Franz Peters

Gas bubbles in simulation and experiment

An experimental setup for the examination of single bubbles, rising in a liquid, is presented. Its main part is a rotating chamber, in which the bubble is spatially stabilized by a balance of buoyancy, drag, and lift forces. This allows for long observation periods in time. Experimental results are presented for air bubbles in silicone oil. The experimental results are validated by a comparison with numerical simulations. A modified, mass-conserving level-set method is used for the representation of the free interface, while an immersed-boundary formulation is engaged for the conservation equations. The agreement between experiment and simulation, and to available correlations from literature, is found to be perfect. It is shown that the influence of the liquid shear due to the rotation is negligible. Also, for the presented liquid system, no influence by Marangoni stresses could be found, which makes the system of air and silicone oil a good choice for validation purposes.

The liquid temperature in diffusion controlled vapor condensation: analysis and experimental verification

This work deals with the temperature of a liquid film that forms on a thin wire by condensation of vapor present in ambient gas at a small fraction. Based on heat and mass transfer theory in the continuum regime, it is shown that the liquid temperature is steady when the ambient state is steady while a boundary layer in-between may develop in time. An expression for the calculation of the liquid temperature is derived being valid for the geometry of the wire and two other 1-D geometries, namely a plane wall and a sphere. Experiments are carried out with a cylindrical thin wire, the electrical resistance of which was measured in order to obtain the liquid temperature. Results are provided for condensing alcohol and water films showing good agreement with the calculated temperature.

Retaining talent, addressing diverse requirements: Academic writing for engineering students

In the light of the anticipated and much-discussed shortage of skilled labor and university graduates in the STEM fields, attracting students for technological studies is an important strategic aim of German universities and engineering faculties. Currently, the so-called “doppelter Abiturjahrgang” (a result of reducing school years from 13 to 12) plays into their hands by releasing twice the amount of high school graduates into the study and work market in Germany. Still, dropout rates among students of engineering sciences are higher than in other subject fields and, consequently, universities and employers are losing potential engineering graduates despite the fact that the total number of enrollments increased. Among the factors considered to be responsible for university dropout are lacks of social and academic integration. Furthermore, formats such as mass lectures that are often part of undergraduate studies in engineering prevent teaching approaches that deal with individual requirements of students. However, a more individual approach is what the increasing diversity among university students would call for. Against the backdrop of an increasingly heterogeneous student body, academic writing presents one of the biggest challenges in engineering studies. In order to support students during their first semesters but also in writing their final theses, the project ELLI is currently implementing a multi-level concept to foster academic writing in engineering studies. At the Ruhr-University Bochum, quantitative and qualitative research was done among students and faculty members of the three engineering faculties, based on which a guidebook for writing final theses was developed. Moreover, a seminar for academic writing for undergraduate engineering students was designed, which will take place for the first time in the winter term of 2014.

Computer aided approximation of flow rate through systemic-pulmonary arterial shunts (SPAS)

Abstract The discrimination of flow rates through bronchial arteries that are affected by pathological SPAS today still happens solely qualitatively. A reproducible quantification of flow rates, however, would enable the comprehension of phenomena like the intensified shunt perfusion seen in cases of chronic inflammations or the characterization of SPAS that may cause cardiovascular problems. A computational program is developed, that allows the modeling of individual bronchial arteries on the basis of the information provided by angiography. Angiographic images are available from the standard clinical assessment of SPAS. The flow through continuous and geometrically measurable vessel segments and SPAS is given by the law of Hagen-Poiseuille. The discharge through healthy branches is calculated by means of allometric scaling laws. The simulation results are verified by flow experiments in artificial vessel networks made of glass and PE tubing. The experimental set-up mimics realistic, pulsating pressure and flow conditions. When applied to the artificial vessel networks, the model described herein provides results for the volumetric flow rate that differ from values measured in laboratory experiments by <6%. The computer model is also applied to real angiographic images. Due to inaccuracies during the deduction of the geometry and due to necessary simplifications of the model, we expect significant deviations between calculated and real flow rates in bronchial systems. Nevertheless, the presented method enables the physician to objectively estimate the order of magnitude of volumetric flow through individual SPAS fairly independently from his experience and without the need of measurements additional to the mandatory angiography.

Diffusional Mass Transfer at the Surface of Small Gas Bubbles

A method apt to observe small levitated bubbles in the sub-mm range in liquid is described. The method combines a rotary chamber with a Mie scattering method. The key result of the measurements with this method is the rate at which gas diffuses from the bubble into the liquid. A remarkable result is found for five different gases in water. It turns out that the mass flux density is independent of the radius. This suggests that convective mass transport is not the controlling mechanism.Copyright

Homogeneous nucleation rates from the piston-expansion tube using a digital camera

Homogeneous nucleation rates of n-pentanol in nitrogen are obtained from a piston-expansion tube (pex-tube) involving the nucleation pulse method which generates a limited number of nuclei that grow into droplets. The detection of the droplets is achieved by a new counting method developed on the basis of a CCD camera in combination with a laser light sheet. Nucleation rates between 104 and 109 cm−3 s−1 are covered for the nucleation temperature 260 K. The rates are plotted as isotherms versus supersaturation. An influence of the initial expansion temperature on the nucleation rate is identified. Literature data from other expansion experiments agree with our finding.