901 publications from this institution
The rotating superconducting windings of large ac machines must be supplied with enough refrigeration at an adequate temperature. The problems associated with bringing liquid helium to the rotating winding structure are analysed. Existing designs as well as newly proposed systems are used to draw the general guidelines for the design concepts of cooling systems for rotating superconducting windings.
No abstract is provided for this article.
TheScience and freedom physics of freedomFreedom is the universal phenomenon of evolutionEvolution, with its many familiar manifestations. This program arches back to scienceScience itself—scienceScience as an evolutionary and self-correcting add-on that...
In this paper the optimization of fluid networks is based on the minimization of pumping power requirement. The total pipe network volume is constrained. It is shown that only in special cases the minimization of pumping power leads to the same architecture as the minimization of pressure drop or flow resistance. Fundamentals of fluid network optimization are developed for both spanning networks and networks where new non-consumer points are added (Gilbert–Steiner points). It is shown that networks with minimum pumping power must not contain loops. The influence of gravity on the optimization of flow configuration is also addressed. The principles developed in the paper are illustrated with an example representing a set of ten vertices to be connected with pipes. The paper provides designers with more effective basic tools for the conceptual design of fluid networks.
The optimum thermodynamic match between two streams at different temperatures is determined by maximizing the power generation (or minimizing the entropy generation) associated solely with the stream-to-stream interaction. Each stream experiences a change of phase. It is shown that the optimum is marked by an optimal ratio between the stream mass flow rates, and an optimal ratio between the two heat exchanger sizes when the total heat transfer area is fixed. The sensitivity of the optimum relative to the various physical parameters of the two-stream arrangement is documented systematically. The study shows that the optimum is “robust” relative to changes in several parameters such as the distribution of heat transfer coefficient along the hot-end heat exchanger, and the model used for the thermodynamic behavior of steam.
This paper describes a comprehensive study of the natural convection phenomenon occurring inside a porous layer with both heat and mass transfer from the side. The natural circulation is driven by a combination of buoyancy effects due to both temperature and concentration variations. The first part of the study consists of an extensive series of numerical simulations conducted in the range 0.01 ⩽ Le ⩽ 100, 50 ⩽ Ra H ⩽ 104, − 5 ⩽ N ⩽ +3 and H/L = 1, where Le, Ra H , N and H/L are the Lewis number, Dareymodified Rayleigh number, buoyancy ratio and geometric aspect ratio of the porous layer. In the second part, the phenomenon is studied based on scale analysis : the chief conclusions of this part, namely, the order-ofmagnitude predictions for the overall heat and mass transfer rates and their respective domains of validity, are shown to be in agreement with the results produced by discrete numerical experiments. Furthermore, the scale analysis is used to sort out the many effects that influence the overall heat and mass transfer results of numerical experiments.
No abstract is provided for this article.
Here we report the heat and fluid flow characteristics of counterflow heat exchangers with tree-shaped line-to-line flow channels. The flow structures of the hot and cold sides are sequences of point-to-line trees that alternate with upside-down trees. The paper shows under what conditions the tree vascularization offers greater heat flow access than corresponding conventional designs with parallel single-scale channels. The analytical part is based on assuming fully developed laminar flow in every channel and negligible longitudinal conduction in the solid. The numerical part consists of simulations of three-dimensional convection coupled with conduction in the solid. It is shown that tree vascularization offers greater heat flow access (smaller global thermal resistance) than parallel channels when the number of pairing levels increases and the available pumping power or pressure drop is specified. When the solid thermal conductivity increases, the heat transfer effectiveness decreases because of the effect of longitudinal heat conduction. The nonuniformity in fluid outlet temperature becomes more pronounced when the number of pairing levels increases and the pumping power (or pressure drop number) increases. The nonuniformity in outlet fluid temperature decreases when the solid thermal conductivity increases.
Technical Briefs The Pressure Melting of Ice Due to an Embedded Cylinder P. A. Tyvand, P. A. Tyvand Department of Agricultural Engineering, Agricultural University of Norway, 1432 A˚s-NLH, Norway Search for other works by this author on: This Site PubMed Google Scholar A. Bejan A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706, USA Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information P. A. Tyvand Department of Agricultural Engineering, Agricultural University of Norway, 1432 A˚s-NLH, Norway A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706, USA J. Heat Transfer. May 1992, 114(2): 532-535 (4 pages) https://doi.org/10.1115/1.2911311 Published Online: May 1, 1992 Article history Received: March 11, 1991 Revised: August 11, 1991 Online: May 23, 2008
Abstract : This project relied on constructal theory to develop novel flow architectures for aircraft thermal management, in particular for the cooling of skins and leading surfaces ofhigh speed aircraft, high-temperature gas turbine blades, etc. The following milestones were reached: 1. The concept of vascular architecture embedded in a wall subjected to intense heating, which showed that tree-shaped channels are more effective than parallel channels oriented across the wall, 2. The concept of bathing a volume with one stream flowing as two trees matched canopy to canopy, which showed that this dendritic architecture is dramatically more effective than parallel channels, 3. The concept of ofcooling a wall with tree-shaped channels that run against the intense heating striking the wall, and 4. The concept of dendritic vascularization of a volume by using one stream. The tree-tree architecture exhibits sharp transitions toward greater complexity as the size of the bathed volume increases. These concepts are essential for future vascular design, and for scaling up to realistic dimensions the results obtained based on small-scale models.
