Ignition chain

In Chapter 3, the conditions for a chain branching explosion were developed on the basis of a steady-state analysis. It was shown that when the chain branching factor a at a given temperature and pressure was greater than some critical value acrit, the reacting system exploded. Obviously, in that development no induction period or critical chain ignition time rc evolved. 

Fundamental theoretical and experimental research on systems capable of ignition has been done by Semenov and his school, especially on the critical conditions of thermal and chain ignition. Later papers studied the process in... 

Figure 5.9 The dependence of methane oxidation rate with hydrogen peroxide on the chain ignition rate.

By detailed studies of the structure of the transitions from to p, and of the accompanying pressure changes, Ashmore and Tyler [276] were able to show that, near the lower sensitizer limits, the ignitions were thermal in nature, while close to the upper limit they were nearly isothermal, branched chain ignitions. This was indicated by the observation that the pressure decrease near the lower limit was preceded by a pressure pulse or increase which could only be reasonably explained by self-heating in the early part of the reaction prior to the ignition. Such pressure increases near the upper limit were less pronounced and occurred much less frequently. In addition, the rates of reaction just outside the upper limit were much smaller than at the lower boundary. The rates near the limits for 100 torr 2H2 + O2 at 360 °C are shown in Fig. 47. 

In addition to showing branched-chain ignition, with (first and second) p-Ta limits similar to those of the H2 + O2 system, the CO -I- O2 reaction supports a response known as glow. This is a spontaneous chemiluminscent state, with a weak, pale-blue emission arising from electronically-excited... 

Type of Smoke Pot Burning Time (Minutes) Ignition Chain Ignition Floating Weight IPounds) Fuel Filler... 

Formation of CO and of the chain centres OH, H and O demands an even greater contribution from dissociation. The overshoot in number of particles which occurs in branched chain ignition in these systems is then determined by the particular intermediates which are formed, and... 

At T < I m the chain branched reaction does not occur, that is, Tf is the critical temperature below which chain ignition is impossible. At T> 2m chains terminate on the surface and in the bulk more rapidly than branching and, therefore, the progres-... 

It follows from this formula that a critical size (diameter) of the reaction vessel also exists chain ignition is observed in a large reactor, and it is not in a small reactor. As in the case of chain termination in the kinetic regime, critical conditions depend on the shape of the vessel (for a planar vessel g = 9.9DRT(f for a spherical vessel g = 39.5DRTd ). 

Since the reaction proceeds with autoacceleration and its occurrence can be noticed experimentally only when some rate v = v, is achieved, the induction period T is observed, which can be determined as the time of achievement of v = v, , and since v is expressed by equation (12.9), then x =

other hand, the limits of chain ignition p, and are the roots of the equation <(> = 0 and, hence,

[Pg.373]

---