Hydrocarbon-air Mixtures Research Articles

How to attack complex gas phase combustion systems

Starting from detailed investigations of elementary chemical reactions a strategy for experimental and theoretical analysis of complex combustion systems is described. Experimental results on the effect of selective translational excitation of reactants in the reaction H + O 2 → OH + O are compared with theoretical studies on ab initio potential energy surfaces. For the reaction CN + H 2O → HCN + OH direct measurements of thermal rates in shock tubes are compared with data obtained from a laser-photolysis-laser-induced fluorescence technique. The attack of practical combustion problems with homogeneous models including detailed chemistry is described for the NO x reduction in power plants and the autoignition of hydrocarbonair mixtures in relation to engine knock. Next the interaction of elementary chemical processes with steady and unsteady laminar flows is considered. Laminar counterflow diffusion flames constitute an important basis set for the simulation of more complicated turbulent combustion processes. Having a library of precalculated “flamelets” of different strain rate and fueloxidizer mixture compositions the numerical treatment of turbulent flame structures can be performed on existing computers. Detailed calculations on pure and partially premixed CH 4air counterflow diffusion flames are compared with results of nonintrusive CARS measurements for various strain rates. For the unsteady case as a simple test system the ignition of O 2O 3 mixtures by irradiation with a CO 2 laser along the axis of a cylindrical vessel is considered. Mathematical simulation of the ignition process is done by solving the corresponding system of conservation equations. Spatial retization using definite differences leads to a system of ordinary differential and algebraic equations that can be solved numerically. Experimental data are presented for velocity components of the flame front from IR-UV double resonance experiments. The last part describes two approaches for linking complex chemistry to models of turbulent flow field predictions applied to an enclosed turbulent jet diffusion flame. One is a conserved-scalar approach employing a stretched-laminar-flamelet model, and the other involves a direct closure of the chemical production terms by a pdf method. Numerical results for turbulent COair diffusion flames and experimental results on imaging of laminar and turbulent flame fronts by OH radical and acetaldehyde fluorescence in an internal combustion engine are presented.

Prediction of knock occurrence in a spark-ignition engine

It is generally accepted that knock in a spark-ignition engine is caused by end-gas autoignition. Hu and Keck have recently developed a reduced kinetic model which simulates the kinetic processes involved in autoignition of hydrocarbon-air mixtures. The model was used successfully to correlate measurements of explosion limits in a constant volume bomb and ignition delay times in rapid compression machines. In this paper, the ability of Hu and Keck's model to predict the onset of knock in a spark-ignition engine is evaluated Experimental data from a large number of individual cycles were generated from a single-cylinder engine over a range of operating conditions where knock occurred. The unburned gas temperature used in the kinetic model was calculated from the measured cylinder pressure data assuming that knock originates in that part of the end-gas region which is compressed adiabatically. The model indicates that autoignition under knocking conditions is a two stage process. The model predictions of when knock occurs generally match well with measured knock angle on a cycle-by-cycle basis for a range of fuel (isooctane, primary reference, and indolene) and engine operating conditions. Agreement is least satisfactory when knock occurs close to the end of the burning process when the end-gas mass fraction is small and its temperature is not known precisely.

Equilibrium Analysis of Combustion/Incineration

ABSTRACT Equilibrium product distributions for a variety of hydrocarbon-air mixtures containing hetero-atoms such as chlorine, nitrogen, and sulfur were calculated. These studies indicated that the...

Normal ammonia flame propagation velocities at elevated pressures

The aim of the investigation was to determine the normal flame propagation velocities S /SUB n/ in ammonia-oxygen and ammonia-oxygen-nitrogen mixtures at atmospheric and elevated pressures p. For investigating the dependence S /SUB n/ (p), the authors chose compositions well away from the limits. This made it possible to disregard the lift of the flame due to free convection, which is especially strongly expressed at low flame propagation velocities near the limits. The mixture was ignited at the center of the vessel by melting a Nichrome wire. The results, presented in a table, show that for near-stoichiometric mixtures the ratio P /SUB max/ /P /SUB O/ exceeds the values typical of the deflagration of fuel-oxygen compositions and in some experiments reaches 17. The normal flame propagation properties were calculated by the Lewis method. A comparison of the values obtained for m (subscript of m indicates pressure range) with the data for slowly burning hydrocarbon-air mixtures shows that they closely complement each other.