The melting rates due to close-contact heating of a block of phase-change material have been analyzed in the past based on thin-film lubrication theory in several internal and external configurations. The scale analysis of close-contact melting in a region of general shape shows that the melting rate in all configurations is anticipated by the expression in which V is the speed with which the solid advances into the melting front (the melting rate), ϱ/ϱ s is the liquid/solid density ratio, Ste is the Stefan number for liquid superheating (Ste >>1), and α and υ are the viscosity and the t hermal diffusivity of the liquid phase. The film exess pressure scale Δ is defined as the net weight of the object surrounded by liquid divided by the horizontal projected area of that object. The flow length scale of liquid film, ℓ, stands for the diameter of the cylinder or the sphere, or the smaller of the two sides of the rectangular shape of the contact area during melting against a flat heater.
This is a review of the evolution of thermodynamics during the past four decades, from classical (engineering) thermodynamics, to thermodynamic optimizatio
This paper describes the thermodynamic optimization of a class of refrigerators without work input, which are driven by heat transfer from a solar collector. The model consists of a finite-size solar collector with heat loss to the ambient, and a refrigerator with three finite-size heat exchangers, namely, the evaporator between refrigeration load and refrigerant, the condenser between the refrigerant and the ambient, and the heat exchanger between the solar collector and the refrigerant. The total thermal conductance of the three heat exchangers is fixed. The solar collector heat loss to the ambient is proportional to the collector-ambient temperature difference. The first part of the paper reports the operating conditions for maximum refrigeration effect, specifically, the optimal collector temperature, and the optimal way of allocating the thermal conductance inventory to the three heat exchangers. For example, the optimal condenser conductance is equal to half of the total thermal conductance, and is independent of other operating parameters. The second part of the paper examines the changes in the optimal design when the price of the refrigeration load (pL) is different (higher) than the price of the heat input provided by the collector (pH). The optimal collector temperature and the optimal three-way allocation of the thermal conductance inventory are reported as functions of the price ratio pH/pL.
This is a review of several key ideas and pioneers in the founding history of thermodynamics, fluid dynamics and heat transfer. Ideas treated in detail are the mechanical equivalent of heat, the difference between heat transfer and work transfer, the Navier-Stokes equations, natural convection in a fluid and a saturated porous medium, the gas bubble rising in a vertical tube filled with liquid, and fluid friction in duct flow. The review shows that good ideas spread and, at the same time the language and national preferences of the followers play a role in whether the idea creators are remembered or forgotten. The forgetting of the origin of ideas and their authors threatens to become a real problem during the digital era. This danger is exacerbated by the enormous increase in the number of publications most of which are not carefully reviewed or read.
Papers on convection in porous media continue to be published at the rate of over 100 per year. This indication of the continued importance of the subject, together with the wide acceptance of the first edition, has encouraged us to prepare an expanded second edition. We have retained the basic structure and most of the text of the first edition. With space considerations in mind, we have been selective in our choice of references, but nevertheless there are over 600 new references. We also made an effort to highlight new conceptual developments and engineering applications. In the introductory material, we judged that Chapters 2 and 3 needed little alteration (though there is a new Section 2.6 on other approaches to the topic), but our improved understanding of the basic modeling of flow through a porous medium has led to a number of changes in Chapter 1, both within the old sections and by the addition of a section on turbulence in porous media, and a section on fractured media, deformable media, and complex porous structures. In Chapter 4, on forced convection, we have added major new sections on compact heat exchangers, on heatlines for visualizing convection, and on constructal tree networks for the geometric minimization of the resistance to volume-to-point flows in heterogeneous porous media.
No abstract is provided for this article.
: The objective of this research effort is to construct a purely theoretical foundation for the phenomenon of turbulent heat transfer. In this first annual report, the current methodology of theoretical research is reexamined critically. The report outlines in three self-standing units (papers) the new theoretical viewpoint that turbulence is the result of inviscid flow buckling. This viewpoint leads to first examples of theoretical prediction of turbulence parameters, for instance, the transition to turbulence, the flapping motion of turbulent jets, the frequency of vortex shedding and the meandering and mixing mechanism of thermal plumes. (Author)
Constructal theory is the view that (i) the generation of images of design (pattern, rhythm) in nature is a phenomenon of physics and (ii) this phenomenon is covered by a principle (the constructal law): ‘for a finite-size flow system to persist in time (to live) it must evolve such that it provides greater and greater access to the currents that flow through it’. This law is about the necessity of design to occur, and about the time direction of the phenomenon: the tape of the design evolution ‘movie’ runs such that existing configurations are replaced by globally easier flowing configurations. The constructal law has two useful sides: the prediction of natural phenomena and the strategic engineering of novel architectures, based on the constructal law, i.e. not by mimicking nature. We show that the emergence of scaling laws in inanimate (geophysical) flow systems is the same phenomenon as the emergence of allometric laws in animate (biological) flow systems. Examples are lung design, animal locomotion, vegetation, river basins, turbulent flow structure, self-lubrication and natural multi-scale porous media. This article outlines the place of the constructal law as a self-standing law in physics, which covers all the ad hoc (and contradictory) statements of optimality such as minimum entropy generation, maximum entropy generation, minimum flow resistance, maximum flow resistance, minimum time, minimum weight, uniform maximum stresses and characteristic organ sizes. Nature is configured to flow and move as a conglomerate of ‘engine and brake’ designs.
