Michael Engler

Fluid Dynamics and Transfer Processes in Bended Microchannels

Abstract The understanding of the flow processes in microchannels and micromixers is essential for the design of microfluidic devices like microreactors or analytical equipment. We have performed a systematic numerical CFD-study of mixing and mass transfer in sharp 90° bends and heat transfer in T-joints to obtain a detailed insight into the flow patterns and corresponding transfer processes in a wide range of Reynolds numbers. With increasing flow velocity, the straight laminar flow starts to form symmetrical vortices in the bend, at the entrance of the mixing channel, and in T-joints. The vortices enhance the transport processes like heat and mass transfer in the channels significantly. The influence of the geometry and the flow conditions is shown by an analytical estimation of the relevant forces. The appearance of convective transport processes is used for the definition of microflows, which are controlled by viscous forces and diffusive transfer processes.

Convective Mixing and Its Application to Micro Reactors

This work shows the application of convective fluid flow caused by flow-induced secondary vortices to fluidic single-phase micro mixers. As an example we used simple static T-shaped micro mixers. The convective flow was observed both by simulations and by experiments and is suitable for enhancing the mixing quality. Concerning micro reactors, it is necessary that the mixing is faster than the chemical reaction to be induced so that the creation of unwanted side products is minimized. The mixing model by Bourne is slightly modified for continuous flow reactors and applied to our mixers. Using this model, timescales for the mixing in our micro mixers are calculated. A first test reaction — the iodide-iodate reaction by Villermaux and Dushman — to check the validity of the timescales is outlined. These overall results will help to achieve a deeper understanding of micro reactors.Copyright

Liquid Mixing in Static Micro Mixers With Various Cross Sections

Micro mixers are an integral part of several micro fluidic devices like micro reactors or analytical equipment. Due to the small dimensions, laminar flow is expected a priori in those devices while the mass transfer is supposed to be dominated by diffusion. A detailed numerical CFD-study by CFDRC-ACE+ of simple static mixers shows a significant deviation from strictly laminar flow in a wide range of Reynolds numbers Re, channel dimensions, and types of cross sections (square, rectangular, trapezoidal). With increasing flow velocity and Re number the flow starts to form vortexes at the entrance of the mixing channel. The vortexes are symmetrical to the symmetry planes of the mixing channel, both for the rectangular and the trapezoidal cross sections investigated here. With further increasing velocity the flow tends to instabilities, which causes a breakup of the flow symmetry. These instabilities are generally found in T-shape mixers with symmetrical flow conditions, but not always in Y-shape mixers or with asymmetrical flow conditions. Within the laminar flow regime diffusive mass transfer is dominant. In this case the mixing quality at constant channel length becomes worse with increasing velocity. This effect can almost be equalized by the onset of the vortex regime, which enhances the mass transfer by convective transport. This paper shows the mixing quality at a certain length for different geometrical parameters and flow conditions.Copyright

Convective mixing and chemical reactions in microchannels with high flow rates

This study presents a theoretical and experimental investigation on convective micro mixing in different mixing structures and their combinations. Different mixing elements have been integrated on a silicon chip to achieve a device for a high mass flow above 20 kg/h. These test structures were fabricated and characterized concerning their flow behavior and mixing performance. Flow measurements with pH neutralisation and indication by bromothymol blue confirm the numerical simulations of the flow characteristics and mixing behaviour. The integral mixing quality in the micro mixer is measured with the iodide-iodate-reaction (Villermaux-Dushman) and shows excellent values for high Re numbers. This opens the potential to use microstructures for new applications in the production of chemicals.

Theoretical and Experimental Investigations of Convective Micromixers and Microreactors for Chemical Reactions

Convective micromixers allow fast mixing with characteristic times below 1 ms by creating vortices in bends or junctions of microchannels. Their robustness and comparatively high throughput make them suitable for process intensification purposes. In this contribution, T-shaped micromixers with rectangular cross section and meandering structures as basic elements are numerically investigated for Reynolds numbers from 0.01 to 1000 in the mixing channel. Static and transient mixing regimes in T-mixers are described up to the transition to turbulence, where a worsening of mixing is found. Different experimental results confirm the simulations and lead to the convective lamination model. This model links the energy dissipation to mixing similar to a modeling of turbulent mixing. Additional to mixing of two fluids, also wall contact of fluids in channel flow is investigated with a catalyzed chemiluminescence reaction. With this method, dead zones can be made visible.

