Showing papers on "Autoignition temperature published in 1993"

Heat injection process and apparatus

Abstract: A method for heat injection into a subterranean formation is provided. The method utilizes flameless combustion and a gas fired heater having an electrical heated surface for ignition of the gas. The absence of a flame eliminates the flame as a radiant heat source and results in a more even temperature distribution throughout the length of the burner. Flameless combustion is accomplished by preheating the fuel and the combustion air to a temperature above the autoignition temperature of the mixture. Preheating hydrocarbon fuel requires the inclusion of a carbon formation suppressant such as carbon dioxide or steam to prevent carbon formation.

Multidimensional Modeling of Diesel Ignition and Combustion Using a Multistep Kinetics Model

Abstract: Ignition and combustion mechanisms in diesel engines were studied using the KIVA code, with modifications to the combustion, heat transfer, crevice flow, and spray models. A laminar-and-turbulent characteristic-time combustion model that has been used successfully for spark-ignited engine studies was extended to allow predictions of ignition and combustion in diesel engines. A more accurate prediction of ignition delay was achieved by using a multistep chemical kinetics model. The Shell knock model was implemented for this purpose and was found to be capable of predicting successfully the autoignition of homogeneous mixtures in a rapid compression machine and diesel spray ignition under engine conditions. The physical significance of the model parameters is discussed and the sensitivity of results to the model constants is assessed. The ignition kinetics model was also applied to simulate the ignition process in a Cummins diesel engine. The post-ignition combustion was simulated using both a single-step Arrhenius kinetics model and also the characteristic-time model to account for the energy release during the mixing-controlled combustion phase. The present model differs from that used in earlier multidimensional computations of diesel ignition in that it also includes state-of-the-art turbulence and spray atomization models. In addition, in this study the model predictionsmore » are compared to engine data. It is found that good levels of agreement with the experimental data are obtained using the multistep chemical kinetics model for diesel ignition modeling. However, further study is needed of the effects of turbulent mixing on post-ignition combustion.« less

Synthesis of YBa2Cu3O7-x powder by autoignition of citrate-nitrate gel

Abstract: A simple and convenient method for the synthesis of YBa2Cu3O7−x powder is described. The technique involves autoignition of a citrate-nitrate gel resulting from a thermally induced oxidation-reduction reaction to yield an ash that upon calcination produces the desired compound. The resulting powder is pure, homogeneous, and possesses a reasonably fine particle size. The autoignition is restricted to a particular range of citrate-nitrate ratio in the gel. Attempts have been made to understand the ignition process with the help of Thermogravimetry (TG) and Differential Thermal Analysis (DTA) of the samples. The process appears to have a higher degree of reproducibility and a good potential for large-scale production.

Autoignition of Hydrocarbon/Air Mixtures in a CFR Engine: Experimental and Modeling Study

Abstract: In order to see the accurate role of low temperature reactions in autoignition nd nock phenomena, oxidation of stoichiometric hydrocarbons/air mixtures has been performed in a CFR engine and in a flow system. The results show that reactions of isomerization of R02 radicals areimponant for hydrocarbons containing five or more carbon atoms. The pernxidic compounds formed through these isomerization reactions are alkylketohydroperoxides. A correlation is made between fuel structure, octane number. isomerization reactions and autoignition. A chemical kinetic model. created to interpret autoignition and knock. reproduces the results obtained with n-heptane and the influence of engine parameters as speed, intake temperature and pressure.

Slow-Combustion of n-Heptane, iso-Octane and a Toluene/n-Heptane Mixture

Abstract: The oxidation of n-heptane, iso-octane and a toluene/n-heptane mixture was studied in a jet-stirred flow reactor operating under pressure at stoichiometric feed ratio in the low-temperature range. In particular, the transition from slow-combustion to a higher reactivity condition was examined by analyzing the composition of the reaction products as the residence time was increased. It was found that high-octane-number fuels, such as iso-octane and toluene, have a high tendency to add O2 keeping intact the original fuel structure forming 2,2,4,4-tetramethyltetrahydrofuran and benzaldehyde, respectively. Conversely, n-heptane has a high tendency to fragment in olefins and light oxygenated species already in slow-combustion. The different tendency to form oxygenated fuel-skeleton retaining species could be responsible for their different reactivity and consequently autoignition tendency. In the oxidation of the toluene/n-heptane mixture the slow-combustion is essentially characterized by benzaldehyd...

