Marko Hoffmann

Experimental Analysis and Modeling of Micromixing in Microreactors

For the investigation of chemical reactions in micro- and minichannels the threedimensional velocity field and three dimensional concentration field of an inert and reactive tracer was measured in a T-shaped micromixer. For this purpose a measurement method for micro particle image velocimetry and confocal laserscanning microscopy has been developed. By solving the continuity equation the calculation of three dimensional streamlines, residence time distribution and local energy dissipation is possible. It becomes out that a regime transition to engulfment flow occurs at a certain Reynolds number dependent on reactor dimensions that can be predicted by the modified model of Soleymani. This transition leads to a different efficiency of micromixing measurable by the reactionproduct of a fast chemical reaction by confocal laserscanning microscopy. The influence of the flow regime on the yield and selectivity has been investigated by means of a parallel consecutive reaction system.

Experimental Investigation and Modelling of Local Mass Transfer Rates in Pure and Contaminated Taylor Flows

In many industrial applications of chemical and bio-chemical engineering, new insights into mass transfer processes across fluidic interfaces are of high interest. Mass transfer processes across gas-liquid interfaces have been investigated for decades to understand the coupling of hydrodynamics and mass transport processes and to describe and correlate them for various gas-liquid flow apparatus and process parameters. The investigation of the linked transport processes and the understanding of their interaction is fundamental for the optimization of multiphase reactors and for the validation of numerical simulations, which are pointing at problems of higher complexity during the last years. One challenge for the investigation of gas-liquid flows is the highly stochastic behaviour of gas bubbles rising in liquids under turbulent flow conditions. For the investigation of local mass transfer processes at fluidic interfaces and the validation of numerical simulations, more well-defined and reproducible conditions are necessary. A suitable setup to study mass transfer at fluidic interfaces under well-defined and reproducible conditions is the gas-liquid flow through a small, straight capillary, called “Taylor bubble” for single bubbles and “Taylor flow” for bubbles in a chain. Taylor flows and Taylor bubbles have ideal properties for detailed investigation on the influence of hydrodynamics and mass transfer at clean and contaminated interfaces, where the shape oscillations are suppressed and the Taylor bubbles are self-centering within vertical channels. Therefore, in this work the local hydrodynamics and mass transfer processes in Taylor flows and at Taylor bubbles have been investigated with laser measurement techniques, to obtain a deeper insight into mass transfer processes at fluidic interfaces. Furthermore, experimental data for the guiding measure “Taylor flow” has been provided. The guiding measure has been established within the SPP 1506 to generate a reliable data basis for the validation of numerical simulations.

Characterization of Micro Fluidic Devices by Optical Measurements

Micro fluidic devices are successfully in use for several applications in chemical engineering and biotechnology. Nevertheless, there is still no breakthrough for micro process engineering because of a lack in understanding the mechanisms for local hydrodynamics and mass transfer on micro scales. Micro Particle Image Velocimetry (μ-PIV) combined with Confocal Laser Scanning Microscopy (CLSM) enables the measurement of three-dimensional flow and concentration fields in micro devices for common stationary cases. By quantitative analysis of pressure drops, mixing qualities and residence time distributions an adjustment of micro reactor devices for the demands of chemical and biochemical reactions becomes possible.Copyright

Experimental Characterization of Micro Mixers Using Microscale-Laser Induced Fluorescence and Particle Image Velocimetry

Microreactors are basic components of microfluidic systems for chemical and biochemical applications and the large area-to-volume ratio of micro-reactors enables a higher yield and selectivity than conventionally designed processes. To take advantage of the full potential of this ambitious technology, a fundamental understanding of the transport processes on the relevant time and length scales is necessary. Besides the approach of using commercial CFD programs for numerical flow visualization, the microscale fluid flow visualization is an important tool for acquiring localized flow information within these microreactors. To get a deeper insight the mixing characteristic of different T-shaped micro mixers with rectangular cross sections (dimensions: 100–400 micron) has been investigated by means of the non-invasive measurement techniques micro-Laser induced fluorescence (micro-LIF) and micro-Particle Image Velocimetry (micro-PIV). The analysis of the concentration fields proves that with a higher Re a stretching and thinning of liquid lamellae (vortex generation) occurs, causing an enlarged interfacial surface area and consequently leading to a better mixing performance by diffusion. The analysis of the velocity fields shows further the existence of a three dimensional flow in the entrance region of the mixing channel of a T-shaped micro mixer.© 2006 ASME