This paper reports experimental observations on transient natural convection in enclosures at high Rayleigh numbers (1.28×109, 1.49×109) and low aspect ratios (0.0625, 0.112). The phenomenon consists of the establishment of thin intrusion layers along the horizontal adiabatic surfaces; in time, the intrusion layers exchange heat with the isothermal core of the cavity, leading to the thermal stratification of the core. The approach to steady state is gradual, contrary to the theoretical prediction of Brunt-Vaisala wave motion (Patterson and Imberger [6]). The measured durations of the observed transients agree very well with theoretical estimates.
No abstract is provided for this article.
Here we show that bodies of the same size suspended uniformly in space constitute a system (a “suspension”) in a state of uniform volumetric tension because of mass-to-mass forces of attraction. The system “snaps” hierarchically, and evolves faster to a state of reduced tension when the bodies coalesce spontaneously nonuniformly, i.e., hierarchically, into few large and many small bodies suspended in the same space. Hierarchy, not uniformity, is the design that emerges, and it is in accord with the constructal law. The implications of this principle of physics in natural organization and evolution are discussed.
Here we show theoretically that the history of solid growth during "rapid" solidification must be S-shaped, in accord with the constructal law of design in nature. In the beginning the rate of solidification increases and after reaching a maximum it decreases monotonically as the volume of solid tends toward a plateau. The S-history is a consequence of four configurations for the flow of heat from the solidification front to the subcooled surroundings, in this chronological order: solid spheres centered at nucleation sites, needles that invade longitudinally, radial growth by conduction and finally radial lateral conduction to interstices that are warming up. The solid volume (Bs) vs time (t) is an S-curve because it is a power law of type Bs ~ tn where the exponent n first increases and then decreases in time (n = 3/2, 2, 1, …). The initial portion of the S curve is not an exponential.
In this paper we propose a constructal approach to understanding and predicting the morphology of agglomerates of aerosol particles and also to the design of air-cleaning devices. We show that particle agglomeration begins in spherical shape and switches to needle-shaped agglomeration, which performs best as a particle collector at long times. The shape of the needle depends on the dipolar moment of the particles, while the critical number of particles prior to switching to needle shape does not depend on the particle properties.We also show how constructal theory leads to the design of air-cleaning devices that achieve maximum performance per unit volume, under the imposed global constrains. The optimal geometry (internal spacing) of devices composed of parallel-plate channels or tubes and porous filters is shown to depend on known non-dimensional numbers that characterize particle deposition. The work reported in this paper adds to many studies that show how constructal theory provides a deterministic basis for the generation of flow configuration everywhere.
The Ranque–Hilsch vortex tube splits a single high pressure stream of gas into cold and warm streams. Simple models for the vortex tube combined with regenerative precooling are given from which an optimization can be undertaken. Two such optimizations are needed: the first shows that at any given cut or fraction of the cold stream, the best refrigerative load, allowing for the temperature lift, is nearly half the maximum loading that would result in no lift. The second optimization shows that the optimum cut is an equal division of the vortex streams between hot and cold. Bounds are obtainable within this theory for the performance of the system for a given gas and pressure ratio.
This paper documents the relative merits of using more than one type of phase-change material for energy storage. In the case of two phase-change systems in series, which are melted by the same stream of hot fluid, there exists an optimal melting point for each of the two materials. The first (upstream) system has the higher of the two melting points. The second part of the paper addresses the theoretical limit in which the melting point can vary continuously along the source stream, i.e., when an infinite number of different (and small) phase-change systems are being heated in series. It is shown that the performance of this scheme is equivalent to that which uses an optimum single phase-change material, in which the hot stream remains unmixed during the melting process. The time dependence, finite thickness and longitudinal variation of the melt layer caused by an unmixed stream are considered in the third part of the paper. It is shown that these features have a negligible effect on the optimal melting temperature, which is slightly higher than (T∞Te)1/2.
This chapter reviews a series of developments that point in the direction of applying porous media concepts to the description, simulation and optimization of compact systems with complex flow structures, see Bejan et al. [6]. Compact and miniaturized heat exchangers...
In the first part of the paper, Ernst Schmidt's intuitive argument for choosing the optimum fin shape for least material is translated into analysis using the method of variational calculus. It is shown that even when the thermal conductivity of the fin is a function of temperature, the optimum shape of an individual ‘heat tube’ of the fin is the uniform shape, i.e. the tube the geometry of which does not vary with the longitudinal position. This generalization of Schmidt's argument is used in the search of optimum shapes for fins the materials of which have temperature-dependent conductivities. In the second part of the paper the analutical generalization of Schmidt's argument is applied to the design of ducts for fluid flow. It is shown that the shape of a duct the flow pattern or temperature of which varies with the longitudinal position can be selected optimally such that the overall flow resistance of the duct is minimized. The optimization of the duct shapes illustrated in this paper is conducted subject to one of two constraints, constant total duct volume or constant total duct wall surface.