Chemical aspects of the autoignition of hydrocarbonair mixtures

Some aspects of the chemistry of knock in internal combustion engines have been examined within the context of a model of autoignition of hydrocarbonair mixtures at high pressure. The basic model follows an earlier one developed by workers at Shell Research Ltd., which has been refined and extended using reactions and rate parameters which are compatible with the expanding body of kinetics information on the type of elementary processes involved. This model was able to predict moderately well the temperature, fuel, and O 2 dependence of ignition delay time observed in previous experiments in a rapid compression machine, thus providing further support for the validity of the knock mechanisms. Fuel-structure dependent parameters were identified, and first indications of a rational basis for the stability ranking of alkane fuels with respect to autoignition have been found.

Spontaneous ignition characteristics of gaseous hydrocarbon-air mixtures

Measurements of spontaneous ignition delay time are conducted using gaseous fuels and fuel vapors at normal atmospheric pressure. The apparatus comprises a test section which is constructed from several lengths of 30 mm diameter stainless steel tubing to form a maximum total length of 2.2 m. The method employed to determine ignition delay times is to establish certain predetermined values of velocity and equivalence ratio within the test section and then gradually increase the temperature of the mixture until autoignition occurs. All measurements are taken, at various levels of velocity, with the flame located a few centimeters downstream of the end of the test section, where it assumes a conical form with the apex pointing upstream. The results show that ignition delay time can be expressed in terms of mixture temperature, fuel concentration, and oxygen concentration, by the equation τ⧜ exp[ E RT m ] [Fuel] m [Oxygen] n . For the fuels employed in this study, which include propane, aviation kerosine, and n-heptane, the results show that E is fairly close to 40,000 kcal/kg mol. Ignition delay times are found to be only slightly dependent on fuel concentration, but strongly dependent on oxygen concentration. For example, for propane n = −0.59, while for kerosine n = −0.65.

The critical tube diameter for detonation failure in hydrocarbon-air mixtures

Critical tube diameters d c for the successful transformation of a planar to a spherical detonation have been measured in nine gaseous fuels (CH 4, C 2H 2, C 2H 4, C 2H 6, C 3H 6, C 3H 8, C 4H 10, MAPP and H 2) in stoichiometric fuel-oxygen mixtures diluted with nitrogen at atmospheric initial pressure. In agreement with the previous work of Matsui and Lee. acetylene is found to have the smallest critical tube diameter of 12 cm at stoichiometric composition with air. However, in contrast with the earlier estimate of Matsui and Lee, hydrogen is now found to be the second most sensitive fuel with a critical tube diameter of 20 cm. Both ethylene and MAPP, with critical diameters of about 38 and 53 cm, respectively, have nearly the same sensitivity. The heavier hydrocarbons (i.e., C 4H 10, C 2H 6, C 3H 6, and C 3H 8) are found to belong to the same sensitivity class with critical diameters of 64, 67, 70, and 70 cm, respectively. Direct measurements of detonation cell sizes λ in both atmospheric fuel-oxygen-nitrogen mixtures and subatmospheric fuel-oxygen mixtures demonstrate that the observation of Mitrofanov and Soloukhin in low-pressure ( p 0 ⋍ 80 Torr) C 2H 2O 2 mixtures that d c3- 13 λ is valid for other fuels as well and can be used as an empirical law to estimate critical tube diameter from cell size data. Based on the preseent results for the critical tube diameter, initiation energies have been estimated using the work-done formula of Lee and Matsui to evaluate the detonation hazard numbers D H as proposed previously by Matsui and Lee. For stoichiometric fuel-air mixtures at 1 atm initially, acetylene ranks first ( D H = 8.4 × 10 5), followed by hydrogen ( D H = 3.4 × 10 6) and ethylene oxide ( D H = 1.1 × 10 7) and MAPP ( D H = 7.2 × 10 7) have nearly the same sensitivity. The other alkanes ( D H = 1.5 × 10 8 for ethane, D H = 1.7 × 10 8 for n-butane, D H = 1.73 × 10 8 for propane) and the alkenepropylene ( D H = 1.6 × 10 8) belong to the same sensitivity class.

