Publications

The objective of the present article is to compare previous experimental data of Gao et al. [20] to the predictions of Bejan and Sciubba's analysis [7] on the optimal spacing for maximum heat transfer from a package of parallel plates. Experimental investigations of the flow and the associated heat transfer were conducted in two-dimensional microchannels in order to test possible size effects on the laws of hydrodynamics and heat transfer and to infer optimal conditions of use from the measurements. The test section was designed to modify easily the channel height e between 1 mm and 0.1 mm. Measurements of the overall friction factor and local Nusselt numbers show that the classical laws of hydrodynamics and heat transfer are verified for e > 0.4 mm. For lower values of e, a significant decrease of the Nusselt number is observed, whereas the Poiseuille number continues to have the conventional value of laminar developed flow. The transition to turbulence is not affected by the channel size. The experimental data were processed by using the dimensionless parameters of Bejan and Sciubba's analysis [7]. For fixed pressure drop across the channel, a maximum of heat transfer rate density is found for a particular value of e. The corresponding dimensionless optimal spacing and heat transfer rate density are in very good agreement with the predictions of Bejan and Sciubba. This article reports the first time that the optimal spacing between parallel plates is determined experimentally. Keywords: microfluidicsheat transfermicrochannelsoptimization

This paper reports two new advances on constructal design, or the generation of flow architecture in the pursuit of global thermodynamic performance subject to constraints. This is thermodynamic optimization with an emphasis on its product: flow geometry, as a mechanism for the achievement of performance under constraints. The first part of the paper relies on this principle in the design of dendritic (tree-shaped) networks that distribute a stream optimally from one source to a disc-shaped area for which the source is the center. One global objective is (i) the minimization of source-area flow resistance, which, when the overall flow rate is fixed, is the same as the minimization of the destruction of exergy. The alternative (ii) is to design the flow architecture such that the flow path followed by each stream is the shortest. Optimal dendritic flow architectures are reported based on both methods. It is shown that the performance of designs (ii) is nearly the same as that of the optimal designs (i). Examples of dendritic patterns of high-conductivity blades used for cooling a disc-shaped body show that the overall thermal resistance of the construct decreases as the size and complexity of the flow architecture increase. The same principle of maximizing global performance can be used to select the optimal sizes of flow components in all complex energy flow systems (e.g., vehicles, animals). The size of a heat transfer surface and the diameter of a tube with fluid are given as examples.

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.

This paper outlines a combined theoretical and numerical study of the mass transfer effected by high Rayleigh number Bénard convection in a two-dimensional saturated porous layer heated from below. The focus of this study is on the Darcy flow, heat transfer and mass transfer scales of a single cell (roll) that exists in the steady two-dimensional convection regime. The numerical solutions are based on the complete governing equations for two-dimensional flow, and cover the Rayleigh number range 50–2000. The numerical results compare favorably with the theoretical conclusions of a scale analysis that is based on the recognition of 1. (i) two temperature difference scales in the cell, 2. (ii) a flow field without horizontal boundary layers, and 3. (iii) thermal top and bottom end-regions that are not slender enough to be boundary layers. Writing Le for the Lewis number, the overall mass transfer rate or Sherwood number is shown to scale as Le 1 2 Ra 7 8 if Le > Ra 1 4 , as Le2 Ra 1 2 if Ra− 1 4 < Le < Ra 1 4 , and as O(1) if Le < Ra− 1 4 . The transition from the Darcy flow to the inertia-dominated Forschheimer flow and the scales of the Forschheimer regime are discussed in the closing section.

In this paper we explore the use of vascular design that provides cooling and mechanical strength at the same time. We illustrate the concept with a circular plate vascularized with embedded channels. The cooling fluid enters to the plate from the center or from the rim, and leaves after it cools the plate down to an allowable temperature level. The vascular cooling channels also affect the mechanical strength of the plate. We simulated numerically the thermofluid and mechanical behavior for three different structures; radial, dendrites with one pairing level and dendrites with two pairing levels. We found that for a given set of conditions (applied pressure difference, coolant inlet position, and number of the cooling channels) there is one configuration that is best; however, there is no single configuration that is best for all conditions.

