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Networks of microdevices can be used to form, sort, split and recombine droplets by varying the fluidic resistance of the channels. To facilitate design of such systems we propose two models to calculate pressure drop of liquid-liquid slug flow in square microchannels. By approximating a droplet surrounded by liquid film as annular flow, the moving film model includes the velocity profile in the lubricating layer. In the no-film model, the presence of liquid film is neglected. Pressure drop measurements of water-toluene and water-silicone oil slug flow generated in glass/silicon devices of 200 and 400 μ m width serve to validate these two models. We found that the droplet viscosity influences the applicability of the models. Good agreement is obtained for water-toluene slug flow, with the average error of 15–20% between the measured and calculated values for both models, proving that the presence of the lubricating film may be neglected for flows with similar viscosities of both phases. On the contrary, both predictions agree poorly with experiments for silicon oil-water flow which we attribute to the lower than expected liquid film thickness around the silicon oil droplets. Therefore we use the moving film model to estimate the thickness of the lubricating layer which we find to lie between 0.5% and 0.75% of the channel width, lower than the assumed 2%. Finally we compare the performance of our models against two correlations adapted from literature. We thus demonstrate that our approach allows reliable pressure drop prediction for liquids with similar viscosities and delivers important information on flow hydrodynamics.

Electrochemical micro-probes are used to study the flow regime and liquid–solid mass transfer inside a designed porous structure. In order to place the electrodes inside the porous structure avoiding gluing and polishing, the electrodes are glued flush with pins. These pins are then simply plugged into the structure. In a first step, the influence of the pin on the local flow behavior is studied. It is shown, that the departure from the laminar flow regime can be determined within 10% accuracy in empty pipes. Additionally, the influence of the pin length on the point of transition is studied inside the porous structure showing errors below 10% for pin lengths with an accuracy of ±0.5 mm. In a second step, the flow regime is determined with a simple algorithm, allowing a reproducible detection of the laminar plateau and the onset of fluctuations. The departure from the laminar regime is found at Re lam , end = 162 ± 3 . In the third and last step, the liquid–solid mass transfer is studied for a single-phase liquid flow and gas–liquid two-phase flow. It is shown, that the second phase is enhancing the mass transfer at low volumetric transport fractions but degrading the mass transfer at volumetric transport fractions higher than 6%. The Sherwood number is compared to the literature and correlations are presented to describe the Sherwood number in the laminar and turbulent flow regime.

Highly porous structures (porosity ≥75%), as metal foams or designed foam-like structures, are often used in industry to augment heat and mass transfer. In the present study, we focus on the mixing in highly porous structures in continuous millireactor characterized with a relatively low Reynolds number (ReD=240 – based on the ligament diameter and bulk velocity) and a high Schmidt number fluid (Sc=2400). We apply a combined numerical and experimental approach. The numerical simulations employ a Large Eddy Simulation (LES) method with dynamic Lagrangian approach for subgrid-scale turbulent stress and turbulent mass flux. The results of the numerical simulation are compared with our own combined Particle Imaging Velocimetry (PIV) and Laser Induced Fluorescence (LIF) measurements. The application of combined PIV/LIF makes it possible to simultaneously measure velocity components, turbulent stresses, concentration and concentration fluxes. The mechanism of the mixing is analyzed in detail with specific focus on validity of a simple gradient diffusion hypothesis (SGDH) in modeling of the turbulent mass transfer in complex porous structure.

This study compares a two-phase flow through a foam-like porous structure and a Sulzer SMX TM static mixing element. Refractive index matching between two immiscible fluids and the internal structures enables optical measurements to be performed. The droplet size and position within the internal structures were observed by laser-induced fluorescence. The results show that at low flowrates, droplets follow preferred paths, whereas at higher flowrates, they are more homogeneously distributed within the structures. The droplet size distribution was found to be well represented by the Sauter mean diameter. Measuring along the axis of the two static mixers, we found that droplets disintegrate more quickly in the foam-like porous structure. As both geometries have the same porosity and hydraulic diameter, we conclude that the change in the free cross section is also an important parameter. We observed that in the geometry with large changes in the free cross section, the droplets are smaller.

Highly porous structures (porosity ≥ 75 % ) used as inserts in continuous chemical reactors with characteristic length scale in the millimeter range exhibit at industrial relevant throughputs the transport phenomenon vortex shedding. The dependency of the ligament shape and thickness of the porous media on vortex shedding is investigated in two geometries: a highly porous structure consisting of round ligaments and a highly porous structure whose ligaments are thicker and have a square cross section. The contribution of vortex shedding to the total turbulent kinetic energy is determined from proper orthogonal decompositions (POD) of fluctuating velocity fields measured with particle image velocimetry (PIV). The spatial structure of the vortex shedding phenomenon is addressed by POD eigenfunctions and by the wake length extracted by the position of zero mean streamwise velocity along the wake axis. Shedding frequencies are evaluated for a large Reynolds number range ( 6 ≤ Re D ≤ 1500 ) by a dye injection technique. The vortex shedding phenomenon is more intense in case of the highly porous structure with square and thicker ligaments. This means that larger vortices are created and detached, Strouhal numbers are larger and the contribution of vortex shedding to the total turbulent kinetic energy is more substantial in this case compared with the structure with round and thinner ligaments.

