J. Brandner
Karlsruhe Institute of Technology (KIT), IMVT, Faculty Member
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ABSTRACT A new micro Molecular Tagging Velocimetry (µMTV) setup has been developed to analyze velocity fields in confined internal gas flows. MTV is a little-intrusive velocimetry technique. It relies on the properties of molecular... more
ABSTRACT A new micro Molecular Tagging Velocimetry (µMTV) setup has been developed to analyze velocity fields in confined internal gas flows. MTV is a little-intrusive velocimetry technique. It relies on the properties of molecular tracers which can experience relatively long lifetime luminescence once excited by a laser beam with an appropriate wavelength. The technique has been validated for acetone seeded flows of argon inside a 1 mm depth rectangular minichannel, with a multilayer design offering two optical accesses. Velocity profiles have been obtained using a specific data reduction process, with a resolution in the order of 15 micrometers. The experimental data are compared to theoretical velocity profiles of compressible pressure-driven flows. A good agreement is observed, except close to the walls, where the accuracy would still need to be improved. Following these first results obtained at atmospheric pressure, the influence of pressure on the luminescence intensity of acetone molecules is analyzed. The obtained data lead to a discussion of MTV applicability to rarefied flows and its possible use for a direct measurement of velocity slip at the channel walls.
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Fast temp. cycling of catalytic reactions using microreactors is discussed. [on SciFinder (R)]
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The forced variation of reaction parameters is a known method to improve the performance of catalytic reactors leading to process intensification. The most often experimentally varied parameters so far were the reactant concentrations or... more
The forced variation of reaction parameters is a known method to improve the performance of catalytic reactors leading to process intensification. The most often experimentally varied parameters so far were the reactant concentrations or pressure. Due to the high thermal inertia of conventional reactors it was almost impossible to achieve fast periodic reproducible temperature changes. However, it has been proven theoretically that fast periodic temperature variations may increase the reaction rate compared to the stationary temperature conditions.The possibility to thermally cycle microstructured stainless steel reactors in a periodic way with temperature differences of up to 60 K and a frequency as high as 0.06 Hz has been demonstrated. This gives the opportunity to study the influence of fast temperature changes on heterogeneously catalyzed gas phase reactions. The catalytic CO oxidation over Pt supported on Al2O3 was chosen as a test reaction. The concentrations of CO, O2 and CO...
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Research Interests: Mechanical Engineering, Chemical Engineering, Materials Science, Chemistry, Microstructure, and 13 moreKinetics, Porosity, Platinum, Stainless Steel, Oscillations, Microstructures, Chemical Reaction, Chemical Engineering Science, Reaction Rate, Water Flow, Experimental Measurement, Amplitude, and thermal behaviour
ABSTRACT Pulsation and maldistribution effects (hot spot formation) with characteristic frequencies below 100 Hz occurring in electrically powered microchannel array heat exchangers used as evaporators for water were investigated... more
ABSTRACT Pulsation and maldistribution effects (hot spot formation) with characteristic frequencies below 100 Hz occurring in electrically powered microchannel array heat exchangers used as evaporators for water were investigated primarily with the aid of visualisation techniques. Pulsation at subaudio frequencies was found to be dominated by the consequences of boiling in the inlet plenum, creating large vapour bubbles that intermittently entered the microchannel array, pushing liquid water at velocities too high to achieve complete evaporation. A new design minimising the residence time in the inlet and comprising an intermediate void was found to produce a two phase mixture that could be evaporated in an array of 68 microchannels, each 200 μm wide, 100 μm deep, and 20mm long, at a mass flux of 60 kgm−2 s−1 at an average surface temperature of 220°C. The redesign led to a change in characteristic flow patterns in the microchannel arrays from plug and slug flow to film flow and drops moving along the walls of the microchannels, as evident from high speed (103 fps) video sequences. By means of infrared thermography of the surface of a metallic device comprising four layers of microchannel arrays, maldistribution between the microchannel array layers leading to hot spot formation was observed when the device was operated in constant power mode. The formation of these hot spots could be avoided by the use of simple temperature control electronics operating at characteristic frequencies below 10−1 Hz.
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In this paper the feasibility of obtaining optical access for micro devices made by stereolithography is investigated, to meet demands set by microfluidic research efforts. A commercially available resin SL5530 was selected for the UV... more
In this paper the feasibility of obtaining optical access for micro devices made by stereolithography is investigated, to meet demands set by microfluidic research efforts. A commercially available resin SL5530 was selected for the UV polymerisation process. By optimising the design of the CAD models and the fabrication process, the suitability of the final product was improved for optical metrology. Transparent T-shaped microchannels were manufactured for future application in gas mixing study on interferometric system. Fringes patterns were first obtained from interferometric experiments and then the signals were extracted from the fringes pattern to analyse its suitability.
