•A novel analytical method of Free-Piston Brayton Generator was proposed.•The distribution of temperature and exergy loss was demonstrated.•The system has high system efficiency at high frequencies ...and small displacements.•System efficiency and power are sensitive to piston diameters.
The escalating energy crisis and environmental issues have compelled countries worldwide to advance energy transitions and vigorously develop green and clean energy technologies. External combustion devices can effectively utilize hydrogen-based fuels, biomass, solar energy, and other sources, playing a critical role in achieving carbon reduction. As a type of external combustion device, the free-piston Brayton generator, with its strong energy adaptability, stands as a highly promising power generation device. Therefore, the design optimization methods and operational characteristics of the free-piston Brayton generator system are worthy of thorough investigation. This paper introduces a novel method for constructing and structurally optimizing a free-piston Brayton generator model. Sage software is utilized to construct a closed-loop free-piston Brayton generator system in the time domain, followed by numerical analysis. The results show that within the frequency range of 35 ∼ 80 Hz, the system maintains a thermal-to-electric efficiency exceeding 40.00 %, demonstrating the system’s ability to operate efficiently at high frequencies. The piston diameter influences the dynamic behavior of the system’s piston, emphasizing the necessity of gradually adjusting the piston diameter to achieve optimal efficiency during structural parameter optimization. System efficiency and power can be balanced by adjusting the compressor inlet pressure. Under design conditions, the system can output 2046 W of electrical power. The thermal efficiency and thermal-to-electric efficiency are 46.84 % and 41.36 %, respectively. Additionally, an innovative analysis of the temperature distribution of the system and the distribution of exergy losses in major components under conditions of high frequency and small piston displacement is conducted. This work provides a new method and guidance for the subsequent design and optimization of free-piston Brayton generators.
Selective laser melting (SLM) produces a large amount of residual stress which limits its applicability. Despite the increasing number of studies to understand the relationship between the process ...parameters and the residual stress, the underlying complex mechanisms are yet to be understood. In this work, a 3D finite element model (FEM) is established to investigate the relationship between the process parameters and the residual stress during SLM. When the process parameters are in different ranges, the residual stress behavior is different. The complex relationship between the residual stress and the process parameters could be the reason for the contradictory parametric effects reported in the literature. The magnitude of the parametric effect on the residual stress is obtained. The influence of the scanning speed is stronger than that of the laser power, which is in turn stronger than that of the hatch spacing. These results can bring insights into the understanding of the residual stress during SLM.
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•The stress generation and evolution are investigated through the temperature and stress history during SLM.•The evolution of the stress can be divided into four areas. The residual stress is decided by the four stages.•When the process parameters are in different range, the residual stress behavior is different.•The magnitude of the parametric effect on the residual stress is scanning speed > laser power > hatch spacing.
Aluminum foam, carbon fiber reinforced plastics (CFRP) and foam-filled structures have been drawn growing attention for their outstanding lightweight and energy absorption capacity; therefore, ...crushing characteristics of a hybrid system involving these components would be of particular interest. In this study, quasi-static compression tests were carried out to experimentally investigate the crushing behaviors of foam-filled aluminum/CFRP hybrid tube subject to transverse loading condition. Based upon the experimental tests and numerical modeling, the interactive effects in between the aluminum foam filler and aluminum/CFRP hybrid tube was explored. It is found that the load carrying and energy absorption capacities of foam-filled hybrid tube were significantly improved in comparison with the summation of net foam filler and empty hybrid tube. The parametric study was carried out for exploring the effects of the aluminum foam density, aluminum tube thickness and ply number of CFRP tube on the crushing behaviors of foam-filled hybrid tube. It is found that both the total energy absorption (EA) of foam-filled hybrid tubes and the EA contributions of the aluminum foam (or aluminum tube, or CFRP tube) were enhanced with increase in density (or thickness, or ply number). The specific energy absorption (SEA) of foam-filled hybrid tubes increased from 3.45 J/g to 9.24 J/g with the foam density changed from 0.23 g cm−3 to 0.70 g cm−3; nevertheless, increase in aluminum tube thickness (or ply number of CFRP tube) has no evident influence on the SEA of foam-filled hybrid tubes. Finally, discrete design optimization was further performed to obtain the best possible foam-filled hybrid configuration for the transverse crushing characteristics. The optimum results showed that the SEA was largely improved by 213% in comparison with the baseline design.
•An innovative bolted connection with dual-slot hole was proposed.•Four full-scale specimens were tested and seismic behavior was obtained.•The FEM was validated using test data and different ...parameters were discussed.•A hysteresis model that compares well with the test curves was given.•Designs to improve the connection’s fast splicing and energy dissipation capacity were found.
Although a significant advantage for modular steel buildings (MSBs) is suitable for rapid construction, the unexpected installation error and initial defects of components in the construction site may lead to the failure of rapid assembly or even connection between modules. Aiming at the error sensitivity problem of module splicing, this paper proposes a new type of bolted connection with dual-slot hole and two kinds of connection strengthening methods, including diagonal brace strengthening and flange strengthening. To proceed, tests on four full-scale connections with cyclic loads were performed for seismic behavior. The test results presented the specimens’ failure mode, strength and stiffness properties, ductility, and energy dissipation capacity. Following that, the finite element model was validated against test data, and the influence of slot hole size and displacement effect was discussed. Finally, the hysteretic model was given. The results indicated that the proposed connection has reliable load-bearing, stable energy dissipation, and fast splicing capacity. The connection may be classified as semi-rigid type according to the stiffness feature for the structural behavior of MSBs. Both diagonal brace and flange strengthening can effectively improve the connections’ load-bearing capacity but weaken the energy dissipation capacity. The research results could serve as an effective reference for the inter-module bolted connection design of MSBs.
