This paper reports a new concept for maximizing heat transfer density in assemblies of cylinders in cross-flow: the use of cylinders of several sizes, and the optimal placement of each cylinder in the assembly. The heat transfer is by laminar forced convection with specified overall pressure difference. The resulting flow structure has multiple scales that are distributed nonuniformly through the available volume. Smaller cylinders are placed closer to the entrance to the assembly, in the wedge-shaped flow regions occupied by fluid that has not yet been used for heat transfer. The paper reports the optimized flow architectures and performance for structures with 1, 2 and 3 cylinder sizes, which correspond to structures with 1, 2 and 4 degrees of freedom. The heat transfer rate density increases (with diminishing returns) as the optimized structure becomes more complex. The optimized cylinder diameters are relatively robust, i.e., insensitive to changes in complexity and flow regime (pressure difference). The optimized spacings decrease monotonically as the driving pressure difference increases. The multi-scale flow architectures optimized in this paper have features and qualities similar to tree-shaped (dendritic) designs, where the length scales are numerous, hierarchically organized, and nonuniformly distributed through the available space.
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 (1978, 1985b). On the other hand, geothermal reservoir...
The phenomenon of natural convection in a horizontal pipe with different end temperature was studied experimentally in a cylindrical cavity with the ratio (diameter) (length) = 0.112 . The Rayleigh number based on diameter was in the range 108 < Ra < 1010. It was concluded that in this range the natural convection mechanism departs considerably from the pattern known in the limit Ra → 0. Specifically, the end-to-end heat transfer is effected via two thin horizontal jets, the upper (warm) jet proceeding along the top of the cylinder toward the cold end and the lower (cold) jet advancing along the bottom in the opposite direction. The region sandwiched between the two jets is filled with nearly stagnant fluid. In this region the temperature varies linearly with depth. In each vertical cross-section, the top-bottom temperature difference is of the same order of magnitude as the end-to-end temperature difference. The Nusselt number for end-to-end heat transfer was shown to vary weakly with the Rayleigh number.
This paper completes the description of geometry optimization in stacks of parallel plates that generate heat. The spacing between plates, or the number of plates in a fixed volume, has been maximized in two limits: pure natural convection and pure forced convection. In this paper, the in-between regime of mixed convection is modeled numerically. After simulating the flow and temperature fields in configurations with a variety of spacings, this paper reports the optimal spacings and the dimensionless groups that govern them (Rayleigh number, pressure drop number, mixed convection ratio). It shows that the numerical results match the results in the limits of natural convection and forced convection. The paper constructs a correlation that bridges the gap between the two limits, and provides a single formula for optimal spacings covering the entire domain, from natural convection to forced convection.
Comments on viscus buckling of thin fluid layers are pretented. (AIP)
This paper describes the optimization of the distribution of heat transfer (cooling) during the process of gas compression. The coolant is a stream of cold liquid. There is a fundamental tradeoff between the savings in compressor power, which are due to distributed cooling, and the pumping power required to circulate the coolant. The tradeoff is revealed on the basis of a combined model of multi-stage gas compression, resistance to fluid flow, and area-constrained counter- or co-current heat exchange between the gaseous stream and the liquid stream. The results are illustrated for the compressor of an actual ammonia refrigeration plant, for which the distributed-cooling design is highly recommended because the compressor discharge temperature in such units is high. It is shown that there is an optimal coolant (water) flow rate such that the total power requirement is minimized. The optimized distribution of gas compression and cooling is robust with respect to the selection of the water flow rate.
Thermodynamic optimization is, literally, the search for the best thermodynamic performance subject to present-day constraints. This philosophy is very old. It has been with us throughout the history of the heat engine. Its reach however, is much broader and more...
A linear stability analysis of the large-scale structure of a round jet surrounded by an annular shear layer is presented. The study is limited to the developing region near the jet nozzle in the limit Re→∞. The radial dependence of the amplitudes of growing disturbances are examined in order to illustrate the extent to which the disturbances penetrate into the jet and its surroundings. The region influenced by a disturbance is found to be directly proportional to the wavelength of the disturbance. Disturbance measurements made on the jet centerline tend to select for long wavelength disturbances, while measurements made in the shear layer tend to select for short wavelength disturbances. When the shear layer thickness is small compared with the jet radius, the wavelength of the most amplified disturbance scales with the shear layer thickness. As the shear layer thickness increases, this scaling quickly breaks down. This change in scaling appears to be responsible for the transition between the “ripples” which occur near the jet nozzle and the “puffs” which are observed further downstream. Amplified disturbances exhibit a phase lag across the shear layer, which may account for the spade-like structures evident in flow visualizations of turbulent jets.
