901 publications from this institution
This paper describes human application of their knowledge and technological ability to fly and continue to enhance that power. Through application of their knowledge and technological ability, human beings have evolved the ability to fly and continue to enhance that power. Aircraft technology evolution is about the evolving design of the human movement on the Earth’s surface: people, goods, materials, and everything else. As the whole vehicle or animal evolves toward becoming better at moving mass on the landscape, the organs remain imperfect, because each represents a compromise. The whole vehicle or animal is a construct of organs that are ‘imperfect’ only when examined in isolation. The vehicle design evolves over time and becomes a better construct for moving the vehicle mass on the world map. Flow architectures are evolving right now, throughout nature and in technologies. The legacy of all flow systems (animate and inanimate) is: they have moved weight horizontally and improved the efficiency of that movement because of design evolution.
This paper documents the relative merits of using more than one type of phase-change material for energy storage. In the case of two phase-change systems in series, which are melted by the same stream of hot fluid, there exists an optimal melting point for each of the two materials. The first (upstream) system has the higher of the two melting points. The second part of the paper addresses the theoretical limit in which the melting point can vary continuously along the source stream, i.e. when an infinite number of different (and small) phase-change systems are being heated in series. It is shown that the performance of this scheme is equivalent to that which uses an optimum single phase-change material, in which the hot stream remains unmixed during the melting process.
The phenomenon of natural convection in a square enclosure heated and cooled in the horizontal direction was investigated numerically in the Prandtl number range 0.01-10 and the Rayleigh number range 102-1011. The numerical method relied on the full governing equations for time-dependent flows. The study focused on the detection of inertia-sustained fluctuations in the flow field and on the highest Rayleigh number where steady-state laminar flows are possible. It was found that the highest Rayleigh number decreases dramatically as the Prandtl number decreases. This finding agrees qualitatively with experimental observations of transition to turbulent natural convection and with the "local Reynolds number" criterion of transition to turbulence recommended by the buckling theory of turbulent flow.
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 article sheds light on the fundamental physics that accounts for the similar behaviors of heat transfer and fluid friction as the configuration of the convective flow system changes. Three methods are used, and as the methods change so does the configuration (design): complicated (bundle of finned tubes in cross flow), elemental (single body in uniform flow), and even more complicated such that it resembles a saturated porous medium. The configuration evolves such that it conveys the explanation better, more effectively. The article illustrates the essential role of evolving configurations in thermal-science fundamentals and practice. It shows that three methods (i.e. three designs, from purposeful changes in design) are better than a single method (one design). Science marches forward in this manner, and, at the same time, there is a creative reassessment of past advances. The purpose of looking back is to construct a single edifice that is general, simpler and easier to use.
In the design of packages of electronic components there are strong incentives to mount as much circuity as possible in a given space. This can be achieved by judiciously selecting the geometry of the package, i.e., the way in which the components are arranged relative to the coolant and to each other in the fixed space. In the electronics industry there is a great diversity of components, packages, and cooling techniques. The chapter systematically reviews the progress made in the newly emerging field of Geometric Optimization of Cooling Techniques. The only degree of freedom in choosing the best cooling arrangement is the position of the heated plate inside the channel. Microelectronics packaging is subject to several conflicting requirements such as electromagnetic compatibility, acoustic limits, and adequate cooling. The optimization of the spacing between boards with discrete heat sources was performed numerically. This was an extensive, systematic study where both flush-mounted and protruding sources were considered.
The main objective of this chapter is to review the fundamentals of three simple methods of problem solving and presentation of convection in porous media: scale analysis, heat lines, and the intersection of asymptotes method. Another objective is to draw attention...
