Third body efficiency

The reactivity of H02 is much lower than that of OH, H, or O therefore, somewhat higher temperatures are necessary for sequence [Eq. (3.22)] to become effective [6a], Water vapor tends to inhibit explosion due to the effect of reaction (3.21) in that H20 has a high third-body efficiency, which is most probably due to some resonance energy exchange with the H02 formed. 

Even though it is generally considered inert, the carrier gas may have a significant impact on flame behavior. A change in the carrier gas alters the thermal diffusivity as well as the heat capacity of the fuel-air mixture, and it may also affect the reaction rate of pressure dependent reactions through differences in third-body efficiency. 

Pressure dependence at 298 K. Third-body efficiencies 810 and discussion of reaction mechanism... 

In this expression, the term [M] is a representation of the concentration of third body species M. The role of M can be played by any molecule or radical in the vessel and is essentially to stabilize the HO2 by energy transfer. In general, only species present in significant concentrations will be important here, but different species have different effectivenesses at facilitating the energy transfer. This is accommodated by assigning different values of third body efficiencies Oi to different species i. The termination rate can then be written as... 

If /C3 and can be taken as Langevin collision rate constants, then to the extent that t (= V3 ) can be taken from (7) (or other, more refined expressions) and p can be assumed constant (or otherwise approximated), one has a theory for three-body association reactions. If and/or deviate from Langevin rate constants, the factor p must absorb this effect also. Variations in third-body efficiency are commonly observed, although not generally very large ones. This establishes that k j) cannot be equal to /cl for the third-body collisions, i.e., either k ki ov p 1. 

For pressure-dependent reactions, both low- and high-pressure-limit rate coefficients k and k are presented where available. For the low-pressure case, collision efficiencies have been reported for several of the third-order reactions in the H2/O2/CO system. From these data it can be concluded that the temperature dependences of third-body efficiencies in this system are roughly equal for the different reactions, taking the limits of experimental errors into consideration. Exceptions are collision efficiencies in reactions where the third body interacts chemically with the collison complex, e.g.. 

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