Browsing by Author "Bejan, A"

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Open Access
Arrays of flow channels with heat transfer embedded in conducting walls
(International Journal of Heat and Mass Transfer, 2016-08-01) Bejan, A; Almerbati, A; Lorente, S; Sabau, AS; Klett, JW
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.
Open Access
Artificial intelligence evolution in smart buildings for energy efficiency
(Applied Sciences (Switzerland), 2021-01-02) Farzaneh, H; Malehmirchegini, L; Bejan, A; Afolabi, T; Mulumba, A; Daka, PP
© 2021 by the authors. Licensee MDPI, Basel, Switzerland. The emerging concept of smart buildings, which requires the incorporation of sensors and big data (BD) and utilizes artificial intelligence (AI), promises to usher in a new age of urban energy efficiency. By using AI technologies in smart buildings, energy consumption can be reduced through better control, improved reliability, and automation. This paper is an in‐depth review of recent studies on the application of artificial intelligence (AI) technologies in smart buildings through the concept of a building management system (BMS) and demand response programs (DRPs). In addition to elaborating on the principles and applications of the AI‐based modeling approaches widely used in building energy use prediction, an evaluation framework is introduced and used for assessing the recent research conducted in this field and across the major AI domains, including energy, comfort, design, and maintenance. Finally, the paper includes a discussion on the open challenges and future directions of research on the application of AI in smart buildings.
Open Access
Cell and extracellular matrix growth theory and its implications for tumorigenesis.
(Bio Systems, 2021-03) Sauer, TJ; Samei, E; Bejan, A
Cells associated with an abnormal (cancerous) growth exchange flows, morph freely and grow hand-in-glove with their immediate environment, the extracellular matrix (ECM). The cell structure experiences two mass flows in counterflow. Flowing into the structure are nutrients and flowing out is refuse from the metabolically active biomass within. The physical effect of the evolution of the cell and extracellular structure is more flow and mixing in that space, that is, more mixing than in the absence of a biological growth in that space. The objective of the present theory is to predict the increase in the size of the cell cluster as a function of its structure, and also to predict the critical cluster sizes that mark the transitions from one distinct cluster configuration to the next. This amounts to predicting the timing and the main features of the transitions from single cell to clusters with two, four, eight and more cells, including larger clusters with cells organized on its outer surface. The predicted evolution of the size and configuration of the cell cluster is validated successfully by comparison with measurements from several independent studies of cancerous and non-cancerous growth patterns.
Open Access
Constructal dendritic configuration for the radiation heating of a solid stream
(Journal of Applied Physics, 2010-06-01) Kang, DH; Lorente, S; Bejan, A
Here we show that the configuration of a slender enclosure can be optimized such that the radiation heating of a stream of solid is performed with minimal fuel consumption at the global level. The solid moves longitudinally at constant rate through the enclosure. The enclosure is heated by gas burners distributed arbitrarily, in a manner that is to be determined. The total contact area for heat transfer between the hot enclosure and the cold solid is fixed. We find that minimal global fuel consumption is achieved when the longitudinal distribution of heaters is nonuniform, with more heaters near the exit than the entrance. The reduction in fuel consumption relative to when the heaters are distributed uniformly is of order 10%. Tapering the plan view (the floor) of the heating area yields an additional reduction in overall fuel consumption. The best shape is when the floor area is a slender triangle on which the cold solid enters by crossing the base. These architectural features recommend the proposal to organize the flow of the solid as a dendritic design, which enters as several branches, and exits as a single hot stream of prescribed temperature. The thermodynamics of heating is presented in modern terms in the Sec. (exergy destruction, entropy generation). The contribution is that to optimize "thermodynamically" is the same as reducing the consumption of fuel. © 2010 American Institute of Physics.
