with the first catalytic reactor (2), wherein the first catalytic reactor (2) is heated to a temperature which exceeds or is equal to the ignition temperature of the first catalytic reactor (2), whereafter a mixture (8) of fuel and air is introduced to the catalytic reactor (2), whereby catalytic combustion is started in the first catalytic reactor (2). The mass flow through the catalytic combustor (1) is increased after ignition of the first catalytic reactor (2), whereafter combustion of the fuel/air mixture (8) partly takes place in gas phase in an intermediary chamber (14) between the first catalytic reactor (2) and the second catalytic reactor (3). The invention also pertains to a combustor (1) and its use for heating in a vehicle.

Face-mixing fluid bed process and apparatus for producing synthesis gas

Abstract: A novel fluidized bed syngas (FBSG) injector/reactor apparatus and an efficient process for the partial oxidation and steam reforming of light hydrocarbon gases such as methane, to convert such gases to useful synthesis gas containing CO and H 2 for recovery and/or subsequent hydrocarbon synthesis. Sources of a light hydrocarbon gas, such as methane, and oxygen or an oxygen-containing gas are preheated and pressurized and injected through gas orifices of an injector at high velocity and comparable momentums into admixture with each other in the desired proportions, at a plurality of mixing chambers or recessed cups which are open to the fluidized bed reaction zone of a reaction chamber and are spaced over the face of the injector, to form a reactant gas premix having a pressure drop of at least 1% through the injector. The gaseous premix is injected in a time period which is less than its autoignition time, preferably less than 9 milliseconds, at a velocity between about 25 to 1000 feet/second, into a partial oxidation reaction zone comprising a fluid bed catalyst so that the gas mixture reacts in the catalyst bed, to reduce the amounts of CO 2 , H 2 O and heat produced by the partial oxidation reaction to favor the desired stoichiometry. The formed syngas is cooled and recovered, such as use in further synthesis processing.

Homogeneous charge compression ignition engine with optimal combustion control

Abstract: An improved HCCI engine and control scheme is provided which produces stable HCCI combustion while optimally minimizing emissions and maximizing efficiency. In the present invention, the fuel/air mixture is thoroughly mixed to form a very lean homogeneous mixture, or is mixed in a manner to form a desired air/fuel stratification, to ensure relatively even, low flame temperatures which result in extremely low NOx emissions. The control system senses the start of combustion, the combustion rate and/or the combustion duration and, based on the sensed condition, actively controls the temperature, pressure, equivalence ratio and autoignition properties to continuously maintain optimum combustion.

Experimental Investigation of the Liquid Fuel Evaporation in a Premix Duct for Lean Premixed and Prevaporized Combustion

Abstract: Liquid fuel evaporation was investigated in a premix duct, operating at conditions expected for lean premixed and prevaporized combustion Results from a flat prefilming airblast atomizer are presented Kerosine Jet A was used in all experiments Air pressure, air temperature, and liquid fuel flow rate were varied separately; their relative influences on atomization, evaporation, and fuel dispersion are discussed The results show that at pressures up to 15 bars and temperatures up to 850 K, nearly complete evaporation of the fuel was achieved, without autoignition of the fuel For the configuration tested, the fuel distributions of the liquid and evaporated fuel show very little difference in their dispersion characteristics and were not much affected by a variation of the operating conditions

Chapter 7 Autoignition in spark-ignition engines

Abstract: Publisher Summary Research on autoignition probably has its most important application in understanding knock in spark-ignition engines. This chapter describes how the autoignition chemistry is applied and shows the progress that has been made in understanding and predicting knock in engines. The goal of knock prevention has resulted in a number of chemical models for autoignition, and, whatever their complexity, they provide a means of extrapolating chemical behavior from idealized laboratory to practical engine conditions. Knock arises, first because the autoignition delay-time has elapsed and second because the volumetric heat release rate at a hot spot is sufficient for generating a pressure pulse that compresses the unburned gas and reduces the effective autoignition delay-time. Comprehensive schemes provide the best description; however, simplified schemes are currently the most appropriate for application in engine models. Reduced schemes have not as yet made much impact in autoignition modeling but are expected to do so in the future. Whether, or not, autoignition occurs, depends upon a race between the reactions in the end gas and its consumption by the propagating flame.

