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

3 Experiments on Droplet Array Combustion
Measurement of Pressure
The pressure inside the DCU vacuum protection is controlled in the range of
0 to 2bar at 5Hz. The responsible pressure control unit is activated just after
lift-off of the sounding rocket module. The most critical event is a pressure
drop in the whole DCU during exhaust gas sampling and air displacement
in the combustion chamber, caused by a leak of the combustion chamber
shutter (cf. Chap. 3.1.3). Therefore, pressure control is disabled during this
crucial event to avoid an excess consumption of air. Apart from that, compensation
is regularly started when the pressure is out of its nominal range
of (1.000±0.025)×10 5 Pa. Besides this direct control, three further sensors are
employed for monitoring purposes in the pressurized air supply system.
A dynamic pressure sensor (manufacturer: PCB Piezotronics; type: M112A05)
is installed in the combustion chamber in order to capture pressure fluctuations
for scientific purposes at a sampling rate of 500 Hz. It has a sensitivity
of 0.160 pC kPa −1 and its signal is amplified by an in-line charge converter
and an ICP ® amplifier. Two different models of the in-line charge converter
(both also manufactured by PCB Piezotronics) were used: the 422E36 and the
422E12 model in the drop tower and the sounding rocket setup, respectively.
Both of them have a charge conversion sensitivity of 10mV pC −1 [330–333].
The exhaust gas sampling (EGS) system including its four sample cylinders is
repeatedly evacuated during experiment preparation and countdown to keep
contamination low and re-evacuation times short. The vacuum level is monitored
directly at the central EGS backbone (cf. Fig. 3.9, D) by a compact Full-
Range gauge (manufacturer: Pfeiffer Vacuum; type: PKR 251). It combines a
cold cathode and Pirani gauge to cover a vacuum/pressure range from 5×10 −9
to 1000 mbar. Thus, the associated leakage rates of the whole EGS system or
single sample cylinders became calculable from the given volumes and the
pressure readings at fixed time intervals. The pressure readings themselves
were included in the data logging of the drop tower campaign but not in the
one of the sounding rocket campaign. In the case of the latter, monitoring
by an external program was performed until −90 min before launch, using a
separate control unit (manufacturer: Pfeiffer Vacuum; type: TPG 261) and access
through the late access panel of the sounding rocket module. Afterwards,
the remaining vacuum level of the sample cylinders was extrapolated for the
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