Wave Structure and Heat Transfer

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 . 

The heat flux transferred back from zone III to zone II, Am, is given by 

The temperature gradient in zone III, (dTjdx)in, increases as the pressure is increased, according to dT/dx)in po.7 However, Tj remains relatively constant (-700 K) in the pressure range between 0.1 MPa and 0.5 MPa. Using the physical parameter values of HMX, p, = 1700 kg mr, Ci = 1.30 kj kg and Xm = 8.4 x 10 kW m K On is determined as 300 kJ kg k Fig. 5.6 shows the heat flux produced in zone II and the heat flux transferred back from zone III to zone II as a function of pressure, n is approximately equal to Am, both of which increase with increasing pressure according to Ani and n 

The temperature gradient in zone III, (dTldx)in, increases as the pressure is increased, according to (dTldx)iu However, remains relatively constant 

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High-intensity ultrasound has the ability to alter a material s characteristics due to interactions between the sound wave and the food product at a structural level. Transmission of the ultrasonic wave takes place by the oscillation of medium particles, and the damping of ultrasound energy in deeper tissue layers forms the basis of the modification of tissue structures, leading to an impact on simultaneously occurring mass and heat transfer processes during drying. 

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). 

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... 

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. 

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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