In this article, overall net heat transfer maximization for a cooler and reheater in the wet flue gas desulfurization equipment of coal-firing thermal power plant was explored. Constructal design was implemented such that optimal flow architectures in a finite volume of the cooler and reheater leading to maximal thermal performance were discovered. The flue gas outlet temperatures at the cooler and reheater are constrained between and in order to avoid acid condensation on the heat exchanger surface. A single water stream circulates through the cooler and reheater in order to absorb heat from the high temperature flue gas and to return heat to the flue gas again, respectively. It is demonstrated that the maximum overall net heat transfer is achievable and the relevant allocation of the heat exchangers can be configured depending on design constraints. The design constraints based on the flue gas temperatures are essential because acid condensation in the flue gas that takes place below the acid dew point can corrode the heat transfer surface and cause performance deterioration. Better performance is achieved when the design is endowed with more degrees of freedom.

Here, we explored the effect of surface wettability on nucleate pool boiling heat transfer performance by employing hydrophilic titanium oxide (TiO2) layer coated on a cylindrical plain aluminum 6061 (Al 6061) as a reference. The comparative heating surfaces are the plain, TiO2 coated and TiO2-water repellent coated surfaces. The water repellent was superimposed on the reference heater before TiO2 coating on it to depress hydrophilicity nature of TiO2. The boiling tests were carried out in steady-state condition and in saturated water under one atmosphere during the entire experiment. The nucleate boiling performance depends on the level of surface wettability. Results showed that TiO2 coated surface contributed to the increasing of nucleate pool boiling performance by 64.1% in comparison to that of the plain surface. The TiO2-water repellent coated surface gave a less enhanced performance than the TiO2 coated surface, and it is underperformed over the plain surface when the wall superheat (AT) > 12.3 degrees C. Boiling visualization demonstrated the augmented heat transfer mechanism by showing more active nucleation sites and more bubble departure frequency on the TiO2 coated surface. (C) 2018 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. (C) 2015 Elsevier Ltd. All rights reserved.

Condensers are flow architectures needed to provide high rates of condensation (or cooling) per unit volume, in enclosures with fixed volume. Their design has not changed from configurations consisting of the banks of horizontal tubes. In this paper, we outline a free path to evolving the design by exploring new features of flow configuration: flattened tubes, multiple tube sizes, arrays of flattened tubes, vertical tubes with turbulent film flow, forced convection condensation instead of gravity driven condensation, and the optimal length of a horizontal tube, i.e., the number of tubes in a column aligned with vapor cross flow. We show that the condensation density can be increased sizably by varying freely and without bias the morphology of the flow system: the shapes and arrangement of the cooled surfaces on which condensation occurs. The evolution of technology is described in terms of the special time direction of the useful (purposeful) changes in the configuration (shapes, arrangements) of surfaces on which flow/condensation occurs. This explains what “evolution” means. It is an important step for physics, not just technology.

In this paper we determine the fundamental relation between global performance and flow configuration (constructal design) in the case of steam power generation with superheater and reheater placed in parallel in the same stream of hot gases of combustion. The superheater heats the steam for the high pressure turbine, and the reheater supplies steam to the low pressure turbine. Both turbines operate irreversibly. We consider superheaters and reheaters with balanced counter flows and unbalanced counter flows. The total heat transfer area (or the overall number of heat transfer units) is finite. We show that the heat transfer area can be allocated to the superheater and reheater such that the overall power output is maximum. The optimal area allocation ratio is reported for two scenarios: designs with and without a maximum allowable steam temperature.

The present work shows how to minimize the global pressure drop in a comb-like channel network with self-healing and self-cooling functionalities implementing the concept of constructal design. A thick channel distributes the fluid flow to many thin channels that are perpendicular to the thick channel. The flow regime throughout the network is assumed to be fully developed laminar flow with negligible local losses. We systematically investigated the degrees of freedom of the fluid channel network, and determined the optimal internal and external aspect ratios of the flow architecture such that the total pressure drop is minimum.