This paper reports the results of an experimental and numerical study of the optimal geometric arrangement of staggered parallel plates in a fixed volume with forced convection heat transfer. The objective of the geometric optimization effort is to maximize the total heat transfer rate between the given volume and the given external flow, when the maximum temperature at a point inside the volume cannot exceed a certain level. The geometric arrangement was varied systematically, by changing the spacing between plates, the number of plates installed in one row, the plate swept length, and the degree to which the plates are staggered. In the first part of the study, it is demonstrated experimentally that there exists an optimal spacing between two adjacent row of plates. Experimental results are reported for air in the range 1000 ⩽ Re L ⩽ 6000, where L is the swept length of the fixed volume. In the second part of the study, extensive numerical results support and extend these findings to 100 ⩽ Re L ⩽10000. In addition, it is shown that there is an optimal way to stagger the plates. In the concluding part of the paper, the optimal spacing and maximum heat transfer rate results are correlated based on the theoretical method of intersecting the two asymptotes (small spacing, large spacing) of the geometric arrangement.
This is a review of recent theoretical developments toward predicting macroscopic organization (the occurrence of shape and structure) in natural flow systems, animate and inanimate. The starting point is the question of how to optimize the access between one point and a finite volume (i.e., an infinite number of points). If the volume is an electronic device that generates heat uniformly, then access optimization means minimum thermal resistance between the volume and a point-size heat sink. Similarly, if the volume must be bathed at every point by a flow (e.g., air flow in the lung, or blood flow in a capillary bed), optimal access means minimum flow resistance between the volume and a source or sink. The main discovery is purely geometric: any finite-size portion of this composite can have its shape optimized such that its overall resistance to flow is minimal. Consequently, the optimal-access solution for the total volume is obtained by optimizing volume shape at every length scale, in a sequence that begins with the smallest building block (elemental system), and proceeds toward larger building blocks (assemblies, constructs). The solution is constructed, hence the “constructal” name of the associated theory. The paths form a tree network in which every single geometric detail is determined theoretically. The tree network cannot be determined theoretically when the time direction is reversed, from large elements toward smaller elements. The constructal principle is further illustrated for fluid flow between a volume and one point, for minimum-time travel between an area and one point, and for minimum-cost economics structures.
This article proposes “distributed energy storage” as a basic design problem of distributing energy storage material on an area. The energy flows by fluid flow from a concentrated source to points (users) distributed equidistantly on the area. The flow is time-dependent. Several scenarios are analyzed: sensible-heat storage, latent-heat storage, exergy storage vs energy storage, and the distribution of a finite supply of heat transfer surface between the source fluid and the distributed storage material. The chief conclusion is that the finite amount of storage material should be distributed proportionally with the distribution of the flow rate of heating agent arriving on the area. The total time needed by the source stream to “invade” the area is cumulative (the sum of the storage times required at each storage site) and depends on the energy distribution paths and the sequence in which the users are served by the source stream. Directions for future designs of distributed storage and retrieval are outlined in the concluding section.
In this paper the global design of energy production, distribution and sustainability is approached based on constructal theory.The global energy system is viewed as a conglomerate of flow systems morphing in time such that it provides greater and greater access to its currents (the constructal law).We show that from this principle follows the global design -the distribution of areas (users) allocated to nodes of power generation.Like the energy consumers themselves (e.g.vehicles, cities), the distribution of power generation on earth should be non uniform.Few large nodes should be interspaced with more numerous smaller nodes on a given territory.Together, this tapestry of areas and nodules serve the entire globe best.They also represent the best equilibrium between the flows of humanity and those of the environment.The paper builds an analogy between this design and the multi-scale design of the forest floor (the distribution of tree sizes) and the design of demography (the distribution of city sizes).
This is a review of two new and important developments in thermal science. First, there exist fundamental optima in the constitution and operation of flow (nonequilibrium) systems, man-made and natural. These optima can be identified based on the simplest models that still retain the essential features of the real systems. Examples are the spatial allocation of heat transfer area in a power plant, and the temporal optimization of on & off processes. The second development is that the engineering method of modeling and optimization has been extended to natural systems, animate and inanimate (e.g., tree networks). This step has been named constructal theory for the reasons given in Section 3. The objective of such work is to predict the macroscopic spatial and temporal structure (organization) that is everywhere. It is to inject a dose of determinsm (theory) in a field that until recently considered natural structures to be nondeterministic: results of chance and necessity. These developments bring to mind the advice left to us by J. W. Gibbs more than one hundred years ago: "One of the principal objects of theoretical research in any department of knowledge is to find the point of view from which the subject appears in its greatest simplicity."
