Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

168 EXPERIMENTAL METHODS FOR FAST REACTIONSare discernible, but the first of these is not very important since the reaction isstarted in the unburnt fuel by active species which have diffused against the flow.In principle there is nothing other than experimental convenience to distinguishsuch a fixed flame from one moving along a tube of fuel.What type of system is suitable for studying in this way? Firstly'03*'04*'05, thesituation is simplified considerably if the reactants are gases. Work has been done onsuspensions of liquid droplets and even fine solid particles, but complications ariseassociated with the volatilization steps prior to the reaction. Secondly, and far moreimportantly, the system must have the rather delicate balance of properties necessaryto maintain a steady-state flame. Active species and heat must be produced andtransported back against the flow of fresh reactants sufficiently rapidly to be ableto sustain the reaction, but not so fast as to produce an explosion. Chemical systemswhich have the necessary properties are quite numerous and rather varied in nature.The simplest flames are "decomposition flames", in which a single substance (e.g.,ozone, hydrazine) undergoes decomposition exothermally. Far more common areflames in which there are two initial components, which are termed fuel and oxidizer.This category includes such well-known examples as hydrocarbon and oxygen,hydrogen and oxygen or hydrogen and bromine, as well as some rather less likelyones such as diborane and hydrazine or atomic nitrogen and oxygen.The size of the combustion zone is determined by the rates of the reactions andby the gas flow rate, which in turn depends on the pressure of the gas-the largerthe pressure and flow rate the smaller the combustion zone. As in other flow systems,the time parameter is replaced by a distance, so it is generally true that thecombustion zone should be as large as possible to give the greatest spatial resolution.A flame can be looked at theoretically either in terms of a continuous fluidor in terms of individual molecules; the theory most often used is the Chapman-Enskog Theory of dilute gasesLo6. It is simplified considerably if the geometry ofthe flame is kept relatively simple, which is the case, especially, when it can betreated as uni-dimensional, e.g., as flat and circular or as spherical. The use of suchshapes also simplifies the experimental problems significantly since the flame dimensionsmust be measured. In practice it is usually possible to define a system completelyfrom one independent and (n + 1) dependent variables, where n is the numberof chemical components involved in the reaction'.Normally the measured quantities will be the weight fractions of (n- 1) species,the flame velocity (for a one-dimensional flat flame this is the velocity componentof the cold gas normal to the front), the temperature profile of the flame and thedistance, although in certain cases other parameters may be substitutedLo3. Theseare the quantities whose measurement is difficult and produced the problems whicht It is generally necessary to draw a dividing line between those species which are present in significantconcentration and those which are not. The position of this line depends to a large extenton the nature of the system.