Company
IGTE Institute for Building Energetics, Thermotechnology and Energy Storage. The IGTE researches and teaches for comfortable living and working conditions in buildings and cities in harmony with energy efficiency, sustainability and technology. Particular emphasis is placed on energy storage, renewable energies, hydrogen technology and indoor climate technology.
Task
If dipole molecules, such as water, are passed through an inhomogeneous electric field, the dielectrophoretic force acts on them and causes them to move in the direction of the vector gradient of the field strength. An inhomogeneous electric field may be generated with, for example, a cylindrical or spherical capacitor. In the E-Drop-Sep research project of the Institutes of Building Energetics, Thermotechnology and Energy Storage (IGTE), and the Institute of Energy Transmission and High Voltage Technology (IEH) at the University of Stuttgart, Germany, intend to develop a new type of energy-efficient droplet separator based on this operating principle.
In order to be able to make qualitative statements about the dependence of the drop trajectories on various parameters (e.g., drop size, field strength, etc.), the trajectories of monodisperse water drops (homogeneous size distribution: in the project between D = 50 μm and D = 100 μm) introduced into the E-field are to be recorded and analysed using optical measurement technology (see experimental setup Fig. 1).
The drop generator is assembled on the test stand just before the test section (see Fig. 2). The test section is 1 m long and has a high voltage (upper, up to 15 kV) and a counter electrode (lower, earth potential). Because a closed-form cylindrical capacitor does not allow optical access for recording the trajectories, a specially developed electrode geometry is used that, according to simulations, has the same electricfield in selected areas as a cylindrical capacitor.
Solution
Use of the RHEINTACHO A4-3505 stroboscope:
The RHEINTACHO stroboscope is used in conjunction with a camera to document the drop movement. In terms of single exposure image, the stroboscope is used like a flash. The low exposure time of the flash in an otherwise darkened room reduces the need for the lowest exposure time of the camera. The intensity of the flashes may be adjusted to the appropriate value on the stroboscope. In addition, multiple exposures are also possible if the exposure time of the camera is set so that a large number of flashes expose the image (and/or the camera sensor). This approach means that each drop may be seen several times throughout the image.
When recording videos, the stroboscope is used to seemingly “freeze” the frequency-dependent emerging drops or to slow down their movement. As a result, changes in the drop frequency become visible. Specifically, two experiments explained below are performed using the stroboscope.
Preliminary tests using the drop generator (experimental setup, Fig. 1):
The drops may be imaged on single-exposure shots by matching the stroboscope frequency to the exposure time. Due to the lack of synchronisation between triggering the stroboscope’s flashes and the camera, continuous images are taken until, by chance, a correctly exposed image is produced.
The images in Fig. 3 show the chain of droplets. They were created using the experimental setup in Fig. 1. The regularity as well as the inhomogeneities of the chain of droplets may be observed in the images. To ensure that the drop generator is properly functioning, the stroboscope frequency is adjusted to the exit frequency of the drops. An almost standing chain of droplets is created within this frequency range. Any changes to the excitation frequency on the drop generator may be detected under the light of the stroboscope by the changing perception of the drop movement. Thus, the ideal excitation frequency may be found and/or checked.
Drop deflection in the inhomogeneous electric field (experimental setup, Fig. 2):
The test stand has an optical access within the test section. The camera is positioned on one side, with the stroboscope diagonally opposite (angle of approx. 40°). If the drops are illuminated with the correct stroboscope frequency, the deflection of the drops in the E-field may be documented using the camera. Both videos and multiple exposure images are possible.
Single images are possible in principle, as described, but this is impractical due to the lack of synchronisation between the stroboscope and camera, because each image of the side view (see Fig. 4) must be composed of several individual exposures and also by moving the camera. For each individual image, therefore, continuous images would have to be taken until a correctly exposed image is produced.
As can be seen in Fig. 4 and Fig. 5, thanks to the images taken with the A4-3505 stroboscope from RHEINTACHO, statements may be made about the deflection of the drops within the electric field, regarding the stationarity of the movement (due to the multiple exposure) and regarding the distribution of the drops at a specific point within the image area. This has provided the research project with new insights into droplet deflection within the E-field. These may be used in the further course of the project in order to construct a prototype of the droplet separator.