901 publications from this institution
Here we draw attention to the development of smart materials with embedded vasculatures that provide multiple functionality: volumetric cooling, self-healing, mechanical strength, etc. Vascularization is achieved by using tree-shaped (dendritic) and grid-shaped flow architectures. As length scales become smaller, dendritic vascularization provides dramatically superior volumetric bathing and transport properties than the use of bundles of parallel microchannels. Embedded grids of channels provide substantially better volumetric bathing when the channels have multiple diameters that are selected optimally and put in the right places. Two novel dendritic architectures are proposed: trees matched canopy to canopy, and trees that alternate with upside down trees. Both have optimized length scales and layouts. Flow architectures are derived from principle, in accordance with constructal theory, not by mimicking nature.
No abstract is provided for this article.
In this paper we develop flow architectures for “vascularizing” smart materials that have self-healing capabilities. The flow architectures are configured as two trees matched canopy to canopy. A single stream flows through both trees and bathes every subvolume (crack site) of the material. Several types of tree-tree configurations are optimized. Trees that have only one level of branching and bathe a rectangular domain have optimal external shapes that are nearly square. They also have optimal ratios of channel sizes before and after branching. Trees optimized on square domains perform nearly as well as trees on freely morphing rectangular domains. The minimized global flow resistance decreases slowly as the number of subvolumes increases. It is more beneficial to bathe the entire volume with a single (optimized) one-stream architecture than to bathe it with several streams that serve small clusters of volume elements. These conclusions are reinforced by an analytical optimization of the same class of architectures in the limit of a large number of assembled subvolumes. We also show that the freedom to morph the design and to increase its performance can be enhanced by using tree-tree architectures with more than one level of branching.
This paper outlines recent thermodynamic optimization work on the geometric layout of schemes for distributing hot water and exergy over a large system. Constrained are the amount of insulation material, the volume of all the pipes, and the amount of pipe wall material. Unknown are the distribution of insulation over all the links of the network, and the configuration of the network itself. The main focus is on how the geometric configuration may be selected in the pursuit of maximized global performance, and how closely a non-optimal configuration performs to the highest level. Maximum global performance means minimum heat loss to the ambient, minimum pressure loss, and minimum exergy destruction. Three configurations are optimized: (a) an area covered by a coiled steam, where all the users are aligned on the same stream, (b) a sequence of tree-shaped flows on square areas in which each area construct is made up of four smaller area constructs, and (c) a sequence of tree-shaped flows where each area construct is made up of two smaller area constructs. It is shown that the tree-shaped designs (b), (c) outperform significantly the coiled stream design (a). The tree designs obtained by pairing (c) are better than the square tree constructs (b) and, in addition, they deliver water at the same temperature to all the users spread over the territory. The fundamental trade off between minimum heat loss and pressure drop, in the pursuit of minimum exergy destruction, pinpoints the optimal size of each duct and insulation shell.
Most of the studies of convection in porous media published before 1970 were motivated by geophysical applications and many published since have geophysical ramifications; see, for example, the reviews by Cheng (1978a, b, 1985b). On the other hand, geothermal...
This paper documents the thermal insulation effect of a screen installed inside a vertical rectangular enclosure (e.g., double-glazed window). The screen is a venetian blind system made out of horizontal strips that can be rotated. The focus is on the “closed” position, where the strips almost touch. The effect of this permeable screen on the temperature field, the flow field, and the overall heat transfer rate is determined numerically. The study shows that there exists a ceiling (critical) conductance for the air leakage through the screen, above which the screen does not cause a significant drop in the overall heat transfer rate. A numerical example shows how this critical conductance can be used to calculate the critical spacing that can be tolerated between two consecutive strips in the screen.
No abstract is provided for this article.
In the examples of forced and natural convection discussed until now, the fluid that flowed through the pores did not experience a change of phase, no matter how intense the heating or cooling effect. In this chapter, we turn our attention to situations in which a...
This paper draws attention to the 1996 constructal theory of the generation of geometric form in flow systems. Flow architecture can be reasoned on the basis of principle: the maximization of global performance subject to finite-size constraints. One example is the generation of tree-shaped flow patterns, as optimized paths between one point (source, sink) and an infinity of points (area, volume), The optimized tree-flow architecture accounts for allometric laws, for example, the proportionality between metabolic rate and body size raised to the power 3/4, and the proportionality between breathing and heart beating times and body size raised to the power 1/4. Another example is the proportionality between the cruising speed of flying bodies (insects, birds, airplanes) and body mass raised to the power 1/6, The “thermodynamics law” status of the constructal principle is discussed. 1 Constructal theory versus biomimetics Geometric similarities and patterns abound in flow systems in engineering and in nature. For example, tree-shaped flows are everywhere, in computers, lungs, dendritic crystals, urban street patterns, and communication links, In a new book [1], I started from the design and optimization of engineered systems and developed a deterministic principle for the generation of geometric form in natural systems. In flow systems for fluid, heat, mass, electricity, goods, and traffic, better performance means improved access: minimal flow resistance, minimal travel time, minimal cost. This observation led to constructal theo~,
This paper illustrates the morphing of flow architecture toward greater performance in a counterflow heat exchanger. The architecture consists of two plenums with a core of counterflow channels between them. Each stream enters one plenum and then flows in a channel that travels the core and crosses the second plenum. The volume of the heat exchanger is fixed while the volume fraction occupied by each plenum is variable. Performance is driven by two objectives, simultaneously: low flow resistance and low thermal resistance. The analytical and numerical results show that the overall flow resistance is the lowest when the core is absent, and each plenum occupies half of the available volume and is oriented in counterflow with the other plenum. In this configuration, the thermal resistance also reaches its lowest value. These conclusions hold for fully developed laminar flow and turbulent flow through the core. The curve for effectiveness vs number of heat transfer units (N tu) is steeper (when N tu <1) than the classical curves for counterflow and crossflow.
