Flow Micro Research Articles

Micro- and nanoscale non-ideal gas Poiseuille flows in a consistent Boltzmann algorithm model

The direct simulation Monte Carlo method in the consistent Boltzmann algorithm model has been developed and expanded for non-ideal gas predictions. The enhanced collision rate factor is determined by considering the excluded molecular volume and shadowing/screening effects based on the Enskog theory. The parameter for the attraction strength is also determined by comparison with the classical thermodynamics theory. Different pressure-driven gas Poiseuille flows in micro- and nanoscale channels are investigated. The van der Waals effect leads to a higher mass flow rate and different friction and heat transfer characteristics on the wall surface, compared to the results in the perfect gas model. The results also show that the van der Waals effect is dependent not only on the pressure but also on the channel size. A higher driving pressure or a smaller channel size will result in a larger van der Waals gas effect.

Crystallography and porosity effects of CO conversion on mesoporous CeO 2

Catalytic properties and thermal stability were studied for samples of mesoporous ceria with different BET specific surface area. The catalytic conversion of carbon monoxide to carbon dioxide and how thermal treatments of the catalysts influence the catalytic properties have been investigated. The materials were studied by transmission electron microscopy and by conversion profile measurements of CO versus temperature using a plug flow micro reactor made in quartz glass only. In order to compare the catalytic properties associated with a specific structure or morphology directly, aliquots of surface area (0.6 m 2) of the catalyst was used. Scanning electron microscopy and X-ray energy-dispersive spectrometry (XEDS) were used for surface morphology studies and elemental analysis. It was found that the proportion of {1 0 0} surfaces determine the catalytic properties of the material and these surfaces become important at calcination temperatures between 773 and 973 K. The internal mesoporous structure is destroyed at calcination temperatures around 873 K.

Separation of lipids from blood utilizing ultrasonic standing waves in microfluidic channels.

A method to continuously separate different particle types in a suspension is reported. Acoustic forces in a standing wave field were utilized to discriminate lipid particles from erythrocytes in whole blood. The presented technology proposes a new method of cleaning, i.e. removing lipid emboli from, shed blood recovered during cardiac surgery. Blood contaminated with lipid particles enter a laminar flow micro channel. Erythrocytes and lipid particles suspended in blood plasma are exposed to a half wavelength standing wave field orthogonal to the direction of flow as they pass through the channel. Because of differences in compressibility and density the two particle types move in different directions, the erythrocytes towards the centre of the channel and the lipid particles towards the side walls. The end of the channel is split into three outlet channels conducting the erythrocytes to the centre outlet and the lipid particles to the side outlets due to the laminar flow profile. The separation channel was evaluated in vitro using polyamide spheres suspended in water, showing separation efficiencies approaching 100%. The system was also evaluated on whole blood using tritium labelled lipid particles added to bovine blood. More than 80% of the lipid particles could be removed while approximately 70% of the erythrocytes were collected in one third of the original fluid volume. The study showed that the further reduced micro channel dimensions provided improved performance with respect to; (i) separation efficiency, (ii) actuation voltage, and (iii) volumetric throughput as compared to earlier work.

New directions in fluid dynamics: non-equilibrium aerodynamic and microsystem flows.

Fluid flows that do not have local equilibrium are characteristic of some of the new frontiers in engineering and technology, for example, high-speed high-altitude aerodynamics and the development of micrometre-sized fluid pumps, turbines and other devices. However, this area of fluid dynamics is poorly understood from both the experimental and simulation perspectives, which hampers the progress of these technologies. This paper reviews some of the recent developments in experimental techniques and modelling methods for non-equilibrium gas flows, examining their advantages and drawbacks. We also present new results from our computational investigations into both hypersonic and microsystem flows using two distinct numerical methodologies: the direct simulation Monte Carlo method and extended hydrodynamics. While the direct simulation approach produces excellent results and is used widely, extended hydrodynamics is not as well developed but is a promising candidate for future more complex simulations. Finally, we discuss some of the other situations where these simulation methods could be usefully applied, and look to the future of numerical tools for non-equilibrium flows.

Nonideal gas flow and heat transfer in micro- and nanochannels using the direct simulation Monte Carlo method.

Subsonic nonideal gas flow and heat transfer in micro- and nanochannels for different Knudsen numbers are investigated numerically using the direct simulation Monte Carlo method modified with a consistent Boltzmann algorithm. The van der Waals equation is used as the equation of state. The collision rate is also modified based on the Enskog theory for dense gas. It is shown that the nonideal gas effect becomes significant when the gas becomes so dense that the ideal gas assumption breaks down. The results also show that the nonideal gas effect is dependent not only on the gas density, but also on the channel size. A higher gas density and a smaller channel size lead to a more significant nonideal gas effect. The nonideal gas effect also causes lower skin friction coefficients and different heat transfer flux distributions at the wall surface. The simulations presented in this work are helpful for a better understanding of micro- and nanoscale gas flows.

