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Study of a micro-structured PHE for the thermal management of a fuel cell
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.In the constantly growing market of fuel cells, the heat management of the system is a crucial area of research, since it affects the efficiency, operability and lifetime of the fuel cell. The CFD simulations performed for a plate heat exchanger (PHE) with flat plates clearly demonstrate temperature non-uniformity across the membrane of the unit cell. The CFD code was successfully validated with experimental data acquired from a setup that reproduces the geometry and simulates the thermal behaviour of a typical unit PEMFC. Additionally, the performance of a novel PHE, with micro-structured corrugations on its plates, was studied with the previously validated CFD code. The results clearly show that the proposed plate modifications can increase temperature uniformity across the membrane more than 20% compared to that of the flat plate
DSMC investigation of rarefied gas flow through diverging micro- and nanochannels
Direct simulation Monte Carlo (DSMC) method with simplified Bernoulli-trials
(SBT) collision scheme has been used to study the rarefied pressure-driven
nitrogen flow through diverging microchannels. The fluid behaviours flowing
between two plates with different divergence angles ranging between 0
to 17 are described at different pressure ratios
(1.52.5) and Knudsen numbers (0.03Kn12.7). The
primary flow field properties, including pressure, velocity, and temperature,
are presented for divergent microchannels and are compared with those of a
microchannel with a uniform cross-section. The variations of the flow field
properties in divergent microchannels, which are influenced by the area change,
the channel pressure ratio and the rarefication are discussed. The results show
no flow separation in divergent microchannels for all the range of simulation
parameters studied in the present work. It has been found that a divergent
channel can carry higher amounts of mass in comparison with an equivalent
straight channel geometry. A correlation between the mass flow rate through
microchannels, the divergence angle, the pressure ratio, and the Knudsen number
has been suggested. The present numerical findings prove the occurrence of
Knudsen minimum phenomenon in micro- and Nano- channels with non-uniform
cross-sections.Comment: Accepted manuscript; 25 Pages and 11 Figures; "Microfluidics and
Nanofluidics
Structure and entrainment in the plane of symmetry of a turbulent spot
Laser-Doppler velocity measurements in water are reported for the flow in the plane of symmetry of a turbulent spot. The unsteady mean flow, defined as an ensemble average, is fitted to a conical growth law by using data at three streamwise stations to determine the virtual origin in x and t. The two-dimensional unsteady stream function is expressed as ψ=U^2_∞tg(ξ,η) in conical similarity co-ordinates ζ = x/U_∞t and η = y/U_∞t. In these co-ordinates, the equations for the unsteady particle displacements reduce to an autonomous system. This system is integrated graphically to obtain particle trajectories in invariant form. Strong entrainment is found to occur along the outer part of the rear interface and also in front of the spot near the wall. The outer part of the forward interface is passive. In terms of particle trajectories in conical co-ordinates, the main vortex in the spot appears as a stable focus with celerity 0·77U_∞. A second stable focus with celerity 0·64U_∞ also appears near the wall at the rear of the spot.
Some results obtained by flow visualization with a dense, nearly opaque suspension of aluminium flakes are also reported. Photographs of the sublayer flow viewed through a glass wall show the expected longitudinal streaks. These are tentatively interpreted as longitudinal vortices caused by an instability of Taylor-Görtler type in the sublayer
Mass transfer, light pulsing and hydrodynamic stress effects in photobioreactor development
Photobioreactor scalability involves multiple different interacting aspects including mass transfer, light pulsing
and hydrodynamic stress. An efficient carbon dioxide supply and a frequent displacement of cells from poorly
to highly illuminated zones is desired to maximise the achieveable specific growth rate. However, a strong
mixing is energy consuming and may reduce the specific growth rate because of induced cell damage.
The current work examines mass transfer effects in photobioreactor development and estimates their
relationship to light pulsing and hydrodynamic stress effects with a special reference to the novel inclined, thinlayer,
wavy-bottomed cascading photobioreactor
Methodology for tidal turbine representation in ocean circulation model
The present method proposes the use and adaptation of ocean circulation models as an assessment tool framework for tidal current turbine (TCT) array layout optimization. By adapting both momentum and turbulence transport equations of an existing model, the present TCT representation method is proposed to extend the actuator disc concept to 3-D large-scale ocean circulation models. Through the reproduction of experimental flume tests and grid dependency tests, this method has shown its numerical coherence as well as its ability to simulate accurately both momentum and turbulent turbine-induced perturbations in both near and far wakes in a relatively short period of computation time. Consequently the present TCT representation method is a very promising basis for the development of a TCT array layout optimization tool
Design, Construction, Operation and Performance of a Hadron Blind Detector for the PHENIX Experiment
A Hadron Blind Detector (HBD) has been developed, constructed and
successfully operated within the PHENIX detector at RHIC. The HBD is a
Cherenkov detector operated with pure CF4. It has a 50 cm long radiator
directly coupled in a window- less configuration to a readout element
consisting of a triple GEM stack, with a CsI photocathode evaporated on the top
surface of the top GEM and pad readout at the bottom of the stack. This paper
gives a comprehensive account of the construction, operation and in-beam
performance of the detector.Comment: 51 pages, 39 Figures, submitted to Nuclear Instruments and Method
On turbulent friction in straight ducts with complex cross section: the wall law and the hydraulic diameter
We develop predictive formulas for friction resistance in ducts with complex
cross-sectional shape based on the use of the log law and neglect of wall shear
stress nonuniformities. The traditional hydraulic diameter naturally emerges
from the analysis as the controlling length scale for common duct shapes as
triangles and regular polygons. The analysis also suggests that a new effective
diameter should be used in more general cases, yielding corrections of a few
percent to friction estimates based on the traditional hydraulic diameter. Fair
but consistent predictive improvement is shown for duct geometries of practical
relevance, including rectangular and annular ducts, and circular rod bundles
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