Inhibition of hydrocarbon oxidation in laminar flames and detonations by halogenated compounds

A detailed chemical kinetic reaction mechanism is developed to describe the inhibition ofhydrocarbon fuel oxidation by halogenated species. Included in this mechanism are the halogen acids HCl, HBr, and HI, as well as halogenated hydrocarbons formed by combining together Cl, Br, or I atoms with methyl, ethyl, or vinyl radicals. Also included is a submechanism describing the kinetics of the combustion of CF3Br. The resulting mechanism is combined with a complete hydrocarbon oxidation mechanism and used to study inhibition of laminar flames and detonations in hydrocarbon-air mixtures. In all of the cases examined, the halogenated species act by catalyzing the recombination of H atoms into relatively nonreactive H2 molecules, reducing the available radical pool and lowering the overall rate of chain branching. In agreement with experimental observations, the present modeling study indicates that I atoms are the most effective, Br atoms are slightly less effective than I atoms, and Cl atoms are very much less effective as kinetic inhibitors. The additional fuel content of the halogenated hydrocarbons makes their inhibition efficiency vary with equivalence ratio, and for all of the inhibitors increased pressure also increases the inhibition efficiency. Discussions of the kinetic factors responsible for this behavior are presented.

Influence of exothermicity on the shape of a diffusion flame

The effects of exothermicity of combustion on the contours of gaseous, unconfined, planarand axi-symmetric, jet diffusion flames, as well as on the structures of their flow fields, are analyzed. For the sake of clarity, the problem is formulated in the simplest possible way. The flame is treated as essentially laminar, the reaction rates are considered infinite, the medium is assumed to behave as a perfect gas with constant specific heats, its Schmidt and Prandtl numbers being unity and viscosity proportional to the temperature, while the buoyancy effects are neglected. Under such circumstances the problem lends itself to treatment by the Shvab-Zeldovich technique. The results, derived as an inverse to self-similar solutions in the incompressible domain obtained by a Dorodnitsyn-Howarth transformation of the governing equations for compressible flow, are expressed in terms of algebraic formulae. In applications to hydrogen-air and a number of hydrocarbon-air mixtures, they turn out to be in reasonable agreement with experimental data under zero gravity conditions.

On the design of venting systems against gaseous explosions

A lumped parameters mathematical model for the venting of gaseous explosions has been used to analyze a wide set of experimental data taken from the literature. The values of a “turbulence factor”, that account for the increase in the laminar burning velocity due to the flow field generated inside the vessel by the effluent gases, have been computed by fitting the model to the experimental maximum explosion pressure. These values have been satisfactorily correlated to the breakout pressure and to the vent area. A diagram, based on this correlation and on the mathematical model, is proposed for the evaluation of the required vent area as a function of the breakout pressure and of the maximum allowable overpressure. This diagram may be used with confidence for hydrocarbon—air mixtures over a wide range of breakout pressures and vent areas.

Flow field induced by unconfined spherical accelerating flames

An experimental study of the flow field ahead of a spherical flame expanding in unconfined hydrogen-air and hydrocarbon-air mixtures is presented. Particular attention is devoted to the influence of flame acceleration on the characteristics of the flame-induced flow field. To specify this influence, a simple theoretical model is proposed which explains quite well the general trends observed for maximum overpressure and impulse behavior. Several conclusions of practical relevance in predicting pressure effects due to accidental vapor cloud explosions are underlined. In particular, it is demonstrated that the maximum overpressure is not always overestimated, using the pressure field solution for a constant velocity flame and the maximum velocity reached by the actual accelerating flame.

Decomposition of hydrogen telluride that proceeds as deflagration combustion

1. We determined the values of the quenching distance, minimum ignition energy, normal combustion rate, and limiting pressure for the flame decomposition of H2Te. 2. H2Te is a more explosive compound than GeH4, and it approximates hydrocarbon-air mixtures in its characteristics. 3. The critical value of the Peclet number has an abnormally low Value in the case of flame propagation for the decomposition of H2Te in narrow channels. A qualitative explanation of this result is proposed.

Detonation limits of hydrocarbon-air mixtures in tubes

Detonation limits of hydrocarbon-air mixtures in tubes

Swirling Flow Combustion

Tests in a combustion centrifuge demonstrated that the normal progression from laminar flamespeeds of about 0.3 m/s (in hydrocarbon-air mixtures) to turbulent flamespeeds of up to 6.1 m/s can be advanced to a third stage, where flamespeeds in excess of 19 m/s were measured. A theory explaining how centrifugal-force-enhanced buoyancy produces the increased flamespeeds was developed. This theory was applied to the results of a combustion centrifuge experiment, the design of a 38-cm-dia afterburner research rig, and eventually to the design of a full-scale afterburner, which was tested on a modern turbofan engine. The experimental results confirmed the theory and indicate a potential 2 percent reduction in specific fuel consumption. 16 figures.

