Oscillatory cool-flames

Spectroscopic methods are required for free radical intermediates. Laser induced fluorescence of hydroxyl radicals has been used successfully to determine elementary rate parameters associated with the isomerization reaction RO2 QOOH [113]. Laser perturbation of hydroxyl radical concentrations in stabilized cool-flames has been used to obtain global kinetic data for chain-branching rates at temperatures of importance to the low-temperature region [79]. These methods appear to be most suited at present to combustion studies in flow systems. There are also several studies of the relative intensity from OH radical fluorescence during oscillatory cool-flames [58,114]. 

Spontaneous ignition and oscillatory cool-flames in closed vessels... 

Fig. 6.10. Idealized (p-T ) ignition diagram which is typical of that obtained for the combustion of many hydrocarbons in oxygen or air in closed vessels and well-stirred flow systems. Ignition, oscillatory cool-flame and slow reaction regions are shown.

In well-stirred flow reactors, oscillatory cool-flames are sustained indefinitely as long as the reactor temperature and flow control are held constant. 

A complementary ignition diagram to that for n-C4Hio in an unstirred closed vessel was obtained in a jet-stirred flow reactor by Proudler et al. [58]. The reactor was a 500 cm stainless steel cylinder, operated at a mean residence time of 9.4 0.4 s with a reactant mixture [n-C4Hio] [O2] = 1.13 1 (Fig. 6.16). Oscillatory cool-flames and ignitions were detected within narrow temperature ranges, but comparable with those of the closed... 

Fig. 6 16. p-Ta) ignition diagram obtained for the combustion of n-butane + oxygen in a well-stirred flow reactor at a mean residence time of 9.4 0.4 s. Different phenomena are identified as follows L, low-temperature stationary states H, high-temperature stationary states CF, oscillatory cool-flames I, oscillatory ignitions. A hysteresis of the boundaries was measured as a result of increasing (solid lines) and reducing (broken lines) the vessel temperature. (After Proudler et al. [58].)... 

The application of the basic ideas to real combustion systems is then taken up in Chapters 6 and 7. In Chapter 6, experimental and modelling studies are described which link the mechanistic observations of Chapter 1 to combustion characteristics of fuels studied under laboratory conditions. The experimental emphasis is initially on global combustion phenomena - ignition and oscillatory cool-flames - for a range of hydrocarbons. Section 6.5 then addresses the distribution of products in hydrocarbon oxidation this discussion differs from that in Chapter 1 where the conditions were optimized to allow the investigation of specific reactions. The focus is now on studies of oxidation products over a range of isothermal and non-isothermal conditions, the interpretation of the results in terms of elementary reactions and the use of the experimental data as a detailed test of combustion models. The chapter provides an overview of the success of detailed models in describing combustion phenomena and combustion... 

Skrumeda, L. L. Ross, J. Further measurements on the oscillatory cool flame oxidation of acetaldehyde and comparison with reaction-mechanism models. J. Phys. Chem. 1995, 99, 12835-12845. 

The existence of thermoKinetic oscillations is demonstrated most convincingly under well-stirred flowing conditions [1-4] and the deepest insights have been obtained, within the last decade, using continuously-stirred tanK reactors (cstr). But flowing conditions are not a pre-requisite for the existence of thermoKinetic oscillations even if their existence in closed conditions is ephemeral, and there is a very substantial history of the study of oscillatory cool flame phenomena in closed, unstirred vessels [5-0]. 

The extent of conversion of reactants and the product composition is measured by continuous withdrawal of samples via a very fine probe to a mass spectrometer. Light output associated with oscillatory "cool flames" (a chemiluminescent emission from excited formaldehyde] is detected by a photomultiplier through a window in the wall of the oven. Continuous and simultaneous measurements are thus made, and since the system is "well-stirred", what is measured at one location by the thermocouple, or by withdrawal to the mass spectrometer, is the same as that at any other point. These measurements are time-dependent and so at a stationary state (sn or sf in the phase-plane] they will be invariant at an oscillatory state (uf in the phase-plane] periodic phenomena will be observed. 

When there are several episodes of cool flames the reaction is oscillatory. The crucial feature is the reversible reaction... 

