1,683 research outputs found
Thermodynamic optimization of global circulation and climate
The constructal law of generation of flow structure is used to predict the main features of global circulation
and climate. The flow structure is the atmospheric and oceanic circulation. This feature is modelled as
convection loops, and added to the earth model as a heat engine heated by the Sun and cooled by the
background. It is shown that the dissipation of the power produced by the earth engine can be maximized
by selecting the proper balance between the hot and cold zones of the Earth, and by optimizing the thermal
conductance of the circulation loops. The optimized features agree with the main characteristics of global
circulation and climate. The robustness of these predictions, and the place of the constructal law as a selfstanding
principle in thermodynamics, are discussed
Constructal theory of global circulation and climate
The constructal law states that every flow system evolves in time so that it develops the flow architecture that maximizes flow access
under the constraints posed to the flow. Earlier applications of the constructal law recommended it as a self-standing law that is distinct
from the second law of thermodynamics. In this paper, we develop a model of heat transport on the earth surface that accounts for the
solar and terrestrial radiation as the heat source and heat sink and with natural convection loops as the transport mechanism. In the first
part of the paper, the constructal law is invoked to optimize the latitude of the boundary between the Hadley and the Ferrel cells, and the
boundary between the Ferrel and the Polar cells. The average temperature of the earth surface, the convective conductance in the horizontal
direction as well as other parameters defining the latitudinal circulation also match the observed values. In the second part of the
paper, the constructal law is invoked in the analysis of atmospheric circulation at the diurnal scale. Here the heat transport is optimized
against the Ekman number. Even though this second optimization is based on very different variables than in the first part of the paper, it
produces practically the same results for the earth surface temperature and the other variables. The earth averaged temperature difference
between day and night was found to be approximately 7 K, which matches the observed value. The accumulation of coincidences
between theoretical predictions and natural flow configuration adds weight to the claim that the constructal law is a law of nature
Optimal behavior of viscoelastic flow at resonant frequencies
The global entropy generation rate in the zero-mean oscillatory flow of a
Maxwell fluid in a pipe is analyzed with the aim at determining its behavior at
resonant flow conditions. This quantity is calculated explicitly using the
analytic expression for the velocity field and assuming isothermal conditions.
The global entropy generation rate shows well-defined peaks at the resonant
frequencies where the flow displays maximum velocities. It was found that
resonant frequencies can be considered optimal in the sense that they maximize
the power transmitted to the pulsating flow at the expense of maximum
dissipation.Comment: Paper accepted to be published in Phys. Rev.
Architecture of optimal transport networks
We analyze the structure of networks minimizing the global resistance to flow
(or dissipated energy) with respect to two different constraints: fixed total
channel volume and fixed total channel surface area. First, we determine the
shape of channels in such optimal networks and show that they must be straight
with uniform cross-sectional areas. Then, we establish a relation between the
cross-sectional areas of adjoining channels at each junction. Indeed, this
relation is a generalization of Murray's law, originally established in the
context of local optimization. Moreover, we establish a relation between angles
and cross-sectional areas of adjoining channels at each junction, which can be
represented as a vectorial force balance equation, where the force weight
depends on the channel cross-sectional area. A scaling law between the minimal
resistance value and the total volume or surface area value is also derived
from the analysis. Furthermore, we show that no more than three or four
channels meet in one junction of optimal bi-dimensional networks, depending on
the flow profile (e.g.: Poiseuille-like or plug-like) and the considered
constraint (fixed volume or surface area). In particular, we show that sources
are directly connected to wells, without intermediate junctions, for minimal
resistance networks preserving the total channel volume in case of plug flow
regime. Finally, all these results are illustrated with a simple example, and
compared with the structure of natural networks
Climate change, in the framework of the Constructal Law
Here we present a simple and transparent alternative to the complex models
of earth’s thermal behaviour under time-changing conditions. We show the one-to-one relationship between changes in atmospheric properties and time-dependent changes in temperature and its distribution on earth. The model accounts for convection and radiation, thermal inertia and changes in albedo (ρ) and greenhouse factor (γ). The
constructal law is used as the principle that governs the evolution of fl ow confi guration on earth. The model showed that for two time-dependent scenarios, (δρ = 0.002;
δγ = 0.011) and (δρ = 0.002; δγ = 0.005) the predicted equatorial and polar temperature
increases and the time scales are (ΔTH = 1.16 K; ΔTL = 1.11 K; 104 years) and (0.41 K;0.41 K; 57 years), respectively. A continuous model of temperature variation was used to predict the thermal response of the Earth’s surface to changes bounded by δρ = δγ and δρ = −δγ. The poleward heat current reaches its maximum in the vicinity of 35° latitude,accounting for the position of the Ferrel cell between the Hadley and Polar Cells
