901 publications from this institution
This is a theoretical and experimental study of the time-optimisation of condensation on a vertical wall when the film is removed periodically by a mechanical device. The objective is to fine-tune the periodic process so that the production of condensate and the heat transfer rate are maximal. The first part of the paper develops the scales of the periodic condensation process, and predicts the existence of an optimal condensation time interval. The analysis also predicts that the augmentation of condensation increases as the mechanical cleaning time decreases. These design optimisation opportunities are confirmed in the second part of the paper, which reports measurements of steam condensation on a vertical surface scraped periodically.
A comprehensive assessment of the methodologies of thermodynamic optimization, exergy analysis and thermoeconomics, and their application to the design of efficient and environmentally sound energy systems. The chapters are organized in a sequence that begins with pure thermodynamics and progresses towards the blending of thermodynamics with other disciplines, such as heat transfer and cost accounting. Three methods of analysis stand out: entropy generation minimization, exergy (or availability) analysis, and thermoeconomics. The book reviews current directions in a field that is both extremely important and intellectually alive. Additionally, new directions for research on thermodynamics and optimization are revealed.
ABSTRACT Preterm birth is a syndrome that is triggered by diverse biological pathways and presents with many comorbid diseases. Although twin studies reveal a substantial heritable component, the genetic mechanisms of preterm birth remain poorly understood. We hypothesize that refining the preterm birth phenotype will reveal sub-phenotypes associated with distinct genetic risk factors and potential treatments. Here, we leverage rich longitudinal data from electronic health records (EHRs) from over 60,000 individuals from two clinical sites. Using tensor decomposition, we uncover several latent factors (LFs) that capture coherent combinations of comorbidities (e.g., metabolic, inflammatory, and mental health) and temporal trajectories of preterm and term births. Similar LFs are discovered between the two sites, underscoring their interpretability. Machine learning models trained on LFs accurately predict preterm birth and perform comparably to models trained on the full EHR data. Integrating genome-wide genotyping for >2,200 individuals, we find robust associations of preterm birth risk with high polygenic burden for cardiovascular disease, type 2 diabetes and body mass index. Using LFs, we discover that these genetic signals are strongly and specifically associated with different subsets of the preterm birth cohort. For example, the polygenic diabetes risk is associated with a LF characterized by relevant metabolic disorders. In summary, our study integrates latent phenotypes discovered from large EHR datasets with genetic data to predict preterm birth risk, uncover disease subtypes and comorbidities that drive genetic associations, and delineate the mechanisms underlying the heterogeneity of this complex trait.
Here we document the effect of flow configuration on the heat transfer performance of a spiral shaped pipe embedded in a cylindrical conducting volume. We considered several configurations with fixed volumes of fluid and solid. First, we optimized the geometry of two spiral pipes with varying spacing between the spiral turns and the vertical spacing between the two spirals. Next, we extended the method to three spiral pipes with varying the spacing between the spiral turns and the spacing between the spiral pipes, and changing the dimensions of the conducting volume. We found that optimal spacings between the spiral turns and spire planes exist, such that the volumetric heat transfer rate is maximal. The optimized features of the heat transfer architecture are robust.
Numerical calculation from the full differential equations for convection in an unbounded region is expensive, and hence approximate solutions are important. For small values of the Rayleigh number Ra, perturbation methods are appropriate. At large values of Ra...