Fluid Dynamics and Transfer Processes in Bended Micro Channels

The understanding of the flow processes in microchannels and micro mixers is essential for the design of micro fluidic devices like micro reactors or analytical equipment. We have performed a systematic numerical CFD-study of mixing and mass transfer in sharp 90° bends and heat transfer in T-joints to obtain a detailed insight into the flow patterns and corresponding transfer processes in a wide range of Reynolds numbers. With increasing flow velocity the straight laminar flow starts to form symmetrical vortices in the bend, at the entrance of the mixing channel, and in T-joints. The vortices enhance the transport processes like heat and mass transfer in the channels significantly. The influence of the geometry and the flow conditions is shown by an analytical estimation of the relevant forces. The appearance of convective transport processes is used for the definition of microflows which are controlled by viscous forces and diffusive transfer processes.Copyright

Aerosol Particle Deposition in a T-Shaped Micro Mixer

Micro technology supported aerosol processes provide a basis for integrated control of complex processes and therefore a promising research subject. A key aspect in aerosol technology is to control particle deposition, either to avoid clogging or to achieve a well defined coating of surfaces. As a first step we conducted an experimental and theoretical study of the particle deposition in a simple static T-shaped micro mixer. For the experiments monodisperse sodium chloride particles in the particle size range between 10 nm and 700 nm were used. The aerosol was introduced into one branch of the micro reactor and mixed with a particle-free air stream. The predominant particle deposition effect within the mixer is due to impaction, which is induced by the high curvature of stream lines at the inlet and in the mixing zone. Additional CFD calculations confirm the experimental results and show ways of optimizing the inlet geometry of the mixer, which should result in a significant reduction in impaction losses.© 2005 ASME

Silicon Microstructures for High Throughput Mixing Devices

Convective mixing in microstructures gives good mixing results in a very short time. In this work a theoretical and experimental study was performed on convective micro mixing in different mixing structures and their combinations. Various mixing elements had been integrated on a silicon chip to achieve a device for a high mass flow above 15 kg/h. These test structures were fabricated and tested concerning their flow behavior and mixing characteristics. Flow measurements with pH neutralisation and indication by Bromothymol Blue confirm the numerical simulations of the flow characteristics and mixing behaviour. The integral mixing quality in the micro mixer is measured with the iodide-iodate-reaction (Villermaux-Dushman) and shows excellent values for high Re numbers. This opens the potential of microstructures for new applications in the production of chemicals.Copyright

Simulation and Characterization of Microreactors for Aerosol Generation

This paper shows the application of T-shaped micromixers for the generation of nanoscale aerosols by the mixing of a hot gas-vapor-mixture with a cold gas. The fast mixing within a T-shaped micromixer leads to a quasi-instantaneous and high saturation of the vapor and therefore to homogeneous nucleation. After nucleation, the particles grow to their final size until the vapor is saturated. Different mixer geometries, mixing ratios, and gas temperatures have been investigated by numerical simulation to yield optimum mixing results over a wide range of operational parameters. The main selection parameters are the mixing time, the mixing quality, and the flow regimes in the mixer. Six different microreactor geometries were designed and fabricated in silicon and covered with a Pyrex glass lid for optical observation. Special attention was paid to thermal insulation and particle deposition at the channel walls. This concerns not only the entire mixing chip, but also the design of the fluidic mount with only few bends and corners. First experimental results for particle deposition (prefabricated NaCl nanoparticles in a nitrogen carrier stream,) and aerosol generation (Vitamin E droplets in nitrogen) are presented. High temperature gradients up to 1 Mio. K/s lead to a rapid condensation and forming of nanosized particles with a mean diameter of 20 to 50 nm and a narrow size distribution.Copyright