OCR Text |
Show - 16 - reduced. This is in distinct contrast to the case of sensitization of methane ignition by addition of ethane or propane or the opposite case of kinetic inhibition by the addition of selected halogenated species [18], both processes which are primarily kinetic in nature rather than thermal as in the case of dilution by inert species. PHYSICAL SYSTEM MODIFICATION If the operating limits to composition which can support normal pulse combustor performance can be related to ignition delay times in the way described above, then the results of Figure 2 suggest another possible strategy for slightly modifying the lean limit. The stability limit, the shaded band at x = 1 msec, is assumed to be a fixed fraction of the natural period of the pulse combustor. If that fraction remains constant as the geometry of the pulse combustor is changed, then acoustic changes in the combustion chamber to decrease the pulse frequency (increase the time interval between successive reignitions) might result in a small reduction in the lean limit for stable operation. Conversely, an increase in the acoustically determined operating frequency of an order of magnitude would result in a shift in the lean limit from * = 0.5 to a higher value of approximately * = 0.6. CONCLUSIONS The simple characteristic time analysis described in this paper must be considered within the context of current research on pulse combustors. On the one hand, this type of analysis is deceptively powerful alone, identifying the key system parameters and physical processes which control the combustor performance, even in those situations where the chemical kinetic factors are not limiting. The predictions about modifications of |