Multistage ignition in hydrocarbon combustion: Temperature effects and theories of nonisothermal combustion

In a study of self-heating during the low-temperature oxidation of propane, attention is given to direct measurement of temperature in the gas reacted under both unstirred and well-stirred conditions in a static reactor. During slow reaction, unstirred, the spatial temperature profile that develops is intermediate between that predicted for either purely conductive or strongly convective heat transfer. With stirring, the temperature and concentration distributions become nearly uniform in space. The P-T₀ ignition diagram has been mapped in the unstirred reactor: apart from the familiar slow reaction, cool-flame oscillations two-stage ignition, lobe and single-stage ignition, a region of multistage ignitions is characterized (cool flames precede the final ignition). Temperature-time profiles for multiple oscillations (≤5) and complex ignitions are given. Often, the cool flame propagates with a feeble front from the hottest zone of the reaction, and this leads to complex temperature-time curves. In the stirred reactor, each cool flame propagates homogeneously. Consequently, temperature-time traces are modified and become more easily interpreted. Thus, inhomogeneity during reaction perturbs the nonisothermal behavior, and this may vitiate conclusions that do not take the effect into account. Res^ults are discussed in terms of unified chain-thermal theory invoking two time-dependent variables (temperature and concentration of one intermediate x). With the exception of multistage ignitions, all physical aspects are explained completely. The former are consistent only with a theory invoking three dependent variables. The new variable may be either another important chemical intermediate or a parameter of the system, such as fuel consumption, or a property controlling the rate of heat loss.