Among the areas not covered here is that of intrinsic instabilities associated with chemical-kinetic mechanisms, as exhibited in cool-flame phenomena, for example these subjects are touched briefly in Section B.2.5.3. Intrinsic instabilities of detonations were considered in Section 6.3.1 and will not be revisited. Certain aspects of intrinsic instabilities of diffusion flames were mentioned briefly in Section 3.4.4 diffusion flames appear to exhibit fewer intrinsic instabilities than premixed flames, although under appropriate experimental conditions their effects can be observed, as indicated at the end of Section 9.5.2. Certain chamber instabilities that are not related to acoustic instabilities (such as Coanda effects—oscillatory attachment of flows to different walls) will not be discussed here, but reviews are available [1]. 

The form of the solutions to the simplified model were analysed by examining the existence and types of the pseudo-stationary points of the equations for d0/dr = d 3/dr = 0 and values of e in the range 0—1 (r = Figure 29 shows the oscillation of a multiple-cool-flame solution about the locus of such a pseudo-stationary point, Sj. The initial oscillation is damped while Si is a stable focus. The changing of Si into a unstable focus surrounded by a stable limit cycle leads to an amplification of the oscillation which approaches the amplitude of the limit cycle. When Si reverts to a stable focus, and then a stable node, the solution approaches the locus of the pseudo-stationary point. In this way an insight may be gained into the oscillatory behaviour of multiple cool flames. 

Exotic oscillatory and other types of non-linear behaviour are also features of most hydrocarbon oxidations [71-74]. The next chapter will provide a detailed mechanistic description of the basis for cool-flames etc., and their relevance in various situations. It is interesting, however, to apply the classification system developed in the previous sections to the global behaviour in these systems. We start with a description of the oxidation of acetaldehyde (ethanal) and again concentrate on modern studies in flow reactors where the effects of reactant consumption (which are much more significant in closed systems for these cases than for CO) are not a feature. 

Gray, R Griffiths, J. F. Hasko, S. M. Lignola, P. G. Oscillatory ignitions and cool flames accompanying the non-isothermal oxidation of acetaldehyde in a well stirred, flow reactor. Proc. Roy. Soc. Lond. A 1981, 374, 313-339. 

The first steps towards an explanation of these extra modes of hydrocarbon oxidation were taken by Frank-Kamenetskii who proposed that the multiple cool flames observed were oscillatory and that the period of oscillation reflected the underlying chemistry. Experimental investigations continued to concentrate upon traditional measurements of pressure versus time, of induction period, and o establishing the identity of stable intermediates and reaction pathways. Interpretations of these results continued to be made on isothermal degenerate branched-chain reactions direct measurements of temperature were not made. These interpretations were very incomplete, and much better understanding has emerged from application of thermokinetic theory. 

At present it would seem as if detailed modelling is most profitable after vindication of a generalized approadi and is not a realistic indepraident altmiativc. Progress 1 both paths has deepened understanding in the study of hydrocarbon oxidation. From being the most mysterious, periodic cool flames have become the best undostood of oscillatory chemical reactions. 

The study of the response of nonlinear systems to external periodic perturbations leads to interesting information.Cool-flame, 9 oscillations occur in a number of combustion reactions, and we discuss an experimental study of the effect of external periodic perturbations on such systems. The application of perturbations to a chemical reaction can reveal important information about the stability, kinetics, and dynamics of the reaction. This technique is well known in the field of relaxation kinetics, in which perturbations are applied to a chemical system at equilibrium. In our work, periodic perturbations are first applied to the input rates of acetaldehyde and oxygen, one at a time, in the combustion of acetaldehyde in a CSTR. We measure periodic responses in five entrainment bands as we vary the frequency and amplitude of the external periodic perturbation. Outside of entrainment bands we find quasi-periodic responses. Next-phase rnapslO, of the experimental results are constructed in real time and used in the observation and interpretation of entrainment and quasi-periodic behavior. Within the fundamental entrainment band, we measure critical slowing down and enhancement of the response amplitude. As the bath temperature is increased, so that the oscillatory system approaches a Hopf bifurcation, we observe an increase in the amplitude enhancement. The predictions of a five-variable thermokinetic model agree well with the experimental results. 

It is the occurrence of a fast steady-state branched-chain reaction that enables to realize a noncatalytic gas-phase process and create a stationary technological process on its basis. The quasi-steady-state branched-chain mode provides a high rate of a noncatalytic reaction at relatively low temperatures, whereas the absence of a solid phase (catalyst) minimizes the influence of heterogeneous processes, which lead to the formation of deep oxidation products. In addition to the critical transition between the oxidation modes, other manifestations of the nonlinear nature of the process, such as cool flames, NTC region, reaction rate temperature hysteresis, and oscillatory regimes, have been observed. 

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