Lean limit

Hydrocarbons and carbon monoxide emissions can be minimised by lean air/fuel mixtures (Fig. 2), but lean air/fuel mixtures maximize NO emissions. Very lean mixtures (>20 air/fuel) result in reduced CO and NO, but in increased HC emissions owing to unstable combustion. The turning point is known as the lean limit. Improvements in lean-bum engines extend the lean limit. Rich mixtures, which contain excess fuel and insufficient air, produce high HC and CO concentrations in the exhaust. Very rich mixtures are typically used for small air-cooled engines, needed because of the cooling effect of the gasoline as it vaporizes in the cylinder, where CO exhaust concentrations are 4 to 5% or more. 

Flammability Limits There are both upper (or rich) and lower (or lean) limits of flammability of fuel-air or fuel-oxygen mixtures. Outside these hmits, a self-sustaining flame cannot form. Flammability limits for common fuels are listea in Table 27-18. 

Figure 10-24 shows a schematic of an actual dry low emission NO combustor used by ALSTOM in their large turbines. With the flame temperature being much closer to the lean limit than in a conventional combustion system, some action has to be taken when the engine load is reduced to prevent flame out. If no action were taken flame-out would occur since the mixture strength would become too lean to burn. 

It can be seen in Figure 3.1.1 that the total surface area of the propane lean limit flame is much less than that of the methane one. This is because the laminar burning velocity for the limit mixture is much higher for propane than for methane. 

The flow structures of lean limit methane and propane flames are compared in Figures 3.1.2 and 3.1.3. The structure depends on the Lewis number for the deficient reactant. A stretched lean limit methane flame (Lepreferential diffusion, giving it a higher burning intensity. Hence, the flame extinction limit is extended. On the other hand, for a stretched lean limit propane flame (Le>l), the same effect reduces the burning intensity, which can... 

Flow velocity field determined by PIV. Lean limit flames propagating upward in a standard cylindrical tube in methane/air and propane/ air mixtures, (a) Methane/air—laboratory coordinates, (b) propane/air—laboratory coordinates, (c) methane/air—flame coordinates, and (d) propane/air—flame coordinates. 

On comparing the two flames, it is evident that the flow structure of the lean limit methane flame fundamentally differs from that of the limit propane one. In the flame coordinate system, the velocity field shows a stagnation zone in the central region of the methane flame bubble, just behind the flame front. In this region, the combustion products move upward with the flame and are not replaced by the new ones produced in the reaction zone. For methane, at the lean limit an accumulation of particle image velocimetry (PIV) seeding particles can be seen within the stagnation core, in... 

The structure of a bubble-shaped lean limit propane flame (Le > 1) is different. At the flame front inflow, the streamlines are much less divergent and soon converge again. 

It is also interesting to examine the global gas dynamic structure of upward propagating flames. Figure 3.1.6 gives an example of the global velocity field for the lean limit methane flame in the flame coordinates. The velocity distributions for all near limit flames studied share certain features. The central part of the bubble-shaped flame is... 

Streamlines of lean limit flames propagating upward in (a) methane/air and (b) propane/air mixtures (flame coordinates). 

PIV image of upward propagating lean limit methane flame,... 

PIV velocity measurements made it possible to evaluate the flame temperature field [23], following the method demonstrated in Ref. [25]. The calculated thermal structure of lean limit methane flame is shown in Figure 3.1.7. The differences between the structures of lean limit methane and propane flames are fundamental. The most striking phenomenon seen from Figure 3.1.7 is the low temperature in the stagnation zone (the calculated temperatures near the tube axis seem unrealistically low, probably due to very low gas velocities in the stagnation core). 

Thermal structure of lean limit methane flame. Isotherms calculated from the measured gas velocity distribution. 

The shape of the stretch rate distribution curve for the lean limit propane flame, shown in Figure 3.1.9b, is very similar. However, for this flame, the estimated contribution of the curvature is more important than with the methane flame, reaching about 40% of the maximum stretch rate. 

In contrast to the lean propane flame, the burning intensity of the lean limit methane flame increases for the leading point. Preferential diffusion supplies the tip of this flame with an additional amoxmt of the deficient methane. Combustion of leaner mixture leads to some extension of the flammability limits. This is accompanied by reduced laminar burning velocity, increased flame surface area (compare surface of limit methane... 

Stretch rate as a function of distance, from the top leading point along the flame front, (a) for a lean limit methane flame and (b) lean limit propane flame propagating upward in a standard cylindrical tube. 

In the case of flame propagation in the lean limit methane/air mixture, the local laminar burning velocity at... 

History of upward flame propagation and extinction in lean limit methane/air mixture. Square 5 cm x 5 cm vertical tube. Green color frames indicate PIV flow images. Red color represents direct photography of propagating flame. Extinction starts just after frame c. Framing rate... 

Evolution of flow velocity field in flame coordinates during extinction of upward propagating lean limit methane flame. Frames selected from Figure 3.1.12. 

Jarosinski ]., Strehlow R.A., and Azarbarzin A., The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube, Proc. Combust. Inst., 19 1549-1557,1982. 

Shoshin Y. and Jarosinski ]., On extinction mechanism of lean limit methane-air flame in a standard flammability tube, paper accepted for publication in the 32nd Proceedings of the Combustion Institute, 2009. 

Lean limit propane flames propagate under conditions when heat conduction dominates over the molecular... 

The characteristic properties of lean limit flames propagating upward are approximately the same as those of flames propagating down. 

For a long time there was no interest in flame speed measurements. Sufficient data and understanding were thought to be at hand. But as lean bum conditions became popular in spark ignition engines, the flame speed of lean limits became important. Thus, interest has been rekindled in measurement techniques. 

That additives affect the rich limit more than the lean limit can be explained by the important competing steps for possible chain branching. When the system is rich [reaction (3.23)],... 

Most early studies of flammability limits at reduced pressures indicated that the rich and lean limits converge as the pressure is reduced until a pressure is reached below which no flame will propagate. However, this behavior appears to be due to wall quenching by the tube in which the experiments were performed. As shown in Fig. 4.26, the limits are actually as wide at low pressure as at 1 atm, provided the tube is sufficiently wide and an ignition source can be found to... 

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