The constructal law was postulated in 1996 as the law of physics that accounts for the phenomenon of time evolution of configuration in all flow systems that have the freedom to morph: animate and inanimate, human made and not human made, social and individual. The constructal‐law field has grown rapidly during the past two decades, particularly with applications to evolutionary design in technology, and with predictions of evolutionary design in biology, geophysics, and social organization. In this review, we highlight some of the current research trends in this field. The topics chosen are from the past 5 years from the activity in our own group, for example, energy storage, heat exchangers, porous media, and technology evolution. The review ends with future directions for research in this very active field.
Background: Myelofibrosis (MF) is a devastating myeloproliferative neoplasm that is hallmarked by marrow fibrosis, symptomatic extramedullary hematopoiesis, and risk of leukemic transformation, most commonly driven by janus kinase 2 (JAK2) pathway mutations. MF risk classification systems guide prognosis, decisions regarding allogeneic stem cell transplantation, and disease modifying agents. Key systems include the Dynamic International Prognostic Scoring System (DIPSS) 2009, DIPSS plus 2010, Genetics-Based Prognostic Scoring System (GPSS) 2014, and Mutation-Enhanced International Prognostic Scoring System (MIPSS) 2014. System contributions include dynamic scoring (DIPSS), cytogenetics (DIPSS Plus), and high risk molecular mutations (GPSS and MIPSS). To power the next generation of MF risk prognostication, and ascertain new prognostic factors, large scale electronic health record (EHR) and genomic data will need integration. As a proof of concept, we leveraged our de-identified research EHR (2.9 million records) and linked genomic biobank (288,000 patients) to develop an all-inclusive phenotype-genotype-prognostic system for MF and recapitulate DIPSS, DIPSS Plus, GPSS and MIPSS. Methods: Our previously described methods (Bejan et al. AACR 2018) utilized natural language processing to algorithmically identify 306 MF patients. A subset (N=125) had available DNA for genotyping. We automatically extracted: age greater than 65, leukocyte count (WBC) greater than 25x109/L, hemoglobin (Hgb) less than 10g/dL, platelets (PLT) less than 100 x 109/L, circulating myeloid blasts ≥ 1%, and 10% weight loss compared to baseline as a proxy for constitutional symptoms. Transfusion data was not included. Karyotype data was manually reviewed. Next generation sequencing (NGS) was performed on biobanked peripheral blood DNA with the Trusight Myeloid Panel (Illumina®). Genotyped samples were restricted to dates after MF diagnosis. Multivariate Cox proportional hazard analysis was performed on all clinical and genomic variables. DIPSS plus was calculated without adjustment but lacked transfusion data. DIPSS, GPSS and MIPSS scores were calculated by published methods. Results: Multivariate Cox proportional hazard regression identified Hgb (HR=6.4; P=0.006), myeloid blasts (HR=3.8; P=0.03), and ASXL1 (HR=5.2; P=0.02) as significant in our cohort with regard to overall survival (OS). We noted a strong trend for high risk karyotype (HR=5.6; P=0.07). Our DIPSS model median survival (N=120) for each subgroup; low risk (median survival not met), intermediate-1 (108 months), intermediate-2 (47 months) and high risk (6 months) P=0.0002 (Figure 1a). DIPSS Plus (N=122) integrated karyotype data and PLT count with similar survival with the exception of high risk (4 months) P=0.00003 (Figure 1b). The percentage of patients with driver mutations in JAK2V617F (57%), CALR (3%) and MPLW515 (7.2%); JAK2WT , CALRWT and MPLWT triple negative (34%); high molecular risk ASXL1 (15%), EZH2 (6%), IDH1/2 (7%), SRFS2 (17%); other variants of interest TET2 (9.6%), TP53 (29%) and DNMT3A (16.8%). MIPSS (N=125; 48 months follow up) noted low risk, intermediate-1, and intermediate-2 (median survival not met) and high risk (32 months) P=0.0001 (Figure 1c). GPSS (N=125; 48 months follow up) did not demonstrate statistical separation among groups (Figure 1d). Discussion: This proof of concept transformed raw EHR records into clinical risk scores for MF. The addition of retrospective DNA analysis via NGS opens the possibility of multi-institutional EHR-biobank studies to most accurately create a system to define MF risk. Our sample size limited the significance of age, PLTs, poor risk mutations and other variables previously shown to impact OS. Likewise, we lacked the capacity to track transfusion dependence, previously shown to have prognostic relevance. Still, prognostication via the EHR mimics common scoring systems in MF and supports correct MF case selection, accurate laboratory extraction and reproducible genotyping of biobanked samples. Similar to the original GPSS report, our low risk cohort was small (N=2) and will benefit from expansion of genotyping underway. Finally, this phenotype-genotype-prognostic paradigm represents a technical advance and a unique opportunity to deploy patient specific comorbidities from lifetime EHR records to further refine risk across all myeloid disease. Disclosures Savona: Boehringer Ingelheim: Consultancy; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees, Research Funding.
No abstract is provided for this article.
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.