Effects of compression and expansion ramp fuel injector configuration on scramjet combustion and heat transfer

Abstract: A scramjet combustor with four wall-ramp injectors containing Mach-1.7 fuel jets in the base of the ramps was investigated experimentally. During the test program, two swept ramp injector designs were evaluated. One swept-ramp model had 10-deg compression-ramps and the other had 10-deg expansion cavities between flush wall ramps. The scramjet combustor model was instrumented with pressure taps and heat-flux gages. The pressure measurements indicated that both injector configurations were effective in promoting mixing and combustion. Autoignition occurred for the compression-ramp injectors, and the fuel began to burn immediately downstream of the injectors. In tests of the expansion ramps, a pilot was required to ignite the fuel, and the fuel did not burn for a distance of at least two gaps downstream of the injectors. Once initiated, combustion was rapid in this configuration. Heat transfer measurements showed that the heat flux differed greatly both across the width of the combustor and along the length of the combustor.

Autoignition of Alcohols and Ethers in a Rapid Compression Machine

Abstract: The autoignition characteristics of methanol, ethanol and MTBE (methyl tert-butyl ether) have been investigated in a rapid compression machine at pressures in the range 20-40 atm and temperatures within 750-1000 K. All three oxygenated fuels tested show higher autoignition temperatures than paraffins, a trend consistent with the high octane number of these fuels. The autoignition delay time for methanol was slightly lower than predicted values using reported reaction mechanisms. However, the experimental and measured values for the activation energy are in very good agreement around 44 kcal/mol. The measured activation energy for ethanol autoignition is in good agreement with previous shock tube results (31 kcal/mol), although ignition times predicted by the shock tube correlation are a factor of three lower than the measured values. The measured activation energy for MTBE, 41.4 kcal/mol, was significantly higher than the value previously observed in shock tubes (28.1 kcal/mol). The onset of preignition, characterized by a slow energy release prior to early ignition was observed in some instances. Possible reasons for these ocurrences are discussed. © Copyright 1993 Society of Automotive engineers, Inc.

Ignition criteria for self-propagating combustion synthesis

Abstract: Ignition criteria for gasless self-propagating combustion synthesis have been investigated through an ignition temperature analysis. The calculations were based on the dimensionless energy and mass continuity equations where the dimensionless parameters associated with the rate of local heat generation (β), activation energy (γ), the rate of surface heat loss by convection (ω), the rate of surface heat loss by radiation (δ), and the rate of reaction (λ) were incorporated. The relative significance of each of these parameters on the ignition of the self-propagating combustion reaction was evaluated to be γ > β > δ > ω. The ignition region, transition region, and nonignition region were identified for selected conditions. The correlations between ignition behavior and the material properties, the thermodynamic and kinetic properties, as well as the experimental conditions were discussed. The calculations indicated that only those systems with δH/Cp > 1.5 × 103 (K) will give rise to a self-propagating combustion reaction without external energy input. Thus, this value can be used as an approximate guide for the existence of self-sustaining combustion. The calculations provide a sound basis toward interpreting experimental observations and developing a fundamental understanding of the process.

On the Derivation of Global Ignition Kinetics from a Detailed Mechanism for Simple Hydrocarbon Oxidation

Abstract: A method for deriving global ignition kinetics from a detailed chemical mechanism is described, with the lime-temperature range of applicability assessed. The computed ordering of simple hydrocarbons by ignition temperature is consistent with published data. Values of the global rate parameters were found to be only weakly dependent on gas mixing intensity, as determined by a comparison of results from a perfectly-stirred reactor model and plug flow reactor model. Major reaction pathways prior to and at ignition are presented for methane, ethane, ethylene and acetylene in stoichiometric air. Radical chain mechanism analysis of reduced reaction sets demonstrates that values of effective ignition activation energies are dependent almost entirely on one or two chain-branching reactions. These results suggest that the global chemistry representing the weakly reacting regime up to ignition may be determined independent of reactor fluid mechanics and utilized in the prediction of hydrocarbon auto-ignition.

Gas-enriched light-off

Abstract: An apparatus for reducing the amount of electrical power needed to attain light-off of a catalytic converter, particularly a catalytic converter for internal combustion engines. In addition to a resistance-heated catalytic converter, means are provided for varying the air/fuel ratio in the exhaust gas system for the purpose of reducing the ignition temperature of the exhaust gas stream, thereby further reducing the electrical energy required to attain light-off during cold-start.