In this paper we explore the opportunity to maximize the production of power in a steam-turbine power plant by properly configuring the hardware at the interface between the stream of hot gas produced by the furnace and the steam that circulates through the power producing cycle. The interface consists of four heat exchangers, superheaters and reheaters, in parallel flow and counter flow. The search for better configurations is based on constructal design, and consists of searching for the distribution of heat exchanger surface (number of heat exchangers, types, sizes) such that the total power output of the turbines is maximum, subject to fixed total size for the heat transfer surface, and fixed maximum allowable steam heat exchanger wall temperature. The emergence of the more effective configurations is documented along with the migration of the spot of maximum temperature along the flow path.

An analytical study on the flow resistance of tree-shaped channel-flow architectures was carried out based on the principle of the constructal law; the evolutionary increase in the access to currents that flow through the channels with improvements in the flow configurations were studied in a square domain using two diameters. Two types of tree-shaped configurations were optimized The minimized global flow resistance decreased steadily as the system size N-2 increased. From the two channel configurations, the one that resulted in better pressure drop was selected. Further, it was shown that the system performance can be enhanced by adopting the second tree-shaped configurations when the system size is greater than 18(2).

Here we explore new vascular designs for volumetric bathing of smart structures with volumetric functionalities (cooling, self healing). One stream bathes the volume, and the vasculature is configured as two trees matched canopy to canopy. Several architecture types are optimized: one channel size versus two channel sizes, increasing complexity (first, second, and third constructs) and increasing size (up to 50×50 elemental volumes). Vasculatures can be optimized for two objectives: low flow resistance and low flow nonuniformity. Tradeoffs are discovered with respect to complexity: for example, the optimized second construct has a lower global resistance than the first construct when the system size exceeds 20×20. The flow nonuniformity in the third construct is lower than in the second and first constructs.

In this study, the hydrodynamic and thermal characteristics of new vascular designs for the volumetric bathing of the smart structures were investigated numerically by addressing three-dimensional continuity, momentum, and energy conservation as a conjugate heat flow phenomenon. The numerical work covered the Reynolds number range of 50–2000, cooling channels volume fraction of 0.02, pressure drop range of 20–2×105Pa, and six flow configurations: first, second, and third constructal structures with optimized hydraulic diameters and non-optimized hydraulic diameter for each system size 10×10, 20×20, and 50×50, respectively. The numerical results show that the optimized structure of cooling plates could enhance heat transfer significantly and decrease pumping power dramatically compared with the traditional channels. The difference in thermal resistance performance between optimized and non-optimized structures was found to increase and manifests itself clearly as the system size increased. The channel configurations of the first and second constructs are competitive in non-optimized configurations, whereas the best architecture was the third construct across all working conditions in non-optimized configurations.

Here we show that there is a complete analogy between the configuring of heat and fluid flow and the configuring of the “flow of stresses”. The common principle is the avoidance of flow strangulations. This endows the flow systems (fluid, heat, stresses) with high density: for example, strongest for a fixed volume, or lightest for a fixed load. Examples treated in detail are “ducts” for the flow of stresses (slender bars in tension of compression), bifurcated bars shaped as Y and V, and slender and tapered plates (adhesive joints, tendons, ligaments), which are shown to be analogous to convection boundary layers. The design value of the “flow of stresses” concept is emphasized: to facilitate the flow of stresses (and the flow of fluid and heat) is to generate the configuration of the flow system, in accord with the constructal law.

Vascular structures are contemplated for cooling the skins and leading surfaces of future high speed aircraft. This paper evaluates the proposal to cool with a flow architecture shaped as trees (dendritic) a parallelepipedic body that is heated uniformly. The coolant enters the body through one face and exits through the opposite face. The vasculature connects the two faces, and consists of trees that alternate with upside down trees. The fields for fluid flow and heat transfer are determined numerically in three dimensions. The effect of local pressure losses at bends, junctions and entrances is documented. Designs with tree-shaped architectures having up to four levels of bifurcation are evaluated for fluid flow and heat transfer performance, and are compared with the performance of a design with a single sheet of fluid sweeping the upper surface of the body. The fluid flow conductance of the tree designs increases when the number of bifurcation levels increases. The thermal performance of tree designs can be improved by endowing the tree design with more freedom such that the bifurcations generate asymmetric daughter channels. The tree designs outperform the fluid sheet design dramatically: the global thermal resistance of the tree designs is roughly one tenth of the global thermal resistance of the fluid sheet design.

