The gravity-induced flow in a long horizontal space of annular cross-section insulated laterally and with the two ends maintained at different temperatures was studied analytically. The velocity and temperature distribution in the central portion of the space was derived based on a perturbation analysis in the Rayleigh number. The Nusselt number for axial heat transfer was shown to depend on the Rayleigh number, the two geometric aspect ratios ri ro and L ro (inner radius/outer radius and length/outer radius) plus the conducting properties of the two cylindrical walls. The analysis was performed for the case where the horizontal space is filled with incompressible fluid and the case where the space is filled with a porous material saturated with an incompressible fluid. The limit r i → r o was used finally to derive expressions for the flow and temperature fields between two vertical planes subjected to a net temperature difference in the horizontal direction parallel to the planes.
Here we report the discovery that even the simplest, oldest and most prevalent forms of evolutionary movement--rolling bodies and whirls of turbulence--exhibit the same body-size effect on life time and life travel as the evolutionary movement united by the body-size effect so far: animals, rivers, vehicles, jets and plumes. In short, the bigger should last longer and travel farther. For rolling bodies, the life span (t) and the life travel (L) should increase with the body mass (M) raised to the powers 1/6 and 1/3, respectively. The number of rolls during this movement is constant, independent of body size. For an eddy of turbulence, t should increase with the eddy mass (M) raised to the power 2/3, while L should increase with M(2/3) times the bulk speed of the turbulent stream that carries the eddy. The number of rolls during the eddy life span is a constant independent of eddy size.
This is a theoretical, numerical and experimental study that shows how to optimize the performance of on and off pulsating heaters in forced convection. Scale analysis shows that there exists an optimal heat pulse interval or frequency that maximizes the overall thermal conductance between the heater and the free stream U ∝. Numerical results for a flat plate heater and experimental results for a cylinder in cross-flow validate the theory. Numerically it is shown that the optimal pulsating regime can be identified accurately by using a complete simulation of the flow and temperature field around the heater. Boundary layer simplified numerical methods fail to simulate the short-times (high frequency) range of the process. The maximized overall conductance of a pulsating heater does not exceed the conductance associated with a steady (continuous) heater. The experimental and numerical results are nondimensionalized and correlated by using the scales recommended by theory. When the ‘on’ and ‘of’ intervals are comparable, the optimal heat pulse interval is approximately 0.1 L U∝ , where L is the scale of the swept length of the heater shape. This conclusion also applies to a pulsating heater embedded in a porous medium with uniform flow.
In this article we rely on constructal theory to show that the hierarchy of universities is rigid, and that the explanation lies in the nature of education (science, news, information) as a natural fl ow system that bathes the globe most effectively. The article begins with two observations: (i) the rankings of the best engineering universities in the USA closely mirror the rankings of the universities that have the most names of researchers on the list of the most highly cited authors; and (ii) the log‐log plot of the number of highly cited authors of one school versus the rank of that school is nearly a straight line with slope between ‐1/2 and ‐1. The straight line is the same as the distribution of city sizes versus city rank throughout the history of Europe. From this follows the argument that the hierarchy of universities is tied to geography, to how each nodule of knowledge generation serves the area allocated to it. Education fl ows from point to area. The compounding of areas to cover the landscape is the origin of the hierarchical and stable arrangement of universities. The rank of a university is closely related to the visibility of its producers of ideas. The tapestry of a university on the landscape is predicted. All universities grow and improve in time (like all the river channels during the rain), but their hierarchy remains the same.
This paper describes the time evolution of the temperature and heat transfer in the vicinity of a flat wall embedded in a parallel flow through a saturated porous medium. The flow is uniform and steady, while the wall is suddenly subjected to heating or cooling. Two distinct problems are considered: the wall with suddenly imposed uniform temperature, and the wall with suddenly imposed uniform heat flux.
