In recent years, the great potential of microfluidic devices for the design and implementation of more efficient and cost effective processes in various scientific and engineering disciplines has lead to a rapid growth of the field. In many microfluidic systems, convective heat transfer and the complex interaction between a fully three-dimensional flow field and the superimposed temperature field play a major role. A better understanding of these phenomena is a key factor for future developments e.g. in the fields of biotechnology, transportation engineering, process engineering, electrochemical energy conversion, heat transfer solutions and many more. As the application of numerical simulations becomes difficult in the case of complex geometries, unknown boundary conditions and multi physics phenomena, highly resolved and accurate measurements of the velocity and temperature field are needed. In this work, novel optical
measurement techniques are introduced and validated which enable the simultaneous determination of the three-dimensional temperature field and the three components of the three-dimensional velocity field in microfluidic applications. Two tracer particle based approaches of determining the temperature distribution in transparent fluids are analyzed. On the one hand, the signal intensity of individual luminescent polymer seeding particles is correlated to temperature. On the other hand, the temperature sensitive luminescence lifetime of the particles is evaluated. The velocity field can be calculated simultaneously from the flowinduced shift of individual tracer particle images in time. To acquire the depth information, the wellestablished astigmatism particle tracking velocimetry technique is employed. With this method, systematic errors caused by volume illumination and the reduced spatial resolution due to ensemble averaging as typical in micro particle image velocimetry or laser-induced fluorescence can be avoided. The techniques offer an exceptionally high spatial resolution and accuracy over a wide range of velocity and temperature scales.
The lifetime based technique is applied to investigate two research topics of current interest from the field of electrochemical energy conversion. In the first study, a novel simple and effective heating system for microfluidic direct methanol fuel cells is characterized experimentally. A semi-conductive indium tin oxide heating layer of nanometer thickness is applied to the anode and cathode cover plates and subjected to a short boost of high electrical power. Within only 25 s, temperatures of up to 70 °C can be reached in the vicinity of the membrane, which is verified by lifetime based temperature measurements in the anode flow field. With this system the start up time of the cell necessary to reach approx. 95 % of its maximum output power can be reduced to less than 5 min, thus overcoming the well known problem of long start up times of this type of fuel cells. Furthermore, deeper insight into the role of the convective heat transfer in the anode and cathode flow field is given. The experimental results prove a significant cooling effect of the liquid anode flow, whereas the effect of the gaseous cathode flow is negligible. Finally, various possible future improvements are identified to increase the efficiency of the heating system.
In the second study, the origin of strong electrolyte flow during water electrolysis is investigated, that arises at the interface between electrolyte and hydrogen bubbles evolving at microelectrodes. This so called Marangoni convection was unveiled only recently and is supposed to be driven by shear stress at the gas-liquid interface caused by thermal and concentration gradients. The present work firstly allows a quantification of the thermocapillary effect and discusses further contributions to the Marangoni convection which may arise also from the electrocapillary effect. Hydrogen gas bubbles are electrolytically generated at a horizontal Pt microelectrode in a 1 M H2SO4 solution. Simultaneous measurements of the velocity and the temperature field of the electrolyte close to the bubble interface are performed. Additionally, corresponding numerical simulations of the temperature distribution in the cell and the electrolyte flow resulting from thermocapillary stress only are performed. The results confirm significant Ohmic heating near the microelectrode and a strong flow motion driven along the interface away from the microelectrode. The results further show an excellent qualitative match between simulation and experiment for both the velocity and the temperature field within the wedge-like electrolyte volume at the bubble foot close to the electrode, thus indicating the thermocapillary effect as the major driving mechanism of the convection. Further away from the microelectrode, but still below the bubble equator, however, quantitative differences between experiment and simulation appear in the velocity field, whereas the temperature gradient still matches well. Thus, additional effects must act on the interface, which are not yet included in the present simulation. The detailed discussion tends to rule out solutal effects, whereas electrocapillary effects are likely to play a role. Finally, the thermocapillary effect is found to exert a force on the bubble which is retarding its departure from the electrode. «
In recent years, the great potential of microfluidic devices for the design and implementation of more efficient and cost effective processes in various scientific and engineering disciplines has lead to a rapid growth of the field. In many microfluidic systems, convective heat transfer and the complex interaction between a fully three-dimensional flow field and the superimposed temperature field play a major role. A better understanding of these phenomena is a key factor for future developments... »
Fakultät für Luft- und Raumfahrttechnik
LRT 7 - Institut für Strömungsmechanik und Aerodynamik