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In this paper investigations on the design of a gas flow microchannel heat exchanger are described in terms of hydrodynamic and thermal aspects. The optimal choice for thermal conductivity of the solid material is discussed by analysis of... more
In this paper investigations on the design of a gas flow microchannel heat exchanger are described in terms of hydrodynamic and thermal aspects. The optimal choice for thermal conductivity of the solid material is discussed by analysis of its influences on the thermal performance of a micro heat exchanger. Two numerical models are built by means of a commercial CFD
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ABSTRACT In this work, a double-layered microchannel heat exchanger is designed for investigation on gas-to-gas heat transfer. The micro-device contains 133 parallel microchannels machined into a polished polyether ether ketone plate for... more
ABSTRACT In this work, a double-layered microchannel heat exchanger is designed for investigation on gas-to-gas heat transfer. The micro-device contains 133 parallel microchannels machined into a polished polyether ether ketone plate for both the hot side and cold side. The microchannels are 200 μm high, 200 μm wide, and 39.8 mm long. The design of the micro-device allows tests with partition foils in different materials and of flexible thickness. A test rig is developed with the integration of customized pressure and temperature sensors for in situ measurements. Experimental tests on the counter-flow micro heat exchanger have been carried out for five different partition foils and various mass flow rates. The experimental results, in terms of pressure drop, heat transfer coefficients, and heat exchanger effectiveness are discussed and compared with the predictions of the classic theory for conventionally sized heat exchangers.
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ABSTRACT A new setup was developed for gas mixing analysis in T-shaped microchannels. The principle of the flow rate measurement was based on the Constant Volume (CV) method [1]. The mass flow rate measurements of two gases N2 / CO2... more
ABSTRACT A new setup was developed for gas mixing analysis in T-shaped microchannels. The principle of the flow rate measurement was based on the Constant Volume (CV) method [1]. The mass flow rate measurements of two gases N2 / CO2 mixing in a T mixer were carried out in the slip flow regime and followed by a simulation work for comparison. The mass flow rate has a magnitude of 10−8 or 10−7 kg/s and has good agreement with simulation for the lowest inlet over outlet pressures ratios and moderate agreement for the highest inlet over outlet pressures ratios.
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This paper presents the results of experimental and numerical investigation of forced convection of gas flows through stainless steel microtubes having inner diameters of 750 μm, 510 μm and 170 μm. The study covers both transitional and... more
This paper presents the results of experimental and numerical investigation of forced convection of gas flows through stainless steel microtubes having inner diameters of 750 μm, 510 μm and 170 μm. The study covers both transitional and turbulent flow regimes (3000
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The promising performance of microchannels has given rise to intensive research on pressure drop and heat transfer characteristics of flows at the small- validate new ones, experiments need to be conducted, which are particularly... more
The promising performance of microchannels has given rise to intensive research on pressure drop and heat transfer characteristics of flows at the small- validate new ones, experiments need to be conducted, which are particularly difficult given the characteristic dimensions involved and the magnitude of the fluxes to be measured. Although more care has been devoted lately to the design of experiments in terms of control of geometry and boundary conditions, the uncertainties which inevitably affect each measurement do not seem to have been given the proper consideration. Correctly calculating uncertainties not only allow to a correct assessment of the experimental data obtained, but can also be used to decide which measurements need to have the highest precision to achieve a certain accuracy, thus saving money on the others. In this paper, a quantitative criterion is given to assess the accuracy achievable in the determination of the friction factor in the laminar regime for the flo...
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Visualization by high-speed videography and infrared surface thermography was used to compare the spatial and temporal maldistribution of flow, manifesting itself in pulsation and hot spot formation, respectively, in water evaporators... more
Visualization by high-speed videography and infrared surface thermography was used to compare the spatial and temporal maldistribution of flow, manifesting itself in pulsation and hot spot formation, respectively, in water evaporators consisting of either a single metallic foil with an array of mechanically micromachined microchannels or of several such foils assembled into an electrically powered micro heat exchanger. In the
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An analysis of the detailed operation for the tube element is proposed for an orifice pulse tube cryocooler. This is achieved through phasor analysis using basic thermodynamic relations to estimate the approximated cooling power... more
An analysis of the detailed operation for the tube element is proposed for an orifice pulse tube cryocooler. This is achieved through phasor analysis using basic thermodynamic relations to estimate the approximated cooling power associated with this machine. Moreover, the effect of the phase shift angle is illustrated by forming an analogy between the phase shift mechanism and a series RLC circuit model. Next, a one-dimensional model based on the conservation equations of mass and energy is presented; the reduced model is solved numerically, for the temperature and velocity of the gas along the tube, to determine the mass flow and time-averaged enthalpy flows at the cold and hot ends of the tube. The findings from the one-dimensional analysis are compared with the phasor analysis results and validated by comparison with similar studies in the literature.