Pressure waves generated by spherical flames

The present paper describes an approximate solution for the description of the shock-hydrodynamic flow structure corresponding to constant velocity expanding cylindrical or spherical flames. The solution is based on a perturbation procedure following the PLK technique. By matching the solutions coming from the inner boundary of the flame (or equivalently a piston) to those from the outer boundary of the shock front, closed form analytical expressions are obtained relating the shock strength to the piston velocity as well as for the various flow distributions. Compared with the exact solution obtained via numerical integration of the self-similar equations. the present approximate solution is found to give excellent results in the range of normal burning velocity of S ≲ 50 m/sec for most common hydrocarbonair mixtures.

Electrostatic charge generation during tank washing. Spark mechanisms in tanks filled with charged mist

Under conditions of tank washing several possible sparking mechanisms were investigated involving conducting objects not bonded to the tank. Some of the tests were done in a full-scale 12,000 m 3 shore tank containing an electrostatically charged mist generated by a tank washing machine. Sparking mechanisms were identified associated with the formation of unbonded conductors in the form of water masses (“water slugs”) by the water jets used for tank cleaning. Ignition experiments were performed using masses of water as unbonded conductors. They produced sparks of sufficient intensity to ignite hydrocarbon-air mixtures under conditions relevant to the practical situation in tankers.

An investigation of the minimum ignition energies of some C1 to C7 hydrocarbons

Minimum ignition energies have been measured by means of the expanding capacitor plate technique for a number of hydrocarbon-air mixtures over the temperature range of 22–125°C and at two pressures. The results are presented in the form E1 = APαTβ where α = 2 and β = −2.The data obtained relate to an ignition probability of 0.8 and the values obtained for methane are consistent with previous measurements. An interesting characteristic of the experimental technique is that these results relate to a sharp boundary below which ignition could not be observed.

Detonation initiation in unconfined fuel-air mixtures

The critical energy threshold for detonation initiation in unconfined gaseous fuel-air mixtures has been determined as a function of fuel concentration in air for a hydrocarbon fuel, which consists of methylacetylene, propadiene, propane, and propylene. A similar function was found for 6% by volume n-propyl nitrate and 94% hydrocarbon fuel mixture. Detonation limits of the hydrocarbon-air mixture were found to be 3.3 and 9.8%. The minimum critical energy threshold for high explosive blast wave initiation of the hydrocarbon-air mixtures was found to be 19 kcals at stoichiometric. The n-propyl nitrate/hydrocarbon fuel was detonable over the range 2.8–10.1% with a minimum critical energy threshold of 12 kcals. Propylene oxide-air mixtures were detonable with a minimum critical energy threshold of 4 kcals, and the propylene oxide was detonable in a much broader mixture range than the hydrocarbon or hydrocarbon/ n-propyl nitrate fuels. Detonation velocities were measured to be 1830 ± 43 m/sec in 5.0% hydrocarbon fuel-air mixtures, which is in good agreement with calculated Chapman-Jouguet velocities. Measured detonation pressures of 250 ± 20 psi were lower than the calculated pressures of 270 psi. The experimentally measured radii at which wave velocities from spherical initiating charges decayed to Chapman-Jouguet velocities were approximately the same as the calculated critical radii r∗. Several interesting detonation wave initiation and propagation phenomena were observed on high-speed film and recorded by pressure and impulse measurements.

Radio Detection of Low Energy Electrostatic Sparks

WE have carried out laboratory and outdoor studies to see whether radio observations with a tuned loop aerial can be used to monitor low energy electrostatic sparks and to separate spark discharges from corona-type discharges. Since corona is in general non-incendive in inflammable hydrocarbon air mixtures—but sparks may be incendive if sufficiently energetic—the observation of electromagnetic radiation from a discharge could form a simple technique for monitoring potential ignition hazards.

Use of the ESR method to study the combustion of gases at atmospheric pressure

1. The possibility of direct measurements of the H concentration by the ESR method at atmospheric pressure in the combustion zone and in the combustion products of hydrogen-and hydrocarbon-air mixtures was demonstrated. 2. In the combustion zone of the mixtures studied, substantial concentrations of H atoms above the equilibrium values, reaching 1017 cm−3 for diffusion flames of hydrogen with air and 1016 cm−3 for methane-and propane-air flames, were detected in the combustion zone of the investigated mixtures.