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

3 Experiments on Droplet Array Combustion
form the basis of the calibration method. As the lower limit of the time integral
can easily be linked with the obvious increase of the measurement signal
(Fig. 3.16), a reasonable value has to be identified for its upper limit only.
Therefore, different filters define contrasting upper limits. Figure 3.17 displays
resulting calibration curves based on the recorded data basis and linear curve
fits for four exemplary filters (i.e. time integrals). Data are consistent and deviations
from the ideal trend are marginal for all four examples. Nevertheless,
the 10% filter shows the best performance and was selected for the present
study. It stops integration as soon as the measurement signal drops below
10% of its maximum value. The related raw data have a standard deviation
σ of the range of 0.032 to 0.090 , typical error (σ/N 1 2 ) of the range of 0.011 to
0.027 , and coefficient of variation (CV = 1/SNR = σ/µ) of the range of 0.91 to
3.64% [409]. The line equation of the linear curve fit is: y = 0.3242 x (Fig. 3.17,
10% filter). The respective coefficient of determination is R 2 = 0.9995.
Furthermore, the converter efficiency according to Equation (3.5) was determined
by gas phase titration and totaled 94.5 % during the final measurement
campaign [115]. It is typically in the range of 83 to 97% but depends on the
absolute NO 2 concentration, the age and use history of the converter, and the
gas matrix used for its determination [136].
Calibration of FT-IR Spectrometer
The calibration of a FT-IR spectrometer requires a high temperature and pressure
stability of the calibration gas supplied, as does the final analysis process.
Misinterpretation would be a consequence of varying density in the optical
measurement cell. Ideally, both the reference and final analysis spectra are
recorded with identical hardware setups as well as identical operational parameters.
In order to quantify the gaseous compounds in the exhaust gas, two existing
multi-component gas analysis methods were refined and employed with the
NEXUS ® 470 spectrometer of Thermo Electron [334, 423]. They include corrections
for interferences and are outlined in Tables 3.7 and 3.8. For Standard
No. 1 (Tab. 3.7), the FT-IR measurement cell was controlled at a constant temperature
of T = 458 K and an absolute pressure of p = 866 mbar. Potassium
bromide (KBr) windows were used for optical access to the cell due to their
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