The present article numerically studies the relationship between a heat exchanger's thermal performance and the occurrence of fouling, here represented by the deposition of calcium sulfate (CaSO4) on its surface. For that, a 2D geometry denoting the internal flow of a CaSO4 saturated aqueous solution between two parallel plates is simulated. The flow, which can be laminar or turbulent (k-omega SST model), is promoted by a known pressure difference across the channel, while the CaSO4 fouling is thermally triggered by a heat source in the central part of the channel. The numerical model considers the solution of the conservative equations of mass, momentum, and energy, while the mass transfer rate is determined based on a correlation available in the literature. Two approaches were used to determine the CaSO4 fouling rate: (i) the CaSO4 concentration on the fouling's surface was assumed to be identical to the concentration of the bulk flow, and (ii) the fouling concentration varies spatially, hence requiring the solution of the differential equation for conservation of species. The steady analysis showed that for both approaches, the optimal plate-to-plate spacing that maximizes the thermal performance is similar to the spacing that minimizes the fouling deposition. Finally, the transient analysis, which also considers the effect of the fouling removal rate through shear, shows that deviations might occur between the optimal spacings for maximal thermal performance and minimal deposition, especially for lower pressure drop values.

Recompression Brayton cycles using supercritical CO2 as the working fluid appear as a prominent alternative for thermo-solar power applications. Also, solar energy's natural variability and intermittence make it difficult for solar plants to operate consistently and predictably. Thus, two of the most explored mitigating alternatives are thermal energy storage and auxiliary heating systems. Hence, this paper used actual meteorological data and transient numerical simulations to investigate the power output dynamics of a 10 MW plant. The modeling of an active control system of the working fluid mass inventory allowed the plant to operate in a stable manner while accounting for the significant variations in the fluid's thermophysical properties. Also, the study investigated the effect of the sizes of the thermal energy storage system and solar collectors field on the dynamics of the system. Finally, statistical analyses with actual meteorological data from Florianopolis/Brazil for nine days between 2017 and 2018 supported determining the optimal thermal energy storage system size. Hence, depending on the daily conditions, the results showed the operating settings that minimize the use of auxiliary heating with reductions of fuel consumption larger than 10%.

Investigating supercritical natural fluids for efficient and clean energy production has become a trending research topic due to their technical and environmental advantages. However, on account of the supercritical operational conditions, using specially-developed components increases manufacturing prices, especially when dealing with solar-powered plants assisted by thermal energy storage (TES) systems. This paper assesses the economic and environmental trends of an integrated supercritical carbon dioxide (s-CO2) solar-powered plant. The system is composed of a packed-bed TES system, a solar field, and a power block while considering conventional backup heating. Transient year-around numerical simulations explore several operational conditions relying on detailed cost and typical meteorological year (TMY) data. Also, the modeling accounts for the system's environmental sustainability through a penalization cost regarding CO2 emissions due to auxiliary heating. With parametric analyses, the study assesses the compromise solutions minimizing the levelized cost of energy (LCOE). The results revealed the possible feasibility of the integrated system using such a TES technology for s-CO2 and evidenced several venues for further examination. In the end, a sensitivity analysis investigates the influence of the specific costs and TMY data on the LCOE.

This numerical study investigates supercritical carbon dioxide (s-CO2) as a medium for thermal energy storage (TES) systems operating under natural convection regarding heat transfer coefficient and energy density. The numerical domain comprises a horizontal heated cylinder surrounded by the supercritical fluid. The modeling employs a 2-D formulation due to the axial symmetry while the constitutive equations consider that the thermophysical properties vary spatially with temperature and pressure. The results revealed thermodynamic states that maximize the heat transfer coefficient and energy density for constant-pressure operation. Besides, considering the numerous studies on correlations for the natural convection heat transfer coefficient for s-CO2, the analysis compares simulation data with correlations based on constant and integrated-averaged fluid properties. The results suggested that the constant-property formulation fails in the thermodynamic region where properties change significantly. Thus, this work proposes a criterion for deciding when to consider integrated-averaged properties regarding the behavior of the isobaric thermal expansion coefficient. Finally, the study extended the analysis to air, helium, and nitrogen while employing operational conditions suitable for power cycles using such fluids. The comparisons revealed that, among these competitors, helium presents the highest heat transfer coefficient, but s-CO2 has the highest energy density.