This paper describes the fundamentals of melting when a shell of phase-change material rides on a heated horizontal cylinder. In the first part of the paper, contact melting theory is used to predict the history of the melting process and, in particular, the time when the remaining ice falls off the cylinder. It is shown that the melting process consists of two distinct regimes, first, an early regime when the cylinder is surrounded by ice and, second, a late regime when the cylinder cuts through the top of the ice shell. The second part describes laboratory measurements that validate the theory. The third part of the paper shows that in the complete cycle that starts with freezing the shell and ends with the contact-melting removal of the shell, there exists an optimal frozen shell thickness such that the cycle-averaged production of ice is maximized.
This paper is a proposal to vascularize a solid wall with transverse tree-shaped channels with fluid flow from side to side. The purpose of the fluid flow is to intercept in counterflow the solid heat conduction caused by an intense heat flux that lands on the exit plane of the wall. Tree architectures with 1–4 levels of bifurcation are simulated numerically, as a conjugate (convection and conduction) heat flow phenomenon in three dimensions. The effect of local pressure losses at junctions and entrances is included. The numerical work covers the Reynolds number range 10–550, svelteness number range 2.2–11.5, wall porosity range 0.01–0.1, and pressure drop number range 3×105–1010. The objective of the tree flow is to keep down the peaks of hot spot temperatures and the fraction of the volume occupied by high temperatures while using minimum pumping power. It is shown that the tree design is very effective, and that there is an optimal number of bifurcation levels for a specified porosity and pressure drop number. Tree designs are more effective than designs with parallel channels when the pressure drop number and the porosity are sufficiently large.
Diverse measurements of heat transfer by Benard convection in a fluid saturated porous layer are correlated by considering only the Darcy and Forschheimer asymptotes of the phenomenon. Scale analysis suggests that in the Forschheimer limit the Nusselt number must increase as (Ra Prp ) 1 2 , where Ra is the Darcy-Rayleigh number based on height and Prp is an entirely new group called “the porous medium Prandtl number.” The transition from the Darcy regime to the Forschheimer regime occurs at Ra ∼ Prp. The heat transfer correlation is Nu = {( Ra 40 )n + [c(RaPrp) 1 2 ]n} 1 n with n = −1 65 , c = 1896.4 and Prp = (H/bK)Prm , where H, b, K and Prm are the height, Forschheimer constant, permeability and Prandtl number based on the thermal conductivity of the porous medium.
Die Arbeit berichtet über eine Lösung mit finiten Differenzen der natürlichen Gegenströmung in einem horizontalen adiabaten Kanal mit k
The drive toward vascular smart materials calls for novel flow architectures that bathe and serve entire volumes and areas as uniformly as possible. Here, we show that vascular designs consisting of trees matched canopy to canopy can be configured so that they have two qualities: small flow resistance (ψ) and small volumetric flow nonuniformity (μ). In the past, the only quality sought was small flow resistance. Two classes of architectures are explored: (a) matched trees with diagonal channels through the core and (b) matched trees with orthogonal channels. First, we show that flow architectures can be developed and selected for minimum flow nonuniformity alone. Second, in the ψ − μ design space the best of designs (b) lie close to the best of designs (a), although the best of designs (b) offer slightly better configurations (low ψ and μ) than the best of designs (a). Comparisons with similar architectures generated based on genetic algorithms show that the minimum global flow resistance ψ of designs (a,b) is 2–5 times smaller than the genetic-algorithm values. The flow nonuniformities μ corresponding to the minimum ψ of designs (a,b) are 2–70 times smaller than the flow nonuniformities of the genetic-algorithm results.
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 shows how to correlate the optimal sizes of bodies with specified external forced convection heat transfer, when the objective is to minimize 1) the total rate of entropy generation, or 2) the total cost. Specific results are reported for the cylinder in cross-flow, the sphere, and the flat plate in parallel flow. The results are presented in terms of dimensionless parameters that show that regardless of the body shape, the optimal size increases monotonically with the heat transfer duty parameter. A universal correlation is obtained by nondimensionalizing the body size as a Reynolds number based on the transversal length scale of the flow. The by replacing the body with an equivalent one in two-dimensional flow. The optimal sizes correlated in this manner can be applied to bodies of other shapes.
