Abstract : This project relied on constructal theory to develop novel flow architectures for aircraft thermal management, in particular for the cooling of skins and leading surfaces ofhigh speed aircraft, high-temperature gas turbine blades, etc. The following milestones were reached: 1. The concept of vascular architecture embedded in a wall subjected to intense heating, which showed that tree-shaped channels are more effective than parallel channels oriented across the wall, 2. The concept of bathing a volume with one stream flowing as two trees matched canopy to canopy, which showed that this dendritic architecture is dramatically more effective than parallel channels, 3. The concept of ofcooling a wall with tree-shaped channels that run against the intense heating striking the wall, and 4. The concept of dendritic vascularization of a volume by using one stream. The tree-tree architecture exhibits sharp transitions toward greater complexity as the size of the bathed volume increases. These concepts are essential for future vascular design, and for scaling up to realistic dimensions the results obtained based on small-scale models.
The melting rates due to close-contact heating of a block of phase-change material have been analyzed in the past based on thin-film lubrication theory in several internal and external configurations. The scale analysis of close-contact melting in a region of general shape shows that the melting rate in all configurations is anticipated by the expression in which V is the speed with which the solid advances into the melting front (the melting rate), ϱ/ϱ s is the liquid/solid density ratio, Ste is the Stefan number for liquid superheating (Ste >>1), and α and υ are the viscosity and the t hermal diffusivity of the liquid phase. The film exess pressure scale Δ is defined as the net weight of the object surrounded by liquid divided by the horizontal projected area of that object. The flow length scale of liquid film, ℓ, stands for the diameter of the cylinder or the sphere, or the smaller of the two sides of the rectangular shape of the contact area during melting against a flat heater.
This is a review of the evolution of thermodynamics during the past four decades, from classical (engineering) thermodynamics, to thermodynamic optimizatio
This is a review of several key ideas and pioneers in the founding history of thermodynamics, fluid dynamics and heat transfer. Ideas treated in detail are the mechanical equivalent of heat, the difference between heat transfer and work transfer, the Navier-Stokes equations, natural convection in a fluid and a saturated porous medium, the gas bubble rising in a vertical tube filled with liquid, and fluid friction in duct flow. The review shows that good ideas spread and, at the same time the language and national preferences of the followers play a role in whether the idea creators are remembered or forgotten. The forgetting of the origin of ideas and their authors threatens to become a real problem during the digital era. This danger is exacerbated by the enormous increase in the number of publications most of which are not carefully reviewed or read.
Technical Briefs The Pressure Melting of Ice Due to an Embedded Cylinder P. A. Tyvand, P. A. Tyvand Department of Agricultural Engineering, Agricultural University of Norway, 1432 A˚s-NLH, Norway Search for other works by this author on: This Site PubMed Google Scholar A. Bejan A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706, USA Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information P. A. Tyvand Department of Agricultural Engineering, Agricultural University of Norway, 1432 A˚s-NLH, Norway A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706, USA J. Heat Transfer. May 1992, 114(2): 532-535 (4 pages) https://doi.org/10.1115/1.2911311 Published Online: May 1, 1992 Article history Received: March 11, 1991 Revised: August 11, 1991 Online: May 23, 2008
Here we illustrate the free search for the optimal geometry of flow channel cross-sections that meet two objectives simultaneously: reduced resistances to heat transfer and fluid flow. The element cross section and the wall material are fixed, while the shape of the fluid flow opening, or the wetted perimeter is free to vary. Two element cross sections are considered, square and equilateral triangular. We find that the two objectives are best met when the solid wall thickness is uniform, i.e., when the wetted perimeters are square and triangular, respectively. We also consider arrays of square elements and triangular elements, on the basis of equal mass flow rate per unit of array cross sectional area. The conclusion is that the array of triangular elements meets the two objectives better than the array of square elements.
In this article we show numerically that the entire flow geometry of a vertical diverging or converging channel with laminar natural convection can be optimized for maximal heat transfer rate density (total heat transfer rate per unit of flow system volume). The geometry is free to change in three ways: (1) the spacing between the walls, (2) the distribution of heating along the walls, and (3) 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 H 0 (≤H) on each wall, which is placed either at the bottom (entrance) end of the channel, or at the top (exit) end. The results confirm that the use of upper unheated sections enhances the chimney effect and the heat transfer. The new aspect is that the heat transfer rate density decreases because the unheated sections increase the total volume. It is shown that for maximal heat transfer rate density it is better to place the H 0 sections at the channel entrance. It is also shown that the optimal angle between the two walls is approximately zero when Ra H is large, i.e., for maximal heat transfer rate density the walls should be parallel or nearly parallel. Finally, the optimized spacing (1) developed in the presence of (2) and (3) as additional degrees of freedom is of the same order of magnitude as the optimal spacing reported earlier for parallel isothermal walls, i.e., in the absence of features (2) and (3). The robustness of the optimized flow architecture is discussed. Additional degrees of freedom and global objectives that may be incorporated in this constructal approach are the curvature of the facing walls and the mechanical strength and stiffness of the confining walls. This work was supported by a doctoral fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq-Brazil) and a research grant from the National Science Foundation.