A constructal tree-shaped microchannel network for maximizing the saturated critical heat flux in a disc-shaped planar body has been investigated. n 0 radial channels touch the center and n p channels touch the periphery. Designs with no pairing level ( n 0 = n p ), one pairing level ( 2 n 0 = n p ) and two pairing levels ( 4 n 0 = n p ) have been studied. The fluid simulated is R-134a at a saturation temperature of T sat = 30 °C with no inlet subcooling. The flow enters at the center and exits at the periphery. The theoretical CHF model of Revellin and Thome [R. Revellin, J.R. Thome, A theoretical model for the prediction of the critical heat flux in heated microchannels, Int. J. Heat Mass Transfer 51 (2008) 1216–1225], specially developed for micro and minichannels, has been modified and used for predicting the wall CHF of low pressure refrigerants flowing in microchannels. The constraints are the disc radius R and the total volume of ducts V. The degrees of freedom are n 0 , n p and the mass flow rate m ˙ . In each case, the minimum global fluid flow resistance design has been adopted as proposed by Wechsatol et al. [W. Wechsatol, S. Lorente, A. Bejan, Optimal tree-shaped networks for fluid flow in a disc-shaped body, Int. J. Heat Mass Transfer 45 (2002) 4911–4924]. Maximizing the base CHF means increasing the number of central tubes for a given complexity. Furthermore, it is better to use a simple radial structure (with no pairing level) and 2 n 0 central tubes than a design with one pairing level and n 0 central tubes. On one hand, simplified structures seem to be roughly better for maximizing the base CHF. On the other hand, coupling the base CHF and the pumping power leads to different conclusions. There exists an optimal n 0 and n p to maximize the base CHF for each range of pumping power. For instance, for low pumping power, using radial ducts without pairing level is the best solution for dissipating high base CHF whereas for higher pumping power, a more complex design is beneficial with greater n p . In conclusion, the recommended complexity is modest (not maximal), and high complexity is not necessarily the best solution.
No abstract is provided for this article.
We start with the simplest case, that of zero flow through the fluid-saturated porous medium. For an equilibrium state, the momentum equation is satisfied if $$ - \nabla P + {{\rho}_{\mathrm{ f}}}g = 0. $$...
Design happens everywhere, whether in animate objects (e.g., dendritic lung structures, bacterial colonies, and corals), inanimate patterns (river basins, beach slope, and dendritic crystals), social dynamics (pedestrian traffic flows), or engineered systems (heat dissipation in electronic circuitry). This “design in nature” often takes on remarkably similar patterns, which can be explained under one unifying Constructal Law. This book explores the unifying power of the Constructal Law and its applications in all domains of design generation and evolution, ranging from biology and geophysics to globalization, energy, sustainability, and security. The Constructal Law accounts for the universal tendency of flow systems to morph into evolving configurations that provide greater and easier access over time. The Constructal Law resolves the many and contradictory ad hoc statements of “optimality”, end design, and destiny in nature, such as minimum and maximum entropy production and minimum and maximum flow resistance, and also explains the designs that are observed and copied in biomimetics. Constructal Law and the Unifying Principle of Design covers the fundamentals of Constructal Theory and Design, as well as presenting a variety of state-of-the-art applications. Experts from the biological, physical and social sciences demonstrate the unification of all design phenomena in nature, and apply this knowledge to novel designs in modern engineering, such as vascularization for self-healing and self-cooling materials for aircraft, and tree fins and cavities for heat transfer enhancement.
We start with the simplest case, that of zero flow through the fluid-saturated porous medium.For an equilibrium state the momentum equation is satisfied if 6.1 $$- \nabla P + \rho fg = 0$$...
The universal phenomenon of evolution consists of change after change in flow configuration in a time direction that is perceptible to the observer. This reality clashes with the doctrine of precise optima, minima, and maxima, now rigidly in place because of calculus and computational simulations of all kinds of flowing and changing configurations. With two dissimilar examples, access on an area (a human settlement) and along a line (animal locomotion), it is shown that even a 1-percent imperfection in performance is accompanied by a sizable bandwidth of freedom to hit the ‘target’, that is, an easily accessible design with close to perfect performance. The evolutionary designs reveal the physics behind the phenomenon of diminishing returns in the vicinity of the mathematical optimum. In evolution what works is kept.
