Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

170 EXPERIMENTAL METHODS FOR FAST REACTIONSFig. 22. Schematic arrangement for flame studies.if a uni-dimensional flame is used. A flat, free-burning flame cannot be produced,but a convenient way of obtaining a flat flame is to support it on a circular meshscreen. By acting as a heat sink (and so removing heat from the combustion zone)such a screen also stabilizes the flame. It is usually necessary to enclose the flamein a gas-tight housing to avoid the effects of small fluctuations in atmospheric conditionsand also to allow gas pressures other than atmospheric to be used.(ii) Pressure regulation and measurement. One of the simplest and most satisfactoryways of maintaining a constant pressure on a gas flame is to locate a restriction(effectively a barrier with a small hole) between the burner and the pumping system.The dimensions of this “choking orifice” and the relative pressures above andbelow it are chosen so as to give the emergent gas the velocity of sound at the hole.If such a condition can be achieved the orifice acts rather like a valve; the flow becomesa function of the cross-sectional area and of the upstream pressure, but isindependent of any fluctuations in the downstream pressure. The pressure, whichis usually around atmospheric, can be conveniently measured with a mercury manometer.(iii) Temperature and velocity projile measurements. There are several methods ofmeasuring the temperature profile of a flame. As with the concentration measurementsdiscussed below, the temperature of a flame usually changes very greatlyand very rapidly with distance. Thus, the method used for monitoring it must becapable of covering a wide range, but the spatial discrimination must be good.One common method uses a microthermocouple or a micro resistance probe. Thereare difficulties connected with the use of probes in a flame: firstly the danger ofaffecting its geometry and thus the kinetics of the relevant reactions and, secondly,the possibility of catalyzing these reactions. The latter is often minimized by coatingthe probe with a ceramic material. A technique which is used frequently in aerody-