Critical sooting equivalence ratios

Most of the early work and more recent efforts on soot formation under premixed conditions were conducted with air as the oxidizer. These early results reported data as a critical sooting equivalence ratio trend observed followed the order... 

TABLE 8.5 Critical Sooting Equivalence Ratios (c) Premixed with Air of Various Fuels... 

FIGURE 8.13 Critical sooting equivalence ratios ( />c) based on CO and H20 stoichiometry of various hydrocarbon fuels as a function of calculated adiabatic flame temperature Tf. 

FIGURE 8.14 Critical sooting equivalence ratio l c at 2200K as a function of the number C—C bonds in hydrocarbon fuels. +, 0, and - indicate ethane/l-octane mixtures in molar ratios of 5 to 1, 2 to 1 and 1 to 2, respectively x, acetylene/benzene at a molar ratio of 1 to 3. The O symbol for 2 to 1, falls on top of the butene symbol. 

For premixed fuel-air systems, results are reported in various terms that can be related to a critical equivalence ratio at which the onset of some yellow flame luminosity is observed. Premixed combustion studies have been performed primarily with Bunsen-type flames [52, 53], flat flames [54], and stirred reactors [55, 56], The earliest work [57, 58] on diffusion flames dealt mainly with axisymmetric coflow (coannular) systems in which the smoke height or the volumetric or mass flow rate of the fuel at this height was used as the correlating parameter. The smoke height is considered to be a measure of the fuel s particulate formation and growth rates but is controlled by the soot particle bumup. The specific references to this early work and that mentioned in subsequent paragraphs can be found in Ref. [50],... 

In Fig. 15, the sooting limits of chlorinated methanes/ methane/air mixtures are presented in terms of critical equivalence ratios and as a function of the chlorinated methane/methane molar ratio (7 ). As the chlorine content of the mixture was increased, sooting occurred at lower equivalence ratios. 

Fig. 15 Critical equivalence ratios (onset of soot formation) of premixed methane and chlorinated methane flames. (From Ref...

Low Pressure Flame Apparatus. All experiments were performed on flat, premixed low pressure, 2.7 kPa (20 Torr) C2H2/O2 flames stabilized on a water cooled burner. This 8.6 cm diam burner was constructed of approximately 900, 0.12 cm i.d. stainless steel tubes microbrazed into two stainless steel perforated plates to form a water jacket around the tubes. Gases were metered using calibrated critical flow orifices. The burner was installed in a low pressure vessel pumped by a 140 L s" (300 CFM) mechanical vacuum pump. Unburned gas velocities (298 K and 2.7 kPa) in all cases were 50 cm s . Equivalence ratios from = 1.5 to 4.0 were studied with most emphasis on a sooting ( ) = 3.0 flame. Visible soot emission became apparent at a soot threshold of <() = 2.4 to... 

The ion concentrations at the peaks are plotted as a function of equivalence ratio in Figure 3. Ignoring for the moment any differential between the first and second maxima, it is seen that peak ion concentration in a series of flames of increasing fuel richness beyond (j) = 1.5 decreases well beyond ( 0.5 equivalence units) the critical threshold for soot formation, (p, indicated by the shaded area. This phenomenon is more marked than that observed by Delfau et al. ( 9) in their C2H2/O2/N2 flames where the lowest maximum occurred about 0.1 equivalence unit greater than <1). This observation refutes any argument against the ionic soot formation mechanism based on an increase in peak ionization at the soot threshold equivalence ratio. 

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