Numerical study of stratified charge combustion in wave rotors

Abstract: A wave rotor may be used as a pressure-gain combustor effecting non-steady flow, and intermittent, confined combustion to enhance gas turbine engine performance. It will be more compact and probably lighter than an equivalent pressure-exchange wave rotor, yet will have similar thermodynamic and mechanical characteristics. Because the allowable turbine blade temperature limits overall fuel/air ratio to sub-flammable values, premixed stratification techniques are necessary to burn hydrocarbon fuels in small engines with compressor discharge temperature well below autoignition conditions. One-dimensional, unsteady numerical simulations of stratified-charge combustion are performed using an eddy-diffusivity turbulence model and a simple reaction model incorporating a flammability limit temperature. For good combustion efficiency, a stratification strategy is developed which concentrates fuel at the leading and trailing edges of the inlet port. Rotor and exhaust temperature profiles and performance predictions are presented at three representative operating conditions of the engine: full design load, 40% load, and idle. The results indicate that peak local gas temperatures will result in excessive temperatures within the rotor housing unless additional cooling methods are used. The rotor itself will have acceptable temperatures, but the pattern factor presented to the turbine may be of concern, depending on exhaust duct design and duct-rotor interaction.

Ignition and Extinguishment of Sodium Fires in Air Diluted by Nitrogen

Abstract: Ignition and extinguishment of sodium fires was investigated in air diluted by nitrogen using a coaxial burner. Oxygen concentration and temperature were changed from 0.5% to 21% and from 110oC to 700 oC, respectively. In undiluted air, ignition occurred near the crust of sodium oxides at temperatures below 600oC. At temperatures higher than 625oC, ignition was observed in a vapour layer and a conical diffusion flame appeared on a sodium pool at 700oC. The flame was observed for only one second after the ignition before a dense cloud from burning sodium obscured vision. In this study, the lowest ignition temperature of sodium was 115oC under an air flow rate of 5 L/min. There were extinguishing limits of oxygen concentrations for burning sodium. The limits decreased linearly with increasing sodium temperature up to a temperature of 700oC. Anomalous ignition was found in a region of oxygen concentrations lower than the limits. One example was an observed ignition occurring in the region of oxygen concentrations below 3% and temperatures above 400oC. The ignition was observed on the crust of sodium oxides after relatively long induction periods, but no stable combustion followed. Another anomaly was re-ignition of a cooled combustion residue exposed to fresh air. Such ignition was even observed below 50oC. The oxidized sodium prepared in a low oxygen atmosphere also ignited at the same temperature. Both types of anomalous ignition were attributed to production of sodium peroxide. Thus, the extinguishment of sodium fires by nitrogen requires consideration of methods to block the production of sodium peroxides.

Mechanism of Single Coal Particle Ignition under Microgravity Condition.

Abstract: In order to simplify the complex coal ignition process, a microgravity condition (10–3g), in which natural convection is negligible, has been employed to observe ignition of single coal particles. The particle was heated and ignited by radiation from spot heaters. Three coals with different volatile matter contents were employed, having particle sizes in a range of 0.8 mm to 1.2 mm.As expected, under microgravity, a nearly spherical volatile cloud was found to form around the particle. Results clearly show that regardless of the particle size, the ignition is preceded by homogeneous gas-phase volatile ignition, followed by heterogeneous solid-phase char ignition after burn-out of the volatiles. The heterogeneous ignition temperature and time were higher and longer, respectively, under microgravity than under normal gravity condition. The homogeneous ignition also took place at a higher particle temperature under microgravity than under normal gravity. Further, the temperature for heterogeneous ignition increased with the absolute amount of volatiles contained in a coal particle, whereas the particle temperature for the homogeneous ignition decreased.