Autoignition Conditions Research Articles

Non-premixed and premixed extinction and autoignition of C 2H 4, C 2H 6, C 3H 6, and C 3H 8

Experimental studies are conducted on laminar non-premixed and premixed flames stabilized in the counterflow configuration. The fuels tested are ethene (C2H4), ethane (C2H6), propene (C3H6), and propane (C3H8). Studies on non-premixed systems are carried out by injecting a fuel stream made up of fuel and nitrogen (N2) from one duct and an oxidizer stream made up of air and N2 from the other duct. Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen, and nitrogen from one duct and an inert-gas stream of N2 from the other duct. Critical conditions of extinction are measured by increasing the flow rates of the counterflowing streams until the flame extinguishes. Critical conditions of autoignition are measured by preheating the oxidizer stream of the non-premixed system and the inert-gas stream of the premixed system. Experimental data for autoignition are obtained over a wide range of temperatures of the heated streams. In addition, for premixed systems, experimental data are obtained for a wide range of values of the equivalence ratio including fuel-lean and fuel-rich conditions. Numerical calculations are performed using a detailed chemical-kinetic mechanism and compared with measurements. The present study highlights the influences of non-uniform flow-feld on autoignition in premixed systems that were not available from previous studies using shock tubes. For the premixed system considered here, the changes in the strain rates at extinction with equivalence ratio are found to be similar to previous observations of changes in laminar burning velocities with equivalence ratio. The studies on autoignition in the premixed system show that the temperature of the inert-gas stream at autoignition reaches a minimum for a certain equivalence ratio. For premixed systems, abrupt extinction and autoignition are not observed if the value of the equivalence ratio is less than some critical value.

Critical Autoignition Conditions for Heterogeneous Condensed Systems in the Presence of Phase Transformations

The critical ignition conditions for a single particle which interacts with a melting medium to form an intermetallic compound on its surface were investigated using the model of a quasistationary melting front. For linear and parabolic drag laws, three characteristic regions, which transform with change of Biot's criterion, are distinguished on the diagram of critical parameters. It is shown that for small values of this criterion, the conditions of self‐heating of the particle are identical to the ignition conditions in a medium with constant temperature. For values of Biot's criterion greater than unity, the critical conditions are substantially affected by the parameters determining the melting kinetics. The characteristic features of self‐heating of a single particle can determine the qualitative pattern of self‐heating of the heterogeneous system as a whole.

Longitudinally Stratified Combustion in Wave Rotors

A wave rotor may be used as a pressure-gain combustor, effecting wave compression and expansion, and intermittent cone ned combustion, to enhance gas-turbine engine performance. It will be more compact than an equivalent pressure-exchange wave-rotor system, but will have similar thermodynamic and mechanical characteristics. Because the allowable turbine blade temperature limits overall fuel ‐air ratio to sube ammable values, premixed stratie cation techniques are necessary to burn hydrocarbon fuels in small engines with compressor discharge temperatures well below autoignition conditions. One-dimensional, nonsteady numerical simulations of stratie ed-charge combustion are performed using an eddy-diffusivity turbulence model and a simple reaction model incorporating a e ammability limit temperature. For good combustion efe ciency, a stratie cation strategy is developed that concentrates fuel at the leading and trailing edges of the inlet port. Rotor and exhaust temperature proe les and performance predictions are presented at three representative operating conditions of the engine: full design load, 40% load, and idle. Theresults indicate thatpeak local gas temperatures will causeexcessivetemperaturesintherotorhousingunlessadditionalcoolingmethodsareused. Therotortemperaturewillbeacceptable,but the pattern factor presented to the turbine may be of concern, depending on exhaust duct design and duct ‐rotor interaction.

Extinction and autoignition of n-heptane in counterflow configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.

Effect of dimethyl ether, NO x, and ethane on CH 4 oxidation: High pressure, intermediate-temperature experiments and modeling

The effects of small amounts of dimethyl ether (DME), NO, and NO 2 on the autoignition and oxidation chemistry of methane, with an without small amounts of ethane present, were experimentally studied in a flow reactor at pressures and temperatures similar to those found in spark- and compression-ignition engines (under autoignition conditions). The reactions were studied at pressures from 10 to 18 atm, temperatures from 800 to 1060 K, and equivalence ratios from 0.5 to 2.0. It is found that 1% DME addition is as effective in stimulating the autoignition and oxidative behavior of methane as 3% C 2 H 6 addition, and that NO x at even ppm levels is more effective than hydrocarbon additives. For the same reaction time and temperatures below 1200 K, addition of small amounts of NO x lowered the temperature at which reaction becomes significant by more than 200 K. Chemical kinetic mechanisms in the literature for the interactions of methane, ethane, and NO x do not predict the reported observations well. The most significant rate-controlling reactions for CH 4 autoignition is found to be CH 3 +HO 2 =CH 3 O+OH. Good agreement, with and without NO x perturbations can be obtained by modifying the rate constants of three reactions (CH 3 +HO 2 =CH 3 O+OH; CH 3 +HO 2 =CH 4 +O 2 : CH 2 O+HO 2 =HCO+H 2 O 2 ) and by adding the reaction CH 3 +NO 2 =CH 3 O+NO to the GRI-Mech v2.11 mechanism. These modifications do not significantly affect predictions for shock tube, flame, and other results used in developing GRI-Mech v2.11. Results strongly suggest that exhaust gas residuals and/or exhaust gas recirculation can have as profound an effect as natural gas contaminants on the apparent octane and cetane behavior.

Critical autoignition conditions for a polydisperse gas suspension of solidfuel particles

Critical autoignition conditions for a polydisperse gas suspension of solidfuel particles

Critical autoignition conditions for a system of particles

1. The critical conditions of autoignition for a system of particles have the same form as the critical conditions for the individual particle, but the place of the heat transfer coefficient α is taken by the coefficient αeff, for which an explicit expression has been found. 2. The cooperative effect of the particles is determined by the parameter A, which is inversely proportional to the square of the mean particle diameter. 3. This simple model, with the effective heat transfer coefficient, explains the qualitative aspects of the measured relation between ignition temperature and concentration [5] for particles of different diameter and, taking into consideration the remarks relating to supercritical heating, the complex (with maximum and minimum) dependence of ignition temperature on particle diameter.