The heat transfer principle of power maximization in power plants with heat transfer irreversibilities is extended to fluid flow. It is shown that when a stream flows between two pressure reservoirs (P 1 > P 2) across linear flow resistances, a piston delivers maximum power when the pressure difference across its faces is (P 1 - P 2)/2. The energy conversion efficiency at maximum power is η max = ( 1 2 ) (1 - P2 - P1 ), as an analog to the efficiency for maximum power in power plants, η max = 1 - (T2/T1) 1 2 . These results are generalized to fluid flow with nonlinear relations of pressure drop vs flow rate. Depending on overall size constraints, the power delivery can be further maximized by balancing the flow resistances upstream and downstream of the piston. The paper concludes with applications to steady-flow shaft-power components. It is shown that turbines can be optimized for maximum power output by selecting the inlet or outlet pressure drop, or the flowrate. Compressors and pumps do not have a power input minimum with respect to pressure drop or flowrate.
Experimental investigations of the flow and the associated heat transfer were conducted in two-dimensional microchannels in order to test possible size effects on the laws of hydrodynamics and heat transfer and to infer optimal conditions of use from the measurements. The test section was designed to modify easily the channel height e between 1 mm and 0.1 mm. Measurements of the overall friction factor and local Nusselt numbers show that the classical laws of hydrodynamics and heat transfer are verified for e > 0.4 mm. For lower values of e, a significant decrease of the Nusselt number is observed, whereas the Poiseuille number continues to have the conventional value of laminar developed flow. The transition to turbulence is not affected by the channel size. For fixed pressure drop across the channel, a maximum of heat transfer rate density is found for a particular value of e. The corresponding dimensionless optimal spacing and heat transfer rate density are in very good agreement with the predictions of Bejan and Sciubba (1992). This paper is the first time that the optimal spacing between parallel plates is determined experimentally.
No abstract is provided for this article.
This paper shows that the time needed to discharge a volume to a concentrated sink can be minimized by making appropriate changes in the geometry of the flow path. The time-dependent flow of heat between a volume and one point is chosen for illustration, however, the same geometric optimization method (the constructal principle) holds for other transport processes (fluid flow, mass transfer, conduction of electricity). There are two classes of geometric degrees of freedom in designing the flow path: the external shape of the volume, and the distribution (amount, location, orientation) of high-conductivity inserts that facilitate the volumetric collection of the discharge. The optimization of flow path geometry is executed in a sequence of steps that starts with the smallest volume elements and proceeds toward larger and more complex volume sizes (first constructs, second constructs, etc.). Every geometric feature is the result of minimizing the time of discharge, or the resistance in volume-to-point flow. The innermost details of the structure have only a minor effect on the minimized time of discharge. The high-conductivity inserts come together into a tree-network pattern which is the result of a completely deterministic principle. The interstices are equally important in this optimal design, as they are occupied by the low-conductivity material in which the energy charge was stored initially. The paper concludes with a discussion of the relevance on this deterministic principle—the constructal law—to predicting structure in natural flow, and to understanding why the geometry of nature is not fractal.
The chapter describes the combined heat and mass-transfer natural convection mechanisms, which are considered an important subfield in contemporary heat and mass-transfer research. This subfield essentially brings together the studies concerned with the combined heat and mass-transfer or double-diffusive processes that are driven by buoyancy through porous media saturated with fluid. The density gradients that provide the driving buoyancy effect are induced by the combined effects of temperature and species concentration nonuniformities present in the porous medium. The chapter considers the phenomena of convection through fluid-saturated porous media generally in terms of volume-averaged quantities. There are four conservation principles considered in the study of convection with more than one buoyancy effect. These include conservation of mass, energy, species, and momentum. Heat and mass transfer in the vertical direction and in horizontal direction are discussed in detail. Another category of studies of combined buoyancy effects in porous media deals with the local fields around buried sources of heat and mass. The recent work in this field focuses on the multilayer structure of flows of the boundary-layer or concentrated-source type.