Turbulent curtains of smoke rise initially as flat plumes and, above a certain height, they become round plumes. The same evolution of cross-sectional shape is exhibited by jets issuing from flat nozzles. Here we predict based on principle that all such flows should evolve their cross-sectional shapes from flat to round (and not the other way) at a critical distance downstream, which is predictable. The principle is that the prevailing flow architecture provides greater access to the flow of momentum from the moving core (plume, jet) to the still surroundings. For turbulent plumes and jets, the transition distance scales with the long dimensions (L) of the two-dimensional (flat) heat sources and nozzles that drive them. For laminar jets, the transition distance scales with L Re, where Re is the Reynolds number based on nozzle velocity and the smaller dimension of the nozzle cross section. These predictions are confirmed by full numerical experiments of the three-dimensional flow fields of turbulent and laminar jets covering the Re range 10–104.
This paper reports in closed form the similarity heatfunctions H for laminar boundary layer flow on a flat wall. Plots of the constant-H lines (‘heatlines’) show that the path of convection from a hot free stream to a cold wall is unlike the path of convection from a hot wall to a cold fluid. The true path of convection in laminar boundary layer flow is visualized in charts drawn for both heat transfer modes (cold wall, hot wall), several Prandtl numbers (0.02, 0.72, 7) and isothermal walls and constant-flux walls. The paper stresses the heat transfer features that are brought into view for the first time by the heatline patterns. As a supplementary contribution, the paper reports the exact similarity solution for the wall with uniform flux in the Pr→ 0 limit, and proposes a closed-form local Nusselt number correlation that covers the entire Pr range.
Social and behavioral factors influence health but are infrequently recorded in electronic health records (EHRs). Here, we demonstrate that psychosocial vital signs can be extracted from EHR data. We processed structured and unstructured EHR data using expert-driven queries and Natural Language Processing (NLP), validating results through structured annotation. We found that although these vital signs are present in EHRs, with 681 structured entries identified for psychosocial concepts, NLP identified a nearly 90-fold increase in patients.
“Empire building” is a phenomenon that dominates today’s research landscape. Large groups, national priorities and research centers dwarf the spontaneous individual investigators. Administrators and the thirst for higher rankings encourage this trend. Yet, the individuals do not disappear. This paper explains why. It attributes the emergence of the large group to the pursuit of greater visibility for the institution as a whole. The visibility (V) is modeled as a product of the production (P) of ideas in the institution, and the support (S) that the institution secures for the production of ideas. The coalescence of some investigators into a large group tends to increase S and decrease P. On the other hand, an increase in the number of individual investigators has the opposite effect. From this trade-off emerge the main and well-known features of contemporary research organization: the proportionality between the size of the large group and the size of the entire institution, the strong relationship between the visibility of an institution and its size, and the fact that large groups occurred fi rst in the largest and most research-intensive institutions. The paper also shows that as the incentives for large-group research become stronger, smaller and smaller institutions fi nd it benefi cial to abandon the individual investigator mode and seek a balance between research empires and individual investigators. The individual researcher will not disappear.
This paper reports an increase in the heat transfer rate density by using wrinkled entrance regions in ducts with laminar flow. The heat transfer rate density is increased by taking advantage of the presence of relatively isothermal fluid in the entrance regions. In order to stimulate a more complete thermal interaction between walls and fluid, the square entrances are wrinkled on the perimeter, at the one-third and the two-third positions. The new structure has two degrees of freedom. The fluid flow through the ducts is forced by the imposed pressure difference across the duct. Numerical simulations document the effects of the dimensionless pressure drop on the optimized configurations and show a fifteen percent enhancement in heat transfer rate density.
This paper summarizes an analytical and numerical study of buoyancy-driven convection in a rectangular cavity filled with fluid. The new feature of this study, suggested by architectural applications of the convection phenomenon, is the presence of constant heat flux heating and cooling along the vertical side walls. It is shown analytically that in the boundary layer regime the boundary layer thickness must be constant (independent of altitude), that the core must be motionless and linearly stratified, and that the vertical walls temperature must vary linearly with the same gradient as the core temperature. The Nusselt number (q″/ΔT)H/k was found to be equal to 0.34 (H/L)1/9 Ra2/9, where H/L is the height/length ratio and Ra = gβq″ H4/ (kαν). The second part of the paper presents a numerical study of the same phenomenon: the numerical results agree very well with the analytical predictions made in the first part of the paper.