Isopropylation of naphthalene over modified faujasites: effect of steaming temperature on activity and selectivity

Naphthalene alkylation with isopropyl alcohol (IPA) to obtain 2,6-diisopropylnaphthalene (2,6-DIPN) was studied over HY and steam-treated HY zeolite catalysts in a continuous down flow micro reactor under atmospheric pressure with cyclohexane as the solvent. The catalytic performance of the catalysts for naphthalene conversion and yield for 2,6-DIPN increases with steaming temperature, suggesting higher acid strength is required for the dialkyaltion. Under reaction conditions, 250 °C, with a feed of naphthalene:isopropanol:cyclohexane=1:3:10 (molar ratio) and space velocity WHSV=4.86 h −1, 86% naphthalene conversion and a 2,6-DIPN yield of 40% was obtained with HY-700 catalyst.

Dehydrogenation of Methanol to Anhydrous Formaldehyde in a Microstructured Reactor System

An integrated microreactor system for the direct dehydrogenation of methanol to water free formaldehyde was developed. The endothermic reaction can be homogeneously catalysed with evaporated sodium. At reaction temperatures of up to 1100K nearly complete conversion of methanol can be achieved. Undesired consecutive reactions can be minimized, if the reaction products are rapidly cooled and if the residence time at reaction temperature is precisely controlled. Fast heating and cooling of the gases in the order of a few milliseconds is desired, specifications that can be met by microreactors. Several tests were carried out using conventional and microscaled systems. The results are compared and served as a base for the development of an integrated microsystem for the dehydrogenation of methanol. First, two different kind of mixers were tested. The results clearly showed that fast and homogeneous mixing of the gaseous sodium catalyst and the methanol is indispensable. Second, in order to improve the temperature profile in the reactor a cross flow micro heat exchanger was employed. A special adaptor with internal heater was developed, allowing to keep the gases at reaction temperature close up to the micro heat exchanger. The final setup allowed to obtain a formaldehyde selectivity of 80.3%at methanol conversion higher than 97%.

Estimation of the duodenal flow of microbial nitrogen in ruminants based on the chemical composition of forages: a literature review

The objective of this study was to evaluate the estimation of the duodenal flow of micro- bial nitrogen (N) in ruminants fed forage only, per kilogram of dry matter (DM) intake, which is the yield of microbial protein (YMP). The estimation was based on the chemical composition of forages. A data file of 62 observations was collected from in vivo studies on cattle and sheep fed diets with for- age only. A statistical analysis of YMP was conducted with neutral detergent fibre (NDF), crude pro- tein (CP), non structural carbohydrates (NSC), group of forage species (legumes or grasses), method of conservation, physical form of presentation, level of DM intake, animal species, methodology and references as parameters. After a stepwise regression, CP was significant and the most important pre- dictor. NSC or the method of conservation had an extra effect on YMP. On the basis of these three pa- rameters the best fit equations were found and the influence of all parameters on YMP were discussed. Using the data file of this study, the prediction of YMP from the PDI-system was also vali- dated. The statistics of the validation of the PDI prediction were similar to the statistics of the equa- tions from this study. In conclusion, the chemical composition of forages, with or without the method of conservation, is a poor indication for the duodenal flow of microbial N (g·kg -1 DM intake) in ruminants fed diets with forages only.

Modeling and experimental evaluation of parallel flow micro channel condensers

This paper reports results obtained in a theoretical and experimental study involving micro channel/louvered fin condensers for automotive applications. A simulation model has been developed based upon three zones related to the thermodynamic states of the refrigerant in the condenser. Experiments with condensers have been performed in a set up developed for thermal performance evaluations of automotive air conditioning systems. Comparisons between simulation model and experimental results for refrigerant HFC-134a have been performed with respect to three important condenser parameters: heat rejection rate, refrigerant pressure drop and overall heat transfer coefficient. It has been shown that model results for the particular tested condenser compare very well with the experimental data, with deviations being within an acceptable range.

Design and Fabrication of Six-Degree of Freedom Piezoresistive Turbulent Water Flow Sensor

This paper presents the design concept, theoretical investigation, and fabrication of a six-degree of freedom (6-DOF) turbulent flow micro sensor utilizing the piezoresistive effect in silicon. Unlike other flow sensors, which typically measure just one component of wall shear stress, the proposed sensor can independently detect six components of force and moment on a test particle in a turbulent flow. By combining conventional and four-terminal piezoresistors in Si (111), and arranging them suitably on the sensing area, the total number of piezoresistors used in this sensing chip is only eighteen, much fewer than the forty eight piezoresistors of the prior art piezoresistive 6-DOF force sensor.

Integrated microfluidic system enabling (bio)chemical reactions with on-line MALDI-TOF mass spectrometry.

