493 research outputs found
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Calibration of lubrication force measurements by lattice Boltzmann simulations
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Many experiments explore the hydrodynamic boundary of a surface by approaching a colloidal sphere and measuring the occurring lubrication force. However, in this case many different parameters like wettability and surface roughness influence the result. In the experiment these cannot be separated easily. For a deeper understanding of such surface effects a tool is required that predicts the influence of different surface properties. Here computer simulations can help. In this paper we present lattice Boltzmann simulations of a sphere submerged in a Newtonian liquid and show that our method is able to reproduce the theoretical predictions. In order to provide high precision simulation results the influence of finite size effects has to be controlled. We study the influence of the required system size and resolution of the sphere and demonstrate that already moderate computing ressources allow to keep the error below 1%.This study is funded by DFG priority program SPP 1164
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A general method for calculation of flow boiling and flow condensation heat transfer coefficients in minichannels
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena, however their description with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where bubble generation is not present. Similar picture holds for inside tube condensation, where annular flow structure predominates. In such case the heat transfer coefficient is primarily dependent on the convective mechanism. In the paper a method developed earlier by D. Mikielewicz et al. (2007) is applied to calculations of heat transfer coefficient for inside tube condensation. Satisfactory consistency with well established correlations for condensation has been found
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Boundary condition, an old but not well solved problem, linked from nanometer to macroscale
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.A key issue of the flow and heat transfer in channels is to determine the boundary condition, which is affected by various parameters, such as the solid-fluid potential interactions, solid wall roughness,
surface wettability etc. We summarize the progress that has been made in recent years. In the first part of this paper, we reviewed the three-atom-model, leading to an important criterion number governing the
boundary conditions. The molecular dynamics (MD) simulations verified the effectiveness of the criterion number. The second part of this paper reports the multiscale simulation of the flow field in channels, adjoining the molecular dynamics (MD) simulation and the continuum fluid mechanics. Three types of
boundary conditions (slip, non-slip and locking) were identified over the multiscale channel sizes. The slip lengths are found to be mainly dependent on the interfacial parameters with the fixed apparent shear rate. The channel size has little effects on the slip lengths if the size is above a critical value within a couple of tens of molecular diameters. The slip, non-slip and locking interfacial parameters yield positive, zero and negative slip lengths, respectively. The three types of boundary conditions existing in “microscale” still
occur in “macroscale”. However, the weak dependence of the slip lengths on the channel size yields decreased slip velocities with increases in channel sizes for all three types of interfacial parameters.This study is supported by the National Natural Science Foundation of China (No. U1034004 and 50825603), and the Basic Research Program (973 program) with the contract number of 2011CB710703
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Influence of metallic porous microlayer on pressure drop and heat transfer of stainless steel plate heat exchanger
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The experimental analysis of passive heat transfer intensification in the case of plate heat exchanger has been carried out. On the heat transfer surface of heat exchanger the metallic porous layer was created. The experiment was accomplished in two stages. In the first stage the commercial stainless steel gasketed plate heat exchanger was investigated, while in the second one – the identical heat exchanger but with the modified heat transfer surface. The direct comparison of thermal and flow characteristics between both devices was possible due to the assurance of equivalent conditions during the experiment. Equivalent conditions mean the same volumetric flow rates and the same media temperatures at the inlet of heat exchangers in the corresponding measurement series. Experimental data were collected for the single-phase convective heat transfer in the water-ethanol configuration. The heat transfer coefficients were determined using the Wilson method
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Electrodiffusion Method of Near-Wall Flow Diagnostics in Microfluidic Systems
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The electrodiffusion technique has been mostly used for the near-wall flow diagnostics on large
scales. A novel technique for fabrication of plastic microfluidic systems with integrated metal
microelectrodes (called technique of sacrificed substrate) enables us to produce microfluidic devices with
precisely shaped sensors for wall shear stress measurements. Several micrometer thick gold sensors built-in a
plastic substrate exhibit good mechanical resistance and smoothness. Proper functioning of prepared chips
with microsensors has been first tested in various calibration experiments (polarization curve, sensor
response to polarization set-up, steady flow calibration, temperature dependence of diffusivity). Our first
results obtained for separating/reattaching flow behind a backward-facing step and for gas-liquid Taylor flow
in microchannels then demonstrate its applicability for the detection of near-wall flow reversal, the
