OCR Text |
Show and partial oxidation processes are each described using sets of unidirectional reactions, while elementary reversible reactions describe the completion of oxidation. Soot formation and gasification, and the NO emissions mechanisms, can be X described by unidirectional or detailed mechanisms as warranted by the level of detail desired. _ The development of the quasiglobal formulation for the chemical kinetics of realistic hydrocarbon fuels rests on the assumption that, from a kinetics standpoint, these fuels can, in general, be represented as mixtures of an aromatic and an aliphatic component. Further, the aromatic component is assumed to be representable, again with respect to chemical kinetics, by toluene, while isooctane represents the aliphatic component. This representation of the combustion characteristics of a class of fuels by a model component is -itself the foundation of the quasiglobal approach, which in its original form is based (Ref. 4) on the observation of similarities in the ignition delay characteristics of a number of different hydrocarbon fuels. Details of the formulation of the toluene-isooctane quasiglobal model are given in Ref. 2. For each of these components, the parent fuel is assumed to break down into a secondary fuel which then reacts to produce CO and hL, leading to the wet-CO mechanism for reaction completion. Since jet-stirred combustion experiments using isooctane (Ref. 6 ) , plug-flow experiments using n-octane and propane (Ref. 7), and flat-flame experiments using n-hexane (Ref. 8) all indicated that ethylene is a major intermediate product of aliphatic fuel combustion, ethylene is used as the secondary fuel intermediate in the isooctane (aliphatic) branch of the quasiglobal model. Similarly, acetylene has been shown to be a major intermediate in jet-stirred combustion studies ofrtoluene combustion (Ref. 6) and flat flame studies of benzene combustion (Ref. 9 ) , so acetylene has been taken to be the secondary fuel intermediate in quasiglobal modeling of aromatic fuel combustion. Table 2 presents the complete quasiglobal model for toluene and isooctane mixtures, including submodels for soot and thermal and fuel-bound NO formation. To a x obtain the rates shown in Table 2 for both the toluene and isooctane components of the model, extensive comparisons with jet-stirred and plug-flow reactor data were undertaken, as described in Refs. 2 and 10. For example, Fig. 1 shows a comparison of measured and predicted jet-stirred reactor exit stream temperatures for toluene combustion, as a function of the equivalence ratio at which the reactor was operated. In these experiments, the reaction was operated under nearly adiabatic conditions (Ref. 6). Two sets of temperature data are shown, one as measured in the exit stream and the second as calculated from the exit stream species concentrations (based 1.2.11 |