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Concentration profiles are measured during the continuous mixing of two gas streams in a wind tunnel with a rectangular cross-section. The volume flow rates of both components differ considerably. The kinetic energy of the gas streams is used for the mixing process. The second gas stream with a different concentration is injected by nozzles. The number of nozzles and the direction of injection are varied. The concentration fields are measured with optical tomography using holographic interferometry as the measuring technique. Projection data our of four different angles of view are used for tomographic reconstruction by the ART method. Pressure losses due to mixing phenomena are measured as well. Results are presented which indicate that the quality of mixing can be improved for higher ratios of momentum flow rate and by injecting jets in crossflow, due to a higher exchange of momentum. The pressure differences due to the mixing phenomena increase with increasing ratio of momentum flow rate. For jets in crossflow a higher quality of mixing can be achieved for higher pressure losses compared to jets in coflow.

We investigate the influence of internal electrodes onto the flow field, governed by electroosmosis and electrophoresis in a modular rectangular microchannel. As internal electrodes can be positioned at lower distances, they can be operated at lower voltages and still ensure strong electrical field strength. Even at lower voltages, electrode reactions influence the species concentration fields, and the crucial question arises, whether at the electrodes all species can be kept in dissolution or whether some species are released in gaseous form. The position and charge of multiple internal electrodes is a further focus of our investigations: wall-tangential electrical field components are responsible for pumping, wall-normal electrical field components are responsible for mixing. Hence, an optimized position and charge of all electrodes will lead to an optimized electrical field, designed to fulfill the desired tasks of the modular microchannel. The mathematical model for the numerical treatment relies on a first-principle description of the EDL and the electrical forces caused by the electrical field between the internal electrodes. Hence, the so-called Debye-Huckel approximation is avoided. The governing system of equations consists of a Poisson equation for the electrical potential, the continuity and Navier-Stokes equations for the flow field, species transport equations, based on the Nernst-Planck equation, and a charge transport equation. Further, a model for the electrode reactions, based on the Butler-Volmer equation, is in place. The simulations are time-dependent and two-dimensional in nature and employ a FVM.Copyright © 2009 by ASME

We investigate the influence of flow field and electrode reactions onto an electrical double layer (EDL), which is located in the immediate vicinity of the walls of a rectangular microchannel. The precise knowledge of the EDL appears to be important for many technical applications in microchannels of small width, since the electrokinetic effects, as electroosmosis or electrophoresis, in such cases depend on the detailed charge distribution. The mathematical model for the numerical treatment relies on a first–principle description of the EDL and the electrical forces caused by the electrical field between internal electrodes. Hence, the so–called Debye–Huckel approximation is avoided. The governing system of equations consists of a Poisson equation for the electrical potential, the Navier–Stokes equations for the flow field, species transport equations, based on the Nernst–Planck equation, and a model for the electrode reactions, based on the Butler–Volmer equation. The simulations are time–dependent and two–dimensional (plane) in nature and employ a finite–volume method. It is discussed, e.g., how the thickness of the EDL expands at the stagnation point of a forced flow, as the velocity (or Reynolds number) is increased. Furthermore, the effect of electrode reactions on the ionic strength and, hence, on the EDL and the electroosmotic flow, are discussed.Copyright © 2008 by ASME

Polarforschung

Polarforschung

We studied multiply charged ion (MCI) induced ionization, excitation and fragmentation of the nucleobases uracil and thymine. Ions of different charge state at velocities between 0.2 and 0.4 atomic units were used as projectiles. By means of time-of-flight spectrometry of the collision products in a statistically representative mode as well as in triple coincidence mode, three different interaction types could be identified. Large impact parameters lead to "gentle'' ionization of the molecules, whereas in close collisions mainly multifragmentation occurs. This gives rise to a bimodal mass-spectrum as found in fullerene fragmentation studies. An additional two-body breakup channel leading to intermediate fragment sizes was found for both nucleobases. From the coincidence data, the kinetic energy release of this channel could be obtained.

Polarforschung

Polarforschung

Polarforschung

Polarforschung

Polarforschung