A.K. Oppenheim

Auto-ignition of hydrocarbons behind reflected shock waves*

The paper reports on the study of auto-ignition of hydrocarbon-oxygen mixtures behind reflected shock waves. Because of their bearing on the problem of knock in internal combustion engines, n-heptane and iso-oc??ne were chosen as the combustible species. Their stoichiometric mixtures with oxygen had to be diluted with 70% argon to reduce the influence of the boundary layer. Photographic records demonstrated the existence of two different modes of ignition, as has been previously established for the hydrogen-oxygen system. One, called strong ignition, manifested itself by a shock wave generated in the immediate proximity of the back wall of the shock tube. The other, refered to as mild ignition, had chemical reactions starting at distinct points in the gaseous medium producing essentially constant pressure flames. The pressure-temperature limits between the regions of mild and strong ignition (strong ignition limit) were determined. From the same experimental tests, induction time data was obtained over the pressure range of 1–4 atm and the temperature interval of 1200–1700°K. The strong ignition limits were found to correlate best with the lines of constant partial derivatives of the logarithm of induction time with respect to temperature at constant pressure.

Autoignition in methanehydrogen mixtures

Abstract Induction time and strong ignition limit associated with autoignition are determined experimentally for 11 mixtures of methaneue5f8hydrogenue5f8oxygen using the reflected shock technique. The experimental conditions cover temperatures from 800 to 2400K and pressures from 1 to 3 atm. The induction time data, τ, obtained for the methaneue5f8oxygen and the hydrogenue5f8oxygen mixtures are correlated with conventional Arrhenius-type correlation formulas. The results compare well with those reported in the literature. For the methaneue5f8hydrogenue5f8oxygen mixtures, a heuristic formula based on the correlations for methaneue5f8oxygen and hydrogenue5f8oxygen is proposed. This formula incorporates a parameter expressing the ratio of methaneue5f8hydrogen concentrations and is found to be quite satisfactory. The strong ignition limits for the 11 mixtures are expressed by a specific value of κ = ( δτ δT ) P , which is deduced for each mixture from the induction time correlation. The results show that κ is not highly sensitive to mixture compositions. This leads to the conclusion that the dominating factor in establishing strong ignition is the sensitivity of the induction time to the rate of change in temperature.

On the inadequacy of gasdynamic processes for triggering the transition to detonation

A complete sequence of stroboscopic laser-schlieren records of the nonsteady flow field ahead of an accelerating turbulent flame, in a stoichiometric hydrogen-oxygen mixture contained in a tube, permits an accurate determination of pressure and temperature profiles in time along the particle path leading to the center of the “explosion in the explosion” that promotes the transition to detonation. On the basis of known correlations for the steady-state induction time, the upper bound for the fraction of the induction process that has occurred as a consequence of just the gasdynamic processes of wave compression up to the onset of detonation is then computed, yielding a surprisingly low value of not more than 4 per cent. These results are offered as a clear demonstration that the gasdynamic processes per se are insufficient to bring about the transition to detonation if it takes place in a close vicinity of the flame front. For this purpose, direct effects of the flame must be taken into account. Their evaluation is left, however, for further study.

Introduction to gasdynamics of explosions

Questions concerning the genesis and sustenance of an explosion are investigated, giving attention to the mechanics of explosions, the gasdynamics of explosions, aspects of technological significance, and future prospects. The dynamics of exothermic processes is discussed together with the most prominent effects of explosions. Blast waves are considered, taking into account conservation principles, blast wave transformation, conservative equations in nondimensional form, the equation of state, Eulerian space profiles, Eulerian time profiles, Lagrangian time profiles, boundary conditions and integral relations, and self-similar flow fields.

Coherence theory of the strong ignition limit

Shock-induced ignition of explosive gases can be either “mild” or “strong,” depending on whether the combustion process manifests itself first in the form of flame kernels (or hot spots) or by a blast wave. On the basis of experimental evidence it has been shown that the strong ignition limit delineating the boundary between the two modes of autoignition corresponds to a fixed value of the partial derivative of induction time with respect to temperature at constant pressure. Here a rational theory for this fact is derived on the basis of the postulate that the chemical process is initiated within a set of distinct reaction centers and that for the strong ignition to develop the power pulses of heat released in these centers per unit mass of the reacting medium must be sufficiently coherent in time.

