Influence of chemical kinetics in the self-ignition of nonpremixed supersonic hydrogen-air flows

Abstract

Self-ignition tests of a model scramjet combustor were conducted by using parallel sonic injection of gaseous hydrogen from the base of a blade-like strut into a supersonic vitiated air stream. The range of stagnation pressure and stagnation temperature studied varied from 1.0 to 4.5 MPa and from 1300 to 2200 K, respectively. Experimental results show that the self-ignition limit, in terms of either global or local quantities of pressure and temperature, exhibits a nonmonotonic behavior resembling the classical homogeneous explosion limit of the hydrogen-oxygen system. Specifically, for a given temperature, increasing pressure from a low value can render a non-ignitable mixture to first become ignitable, then non-ignitable again. This correspondence shows that, despite the globally supersonic nonpremixed configuration studied herein, ignition is strongly influenced by the intricate chemical reaction mechanism and assumes a premixed, homogeneous explosion character. Consequently, self-ignition criterion based on a global reaction rate approximating the complex chemistry is inadequate. An auxiliary computational study on counterflow ignition was also conducted to systematically investigate the contamination effects of vitiated air. Results indicate that such contamination effects are expected to be weak for the present experimental data because of the counterbalancing influence of the H2Oinhibition and NO-promotion reactions in effecting ignition. Introduction Considerable fundamental research has been conducted in response to the interest in the development of scramjet engines and ram accelerators [1-3]. Since hydrogen-air is the preferred reactant system for scramjet engines, and because of the significantly reduced residence time available for reaction to proceed subsequent to mixing, a comprehensive understanding of the supersonic ignition phenomena must necessarily include a reasonably realistic description of the associated chemistry and a corresponding exposition of its implications. Recognizing the need to account for finite rate kinetics, Huber et al. [4] developed a criterion for self-ignition by assuming that the ignition time is equal to the t Research Staff, Dept. of Mechanical & Aerospace Engineering. Member, AIAA $ Professor, Institute of Mechanics. Member, AIAA * Robert H. Goddard Professor, Dept. of Mechanical & Aerospace Engineering. Fellow, AIAA Copyright ©1998 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. mixture residence time, and by using a global reaction rate expression to describe the finiterate chemistry. However, while the use of a global reaction expression in combustion modeling has enjoyed a long history of acceptance, and indeed is still the description of preference in many present computational modeling of complex flows, the facts that it compresses the response of a chemical system which is fundamentally described by a multitude of elementary reactions involving a myriad of reaction intermediates, and that each of these elementary reactions has its own characteristic dependence on the local environment of temperature, pressure, and concentration, signifies that the system response described by such an empirical, global expression must be rather limited in the thermodynamic range of applicability. By implication, then, the associated empiricallydetermined global kinetic of activation energy, reaction orders, and frequency factor must also have only limited range of applicability. Furthermore, since elementary reactions can either facilitate or retard the progress of an overall reaction mechanism depending on the local thermodynamic environment, it is then reasonable to anticipate that the use of a given Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. set of global kinetic constants could totally miss possible trend reversals as the kinetic expression is applied beyond the range over which it is determined. A most dramatic example of such kinetic intricacies is the well-known explosion limits of the hydrogen-oxygen system which, coincidentally, is also our system of interest. Figure 1 shows the limit boundary in terms of the temperature and pressure of an enclosed homogeneous mixture. The characteristics of the limit can be considered for either a fixed temperature or a fixed pressure. Thus, it is seen that, over a certain temperature range, as the system pressure is continuously increased, the system changes from no explosion, to explosion, to no explosion, to explosion again, thereby demonstrating the nonlinear effect of the system pressure on the mixture ignitability. In particular, the middle, second explosion limit segment demonstrates that ignition is inhibited with increasing system pressure a response which intrinsically cannot be described by a global reaction mechanism with an overall reaction order which is usually taken to be positive. If we next consider the system response for a given pressure, then Fig. 1 shows that the system igni tabi l i ty changes drastically as the temperature crosses the limit line. Thus we expect that the effective activation energy controlling the system response would change from a large value to a smaller value as the system temperature increases beyond the limit line.