A. K. Oppenheim

Dynamic effects of autoignition centers for hydrogen and C1,2-hydrocarbon fuels

A chemical kinetics model is used to compute the dynamics of a local autoignition center in a combustible mixture. Such centers arise from inhomogeneities in the bulk mixture and ignite earlier than the surrounding mixture. The model of an exothermic center considers a small, homogeneous mass of reacting mixture that is surrounded by an inert mixture. The model combines energy and species equations for the reacting mixture with a gas dynamic constraint for the expansion of the exploding center. The induction time, the time of chemical energy release, and the exothermic energy and power of the centers are evaluated for a variety of stoichiometric fuel-air mixtures. Fuels include hydrogen, methane, acetylene, ethylene, and ethane. An important dynamic effect of the center is the compression wave it produces in the surrounding mixture. The computations show that compression ratios of 1.2 to 1.8 are produced by a center in these fuels at high pressure.

Combustion by Pulsed Jet Plumes - Key to Controlled Combustion Engines

Abstract : Pulsed Jet Combustion (PJC) is introduced here as a key element for engines where the progress of combustion is interactively controlled by a microprocessor system. Practical realization of PJC presented here involves the use of an 18 mm plug containing a cavity, where a rich mixture is ignited by a conventional spark discharge, closed by a tip with a suitable orifice to form the effluent stream. Its performance is determined by tests carried out in a constant volume vessel, simulating the enclosure of a CFR engine at 60 CAD with compression ratio of 7:1, using propane/air mixtures at equivalence ratios of an order of 0.6, in comparison to that of a flame traversing the charge, a so- called FTC mode, upon Ignition by standard spark discharge under identical geometrical and initial thermochemical conditions. The results demonstrate the superiority of PJC for executing the exothermic process of combustion in a lean burn engine.

A parametric study of self-similar blast waves.

The paper presents a comprehensive examination of self-similar blast waves with respect to two parameters, one describing the front velocity and the other the variation of the ambient density immediately ahead of the front. All possible front trajectories are taken into account, including limiting cases of the exponential and logarithmic form. The structure of the waves is analysed by means of a phase plane defined in terms of two reduced co-ordinates F ≡ ( t / r μ) u and Z ≡ [( t / r μ) a ] 2 , where t and r are the independent (time and space) variables, μ ≡ d ln r n / d In t n the subscript n denoting the co-ordinates of the front, and u and a are, respectively, the particle velocity and the speed of sound. Loci of extrema of the integral curves in the phase plane are traced and loci of singularities are determined on the basis of their intersections. Boundary conditions are introduced for the case when the medium into which the waves propagate is at rest. Representative solutions, pertaining to all the possible cases of blast waves bounded by shock fronts propagating into an atmosphere of uniform density, are obtained by evaluating the integral curves and determining the corresponding profiles of the gasdynamic parameters. Particular examples of integral curves for waves bounded by detonations are given and all the degenerate solutions, corresponding to cases where the integral curve is reduced to a point, are delineated.

JET IGNITION OF AN ULTRA-LEAN MIXTURE

A preliminary study was conducted of jets of active radicals used as igniters for lean fuel mixtures. Jets were generated by combustion or electric discharge. Experiments were performed in a cylindrical steel vessel, 9 centimeters in diameter and 9 centimeters in length, filled initially with air or an ultra-lean methane-air mixture (equivalence ratio of 0.5) at atmospheric pressure and room temperature. Observations were made by streak photography of light emitted by the jets taken with a rotating mirror camera and by schlieren photography, using a submicrosecond spark discharge in air as a point light source. Gas dynamic properties of jets were primarily governed by their initial velocity, while the particular process by which they were formed had a secondary role. Jets of radicals invariably appeared as turbulent plumes which were embedded in blast waves headed by hemispherical shock fronts. Three interesting properties of jet ignition are examined: controllable depth of penetration so that combustion can be started at any desired location within the charge; zonal pre-turbulization enhancing the combustion process; and wide dispersion resulting in multipoint ignition so that combustion is initiated throughout a relatively large segment of the medium rather than in the form of a small, laminar flame kernel as it does in an unconfined spark discharge.

