Oscillatory ignitions and cool flames accompanying the non-isothermal oxidation of acetaldehyde in a well stirred, flow reactor

Abstract

Oxidation, cool flames and ignition in equimolar mixtures of acetaldehyde and oxygen have been studied in a well stirred, continuous-flow reactor (an open system), over temperatures from 450 to 625 K and pressures from 5 to 25 kN m -2 . The reactor is a 500 cm 3 , spherical, glass vessel, ca . 10 cm in diameter, and is stirred mechanically at a revolution rate of up to 1200 min -1 . Residence times can be varied down to a few seconds; our work relates to 10, 7, 5 and (mainly) 3 s. These conditions broadly resemble those in which transient cool-flame phenomena can be seen in closed vessels, and for which steady-state calculations have been made for open systems. Open systems, however, allow unlimited numbers of oscillations to be observed, and stationary states to be maintained indefinitely. The emphasis in the present work is on establishing the variety of behaviour, and on characterizing the new modes of reaction possible in open systems by quantitative and continuous measurements. Concentrations of reactants, products and molecular intermediates, and hence rates of reaction, are monitored continuously by a mass spectrometer; light emissions are monitored instrumentally; self-heating and hence rates of heat evolution are detected and recorded by measuring excess temperatures with fine-wire thermocouples. An unprecedented variety of behaviour has been encountered. Nine chemically and physically distinct, stable modes of reaction have been observed. There are stable oscillations (limit cycles) of seven clearly differentiated forms, and two stationary states. The conditions for the occurrence of each mode have been mapped for various residence times on a traditional ignition diagram of reactant pressure and vessel temperature. They occupy five regions (representative, overlapping reactortemperature ranges in parentheses): I, steady reaction without light emission (up to 550 K); II, oscillatory ignition (500-520 K); III, five modes of oscillatory ignition interspersed with cool flames (520-540 K); IV, oscillatory cool flames (500-600 K); V, steady reaction with chemiluminescence (580-650 K). At the various boundaries between the five regions, sharp jumps occur from one kind of behaviour to another. At three segments of the boundary, there is hysteresis, the jumps occurring at different temperatures during heating (I -> II, III or IV) and cooling (II, III or IV -> I) traverses. There are thus regions of bistability, where identical external conditions - vessel temperatures, reactant pressures and flow rates - can give rise to alternative states inside the reactor. The two non-oscillatory, stationary states have different characters: I is a stable node and V is a stable focus. In region I, the reaction rate increases with temperature; but in region V, both reaction rate and extent of self-heating show a near-zero or negative temperature-coefficient.