876 research outputs found
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Rarefied flow between plates of finite length via the coupling approach
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.The coexistence of rarefied continuum flow regime areas and relatively small elements in which rarefaction effects become important is a typical feature of many complex gas flows micro systems. In rarefied domains, the mean free path of gas molecules is comparable or larger than a characteristic scale of the system. These domains are naturally described by kinetic equation for the velocity distribution function, which involve a considerable effort in terms of CPU time and memory requirements, due to the discretization in both physical and velocity space. The continuum domains are best described by the fluid Navier Stokes (NS) equations in terms of average flow velocity, gas density and temperature. These equations are more efficient, but less accurate in critical rarefied areas. Thus, the development of hybrid solver combining kinetic and continuum models is of great interest especially for applications range from gas flows in micro systems to the aerospace applications, such as high altitude flights. The pressure–driven gas flow of rarified monatomic gas through a two-dimensional short microchannel is considered using hybrid solver. The calculations have been carried out for pressure ratios 0.1, 0.5 and 0.9 and fixed relatively large Knudsen number. The applicability of the solver is discussed via comparison with the kinetic and NS solutions.The European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement ITN GASMEMS no 215504
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An experimental study on micro-scale flow boiling heat transfer
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.New experimental flow boiling heat transfer results in micro-scale tubes are presented in this paper. The experimental data were obtained in a horizontal 2.32 mm I.D. stainless steel tube with heating length of 464 mm, R245fa as working fluid, mass velocities ranging from 50 to 500 kg/m2s, heat flux from 5 to 55 kW/m2, exit saturation temperatures of 22, 31 and 41 oC, and vapor qualities from 0.05 to 0.99. Flow
pattern characterization was also performed from images obtained by high speed filming. Heat transfer coefficient results from 2 to 6 kW/m2K were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity and vapor quality. The experimental data were compared against the following micro-scale flow boiling predictive methods from the literature: Bertsch et al. (2008), Saitoh et al. (2007), Kandlikar and Balasubramanian (2004), Zhang et al. (2004), Thome et al. (2004) and Liu and Winterton (1991). Although not satisfactory, Thome et al. (2004) worked the best when predicting
the present database.This study is funded under contract numbers 05/60031-0, 06/52089-1 and 07/53950-5 by FAPESP (The State of SĂŁo Paulo Research Foundation, Brazil)
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The effect of flow coefficient on the design of miniature centrifugal impeller
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 systematic and simple design methodology of miniature centrifugal impeller is proposed. In the design of miniature centrifugal impeller, the flow coefficient Ď•1 plays a significant role. Theoretically, the geometric parameters, including inner radius, blade angles and blade height can be expressed as functions of the flow coefficient. Accordingly, the theoretical head and energy losses are also influenced by the flow coefficient. To investigate the effect of flow coefficient, a series of miniature impellers are designed for different flow coefficients, and CFD simulations are conducted. Both theoretical analysis and CFD simulations show similar trends. Initially, the pressure generated increases with increasing flow coefficient. Upon reaching a maximum, it will subsequently decrease with increasing flow coefficient. Hence, an optimal flow coefficient should be chosen to achieve the best performance. From the theoretical results, the maximum pressure generated occurs when the flow coefficient is approximately 2.8, while for CFD, it is approximately 1.3. The difference between the theoretical analysis and CFD simulation shows that the theoretical model should be further improved to enhance its accuracy
Ischemia with endovascular balloon during disarticulations and amputations of limbs
Objective Limb disarticulation has been widely performed since the
18th century, especially in war surgery. Actually is infrequently done
in orthopaedic and vascular surgery, and it is associated with a high
mortality rate because of frequent comorbidities. Disarticulation
usually is reserved for patients with malignant tumours or gangrene
from severe artherosclerosis. During disarticulation, hemodynamic
stability can be altered by hemorrhagic events in the femoral or
humeral arteries. We propose an endovascular technique for proximal
control of the artery to reduce blood loss during disarticulation. Our
experience today is limited at hip disarticulation.
Material and methods The vascular access was percutaneous at the
common femoral artery of the healthy limb. A 6 French (Fr) introducer
sheath was placed using the Seldinger technique. Under
fluoroscopic control, with a portable vascular C-arm capable of digitally
subtracter angiogram and roadmap angiography, a 0.035 inch
hydrophilic guide wire was crossed aver into the opposite side iliac
artery through a 5F contra angiographic catheter placed at the aortic
bifurcation. After a diagnostic angiography the guide wire was
replaced with an Amplatz 0.0035 inch, 260 cm long, super stiff guide
wire. Then, a 7 9 20 mm Ultra-thinTM SDS balloon catheter was
placed in the external iliac artery and systemic heparinization with
2500 UI was performed. The balloon catheter was inflated and femoral
pulsation ceased immediately. After proximal, endovascular
occlusion, hip disarticulation was accomplished without any hemorrhagic
complication. At the end of procedure, the balloon was deflated
and removed. Hemostasis of the surgical field completed the procedure.
The femoral access in the healthy common femoral artery was
controlled with a 6 Fr Angio-seal percutaneous hemostatic system.
Results and discussion In hip disarticulation, hemostatic tourniquets
cannot be used of the location of the operating field. Therefore,
control of bleeding is a major issue in this procedure. Various techniques
have been proposed, femoral vessels and nerves were attached before the disarticulation. The use of semi-compliant balloon catheters
for endovascular occlusion avoids injury to the endothelium of
the vessel wall during balloon inflation. However preoperative
assessment, with color-duplex scanning and plain abdominal radiographs,
is mandatory; coexisting atherosclerosis often is present
especially in elderly patients, and severe wall calcification can lead to
vessel rupture and retroperitoneal hematoma, or even balloon catheter
rupture. Moreover, color-duplex scanning and radiographs will help
in choosing the landing-zone for balloon inflation.