Pressure waves generated by steady flames

The paper is concerned primarily with the analysis of pressure waves that can be generated by clouds of explosive gas mixtures in a free atmosphere which is initially at a uniform state. Since maximum effects are of prime interest, ignition and initial flame acceleration are considered to be out of scope, and the treatment is restricted only to the final stage of constant flame velocity when the flow field is self-similar. By the introduction of reduced blast-wave parameters as phase-plane coordinates, the problem is resolved into the determination of the appropriate integral curves on this plane. Salient properties of such solutions related to deflagrations are worked out in detail for the regimes of blast waves and acoustic fields. Results, including space profiles of gasdynamic parameters, have been computed for a specific case of a hydrocarbon-air mixture that is characterized by a specific heat ratio of 1.3, sound speed at N.T.P. of 345 m/sec, and volumetric expansion ratio corresponding to constant pressure deflagration of 7, covering a complete range of burning speeds from 0.5 m/sec to the Chapman-Jouguet deflagration of about 120 m/sec. Maximum overpressure ratios that can be generated by such flames in point-and line-symmetrical waves range from 0.53×10 −3 , for the lower bound in the burning speed, up to 6 for the deflagration, while, for the average speeds of 5 to 10 m/sec, they are at a level of 0.05 to 0.10.

Initiation of Combustion in Lean Mixtures by Flame Jets

Flame jets produced by Pulsed Combustion Jet (PCJ) systems are particularly suited for initiating combustion in lean fuel-air mixtures. Presented here is a detailed investigation of performance characteristics of a PCJ system. The progress of combustion in the cavity of a generator was observed by schlieren photography of the events occurring in the interior of a rectangular cavity simulator fitted with transparent windows. Electric properties of the jet plume were measured by an electrostatic probe. Schlieren photographs revealed that products of combustion are discharged through the orifice before the combustion process can spread throughout the volume of the cavity. Signals of the electrostatic probe showed that ejection of incomplete combustion products from the cavity is principally governed by the duration of spark discharge. In the jet, the exothermic process decays at first and then becomes reestablished inside large scale vortex structures in the jet plume.

Quest for controlled combustion engines

Upon «harnessing the fire» by the early pioneers, the quest for controlled combustion provided the major incentive for progress in engine technology. At first this involved primarily the problem of knock and later that of pollutant emissions. It appears that both can be solved by treating the engine cylinder not only as a source of power but also as a controllable chemical reactor. The principal concept whereby this can be accomplished involves multi-point ignition combined with charge dilution and stratification. Means for this purpose include jet ignition, product recirculation, and chemical additives. The most suitable for the realization of this concept is a direct injection two-stroke engine

On self-similar blast waves headed by the Chapman-Jouguet detonation.

Consideration of the whole class of self-similar solutions for blast waves bounded by Chapman-Jouguet detonations that propagate into a uniform, quiescent, zero counterpressure atmosphere of a perfect gas with constant specific heats. Since such conditions can be approached quite closely by some actual chemical systems at NTP, this raises the interesting possibility of the existence of Chapman-Jouguet detonations of variable velocity. The principal virtue of the results presented is, however, more of theoretical significance. They represent the limiting case for all the self-similar blast waves headed by gasdynamic discontinuities associated with a deposition of finite amounts of energy, and they exhibit some unique features owing to the singular nature of the Chapman-Jouguet condition.

Study of exothermic processes in shock ignited gases by the use of laser shear interferometry.

Abstract The paper reports on the measurements of maximum exothermic power pulses attainable from a given chemical system. Experimental tests involved the use of a shock tube technique whereby the exothermic process of combustion was controlled by reflected shock, so that it occurred under virtually inviscid flow conditions, while the measurements were performed at a resolution commensurate with the actual rate of chemical reaction. Experimental observations were made by means of a novel method of laser shear interferometry—a cross-breed between holography and the conventional means for measuring refractive index fields, in that, on one hand, it was based on the exploitation of the phase coherence of the laser light beam, recording first a diffraction image of the wave fronts which, for the desired final result, had to be optically reconstructed, and, on the other, it yielded eventually either two-dimensional interferograms or schlieren photographs of the observed phenomena. Chemical systems treated in th...

Boundary-layer theory for blast waves

The necessity for developing a boundary-layer theory in the case of blast waves stems from the fact that inviscid flow solutions often yield physically unrealistic results. For example, in the classical problem of the so-called non-zero counterpressure explosion, one obtains infinite temperature and zero density in the centre at all times even after the shock front deteriorates into a sound wave. In reality, this does not occur, as a consequence, primarily, of heat transfer that modifies the structure of the flow field around the centre without drastically affecting the outer region. It is profitable, therefore, to consider the blast wave as a flow field consisting of two regions: the outer, which retains the properties of the inviscid solution, and the inner, which is governed by flow equations including terms expressing the effects of heat transfer and, concomitantly, viscosity. The latter region thus plays the role of a boundary layer. Reported here is an analytical method developed for the study of such layers, based on the matched asymptotic expansion technique combined with patched solutions.