The present paper is a review of a new conceptual design for the maximization of heat transfer density (i.e., heat transfer rate per unit of volume) in channels installed in a fixed volume. The volume is filled optimally with parallel equidistant heated blades. The optimal spacing between two blades is such that the thermal boundary layers merge at the end of the channel. The blades can be cooled either by laminar natural or forced convection. Unheated volumes of fluid near the tips of the boundary layers are eliminated through the insertion of new generations of smaller blades. Based on the same principle, new generation of even smaller blades are added stepwise to the multi-scale structure. The optimal length of each family of new blades is determined based on the assumption that the flow downstream the smallest plates is not disturbed. This allows us to predict the exact height where the thermal boundary layer of two distinct generations will merge. As the number length of scales increase, the volume-averaged heat transfer density increases. The results show that the improvement associated with optimal insertion of the first and second generations of scales is significant. Diminishing returns are also observed as the complexity increases, meaning that the contribution of each smaller scale is less important than the contribution of the preceding scale. The theoretical results (i.e., the optimal spacings, optimal lengths, maximal heat transfer density and the cutoffs) are validated numerically.
Natural porous structures are heterogeneous with multiple scales that are distributed nonuniformly. Few large pores (fissures, channels, and cracks) are accompanied by numerous finer channels. Can this type of flow architecture be attributed to a principle of maximization of global flow access? Features similar to those of multiscale porous structures are exhibited by tree-shaped flow structures. Trees have been deduced from the maximization of flow access between a point and a volume, a point and an area, and a point and a curve (e.g., circle). In this paper we invoke the same principle and consider fundamentally the question of how to bathe with minimal flow resistance a microchannel structure that globally behaves as a porous medium. We develop completely multiscale configurations that guide the flow from one side of the porous structure to the other (line to line and plane to plane) and show analytically the advantages of tree structures over the usual stacks of parallel microchannels. The “porous medium” that has tree-shaped labyrinths is heterogeneous, with multiple scales that are distributed nonuniformly. These features justify comparisons with the design of natural porous structures.
No abstract is provided for this article.
This paper presents a method using which a set of reading and writing agents concurrently read and update the global state of the network. In addition to the consistency of the snapshot and the reset states, our protocol preserves the atomicity of the reads and...
This paper describes the water film that is formed as ice melts under a two-dimensional solid slider. The analysis is based on the contact melting theory, and accounts for the coupling between the heat transfer across the water film and the fluid mechanics of the film. The competition between pressure melting and frictional melting in the creation of the film, and the effect of slider thermal boundary conditions (isothermal vs adiabatic) are documented. The dimensionless groups that govern the film behavior are identified. The film thickness increases in the downstream direction. The water leaves the relative-motion area through the front opening and through the trailing opening. The slenderness ratio (thickness/length) of the water film depends on water properties and contact length, and is fairly insensitive to changes in the applied normal force. It is shown that if the presence of ice asperities is taken into account, the contact melting theory may anticipate the trend and order of magnitude of the coefficient of friction determined experimentally.
In December 1968 the Edison Electric Institute authorized support of a research project on superconductors in large synchronous machines.* The purposes of the program are to demonstrate the practicality of utilizing superconductors for the field windings in large...
The objective of the Journal of Mechanics is to provide an international forum to foster exchange of ideas among mechanics communities in different parts of world.The Journal of Mechanics publishes original research in all fields of theoretical and applied mechanics.The Journal especially welcomes papers that are related to recent technological advances, such as micro/nanomechanics, medical and biological systems, and microscale heat transfer.The contributions, which may be analytical, experimental or numerical, should be of significance to the progress of mechanics.
This article examines the buoyancy induced circulation occurring on both sides of a vertical impermeable partition separating two semi-infinite porous reservoirs maintained at different temperatures. The circulation is found to consist of two counterflowing boundary layers which interact thermally across the partition, transferring heat from the hot side to the cold side. The net heat transfer rate is calculated and the effect of the thickness and conductivity of the partition on the heat transfer rate is determined. It is demonstrated that the insertion of a vertical impermeable partition in the middle of a vertical porous layer reduces significantly the net heat transfer rate through the layer.
