42,264 research outputs found
Why do axons differ in caliber?
CNS axons differ in diameter (d) by nearly 100-fold (∼0.1-10 μm); therefore, they differ in cross-sectional area (d(2)) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d(2). We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ∼1 to >100 Hz; (3) a tract's distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises ≥d(2). To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axon's volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate
Expanded microchannel heat exchanger: design, fabrication and preliminary experimental test
This paper first reviews non-traditional heat exchanger geometry, laser
welding, practical issues with microchannel heat exchangers, and high
effectiveness heat exchangers. Existing microchannel heat exchangers have low
material costs, but high manufacturing costs. This paper presents a new
expanded microchannel heat exchanger design and accompanying continuous
manufacturing technique for potential low-cost production. Polymer heat
exchangers have the potential for high effectiveness. The paper discusses one
possible joining method - a new type of laser welding named "forward conduction
welding," used to fabricate the prototype. The expanded heat exchanger has the
potential to have counter-flow, cross-flow, or parallel-flow configurations, be
used for all types of fluids, and be made of polymers, metals, or
polymer-ceramic precursors. The cost and ineffectiveness reduction may be an
order of magnitude or more, saving a large fraction of primary energy. The
measured effectiveness of the prototype with 28 micron thick black low density
polyethylene walls and counterflow, water-to-water heat transfer in 2 mm
channels was 72%, but multiple low-cost stages could realize the potential of
higher effectiveness
Full Hydrodynamic Simulation of GaAs MESFETs
A finite difference upwind discretization scheme in two dimensions is
presented in detail for the transient simulation of the highly coupled
non-linear partial differential equations of the full hydrodynamic model,
providing thereby a practical engineering tool for improved charge carrier
transport simulations at high electric fields and frequencies. The
discretization scheme preserves the conservation and transportive properties of
the equations. The hydrodynamic model is able to describe inertia effects which
play an increasing role in different fields of micro- and optoelectronics,
where simplified charge transport models like the drift-diffusion model and the
energy balance model are no longer applicable. Results of extensive numerical
simulations are shown for a two-dimensional MESFET device. A comparison of the
hydrodynamic model to the commonly used energy balance model is given and the
accuracy of the results is discussed.Comment: 18 pages, LATE
Exergy optimization in a steady moving bed heat exchanger
Proceedings of: Interdisciplinary Transport Phenomena V: Fluid, Thermal, Biological, Materials and Space Sciences (ITP 2007), 14-19 of October, 2007, Bansko, Bulgaria (Oral paper nº 70)This work provides an exergy analysis of a moving bed heat
exchanger to obtain for a range of incoming fluid flow rates the
operational optimum and the incidence on it of the relevant
parameters such as the dimensions of the exchanger, the
particle diameter and the flow rate of the fluid. The MBHE
proposed can be analyzed as a cross flow heat exchanger where
one of the phases is a moving granular medium. In the present
work the exergy analysis of the MBHE is carried out over
operation data of the exchanger obtained in two ways: a
numerical simulation of the steady state problem and the
analytical solution of the simplified (avoiding conduction
terms) equations. The numerical simulation is carried over the
two steady energy equations (fluid and solid), involving for the
solid the convection heat transfer to the fluid and the diffusion
term in both directions, and for the fluid only the convection
heat transfer to the solid. The analytical solution is the wellknown
solution of the simplified problem neglecting
conduction effects.Publicad
Coupling of energy conversion systems and wellbore heat exchanger in a very deep oil well
The conventional geothermal power plants use the reinjection wells mostly to avoid the depletion of the geothermal reservoir gathering in the underground of the produced brine. Nevertheless, reinjection operations entail high economic costs and some risks. An alternative is the extraction of the heat without geothermal fluids production, the wellbore heat exchanger. The goal of the present paper is the analysis of the power production of the wellbore heat exchanger (WBHX) in time and the comparison between two different conversion systems of the thermal energy into electrical: the organic ranking cycle (ORC) plant and the Stirling motor. The selected case study is the oil field of Villafortuna Trecate, a medium enthalpy geothermal resource. The simulation results show a substantial decrease of the wellhead temperature in the first 6 months. After 1 year, the thermal power extracted with the WBHX is greater than 1.3 MW. The
design parameters are 20 m3/h for the flow rate, outlet temperature 100.38 °C and the inlet temperature is 40 °C. The R-C318 has been selected as working fluid in the ORC plant: the net electrical power is 121 kW. The air is the working fluid in the Stirling motor: the evaluated net electrical power is 152 kW. The Stirling engine has an efficiency greater than 41 % compared to a system ORC
Evolution and Modern Approaches for Thermal Analysis of Electrical Machines
In this paper, the authors present an extended survey on the evolution and the modern approaches in the thermal analysis of electrical machines. The improvements and the new techniques proposed in the last decade are analyzed in depth and compared in order to highlight the qualities and defects of each. In particular, thermal analysis based on lumped-parameter thermal network, finite-element analysis, and computational fluid dynamics are considered in this paper. In addition, an overview of the problems linked to the thermal parameter determination and computation is proposed and discussed. Taking into account the aims of this paper, a detailed list of books and papers is reported in the references to help researchers interested in these topics
Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation
Among the many additive manufacturing (AM) processes for metallic materials,
selective laser melting (SLM) is arguably the most versatile in terms of its
potential to realize complex geometries along with tailored microstructure.
However, the complexity of the SLM process, and the need for predictive
relation of powder and process parameters to the part properties, demands
further development of computational and experimental methods. This review
addresses the fundamental physical phenomena of SLM, with a special emphasis on
the associated thermal behavior. Simulation and experimental methods are
discussed according to three primary categories. First, macroscopic approaches
aim to answer questions at the component level and consider for example the
determination of residual stresses or dimensional distortion effects prevalent
in SLM. Second, mesoscopic approaches focus on the detection of defects such as
excessive surface roughness, residual porosity or inclusions that occur at the
mesoscopic length scale of individual powder particles. Third, microscopic
approaches investigate the metallurgical microstructure evolution resulting
from the high temperature gradients and extreme heating and cooling rates
induced by the SLM process. Consideration of physical phenomena on all of these
three length scales is mandatory to establish the understanding needed to
realize high part quality in many applications, and to fully exploit the
potential of SLM and related metal AM processes
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