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

3 Experiments on Droplet Array Combustion
generator reaches the apex of its upward trajectory, a state of single droplet
levitation can be achieved by releasing the whole experimental setup into free
fall. Subsequently, Jackson and Avedisian [192] and Avedisian and Callahan
[28] employed the same piezoelectric droplet generator in combination with
single as well as crossed pairs of silicon carbide (SiC) fibers. As a droplet hits
the single fiber or the fiber intersection, it sticks to it. The droplet diameter
of interest is built up by impingement and coalescence of successive droplets.
Nevertheless, a technical limit is also reached for this latter droplet generation
technique, when investigating droplet arrays. Compact droplet generators
operating at a high success rate and with excellent reproducibility characteristics
become indispensable in this case. The most common technique,
after all, is droplet generation on a single quartz fiber by fuel supply through
a hypodermic needle. It allows a defined droplet positioning in space. Still,
the fiber can affect droplet vaporization and combustion in a negative way,
as it is typically only one order of magnitude smaller than the droplet diameter
itself. Additionally, the droplet shape is spheroidal, which is a result of
droplet elongation in the direction of the suspending fiber axis [129, 222, 278–
280, 282, 317, 376, 429, 431–433].
Here, a new droplet array is generated by the droplet array generation device
for every combustion run (Fig. 3.4). For this purpose, the droplet array
generation device needs to interoperate with the droplet array holder. The
utilized technique attempts to overcome as many disadvantages of the pervi-
E
D
C
F
B
G
A
Figure 3.4: Droplet Array Generation Device. A: ball screw, B: fine glass tube, C: fuel supply,
D: fixation of fuel supply block, E: shaft coupling, F: stepper motor SM1, G: linear
bearing [196, 208].
62