Thermal energy storage is extremely important to power plants that rely on intermittent heat sources. Additionally, the interest in power cycles operating with supercritical carbon dioxide (s-CO2) is increasing recently. Therefore, this study investigates the performance of a packed-bed thermal energy storage (TES) system using s-CO2. The 1-D computational porous medium-based model accounts for the heat exchange between the solid matrix and heat transfer fluid through a two-temperature formulation, thus allowing the calculation of the pressure drop and the heat loss to the environment, which are derived from the fluid flow through the thermal energy storage tank. The thermal-hydraulic analysis investigates, from parametric and optimization standpoints, the effects on the performance of the TES system of design and operational parameters, such as the storage tank volume and length/diameter ratio, porous medium particle size and porosity, charging and discharging mass flow rates, charging temperature, and the charging-discharging cycling. The results clearly suggest design trends for s-CO2-based systems, as well as show that not only the thermal-hydraulic charging-discharging combined efficiency of the TES unit is highly dependent on the above-mentioned parameters, but also and more importantly, that it can be optimized with respect to dimensional and operational parameters.

This article discusses the use of quasi-steady and transient modeling approaches for thermal systems subjected to periodic operating conditions. Specifically, the study quantifies the outlet bulk temperature deviation, between approaches, of an internal flow subjected to time-variable inlet temperature and/or external heat flux - which are common conditions in subsystems of solar-powered cycles. The analysis uses an in-house numerical routine based on the Fourier series and the superposition principle, also considers specific sets of boundary conditions (BCs) as well as laminar and turbulent flow regimes. The results show the contribution to the total deviation of each frequency composing the time-dependent BCs. Moreover, this study discusses how such contributions depend on the flow characteristic time-scale, which is affected by the fluid properties. Through regression analysis, the investigation develops generalized correlations for directly determining the maximal deviation between both modeling approaches without solving temperature fields, i.e., with the appropriate expressions, the method only requires the BCs and the fluid properties. Finally, the method is applied to a simplified solar absorber using real irradiation data, results compared with the fully transient ones, and a cutoff-based adaptation proposed for balancing frequency interferences.

Daily minute-based (i.e., short-term) and typical meteorological year (TMY) hourly-based (long-term) analyses are presented for a recompression cycle with direct CO2 heating in a parabolic collector field. The cycle is assisted by a backup heating unit, which is composed of a two-tank molten salt thermal storage and an auxiliary heating process representing, for example, a fossil fuel burner. The analyses also considered that the cycle is subjected to two distinct demand profiles, one that is constant throughout the day and another that varies hourly. The multitude of conditions modeled allowed the results to explore the effect of different solar irradiation profiles, seasonality and solar multiple factor. The analysis revealed that for short and long-term analysis the proper size of the solar collector field, here presented by the solar multiple, is strongly related to the demand profile as well as to the time frame considered within the analysis, i.e. short-term or TMY. The results also indicated that when the recompression cycle operates under same solar multiple factor with either a variable or a constant demand, the latter configuration is less dependent of auxiliary heating while the former presents higher performance values. Ultimately, the analysis reinforces the need for considering realistic inputs (e.g., irradiation, demand, etc.) when dealing with renewable-based power plants.

The theoretical transient analysis of a recompression cycle is considered while directly heating the working fluid, CO2, in a solar trough field. The analysis explores the temporal behavior of the system's performance for three distinct, minute-based, solar irradiation profiles: (i) daylong perfectly clear skies, (ii) instantaneous on-off cloud shading and (iii) partially cloudy day. The key objective is to study the effect of the solar intermittency on the heat exchangers, solar collector and on the overall cycle's behavior - the thermal capacitance of the two first components is considered. The calculations reveal that, even though auxiliary heating systems was not considered, the heat capacitance of the working fluid and parabolic collector assembly is capable of instantaneously smoothing out the cycle's performance for on-off clouding conditions, such as the net power and thermodynamics efficiencies - note that the cycle's performance disregards the parabolic trough efficiency. Differently, the overall system's efficiency, which is regarded as the product of the efficiencies of the cycle and parabolic trough, is much more sensitive to irradiation changes. The study also specifically considers the effect of the clouding intensity and time, with the former being measured as fraction of clear sky irradiation on the system's performance. The analysis shows that it is possible to quantify the time scale available such that an auxiliary heating system delivers heat to the working fluid during a clouding scenario allowing the plant to work steadily.