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Government regulations and environmental conditions are pushing the development of improved miniaturized gas analyzers for volatile organic compounds. One of the many detectors used for gas analysis is the photoionization detector (PID).... more
Government regulations and environmental conditions are pushing the development of improved miniaturized gas analyzers for volatile organic compounds. One of the many detectors used for gas analysis is the photoionization detector (PID). This paper presents the design and characterization of a microfluidic photoionization detector (or µPID) fabricated using micro milling and electrical discharge machining techniques. This device has no glue and facilitates easy replacement of components. Two materials and fabrication techniques are proposed to produce a layer on the electrodes to protect from ultraviolet (UV) light and possible signal noise generation. Three different microchannels are tested experimentally and their results are compared. The channel with highest electrode area (31.17 mm²) and higher volume (6.47 µL) produces the highest raw signal and the corresponding estimated detection limit is 0.6 ppm for toluene without any amplification unit.
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Microstructure devices are well known for their excellent performance with regard to heat and mass transfer. Microstructured heat exchangers show significant advantages in comparison with conventional heat exchangers. The unique... more
Microstructure devices are well known for their excellent performance with regard to heat and mass transfer. Microstructured heat exchangers show significant advantages in comparison with conventional heat exchangers. The unique properties of a microreaction system show high overall heat transfer coefficients for example. Small characteristic dimensions are in the order of a few hundred μm (Schubert et al., 1998, 2001;
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Metabolomics, understood as the quantification of intracellular metabolites, is a powerful tool that has proven its benefit in microbial strain and fermentation process development. When doing metabolome analysis the crucial step is the... more
Metabolomics, understood as the quantification of intracellular metabolites, is a powerful tool that has proven its benefit in microbial strain and fermentation process development. When doing metabolome analysis the crucial step is the rapid quenching of the cellular ...
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We present a high power density UV-LED module for a wavelength of 395 nm with an optical power density of 13.1 W/cm2. The module consists of 98 densely packed LED chips adhesively bonded to an Al2O3-ceramic board. Thermal simulations and... more
We present a high power density UV-LED module for a wavelength of 395 nm with an optical power density of 13.1 W/cm2. The module consists of 98 densely packed LED chips adhesively bonded to an Al2O3-ceramic board. Thermal simulations and measurements as well as optical measurements were conducted. The module was cooled by a forced air heat sink for the
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Micro heat exchangers, which until recently have been implemented only at laboratory scale, are now being available for industrial applications. They are well known for their superior heat transfer properties due to the large... more
Micro heat exchangers, which until recently have been implemented only at laboratory scale, are now being available for industrial applications. They are well known for their superior heat transfer properties due to the large surface-to-volume ratio. But there are little data available on the long term stability of these devices. In this paper application several application examples for micro heat exchangers made of stainless steel are presented. The devices consist of stainless steel foils providing numerous micro channels generated by mechanical micromachining or wet chemical etching. A number of the foils are arranged in a specific way and bonded together. Device property descriptions as well as some possible application examples show the potential of metallic microstructure devices. Results on two crossflow microstructure heat exchangers running in long term tests are presented. Both devices have been tested for more than 8000 hours each, using deionised water as test fluid. Ex...
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Microstructure heat exchangers have unique properties that make them useful for numerous scientific and industrial applications. The power transferred per unit volume is mainly a function of the distance between heat source and heat... more
Microstructure heat exchangers have unique properties that make them useful for numerous scientific and industrial applications. The power transferred per unit volume is mainly a function of the distance between heat source and heat sink—the smaller this distance, the better the heat transfer. Another parameter governing for the heat transfer is the lateral characteristic dimension of the heat transfer structure;
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ABSTRACT Two techniques for the integration of temperature sensors in microchannels have been developed and compared. The two designs rely on different measuring principles, the first deriving from integrated circuit silicon... more
ABSTRACT Two techniques for the integration of temperature sensors in microchannels have been developed and compared. The two designs rely on different measuring principles, the first deriving from integrated circuit silicon microfabrication and the second being based on conventional commercial sensors. The proposed experimental approach offers a broad flexibility of applications, overcoming the material limitations typical of microelectronic integrated sensors. Heat transfer of gases in microchannels was studied to validate and characterize the integrated sensor systems, and the results were also compared with computational fluid dynamics (CFD) simulations. Whereas conventional sensors result as a simpler and robust technology, integrated circuit-based sensors offer higher sensitivities and potentials.