delimitation of flow-recirculation zones, and the determination of wall shear stress response to moving
bubbles. Other applications of these sensors in microfluidics (e.g. characterization of liquid films, capillary
waves, bubbles or drops) can be also envisaged
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Numerical Studies on Geometric Features of Microchannel Heat Sink with Pin Fin Structure
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The manifold microchannel (MMC) heat sink is an effective method of electronic device cooling featured by the advantages such as low thermal resistance, compact structure, low flow rate and uniform temperature distribution along the flow direction. To improve the performance of MMC, numerical studies on geometry features of pin fin microchannel were carried out and the characteristics of flowing and heat transfer in two types of MMC structures which are respectively optimized through porosity with different pin-fin distribution and pin-fin located angle were investigated. Numerical results indicated that there are apparent influences of both above geometry features on the characteristics of flowing and heat transfer in MMC. Based on cooling
performance factor, an optimum value of porosity with specific pin-fin distribution was obtained. Moreover, better effect on heat transfer could be achieved at a 30 degree around of rectangular pin fin located angle in our work
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On analysis of chemical reactions coupled gas flows in SOFCs
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Solid oxide fuel cell (SOFC) is among others one of the most promising technologies for electricity energy generation. A recent new trends is to reduce its operating temperature from 1000oC to 800oC by
employing a thick porous layer as the supporting structure. Various transport processes occurred are strongly affected by catalytic chemical/electrochemical reactions appearing in nano- or/and microstructured and
multi-functional porous electrodes. It is particularly true if methane is used as the fuel, and internal reforming reactions within the microstructured porous anodes enable the conversion of the methane into H2
and CO. To deeply understand the chemical reaction coupled gas flow and heat transfer in the microstructured porous anode, a fully three-dimensional numerical calculation procedure (CFD) is developed and applied. The species mass/heat generation and consumption related to the internal reforming reactions and the electrochemical reaction have been identified and employed in the study. The variable thermalphysical
properties and transport parameters of the fuel gas mixture have also been taken into account. Furthermore, the heat transfer due to the fuel gas flow is implemented into the energy balance based on multi-component diffusion models. Finally, various issues connecting to the micro models of the surface
reactions are discussed and reviewed.Thi study is supported by the Swedish Research Council (VR) and the National Natural Science Foundation of China (NSFC-50706004)
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Numerical optimization of passive chaotic micromixers
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Due to the lack of turbulence in micromixers diffusion is the main process contributing to microfluidic mixing. Especially mixing of
uids with low diffusivity is a difficult task. The recently discovered mechanism of "chaotic-advection" enhances the diffusion process by stretching and folding the fluid interfaces in order to provide a larger interface. Certain passive micromixers like the staggered herringbone mixer (SHM) apply this concept and succeed in enhancing the mixing process considerably. The optimization of such micromixers is a time consuming and often expensive process. We demonstrate that the application of the lattice Boltzmann (LB) method to study advection and diffusion processes can be an efficient tool to optimize micromixers. By combining finite time Lyapunov exponents to study chaotic advection and Danckwert's intensity of segregation to study the diffusion, we demonstrate how optimal geometrical parameters for the SHM can be found and
how diffusion is improved by the complex
ow pattern inside the mixer. The current article provides a review of our results published in [1] together with additional studies on modelling diffusive mixing
with the LB method.This work was financed within the DFG priority program "nano- and microfluidics", the DFG collaborative research center 716, and by the NWO/STW VIDI grant of J. Harting
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An experimental study of dynamic flow of nanofluid with different concentrations
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Current reported data of nanofluid concentration is almost all based on TEM observation, which is in a static situation. No data of dynamic concentration during flow is reported. In the present study, an experimental measurement based on nuclear magnetic resonance (NMR) of monitoring the dynamic concentrations of nanofluid flow is carried out. It is demonstrated that the ferrofluid with Fe3O4 as its nanoparticles coated with surfactant as a special type of nanofluid can be used as T2 contrast agent in NMR scanning as well as a magnetic and thermal sensitive nanoparticle solution that would enhance heat transfer
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Simulation of gas micro flows based on finite element and finite volume method
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The Regularized 13-Moment Equations state a model for describing rarefied gas flows. The equations
are based on a moment approximation of the Boltzmann equation and aim at an accurate prediction up
to Knudsen numbers of 0.5. This paper is concerned with the numerical treatment of the PDE system, with
special focus on slow flows. Finite elements and finite volumes are applied and the results of both approaches
are discussed and the pros and cons are outlined
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