Thermokinetic feedback

THERMOKINETIC FEEDBACK OSCILLATIONS AND LOCAL STABILITY ANALYSIS... 

Another model studied under the name of thermokinetic feedback is due to Salnikov [14,15]. This has two first-order reaction steps which, in general, could both be exothermic and both have an Arrhenius temperature dependence ... 

In 1969 Gray and Yang [70] formulated an extremely simple scheme which could reproduce these phenomena and it is described in detail in Chapter 5. Its importance lay, not so much in its application to the modelling of practical systems, but in its provision of a conceptual base for further development. It incorporated the essential features of the science, particularly thermokinetic feedback - the interaction between a branched radical chain and the reaction-generated temperature rise. 

The cool flames associated with the low-temperature oxidation of hydrocarbon have great relevance, being the fundamental cause of knock in internal combustion engines. Their mechanistic origin arises through a thermokinetic feedback [2]. The crucial feature is the reaction through which O2 reacts by addition to an alkyl... 

This chapter and chapter 5 study the prototypical thermokinetic oscillator. Thermal feedback replaces autocatalysis, and the Arrhenius temperature dependence of rate coefficients supplies non-linearity in the scheme P - A - B + heat. After careful study of this chapter the reader should be able to ... 

Basis of mechanistic interpretation A full mechanistic account follows in the next chapter. Here we simply indicate the important features of the currently-accepted interpretation of the above facts. The key feature in these thermokinetic phenomena is that there are both thermal and chemical feedback processes combining to produce the various exotic responses, including the ntc. At the heart of the clockwork is the equilibrium involving the methyl radical CH3 and molecular oxygen [78]... 

Since the main reactants, bromate and malonic acid, are in surplus, inhibition and negative feedback in the BZ reaction are quite different from that in the previous example of thermokinetic oscillations. Inhibition is provided in part by a direct decomposition of HBr02 (analogous to heat removal in the previous example) but mainly by a chain of reactions of oxidized ion catalyst with brominated malonic acid (BrMA), summarized as... 

The pathway from Ce + to Br is the key to negative feedback on HBr02, which helps to remove this species after being accumulated due to autocatalysis. However, different roles are associated with the two species while Br is an inhibitor that directly removes HBr02, Ce" " " is a controlling species that provides a delay allowing the autocatalysis to advance considerably before inhibition by Br causes the concentration of HBr02 to drop hence oscillations may appear. As before, proper time scales of reaction steps are necessary for oscillations to appear. Clearly, this example of isothermal oscillations is more involved than the thermokinetic one. In particular, there are three main (or essential) types of variables rather than two this observation prompts for a classification of oscillatory reaction mechanisms—one of the main topics of this chapter. 

The first sdf-consistent thermokinetic scheme for an oscillatory reaction was discussed by Salnikov in 1949. He proved that oscillations would arise under the infiuenoe of thermal feedback in the simple reaction sdione... 

The fundamental property of any combustion process is that heat is released during chemical conversion and that some of this energy (or, in adiabatic conditions, all of it) is retained by the system, causing an internal temperature rise. This constitutes the thermal feedback that is essential to self-sustained combustibn. Chemical feedback through atoms and radicals produced in chain branching processes is also a common feature, though not necessarily pre-requisite. (It is, however, a pre-requisite for thermokinetic oscillatory phenomena.)... 

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