Flame luminous front

The release of chemical energy during combustion of gases produces a luminous, radiating zone which is seen as the flame or flame front. ... 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

If the reactive gas produced at the burning surface of an energehc material reacts slowly in the gas phase and generates a luminous flame, the distance Lg between the burning surface and the luminous flame front is termed the flame stand-off distance. In the gas phase shown in Fig. 3.9, the temperature gradient appears to be small and the temperature increases relatively slowly. In this case, heat flux by conduction, the first term in Eq. (3.41), is neglected. Similarly, the rate of mass diffusion, the first term in Eq. (3.42), is assumed to be small compared with the rate of mass convection, the second term in Eq. (3.42). Thus, one gets... 

Fig. 5.11 shows a flame photograph ofTAGN burning at 0.2 MPa. The luminous flame ofTAGN stands some distance from the burning surface, but the luminous flame front approaches the burning surface when the pressure is increased, similarly to the luminous flame of HMX described in Section 5.1.3. As for double-base propellants and nitramines, the flame stand-off distance is represented by... 

Fig. 5.11 Flame structure of TAGN showing a luminous flame standing above the burning surface the flame front approaches the burning surface as the pressure is increased (not shown).

Since the final gas-phase reaction to produce a luminous flame zone is initiated by the reaction in the dark zone, the reaction time is determined by the dark zone length, L4, i. e., the flame stand-off distance. Fig. 6.8 and 6.9 show data for the dark zone length and the dark zone temperature, T, respectively, for the propellants listed in Table 6.3. The luminous flame front approaches the burning surface and... 

Fig. 6.29 shows the effect, or lack thereof, of the addihon of Ni particles on the burning rate of a double-base propellant. The double-base propellant is composed of Nc(0-44), ng(0.43), i3gp(0.11), and Iec(0-02) as a reference propellant. This propellant is catalyzed with 1.0% Ni particles (2 pm in diameter). No burning rate change is seen upon the addition of Ni particles.F However, the flame structure is altered significantly by the addition of Ni. The flame stand-off distance between the burning surface and the luminous flame front is shortened, as shown in Fig. 6.30. Though the flame stand-off distance of the reference propellant is about 8 mm at 1.5 MPa and decreases rapidly with increasing pressure (1 mm at 4.0 MPa), the flame stand-off distance of the Ni-catalyzed propellant remains unchanged (0.3 mm) when the pressure is increased.

Fig. 7.34 Flame photographs of an AP composite propellant (a) and an RDX composite propellant (b) showing that the luminous flame front of the RDX composite propellant is distended from the burning surface ...

The combustion wave structure of RDX composite propellants is homogeneous and the temperature in the solid phase and in the gas phase increases relatively smoothly as compared with AP composite propellants. The temperature increases rapidly on and just above the burning surface (in the dark zone near the burning surface) and so the temperature gradient at the burning surface is high. The temperature in the dark zone increases slowly. However, the temperature increases rapidly once more at the luminous flame front. The combustion wave structure of RDX and HMX composite propellants composed of nitramines and hydrocarbon polymers is thus very similar to that of double-base propellants composed of nitrate esters.[1 1... 

Fig. 7.45 shows a set of flame photographs of HMX-GAP propellants with and without catalysts. The luminous flame front of the non-catalyzed propellant is almost attached the burning surface at 0.5 MPa (a). When the propellant is catalyzed, the luminous flame is distended from the burning surface at the same pressure (b). Since the heat flux transferred back from the gas phase and the heat of reaction at... 

The combustion wave structure of HMX propellants catalyzed with LiF and C is similar to that of catalyzed nitropolymer propellants the luminous flame stands some distance above the burning surface at low pressures and approaches the burning surface with increasing pressure. The flame stand-off distance from the burning surface to the luminous flame front is increased at constant pressure when the propellant is catalyzed. The flame stand-off distance decreases with increasing pressure for both non-catalyzed and catalyzed propellants. 

Fig. 8.22 The luminous flame front of the platonized propellant approaches the burning surface more rapidly than that of the non-catalyzed propellant when the pressure is increased in the plateau-burning pressure region.

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