The constructal law accounts for the universal phenomenon of generation and evolution of design (configuration, shape, structure, pattern, rhythm). This phenomenon is observed across the board, in animate, inanimate and human systems. The constructal law states the time direction of the evolutionary design phenomenon. It defines the concept of design evolution in physics. Along with the first and second law, the constructal law elevates thermodynamics to a science of systems with configuration. In this article we review the more recent work of our group, with emphasis on the advances made with the constructal law in the natural sciences. Highlighted are the oneness of animate and inanimate designs, the origin of finite-size organs on animals and vehicles, the flow of stresses as the generator of design in solid structures (skeletons, vegetation), the universality and rigidity of hierarchy in all flow systems, and the global design of human flows. Noteworthy is the tapestry of distributed energy systems, which balances nodes of production with networks of distribution on the landscape, and serves as key to energy sustainability and empowerment. At the global level, the constructal law accounts for the geography and design of human movement, wealth and communications.
Technical Briefs Predicting the Pool Fire Vortex Shedding Frequency 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 Author and Article Information A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 J. Heat Transfer. Feb 1991, 113(1): 261-263 (3 pages) https://doi.org/10.1115/1.2910540 Published Online: February 1, 1991 Article history Received: January 5, 1990 Revised: June 6, 1990 Online: May 23, 2008
Chambadal, Novikov, Curzon and Ahlborn have shown that the efficiency of an irreversible heat engine at maximum power output is 1−(TL/TH)1/2. This article describes several heat engine models in which the same efficiency formula can be derived by minimizing the rate of entropy generation. Included in the entropy generation is the usually overlooked contribution made by the components that provide a freely varying heat input to the power cycle.
When an amount of high-temperature molten material is suddenly and finely mixed with a body of liquid water, the ensuing mixture expands because of the volumetric generation of steam. At the same time, the expanding mixture is accelerated away from the surfaces with which it comes in contact. In this paper, we address the fundamental thermodynamic aspects of the energy-conversion process, with emphases on the energy-conversion efficiency and the impact of the intensity of the heat-transfer irreversibility on decreasing the efficiency. The expanding mixture is modeled as a conglomerate of spherical drops of molten material distributed uniformly throughout a body of water. At the elemental level, steam annuli develop around the spherical drops as time increases. At the mixture level, the density decreases while the pressure and velocity increase. The energy-conversion process is simulated numerically, and results are reported for the evolution of a mixture layer bounded on one side by an impermeable plane wall. The energy conversion efficiency is in the 10−2–10−3 range. The effects of physical parameters such as droplet and waterlayer sizes are discussed.
This paper reports an analytical solution and a numerical study of natural convection in a rectangular porous layer heated and cooled with uniform heat flux along the vertical side walls. It is shown analytically that in theboundary layer regime the vertical boundary layer thickness isconstant (independent of altitude) and the core region is motionless. The vertical temperature gradient is the same constant everywhere in the porous layer. Numerical results are reported in the range 100 < Ra < 5000, 1 ≤ H/L ≤ 10, where Ra = KgßH2q″/(xvk). The boundary layer analytical solution is shown to agree well with the numerical results.
No abstract is provided for this article.
This paper is a proposal to embed tree-shaped vasculatures in a wall designed such that the wall withstands without excessive hot spots and peak stresses the intense heating and pressure that impinge on it. The vasculature is a quilt of square-shaped panels, each panel having a tree vasculature that connects the center with the perimeter. The vascular designs for volumetric cooling can be complemented by the shaping and distributing of channels for maximum strength and thermal performance at the same time. Numerical simulations of heat flow and thermal stresses in three directions show that it is possible to determine the optimal geometric features of configurations with radial channels and trees with radial and one level of bifurcations. The global performance is evaluated in terms of the overall thermal resistance and peak von Mises stresses. The dendritic design is superior under the studied thermal condition.
