Unstable flame

Emissions Control. From the combustion chemistry standpoint, lean mixtures produce the least amount of emissions. Hence, one pollution prevention alternative would be to use lean premixed flames. However, lean mixtures are difficult to ignite and form unstable flames. Furthermore, thek combustion rates are very low and can seldom be appHed dkectly without additional measures being taken. Consequently the use of lean mixtures is not practical. 

Y. Huang and V. Yang. Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor Transition from stable to unstable flame. Combust. Flame, 136(3) 383-389, 2004. 

Table 12.1 Calculated characteristic reaction time tc, residence time tr, and the Mikhelson number Mi for the combustion of the stoichiometric methane-air mixture in a combustor with the open-edge V-gutter flame holder of height H and apex angle 60° at the mean inlet velocity Uin. Also presented is the maximum approach-stream velocity Um- Signs and correspond to stabilized flame and unstable flame, respectively... 

Where the element of interest is present in the sample as a soluble complex then it is often sufficient to dilute the sample with a suitable organic solvent. The sample may be diluted to the desired concentration level and any viscosity effects reduced. Not all solvents are suitable for flame AAS. Some, such as n-hexane and benzene, give smokey, unstable flames and should be avoided. Solvents such as white spirit, MIBK, n-heptane and xylene are acceptable. A dilution ratio should be chosen to bring the metal concentration into the region 0.2—0.8 absorbance units (or approximately fifty times the manufacturers quoted sensitivity). 

The first objective of the following analysis will be to compute the burning velocity for a simplified model of a plane, one-dimensional, non-adiabatic flame. Under certain conditions involving nonvanishing heat losses, it will be found that no solution for the burning velocity exists, thus indicating the presence of flammability limits. Under other conditions, two burning velocities will be obtained, one of which presumably corresponds to an unstable flame. 

Due to high viscosity, heavy oils are often heated by an electrical heater or steam before they are pumped to delivery lines. The resulting temperature is required to be high enough so that the oil can be easily atomized. Otherwise, a spray of undesirably large oil droplets will be formed. It leads to an unstable flame and carbon buildup on the atomizer and burner block. The American Society for Testing and Materials (ASTM) reported the relationship between the viscosity and temperature for different fuel oils, as shown in Figure 11.3.18... 

Lewis number based on the limiting reactant is sufficiently large [3]. Therefore, the Darrieus-Landau instability is expected to result in enhanced flame-surface wrinkling only when it is able to overcome stabilizing influences of buoyancy and thermal diffusion. Additionally, a flame under confinement may be influenced by acoustic waves, and coupling between the flame and acoustic-wave dynamics may result in unstable flame propagation as well. 

The test furnace (Figure 23.10) has been used not only for the single nozzle burner described above, but also for a six-nozzle burner (refer to Figure 23.9). Measurements with the six-nozzle burner showed very similar characteristics as above. Flere the fuel in flame mode was stabilized upstream of the nozzle plane and six small flames were injected into the combustion chamber. In flameless mode the fuel gas is injected axially and the combustion air is injected in axial direction via the six nozzles. By adjustment of defined parameters, this burner could also be operated with lifted unstable flames. Some results of the operating range are schematized in Figure 23.5 ... 

A striking difference with simple chemistry results is that complex chemistry flames are more sensitive to strain than simple chemistry flames. Their thickness changes strongly with strain and the local consumption speed Sc correlates rather well with strain, a result which is opposite to the one obtained with simple chemistry computations. However, like simple chemistry flames, the space-averaged flame speed 5 remains of the order of the laminar flame speed si, except for unstable flames. 

Flue-Gas Recirculation (FGR). In this technique, a portion of the flue gas is withdrawn from the boiler stack and introduced with the combustion air. The oxygen avaUabiUty is reduced, thus reducing the PFT and NO j. Most applications recirculate roughly 20 percent of the flue gas to achieve less than 40 ppmvd NO, . However, excessive FGR can produce unsafe, unstable flames and result in an increase in CO emissions. 

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