Vector polar method for the analysis of wave intersections

The Vector Polar Method is a graphical technique for the solution of wave interaction problems that are concerned with overall results, rather than with the progress of the process. The paper describes the application of this method to wave intersections, that is interactions in steady flow. The fundamental concepts for the method are developed from basic principles, and its use is illustrated by a number of multi-wave intersection processes, including triple front intersections and their reflections.

The onset of retonation

Abstract The onset of the retonation wave in a stoichiometric hydrogen-oxygen mixture contained in a 1 in. × 1·5 in. tube has been observed by means of schlieren streaks and instantaneous photographs. Some interesting details of transverse oscillation characteristics of spin which accompany this event have been revealed. Experimental records have been interpreted by means of a wave dynamic analysis to determine the state of the medium where the oscillations are set in. Their frequency has been found to be in agreement with the eigenvalue solution of the linear wave equation. It appears that the process has been initiated by a point explosion that is preceded by a deflagrative implosion. In this respect the phenomenon bears an interesting similarity to high frequency combustion instabilities in rocket thrust chambers, promising thus to serve as a useful tool for the study of their physicochemical aspects.

A computational technique for the evaluation of dynamic effects of exothermic reactions

Abstract The paper presents a computational technique for the analysis of nonsteady flow fields generated by exothermic reactions in a compressible medium. This is obtained by numerical integration of the set of rate equations of chemical kinetics combined with the set of conservation equations of nonsteady gasdynamics expressed in a Lagrangian form. Under the hypothesis that the power pulse of exothermic energy is so short that the effects of diffusion, viscosity, and conductivity are, during its effective life span, negligible, the place where it is generated can be restricted to a discrete Lagrangian cell, and, consequently, the chemical kinetic set expressed in terms of ordinary differential equations. The computational model devised in this manner is referred to as the exothermic center. Conceptually, it is a simple flow field, which consists of a kernel, confined within a single Lagrangian cell around the center where the exothermic reaction takes place, and of inert surroundings through which the pressure wave generated by the expanding kernel propagates, the two separated from each other by an impermeable interface. The computations yield the characteristic features of the power pulse of work done by the kernel on the surroundings. On the basis of this information, complex flow fields, where exothermic processes occur, can be analyzed as a nonsteady (diabatic) flow subject to energy (heat) supply that is furnished at a predetermined rate and given as a function of local thermodynamic state parameters, as demonstrated in the companion paper presented at the 15th Symposium (International) on Combustion.

Onset of detonation

Abstract What previously has been believed to be the onset of retonation is now shown to be the onset of detonation as well. The proof is experimental in nature and it is based on the use of two completely independent techniques. One, quite unsophisticated, utilizes the unique ability of the self-sustained detonation wave to ‘write on the walls’, the record being obtained on a carbon-soot covered film. The other exploits recent developments in laser technology, permitting the attainment of high repetition rate, extremely high resolution schlieren stroboscopic records of wave phenomena.

Flames, Lasers and Reactive Systems

The gasdynamics of flames, lasers, and other reactive systems are explored in contributions (about half by Soviet scientists) to the Eighth International Colloquium on the Gasdynamics of Explosions and Reactive Systems held in Minsk, USSR, in August, 1981. Major areas covered are laminar flames, turbulent flames, combustion of solids, ignition and extinction, nonequilibrium systems, and lasers. Topics discussed include the resonant response of a flat flame near a flame holder, concentration and velocity measurements in a turbulent reacting mixing layer, the turbulent combustion zone in a tubular reactor, the unsteady burning of double-base propellants, surface-layer destruction during the combustion of homogeneous powders, the extinction of in-flight engine-fuel-leak fires with dry chemicals, linearized kinetic models for polyatomic gases and mixtures, a gasdynamic laser using products of acetylene explosions, the influence of flow structure on optical gain in gasdynamic lasers, and an arc-heated gasdynamic CO2 laser operating at 16.4-18.6 microns. For individual items see A84-28388 to A84-28411