Combustion Wave Structure and Heat Transfer

The AN particles incorporated into GAP-AN pyrolants form a molten layer on the burning surface and decompose to form oxidizer fragments. The fuel-rich gas produced by the decomposition of GAP interdiffuses with these oxidizer fragments on and above the burning surface and produces a premixed flame. A luminous flameis formed above the burning surface. 

When AP particles are added to GAP-AN pyrolants, a number of luminous flame-lets are formed above the burning surface. These flamelets are produced as a result of diffusional mixing between the oxidizer-rich gaseous decomposition products of the AP particles and the fuel-rich gaseous decomposition products of the GAP-AN pyrolants. Thus, the temperature profile in the gas phase increases irregularly due to the formation of non-homogeneous diffusional flamelets. 

When A1 particles are added to GAP-AN pyrolants, agglomerated A1 fragments are formed on the burning surface. However, when A1 particles are mixed with pyrolants composed of GAP, AN, and AP, numerous flame streams are formed in the gas phase. The A1 particles are oxidized by the gaseous decomposition products evolved by the AP particles. The combustion efficiency of the A1 particles is improved significantly by the addition of the AP particles. 

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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 heat transfer in the combustion wave structure of an energetic material is illustrated in Fig. 3.10. The heat flux feedback from zone III to zone II by conductive heat transfer, = kg (dTIdx), is given by Eq. (3.46), and the heat flux feedback from zone II to zone I by conduction heat transfer, dT/dx), is given by... 

No doubt, an extensive investigation of the combustion wave structure under different conditions would permit to verify many conceptions of the current flame spread theories, and also to determine the applicability limits of the latter. Even now, since more experimental investigations of the rate of flame spread over polymer material surfaces as a function of various factors are bdng carried out, it is becoming increasingly clear that the mechanism of heat transfer from the flame to the combustible surface can change radically as the size of the combustion zone increases. 

For this study mixts of CeH6 O and H O were detonated in a tube either by a shock wave or by a spark. The arrival of the pressure step was detd by a thin-film, heat-transfer probe with a rise time of 0.5 microsecs. The spectrograph viewed the passing deton wave thru a window slit and lens arrangement. Recording was accomplished by photomultiplier tubes. The deton waves observed consisted of a shock front followed by a combustion front and were classed as "strong , which is equiv to "unsteady or "decelerating detonation. Detailed structure of the detonations could not be resolved... 

Because it is difficult to account for changes in the properties of the reaction medium (e.g., permeability, thermal conductivity, specific heat) due to structural transformations in the combustion wave, the models typically assume that these parameters are constant (Aldushin etai, 1976b Aldushin, 1988). In addition, the gas flow is generally described by Darcy s law. Convective heat transfer due to gas flow is accounted for by an effective thermal conductivity coefficient for the medium, that is, quasihomogeneous approximation. Finally, the reaction conditions typically associated with the SHS process (7 2(XX) K and p<10 MPa) allow the use of ideal gas law as the equation of state. 

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