901 publications from this institution
IfDiminishing returns evolutionEvolution is so ongoing and everywhere, why do so many things look as if they are stuck in time? The cat and the dog, the milk and the four limbs, they are the same as they ever were. Even in technologyTechnology evolutionEvolution,...
This paper considers the fundamental problem of optimizing the geometry of the interface between two conductive bodies, with the objective of minimizing the thermal resistance. The interface geometry is free to change. For simplicity, the geometry is assumed to be two-dimensional with equidistant tooth-shaped features. The tooth shape varies from triangles, to trapezoids and rectangles. The aspect ratio (height/width) of the tooth also varies. The third degree of freedom of the interface architecture is the volume fraction of the higher-conductivity tooth material that is present in the interface region. It is shown that the interface geometry can be optimized with respect to tooth shape. The global thermal resistance minimized with respect to tooth shape varies monotonically with the tooth aspect ratio and volume fraction. The optimized geometry and performance are reported graphically as functions of the physical properties and geometric parameters of the interface region.
Here we present a simple and transparent alternative to the complex models of earth's thermal behaviour under time-changing conditions. We show the one-to-one relationship between changes in atmospheric properties and time-dependent changes in temperature and its distribution on earth. The model accounts for convection and radiation, thermal inertia and changes in albedo (rho) and greenhouse factor (gamma). The constructal law is used as the principle that governs the evolution of flow configuration on earth. The model showed that for two time-dependent scenarios, (delta(rho)=0.002; delta(gamma)=0.011) and (delta(rho)=0.002; delta(gamma)=0.005) the predicted equatorial and polar temperature increases and the time scales are (Delta T-II=1.16 K; Delta T-L=1.11 K; 104 years) and (0.41K; 0.41 K; 57 years), respectively. A continuous model of temperature variation was used to predict the thermal response of the Earth's surface to changes bounded by delta(rho)=delta(gamma) and delta(rho)=-delta(gamma) The polcward heat current reaches its maximum in the vicinity of 35 latitude, accounting for the position of the Ferrel cell between the Hadley and Polar Cells.
The heat transfer by natural convection and radiation in a two-dimensional cavity with one side heated, one side and bottom insulated, and the top open was investigated numerically. The first part of the study focused on natural convection alone, and concluded with closed-form correlations for the heat transfer from each of the side walls. It was found that the fluid rises along both side walls in the boundary layer regime. In the second part of the study, the natural convection correlations were incorporated into a seven-equation model for combined convection and radiation. The procedure needed for calculating the floating temperature of the insulated wall was illustrated numerically. The individual effects of the dimensions of the cavity, the temperature of the heated wall, and the emissivities of the two side walls were also documented numerically.
The natural convection flow and heat transfer between two enclosures that communicate through a vertical opening is studied by considering the evolution of an enclosed fluid in which the left half is originally at a different temperature than the right half. Numerical experiments show that at sufficiently high Rayleigh numbers the ensuing flow is oscillatory. This and other features are anticipated on the basis of scale analysis. The time scales of the oscillation, the establishment of thermal stratification, and eventual thermal equilibrium are determined and tested numerically. At sufficiently high Rayleigh numbers the heat transfer between the communicating zones is by convection, in accordance with the constant-Stanton-number trend pointed out by Jones and Otis (1986). The range covered by the numerical experiments is 102 < Ra < 107, 0.71 < Pr < 100, and 0.25 < H/L < 1.
This note contains a set of charts for the enthalpy change function Δh(T), the absolute entropy at atmospheric pressure s°(T), and the flow exergy at atmospheric pressure e°x(T) of 12 gases in the ideal-gas limit. The gases considered are CO, CO2, H, H2, H2O, N, N2, NO, NO2, O, O2 and OH. These charts and the ideal-gas model can be used to calculate the enthalpy h(T), absolute entropy s(T,P) and flow energy e x(T,P) of the same gases.
No abstract is provided for this article.
No abstract is provided for this article.
This article communicates a new problem and solution: Fluid is initially motionless in a rectangular space the shape of which is free to vary. A blade enters along one of the sides of the rectangle, moves at constant speed, and sets the fluid in motion by viscous diffusion. If the constructal law of design evolution is invoked, then the evolutionary direction is toward the shape of the rectangular space such that the entire fluid is set in laminar flow more easily, during the shortest time. The shape is found via scale analysis, and, coincidentally, it matches the shape of the boundary layer known from classical boundary layer theory. The difference between the two methods is fundamental: with the constructal law, the shape comes from the evolution of all the possible shapes toward greater access for the flow of momentum; with boundary layer theory, the shape is unique and comes from solving the simplified momentum equations for an assumed slender flow region.
