901 publications from this institution
No abstract is provided for this article.
No abstract is provided for this article.
This paper reports a theoretical and experimental study of the fundamental mechanism responsible for transition in natural convection plume flow. Theoretically, it is argued that the transition occurs when the time of viscous penetration normal to the plume becomes comparable with the minimum time period with which the plume can fluctuate as an unstable inviscid stream. It is also argued that at transition the plume wavelength must always scale with the local plume diameter. The experimental part of the study focused on transition in the axisymmetric air plume above a point heat source. Smoke visualization of the plume shape at transition led to extensive observations that support strongly the transition mechanism proposed theoretically. The transitional plume is seen to meander in a plane (two-dimensionally) and with a wavelength which scales with the plume diameter. If excited externally by many such wavelengths, the plume has the property to select the natural wavelength proposed theoretically. The equivalence between the present transition mechanism and the transition predicted by the buckling theory is discussed.
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.
No abstract is provided for this article.
Recent developments in thermodynamic optimization are reviewed by focusing on the generation of optimal geometric form (shape, structure, topology) in flow systems. The flow configuration is free to vary. The principle that generates geometric form is the pursuit of maximum global performance (e.g., minimum flow resistance, minimum irreversibility) subject to global finiteness constraints (volume, weight, time). The resulting structures constructed in this manner have been named constructal designs. The thought that the same objective and constraints principle accounts for the optimally shaped flow paths that occur in natural systems (animate and inanimate) has been named constructal theory. Examples of large classes of applications are drawn from various sectors of mechanical and civil engineering: the distribution of heat transfer area in power plants, optimal sizing and shaping of flow channels and fins, optimal aspect ratios of heat exchanger core structures, aerodynamic and hydrodynamic shapes, tree-shaped assemblies of convective fins, treeshaped networks for fluid flow and other currents, optimal configurations for streams that undergo bifurcation or pairing, insulated pipe networks for the distribution of hot water and exergy over a fixed territory, and distribution networks for virtually everything that moves in society (goods, currency, information). The principle-based generation of flow geometry unites the thermodynamic optimization developments known in mechanical engineering with lesser known applications in civil engineering and social organization. This review extends thermodynamics, because it shows how thermodynamic principles of design optimization account for the development of optimal configurations in civil engineering and social organization.
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This paper considers the fundamental problem of how to shape rectangular high-conductivity inserts (fins) that are mounted on the rim of and protrude into a disc-shaped body that generates heat. The objective is to minimize the global thermal resistance by optimizing geometrically the distribution of a fixed amount of high-conductivity material through the material of lower conductivity. In addition to the fin geometry, three other design parameters are considered: the ratio between the high conductivity and low conductivity k ̃ , the relative amount of high conductivity material φ, and the number of sectors of the disc-shaped body, N. It is shown analytically and numerically that the thermal resistance can be minimized with respect to the fin aspect ratio, λ. The optimized geometry and performance are reported graphically as functions of k ̃ , φ and N. Good agreement is found between the analytical solution and the numerical results.
Superconducting magnets are energized through helium vapour-cooled cryogenic current leads operating at high ratios of current to mass flow. The high current operation where lead temperature, runaway, and eventual burn-up are likely to occur is investigated. A simple criterion for estimating the burn-up operation conditions (current, mass flow) for a given lead geometry (cross-sectional area, length, heat exchanger area) is presented. This article stresses the role played by the available heat exchanger area in avoiding burnup at high ratios of current to mass flow.
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 bys Cheng (1978, 1985b). On the other hand, geothermal reservoir...
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This paper addresses the fundamental heat transfer augmentation question of how to arrange a stack of parallel plates (e.g. fins of heat sink, printed circuit boards) in a free stream such that the thermal resistance between the stack and the stream is minimum. It is shown that the best way of positioning the plates relative to one another is by spacing them equidistantly. When the overall dimensions of the stack are specified, there is an optimal number of plates for minimum thermal resistance. The optimal number and minimum resistance are anticipated theoretically and correlated into compact formulas that agree with numerical and experimental results in the ReL range 102-104. Finally, it is shown that a stack with more plates than the optimal number can be modeled more expediently as a porous block immersed in a free stream.
This note summarizes the results of a numerical study designed to question (i.e., refute or validate) Chao et al.'s1 and Bertin and Ozoe's2 conclusion that the critical Rayleigh number increases substantially as the Prandtl number becomes very small. The numerical method is based on the finite-difference control volume formulation and the complete equations for two-dimensional (2-D) time-dependent flow. The present results show that the lowest attainable Rayleigh number for numerically simulated convection increases as Pr decreases below 0.1. These results also extend the Prandtl number domain of the observations down to Pr=10−4 and indicate that the natural shape of a single roll in this Pr range is approximately square. The discrepancy between these observations and the constant Rac=1,707.8 of the linear stability analysis is attributed to the extrapolation method on which the numerical convection-onset Ra data1,2 were based. It is shown that the numerical results agree with the linear stability constant Rac=1,707.8 and Schlüter et al.'s9 small amplitude perturbation analysis.