Conclusions Endovascular balloon assistance is a simple, safe and
effective technique in preventing major arterial bleeding during
amputation or disarticulation and can be routinely used
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Performance comparison between planar and pyramidal microdiffuser for valveless micropump
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.The microdiffuser is the most important component of the valveless micropump and its design plays a role in the valveless micropump performance to direct the flow in a proper direction. A planar microdiffuser valveless micropump has been compared with a pyramidal microdiffuser valveless micropump using 3-D CFD simulations. Both planar and pyramidal microdiffuser has a throat hydraulic diameter of 0.6865mm and diffuser half angle of 6.65 degrees. The dynamic mesh was applied under different actuation frequency of the micropump diaphragm (8, 50, 100, 200, 500, 1000, and 2000Hz). The net flow rate and the rectification efficiency were calculated for the two valveless micropumps. The results showed that the pyramidal microdiffuser performance was better than the planar microdiffuser for frequency f ≥ 200Hz as the net flow rate generated by pyramidal microdiffuser was higher than that by planar microdiffuser. The highest net flow rate of 18.3μL/min was achieved by the pyramidal microdiffuser at rectification efficiency of 0.35% and actuation frequency of 2000Hz
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Experimental Apparatus for the Study of micro Heat Exchangers with Inlet Temperatures between -200 and 200 °C and Elevated Pressures
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 current paper presents a test bench for micro-fabricated Recuperative Counter Flow Heat
Exchanger (RCFHE). The bench is suitable for up to 200 K difference between inlets temperatures and
operating pressures up to 32 MPa. The experimental setup allows controlling the physical state of the gas
(i.e. temperature, pressure and flow rate) at the RCFHE inlets. The bench has 5 controlled parameters and 5
more that are monitored and enables studying each of the hot and cold channels separately. We demonstrate
a steady supply of liquid nitrogen into the device for 10 minutes without thermal insulation of the specimen.
Another run is a steady state experiment with a temperature difference of about 20-30 K between inlets.
These show that the apparatus is capable of characterizing heat exchangers and serve as preliminary results
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Time dependent superhydrophobicity of drag reducing surfaces
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.Air can be trapped on the crevices of specially textured hydrophobic surfaces immersed in water. This heterogenous state of wetting in which the water is in contact with both the solid surface and the entrapped
air is not stable. Diffusion of air into the surrounding water leads to gradual reduction in the size and numbers of the air bubbles. The sustainability of the entrapped air on such surfaces is important for many underwater applications in which the surfaces have to remain submersed for longer time periods. In this paper we explore the suitability of different classes of surface textures towards the drag reduction application by evaluating the time required for the disappearance of the air bubbles under hydrostatic conditions. Different repetitive textures consisting of holes, pillars and ridges of different sizes have been generated in silicon, aluminium and brass by isotropic etching, wire EDM and chemical etching respectively. These surfaces were rendered hydrophobic with self-assembled layer of fluorooctyl trichlorosilane for silicon and aluminium surfaces and 1-dodecanethiol for brass surfaces. Using total internal reflection the air bubbles are visualized with the help of a microscope and time lapse photography. Irrespective of the texture, both the size and the number of air pockets were found to decrease with time gradually and eventually disappear. In an attempt to reverse the diffusion we explore the possibility of using electrolysis to generate gases at the
textured surfaces. The gas bubbles are nucleated everywhere on the surface and as they grow they coalesce with each other and get pinned at the texture edges
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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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Behaviour of the von Willebrand Factor in Blood Flow
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 von Willebrand factor (vWF), a large multimeric protein, is essential in hemostasis. Under
normal conditions, vWF is present in blood as a globular polymer. However, in case of an injury, vWF is able
to unwrap and bind to the vessel wall and to flowing platelets. Thus, platelets are significantly slowed down
and can adhere to the wall and close the lesion. Nevertheless, it is still not clear how the unwrapping of the
vWF is triggered. To better understand these complex processes, we employ a particle-based hydrodynamic
simulation method to study the behaviour of vWF in blood flow. The vWF is modelled as a chain of beads
(monomers) connected by springs. In addition, the monomers are subject to attractive interactions in order to
represent characteristic properties of the vWF. The behaviour of vWF is investigated under different conditions
including a freely-suspended polymer in shear flow and a polymer attached to a wall. We also examine the
migration of vWF to a wall (margination) depending on shear rate and volume fraction of red blood cells
(RBCs). Furthermore, the stretching of the vWF in flow direction depending on its radial position in a capillary
is monitored. Our results show that attractive interactions between monomer beads increase margination
efficiency and significantly affect the extension of vWF at different radial positions in blood vessels
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Aerodynamic behavior of the bridge of a capacitive RF MEMS switch
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 present paper proposes a coupled 3D multi-physics model and presents the results of its
transient simulation, for a RF MEMS capacitive switch of bridge-type. The fluid structure interaction (FSI)
simulation sustains time-varying viscous damping and modified time response of the bridge deflection
compared to the actuation modulation above closing of the switch. Complex 3D geometries of the bridge
were rarely taken into account in viscous damping assessment much less in the simulation of the full flow
around the bridge of the switch. The final goal of the paper is to obtain the dependency of an equivalent
damping coefficient with respect to time, to be used in subsequent reduced order models for the switch, that
include the aerodynamic behaviour of the switch
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