Dark zone reaction

This reduction of NO is highly exothermic but relatively slow at low pressure, because it appears to be a third-order reaction, similar to the dark-zone reaction of nitropolymer combustion. The overall reaction of HNF is represented by... 

V) Flame zone When the dark-zone reactions occur rapidly after an induction period, they produce a flame zone in which the final combustion products are formed and attain a state of thermal equiUbrium. When the pressure is low, below about 1 MPa, no flame zone is produced because the reduction of nitric oxide is too slow to produce nitrogen. 

The soUd-phase reaction zone is also termed the subsurface reaction zone or condensed-phase reaction zone . As the dark zone reaction represents an induction zone ahead of the flame zone, the dark zone is also termed the preparation zone when it produces a luminous flame. Since the flame zone is luminous, it is also termed the luminous flame zone . 

MPa and 3.0 MPa. The reaction time defined in Eq. (6.3) remains relatively unchanged and no significant effect on the dark zone reaction is seen by the addition of LiF and C. 

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 combustion of H2, CO, and hydrocarbons with NO is important in both the dark zone and the flame zone of nitropolymer propellants. It is well known that NO behaves in a complex way in combustion processes, in that at certain concentrations it may catalyze a reaction to promote a process, while at other concentrations it may inhibit the reaction. Sawyer and GlassmanP attempted to estabUsh a measurable reaction between H2 and NO in a flow reactor at 0.1 MPa. Over a wide range of mixture ratios, they found that the reaction did not occur readily below the temperature of NO dissociation, except in the presence of some radicals. Mixtures of CO and NO are also difficult to ignite, and only mixtures rich in NO could be ignited at 1720 K. 

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 ]... 

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 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... 

The probable set of chemical reactions taking place in the dark zone has been analyzed theoretically by Sotter.Ii Sixteen reversible and four irreversible chemical reactions involving twelve chemical species were considered. The following reachons were taken to be the most important ... 

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). 

Since the final gas-phase reaction to produce a luminous flame zone is initiated by the reaction in the dark zone, the reaction time is determined by the dark zone length, L4, i. e., the flame stand-off distance. Fig. 6.8 and 6.9 show data for the dark zone length and the dark zone temperature, T, respectively, for the propellants listed in Table 6.3. The luminous flame front approaches the burning surface and... 

Fig. 6.10 Reaction time in the dark zone decreases with increasing (NO) at constant pressure.

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... 

Table 6.10 Overall reaction order, m, in the dark zone determined from the pressure exponent of burning rate, n, and the dark zone index, d.

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. 

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 dark zone length of liF-catalyzed propellants is increased by the addition of LiF in the region of super-rate burning, similar to the case of Pb-catalyzed propellants, as shown in Fig. 6.28. Table 6-11 shows the dark zone lengths and reaction times Xg in the dark zone producing the luminous flame at two different pressures,... 

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