Gas-phase reaction zone

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... 

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. 

Our data can be used to estimate the effective temperatures reached in each site through comparative rate thermometry, a technique developed for similar use in shock tube chemistry (32). Using the sonochemical kinetic data in combination with the activation parameters recently determined by high temperature gas phase laser pyrolysis (33), the effective temperature of each site can then be calculated (8),(34) the gas phase reaction zone effective temperature is 5200 650°K, and the liquid phase effective temperature is 1900°K. Using a simple thermal conduction model, the liquid reaction zone is estimated to be 200 nm thick and to have a lifetime of less than 2 usee, as shown in Figure 3. 

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). 

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. 

I First-stage gas-phase reaction zone (A/PA flame)... 

Condition at end of first-stage gas-phase reaction zone (A/PA flame) 2 Condition at end of second-stage gas-phase reaction zone (O/F flame)... 

The effective local temperatures in both sites were determined. By combining the relative sonochemical reaction rates for equation 5 with the known temperature behavior of these reactions, the conditions present during cavity collapse could then be calculated. The effective temperature of these hotspots was measured at 5200 K in the gas-phase reaction zone and 1900 K in the initially liquid zone (6). Of course, the comparative rate data represent only a composite temperature during the collapse, the temperature has a highly dynamic profile, as well as a spatial temperature gradient. This two-site model has been confirmed with other reactions (27,28) and alternative measurements of local temperatures by sonoluminescence are consistent (7), as discussed later. 

The combustion wave of GAP copolymer is divided into three zones zone I is a non-reactive heat conduction zone, zone II is a condensed phase reaction zone, and zone III is a gas phase reaction zone in which final combustion products are formed. Decomposition reaction occurs at Tu in zone II, and gasification reaction is complete at Ts in zone II. This reaction scheme is similar to that of HMX or TAGN shown in Fig. 5-5. 

Fig. 1 Problem schematic for burning homogeneous propellant showing condensed phase (surface) reaction zone, gas phase reaction zone and corresponding steady-state temperature profiles. Propellant is fed from left at surface regression rate r. Simplified kinetics description has propellant (A) decomposing in condensed phase to intermediate species (B) via zero-order, high activation energy, irreversible single-step reaction, and (B) reacting to (C) in gas phase via second-order (overall), irreversible single-step reaction.

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... 

Fig. 3. Mole fraction of gas species in gas phase reaction zone.

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