Flame stand-off distance

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 

The reaction rate for an m th order reaction (ignoring the temperature dependence of pg) is given by 

The mass flow continuity relationship between the gas phase and solid is % = -p ,/pg (3.66) 

The burning rate of an energehc material is represented by Vieille s law (Saint Robert s law) according to 

The temperature gradient, dT/dx, in the gas phase is approximately equal to ATg/Lg, where ATg is the temperature change across the gas-phase zone. Thus, the flame stand-ofT distance Lg is represented by 

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The quenching distance is also related to the flame stand-off distance depicted in Figure 4.14. This is the closest distance that a premixed flame can come to a surface. 

Global feature of the spray and spray flames were observed using planar Mie scattering. A CW Argon-ion laser sheet was used to illuminate vertical cross-sections of the spray. A 35-millimeter camera placed nearly normal to the illuminated spray cross-section was used to record the results using a narrow depth of held and short exposure times ( 1/125 s). This also provided the flame stand-off distance, which is defined as the distance between the fuel nozzle exit and mean upstream position of the flame. 

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... 

Table 6.1 Measured parameter values of the flame stand-off distance in the fizz zone.

Fig. 6.8 Dark zone length (flame stand-off distance) decreases with increasing pressure.

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. 6.28 Flame stand-off distance is increased by the addition of LiF in the super-rate burning region.

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. 6.30 Flame stand-off distance is decreased significantly by the addition of nickel powder even though the burning rate remains unchanged (see Fig. 6-29).

The flame stand-off distance, L4, defined in Eq. (3.70), decreases with increasing pressure, and the pressure exponent of the flame stand-off distance, d, ranges from -1.9 to -2.3 for RDX and HMX propellants. The overall order of the reaction in the dark zone is determined to be m = 2.5-2.8. This is approximately equal to the overall order of the reaction in the dark zone in the case of double-base propellants, m = 2.5, which would suggest close similarity of the reaction pathways in the dark zone for nitramine composite propellants and double-base propellants. 

Fig. 7.39 Burning rates and flame stand-off distances of non-catalyzed and catalyzed HMX-GAP composite propellants.

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. 

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