Sensitization of the hydrogen—oxygen system

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. 

During the initial induction period the net branching factor 0 is n ative, giving a non-branched chain system (cf. Sect. 8.2). However, if j) increases during the induction period, because of the changes in concentration of NO2 and NO, then a sudden increase in chain centre concentration would occur as 0 passes through zero and becomes positive. This is taken to occur at Pe. 

The complete reaction scheme which Ashmore and Tyler [276] r ard as most satisfactory for explaining the sensitization phenomena, including 

During the initial induction period, when 0 is n ative, the major reactions are those of the H2—NO2 system. The branching reaction (ii) is outweighed by the fast reaction (xxxix) of O atoms, and by reaction (iv) and its successors. Reaction (xxxix) becomes less important as [NO2 ] decreases, and at the same time the increasing concentration of NO favours reaction (xxxvii) of HO2 at the expense of (v) or (x). Since reactions (xxxiii) and (xxxiv) have similar rates (see Sect. 8.2) the effect of replacement of NO2 by NO on these termination steps is negligible. 0 therefore increases during the induction period. 

Regarding the chain termination by reactions of HO2, the lack of dependence of Pe on vessel diameter suggests that reaction (x) rather than a surface reaction (v) is the major contributor. This also accounts for the thermal nature of the ignitions at the lower sensitizer limits, where the NO and NO2 concentrations are also low. Near the upper sensitizer limit the NO concentration at the end of the induction period will be comparatively high. This favours the removal of HO2 by reaction (xxxvii) rather than (x), and at the same time favours reaction (xxxiv) rather than (x) as the chain terminating step. With the replacement of quadratic by mainly linear termination, the nature of the ignition changes from purely thermal to nearly isothermal. 

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E. Oran, J. Boris, Weak and strong ignition. Sensitivity of the hydrogen-oxygen system. Combust. Hame 48(1), 149-161 (1982)... 

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