Preface. Contributors. 1. Basic Concepts (Allan D. Kraus). 2. Thermophysical Properties of Fluids and Materials (R. T Jacobsen, E. W. Lemmon, S. G. Penoncello, Z. Shan, and N. T. Wright). 3. Conduction Heat Transfer (A. Aziz). 4. Thermal Spreading and Contact Resistances (M. M. Yovanovich and E. E. Marotta). 5. Forced Convection: Internal Flows (Adrian Bejan). 6. Forced Convection: External Flows (Yogendra Joshi and Wataru Nakayama). 7. Natural Convection (Yogesh Jaluria). 8. Thermal Radiation (Michael F. Modest). 9. Boiling (John R. Thome). 10. Condensation (M. A. Kedzierski, J. C. Chato, and T. J. Rabas). 11. Heat Exchangers (Allan D. Kraus). 12. Experimental Methods (Jose L. Lage). 13. Heat Transfer in Electronic Equipment (Avram Bar-Cohen, Abhay A. Watwe, and Ravi S. Prasher). 14. Heat Transfer Enhancement (R. M. Manglik). 15. Porous Media (Adrian Bejan). 16. Heat Pipes (Jay M. Ochterbeck). 17. Heat Transfer in Manufacturing and Materials Processing (Richard N. Smith, C. Haris Doumanidis, and Ranga Pitchumani). 18. Microscale Heat Transfer (Andrew N. Smith and Pamela M. Norris). 19. Direct Contact Heat Transfer (Robert F. Boehm). Author Index. Subject Index. About the CD-ROM.
This paper develops a mathematical model and a structured procedure to optimize the internal structure (relative sizes, spacings) and external shape (aspect ratios) of a unit PEM fuel cell so that net power is maximized. The optimization of flow geometry is conducted for the smallest (elemental) level of a fuel cell stack, i.e., the unit PEM fuel cell, which is modeled as a unidirectional flow system. The polarization curve, total and net power, and efficiency are obtained as functions of temperature, pressure, geometry and operating parameters. The optimization is subjected to fixed total volume. There are two levels of optimization: (i) the internal structure, which basically accounts for the relative thicknesses of two reaction and diffusion layers and the membrane space, and (ii) the external shape, which accounts for the external aspect ratios of a square section plate that contains all unit PEM fuel cell components. The available volume is distributed optimally through the system so that the net power is maximized. Temperature and pressure gradients play important roles, especially as the fuel and oxidant flow paths increase. Numerical results show that the optimized internal structure is “robust” with respect to changes in external shape. The optimized internal structure and external shape are results of the optimal balance between electrical power output and pumping power required to supply fuel and oxidant to the fuel cell through the gas channels. Directions for future improvements at the PEM fuel cell stack level in constructal geometric optimization are discussed.
This paper reports an experimental study of the nonaxisymmetric flow of a fast liquid jet discharging into the atmosphere. The nonaxisymmetric shape of the jet was photographed and subjected to a wavelength analysis. The results of the wavelength analysis demonstrate that the jet shape is governed by a narrow band of wavelengths associated with a characteristic value λmax which scales with the jet diameter D. It is shown that the experimental observations are in agreement with predictions based on hydrodynamic stability theory and buckling theory.
This paper shows numerically how to select the optimal spacing between boards mounted in a stack of specified volume, so that the overall thermal conductance between the stack and the forced coolant is maximum. Several configurations are considered: boards with uniform flux, flush-mounted discrete sources, and protruding heat sources. The flow is laminar and the pressure difference across the stack is fixed (Δp). It is shown that for all the board geometries and thermal boundary conditions studied, the optimal board-to-board spacing is correlated by (Dopt/l ∼ 2.7(Δpl2 / μα)−¼where l is the effective longitudinal (flow) distance occupied by the discrete sources and the unhealed patches contained between them, and μ and α are the viscosity and thermal diffusivity of the fluid. If U∞is the free-stream velocity upstream of the stack, the optimal spacing is given by (Dopt/l) ∼ 3.2pr−¼(U∞l / v)−½.
People like to say that energy and water are two problems, two vital commodities in short supply. Here I draw attention to the emerging literature and physics principle (constructal law) that provide the scientific foundation for sustainability. I show that the sustainability need is about flow : the flow of energy and the flow of water through the inhabited space. All the flows needed for human life (transportation, heating, cooling, water) are driven by the purposeful consumption of fuels. This is why the wealth of a country (the GDP) is directly proportional to the annual consumption of fuel in that country. This hierarchical organization happens; it is natural and efficient. Sustainability is the one-word need that covers all the specific needs. Sustainability comes from greater freedom in changing the organization – the flow architecture – that sustains life. Greater freedom to change the design (from water and power to laws and government) leads to greater flow, wealth, life and staying power, i.e. sustainability.
Under certain conditions, the heat transfer rate from a surface covered with hair is due to two competing effects: i) the heat transfer to the slowed air flow, through the bare portions of the skin, and ii) the heat transfer through all the hair strands, which act as...
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 relative 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 along the duct. Numerical simulations document the effects of the dimensionless pressure drop on the optimized configurations, and show a 15% enhancement in heat transfer rate density.
In this chapter we focus on the equation that expresses the first law of thermodynamics in a porous medium. We start with a simple situation in which the medium is isotropic and where radiative effects, viscous dissipation, and the work done by pressure changes are...