Here we show that the morphology of a conducting wall can be ‘evolved’ by design so that its ability to enhance thermal contact with the ambient is maximized. Free to morph is the entire configuration of the conducting volume of the wall: the wall thickness and the population of fins planted on the wall are free to vary. The evolution of the design is pursued systematically. First, the thickness of the base layer is free to morph, and the population of fins is uniform (identical fins are spread equidistantly). Second, the base thickness and the fin-to-fin spacings are free to morph, while the fins are identical. Third, the base thickness and the fin sizes are free to morph, while the fins are distributed equidistantly. The emerging rules for distributing the wall thickness and the fin size and density are reported. The optimal allocation of the conducting material to the longitudinally conducting wall and the finned structure is also reported.
The paper summarizes results from an experimental study of buoyancy-induced motion and heat transfer in a horizontal rectangular cavity with the two vertical ends at different temperatures and the long horizontal walls adiabatic. The cavity height/length ratio is A = 0·0625. The high-Rayleigh-number range reported on in this paper, 2 × 10 8 < Ra < 2 × 10 9 , has not been studied before. It is shown that, contrary to lower-Rayleigh-number behaviour known previously, the core flow structure is non-parallel and is dominated by horizontal intrusions flowing along each of the two insulated horizontal walls of the enclosure. The fluid embraced by the two horizontal jets is practically stagnant and thermally stratified. Flow visualization experiments suggest that adjacent to the two horizontal jets two secondary flat cells are formed by the baroclinic pressure field in an analogous way to what is observed in intrusions in a stratified fluid. Nusselt-number-Rayleigh-number results for the overall end-to-end heat transfer in the horizontal direction are reported and compared with previous experimental and theoretical results available for lower Rayleigh numbers. It is shown also that the transition from a parallel core structure to one dominated by intrusion layers is governed by the parameter Ra ½ A , with Ra ¼ A < 1 as necessary condition for a parallel core flow.
PhysicsInequality impresses all of us with its lawsLaw, which are few, precise, and universally applicable, valid for any imaginable situation and flowFlow system. Can economics be a more precise and predictive scienceScience, like physics? This chapter unveils the...
Here, we document the diversity of commercial aircraft models and bodies in use during the past five decades. Special emphasis is on the models that have moved humanity across the globe during the past three decades. The first objective is to show that the apparent diversity is in fact underpinned (sustained) by organization, which is a distinct hierarchy of “few large and many small” coexisting and moving people harmoniously everywhere. The second objective is to rely on the emerging hierarchy in order to predict for the future how few the even bigger models will be and how more numerous the even smaller models (e.g., drones for package delivery) will be, naturally.
In this study, we describe the constructal-theory search for the geometry of a solar chimney. The objective is to increase the power production over the area occupied by the plant. The ratio height/radius, maximum mass flow rate and maximum power under the constraints of a fixed area and volume are determined. We find that the power generated per unit of land area is proportional to the length scale of the power plant. The analysis is validated by a detailed mathematical model. Pressure losses are reported in terms of the dimensionless length scale of the system, and are illustrated graphically. They indicate that the pressure drop at the collector inlet and at the transition section between the collector and chimney are negligible, and the friction loss in the collector can be neglected when the svelteness (Sv) of the entire flow architecture is greater than approximately 6.
Technical Briefs Transition to Meandering Rivulet Flow in Vertical Parallel-Plate Channels A. Anand, A. Anand Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27006 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 27006 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information A. Anand Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27006 A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27006 J. Fluids Eng. Jun 1986, 108(2): 269-272 (4 pages) https://doi.org/10.1115/1.3242574 Published Online: June 1, 1986 Article history Received: November 5, 1984 Online: October 26, 2009
Gas-cooled cryogenic electric current leads which are operating at high ratios of current to mass flow are commonly used to supply electric current to superconducting magnets. Loss of coolant flow under such operating conditions will result in destruction of the lead and possible damage to the magnet itself. A flow instability, which results in loss of coolant flow, can occur in current leads due to the dependence of the kinematic viscosity of the coolant on the local temperature. This paper discusses the physical basis of the instability and presents an analysis which is used to describe the operation of current leads used in the EPRI-MIT 2000 kVA superconducting alternator. Details of the lead design and test results are included.
Fundamental Principles Laminar Boundary Layer Flow Laminar Duct Flow External Natural Convection Internal Natural Convection Transition to Turbulence Turbulent Boundary Layer Flow Turbulent Duct Flow Free Turbulent Flows Convection with Change of Phase Mass Transfer Convection in Porous Media.