The effect of hydrophilic constructal-like patterns on the condensate mass flow rate production of super-hydrophobic vertical test sections is discussed. The surface patterning, which presents a branching topology, is obtained by coating selected areas of the condensation surface with a commercial product. In total, five different designs were tested, three with a branched topology having coated/clear area fractions of approximately 30%, 50% and 70%, in addition to fully clear and fully coated test sections. The condensation performance of each test section was measured in the presence of non-condensable gases inside a climate-controlled chamber, which allowed independent adjustment of its internal temperature and humidity. Condensation measurements indicate that for some cases, averaged improvements ranged between 7.4% and 17.5% when a test section with an area fraction of 70% is compared with clear and fully coated surfaces, respectively, considering the climate-controlled chamber is kept with a relative humidity of nearly 80%. Additionally, a qualitative image-based analysis of the condensation process revealed that the superhydrophobic/hydrophilic interface arguably aids the droplet detachment and could, potentially, guide/direct the draining flow in real systems.

Undoubtedly, solar energy is one of the most promising renewable sources available. However, the wide range of possible configurations and operational scenarios makes it difficult to decisively select one power plant assembly over another. Therefore, the present analysis proposes a performance-based methodology to assist the design of solar trough power plants. The analysis, which considers carbon dioxide as the only fluid circulating through the cycle, initially determines transition conditions for selecting between transcritical Rankine or transcritical Brayton cycles based on standard thermodynamic performance parameters, such as, the net power delivered and thermodynamic efficiencies. The analysis is followed by a parametric study, which now considers the effect of the collector's area on the net power delivered and thermodynamic efficiencies. The results clearly show the existence of an optimal collector's size, which returns the most favorable tradeoffs for some of the performance parameters mentioned above. The analysis also suggests a Brayton/Rankine transition function in terms of pressure and temperature downstream the solar collector aiming to easy the selection between both types of cycles. At the end, the results explore the relative performance of different arrangements of fully supercritical Brayton cycles. (C) 2017 Elsevier Ltd. All rights reserved.

This paper experimentally considers the use of non-structured (i.e., flexible) metal foams as a way to increase the amount of water vapor recovered from a humidified air stream. For that, a dedicated experimental setup consisting of three sub-systems (i.e., air pumping, air humidification, and air dehumidification) was developed. In the dehumidification section, different test sections (condensation structures) were installed and the amount of water recovered from the saturated air stream was recorded and correlated with each of them. Four main types of test sections were tested: (i) bare copper tube (used as base line), (ii) copper tube assisted by metal foam, (iii) finned tube and (iv) finned tube assisted by metal foam. Refrigerated fluid was circulated through copper tubes within a close loop and served as heat sink, allowing vapor phase change. The results show that the amount of water recovered increases with the surface area of the test section and with the temperature difference between the saturated air stream and the cooled surface. Also, it was shown that, above a certain quantity of metal foam within the test section, the amount of condensate produced is basically unaffected. Differently, the results indicate that the use of fins, which were properly brazed to the copper tubes, increases significantly the amount of water recovered from the saturated air stream. (C) 2014 Elsevier Inc. All rights reserved.

In this article, the thermal behavior of a cylindrical asphalt tank subjected to two different heating configurations: (i) large area serpentine heating and (ii) concentrated heating cartridges, was studied numerically and experimentally. The goal is to investigate the effect of these two heating configurations on the temperature distribution of the molten asphalt inside the tank, as well as to validate temperature-dependent thermal-physical properties of molten asphalt employed in the numerical simulations. For the experimental setup, a scaled tank having a volume of approximately 0.2 m(3) was built, and filled with molten asphalt. The setup was fully instrumented and run at different heat dissipations rates for each of the heating configurations. The experimental results show that the temperature distribution within the tank depends on the heating method and on the heat dissipation level, as expected, however, both heating methods were able to prevent the solidification of the enclosed asphalt and so are feasible for actual application in storage tanks. As for the numerical work, two models representing both experimental heating configurations were also implemented. The numerical results obtained present a good agreement with the experimental measurements, suggesting that a strictly theoretical optimization of the heating mechanics in asphalt tanks can be used in future studies. (c) 2013 Elsevier Ltd. All rights reserved.

This paper uses the Growing Structure Multiple Model System (GSMMS) method for fault detection and precedent-free localization of unwanted heating anomalies in two different configurations of channel flow systems operated under dynamic conditions: (i) straight channel and (ii) straight channel with an internal flow disruptor. Unlike commonly used fault detection methods, the newly proposed approach does not require prior information regarding the fault location, fault severity or data emitted in the presence of a fault to build the model of that fault and recognize it. The new detection mechanism is based only on the models of normal behavior for various portions of the monitored system. The obtained results indicate that the detection and localization of the unwanted heating element (i.e., heat source) can be achieved through distributed GSMMS-based anomaly detection, with multiple anomaly detectors monitoring different parts of each configuration. The results also suggest that fault detection and localization are strongly related to a system’s configuration and operational conditions.