Open Access
Constructal design of evacuation from a three-dimensional living space
(Physica A: Statistical Mechanics and its Applications, 2015-03-15) Lui, CH; Fong, NK; Lorente, S; Bejan, A; Chow, WK
© 2014 Elsevier B.V. All rights reserved. This paper demonstrates the fundamental relation that exists between the configuration of a three-dimensional living space and the time needed for the evacuation of all the inhabitants. The evacuation is treated as a physical flow system consisting of pedestrians who move from a volume to one or two exits. The living space has two variable aspect ratios, the floor shape and the profile shape (or the number of floors). First, the paper reports analytically the optimal floor and profile shapes for which the total evacuation time is minimum. Second, the analytical results are complemented and validated by numerical results obtained based on numerous simulations of pedestrian flow from volume to exits. The numerical results are further validated by performing the simulations of pedestrian movement with two different computational codes (Simulex and FDS + Evac). The fundamental relation presented in this paper can be used in the design of larger and more complex living spaces in modern urban settings.
Open Access
Constructal design of gas-cooled electric power generators, self-pumping and atmospheric circulation
(International Journal of Heat and Mass Transfer, 2015-08-24) Bejan, A; Lorente, S; Lee, J; Kim, Y
© 2015 Elsevier Ltd. All rights reserved. Rotating electric machines generate heat volumetrically, and are cooled by forced convection aided by the self-pumping effect. In this paper we focus on the fundamental relationship between the internal flow architecture of the gas cooled winding and its thermal performance, which is represented by the nearly uniform distribution of peak temperature throughout the winding volume. We show that the cooling passages can be sized such that the volumetric cooling is most effective. From this finding follows the number of passages and their distribution through the heat generating volume. The principle is developed analytically, and it is then validated based on numerical simulations of the cooling architecture. The paper also reports the thermodynamics basis of the self-pumping effect, and its natural occurrence as free convection in general, which includes atmospheric circulation.
Open Access
Constructal design of salt-gradient solar pond fields
(International Journal of Energy Research, 2016-08-01) González, D; Amigo, J; Lorente, S; Bejan, A; Suárez, F
© 2016 John Wiley & Sons, Ltd. Salt-gradient solar ponds (SGSPs) are water bodies that capture and accumulate large amounts of solar energy. The design of an SGSP field has never been analyzed in terms of studying the optimal number of solar ponds that must be built to maximize the useful energy that can be collected in the field, or the most convenient way to connect the ponds. In this paper, we use constructal design to find the optimal configuration of an SGSP field. A steady-state thermal model was constructed to estimate the energy collected by each SGSP, and then a complementary model was developed to determine the final temperature of a defined mass flow rate of a fluid that will be heated by heat exchangers connected to the solar ponds. By applying constructal design, four configurations for the SGSP field, with different surface area distribution, were evaluated: series, parallel, mixed series-parallel and tree-shaped configurations. For the study site of this investigation, it was found that the optimal SGSP field consists of 30 solar ponds of increasing surface area connected in series. This SGSP field increases the final temperature of the fluid to be heated in 22.9%, compared to that obtained in a single SGSP. The results of this study show that is possible to use constructal theory to further optimize the heat transfer of an SGSP field. Experimental results of these configurations would be useful in future works to validate the methodology proposed in this study.
Open Access
Constructal Law: Optimization as Design Evolution
(Journal of Heat Transfer, 2015-06-01) Bejan, A
Copyright © 2015 by ASME. Here, I review the physics meaning of optimization, knowledge and design evolution, and why these concepts and human activities are profoundly useful for human life. A law of physics is a concise statement that summarizes a phenomenon that occurs in nature. A phenomenon is a fact, circumstance, or experience that is apparent to the human senses and can be described. The design in nature phenomenon facilitates access for everything that flows, evolves, spreads, and is collected: river basins, atmospheric and ocean currents, animal life and migration, and technology (the evolution of the "human-and-machine species," wealth, life). This phenomenon is summarized by the constructal law: the occurrence and evolution of designs in nature, its time direction. Based on its record, the constructal law accounts for the design phenomenon and also for all the phenomena that have been described individually (ad-hoc) with end-design (destiny) statements of "optimality" (min, max). Most notably, the constructal law accounts for contradictory end-design statements such as minimum entropy production and maximum entropy production, and minimum flow resistance and maximum flow resistance.
