Emission index

The emission index in general is defined as the mass of pollutant emitted per unit mass of fuel consumed. In quasi-steady diffusion flames, this is the ratio of the mass flux of pollutant out of the flame to the mass rate of consumption of fuel per unit flame area. Depending on the application, it may be more desirable to consider only the flux of pollutant to the air or the sum of the pollutant flux to both air and fuel. The latter definition is selected here, and a pollutant balance for the flame then enables the emission index to be expressed as the ratio of the mass rate of production of pollutant per unit area to the mass rate of consumption of fuel per unit area. In terms of the mass rate of production of species i per unit volume cDj, the mixture fraction, and the magnitude of its gradient VZ, the mass rate of production of species i per unit area is... 

Following existing convention, oxides of nitrogen (NOj,) are considered here to consist of NO and NO2 since N2O generally is treated separately, and in addition, N2O emissions are small compared with those of NO and NO2. The NO, emission index for methane-air flames then is defined here as... 

The contributions of the three production mechanisms and that of the sum of the reburn reactions to the NOj, emission index are plotted in Figs. 25.8 and 25.9, along with the net emission index, as functions of x° for ambient... 

Figure 25.8 The relative contributions of the thermal Et (1), prompt Ep (2), and nitrous oxide En (3) mechanisms as weU as reburn Er (4) to the emission index and the net emission index found from Eq. (25.2) (symbols 5 — Ej o ) as predicted by numerical computations using the starting mechanism for flames at p = 1 bar and oxidizer and fuel stream temperatures of Tf =To = 300 K...

Figure 25.11 Comparison between the predicted net emission index (including all mechanisms) using RRA and numerical calculations for atmospheres 1 — 1.0 bar, 300 K 2 2.0 bar, 300 K and 3 — 1.0 bar, 500 K. Curves with symbols represent numerical computations with the starting mechanism curves without symbols represent the RRA analysis...

Driscoll et al. [5] studied NO emission properties of turbulent partially pre-mrxed hydrogen-air and methane-air flames. The emission results for hydrogen-air flames showed that the emission index decreased monotonically with increasing levels of partial premixing because of the reduction in residence time caused by increasing jet velocity. The results for the methane-air flames were more complicated. 

Gore et al. [8] reported, for the first time, the existence of an optimum level of partial premixing for minimum NO emissions from turbulent jet flames. The optimum equivalence ratio ( 5) for a minimum emission index was found to be f.5, which is less than that found for the laminar flames discussed above. Lyle et al. [f5] confirmed the existence of an optimum level of partial premixing for both confined and unconfined turbulent flames. Lyle et al. [15] established that changes in thermal NO production do not control the emission behavior of partially premixed turbulent flames. More recently, Kemal et al. [10] have shown that a minimum in NO emissions can also be obtained for sudden dump-stabilized turbulent partially premixed flames. 

Blevins and Gore [14, 15] found that low-stretch-rate partially premixed flames involve multiple peaks in the profiles of intermediate hydrocarbon species. In particular, the CH species existing between the premixed and the diffusion flame part of the partially premixed flames were observed to react with NO and create an intermediate NO consumption zone. DuPont et al. [16] for low-stretch-rate flames also found the double peaks of intermediate hydrocarbon species and the NO consumption zone. However, Tanoff et al. [17] used the CH peak to characterize the location of the rich premixed flame and the OH peak to characterize the location of the diffusion flame. The NO concentration profiles showed that the peak NO mole fractions first increased and then decreased with increasing levels of partial premixing. However, the emission index of NO was not reported. 

Blevins and Gore [9] found that, for the low-stretch-rate flames, the NO emission index increased from the diffusion flame value up to = 2.5 in contrast to the experimental data. However, the NO emission index decreased at lower and reached a minimum at = 1.6. Based on this background, the... 

Table 1 List of experiments done in die aircraft assessment (The Emission index E.I.(NOx) is defined as grams of NOx emitted per kg of burnt fuel.)...

TABLE 4. Types of experiments used in the study. For supersonic aircraft, Z (km) indicates the cruising altitude with emission index (EI=g of NO, emitted/kg of burnt fuel). 

Figure 1. Measured aircraft ultrafine aerosol emissions are compared with equivalent model predictions. The aerosol emission index (El) is given as the total number of particles generated for each kilogram of fuel burned, at particle sizes exceeding d>5 nm or d> 14 nm (open and filled symbols, respectively). Data were collected in the SULFUR-5 field campaign. In the simulations (lines), different initial chemiion concentrations, nio, were assumed, as indicated in the legend at the left of the figure (the first number is the value of n in /cmJ, and the second is the lower particle size cutoff diameter, nm. From [84],...

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