Diffusional flame

When a composite propellant composed of ammonium perchlorate (AP) and a hydrocarbon polymer burns in a rocket motor, HCl, CO2, H2O, and N2 are the major combustion products and small amounts of radicals such as OH, H, and CH are also formed. These products are smokeless in nature and the formation of carbon particles is not seen. The exhaust plume emits weak visible light, but no afterburning occurs because AP composite propellants are stoichiometrically balanced mixtures and, in general, no diffusional flames are generated. 

The supersonic air induced into the air-intake is converted into a pressurized subsonic airflow through the shock wave in the air-intake. The fuel-rich gas produced in the gas generator pressurizes the combustion chamber and flows into the ramburner through a gas flow control system. The pressurized air and the fuel-rich gas produce a premixed and/or a diffusional flame in the ramburner. The combustion gas flows out through the convergent-divergent nozzle and is accelerated to supersonic flow. 

Both diffusional flame calculations and detailed spatial mapping indicate that the nondispersed injection mode produces a vapor cloud that is characterized by diffusionally controlled combustion and bulk heating while subjecting the droplets to near isothermal conditions. The soot produced in this cloud is strongly influenced by bulk diffusion limitations and as such represents a bulk soot formation extreme. It was found that fuel changes had little effect on the overall soot yield due to this diffusion control. Lower gas temperatures and richer conditions were found to favor soot formation under bulk sooting conditions, probably due to a decrease in the oxidation rate of the soot. 

Laminar flame instabilities are dominated by diffusional effects that can only be of importance in flows with a low turbulence intensity, where molecular transport is of the same order of magnitude as turbulent transport (28). Flame instabilities do not appear to be capable of generating turbulence. They result in the growth of certain disturbances, leading to orderly three-dimensional stmctures which, though complex, are steady (1,2,8,9). 

S. R. Lee and J. S. Kim, On the sublimit solution branches of the stripe patterns formed in counterflow diffusion flames by diffusional-thermal instability. Combust. Theory Model. 6(2) 263-278,2002. 

The inset of Figure 6.3.7 shows the flame response for rich hydrogen/air mixture of =7.Q. Since the Lewis number of this mixture is sufficiently greater than unity, it is susceptible to diffusional-thermal pulsating instability. Four flames, denoted by Flames 1-lV along the... 

Sivashinsky, G.L, Diffusional-thermal theory of cellular flames. Combust. Sci. Technol., 15,137,1977. 

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. 

Fig. 12.11 shows the structure of a rocket plume generated downstream of a rocket nozzle. The plume consists of a primary flame and a secondary flame.Fil The primary flame is generated by the exhaust combustion gas from the rocket motor without any effect of the ambient atmosphere. The primary flame is composed of oblique shock waves and expansion waves as a result of interaction with the ambient pressure. The structure is dependent on the expansion ratio of the nozzle, as described in Appendix C. Therefore, no diffusional mixing with ambient air occurs in the primary flame. The secondary flame is generated by mixing of the exhaust gas from the nozzle with the ambient air. The dimensions of the secondary flame are dependent not only on the combustion gas expelled from the exhaust nozzle, but also on the expansion ratio of the nozzle. A nitropolymer propellant composed of nc(0-466), ng(0-369), dep(0104), ec(0 029), and pbst(0.032) is used as a reference propellant to determine the effect of plume suppression. The burning rate characteristics of the propellants are shown in Fig. 6-31. Since the nitropolymer propellant is fuel-rich, the exhaust gas forms a combustible gaseous mixture with the ambient air. This gaseous mixture is ignited and afterburning occurs somewhat downstream of the nozzle exit. The major combustion products in the combustion chamber are CO, Hj, CO2, N2, and HjO. The fuel components are CO and H2, the mole fractions of which at the nozzle throat are co(0.47) and iH2(0.24).

Compared with the AP decomposition flame thickness, the fuel-oxidant redox flame extends a much greater distance from the propellant surface and depends on the rate of both chemical reaction and diffusional mixing. 

The relative roles played by diffusional mixing and chemical reaction in determining the reaction rate of the O/F flame are difficult to analyze. Consequently, as was done in the original formulation (92), expressions... 

As in Ref. 92 for a second-order gas-phase reaction where the prevailing pressure is sufficiently low for the O/F flame (Zone II of Figure 1) to be chemical-reaction rate controlled (high diffusional mixing rate), the chemical mass conversion rate in a zone of length LIIt ck at temperature Tg may be expressed as ... 

The Effect of Finite Reaction Rates When the fuel and oxidizer react at a finite rate, the flame front can no longer be considered infinitely thin. The reaction rate is then such that oxidizer and fuel can diffuse through each other and the reaction zone is spread over some distance. However, one must realize that although the reaction rates are considered finite, the characteristic time for the reaction is also considered to be much shorter than the characteristic time for the diffusional processes, particularly the diffusion of heat from the reaction zone to the droplet surface. 

An LCVD system is somewhat similar to a gas flame in which the combustion rate and the gas flow rate establish a steady-state flame. In an LCVD system, the monomer flow rate and the polymer formation rate establish a steady-state polymer-forming luminous gas phase. This situation is expressed schematically in Figure 20.17, where (a) indicates the diffusional transport of the energy-carrying... 

Fig. 32. Computed fluxes of hydrogen atoms in flame of Fig. 25. (a) Convective flux, Afiun (see eqn. (64)) (b) ordinary diffusions flux, j (c) thermal diffusional flux,yjf (d) overall flux, MGyf.

Important results are that the dark zone temperature (Tf) decreases even though the flame temperature (Tg) is increased by the increase of Sj(N02) at constant pressure as shown in Fig. 7-38. Furthermore, (f> decreases also as Sj(N02) increases, and thus the burning rate decreases as Sj(N02) increases, i.e., the burning rate of HMX-CMDB propellants decreases as Ij(HMX) increases at a constant pressure. The observed burning rate characteristics of HMX-CMDB propellants are understood without consideration of the diffusional process and the chemical reaction between the decomposed gases of the base-matrix and the HMX particles. This is a significant difference from the burning rate characteristics of AP-CMDB propellants. 

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