This paper documents the geometric optimization of an assembly of staggered vertical plates that are installed in a fixed volume. The heat transfer is by laminar natural convection. The objective is to maximize the overall thermal conductance between the assembly of plates and the surrounding fluid. The geometric parameters that vary are the horizontal spacing between adjacent columns of plates, the number of plates mounted in each vertical column, the plate dimensions, and the stagger between columns. The optimization is performed numerically and experimentally in the range Pr = 0.72 and 103 ≤ RaL ≤ 106, where RaL is the Rayleigh number based on the vertical dimension of the assembly (L). The results for the optimal horizontal spacing are correlated using formulas derived from the theory of the intersection of asymptotes (Bejan, 1984, 1995).
This paper is an introduction to a new trend in the conceptual design of energy systems: the generation of flow configuration based on the “constructal” principle that the global performance is maximized by balancing and arranging the various flow resistances (the irreversibilities) in a flow system that is free to morph. The paper focuses on distribution and collection, which are flows that connect one point (source, or sink) with an infinity of points (volume, area, curve). The flow configurations that emerge from this principle are tree-shaped, and the systems that employ them are “vascularized”. The paper traces the most recent progress made on constructal vascularization. The direction is from large-scale applications toward microscales. The large-scale tree-shaped designs of electric power distribution systems and networks for natural gas and water are now invading small-scale designs such as fuel cells, heat exchangers and cooled packages of electronics. These flow configurations have several properties in common: freedom to morph, multiple scales, hierarchy, nonuniform (optimal) distribution of scales through the available volume, compactness and finite complexity.
This paper documents the fundamental relation between the maximization of global performance and the maleable (morphing) architecture of a flow system with global constraints. The example is the coaxial two-stream heat exchanger with flow through a porous bed in the annular space. It is shown that the constraints force the design toward heat exchangers with finite axial length, where additional improvements are derived from installing high-conductivity fins across the porous bed. The maximization of global performance is achieved through the optimization of the configuration of plate fins. Configurations with radial fins are optimized analytically and numerically. Configurations with branched fins are optimized numerically. It is shown that the best configuration (radial vs. branched) depends on the size of the heat exchanger cross-section. When the size is small, the best is the radial pattern. When the size exceeds a certain threshold, the best configuration is the optimized branched tree of fins.
No abstract is provided for this article.
No abstract is provided for this article.
Natural convection in a porous medium filling a slender horizontal space with an end-to-end temperature difference is studied analytically. The end-to-end temperature difference gives rise to a horizontal counterflow pattern augmenting the heat transfer rate through the porous medium. Two basic geometries are considered: horizontal layer confined between two adiabatic and impermeable parallel plates, and horizontal cylinder surrounded by an adiabatic and impermeable cylindrical surface. Nusselt number relations are derived in terms of the Rayleigh number and the cavity aspect ratio. The end-wall permeability is shown to affect the heat transfer rate through the medium.
This paper advances a theoretical explanation for the lateral periodicity and geometric similarity of meanders observed in rivers of many sizes. Invoking the static equilibrium of a straight river bed, it is shown analytically that the equilibrium shape of the bed is a unique sinusoid. The theoretical wavelength of the sinusoidal shape is proportional to the width of the river, in agreement with visual observations of rivers of all sizes.
Here, we show that a plate heated by a moving beam can be cooled effectively by fluid that flows through a vasculature of channels embedded in the plate. The vascular designs studied are radial, grid and hybrid (radial + grid). The peak temperature of the plate changes with the path and direction of the moving beam. The strength, size and speed of the beam vary. The peak temperature increases as the beam strength and size increase and as the speed of the beam decreases. The grid and hybrid designs are robust because of loops present in the flow structure. The pressure difference that drives the fluid flow varied. The channel diameter ratios that provide greatest flow access are reported. The cooling performance of the multiscale grid structures is less sensitive to the changes in beam path than the cooling performance of the other structures studied. The effect of adding a vascular structure to the design is dramatic.
