J. Brandner | Karlsruhe Institute of Technology (KIT) (original) (raw)

Papers by J. Brandner

Microsystem Technologies, 2013

ABSTRACT A new micro Molecular Tagging Velocimetry (µMTV) setup has been developed to analyze vel... 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.

Fast temp. cycling of catalytic reactions using microreactors is discussed. [on SciFinder (R)]

International Journal of Chemical Reactor Engineering, 2007

The forced variation of reaction parameters is a known method to improve the performance of catal... 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...

Chemical Engineering Science, 2008

In this publication, several stainless steel microstructure reactors specially designed to obtain... more In this publication, several stainless steel microstructure reactors specially designed to obtain rapid and periodic temperature changes are presented. Different microstructure reactor designs have been manufactured and tested for their thermal behaviour and equally by running a test reaction under stationary and non-stationary temperature conditions. The devices were continuously electrically heated and periodically cooled by a deionized water flow. The objective of the experimental measurements was to demonstrate that non-stationary temperature conditions may lead to an increase in the reaction rate compared to the stationary conditions. The heterogeneously catalysed oxidation of CO was chosen as the test reaction. The catalyst used was a dispersion of platinum on a porous alumina support generated by sol-gel technology. The experiments realized under non-stationary temperature conditions with a temperature oscillation amplitude of 41 K and a period duration of 21 s show an increase in the mean CO 2 concentration of a factor 1.72 compared to the mean concentration under quasi-stationary temperature conditions. The simulations of a simple monomolecular reaction under non-stationary temperature conditions indicate that the presence of a transitional surface coverage generated by the temperature oscillations may be a possible explanation for the observed phenomenon.

ASME 4th International Conference on Nanochannels, Microchannels, and Minichannels, Parts A and B, 2006

ABSTRACT Pulsation and maldistribution effects (hot spot formation) with characteristic frequenci... 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.

ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2011, 2011

In this paper the feasibility of obtaining optical access for micro devices made by stereolithogr... 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.

Journal of Physics: Conference Series, 2012

In this paper investigations on the design of a gas flow microchannel heat exchanger are describe... 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

Experimental Heat Transfer, 2014

ABSTRACT In this work, a double-layered microchannel heat exchanger is designed for investigation... 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.

Volume 6: Fluids and Thermal Systems; Advances for Process Industries, Parts A and B, 2011

ABSTRACT A new setup was developed for gas mixing analysis in T-shaped microchannels. The princip... 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.

Microfluidics and Nanofluidics, 2014

ASME 2012 10th International Conference on Nanochannels, Microchannels, and Minichannels, 2012

This paper presents the results of experimental and numerical investigation of forced convection ... 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

ASME 5th International Conference on Nanochannels, Microchannels, and Minichannels, 2007

The promising performance of microchannels has given rise to intensive research on pressure drop ... 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...

Journal of Physics: Conference Series, 2012

The study of gas flows in microchannels has received considerably more attention in the literatur... more The study of gas flows in microchannels has received considerably more attention in the literature from a simulation perspective than an experimental. The majority of the experimental work has emphasis on the global measurements at the inlet or exit of the microchannel instead locally along it. In this paper some efforts were made to measure the pressure drop along T-shaped micro channel by using interferometry. The two side channels were served as gas entrances and they were both open to air and the channel outlet was being vacuumed during experiments. A Mach-Zehnder interference microscopy was built for the measurement of gas pressure drop along the mixing channel. Some points along the mixing channel were selected for interferometric measurements. Simulations were first developed in unsteady condition by using Ansys Fluent to verify the nonexistence of transient phenomena of gas flow in the defined condition and then run again in steady condition to get the theoretical pressure drop that was would be used for comparison with experimental results.

Journal of Physics: Conference Series, 2012

Heat Transfer Engineering, 2007

Visualization by high-speed videography and infrared surface thermography was used to compare the... 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

Experimental Heat Transfer, 2014

Chemical Engineering Journal, 2008

Two types of stainless steel microstructured reactors for catalytic gas-phase reactions have been... more Two types of stainless steel microstructured reactors for catalytic gas-phase reactions have been developed and characterized with respect to their thermal behaviour under non-stationary temperature conditions. One of the reactors used (FTC-I) allowed periodic temperature changes up to 100 K with a frequency of 0.05 Hz. However, a broad temperature gradient of 80 K developed inside the reactor. A second reactor (FTC-II) enabled periodic temperature variations of maximum 60 K with a frequency of 0.06 Hz while avoiding temperature non-homogeneity. The CO oxidation taken as a test reaction was carried out over a Pt/Al 2 O 3 catalyst in the FTC-II reactor. In this way it was possible to study the effect of non-stationary temperature conditions on the reactor performance. A significant increase in CO conversion was observed with periodic temperature cycling as compared to values obtained under steady-state conditions.

is a noninvasive measurement and based on this technique, the measurement or visualization of cha... more is a noninvasive measurement and based on this technique, the measurement or visualization of changes in physical properties of transparent objects can be achieved by detecting the refractive index changes. Gas pressure and temperature can be related to their refractive index, so interferometry can be used for local measurement of changes of these properties along the channel. A Mach-Zehnder interferometer was built with a laser with a wavelength of 633 nm, a high speed camera and two acousto-optic modulators (AOMs). Due to small channel characteristic length and sometimes low gas pressure, the measurement could be much influenced by noise. The AOMs can introduce frequency shifts into the system by acousto-optic effect, which can avoid mechanical noise generally by translating piezoelectric mirrors. The channel sides with optical access are made from crystalline silica (Quartz) that does not show speckle effects and any laser absorbance. For initial tests, the local gas pressure drop distribution along microchannel is studied at room temperature.