Technical Briefs The “Heatline” Visualization of Convective Heat Transfer S. Kimura, S. Kimura Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80309 Search for other works by this author on: This Site PubMed Google Scholar A. Bejan A. Bejan Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80309 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information S. Kimura Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80309 A. Bejan Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80309 J. Heat Transfer. Nov 1983, 105(4): 916-919 (4 pages) https://doi.org/10.1115/1.3245684 Published Online: November 1, 1983 Article history Received: December 13, 1982 Online: October 20, 2009
This paper describes a two-part study of the time-dependent melting of an enclosed phasechange material heated at a constant rate from the side. The first part describes experimental measurements with n-octadecane conducted in a 74 cm tall enclosure. The heat flux Rayleigh numbers of these experiments are of the order of 1013, and the liquid flow pattern is weakly turbulent. The second part of the study describes the liquid-superheat effect analytically, by means of a ‘matched boundary layers’ solution for the convection regime of the heat transfer and melting process. The predicted overall Nusselt number relationship agrees very well with the empirical correlation based on experiments.
In this chapter we turn our attention to processes of combined (simultaneous) heat and mass transfer that are driven by buoyancy. The density gradients that provide the driving buoyancy force are induced by the combined effects of temperature and species...
In this paper, we optimize the performance of several classes of simple flow systems consisting of T- and Y-shaped assemblies of ducts, channels and streams. In each case, the objective is to identify the geometric configuration that maximizes performance subject to several global constraints. Maximum thermodynamic performance is achieved by minimization of the entropy generated in the assemblies. The boundary condition is fixed temperature of the channel wall. The flow is assumed laminar and fully developed. Every geometrical detail of the optimized structure is deduced from the constructal law. Performance evaluation criterion is proposed for evaluation and comparison of the effectiveness of different tree-shaped design heat exchangers. This criterion takes into account and compare the entropy generated in the system with heat transfer performance achieved.
This paper establishes a theoretical framework for the minimization of entropy generation (the waste of exergy, or useful energy) in extended surfaces (fins). The entropy generation rate formula for a general fin is derived first. Based on this general result, analytical methods and graphic results are developed for selecting the optimum dimensions of pin fins, rectangular plate fins, plate fins with trapezoidal cross section, and triangular plate fins with rectangular cross section.
Nature abounds in examples of evolutionary designs (bio and non-bio) that evolve freely into configurations that provide easier and greater access for movement. The present article considers three seemingly unrelated phenomena that appear to obstruct flow: stick–slip friction, animal jump, and earthquake. The analysis is based on simple models of rhythmic energy store & release motion. In each case, the rhythm is the sole degree of freedom. The analyses show that stick–slip friction facilitates movement because the coefficient of static friction is greater than the coefficient of sliding friction. Next, all forms of animal locomotion under gravity consist of cycles of energy storage (jump to a height) and energy release (forward fall). The rhythm of the cycle is natural such that the forward advance of the animal is economical. Finally, the onset of the earthquake is modeled the same way, as shear stresses at the rock-on-rock interface, which are matched by bending stresses in the bent 'blades' of rock contained between fissures perpendicular to the interface. In sum, naturally evolved store & release rhythm facilitates the movement, contrary to the commonly held impression.
Evolution is a universal phenomenon of nature, bio, and non-bio. Evolution belongs in physics and general education. This step clarifies and enhances the transmission and retention of knowledge. Teaching and learning are the mental activity of grasping a new subject as something familiar, like an older and wider body of knowledge. The new subject is evolution, and the older and wider is physics. The older is accepted because it is based on morphing images available for observation and testing during a human lifetime (e.g., dendrites of river basins, lightning, snowflakes). Physical images can be predicted, tested, confirmed, and condensed into principles with the power to predict directionality and future evolution. The teaching of this method is illustrated with examples from animal locomotion and vehicle configuration, speed, and path.
This paper describes the flow and temperature distribution in the hourglass-shaped cavity formed between two rollers and two parallel runner surfaces. The cavity fluid is heated uniformly over the roller surfaces, and cooled along the two runners, which are modeled as isothermal. The focus is on the characteristics of the flow (the Reynolds number effect) and the temperature distribution in the fluid, especially at the roller surface. These features are determined numerically. The results document the extent to which the Reynolds number, Prandtl number, geometric aspect ratio, and viscous dissipation number influence the steady-state temperature of the roller surface. It is shown that at Reynolds numbers greater than 100, the cooling effect that the cavity fluid provides to the roller approaches the behavior anticipated based on laminar forced-convection boundary layer theory.
No abstract is provided for this article.
