4,399 research outputs found
Reactant pressure differential control for fuel cell gases
A pair of spool valves are described which are balanced between pressures of reactant gases supplied to a fuel cell power plant. The pressure differences are controlled between the gases so as to maintain those pressures substantially in the proportions necessary for operation of the fuel cell
Turbulence and secondary motions in square duct flow
We study turbulent flows in pressure-driven ducts with square cross-section
through direct numerical simulation in a wide enough range of Reynolds number
to reach flow conditions which are representative of fully developed
turbulence. Numerical simulations are carried out over extremely long
integration times to get adequate convergence of the flow statistics, and
specifically high-fidelity representation of the secondary motions which arise.
The intensity of the latter is found to be in the order of 1-2% of the bulk
velocity, and unaffected by Reynolds number variations. The smallness of the
mean convection terms in the streamwise vorticity equation points to a simple
characterization of the secondary flows, which in the asymptotic high-Re regime
are found to be approximated with good accuracy by eigenfunctions of the
Laplace operator. Despite their effect of redistributing the wall shear stress
along the duct perimeter, we find that secondary motions do not have large
influence on the mean velocity field, which can be characterized with good
accuracy as that resulting from the concurrent effect of four independent flat
walls, each controlling a quarter of the flow domain. As a consequence, we find
that parametrizations based on the hydraulic diameter concept, and
modifications thereof, are successful in predicting the duct friction
coefficient
Temperature and finite-size effects in collective modes of superfluid Fermi gases
We study the effects of superfluidity on the monopole and quadrupole
collective excitations of a dilute ultra-cold Fermi gas with an attractive
interatomic interaction. The system is treated fully microscopically within the
Bogoliubov-de Gennes and quasiparticle random-phase approximation methods. The
dependence on the temperature and on the trap frequency is analyzed and
systematic comparisons with the corresponding hydrodynamic predictions are
presented in order to study the limits of validity of the semiclassical
approach.Comment: 9 pages, 4 figure
Materials for Teaching Lean Accounting
Lean production and lean management practices are becoming more widespread, and conventional accounting methods may be incompatible with lean management. Lean accounting methods and performance measures have been developed that support lean management, but these methods receive very little coverage in cost and managerial accounting courses. One reason for the lack of lean accounting coverage is a lack of availability of classroom materials for teaching lean accounting. The Lean Accounting Content Depository hosted by the American Accounting Association has been created to address the lack of lean accounting teaching materials. The material available from the Lean Accounting Content Repository is described. Suggestions for incrementally increasing coverage of lean accounting in cost and managerial courses are presented, and a semester long course devoted entirely to lean accounting is outlined. Selected lean accounting materials suitable for classroom use available from other sources are described
Study of fuel cell powerplant with heat recovery
It was shown that heat can be recovered from fuel cell power plants by replacing the air-cooled heat exchangers in present designs with units which transfer the heat to the integrated utility system. Energy availability for a 40-kW power plant was studied and showed that the total usable energy at rated power represents 84 percent of the fuel lower heating value. The effects of design variables on heat availability proved to be small. Design requirements were established for the heat recovery heat exchangers, including measurement of the characteristics of two candidate fuel cell coolants after exposure to fuel cell operating conditions. A heat exchanger test program was defined to assess fouling and other characteristics of fuel cell heat exchangers needed to confirm heat exchanger designs for heat recovery
Public crowdsensing of heat waves by social media data
Abstract. Investigating on society-related heat wave hazards is a global issue concerning the people health. In the last two decades, Europe experienced several severe heat wave episodes with catastrophic effects in term of human mortality (2003, 2010 and 2015). Recent climate investigations confirm that this threat will represent a key issue for the resiliency of urban communities in next decades. Several important mitigation actions (Heat-Health Action Plans) against heat hazards have been already implemented in some WHO (World Health Organization) European region member states to encourage preparedness and response to extreme heat events. Nowadays, social media (SM) offer new opportunities to indirectly measure the impact of heat waves on society. Using the crowdsensing concept, a micro-blogging platform like Twitter may be used as a distributed network of mobile sensors that react to external events by exchanging messages (tweets). This work presents a preliminary analysis of tweets related to heat waves that occurred in Italy in summer 2015. Using TwitterVigilance dashboard, developed by the University of Florence, a sample of tweets related to heat conditions was retrieved, stored and analyzed for main features. Significant associations between the daily increase in tweets and extreme temperatures were presented. The daily volume of Twitter users and messages revealed to be a valuable indicator of heat wave impact at the local level, in urban areas. Furthermore, with the help of Generalized Additive Model (GAM), the volume of tweets in certain locations has been used to estimate thresholds of local discomfort conditions. These city-specific thresholds are the result of dissimilar climatic conditions and risk cultures
Smartphone-based photogrammetry for the 3D modeling of a geomorphological structure
The geomatic survey in the speleological field is one of the main activities that allows for the adding of both a scientific and popular value to cave exploration, and it is of fundamental importance for a detailed knowledge of the hypogean cavity. Today, the available instruments, such as laser scanners and metric cameras, allow us to quickly acquire data and obtain accurate three-dimensional models, but they are still expensive, require a careful planning phase of the survey, as well as some operator experience for their management. This work analyzes the performance of a smartphone device for a close-range photogrammetry approach for the extraction of accurate three-dimensional information of an underground cave. The image datasets that were acquired with a high-end smartphone were processed using the Structure from Motion (SfM)-based approach for dense point cloud generation: different image-matching algorithms implemented in a commercial and an open source software and in a smartphone application were tested. In order to assess the reachable accuracy of the proposed procedure, the achieved results were compared with a reference dense point cloud obtained with a professional camera or a terrestrial laser scanner. The approach has shown a good performance in terms of geometrical accuracies, computational time and applicability
Second-order equation of state with the full Skyrme interaction: toward new effective interactions for beyond mean-field models
In a quantum Fermi system the energy per particle calculated at the second
order beyond the mean-field approximation diverges if a zero-range interaction
is employed. We have previously analyzed this problem in symmetric nuclear
matter by using a simplified nuclear Skyrme interaction, and proposed a
strategy to treat such a divergence. In the present work, we extend the same
strategy to the case of the full nuclear Skyrme interaction. Moreover we show
that, in spite of the strong divergence ( , where is
the momentum cutoff) related to the velocity-dependent terms of the
interaction, the adopted cutoff regularization can be always simultaneously
performed for both symmetric and nuclear matter with different
neutron-to-proton ratio. This paves the way to applications to finite nuclei.Comment: 15 figure
A New Tensile Test for Aluminum Alloys in the Mushy State: Experimental Method and Numerical Modeling
A fairly simple experimental setup has been designed for testing the resistance of the mushy zone of alloys during solidification under tensile conditions. It has been used to study the effect of coalescence among the solid grains at a late stage of solidification. The experimental approach involves both tensile-strength measurements and scanning electron microscope (SEM) observations of fracture surfaces. Complementary information can be obtained by numerical modeling of this solidification process. The latter takes into account heat flow in the sample, rheology of the mushy alloy, liquid feeding, and porosity formation. All of the available information indicates that the transition from a granular mushy alloy to a coalesced solid-skeleton behavior starts for a solid fraction of approximately 92pc
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