Open Access
Constructal thermodynamics
(International Journal of Heat and Technology, 2016-01-01) Bejan, A
Thermodynamics is brief, simple, unambiguous and improving. Yet, confusion reigns in the field. The word "entropy" is pasted on almost any new thing, without any respect for its proper definition in thermodynamics. Every author bows to his own maximum or minimum principle, even when it contradicts English, not just thermodynamics. Minimizing resistance cannot be the same as maximizing resistance. Minimizing entropy generation cannot be the same as maximizing entropy generation. Because of the word "entropy", many believe that entropy generation minimization and maximization are covered by the second law, which is incorrect, twice. Because for an isolated system (or an adiabatic closed system) the second law states that the system entropy inventory increases during changes inside the system, many believe that the second law accounts for organization, evolution, and the arrow of time. This too is incorrect. It is time for a reality check, and this means to take a look at nature, at the physics, at the science of all the natural things that "happen". Here then is a review of the few, the noble, the laws with which in science we cover the few distinct phenomena that nature is made of.
Open Access
Counterflow heat exchanger with core and plenums at both ends
(International Journal of Heat and Mass Transfer, 2016-08-01) Bejan, A; Alalaimi, M; Lorente, S; Sabau, AS; Klett, JW
This paper illustrates the morphing of flow architecture toward greater performance in a counterflow heat exchanger. The architecture consists of two plenums with a core of counterflow channels between them. Each stream enters one plenum and then flows in a channel that travels the core and crosses the second plenum. The volume of the heat exchanger is fixed while the volume fraction occupied by each plenum is variable. Performance is driven by two objectives, simultaneously: low flow resistance and low thermal resistance. The analytical and numerical results show that the overall flow resistance is the lowest when the core is absent, and each plenum occupies half of the available volume and is oriented in counterflow with the other plenum. In this configuration, the thermal resistance also reaches its lowest value. These conclusions hold for fully developed laminar flow and turbulent flow through the core. The curve for effectiveness vs number of heat transfer units (Ntu) is steeper (when Ntu < 1) than the classical curves for counterflow and crossflow.
Open Access
Energy design for dense neighborhoods: One heat pump rejects heat, the other absorbs heat from the same loop
(International Journal of Thermal Sciences, 2015-06-29) Almerbati, A; Lorente, S; Bejan, A
© 2015 Elsevier Masson SAS. This paper documents the joint performance of heat pumps that are served by a common loop buried in the ground, and which operate simultaneously: one heat pump absorbs heat from the buried loop whereas the other one rejects heat. A background flow is circulated in the underground loop even when the two heat pumps are not operating. The objective is to determine the performance and the manner in which it is affected by the way in which the two heat pumps are connected to the loop. The performance measures are the heat transfer rates into and out of the heat pumps, and the total pumping power required by the assembly. The paper documents the individual performance of the heat pumps, and their relative performance, which is the ratio of heating absorbed by one pump to the heating rejected by the other pump.
Open Access
Entrance-length dendritic plate heat exchangers
(International Journal of Heat and Mass Transfer, 2017-01-01) Bejan, A; Alalaimi, M; Sabau, AS; Lorente, S
© 2017 Elsevier Ltd. Here we explore the idea that the highest heat transfer rate between two fluids in a given volume is achieved when plate channel lengths are given by the thermal entrance length, i.e., when the thermal boundary layers meet at the exit of each channel. The overall design can be thought of an elemental construct of a dendritic heat exchanger, which consists of two tree-shaped streams arranged in cross flow. Every channel is as long as the thermal entrance length of the developing flow that resides in that channel. The results indicate that the overall design will change with the total volume and total number of channels. We found that the lengths of the surfaces swept in cross flow would have to decrease sizably as number of channels increases, while exhibiting mild decreases as total volume increases. The aspect ratio of each surface swept by fluid in cross flow should be approximately square, independent of total number of channels and volume. We also found that the minimum pumping power decreases sensibly as the total number of channels and the volume increase. The maximized heat transfer rate per unit volume increases sharply as the total volume decreases, in agreement with the natural evolution toward miniaturization in technology.
Open Access
EVOLUTION AND THE CITY
(MECHANICAL ENGINEERING, 2017-02) Bejan, A
Open Access
Evolution as Physics: The Human & Machine Species
(European Review, 2016-12-20) Bejan, A
Humans and technology are not in symbiosis. They are one species, not two. Humans, enveloped in artefacts of many kinds and ages (from writing, to airplanes), are evolving as one species, the ‘human & machine species’. This evolution is visible and recorded in our lifetime. Here, I illustrate the evolution of the human & machine species by focusing on commercial aircraft, the cooling of electronics, and modern athletics, which is a special laboratory for witnessing the evolution of animal locomotion. I show that these evolutionary forms of flow organization are in accord with, and can be predicted based on the law of physics that governs evolution in nature, bio and non-bio: the constructal law. Evolution, life and the human & machine species are physics.