The present work explores the formal evolutionary development of complex thermal physical systems using a bio-inspired evolutionary method. The bio-inspired method consists of Lindenmayer systems (L-systems) with its turtle interpretation for the modeling of the complex dendritic structures, the finite element method for the analysis of the structure and an evolutionary algorithm to evolve the topology of the dendritic structure. With this method, we investigate the optimal topology of a highly conductive dendritic structure for draining excessive thermal energy from a fixed area subject to uniform heat generation. The results show that the evolutionary approach can yield complex, dendritic topologies that are highly performing and robust. Moreover, our results demonstrate that a better performance in heat removal implies an increased complexity of the draining system; and that there is an optimal level of complexity beyond which the performance of the system is not substantially improved. Finally, we discuss the robustness of hierarchical, dendritic complex topologies for heat transfer systems.

The present paper addresses the numerical optimization of geometrical parameters of non-Newtonian micro-scale viscous pumps for biomedical devices. The objective is to maximize the mass flow rate per unit of shaft power consumed by the rotor when an external pressure load is applied along the channel that houses the rotor. Two geometric parameters are considered in the optimization process: (i) the height of the channel that houses the rotor (H) and (ii), the eccentricity (epsilon) of the rotor. Three different micro-scale viscous pump configurations were tested: a straight-housed pump (I-shaped housing) and two curved housed pumps (L- and U-shaped housings). The stress-strain constitutive law is modeled by a power-law relation. The results show that the geometric optimization of micro-scale viscous pumps is critical since the mass flow rate propelled by the rotor is highly dependent on epsilon and H. Numerical simulations indicate that mass flow rate is maximized when epsilon approximately 0, namely when the rotor is placed at a distance of 0.05 radii from the lower wall. The results also show that micro-scale viscous pumps with curved housing provide higher mass flow rate per unit of shaft power consumed when compared with straight-housed pumps. The results are presented in terms optimized dimensions of all three configurations (i.e., H(opt) and epsilon(opt)) and for values of the power-law index varying between 0.5 (shear thinning fluids) and 1.5 (shear-thickening fluids).

This paper reviews recent constructal-theory advances: the optimal distribution of discrete heat sources cooled by laminar natural convection. Three scenarios are investigated: (i) many small heat sources mounted on a vertical wall, (i) a few small finite-size heat sources mounted on the side wall of a two-dimensional enclosure, and (iii) one heated area on the wall of a vertical diverging or converging channel with chimney flow. In (i) and (ii), the optimally distributed heat sources are not equidistant. In (iii), the geometry changes by varying the space between the walls, the distribution of heating along the walls, and the angle between the two walls. Numerical simulations in the Rayleigh number range 105≤RaH≤107 show that for maximal heat transfer rate density it is better to install heated sections at the channel entrance. The optimal angle between the two walls is approximately zero when RaH is large. The robustness of flow architectures with optimized distribution of heat sources is discussed.

The paper reports the performance of balanced two-stream parallel flow heat exchangers, in which each stream flows as a tree network through its allotted space. The two trees are in parallel flow, and are arranged like two palms pressed against each other. The relationships between effectiveness and number of heat transfer units are developed for several parallel tree flow configurations: (i) constructal dichotomous trees covering uniformly a rectangular area, (ii) trees on a disk-shaped area, and (iii) trees on a square-shaped area. In configurations (ii) and (iii) each stream flows between the center and the periphery of the area. Configurations (i) and (ii) are trees with minimal resistance to fluid flow. Configuration (iii) is designed by minimizing the length of each duct in the network. The performance of the parallel flow configurations is compared with the performance of counterflow configurations. The future use of dendritic heat exchangers in devices with maximal heat transport density is proposed.

The present paper addresses the effect of geometric parameters such as channel height and rotor eccentricity on the mass flow rate and power consumption of a two-dimensional microscale viscous pumps. The objective is to maximize the mass flow rate and at the same time minimize shaft power consumption when an external pressure load is applied along the channel that houses the rotor. Three different viscous micropump configurations were considered, a straight housed pump (I-shaped housing) and two curved housed pumps (L- and U-shaped housings). Because the performance of a microviscous pumps are based on the asymmetric placement of the rotor within the surrounding housing, the numerical results show that the rotor eccentricity and the channel height have a major effect on the mass flow rate generated by the rotor and on the shaft power demanded by the rotor. Preliminary simulations showed that mass flow rate is maximized when the eccentricity is small. The results also show that micropumps with curved housing (i.e., L- and U-shaped configurations) not only provide higher mass flow rates when compared with straight housed pumps, but also demand less shaft power to operate. Optimized geometric dimensions of all three configurations are presented for several values of the Reynolds number and pressure load.

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.