Combustion Waves of Energetic Materials

Combustion Waves of Energetic Materials 67 The heat generated in the gas phase is then given by... 

Fig. 5.5 Heat transfer model of the combustion wave of an energetic material.

When an energetic material burns in a combustion chamber fitted with an exhaust nozzle for the combustion gas, oscillatory combustion occurs. The observed frequency of this oscillation varies widely from low frequencies below 10 Hz to high frequencies above 10 kHz. The frequency is dependent not only on the physical and chemical properties of the energetic material, but also on its size and shape. There have been numerous theoretical and experimental studies on the combustion instability of rocket motors. Experimental methods for measuring the nature of combustion instability have been developed and verified. However, the nature of combustion instability has not yet been fully understood because of the complex interactions between the combustion wave of propellant burning and the mode of acoustic waves. 

Equation (3.89) is the expression of temperature sensitivity of energetic materials based on the analysis of a one-dimensional one-step reaction in the combustion wave. 

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). 

Fig.3.15 Temperature profiles in combustion waves at different initial temperatures of an energetic material.

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... 

In Chapter 1.3 we defined a detonation as the propagation of a chemical reaction through an energetic material under the influence of a shock-wave at speeds faster than the speed of sound in the material. The velocity at which the energetic material decomposes is therefore only dependent on the velocity of the shock-wave. It is not determined by a heat-transfer process as it is the case for deflagration or combustion. 

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