A continuous flow micro total analysis system (micro-TAS) consisting of an on-chip microfluidic device connected to a matrix assisted laser desorption ionization [MALDI] time-of-flight [TOF] mass spectrometer (MS) as an analytical screening system is presented. Reaction microchannels and inlet/outlet reservoirs were fabricated by powderblasting on glass wafers that were then bonded to silicon substrates. The novel lab-on-a-chip was realized by integrating the microdevice with a MALDI-TOFMS standard sample plate used as carrier to get the microfluidic device in the MALDI instrument. A novel pressure-driven pumping mechanism using the vacuum of the instrument as a driving force induces flow in the reaction microchannel in a self-activating way. Organic syntheses as well as biochemical reactions are carried out entirely inside the MALDI-MS ionization vacuum chamber and analyzed on-line by MALDI-TOFMS in real time. The effectiveness of the micro-TAS system has been successfully demonstrated with several examples of (bio)chemical reactions.

SCALAR MIXING AND CHEMICAL REACTION SIMULATIONS USING LATTICE BOLTZMANN METHOD

We simulate scalar mixing and chemical reactions using the lattice Boltzmann method. Simulations of initially non-premixed binary mixtures yield scalar probability distribution functions that are in good agreement with direct numerical simulation (of Navier-Stokes equation) data. One-dimensional chemically-reacting flow simulation of a premixed mixture yields a flame speed that is consistent with experimentally determined value. These results serve to establish the feasibility of the lattice Boltzman method as a viable tool for computing turbulent combustion.

Gas and Liquid Transfer in Narrow Pores

A microhydrodynamic approach is formulated to describe the molecular flows in micro- and mesoporous systems, including the region of capillary phenomena, where one needs a single set of equations characterizing the flows of both dense gases and liquids. The set of equations of the transfer of dense fluids in narrow pores is closed by using the simplest molecular model, specifically, the lattice gas model, which describes the behavior of fluids over wide ranges of concentrations (from the gas to the liquid state) and temperatures, including the critical region. This model takes into account the volume of molecules and the intermolecular interactions in a quasi-chemical approximation, which retains short-range order correlations. The model reflects the strong anisotropy of the molecular distributions along a normal to the pore wall (because of the adsorption forces) and along the pore axis if there is capillary condensation. The derived set of equations is a set of finite-difference equations in increments of coordinates (instead of derivatives). The dissipative coefficients have a nonlocal anisotropic character. The equations of the transfer in near-wall cells and the prospects of the microhydrodynamic approach are discussed.

Potentials of separation membranes in the sugar industry

There are described some applications of membrane separation, e.g. reverse osmosis in the treatment of sugar beat press water, recovery of salt from waste brine at a sugar decolorization plant or microfiltration and ultrafiltration of raw sugar juice, as an alternative to chemical purification process. The last case is completed by original experimental work. Samples were treated with cross flow micro- and ultrafiltration on ceramic membranes having porosity 20 and 50 nm. The changes in the content of sucrose, invert sugar and lactic acid were observed, resulting in increasing the juice purity from the initial value 89% to the final value 91‐92%. In addition, permeate can be a subject for direct crystallisation after thickening to produce the final product. Retentate purity decreased to 87‐88%. For nanofiltration tests (NF) there has been designed a special stainless steel cross-flow dynamic cell for pressures up to 4 MPa. The preliminary NF tests were focused on testing different commercial flat polymeric membranes (Hydronautics-Nitto Denko, Osmonics-Desal). © 2002 Elsevier Science B.V. All rights reserved.

3D-flow simulation and optimization of a cross flow filtration with rotating discs

The 3D-flow regime of a cross flow filter with horizontal rotating discs was examined. The influence of different shaped scrapers between the rotating discs on parameters like turbulence (k), dissipation (e) and overflow velocities were investigated by the commercial cfd program Fluent 5.0.2®. The main focus of the scrapers is to trigger off certain amounts of turbulence between the discs, that are necessary to avoid cake formation on the filter disc surface. Turbulence depends strongly on the rotation velocity of the discs. By modifying the cross sectional shape and the curvature of the scraper turbulence between two filter discs can also be influenced. The input parameters like the geometry and rotation velocity of the discs were altered. To understand the appearing swirls and flow characteristics of the model representing the real filter, simple models were simulated first. For all simulations the existing symmetries of the horizontal arranged filter discs could be used to simplify the geometrical models. The results of the simulation reflect the flow conditions in a rotating cross flow micro filter with cake limitation by scrapers with satisfying closeness. The usage of bent scrapers and the variation of scraper profiles lead to considerable improvements in the passing flow and turbulence situation in the region behind the scrapers.