Biologists have treated the view that fundamental differences exist between running, flying and swimming as evident, because the forms of locomotion and the animals are so different: limbs and wings vs body undulations, neutrally buoyant vs weighted bodies, etc. Here we show that all forms of locomotion can be described by a single physics theory. The theory is an invocation of the principle that flow systems evolve in such a way that they destroy minimum useful energy (exergy, food). This optimization approach delivers in surprisingly direct fashion the observed relations between speed and body mass (M(b)) raised to 1/6, and between frequency (stride, flapping) and M(b)(-1/6), and shows why these relations hold for running, flying and swimming. Animal locomotion is an optimized two-step intermittency: an optimal balance is achieved between the vertical loss of useful energy (lifting the body weight, which later drops), and the horizontal loss caused by friction against the surrounding medium. The theory predicts additional features of animal design: the Strouhal number constant, which holds for running as well as flying and swimming, the proportionality between force output and mass in animal motors, and the fact that undulating swimming and flapping flight occur only if the body Reynolds number exceeds approximately 30. This theory, and the general body of work known as constructal theory, together now show that animal movement (running, flying, swimming) and fluid eddy movement (turbulent structure) are both forms of optimized intermittent movement.
This paper addresses the fundamental problem of optimizing the geometry of an electromagnet by maximizing at the same time its magnetic performance and thermal performance. The solenoid has a cylindrical shape and a uniform current density. Cooling discs made of high thermal conductivity material are inserted in the coil to collect the heat generated by Joule heating. The collected heat is evacuated to the ambient. For a given magnetic performance and volume, the maximum temperature inside the electromagnet is minimized by selecting the shape of the coil (length and outer radius), the number of cooling discs, and the amount of high thermal conductivity material. The two objectives pursued in this geometric optimization (magnetic and thermal) show that the constructal design method can be extended to the generation of architecture for multi-objective systems.
The design of thermally efficient supports for cryogenic vessels is approached with renewed interest in view of today’s large-scale applications of superconductivity [1]. In large superconducting systems such as electric machines and magnetic energy storage...
The floating constants in Weber's boundary layer solution for free convection in a differentially heated vertical porous slab are reevaluated with a new approach. This approach uses Weber's solution to calculate the net vertical heat flux which is equated to zero near the top and bottom ends of the enclosure. It is shown that the Nusselt numbers predicted with the new constants are in excellent agreement with experimental and numerical results.
This paper is a review of current work on the optimal distribution of discrete heat sources cooled by laminar natural convection. The global objective is to maximize the global conductance between the wall and the fluid, or to minimize the hot-spot temperatures when the total heat generation rate and global system dimensions are specified. Three scenarios are investigated: (i) a large number of small heat sources mounted on a vertical wall facing a fluid, (i) a small number of finite-size heat sources mounted on the side wall in a two-dimensional enclosure, and (iii) a heated area on the wall of a vertical diverging or converging channel with chimney flow. For (i) and (ii), it is shown that the optimal distribution is not uniform (the sources are not equidistant), and that as the Rayleigh number increases the heat sources placed near the tip of a boundary layer should have zero spacings. In configuration (iii), the geometry is free to change in three directions: by varying the space between the walls, the distribution of heating along the walls, and the angle between the two walls. Numerical simulations cover the Rayleigh number range 105 ≤ RaH ≤ 107, where H is the channel height. Nonuniform wall heating is modeled as an isothermal patch of varying height, which is placed near the entrance of the channel, or near the exit. It is shown that for maximal heat transfer rate density it is better to place the heated sections at the channel entrance. The optimal angle between the two walls is approximately zero when RaH is large. The optimized spacing is of the same order of magnitude as the optimal spacing reported earlier for parallel isothermal walls. The robustness of flow architectures with optimized distribution of heat sources is discussed.
This is a review of recent analytical and numerical work on the generation and flow of methane gas through a layer of porous medium impregnated with solid clathrate hydrates. The porous layer is depressurized suddenly on its lower plane and the phase-change front advances under the influence of heat conduction and convection. The first part of the chapter describes a simplified analytical solution based on a unidirectional phase-change model in which the conduction in the gas-filled region behind the front is neglected. The chapter continues with numerical results for the evolution of the unidirectional phase-change process. Both methods lead to the conclusion that the rate of gas flow through the depressurized (bottom) plane of the layer decreases approximately as t −1/2. Further numerical modeling shows that the presence of a vertical geothermal gradient has a significant effect on the rate of gas generation. Numerical results for phase change and gas generation in a porous sediment with non-uniform porosity and permeability are also reported.
