No abstract is provided for this article.
This paper reviews a recent development in the integrative optimization of complex energy systems: the geometric optimization of tree-shaped networks that facilitate volume-point and area-point flows with minimum resistance, time, or cost. This method was originally...
This article addresses two questions, why certain animals (frogs, breaststroke swimmers, hovering fliers, jellyfish) push rapidly against the surrounding fluid and then reach forward slowly, and whether this rhythm of propulsion is a manifestation of the universal phenomenon of design evolution in nature. Emphasis is on the distribution of time periods of locomotion in which, during the driving phase of cyclic movement (the motive stroke, phases 1 and 2, in alternating sequence with the dissipative stroke, phase 3), the work is generated (phase 1) and dissipated (phase 2). The relative lengths of the characteristic times t1 and t2 of the phases 1 and 2, are predicted. The relative duration of the proposed three phases of a cycle is the 'rhythm'. The analysis is based on a model of how the effective cross-sections of the stroking body parts impact the surrounding medium, water, or air, and the total power required to account for the kinetic energy losses during phases 2 and 3, which are due to drag forces posed by the surrounding medium. The body configuration (limbs' cross-sections) determines the limbs' velocities that maximize mean power, and the times t1 and t2 within the motive stroke. Emphasis is placed on the freedom to change the evolving design. Freedom is represented in two ways: the number of degrees of freedom in changing the dimensions of the model and its deformation in time, and the effect that evolutionary changes have on the access that the body has to its available space. Freedom to change the locomotion design leads to greater power and speed.
No abstract is provided for this article.
Technical Briefs The Prandtl Number Effect on the Transition in Natural Convection Along a Vertical Surface A. Bejan, A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 Search for other works by this author on: This Site PubMed Google Scholar J. L. Lage J. L. Lage Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 J. L. Lage Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 J. Heat Transfer. Aug 1990, 112(3): 787-790 (4 pages) https://doi.org/10.1115/1.2910457 Published Online: August 1, 1990 Article history Received: September 25, 1989 Revised: January 4, 1990 Online: May 23, 2008
Here we show that the configuration of a slender enclosure can be optimized such that the radiation heating of a stream of solid is performed with minimal fuel consumption at the global level. The solid moves longitudinally at constant rate through the enclosure. The enclosure is heated by gas burners distributed arbitrarily, in a manner that is to be determined. The total contact area for heat transfer between the hot enclosure and the cold solid is fixed. We find that minimal global fuel consumption is achieved when the longitudinal distribution of heaters is nonuniform, with more heaters near the exit than the entrance. The reduction in fuel consumption relative to when the heaters are distributed uniformly is of order 10%. Tapering the plan view (the floor) of the heating area yields an additional reduction in overall fuel consumption. The best shape is when the floor area is a slender triangle on which the cold solid enters by crossing the base. These architectural features recommend the proposal to organize the flow of the solid as a dendritic design, which enters as several branches, and exits as a single hot stream of prescribed temperature. The thermodynamics of heating is presented in modern terms in the Sec. VII (exergy destruction, entropy generation). The contribution is that to optimize “thermodynamically” is the same as reducing the consumption of fuel.
This is a review of two new and important developments in thermal science. First, there exist fundamental optima in the constitution and operation of flow (nonequilibrium) systems, man-made and natural. These optima can be identified based on the simplest models that still retain the essential features of the real systems. Examples are the spatial allocation of heat transfer area in a power plant, and the temporal optimization of on & off processes. The second development is that the engineering method of modeling and optimization has been extended to natural systems, animate and inanimate (e.g., tree networks). This step has been named constructal theory for the reasons given in Section 3. The objective of such work is to predict the macroscopic spatial and temporal structure (organization) that is everywhere. It is to inject a dose of determinsm (theory) in a field that until recently considered natural structures to be nondeterministic: results of chance and necessity. These developments bring to mind the advice left to us by J. W. Gibbs more than one hundred years ago: "One of the principal objects of theoretical research in any department of knowledge is to find the point of view from which the subject appears in its greatest simplicity."
This paper documents the conjugate heat transfer through a wall with nonuniform thickness, which is lined on one side by a boundary layer. In the first part, variational calculus shows that the total heat transfer rate is minimized w hen the wall thickness decreases in an optimal manner in the direction of flow. The reductions in total he; t transfer rate are significant when the Biot number is smaller than 1. In the second part of the study, th e complete problem of a laminar forced convection boundary layer coupled with conduction through a variiible-thickness wall is solved numerically. Means for calculating the total heat transfer rate are reported graphically. It was again found that the total heat transfer rate decreases when the wall profile is tapered so that the wall thickness decreases in the direction of flow.
No abstract is provided for this article.
