901 publications from this institution
In this paper, we demonstrate that asymmetry in fluid distribution tree networks emerges from power requirement minimization, under global volume constraint. We have discovered several levels of asymmetry in optimal trees: different pipe lengths at the same level of branching, different mass flow rates at junctions or bifurcations, and different main branches to build the optimal dendrite. The emergence of asymmetry in optimal tree networks (man-made or natural) is a result of the optimization: it is not an assumption or a modeling feature. The constructal method that we used to discover asymmetry is predictive, and this distinguishes it from descriptive methods such as fractal geometry.
This paper addresses the fundamental question of how to position a heat-generating board inside a parallel-plate channel, where it is cooled by forced convection. It is shown that when the board substrate is a relatively good thermal conductor, the best board position is near one of the channel walls, and the worst position is in the middle of the channel. The best and worst positions switch places when the board substrate is a relatively poor conductor. The optimal spacing between a heat-generating surface (uniform temperature, or uniform heat flux) and the insulated wall that completes a parallel-plate channel is reported. Finally, it is shown under what conditions it is advantageous to divide a heat-generating board into two or more equidistant boards inside the same channel, when the total rate of heat generation of all the boards and the channel spacing are fixed.
This paper outlines the thermodynamic optimization of a combined power and refrigeration system subject to constraints. In the first part, the system operates in the refrigeration mode and is driven by a hot stream of single-phase fluid that is subsequently discharged into the ambient. The irreversibility is due to three heat exchangers and the discharging of the used stream. It is shown that the thermodynamic optimum is pinpointed by an optimal ratio between the mass flow rates of the hot stream and the stream that is heated by the hot stream, and by an optimal distribution of the heat exchanger inventory among the three heat exchangers of the installation. The second part of the paper considers the more general situation where the system delivers power and refrigeration, and where the irreversibility is due additionally to the internal parts of the system. It is shown that the thermodynamic optimum is reached by distributing optimally the heat exchanger inventory among the three heat exchangers, and that this optimum is sensitive to the total inventory and the degree of irreversibility of the internal parts. It is also shown that the optimum is robust with respect to changes in several physical parameters.
Here we show how the size of a body affects its maximum average speed of movement through its environment. The theoretical challenge was to predict that ‘outliers’ must exist, such as the cheetah for terrestrial animals and the jet fighter for airplanes. We show that during a travel that starts from rest and continues at cruising speed, the body size for minimum travel time, or maximum average speed, is not the biggest. The results are compared with extensive data for military aircraft for chase, attack and reconnaissance, in addition to data for commercial aircraft. The paper also explains why in earlier studies of flying (animals, airplanes) the airplane data deviated upward (toward greater speeds) relative to the theoretical trend followed by flying animals, and why the fastest animal flyers are one thousand times smaller than the fastest swimmers. Unlike the biggest animals and airplanes (elephant, whale, commercial jet), which move constantly, the fastest animals and airplanes spend most of their lives at rest. When judged for speed averaged over lifetime, the fastest ‘sprinters’ are in fact the slowest movers (as in Aesop’s fable ‘The Tortoise and the Hare’).
No abstract is provided for this article.
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.
This paper draws attention to a series of misconceptions and misstatements regarding the origin and meaning of some of the most basic concepts of engineering thermodynamics. The six examples exhibited in the paper relate to the concepts of reversibility, entropy, mechanical equivalent of the calorie, the first law of thermodynamics for open systems, enthalpy and the Diesel cycle. A complete list of the pioneering references concludes the paper.
No abstract is provided for this article.
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.
No abstract is provided for this article.
The MIT-EPRI 3 MVA superconducting alternator has undergone a number of successful electrical and mechanical tests. During these tests a great deal of information on the thermal performance of the rotor has been collected. This information has not been presented in the literature or discussed generally. Thermal performance data of interest to generator designers such as: helium flow circuit behavior and conceptual design, electrical lead performance following reconstruction and shield thermal performance is discussed.
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."
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.
Here, we report the numerical study of time dependent storage of energy by melting a phase change material. The heating is provided along invading lines, which change from single-line invasion to tree-shaped invasion. The numerical simulations show that the history of the amount of melted material is S-shaped. We also found that the fastest melting (i.e., the steepest S curve) is discovered by allowing the tree architecture to morph freely, toward greater access over time. The stem length and branching angle of invading trees can be selected such that the complete melting process is shorter. The melting process can also be made faster by increasing the complexity of the tree structure.
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.
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).
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.
A summary is not available for this content so a preview has been provided. Please use the Get access link above for information on how to access this content.
It is shown that to maximize the power output of a power plant is equivalent to minimizing the total entropy generation rate associated with the power plant. This equivalence is illustrated by using two of the oldest and simplest models of power plants with heat transfer irreversibilities. To calculate the total entropy generation rate correctly, one must recognize that the optimization process (e.g., the variability of the heat input) requires “room to move,” i.e., an additional, usually overlooked, contribution to the total entropy generation rate.