Efficiency at maximum power of low dissipation Carnot engines
We study the efficiency at maximum power, , of engines performing
finite-time Carnot cycles between a hot and a cold reservoir at temperatures
and , respectively. For engines reaching Carnot efficiency
in the reversible limit (long cycle time, zero dissipation),
we find in the limit of low dissipation that is bounded from above by
and from below by . These bounds are reached when
the ratio of the dissipation during the cold and hot isothermal phases tend
respectively to zero or infinity. For symmetric dissipation (ratio one) the
Curzon-Ahlborn efficiency is recovered.Comment: 4 pages, 1 figure, 1 tabl
Constructal blade shape in nanofluids
Blade configuration of nanofluids has been proven to perform much better than dispersed configuration for some heat conduction systems. The analytical analysis and numerical calculation are made for the cylinder--shaped and regular-rectangular-prism--shaped building blocks of the blade-configured heat conduction systems (using nanofluids as the heat conduction media) to find the optimal cross-sectional shape for the nanoparticle blade under the same composing materials, composition ratio, volumetric heat generation rate, and total building block volume. The regular-triangular-prism--shaped blade has been proven to perform better than all the other three kinds of blades, namely, the regular-rectangular-prism--shaped blade, the regular-hexagonal-prism--shaped blade, and the cylinder--shaped blade. Thus, the regular-triangular-prism--shaped blade is selected as the optimally shaped blade for the two kinds of building blocks that are considered in this study. It is also proven that the constructal cylinder--regular-triangular-prism building block performs better than the constructal regular-rectangular-prism--regular-triangular-prism building block
Single-phase laminar flow heat transfer from confined electron beam enhanced surfaces
An experimental investigation of the thermal-hydraulic characteristics for single-phase flow through three electron beam enhanced structures was conducted with water at mass flow rates from 0.005 kg/s to 0.045 kg/s. The structures featured copper heat transfer surfaces, approximately 28 mm wide and 32 mm long in the flow direction, with complex three-dimensional (3D) electron beam manufactured pyramid-like structures. The channel height varied depending on the height of the protrusions and the tip clearance was maintained at 0.1-0.3 mm. The average protrusion densities for the three samples S1, S2, and S3 were 13, 11, and 25 per cm2 with protrusion heights of 2.5, 2.8, and 1.6 mm, respectively. The data gathered were compared to those for a smooth channel surface operating under similar conditions. The results show an increase up to approximately three times for the average Nusselt number compared with the smooth surface. This is attributed to the surface irregularities of the enhanced surfaces, which not only increase the heat transfer area but also improve mixing, disturb the thermal and velocity boundary layers, and reduce thermal resistance. The increase in heat transfer with the enhanced surfaces was accompanied by an increase of pressure drop, which has to be considered in design.The authors would like to acknowledge Dr Anita Buxton and Dr Bruce Dance of TWI for their contribution to this project and also EPSRC and TSB for funding the EngD programme and sponsoring the ASTIA collaborative research project that helped to develop the Electron Beam enhanced surfaces respectively
Local energy balance, specific heats and the Oberbeck-Boussinesq approximation
A thermodynamic argument is proposed in order to discuss the most appropriate
form of the local energy balance equation within the Oberbeck-Boussinesq
approximation. The study is devoted to establish the correct thermodynamic
property to be used in order to express the relationship between the change of
internal energy and the temperature change. It is noted that, if the fluid is a
perfect gas, this property must be identified with the specific heat at
constant volume. If the fluid is a liquid, a definitely reliable approximation
identifies this thermodynamic property with the specific heat at constant
pressure. No explicit pressure work term must be present in the energy balance.
The reasoning is extended to the case of fluid saturated porous media.Comment: 14 pages, 2 figures, 1 table, submitted for publicatio
Lifetime Adherence to Physical Activity Recommendations and Fall Occurrence in Community-dwelling Older Adults: a Retrospective Cohort Study
Falling is a major health concern for community-dwelling older adults. Regular physical activity has been proposed to prevent falls. The aim of this study was to assess whether the achievement of the 2004 UK Department of Health physical activity recommendations over a lifetime had a protective effect against falling in older people. 313 community-dwelling older adults completed a questionnaire about lifetime physical activity and fall occurrence. There were significantly fewer falls in those who had led an active lifestyle compared to those who had not (χ2Yates=4.568, p=0.033), with a lower relative risk of fall occurrence for the active respondents (RR=0.671) compared to the inactive (RR=1.210). Of those who were sufficiently active in their early adulthood, the decade where there was the biggest decrease in remaining active enough was in the 60s. It is concluded that an active lifestyle may have decreased the likelihood of having a fall in older ag
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