Search Results (338)

Vibration energy is widely present in the natural environment. In the development of wearable self-powered systems, how to efficiently harvest the low-frequency mechanical energy of human motion has always been a core challenge. The piezoelectric–electromagnetic hybrid energy harvester designed in this paper consists of two units: a piezoelectric unit and an electromagnetic unit. The piezoelectric unit is composed of two arched plates, a piezoelectric layer, and an end magnet. The two sides of the piezoelectric unit are completely symmetrical. The electromagnetic unit is composed of a hollow tube, a central magnet, and a coil. The coil is wound around the outside of the center of the hollow tube to ensure that the central magnet can cut more magnetic flux lines. The two units output voltage through an external load. Firstly, based on a physical model, the force–electricity coupling mechanism is derived, and the dynamic response of the harvester at different frequencies is systematically tested. Secondly, through simulation and experiment, the influencing factors of the output voltage are deeply studied, and it is concluded that at medium and low frequencies (5 Hz–15 Hz), the harvester can provide efficient voltage output. The electromagnetic unit dominates at low frequencies and can output a larger voltage, but the voltage drops significantly after a certain frequency. The piezoelectric unit can supplement after the electromagnetic voltage drops, and the two have a synergistic effect. In addition, the output characteristics of the system mainly depend on frequency, initial distance, coil turns, and magnet mass. This paper clarifies the inherent physical mechanism of the hybrid energy harvester and provides an effective scientific reference for practical human motion energy conversion applications. Full article

Waste electronic components are valuable secondary resources containing various metals. Analyzing their elemental distribution is crucial for developing recycling methods. Micro- X-ray fluorescence (μ-XRF) is commonly used for this purpose, but traditional polychromatic X-ray excitation creates high background scattering. This masks trace element signals, impairing detection limits and accurate identification of minor valuable or hazardous elements. To address this, this study developed a monochromatic μ-XRF spectrometer using a low-power molybdenum-target X-ray tube. The system integrates polycapillary lenses for X-ray regulation and a flat crystal for monochromatization, producing a micron-sized monochromatic X-ray spot with high power density. This design eliminates scattered background from the primary continuous spectrum and enhances excitation efficiency by concentrating photon flux, enabling high-brightness monochromatic beams even at low tube power. The spectrometer was validated by analyzing a waste printed circuit board. High-resolution elemental mapping successfully revealed clear distribution patterns of major elements like copper, nickel, and iron, consistent with their physical structures. These images allowed intuitive differentiation of compositional differences across functional regions. This technique effectively overcomes the background interference caused by polychromatic excitation and is expected to further enhance the quality and reliability of elemental distribution imaging. It provides a powerful tool for formulating precise, scientific recycling strategies for waste electronics. Full article

Supercritical carbon dioxide (CO₂) is a promising working fluid for advanced power cycles. However, under high heat flux and low mass flux, its heat transfer performance can deteriorate severely, posing significant risks to system safety and efficiency. Inserting obstacles into flow channels is an effective way to suppress such heat transfer deterioration (HTD). In this study, the body-centered cubic (BCC) lattice structure is taken as an example to investigate the effects of the number and arrangement of BCC units on the flow and heat transfer of supercritical CO₂ using numerical simulation, deep learning, and genetic algorithms. The results show that placing a BCC lattice structure upstream of the HTD temperature peak effectively improves local heat transfer, and the deterioration zone is shifted downstream. For a fixed number of BCC units, different spatial arrangements have little impact on pressure drop and only a limited effect on heat transfer enhancement. However, their influence on the suppression of HTD is very significant. Based on the analysis of the optimal arrangement, an approximate optimal method is obtained, in which BCC structures are inserted sequentially at locations 0.5 to 2 tube diameters (D) upstream of each wall temperature peak. A simplified yet effective design strategy is also proposed: the first BCC structure is placed 0.5 to 2 D upstream of the smooth tube’s temperature peak, and the remaining BCC units are then distributed uniformly along the subsequent flow length. In this way, effective suppression of heat transfer deterioration is achieved. Full article

Heating gases to high temperatures is essential for supplying energy to thermal and thermochemical processes. This study presents the optical–thermal design of a mini heliostat field coupled with a tubular solar receiver equipped with second optics, aiming to heat nitrogen to approximately 850 K. The secondary optical system redistributed up to 40% of the incident solar flux from the front to the rear surface of the receiver, improving radial temperature uniformity and significantly reducing thermal gradients along the tube wall. An overall optical efficiency of 65.25% was achieved, accounting for atmospheric attenuation, shading, blocking, and the cosine effect. A coupled computational model was developed by solving the conservation equations of mass, momentum, and energy, with the spatially resolved solar flux distribution obtained via ray tracing used as a thermal boundary condition. The simulation results, validated with an empirical correlation, include solar flux contours, nitrogen temperature distributions, surface temperatures, and heat transfer coefficients. The configuration with a 12 mm vertex spacing between secondary reflectors demonstrated the best thermal performance, reducing the maximum tube surface temperature by 11% and improving radial symmetry, while maintaining nitrogen outlet temperatures near the design target of 850 K. These results confirm the suitability of the system for high-temperature applications such as solar pyrolysis using nitrogen as the heat transfer fluid to deliver the required thermal energy. Full article

