Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 SHOCK TUBE AND ADIABATIC COMPRESSION 127reaction is discussed below. It is not a straightforward matter to extract reactionrate coefficients from the concentration-time data. For endo- and exothermic reactions,where the temperature rise is large, the enthalpy of reaction alters the measuredparameters significantly. Another complicating factor is the contraction whichoccurs in the time scale. Because the heated gas is moving rapidly along the tubebehind the shock, the gas which is being observed at any one instant will have beenheated not since the shock wave passed the observation point (which would be thecase if there were no net motion) but for a considerably longer time. The temperaturechange, which is related to the strength of the shock, is limited to a certainmaximum value which can be calculated. Various devices have been used to increasethis maximum value. Light driver gases, such as hydrogen and helium, producemuch stronger shocks for a given pressure ratio than heavier ones. This is primarilybecause of their lower molecular weight and the consequent higher velocity ofsound in them. The temperature change may also be increased by pre-heating thedriver gas. Yet another device is to use the shock wave after reflection off the frontwall; this increases the temperature rise by a factor of two. The rate data are generallyobtained from the velocity and concentration measurements with the help of acomputer. Equations representing the various processes are set up and trial solutionsinserted; the resulting profiles should eventually fit the experimental traces.One of the most difficult types of gas reaction to follow kinetically is the explosion.Typically, this has a radical chain mechanism and an induction period, and is thereforeseriously disturbed in any apparatus in which wall effects play a significant role.Explosions may not be studied conveniently in flames (Section 8) since the constantsupply of radicals diffusing back against the gas flow renders the concept of an inductionperiod somewhat irrelevant. The shock tube is suitable for studying themin the micro- to millisecond time range, but for times of 10- 3-1 sec a technique whichuses rapid adiabatic compression is being developed3 '. The apparatus consists essentiallyof a cylinder containing the reactant gases and a compressing piston. It isabout 80 cm long and 5 cm in diameter and is mounted vertically so that the pistonmay be driven by a falling weight of about 100 kg. Because preliminary reaction ofthe gas mixture would be important just before the piston is arrested (when thegas temperature is high but not at its final value), it is important to stop the pistonin as short a space as possible. The pressure in the reaction vessel at known timesmay be recorded, and the gas may be removed for analysis at a given time afterthe start of the reaction through a punctured membrane. The preliminary workusing this technique has been on the combustion of hydrocarbons, but it is alsosuitable for following such reactions as that between hydrogen and oxygen.2.1 THE HYDROGEN/OXYGEN REACTIONA reaction which has been studied in shock tubes as well as by means of flamesReferences pp. 176-1 79