Objective Data in electronic health records (EHRs) is being increasingly leveraged for secondary uses, ranging from biomedical association studies to comparative effectiveness. To perform studies at scale and transfer knowledge from one institution to another in a meaningful way, we need to harmonize the phenotypes in such systems. Traditionally, this has been accomplished through expert specification of phenotypes via standardized terminologies, such as billing codes. However, this approach may be biased by the experience and expectations of the experts, as well as the vocabulary used to describe such patients. The goal of this work is to develop a data-driven strategy to (1) infer phenotypic topics within patient populations and (2) assess the degree to which such topics facilitate a mapping across populations in disparate healthcare systems. Methods We adapt a generative topic modeling strategy, based on latent Dirichlet allocation, to infer phenotypic topics. We utilize a variance analysis to assess the projection of a patient population from one healthcare system onto the topics learned from another system. The consistency of learned phenotypic topics was evaluated using (1) the similarity of topics, (2) the stability of a patient population across topics, and (3) the transferability of a topic across sites. We evaluated our approaches using four months of inpatient data from two geographically distinct healthcare systems: (1) Northwestern Memorial Hospital (NMH) and (2) Vanderbilt University Medical Center (VUMC). Results The method learned 25 phenotypic topics from each healthcare system. The average cosine similarity between matched topics across the two sites was 0.39, a remarkably high value given the very high dimensionality of the feature space. The average stability of VUMC and NMH patients across the topics of two sites was 0.988 and 0.812, respectively, as measured by the Pearson correlation coefficient. Also the VUMC and NMH topics have smaller variance of characterizing patient population of two sites than standard clinical terminologies (e.g., ICD9), suggesting they may be more reliably transferred across hospital systems. Conclusions Phenotypic topics learned from EHR data can be more stable and transferable than billing codes for characterizing the general status of a patient population. This suggests that EHR-based research may be able to leverage such phenotypic topics as variables when pooling patient populations in predictive models.

Technical Briefs The Optimal Spacing for Cylinders in Crossflow Forced Convection A. Bejan A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708-0300 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 27708-0300 J. Heat Transfer. Aug 1995, 117(3): 767-770 (4 pages) https://doi.org/10.1115/1.2822645 Published Online: August 1, 1995 Article history Received: January 1, 1994 Revised: October 1, 1994 Online: December 5, 2007

In this paper, we show how to use constructal design to distribute solar chimney power production on available land area most efficiently. The solar chimney design is used as an example of solar-driven power plant. We found that the power generated per unit of land area is proportional to the length scale of the power plant, as well as to the chimney height, roof radius, and chimney radius. It was also found that the chimney height, roof radius, and chimney radius cannot increase independently and indefinitely. Because of the flow resistances associated with distributing the power over a territory, the land area for each power plant must be finite and allocated optimally. Several patterns of the multi-scale plants on a square area are explored. The global performance of such patterns is greater when more land area is allocated to the largest plant. This performance depends comparatively less on the total land area covered by all power plants.

This paper is a fundamental study of the effect of junction losses on the optimized geometry of tree-shaped flows. Several classes of flows are investigated systematically in a T-shaped construct with fixed internal and external size: laminar with non-negligible entrance and junction losses, and turbulent in tubes with smooth and rough walls. It is shown that in all cases junction losses have a sizeable effect on optimized geometry when Sv 2 <10, where the svelteness Sv is a global property of the entire flow system: Sv =external length scale/internal length scale. The relationship between the global Sv and the slenderness of individual channels is discussed. The study shows that, in general, the duct slenderness decreases as the tree architecture becomes finer and more complex. In conclusion, miniaturization pushes flow architectures not only toward the smaller, finer and more complex, but also toward the domain in which junction losses must be taken into account in the optimization of geometry.

. This review article shows that the occurrence of macroscopic flow configuration is a universal natural phenomenon that can be explained and predicted on the basis of a principle of physics (the constructal law): "For a flow system to persist in time (to survive) it must evolve in such a way that it provides easier and easier access to the currents that flow through it". The examples given in this article come from natural inanimate flow systems with configuration: duct cross-sections, open channel cross-sections, tree-shaped flow architectures, and turbulent flow structure (e.g., eddies, laminar lengths before transition). Other examples that are treated in the literature, and which support the constructal law, are the wedge-shape of turbulent shear layers, jets and plumes, the frequency of vortex shedding, Bénard convection in fluids and fluid-saturated porous media, dendritic solidification, the coalescence of solid parcels suspended in a flow, global atmospheric and oceanic circulation and climate, and virtually all architectural features of animal design. The constructal law stresses the importance of reserving a place for pure theory in research, and for constantly searching for new physics – new summarizing principles that are general, hence useful.

