Freeze out [NO_x] mixing ratio

Zel'dovich et al. [1947] added quantities of NO to N₂:O₂ mixtures that were heated by combustion to greater than 2000 K and found that when NO was added to the initial mixture in large quantities, it decomposed, whereas with addition of small quantities led to an increase in concentration. The NO freeze out mixing ratio can be determined directly by finding the concentration of NO that needs to be added to give no net change in NO concentration. A similar experiment has been conducted in the present work, by adding small quantities of NO to N₂:O₂ mixtures in which an electrical discharge is struck.

Since [NO]_{freeze out} is equal to the equilibrium NO concentration at the freeze out temperature (T_{freeze out}), T_{freeze out} can be determined from calculations of [NO]_equil.(T). In addition, at the freeze out temperature, the time to achieve chemical equilibrium tau_equil. will be equal to the time constant for the cooling of the gas (tau_cooling), so calculations of tau_equil. allow tau_cooling to be determined. Finally, since the total number of NO_x molecules produced by the discharge is determined, the volume of unperturbed gas heated to T_{freeze out} can be calculated.

From figure 3, the equilibrium NO mixing ratio for a pressure of 27 mbar is (2.8±0.4)% (v/v). That an equilibrium was observed is strong direct evidence that NO_x formation is occurring via the Zel'dovich mechanism. From figure 6, this value corresponds to a freeze out temperature of ca. 2900±400 K with a time constant for the cooling of the gas at this temperature of between 10^-2 s and 0.4 s. The time constant for the cooling of the gas in the shock front is ca. 10^-5 s, so again, these results do not support the suggestion that NO_x formation occurs in the shock front. Likewise the time constant for the drop in density in the core during the onset of the discharge will be ca. 10^-6 s, so these results are not consistent with the density freeze out mechanism of Goldenbaum and Dickerson.