Fizz zone reaction

IV) Dark zone In this zone, oxidation reactions of the products formed in the fizz-zone reaction take place. Nitric oxide, carbon monoxide, hydrogen, and carbonaceous fragments react to produce nitrogen, carbon dioxide, water, etc. These exothermic reactions occur only very slowly unless the temperature and/or pressure is sufficiently high. 

The rate of temperature increase in the fizz zone, (dT/dt)f, indicates the heating rate due to the exothermic reaction in the fizz zone. As shown in Fig. 6.12, the heating rate increases linearly in a plot of In dT/dt)f versus KNO2) at 2.0 MPa. The reaction time in the fizz zone, Xf, can be obtained by means of a similar relationship to Eq. (6.5), adapted to the fizz zone reaction. Fig. 6.13 shows Xy versus KNOj) at 2.0 MPa. The reaction time decreases linearly with increasing KNOj) in a plot of In Xj versus (N02) and is represented by... 

Most importantly, the presence of lead compounds results in a strong acceleration of the fizz zone reactions, i. e., those in the gas phase close to the burning surface. Acceleration of the reactions in the subsequent dark zone or in the luminous flame zone is not significant. The net result of the fizz zone reaction rate acceleration is an increased heat feedback to the surface (e. g., by as much as 100 %), which produces super-rate burning. 

The effective overall order of the fizz zone reaction, k, is determined to be zero for plateau burning, and approximately 1.4for super-rate burning. The reaction order for the non-catalyzed propellant is also determined to be approximately 1.7, that is, nearly equal to the order of a conventional gas-phase reaction. 

As described in Section 6.4, lead catalysts act on the condensed phase and fizz zone reactions, not on the dark zone reaction, and increase the burning rate. On the contrary, Ni catalysts act on the dark zone reaction, but not on the condensed phase or fizz zone reactions, and do not increase the burning rate. No luminous flame is produced when double-base propellants burn, for example, at 0.5 MPa. However, when catalyzed propellants with metallic Ni or organic Ni compounds burn, a luminous flame is produced just above the burning surface at the same pressure. 

The burning mechanism of composite propellants differs from that described above. There is no exothermic reaction which can lead to a self-sustaining fizz zone. Instead, the first process appears to be the softening and breakdown of the organic binder/fuel which surrounds the ammonium perchlorate particles. Particles of propellant become detached and enter the flame. The binder is pyrolysed and the ammonium perchlorate broken down, initially to ammonia and perchloric acid. The main chemical reaction is thus in the gas phase, between the initial dissociation products. 

The combustion wave of a double-base propellant consists of the following five successive zones, as shovm in Fig. 6.3 (I) heat conduction zone, (II) soHd-phase reaction zone, (III) fizz zone, (IV) dark zone, and (V) flame zone-l i -i -i ]... 

III) Fizz zone The major fractions of nitrogen dioxide and the aldehydes and other C,H,0 and HC species react to produce nitric oxide, carbon monoxide, water, hydrogen, and carbonaceous materials. This reaction process occurs very rapidly in the early stages of the gas-phase reaction zone, just above the burning surface. 

The thermal structure of the combustion wave of a double-base propellant is revealed by its temperature profile trace. In the solid-phase reaction zone, the temperature increases rapidly from the initial temperature in the heat conduction zone, Tq, to the onset temperature of the solid-phase reaction, T , which is just below the burning surface temperature, T. The temperature continues to increase rapidly from T to the temperature at the end of the fizz zone, T, which is equal to the temperature at the beginning of the dark zone. In the dark zone, the temperature increases relatively slowly and the thickness of the dark zone is much greater than that of the solid-phase reaction zone or the fizz zone. Between the dark zone and the flame zone, the temperature increases rapidly once more and reaches the maximum flame temperature in the flame zone, i. e., the adiabatic flame temperature, Tg. 

Thus, the combustion wave structure of double-base propellants appears to showa two-stage gas-phase reaction, taking place in the fizz zone and the dark zone. The thickness of the fizz zone is actually dependent on the chemical kinetics of the... 

In the dark zone, the temperature increases relatively slowly and so for the most part the temperature gradient is much less steep than that in the fizz zone. However, the temperature increases rapidly at about 50 pm from where the flame reaction starts to produce the luminous flame zone. The gas flow velocity increases with increasing distance due to the increase in temperature. The mole fractions of NO, CO, and Hj decrease and those of N2, CO2, and H2O increase with increasing distance in the dark zone. The results imply that the overall reaction in the dark zone is highly exothermic and that the order of reaction is higher than second order because of the reduction reaction involving NO. The derivative of temperature with respect to time t in the dark zone is expressed empirically by the formulal =l... 

The burning rate of a double-base propellant can be calculated by means of Eq. (3.59), assuming the radiation from the gas phase to the burning surface to be negligible. Since the burning rate of a double-base propellant is dominated by the heat flux transferred back from the fizz zone to the burning surface, the reaction rate parameters in Eq. (3.59) are the physicochemical parameters of choice for describing the fizz zone. The gas-phase temperature, Tg, is the temperature at the end of the fizz zone, i. e., the dark-zone temperature, as obtained by means of Eq. (3.60). 

Fig. 6.13 Reaction time in the fizz zone decreases with increasing...

The combustion wave of an NC-NG-GAP propellant consists of successive two-stage reaction zones.0 1 The first gas-phase reaction occurs at the burning surface and the temperature increases rapidly in the fizz zone. The second zone is the dark zone, which separates the luminous flame zone from the burning surface. Thus, the luminous flame stands some distance above the burning surface. This structure... 

Though it is impossible to formulate a complete mathematical representation of the super-rate burning, it is possible to introduce a simplified description based on a dual-pathway representation of the effects of a shift in stoichiometry. Generalized chemical pathways for both non-catalyzed and catalyzed propellants are shown in Fig. 6.26. The shift toward the stoichiometric ratio causes a substantial increase in the reaction rate in the fizz zone and increases the dark zone temperature, a consequence of which is that the heat flux transferred back from the gas phase to the burning surface increases. 

Reaction zone Subsurface -> Fizz zone Dark zone —> Flame zone... 

The temperature gradient in the fizz zone just above the burning surface increases from 1.9 K pm to 2.5 K pm at 0.7 MPa when 2.4% LiF and 0.1 % C are added. The gas-phase reaction rate in the fizz zone is increased and the heat flux feedback from the fizz zone to the burning surface is increased by the addition of the catalysts. However, the effect of the addition of the catalysts is not seen in the dark zone. 

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