This paper shows that the power output of various power plant configurations can be maximized by properly dividing the fixed inventory of heat exchange equipment among the heat transfer components of each plant. This conclusion is built over a sequence of fundamental power-maximization problems: the solar power plant with total area constraint, the solar power plant with storage by melting, the power plant with two heat exchangers and total area constraint, the combined-cycle power plant, the power plant viewed as an insulation between its heat source and heat sink, and the power plant with by-pass heat leak to the ambient. New diagrams are reported for the power plant at maximum power, and the refrigeration plant at maximum refrigeration load.

A swarm is a temporary structure formed when several thousand honey bees leave their hive and settle on some object such as the branch of a tree. They remain in this position until a suitable site for a new home is located by the scout bees. A continuum model based on heat conduction and heat generation is used to predict temperature profiles in swarms. Since internal convection is neglected, the model is applicable only at low values of the ambient temperature T a . Guided by the experimental observations of Heinrich (1981a–c, J. Exp. Biol. 91, 25–55; Science 212, 565–566; Sci. Am. 244, 147–160), the analysis is carried out mainly for non-spherical swarms. The effective thermal conductivity is estimated using the data of Heinrich (1981a, J. Exp. Biol. 91, 25–55) for dead bees. For T a = 5 and 9°C, results based on a modified version of the heat generation function due to Southwick (1991, The Behaviour and Physiology of Bees, pp. 28–47. C.A.B. International, London) are in reasonable agreement with measurements. Results obtained with the heat generation function of Myerscough (1993, J. Theor. Biol. 162, 381–393) are qualitatively similar to those obtained with Southwick's function, but the error is more in the former case. The results suggest that the bees near the periphery generate more heat than those near the core, in accord with the conjecture of Heinrich (1981c, Sci. Am. 244, 147–160). On the other hand, for T a = 5°C, the heat generation function of Omholt and Lønvik (1986, J. Theor. Biol. 120, 447–456) leads to a trivial steady state where the entire swarm is at the ambient temperature. Therefore an acceptable heat generation function must result in a steady state which is both non-trivial and stable with respect to small perturbations. Omholt and Lønvik's function satisfies the first requirement, but not the second. For T a = 15°C, there is a considerable difference between predicted and measured values, probably due to the neglect of internal convection in the model.

This is a theoretical, numerical and experimental study of how to select the spacing (S) between horizontal cylinders in an array with laminar natural convection, such that the total heat transfer (q) between the array and the ambient is maximized. The volume occupied by the array (height H, width W, cylinder length L) and the cylinder diameter (D) are arbitrary but fixed, while the spacing (or number of cylinders in the array) varies. The optimal spacing and maximum heat transfer results predicted theoretically are developed into accurate and well tested correlations by means of numerical simulations and experimental measurements. The recommended correlations are Sopt/D = 2.72(H/D) 1 3 RaD − 1 4 +0.263 and q ̃ max = 0.448[( H D ) 1 3 Ra D − 1 4 ]− 1 6 where q ̃ max is the dimensionless maximum overall thermal conductance, q ̃ max = q max D2/[HLWk(T w - T∞)]. The optimal spacing is relatively insensitive to whether the cylinders are isothermal or with uniform heat flux.

A Carnot engine operating on a closed cycle and having both external and internal irreversibilities is analyzed. The internal irreversibilities are caused by losses generate by finite piston speed and the external irreversibilities are caused by heat transfer through a temperature difference. The irreversible Carnot cycle is displayed on a original T-S property coordinates in a manner that accurately illustrates the lost work (Exergy losses) due to the irreversibilities (internal and external). A method for calculating the effect of the piston speed on the internal irreversibilities of Carnot cycle is developed and an example of the results is shown for a range of values of cycle high temperature (2000 K). Using the results of this example, the optimal Carnot engine efficiency is determined as a function of piston speed. The example is extended to include the determination of the optimal system temperature for Maximum Carnot engine power over a range of piston speeds.