Radioisotope thermoelectric generators (RTGs) have been widely used as a promising power source for space mission, in which the Multi-Mission RTG (MMRTG) is the state of the art type. However, due to ...the scarcity of the 238Pu fuel and associated cost concerns, there exists an imperative need to increase the efficiency of RTGs. This requires intuitive, detailed and accurate system property predictions of the RTG system. In this work, a comprehensive finite element model of the MMRTG has been developed, by using a commercial software, COMSOL Multiphysics, including a full-size three-dimensional geometry model, temperature dependent materials and the thermoelectric and radiation heat transfer multiphysics fields involved. The calculation results agree well with reported experimental and simulated values. Through the obtained detailed temperature and voltage distribution, thermal and power properties of the MMRTG are characterized and the effects of non-uniform distributions of temperature and power generated in each TE couple are revealed. At last, the parametric analyses of operation conditions, thermal environment and key design factors are performed and several salient findings have been obtained. Through the analysis results, the design parameters and working mechanisms of the MMRTG are clarified, which can help the design and optimization of the future hundred-watt RTG.
•The comprehensive FEM method is first implemented in the MMRTG modeling and analysis.•The non-uniform distributions of temperature and power in TE modules are revealed.•Suitable load and sufficient thermal inventory ensure adequate power output.•The insensitivity of performance to ambient temperature expands MMRTG’s application.
Hierarchical honeycomb topology is one of the most promising bio-inspired ultralight materials with superior specific strength and stiffness. The existing investigations mainly focus on the ...hierarchical structures composed of conventional triangular or hexagonal unit cells; however, the influences of other types of polygonal unit cells and non-uniform wall thickness were less studied. This paper gives an insight into the out-of-plane crushing of vertex based hierarchical honeycombs with polygonal substructures and non-uniform wall thickness. The experimental results report that the deformation mode of hierarchical honeycomb varies from progressive folding to irregular folding at a relative density of 0.0386. With the increase of relative density, the failure mechanism changes from elastic wall bulking to plastic wall buckling. The theoretical solutions suggested that the wall thickness ratio affects the half wavelength and plateau crushing stress more considerably than the edge length ratio. Though the substructures with less edge number decrease the overall plateau crushing stress level of hierarchical honeycombs, they increase the maximum plateau crushing stress due to the raise of the wall thickness ratio achieved. The specific peak values of the maximum plateau crushing stress are increased by 27.7–51.8% compared to those of the uniform wall thickness hierarchical honeycombs.
The performance of a PEMFC (Polymer Electrolyte Membrane Fuel Cell) is greatly influenced by the various operating variables like temperature, pressure, stoichiometry of reactants and humidity. The ...presented work deals with the experimental study of 30 cells low temperature PEM fuel cell stack employing Nafion®-212 membranes. Experimental results are discussed with polarization curves and compared with the steady state model results, developed in MATLAB. The model predicts increase PEMFC performance with increase in operating temperature, pressure and reactant humidity. The comparison shows good agreement between experimental and modeling results with deviations in the range of 7–15% only.
·Experimental evaluation of operating parameters' effect on fuel cell performance.·Steady state mathematical model development in MATLAB.·Model validation by comparison with experimental results.·Identification of optimum operating parameters.
•A study on the transpirative cooling effect of a single row of trees.•The transpirative cooling effect assessed by calculating UTCI around vegetation.•The shading due to trees increases thermal ...comfort more than transpirative cooling.
Vegetation can provide transpirative cooling in cities and is therefore being increasingly integrated as an essential part of Urban Heat Island (UHI) mitigation strategies. However, the behaviour of vegetation must be accurately understood to determine the effectiveness of vegetation based solutions. In this study, vegetation is modelled as a porous medium in a computational fluid dynamics model for flow of moist air, where a leaf energy balance model is used to determine the heat fluxes. We study the cooling effect of a single row of trees at noon with solar altitude at 90° for various environmental factors (wind speed, air temperature, relative humidity and solar radiation intensity) and tree properties (leaf size, stomatal resistance and leaf area density). Furthermore, the influence of tree height and number of tree rows on the cooling effect are studied. The Universal Thermal Climate Index (UTCI) around the trees is estimated to determine the impact of transpirative cooling on pedestrian thermal comfort. The study shows that, at low wind speeds, pedestrians would only perceive a local benefit of transpirative cooling. However, vegetation extracts overall more heat from the flow at higher wind speeds. A study on the influence of environmental conditions quantifies to which extent a single row of trees provide maximum cooling during hot and dry conditions. The shading provided by trees improves thermal comfort more that transpirative cooling of a single row of trees. Furthermore, taller trees are more beneficial as the vegetation canopy with high leaf temperatures is further away from the pedestrian level.
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