Ignition of Flammable Atmospheres by Radiation-Heated Fibrous Agglomerates

Abstract: The conditions under which laser irradiation of loose agglomerates of fine fibres or particles leads to the ignition of a surrounding flammable gas mixture are studied in relation to the hazard associated with the use of optical sensing in explosive atmospheres. Results for stoichiometric mixtures in air of a range of hydrocarbons as well as of diethyl ether, carbon disulphide and hydrogen, are presented in the form of minimum igniting radiation flux as a function of the time to ignition. The minimum igniting fluxes at long induction times prove to be surprisingly low, down to 22 kW m$^{-2}$ for carbon disulphide/air mixtures. Traverses with small thermocouples, along with equilibrium temperature measurements during irradiation in air and cooling curves obtained following switch off, show that the system acts as a two-dimensional slab, though its heat transfer properties differ somewhat from bodies with well-defined boundaries. Irradiation with lasers of widely differing wavelengths and the use of different target surface coatings leads to widely varying equilibrium temperatures. While the minimum igniting flux cannot be correlated simply with any of the obvious ignition criteria, the corresponding equilibrium temperature reached in an inert atmosphere points to a critical ignition temperature for each particular flammable mixture. This is used to propose a tentative hazard assessment based on the concept of black body radiation interacting with a `black' particle which, although not the worst case for monochromatic radiation, proves to be more hazardous than the worst case yet encountered in practice. The variation of ignition lag with radiant power flux is such that the total energy flux varies linearly with time; effective activation energies for various fuels are deduced from the variation of induction time with temperature. A simplified theoretical model is proposed, based on classical thermal explosion theory and the large activation energy idealization. It is shown that the radiant power flux is the sole ignition criterion so long as the irradiated area is large by comparison with the quenching dimensions of the mixture. Measurements on smaller beam diameters carried out by refocusing the beam emerging from an optical fibre indicate that, below these dimensions, minimum igniting power fluxes rise, while hazardous laser powers fall to values of the order of 100 mW.

Mixture effects in deep catalytic oxidation of ethanol over platinum. influence of benzene and ethenylbenzene on rate and selectivity

Abstract: Mixture effects are observed when ethanol is oxidized in mixture with benzene or elhenylbenzene (styrcne). Very lean mixtures (<2000ppmV) were tested over a commercial monolithic catalyst. In the mass transfer controlled regime higher conversions are obtained for ethanol in mixture with aromatic, due to the increased adiabatic temperature rise; but the ignition temperature increased. The hydrocarbon reaction rate is slightly affected, while ethanol oxidation (either complete or partial) is strongly inhibited, especially by elhenylbenzene. The selectivity of the reaction is modified, but the significant reduction in aldehyde emissions all over the temperature range is mainly due to the strong inhibition at low temperature and to the increased ignition temperature

Combustion synthesis of ceramic and metal-matrix composites

Abstract: Combustion synthesis or self-propagating high temperature synthesis (SHS) is effected by heating a reactant mixture, to above the ignition temperature (Tig) whereupon an exothermic reaction is initiated which produces a maximum or combustion temperature, Tc. These SHS reactions are being used to produce ceramics, intermetallics, and composite materials. One of the major limitations of this process is that relatively high levels of porosity, e.g., 50 percent, remain in the product. Conducting these SHS reactions under adiabatic conditions, the maximum temperature is the adiabatic temperature, Tad, and delta H (Tad) = 0, Tad = Tc. If the reactants or products go through a phase change, the latent heat of transformation needs to be taken into account.

Diesel fuel containing an additive which improves the combustion of soot

Abstract: A diesel fuel containing an additive which improves the combustion of soot, for reducing the pollutant emission in the combustion exhaust gases from diesel engines by discontinuous burning-off of soot which has been precipitated in the exhaust gas filter, is described. For this purpose, a lithium, sodium or potassium salt of an aliphatic or aromatic alcohol, of a phenol, of an aliphatic acid or of a naphthoic acid, phenylacetic acid or cinnamic acid is added, singly or as a mixture, to the diesel fuel before the combustion of the latter in the internal combustion engine. As a result of the addition of the alkali metal salts, the ignition temperature of the soot precipitated in the particle filter is reduced, and the soot is oxidized at a temperature which is considerably lower than the normal ignition temperature. The regeneration range for the particle filter is therefore reached much more frequently in real running practice. This avoids a critical filter loading with soot, which can lead to filter damage during burning off. A further advantage of the process described is that, according to present knowledge, no additional substances with a health risk are emitted during running as a result of the addition of these alkali metal salts to the diesel fuel.

Improved 6.8-L Furnace for Measuring the Autoignition Temperatures of Dust Clouds

Abstract: A new US Bureau of Mines 6.8-L ignitability furnace was used to study the thermal autoignitability of dust clouds. This furnace has a quartz window to allow observation of the early stages of the ignition process and to allow measurement of the dust explosion temperature. Thermal ignitability data were obtained for various carbonaceous and metal dust clouds, with particular emphasis on various ranks of coal dusts. One of the reasons for the construction of the new 6.8-L furnace was to evaluate the effect of chamber volume on the measured autoignition temperatures. Therefore, data from the 6.8-L furnace were compared with data measured earlier in a 1.2-L furnace. The conclusion is that the autoignition temperatures measured in the 6.8-L furnace were only slightly lower on average than those from the 1.2-L furnace. These data on the minimum autoignition temperatures of various materials will be useful in analyzing the thermal ignition hazards of dust clouds in the mining industry and other industries that manufacture, process, or use combustible dusts.