Microsystem Technologies, 2013

ABSTRACT A new micro Molecular Tagging Velocimetry (µMTV) setup has been developed to analyze vel... 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.

Fast temp. cycling of catalytic reactions using microreactors is discussed. [on SciFinder (R)]

International Journal of Chemical Reactor Engineering, 2007

The forced variation of reaction parameters is a known method to improve the performance of catal... 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...

Chemical Engineering Science, 2008

In this publication, several stainless steel microstructure reactors specially designed to obtain... more In this publication, several stainless steel microstructure reactors specially designed to obtain rapid and periodic temperature changes are presented. Different microstructure reactor designs have been manufactured and tested for their thermal behaviour and equally by running a test reaction under stationary and non-stationary temperature conditions. The devices were continuously electrically heated and periodically cooled by a deionized water flow. The objective of the experimental measurements was to demonstrate that non-stationary temperature conditions may lead to an increase in the reaction rate compared to the stationary conditions. The heterogeneously catalysed oxidation of CO was chosen as the test reaction. The catalyst used was a dispersion of platinum on a porous alumina support generated by sol-gel technology. The experiments realized under non-stationary temperature conditions with a temperature oscillation amplitude of 41 K and a period duration of 21 s show an increase in the mean CO 2 concentration of a factor 1.72 compared to the mean concentration under quasi-stationary temperature conditions. The simulations of a simple monomolecular reaction under non-stationary temperature conditions indicate that the presence of a transitional surface coverage generated by the temperature oscillations may be a possible explanation for the observed phenomenon.

ASME 4th International Conference on Nanochannels, Microchannels, and Minichannels, Parts A and B, 2006

ABSTRACT Pulsation and maldistribution effects (hot spot formation) with characteristic frequenci... 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.

ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2011, 2011

In this paper the feasibility of obtaining optical access for micro devices made by stereolithogr... 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.

Journal of Physics: Conference Series, 2012

In this paper investigations on the design of a gas flow microchannel heat exchanger are describe... 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

Experimental Heat Transfer, 2014

ABSTRACT In this work, a double-layered microchannel heat exchanger is designed for investigation... 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.

Volume 6: Fluids and Thermal Systems; Advances for Process Industries, Parts A and B, 2011

ABSTRACT A new setup was developed for gas mixing analysis in T-shaped microchannels. The princip... 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.

Microfluidics and Nanofluidics, 2014

ASME 2012 10th International Conference on Nanochannels, Microchannels, and Minichannels, 2012

This paper presents the results of experimental and numerical investigation of forced convection ... 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

ASME 5th International Conference on Nanochannels, Microchannels, and Minichannels, 2007

The promising performance of microchannels has given rise to intensive research on pressure drop ... 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...

Journal of Physics: Conference Series, 2012

The study of gas flows in microchannels has received considerably more attention in the literatur... more The study of gas flows in microchannels has received considerably more attention in the literature from a simulation perspective than an experimental. The majority of the experimental work has emphasis on the global measurements at the inlet or exit of the microchannel instead locally along it. In this paper some efforts were made to measure the pressure drop along T-shaped micro channel by using interferometry. The two side channels were served as gas entrances and they were both open to air and the channel outlet was being vacuumed during experiments. A Mach-Zehnder interference microscopy was built for the measurement of gas pressure drop along the mixing channel. Some points along the mixing channel were selected for interferometric measurements. Simulations were first developed in unsteady condition by using Ansys Fluent to verify the nonexistence of transient phenomena of gas flow in the defined condition and then run again in steady condition to get the theoretical pressure drop that was would be used for comparison with experimental results.

Journal of Physics: Conference Series, 2012

Heat Transfer Engineering, 2007

Visualization by high-speed videography and infrared surface thermography was used to compare the... 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

Experimental Heat Transfer, 2014

Chemical Engineering Journal, 2008

Two types of stainless steel microstructured reactors for catalytic gas-phase reactions have been... more Two types of stainless steel microstructured reactors for catalytic gas-phase reactions have been developed and characterized with respect to their thermal behaviour under non-stationary temperature conditions. One of the reactors used (FTC-I) allowed periodic temperature changes up to 100 K with a frequency of 0.05 Hz. However, a broad temperature gradient of 80 K developed inside the reactor. A second reactor (FTC-II) enabled periodic temperature variations of maximum 60 K with a frequency of 0.06 Hz while avoiding temperature non-homogeneity. The CO oxidation taken as a test reaction was carried out over a Pt/Al 2 O 3 catalyst in the FTC-II reactor. In this way it was possible to study the effect of non-stationary temperature conditions on the reactor performance. A significant increase in CO conversion was observed with periodic temperature cycling as compared to values obtained under steady-state conditions.

is a noninvasive measurement and based on this technique, the measurement or visualization of cha... more is a noninvasive measurement and based on this technique, the measurement or visualization of changes in physical properties of transparent objects can be achieved by detecting the refractive index changes. Gas pressure and temperature can be related to their refractive index, so interferometry can be used for local measurement of changes of these properties along the channel. A Mach-Zehnder interferometer was built with a laser with a wavelength of 633 nm, a high speed camera and two acousto-optic modulators (AOMs). Due to small channel characteristic length and sometimes low gas pressure, the measurement could be much influenced by noise. The AOMs can introduce frequency shifts into the system by acousto-optic effect, which can avoid mechanical noise generally by translating piezoelectric mirrors. The channel sides with optical access are made from crystalline silica (Quartz) that does not show speckle effects and any laser absorbance. For initial tests, the local gas pressure drop distribution along microchannel is studied at room temperature.