Chapter 4 - INTERACTION OF CHAINS

Abstract

The chain termination and branching rates are proportional to the concentrations of the free atoms and radicals formed in the chains; this is equivalent to assuming the chains independent. However, termination or branching involves radical–radical interactions or secondary radical reactions on intermediate products. This chapter discusses negative and positive chain interactions. In negative chain interactions, radical loss occurs by combination, but the rate is usually low compared with that at the wall at low pressures. At higher pressures, branching is usually so rapid that the gas becomes heated; therefore, the chain explosion becomes a thermal one before the free radical concentrations can become high. Positive chain interactions occur when active and inactive radicals react to give two active radicals. The chain ignition occurs isothermally, that is, when all the heat produced is lost. However, in chain reactions where the branching and termination rates are directly proportional to the active particle concentrations, the flame does not spread much if no layer heating occurs. However, the situation changes if the chains interact positively.