Flow characteristics and micro—bubble behaviour in a rotating pipe section with an abrupt enlargement

The flow characteristics and dynamic behaviour of micro-bubbles were investigated experimentally and numerically for swirling flow within rotating pipe sections. In the present investigation, three types of rotating pipe section were used in order to study the effects of geometric configuration upon the flowfield. Experimental data were obtained for locations of the stagnation point and profiles of the parabolic surface. The results obtained from experiments were examined and verified with the aid of numerical analysis. It was found that the air pocket formed at the stagnation point was moved upstream by the effect of the suddenly expanded part when pipe rotation was increased. The reattachment point did not shift downstream as a result of an increase in the Reynolds number. This is due to the fact that the vortex zone at the expanded corner expands or shrinks, depending on the speed of rotation.

A COMPARATIVE ANALYSIS OF STUDIES ON HEAT TRANSFER AND FLUID FLOW IN MICROCHANNELS

The extremely high rates of heat transfer obtained by employing microchannels makes them an attractive alternative to conventional methods of heat dissipation, especially in applications related to the cooling of microelectronics. A compilation and analysis of the results from investigations on fluid flow and heat transfer in micro- and mini-channels and microtubes in the literature is presented in this review, with a special emphasis on quantitative experimental results and theoretical predictions. Anomalies and deviations from the behavior expected for conventional channels, both in terms of the frictional and heat transfer characteristics, are discussed.

Constant-Wall-Temperature Nusselt Number in Micro and Nano-Channels1

We investigate the constant-wall-temperature convective heat-transfer characteristics of a model gaseous flow in two-dimensional micro and nano-channels under hydrodynamically and thermally fully developed conditions. Our investigation covers both the slip-flow regime 0⩽Kn⩽0.1, and most of the transition regime 0.1<Kn⩽10, where Kn, the Knudsen number, is defined as the ratio between the molecular mean free path and the channel height. We use slip-flow theory in the presence of axial heat conduction to calculate the Nusselt number in the range 0⩽Kn⩽0.2, and a stochastic molecular simulation technique known as the direct simulation Monte Carlo (DSMC) to calculate the Nusselt number in the range 0.02<Kn<2. Inclusion of the effects of axial heat conduction in the continuum model is necessary since small-scale internal flows are typically characterized by finite Peclet numbers. Our results show that the slip-flow prediction is in good agreement with the DSMC results for Kn⩽0.1, but also remains a good approximation beyond its expected range of applicability. We also show that the Nusselt number decreases monotonically with increasing Knudsen number in the fully accommodating case, both in the slip-flow and transition regimes. In the slip-flow regime, axial heat conduction is found to increase the Nusselt number; this effect is largest at Kn=0 and is of the order of 10 percent. Qualitatively similar results are obtained for slip-flow heat transfer in circular tubes.

Numerical simulations of fully developed turbulent liquid flowsin micro tubes

Fully developed turbulent liquid flows in micro tubes at various Reynoldsnumbers and tube diameters are numerically simulated using the second-orderscheme of the control volume method. A one-equation turbulent model is introduced and modified toconsider the micro-size effect. Detailed velocity and turbulent kinetic energydistributions are obtained based on the present model. Comparisons between thepresent numerical results and the available experimental measurements in the openliterature show excellent agreement, thus validating the present model for thisclass of flow problems.

A study on the synthesis of diethyl oxalate over Pd/α-Al 2O 3 catalysts

The synthesis of diethyl oxalate from ethyl nitrite and carbon monoxide was studied in a continuos flow micro fixed-bed reactor at atmospheric pressure. Two palladium catalysts supported on α-Al 2O 3 of different pore structures were studied under mild conditions. One catalyst (1 wt.% Pd/α-Al 2O 3-1) showed higher catalytic activity and selectivity than the other catalyst (1 wt.% Pd/α-Al 2O 3-2). X-ray diffraction patterns have confirmed that both supports are well-defined α-Al 2O 3. Palladium dispersions were greater on the 1 wt.% Pd/α-Al 2O 3-1 catalyst than that on the 1 wt.% Pd/α-Al 2O 3-2 catalyst as determined by hydrogen chemisorption. The FTIR spectroscopy study indicated that the 1 wt.% Pd/α-Al 2O 3-1 catalyst adsorbed CO easily in the linear form (band at 2089 cm −1) and bridge form (bands at 1950 and 1880 cm −1), and as carbonate species (at 1628 cm −1). There was little CO adsorption on 1 wt.% Pd/α-Al 2O 3-2. The average pore size of the support α-Al 2O 3-1 was 41.6 Å, and 14.0 Å for α-Al 2O 3-2. This study reveals that the pore structure of the supports remarkably affects the palladium dispersion of the catalysts and alters the CO adsorption behavior, therefore, dramatically affect the catalytic performance. In the case of CO+C 2H 5ONO reactions, diethyl oxalate was efficiently formed over the 1 wt.% Pd/α-Al 2O 3-1 catalyst.