Highly porous media (with a porosity ≥85%), like metal foams, fins or designed porous inserts are widely used in industrial flow applications. Motivated by the use of highly porous inserts in continuous chemical reactors, the time dependent behavior of flows through a highly porous structure is investigated experimentally and numerically for Reynolds numbers ReD=33 and 330 (based on the ligament diameter and the bulk velocity). The experimental studies are based on the Particle Image Velocimetry (PIV), whereas the numerical studies are conducted by using the Large Eddy Simulation (LES) approach with the dynamic Lagrangian subgrid closure. The vortex shedding phenomena is observed for both values of Reynolds number. It is demonstrated that the length of the wake and the orientation of the shedded vortices are dependent on the position within the highly porous structure.

Lignin represents a promising renewable source of chemicals. Valuable aromatic monomers, such as vanillin and methyl vanillate, can be obtained through its acidic oxidation and subsequently have to be separated and purified for further usage. Extraction directly from the aqueous reaction mixture using supercritical carbon dioxide as solvent was evaluated as a possible separation step. Fast screening of different extraction conditions up to pressures of 121 bar at temperatures from 39.8 to 59.3 °C was enabled using a continuous microfluidic device. Distribution ratios of the main five monomeric products were calculated from concentration measurements. With increasing pressure and decreasing temperature higher quantities of monomers were extracted, thereby decreasing selectivity. Overall, selectivity towards specific monomers was high, especially at conditions close to the critical point of supercritical carbon dioxide, confirming the potential of this green and cheap purification method.

We present the characterization of heat transfer in commercial metal foam filled tubular reactors in comparison to a designed laser sintered device. The investigations are performed at empty tube Reynolds numbers ranging from 600 to 7600. Volumetric heat transfer performances up to 4.5 MW/(m 3 K) were estimated by means of a simple calorimetric measurement setup, which is 3 orders of magnitude higher compared to conventional batch reactors. The heat transfer was found to increase with the ligament diameter ascribed to the enhanced turbulent kinetic energy induced. The fixed wall connection of the fully sintered device, realized by the applied manufacturing method, leads to 30% improvement of the heat transfer compared to no connection. Selective laser sintering was found to be an efficient tool for the design of continuous heat exchanger reactors covering a wide range of applications by simply adapting the geometry.

We present a study on the axial dispersion in metal foams and laser sintered reactors. Commercially available metal foams of 20 and 30 ppi are compared to a designed streamwise-periodic structure in terms of axial dispersion coefficients and pressure drops. Therefore tracer pulse experiments were performed and post processed by means of a deconvolution method. The Peclet number Pep based on the pore size is ranging from 5 × 10 4 to 8 × 10 5 which is attributed to the increased velocities due to the high porosity of the material compared to fixed bed reactors. The attained dispersion coefficients ranging from 1.3 × 10 − 4 to 6.7 × 10 − 3 m 2 / s demonstrate the trend of packed beds and common packing materials and increase monotone with the Peclet number Pep. The pressure drop versus the interstitial bulk velocity follows the Forchheimer equation and can be described by the conventional Ergun model for all investigated porous media. The parameters obtained correspond to values found in literature. The results of this study show the high potential of foam reactors for catalyst driven reactions. They provide the same or even a higher surface area per volume of catalyst bed while inducing a much smaller pressure drop than corresponding fixed beds.

We present a dynamic large eddy simulation of flow in a periodic open-cell structure in the turbulent regime. The flow domain is represented by a tube with inner diameter of 7 mm with integrated highly porous media. Simulations were performed for three different Reynolds numbers ranging from 1200 to 4500 based on the empty tube diameter. The obtained results were compared to experimental data for which measurements were performed downstream of the same periodic geometry manufactured from a transparent material by stereolithography. The measured pressure drop and turbulence statistics compare well to the results of the large eddy simulations. The numerical results give an insight into the forced convection mechanism within the porous media and enhance the understanding of flow through foamlike structures. Furthermore specific geometric details like the influence of the ligament shape of the porous material can be investigated applying this technique.

We present an investigation on species mass transfer in a milli-scale plug flow reactor using metal foams to enhance the mixing process. In the current design the reactor consists of a pipe with an inner diameter of 7 mm and metal foam inserts with different pore sizes of 20, 30 and 45 ppi. Simultaneous PIV and LIF measurements were performed in orthogonal planes normal to the radial and axial direction downstream of a foam element of 50 mm length. The investigated Reynolds numbers range from 600 to 7600 defined with the empty tube diameter and the bulk velocity. We discuss the influence of the pore sizes on the scalar mixing efficiency and compare the results to the reference empty tube case. A strongly intermittent flow caused by the metal foam was observed over the whole range of Reynolds numbers. The increased radial velocities lead to an enhanced mixing performance. Coefficients of Variation in the order of 0.1 were achieved.

The influence of the channel radius on the mass transfer in rectangular meandering microchannels (width 200 – 400 μ m and height of 150 μ m ) has been investigated for gas–liquid flow. Laser induced velocimetry measurements have been compared with theoretical results. The symmetrical velocity profile, known from the straight channel, was found to change to an asymmetrical one for the meandering channel configuration. The changes in the secondary velocity profile lead to an enhanced radial mass transfer inside the liquid slug, resulting in a reduced mixing length. In the investigated experimental range (superficial gas velocity 0.08 m / s and superficial liquid velocity 0.01 – 0.07 m / s ) the mixing time was reduced eightfold solely due to changes in channel geometry. An experimental study on the liquid slug lengths, the pressure drop and their relation to the mass transfer have also been performed. Experimental results were validated by a simulation done in Comsol Multiphysics ® . To obtain information for higher velocity rates, simulations were performed up to 0.64 m / s . These velocity variations in the simulation indicate the occurrence of a different flow pattern for high velocities, leading to further mass transfer intensification.