front page - tuprints - Technische Universität Darmstadt

Flexible Construction
The intake tube (between the solenoid valve and the T-conjunction) as well as exit
tube (between the T-conjunction and the outer atmosphere) can be amended for
different lengths. Both of the tubes influence the pressure peak inside the chamber.
A longer intake tube leads to lower pressure peak inside the chamber due to
friction in the tube. The pressure drop is described by Darcy-Weisbach equation as
l ρv 2
∆p = f d
d 2 , (3.26)
where l is the length of the tube, d is the hydraulic diameter of the tube, ρ is density
of the compressed air, v is velocity of the air flow, f d is the Darcy friction factor.
This friction was economically beneficial. The minimum pressure required to force
the liquid out of the pinhole is given by Eq. 3.26 as ∆p = 2σcos(θ)/R jet . This
yields 396 Pa and 247 Pa for φ500 µm and φ800 µm pinhole respectively. Operation
at such low gauge pressures demand very expensive low pressure regulators.
Friction in the intake tube helps to increase the pressure level, making an ordinary
regulator applicable. The finally applied pressure were significantly higher than
the minimum value, as indicated in the caption of Figure 3.7 to Figure 3.11.
The exit tube raises the pressure peak as the length increased [43], likewise according
to Darcy-Weisbach equation. The exit tube provided another possibility of
pressure control by simply changing the tube length. This was helpful on precise
regulation at low pressures.
The intake and exit tubes helped to avoid condensation. Since the atmosphere
around the drop generator and the chilling tube was dry nitrogen gas, the compressed
air with humidity was not allowed to enter this region. The compressed
air was fed into the drop generator with the intake tube and exhausted outside the
enclosure with the exit tube. The two tubes isolated the compressed air from the
dry nitrogen atmosphere.
Adjusting the Pressure Level
The pressure is to be optimized by two conflicting factors. On the one hand, the
pressure peak must be sufficient, or else the liquid is retracted back to the chamber
without detachment, as shown in Figure 3.13.
On the other hand, the pressure should not be too high. An excessive pressure
peak pushes out a long liquid jet, which dissembles into multiple drops, as shown
64 3. Experimental Setup