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 paper documents two methods of improving the performance of volume-to-point tree networks for two-dimensional heat conduction. These improvements are offered relative to the design produced by the constructal method, in which optimized volume elements (building blocks) are presented and grouped into larger constructs, which are also optimized. The first improvement is that each construct is optimized with respect to all its degrees of freedom: unlike in the constructal method, the optimized features of the smaller building blocks are not preserved during the optimization of the construct. The second improvement opportunity is that spacings are allowed between the facing tips of neighboring high-conductivity inserts. All such spacings are optimized. Another new direction pursued in this paper is the optimization of tree networks that must fill a volume with prescribed external shape (e.g., square, or disk). The designs optimized numerically in this paper lower the volume-to-point resistance beyond the levels achieved based on the constructal method, and bring the shape of the conduction tree closer to the shapes seen in nature.
This review covers two aspects of "evolution" in thermodynamics. First, with the constructal law, thermodynamics is becoming the domain of physics that accounts for the phenomenon of evolution in nature, in general. Second, thermodynamics (and science generally) is the evolving add-on that empowers humans to predict the future and move more easily on earth, farther and longer in time. The part of nature that thermodynamics represents is this: nothing moves by itself unless it is driven by power, which is then destroyed (dissipated) during movement. Nothing evolves unless it flows and has the freedom to change its architecture such that it provides greater and easier access to the available space. Thermodynamics is the modern science of heat and work and their usefulness, which comes from converting the work (power) into movement (life) in flow architectures that evolve over time to facilitate movement. I also review the rich history of the science, and I clarify misconceptions regarding the second law, entropy, disorder, and the arrow of time, and the supposed analogy between heat and work.
The hydrogen economy is a possible alternative to the current oil based global economy. The technology to build and operate fuel cells is well advanced. However, cost is the reason why fuel cells are not being installed wherever there is a need for more power. Therefore, optimization is a natural alternative to reduce cost and make fuel cells increasingly more attractive for power generation. This paper discusses the process of determining the internal geometric configuration of a unit fuel cell for maximum power. The optimization of construction (architecture) starts at the smallest (elemental) fuel cell level. The optimization of system architecture must be subjected to a fixed volume constraint. There are several degrees of freedom in the fuel cell configuration, i.e., the thickness of two gas channels (fuel and oxidant), two diffusion layers and two reaction layers (anode and cathode) and the electrolyte solution space. Research perspectives for fuel cells are presented and discussed.
Here we show that the main features of citations history (S-shaped curve, increasing h index, decreasing m quotient) are predictable with the constructal law of how an idea flows over a populated territory. It flows in two ways: fast and long, as “convection” along established channels of researchers interested in the idea, and slow and short, sideways from channels, by “diffusion” to new users. The first regime accounts for the rising portion of the S curve, and the second for the slow-down. Tree-shaped established channels spread the idea faster than single channels over the same area. The complexity of the design of fast channels increases in time due to the evolution of communications technologies.
No abstract is provided for this article.
In this paper we report the main advances made by our research group on the heat transfer performance of complex stream architectures embedded in a conducting solid. The immediate application of this review work deals with ground-coupled heat pumps. Various configurations are considered: U-shaped with varying spacing between the parallel portions of the U, serpentines with three elbows, and trees with T- and Y-shaped bifurcations. In each case the volume ratio of fluid to soil is fixed. We determine the critical geometric features that allow the heat transfer density of the stream-solid configuration to be the highest that it can be. In the case of U-tubes and serpentines, the best spacing between parallel portions is discovered, whereas the vascular designs morph into bifurcations and angles of connection that provide progressively greater heat transfer rate per unit volume. Next we move to more complex underground structures, connecting several heat pumps to the same fluid loop. We conclude by comparing the merits of the two options.
The thermodynamics of solar thermal energy conversion installations is reviewed in the spirit of second law analysis, where the mission of such installations is to extract exergy from solar thermal radiation. The paper begins with a review of the equilibrium...
This paper documents the joint performance of heat pumps that are served by a common loop buried in the ground, and which operate simultaneously: one heat pump absorbs heat from the buried loop whereas the other one rejects heat. A background flow is circulated in the underground loop even when the two heat pumps are not operating. The objective is to determine the performance and the manner in which it is affected by the way in which the two heat pumps are connected to the loop. The performance measures are the heat transfer rates into and out of the heat pumps, and the total pumping power required by the assembly. The paper documents the individual performance of the heat pumps, and their relative performance, which is the ratio of heating absorbed by one pump to the heating rejected by the other pump.
Condensers are flow architectures needed to provide high rates of condensation (or cooling) per unit volume, in enclosures with fixed volume. Their design has not changed from configurations consisting of the banks of horizontal tubes. In this paper, we outline a free path to evolving the design by exploring new features of flow configuration: flattened tubes, multiple tube sizes, arrays of flattened tubes, vertical tubes with turbulent film flow, forced convection condensation instead of gravity driven condensation, and the optimal length of a horizontal tube, i.e., the number of tubes in a column aligned with vapor cross flow. We show that the condensation density can be increased sizably by varying freely and without bias the morphology of the flow system: the shapes and arrangement of the cooled surfaces on which condensation occurs. The evolution of technology is described in terms of the special time direction of the useful (purposeful) changes in the configuration (shapes, arrangements) of surfaces on which flow/condensation occurs. This explains what “evolution” means. It is an important step for physics, not just technology.