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 patterns of heatlines reported in this paper illustrate for the first time the true path of convective heat transfer through a saturated porous medium. Heatline patterns are reported for the following fundamental configurations: the boundary layer near an isothermal wall, the boundary layer near a wall with uniform heat flux, and the two-dimensional porous layer confined by two parallel plates. Emphasis is placed on the convection features that are being visualized for the first time by the heatline method, i.e., not by traditional methods such as the use of isotherms. It is shown that the heatlines of a flow in which the wall serves as heat sink are unlike the heatlines of the same flow with a wall that serves as heat source. The seepage flow with slip at the boundary is visualized by heatlines that leave a hot wall at an angle. The wall heat-flux distribution is visualized by the density of the heatlines that intersect the wall. The heatline pattern in fully developed flow of a pure fluid through a parallel-plate channel is also reported in order to emphasize that the pure-fluid pattern is not exactly the same as the pattern in the corresponding two-dimensional space filled with seepage flow through a porous medium.
No abstract is provided for this article.
This paper reports optimal bifurcation shapes (T and Y) in turbulent regime of tree-shaped flows. Unlike earlier studies of T and Y constructs, here the effect of pressure losses at the junction is taken into account, and the wall roughness and duct cross-sectional shapes are free to vary. The optimal ratio of duct cross-sectional areas (as a generalization of Murray's law), the optimal ratio of duct lengths, and the optimal angle between the branches of the Y are presented. These optimal geometrical features are reported as functions of the flow direction (splitting flow versus merging flow), wall roughness, duct cross-sectional shape, and svelteness. The svelteness, Sv, is a global property defined as the external length scale of the flow construct divided by the internal length scale. It is shown that the effect of junction pressure losses on the optimized architecture can be neglected when Sv3∕2 is greater than approximately 104. Two dimensionless terms are introduced and shown to be useful for the optimization of flow networks.
This is a fundamental theoretical and numerical study of the evolution of tree-shaped flow on one side of the heat transfer surface of a ‘dendritic’ heat exchanger. The flow is shaped by step changes in the dimensions of the available openings. The architecture evolves toward providing easier access to the stream. Two extremes of this flow architecture are treated theoretically, orifices in parallel and equidistant partitions, and slender two-dimensional channels (slits) in sequential blocks of channels. The orifice and channel sizes change stepwise in the downstream direction. Theoretically, the recommended step down ratio for orifice diameters is 0.707, and for the slit size is 0.595. The theoretical step down ratio for the lengths of parallel channels is 0.648. Numerical simulations conducted in a wide parametric domain illustrate the evolution of the flow configuration toward greater access. Remarkable is that the numerical results for the step down ratio in slit sizes covers the very narrow range 0.72–0.75. This study contributes fundamentals that can serve as reliable reference in future designs of dendritic heat exchangers spearheaded by constructal theory.
No abstract is provided for this article.
In this paper we describe the thin-film melting of a block of solid phase-change material (the bearing) around a rotating cylinder (the shaft). We determine the relation between the force applied on the shaft and the speed with which the shaft migrates into the bearing, the relation between the applied force and the torque, and the angle between the applied force and the direction of migration into the bearing. The method is based on contact melting theory, which combines the Reynolds thin-film lubrication theory with an analysis of phase-change heat transfer in the melt. The paper addresses three limiting regimes of the contact melting phenomenon: (1) the long bearing with melting due to frictional heating in the melt layer; (2) the short bearing with melting due to frictional heating in the melt layer, and (3) the short bearing with melting due to a temperature difference imposed between the hot cylinder and the cold phase-change material.
Here we see why humans unwittingly build fires that look the same: edifices of fuel, as tall as they are wide. The pile of fuel is permeable, air invades it by natural convection and drives the combustion. I show that the hottest pile of burning fuel occurs when the height of the pile is roughly the same as its base diameter. Future studies may address the shape effect of wind, material type and packing. Key is why humans of all eras have been relying on this design of fire “unwittingly”. The reason is that the heat flow from fire facilitates the movement and spreading of human mass on the globe.
The focus of this paper is on the fundamentals of the transient mechanism responsible for the establishment of natural convection in a two-dimensional porous layer confined between isothermal vertical walls and insulated horizontal walls. In the first part of the paper, pure scaling arguments are used to identify the characteristic time scales of the phenomenon and, based on these scales, the evolution of the porous system to steady state. The type of steady state is shown to depend on the relative order of magnitude of the characteristic time scales. Distinct vertical boundary layers are shown to be possible if (L/H)Ra1/2&gt;1, and distinct horizontal layers if (H/L)1/2 Ra1/4&gt;1. In the second part of the paper, these and other scaling laws of the transient evolution to steady state are successfully tested against a series of transient numerical experiments conducted in the range H/L=0.2–5 and Ra=10–50.