The objective of this chapter is to review a modern transformation in the teaching, research and practice of energy engineering: the increasingly important roles played by thermodynamics (especially the second law) in problem formulation, modeling and design...
This is an experimental, numerical and analytical study of the optimal spacing between cylinders in cross-flow forced convection. The cylinder array occupies a fixed volume and is exposed to a free stream of given velocity and temperature. The optimal cylinder-to-cylinder spacing is determined by maximizing the overall thermal conductance between all the cylinders and the free stream. In the first part, the optimal spacing and corresponding maximum thermal conductance are determined based on experiments with forced air for H D = 6.2 and in the Re D range 50–4000, where Re D is based on the free-stream approach velocity and cylinder diameter D, and H is the array length in the flow direction. In the second part, similar results are developed based on numerical simulations for Pr = 0.72, 10 ⩽ H D ⩽ 20 and 40 ⩽ Re D ⩽ 200. In the concluding section, the experimental and numerical results for optimal spacing and maximum thermal conductance are explained and correlated analytically by intersecting the small-spacing and large-spacing asymptotes of the thermal conductance function.
What has been accomplished to date on the question of how to minimize and allocate the heat exchanger inventory in power plants and refrigeration plants is summarized in a table. This table also highlights the three objectives of the present study. The first objective is to devise a much simpler model and analysis to reproduce in closed form Ibrahim et al.'s conclusions for fixed power and minimum UA (total heat exchanger inventory). The second objective is to consider the reverse of the refrigeration problem, and to minimize the total UA while holding the refrigeration load fixed. It will be shown analytically that (UA)[sub H] = (UA)[sub L] is once again a feature of the optimal design, the H and L referring to hot and cold ends of the heat exchanger. The third objective is to minimize the total heat exchanger inventory while holding the refrigerator power input fixed, and to show analytically that (UA)[sub H] = (UA)[sub L]. The overriding objective of this note is to demonstrate in the simplest analytical way possible that power plants and refrigeration plants share a common optimization principle. That principle is that the needed heat exchanger inventory is minimum when it is divided evenlymore » between the two ends of the cycle.« less
This article presents a numerical study of the thermal and fluid flow interaction driven by the sudden contact between saturated liquid water (373 K) and a hot (2300 K) spherical or plane surface. It is shown that under these conditions the sudden contact is characterized initially by single-phase interfacial water and supercritical pressure that decays in time as the water is accelerated away from the interface. The sudden contact generates (high) temperature, pressure, and density waves that propagate away from the surface. The water is modeled as an inviscid single-phase fluid that behaves either as an ideal gas or a real gas with properties taken from steam tables. The ideal-gas results are in good qualitative and quantitative (within a factor of order 1) agreement with the results based on the real-gas model. When the hot object is large (radius ≥ 10 mm), the results are insensitive to the geometry of the model (i.e., spherical versus plane).
Why does it feel that the time passes faster as we get older? What is the physical basis for the impression that some days are slower than others? Why do we tend to focus on the unusual (the surprise), not on the ever present? This article unveils the physics basis for these common observations. The reason is that the measurable ‘clock time’ is not the same as the time perceived by the human mind. The ‘mind time’ is a sequence of images, i.e. reflections of nature that are fed by stimuli from sensory organs. The rate at which changes in mental images are perceived decreases with age, because of several physical features that change with age: saccades frequency, body size, pathways degradation, etc. The misalignment between mental-image time and clock time serves to unite the voluminous observations of this phenomenon in the literature with the constructal law of evolution of flow architecture, as physics.
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 paper reports the performance of balanced two-stream parallel flow heat exchangers, in which each stream flows as a tree network through its allotted space. The two trees are in parallel flow, and are arranged like two palms pressed against each other. The relationships between effectiveness and number of heat transfer units are developed for several parallel tree flow configurations: (i) constructal dichotomous trees covering uniformly a rectangular area, (ii) trees on a disk-shaped area, and (iii) trees on a square-shaped area. In configurations (ii) and (iii) each stream flows between the center and the periphery of the area. Configurations (i) and (ii) are trees with minimal resistance to fluid flow. Configuration (iii) is designed by minimizing the length of each duct in the network. The performance of the parallel flow configurations is compared with the performance of counterflow configurations. The future use of dendritic heat exchangers in devices with maximal heat transport density is proposed.
Here we develop vasculatures for smart materials with volumetric cooling capability under time-varying conditions. The objective is for the vascularized composite to survive without coolant flow such that its peak temperature does not overshoot the maximum allowable level. The transient performance of four vasculatures is reported: grids (G) and radial channels (R), and two flow directions, inlet in the center (I) and outlet in the center (O). Designs with outlet in the center offer short response times when the Be number is smaller than 109. Configurations with outlet in the center offer short heat removal times and small hot volume fractions.
