Chemical kinetic factors of hydrocarbon oxidation are examined in a variety of ignition problems. Ignition is related to the presence of a dominant chain-branching reaction mechanism that can drive a chemical system to completion in a very short period of time. Ignition in laboratory environments is studied for problems including shock tubes and rapid compression machines. Modeling of the laboratory systems is used to develop kinetic models that can be used to analyze ignition in practical systems. Two major chain-branching regimes are identified, one consisting of high temperature ignition with a chain branching reaction mechanism based on the reaction between atomic hydrogen with molecular oxygen, and the second based on an intermediate temperature thermal decomposition of hydrogen peroxide. Kinetic models are then used to describe ignition in practical combustion environments, including detonations and pulse combustors for high temperature ignition and engine knock and diesel ignition for intermediate temperature ignition. The final example of ignition in a practical environment is homogeneous charge, compression ignition (HCCI), which is shown to be a problem dominated by the kinetics of intermediate temperature hydrocarbon ignition. Model results show why high hydrocarbon and CO emissions are inevitable in HCCI combustion. The conclusion of this study is that the kinetics of hydrocarbon ignition are actually quite simple, since only one or two elementary reactions are dominant. However, many combustion factors can influence these two major reactions, and these are the features that vary from one practical system to another.

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Chemical kinetics of hydrocarbon ignition in practical combustion systems

A Numerical Study of a Two-Dimensional H2-O2-Ar Detonation Using a Detailed Chemical Reaction Model

We review recent progress in gaseous detonation experiment, modeling, and simulation. We focus on the propagating detonation wave as a fundamental combustion process. The picture that is emerging is that although all propagating detonations are unstable, there is a wide range of behavior with one extreme being nearly laminar but unsteady periodic flow and the other chaotic instability with highly turbulent flow. We discuss the implications of this for detonation propagation and dynamic behavior such as diffraction, initiation, and quenching or failure.

https://shepherd.caltech.edu/EDL/publications/reprints/proci_1374.pdf

Detonation in gases

Publisher Summary Chemical mechanisms have been employed in hydrocarbon combustion as a means of understanding the underlying phenomenology of the combustion process in terms of the elementary reactions of individual species. This chapter provides an introduction to most of the mathematical methods that have been used for the construction, investigation, and reduction of combustion mechanisms. The use of algebraic manipulation in techniques, such as the quasi-steady-state approximation (QSSA) and lumping, make the production of a reduced mechanism essential and make subsequent calculations as simple as possible. Computational singular perturbation (CSP) is an alternative to the rate-of-production and sensitivity methods for mechanism reduction and provides an automatic selection of the important reactions as well as time-scale analysis. The simplest and most widely used technique involving the separation of time scales is the QSSA; however, a possible limitation is that it may not provide the minimum low-order system. Chemical lumping can prove very useful in areas, such as the combustion of hydrocarbon mixtures or soot formation. Several programs are available for the investigation and reduction of combustion mechanisms, including MECHMOD, a code for the automatic modification of CHEMKIN format combustion mechanisms, and KINALC, which is an almost automatic program for the investigation and reduction of gas-phase reaction mechanisms. KINALC is a postprocessor to CHEMKIN-based simulation packages SENKIN, PREMIX, OPPDIF, RUN1DL, PSR, SHOCK and EQLIB. Because models in combustion are expected to cover a wide range of conditions, it is natural to expect that a different approach might be used for different cases.

Chapter 4 Mathematical tools for the construction, investigation and reduction of combustion mechanisms

In this review, we emphasize the recent progress achieved in understanding the behaviour of unstable detonations through the interaction of theoretical, asymp­ totic, and numerical ideas. Theoretical predictions and numerical simulations for unstable one-dimensional detonations are described in detail as an important testing ground for the more complex ideas and phenomena that occur in several space dimensions. The linear and nonlinear theories for unstable detonations are generalized to several space dimensions. A new dedicated numerical method leads to better insight into the physical phenomena of unstable detonations, such as the nature of the turbulence generated in the wake of the front. Simplified models derived through asymptotics and comparisons between theoretical and numerical predictions are stressed throughout this paper.

Theoretical and numerical structure of unstable detonations

A numerical analysis of performed on the nonsteady two-dimensional behavior of self-sustaining detonations. In order to produce realistic two-dimensional structures during propagation in a finite width channel, an initially assumed plane one-dimensional Chapman-Jouguet detonation was perturbed by locating up to five pairs of exothermicity spots in the passage of the detonation. The most interesting feature of the simulation was the number of triple shock waves existing after a certain time of transition from plane to periodically nonsteady two-dimensional structures. In other words, the final number of transverse shock waves becomes mostly two, which are nearly irrelevant to the assumed number of initial exothermicity spots. Details of the disappearance of several transverse waves during their rearrangement were clearly observed and are discussed. In addition, despite considerable deviation of the instantaneous propagation velocity from from the C-J value, the average of such oscillations was close to the C-J value. Thus, the results of the simulation provided the numerous features observed in the present gaseous detonations experiments. The numerical schemes utilized were the explicit first-order Van Leer and second-order MacCormack methods. The effect of artificial viscosity was examined and found to affect the transition to fully developed detonation.

T. Fujiwara

Papers

Numerical simulation of triple shock behavior of gaseous detonation