901 publications from this institution
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.
Accurate transition onset modeling is a fundamental part of modern turbomachinery designs, where bypass transition is the dominant mechanism of transition to turbulence. Despite this situation a range of transition onset models exist primarily based upon both integral and local parameters within the boundary layer. All such transition models have empirical origins. To date the relationships between such models has not been forthcoming and hence lack of physical understanding of the transition process is evident. This paper details a new approach to transition modeling and provides a theoretically based approach to transition onset prediction by invoking a single principle developed within constructal theory. We not only present a new model but also demonstrate the equivalence between existing models by implementing the same theory. Such understanding of the transition onset problem may provide a new perspective towards more theoretically based transition onset models rather than empirical ones, although much work remains to be done in understanding the receptivity mechanisms within a laminar boundary layer.
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 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.
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.
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.
Comments on viscus buckling of thin fluid layers are pretented. (AIP)
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.
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.
No abstract is provided for this article.
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...
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 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 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.
The present study is aimed at bringing to the public and specialists’ attention an archaeological discovery madein 2005, which is of great importance to the complex issue of Sarmatian settlement on the lower course of theMureş river.The archaeological location is well known by the townspeople as a place of provenance for many artefacts.Even so, in modern times it has created confusion in regards to the „Roman Age” settlement at Sânnicolau Mare,which has been perpetuated even in archaeological studies, thus giving rise to a great historiographic debate in thelast century and a half. The discussion of a Roman presence in the western Banat area has its starting point in theXIXth century, based on information that is both very old and ambiguous, regarding accidental discoveries madeby non-specialists. This information was then passed down as certainties in modern historiography.Systematic research on the archaeological site was carried out for 12 years, (1995-2007), by a team lead byProfessor Adrian Bejan, Universitatea de Vest, Timişoara. The research has revealed a very complex archaeologicalsite, with a median thickness of the archaeological deposit of about 2 m (and thicker than 3 m in some areas),containing numerous archaeological complexes which can be dated as early as the Bronze Age and as late as theLate Dark Ages (above ground houses, hovels, a well, workshops etc.). The archaeological artefacts accumulatedfrom the successive years of research are impressive, being mostly composed of ceramic shards, some of which canbe restored, but also metal artefacts (bronze, iron) and even stone (grinding stones made of tufa or andesite, largemica-schist fragments etc.) of diverse in type and usage.Apart from all stand the Sarmatian Age and Dark Age (both Early and Late) burial sites, which containnumerous graves. Many of these graves lack artefacts, but in one, the subject of the present study, gold pendants,a medallion and beads were discovered.The S-N orientation of the grave is typical the Iazig Sarmatians in the Carpathian Basin. It is worth mentioningthat the right forearm and arm bones are missing. Two human bone fragments (possibly forearm) were found inthe head region, accompanied by a bronze bracelet (Fig. 8-9). The grave contained, besides clay pots, adornmentsof gold and glass (a necklace of gold beads and pendants horseshoe and tear shaped, as well as blue and navycolored glass, limestone and glass beads) and a spindle whorl.In our opinion the grave discovered at Sânnicolau Mare-Selişte is proof that Metanast Iazigs had entered to asmall degree the north western corner of the Banat. Other Early Sarmatian Age burials have not been unearthedas of yet in this region; the Sarmatians having settled on the Banat plains during the last third of the 2nd centuryA.D. The grave at Sânnicolau Mare-Selişte dates during the 1st century A.D., based on these gold jewelries beingthe earliest Sarmatian artefacts.Although we do not have an anthropological assessment of the skeleton, based on the grave inventory andanalogies with other burials of women containing jewelry, in the Carpathian Basin, we believe the individualburied at Sânnicolau Mare-Selişte was a woman. She had been buried with all due respect, as a high social statusdemanded, as indicated by the jeweler and food offerings. Questions remain regarding the missing right forearm,which may be related to the fragments in the head area, perhaps as some sort of magic ritual.
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.
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.
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.