Here we report the transient cooling performance of a body vascularized with tree-shaped channels supplied with coolant, which flows from one side of the body to the other. The vasculature consists of trees that alternate with upside down trees. Heat is generated volumetrically through the body at t=0. A time delay td separates the start of the flow of coolant from the start of heating. Three-dimensional simulations of conduction and convection in the solid-fluid composite show the formation and evolution of hot spots in the material. If the delay is not short enough, the maximum temperature of the body overshoots the maximum allowable level. The paper shows how to design the cooling delay time such that the hot-spot temperature does not exceed the safe level represented by the maximum temperature in the limit of steady state operation. The critical delay time is determined as a function of the applied pressure difference and the complexity of the dendritic flow architecture.

The drive toward vascular smart materials calls for novel flow architectures that bathe and serve entire volumes and areas as uniformly as possible. Here, we show that vascular designs consisting of trees matched canopy to canopy can be configured so that they have two qualities: small flow resistance (ψ) and small volumetric flow nonuniformity (μ). In the past, the only quality sought was small flow resistance. Two classes of architectures are explored: (a) matched trees with diagonal channels through the core and (b) matched trees with orthogonal channels. First, we show that flow architectures can be developed and selected for minimum flow nonuniformity alone. Second, in the ψ−μ design space the best of designs (b) lie close to the best of designs (a), although the best of designs (b) offer slightly better configurations (low ψ and μ) than the best of designs (a). Comparisons with similar architectures generated based on genetic algorithms show that the minimum global flow resistance ψ of designs (a,b) is 2–5 times smaller than the genetic-algorithm values. The flow nonuniformities μ corresponding to the minimum ψ of designs (a,b) are 2–70 times smaller than the flow nonuniformities of the genetic-algorithm results.

The vascularization of smart materials with self-healing functionality requires the distribution of fluids continuously and uniformly throughout the material volume. This paper shows how to configure the architecture such that the single stream that flows through the vascularized body has access to every volume element. The configuration is two trees matched canopy to canopy, and has freedom to morph in several directions: channel orientations (diagonal vs. orthogonal), channel sizes, and system sizes. Tree–tree configurations provide greater access when diagonal channels are combined with orthogonal channels, and when multiple and optimized channel sizes are used.

Here, we show that the most basic features of tree and forest architecture can be put on a unifying theoretical basis, which is provided by the constructal law. Key is the integrative approach to understanding the emergence of “designedness” in nature. Trees and forests are viewed as integral components (along with dendritic river basins, aerodynamic raindrops, and atmospheric and oceanic circulation) of the much greater global architecture that facilitates the cyclical flow of water in nature (Fig. 1) and the flow of stresses between wind and ground. Theoretical features derived in this paper are: the tapered shape of the root and longitudinally uniform diameter and density of internal flow tubes, the near-conical shape of tree trunks and branches, the proportionality between tree length and wood mass raised to 1/3, the proportionality between total water mass flow rate and tree length, the proportionality between the tree flow conductance and the tree length scale raised to a power between 1 and 2, the existence of forest floor plans that maximize ground-air flow access, the proportionality between the length scale of the tree and its rank raised to a power between −1 and −1/2, and the inverse proportionality between the tree size and number of trees of the same size. This paper further shows that there exists an optimal ratio of leaf volume divided by total tree volume, trees of the same size must have a larger wood volume fraction in windy climates, and larger trees must pack more wood per unit of tree volume than smaller trees. Comparisons with empirical correlations and formulas based on ad hoc models are provided. This theory predicts classical notions such as Leonardo's rule, Huber's rule, Zipf's distribution, and the Fibonacci sequence. The difference between modeling (description) and theory (prediction) is brought into evidence.