Background:Patients with clonal hematopoiesis (CH) in the absence of WHO-classified myeloid disease are of special interest given their increased prevalence with age, predisposition to morbid cardiovascular complications, and amplified risk of overt hematologic malignancy. Pts are often stratified by normal peripheral blood counts into clonal hematopoiesis of indeterminate potential (CHIP), or those with unexplained cytopenias as clonal cytopenias of undetermined significance (CCUS). However, less is known about pts with elevated counts and clonal hematopoiesis who do not fulfill WHO criteria for any myeloproliferative neoplasia (MPN). We leveraged Vanderbilt University Medical Center's unique biobank, BioVU, to identify the prevalence of JAK2V617F across 48,000 pts to evaluate the clinical changes in progression from CH to overt myeloid disease. Methods:To develop a reference JAKV617F training set, next generation sequencing via Illumina Trusight Myeloid Panel (NGS) was performed on BioVU samples (N=133) from pts with confirmed myeloproliferative malignancy. Of those pts, 78 harbored JAK2V617F with a range of variant allele frequencies (VAF). Matched samples in this training set (N=133) were also analyzed via Infinium® Expanded Multi-Ethnic Genotyping Array (MEGAEX). SNP array JAK2V617F variant intensity was extracted (rs77375493; NM_004972.3(JAK2): c.1849G>T (p.Val617Phe). A regression model was built using NGS VAF as a dependent variable and MEGAEX intensity data as independent variable (r2 =0.9931).Based on this model, we imputed JAK2V617F VAF for all 48,000 pts in our cohort. Pts with JAK2V617F were subdivided into: clinically confirmed myeloid disease, or JAK2V617F without a diagnosis of MPN. Upon review of the EMR, the latter group was further dived into: 1) probable undiagnosedMPN, 2) CHIP, 3) CCUS, or 4) CH with associated elevated peripheral blood counts (CHAPbc). Only lab values after the date of JAK2V617F detection were included. Confirmed malignancy was defined by WHO classification of disease. Pts with evidence of possible WHO classified PV or ET with Hgb >18.5g/dl in men, >16.5g/dl in women, or PLT count >450k/mcl regardless of gender were classified as probable undiagnosed MPN. CHIP was defined as JAK2V617F without abnormal counts across a patient's EMR lifetime, except when confounding events, e.g. trauma surgery or overt iron deficiency anemia, incorrectly skewed values. CCUS was defined as JAK2V617F in the presence of unexplained cytopenias; hemoglobin (Hgb) <13.5g/dl men or <12g/dl women, leukocyte count (WBC) <3.9x10^3/uL or platelet (PLT) <135 x10^3/mcL. We classified pts with elevated blood counts who did not meet the WHO classification of MPN [e.g. WBC >10.7 regardless of gender, Hgb 18-18.5 g/dL in men or 16.0-16.5 g/dL in women with maximum Hgb no greater than 18.5g/dl in men and 16.5g/dl in women, or PLT count between 371-450k/mcl regardless of gender (and no values >450k/mcl)] as CHAPbc. Results:We identified 410 of 48,000 pts who harbored JAK2V617F (0.85% prevalence). Of those, 270/410 had clinically diagnosed hematologic malignancy including primary myelofibrosis (PMF) (79), ET (48), PV (43) and Ph-MPN NOS (29). MDS (29), AML (15), NHL (16), plasma cell dyscrasias (5), CML (3), other (3). There were 19/410 with insufficient clinical data to determine diagnosis. The remaining 121/410 JAK2V617F pts did not have a related diagnosis. Figure 1a demonstrates imputed VAF differences in JAK2V617F between known MPN vs. CH (including undiagnosed MPN) by age (mean VAF 0.44 vs. 0.17 respectively P<0.001). We identified several undiagnosed MPNs (22), which subdivided into polycythemia predominant (1), thrombocythemia predominant (16) or a combination of polycythemia and thrombocythemia (5). The remaining 99/410 cases were CH,with CHIP (55), CCUS (29), and CHAPbc (15). Ranges of blood counts appear to be a continuous variable among JAK2V617F pts with CH or undiagnosed MPN (Fig 1b-1d). Summary: We used an unbiased approach to identify the prevalence of JAK2V617F across all pts at a single institution. In this cohort, hematologic malignancy and CH did stratify by imputed VAF. Further, within JAK2V617F CH, CHAPbc may be differentiated from CHIP by clinical phenotype and further investigation will be required to determine its impact on patient outcomes. Disclosures Savona: Sunesis: Research Funding; TG Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Incyte Corporation: Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Membership on an entity's Board of Directors or advisory committees; Boehringer Ingelheim: Patents & Royalties; Selvita: Membership on an entity's Board of Directors or advisory committees; Karyopharm Therapeutics: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding.
. 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 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 addresses a modern trend in heat transfer education, namely, the increasingly important roles played by ‘purpose’ and thermodynamic principles in problem formulation. The answers to such basic questions represent an emerging class of fundamental optima (extrema) that are redefining thermal science. The combined heat transfer and thermodynamics models and analyses are extremely simple and brief. The method is illustrated by means of six examples: maximum rate of ice production, optimal on & off operation of defrosting refrigerators and power plants with heat exchangers that experience fouling, optimal allocation of heat exchanger equipment in power plants and refrigeration plants, and optimal spacing between elements in a fixed volume with natural or forced convection.
