Solid phase reaction zone

Based on this approach, an approximate energy balance around the solid-phase reaction zone is given as... 

The most notable theoretical analysis of the instability problem has been presented by McClure and Hart (M5). These investigators postulated a generalized combustion zone that includes a temperature-dependent and pressure-independent solid-phase reaction zone, and a temperature- and pressure-dependent gas-phase reaction zone. From this general model, Hart... 

II) Solid-phase reaction zone Nitrogen dioxide and aldehydes are produced in the thermal degradation process. This reaction process occurs endothermically in the solid phase and/or at the burning surface. The interface between the solid phase and the burning surface is composed of a solid/gas and/or soUd/Uquid/gas thin layer. The nitrogen dioxide fraction exothermically oxidizes the aldehydes at the interface layer. Thus, the overall reaction in the solid-phase reaction zone appears to be exothermic. The thickness of the solid-phase reaction zone is very small, and so the temperature is approximately equal to the burning surface temperature, T. ... 

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. 

The combustion wave of a double-base propellant consists of the following five successive zones, as shown in Fig. 6.3 (I) heat conduction zone, (II) solid-phase reaction zone, (III) fizz zone, (IV) dark zone, and (V) flame zone.t To.B-is]... 

The solid-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 . 

The thermal structure of the combustion wave of double-base propellants is understood from the temperature profile traces in the combustion wave. In the solid phase reaction zone, the temperature in the solid phase increases rapidly from the initial temperature T0 to the onset temperature of the solid phase reaction zone, Tu... 

These studies have indicated that the independent parameters controlling the postulated solid-phase reactions significantly affect the resulting acoustic admittance of the combustion zone, even though these reactions were assumed to be independent of the pressure in the combustion zone. In this combustion model, the pressure oscillations cause the flame zone to move with respect to the solid surface. This effect, in turn, causes oscillations in the rate of heat transfer from the gaseous-combustion zone back to the solid surface, and hence produces oscillations in the temperature of the solid surface. The solid-phase reactions respond to these temperature oscillations, producing significant contributions to the acoustical response of the combustion zone. 

Williams (W2) has recently modified the analysis of Hart and McClure by considering in more detail the effect of diffusional processes on the gas-phase reaction zone. The results of his study show that the diffusional processes tend to stabilize the gas-phase combustion process, indicating that the postulated solid-phase reactions are probably the underlying cause of the instability. 

Another contributing mechanism is the direct cooling of hot propellant surface by contact with the injected fluid. The fluid should cause the decomposing surface to reduce its pyrolysis rate to a point where combustion cannot be sustained. In addition, the presence of water on the surface would obstruct heat transfer from the gas-phase reaction zones to the solid surface, thus augmenting the cooling of the surface. Proponents of these two approaches have correlated the injection data on the basis of mass of fluid required per unit area of surface, but theoretical justifications for the use of this particular correlating parameter have not been presented. 

A schematic representation of the combustion wave structure of a typical energetic material is shown in Fig. 3.9 and the heat transfer process as a function of the burning distance and temperature is shown in Fig. 3.10. In zone I (solid-phase zone or condensed-phase zone), no chemical reactions occur and the temperature increases from the initial temperature (Tq) to the decomposition temperature (T ). In zone II (condensed-phase reaction zone), in which there is a phase change from solid to liquid and/or to gas and reactive gaseous species are formed in endothermic or exothermic reactions, the temperature increases from T to the burning surface temperature (Tf In zone III (gas-phase reaction zone), in which exothermic gas-phase reactions occur, the temperature increases rapidly from Tj to the flame temperature (Tg). 

The combustion wave of HMX is divided into three zones crystallized solid phase (zone 1), solid and/or liquid condensed phase (zone 11), and gas phase (zone 111). A schematic representation of the heat transfer process in the combustion wave is shown in Fig. 5.5. In zone 1, the temperature increases from the initial value Tq to the decomposition temperature T without reaction. In zone 11, the temperature increases from T to the burning surface temperature Tj (interface of the condensed phase and the gas phase). In zone 111, the temperature increases rapidly from to the luminous flame temperature (that of the flame sheet shown in Fig. 5.4). Since the condensed-phase reaction zone is very thin (-0.1 mm), is approximately equal to T . 

Note that in contrast to binary systems where only one-phase compound layers can occur at the interface between two elementary substances, in ternary systems, when a two-component alloy or a binary compound reacts with a third metal or non-metal (either solid or liquid), the formation of both compact one-phase layers and two-phase reaction zones is observed. These may have a different morphology, possible types of which were con-sidered, for example, in works by F.J.J. van Loo and J.E. Morrel et al. 

The coordinate system is selected so that the solid-gas interface, where conditions are identified by the subscript i, is maintained at x — 0. By mass conservation, m is independent of x everywhere, but it varies with t in this coordinate system. Boundary conditions for equation (56) are T = Tq, the initial temperature of the propellant, at x = — oo and T = 7 - at x = 0. The interface Arrhenius law, given by equation (7-6) but also interpretable in terms of a distributed solid-phase reaction in a thin zone (as indicated at the end of Section 7.4), is written here in the form... 

I) Heat conduction zone Though a thermal effect is given by heat conduction from the burning surface, no chemical changes occur. The temperature increases from the initial propellant temperature T0 to the onset temperature of the solid phase reaction Tu. 

The stand-off distance characterizes the thickness of the gas phase reaction zone, and it is defined in this work as the distance from the solid surface to the point where 85% of the final temperature is achieved. Fig. 3 shows the mole... 

---