On the Formation of Nitrogen Oxides During the Combustion of ...

3 Experiments on Droplet Array Combustion
The respective intensities of photoemission measured in the presence of nitrogen
and in the presence of a third body species are indicated by I N2 and I M .
The relative quenching efficiency R M may be calculated from the relation
R M = 1+m d ˜X M . (3.9)
Equation (3.9) is a line equation where m is the slope of the first-order curve
fit of measured reference data (Tab. 3.5), d is the dilution constant (a function
of the analyzer used), and ˜X M is the third body concentration in mole
percent (volume percent) in the sample stream. Generally, the quenching effect
is a strong function of the particular analyzer type. The chemiluminescent
NO x analyzer used by Matthews et al. [273] and Tidona et al. [439] had
a 8mil (= 0.203 mm) sample capillary; the CLD units used here have a 7mil
(= 0.178 mm) sample capillary. Thus, the dilution constant presented in these
two publications was also adopted here with d = 0.4545. Table 3.5 lists values
for the slope m of Equation (3.9), as reported by Matthews et al. [273]
and Tidona et al. [439]. They are similar except for Ar and H 2 O. In order to
expound the quenching effect by means of an example, equilibrium concentrations
at equivalence ratios of 0.5, 1.0, and 2.0 are taken as a basis for obtaining
the ratio of actual and indicated NO concentration in Table 3.6. The
equilibrium species concentrations are calculated for atmospheric pressure
and n-decane/air flames. The bottom line results in Table 3.6 clearly indicate
that the chemiluminescence method is sensitive to third body quenching. The
difference between the correction factors at a constant equivalence ratio φ is
primarily due to the difference in the H 2 O correction. Finally, the m-values
provided by Tidona et al. [439] are adopted in the work at hand because they
showed higher consistency in the raw data relevant for the relative quenching
efficiency of water R H2 O.
The utilized CLD units feature a linearity of ±1% of the full scale reading
within each measurement range, which can for instance be set to 0 – 0.1, 0 –
1, or 0 – 10ppm. However, the CLD 700 LEV ht outperforms the CLD 700 EL ht
unit by one to two orders of magnitude regarding minimum detectable concentration
and noise at zero point. This presumes a nominal operation with
a constant analysis pressure of 40±10 mbar [111–114]. Nominal operation of
the NO/NO x analyzer is not feasible, though, as the gas sample is limited to
200ml and the source “FT-IR measurement cell” is subsequently evacuated
during CLD analysis (see Fig. 3.14).
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