Open Access
Heatlines (1983) versus synergy (1998)
(International Journal of Heat and Mass Transfer, 2015-01-01) Bejan, A
© 2014 Elsevier Ltd. All rights reserved. A picture is worth a thousand words. This article is about a picture known as "heatlines" since 1983, and "synergy" since 1998. Both concepts, heatlines and synergy, are about visualizing the physics of convection, which is the combination (superposition) of heat conduction lines and enthalpy flow lines over a material in motion. Heatlines and synergy are reviewed here comparatively. This comparison reveals that synergy is a remake of heatlines, and that synergy has no physical connection with heat transfer enhancement. At bottom, it has become a lot easier to take an existing idea change some key words and drawings and publish the old idea as new.
Open Access
Letter to the editor of renewable and sustainable energy reviews
(Renewable and Sustainable Energy Reviews, 2016-01-01) Bejan, A
Open Access
Morphing the design to go with the times
(International Communications in Heat and Mass Transfer, 2021-01-01) Bejan, A; Gucluer, S
© 2020 Elsevier Ltd The recorded history of technology and science and art shows that the evolutionary path of design is toward a greater number of dimensions, more degrees of freedom, and greater performance, efficiency and economy. Until now, designs have evolved in steps from one to two and three dimensions. The future will bring one more step, to four dimensions: three-dimensional objects that morph in time in accord with their time-changing environmental conditions. This concept is illustrated with the thermo-fluid design and time-behavior of a volume filled with parallel plates cooled by forced convection. When the pressure difference that drives the flow varies stepwise in time, the plate to plate spacing for maximum heat transfer density must change. If the structure is free to morph to maintain its optimal spacings in step with its changing environment, then the time-integrated performance of the morphing object is maximum. If the structure is rigid (sized optimally for one flow condition), its performance is inferior. The general significance and applicability of this future of design activity is discussed.
Open Access
Morphing tree structures for latent thermal energy storage
(Journal of Applied Physics, 2015-06-14) Ziaei, S; Lorente, S; Bejan, A
© 2015 AIP Publishing LLC. Here, we report the numerical study of time dependent storage of energy by melting a phase change material. The heating is provided along invading lines, which change from single-line invasion to tree-shaped invasion. The numerical simulations show that the history of the amount of melted material is S-shaped. We also found that the fastest melting (i.e., the steepest S curve) is discovered by allowing the tree architecture to morph freely, toward greater access over time. The stem length and branching angle of invading trees can be selected such that the complete melting process is shorter. The melting process can also be made faster by increasing the complexity of the tree structure.
Open Access
Nationalism and forgetfulness in the spreading of thermal sciences
(International Journal of Thermal Sciences, 2021-05-01) Bejan, A
© 2020 Elsevier Masson SAS 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.
Open Access
Natural constructal emergence of vascular design with turbulent flow
(Journal of Applied Physics, 2010-06-01) Cetkin, E; Lorente, S; Bejan, A
Here, we show that vascular design emerges naturally when a volume is bathed by a single stream in turbulent flow. The stream enters the volume, spreads itself to bathe the volume, and then reconstitutes itself as a single stream before it exits the volume. We show that in the pursuit of a smaller global flow resistance and larger volumes, the flow architecture changes stepwise from a stack of identical elements bathed in parallel flow (like a deck of cards) to progressively more complex structures configured as trees matched canopy to canopy. The transition from one architecture to the next occurs at a precise volume size, which is identified. Each transition marks a decrease in the rate at which the global flow resistance increases with the volume size. This decrease accelerates as the volume size increases. The emergence of such vasculatures for turbulent flow is compared with the corresponding phenomenon when the flow is laminar. To predict this design generation phenomenon is essential to being able to scale up the designs of complex flow structures, from small scale models to life size models. The constructal law is a bridge between the principles of physics and biology. © 2010 American Institute of Physics.

Browsing by Author "Bejan, A"

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