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ABSTRACT Microstructured heat exchangers are well suited for such phase transition processes as evaporation of liquids due to their heat transfer capabilities, being two to three orders of magnitude higher than those of conventional heat... more
ABSTRACT Microstructured heat exchangers are well suited for such phase transition processes as evaporation of liquids due to their heat transfer capabilities, being two to three orders of magnitude higher than those of conventional heat transfer devices. Controlling liquid evaporation inside micro-channels to provide full evaporation in a stable way is not trivial. In most cases, such instabilities as slug flow, bubbly flow, or vapor clogging occur, based on cross-talk possibilities between the individual micro-channels of a channel array, normally caused by open void inlet structures. Therefore, fluid inlet distribution is inhomogeneous, which results, in the best case, in a parabolic shape of a stable evaporation frontline. The parabolic shape occurs due to the residence time distribution of the fluid, generated by shorter path length in the array center and longer ones in the outer areas of the micro-channel array. Computational fluid dynamics simulation approves this result. Such a frontline can be kept stable when the process parameters are well controlled. Small deviations of the inlet parameters may lead to strong disturbances of the evaporation process, destabilizing it. When changing the inlet fluid distribution system to provide the most equal flow distribution possible, the span of the parabolic shape of the evaporation frontline can be reduced drastically. Finally, a stable evaporation frontline perpendicular to flow direction can be obtained. This status is no longer very sensitive to process deviations.This article presents an optimized micro-channel device for the optical investigation of phase transition phenomena. The device allows the exchange of integrated micro-channel arrays to investigate different designs for their suitability. It is separated into three independent sections, which can be heated or cooled individually. Therefore, very strict and rapid temperature jumps can be obtained within relatively short distances. The micro-channel array foils used for the experiments have been manufactured by mechanical micro-machining. Thus, the cross-sections of the micro-channels are always rectangular. Hydraulic diameter and length of the micro-channels, as well as the shape of the inlet and outlet voids, can be varied. Using a simple triangular or rectangular open inlet void, a stable evaporation line was generated, showing a parabolic shape. Depending on the mass flow and the size and shape of the inlet void, the span of the parabolic arc was influenceable.
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ABSTRACT Microreactor technology has developed various components in recent years—microreactors, micromixers, micro-heat exchangers and others. These components are advantageous with respect to the mixing procedure, heat transfer and also... more
ABSTRACT Microreactor technology has developed various components in recent years—microreactors, micromixers, micro-heat exchangers and others. These components are advantageous with respect to the mixing procedure, heat transfer and also residence time distribution. However, it has hitherto not been possible to measure the residence time distribution of microstructures with gas flow with sufficient accuracy. This situation is very unsatisfactory.In this work, a sensor system was developed for the measurement of the residence time distribution for gas flow through microstructured devices. The sensor is based on a thermal conductivity detector and fulfills the requirements for such measurements.The sensor was tested on microreactors built by Forschungszentrum Karlsruhe. A relatively large dispersion within the microstructure was found, which could be the result of unequally distributed flow through the microreactor channels.
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ABSTRACT The goal of the present work is to implement Molecular Tagging Velocimetry (MTV) for the analysis of internal gas flows in mini-channels. A MTV experimental setup has been designed. Tagging and detecting steps are respectively... more
ABSTRACT The goal of the present work is to implement Molecular Tagging Velocimetry (MTV) for the analysis of internal gas flows in mini-channels. A MTV experimental setup has been designed. Tagging and detecting steps are respectively insured by a UV laser and a CCD camera coupled to intensified relay optics. A specific channel with 1 × 5 mm2 rectangular cross section has been designed and equipped with integrated temperature sensors along its 20 cm length. It has been manufactured in PEEK (PolyEtherEther-Ketone) and Suprasil® optical windows have been integrated for the tagging access. Image processing allows extraction of velocity profiles for a pressure driven steady flow of argon through this channel. These profiles are compared to the theoretical profiles of laminar flows and the accuracy of the method is discussed. The MTV potential for the analysis of internal gaseous flows is commented on, with a discussion on perspectives for velocity measurement in rarefied flows and direct access to slip velocity at the walls.