Metal-chalcogenide compounds with perovskite-type compositions have drawn increasing attention for their optical properties for solar energy conversion. Herein, a new α-type polymorph of the ternary sulfide SrHfS₃ is described, crystallizing in the NH₄CdCl₃ structure type. The yellow-colored plate-shaped crystals were synthesized at 1173 K using an elemental tin flux in an evacuated sealed tube. Its crystal structure was characterized at room temperature using single crystal X-ray diffraction to form in the orthorhombic Pnma space group, with the refined cell parameters of a = 8.5041(4) Å, b = 3.8004(2) Å, c = 13.8935(6) Å, and V = 449.02(4) ų. The structure comprises five independent crystallographic sites, having one Sr, one Hf, and three S sites. The structure can be described as containing one-dimensional chains of distorted HfS₆ octahedra extending down the b-axis to form ${\displaystyle \genfrac{}{}{0pt}{}{1}{\infty }}[$HfS₃]²⁻ strips of edge-sharing octahedra. The Sr atoms act as charge-balancing space fillers in the structure. High-purity bulk samples of α-SrHfS₃ could be prepared for measurement of its bandgap by optical diffuse-reflectance spectroscopy, showing a direct bandgap of 2.1(1) eV. Results of electronic structure calculations are consistent with this bandgap and type. The conduction and valence band edges stem from the respective empty Hf d-orbitals and the filled S p-orbital states. In summary, crystal growth of the α-type polymorph of SrHfS₃ has been demonstrated using a Sn flux approach, which can facilitate future broader synthetic explorations at lower temperatures. Full article

Irradiation creep of engineering alloys in nuclear reactor cores differs from the creep that is observed outside of the irradiation environment. It exhibits characteristics like high temperature thermal creep because it occurs in an environment of elevated vacancy point defect concentrations, but one must also consider the effect of interstitial point defects and the effect of both vacancy and interstitial concentrations, which are greater than the thermal equilibrium values, on an evolving microstructure. Irradiation creep is dependent on the point defect flux to different sinks and can be modelled using conventional rate theory. The net interstitial or vacancy point defect flux to different sinks determines the strain rate in a direction that can be considered perpendicular to the plane of the sink, which is the extra half plane of an edge dislocation or the plane of a grain boundary. There has been increasing evidence that, for complex alloys such as Zr-2.5Nb pressure tubing in CANDU reactors, the irradiation creep is largely dependent on the grain structure (size and shape). While the maximum amount of thermal creep by dislocation slip will be proportional to the distance a dislocation travels, i.e., proportional to the grain dimension in the direction of slip, observations indicate that the magnitude of irradiation creep is inversely proportional to the grain dimensions, indicating a creep mechanism dependent on diffusional mass transport. Mechanistic modelling of irradiation creep based on rate theory is described and used to account for high diametral creep rates observed for pressure tubes with unusual microstructures fabricated by non-standard fabrication routes. Full article

Direct steam generation in parabolic trough collectors presents challenges due to the non-uniform distribution of heat flux and the appearance of flow patterns. These conditions can induce stresses, deformations, and deflections that compromise the structural integrity of the absorber tube; therefore, this study developed a coupled numerical model (optical, thermohydraulic, thermal, and thermoelastic) capable of reproducing the absorber tube’s behavior under real operating conditions. The methodology includes the following: (i) an optical model using Monte Carlo ray tracing to obtain the non-uniform distribution of solar heat flux and the local concentration ratio; (ii) a two-fluid thermohydraulic model to describe the transition from subcooled liquid to superheated vapor; (iii) a thermal conduction model; and (iv) an analytical thermoelastic model to quantify stresses, deformations, and deflections. The results identify the region near 421.35 m as the most critical, where circumferential temperature differences reached 28.38 K, generating maximum deformations between 600 and 800 με and deflections up to 18 mm along a 25 m section, 1 mm about to touch the glass cover. These findings demonstrate that this model facilitates the identification of critical conditions and the assessment of structural risks, contributing to improved reliability and safety in parabolic trough solar thermal power plants. Full article

Supercritical hydrogen has attracted much attention due to its convenience for storage and transportation. However, its thermophysical properties undergo significant changes within a narrow temperature range under ultra-low temperature and high-pressure conditions, resulting in significant differences in its heat transfer characteristics compared to normal-pressure hydrogen. So, it is urgent to clarify the heat transfer characteristics of supercritical hydrogen under the effects of various factors. For this, numerical simulations were conducted to study the heat transfer characteristics of supercritical hydrogen flow in a vertical upward tube under uniform heat flux conditions. Based on the NIST database, the drastic changes in the thermophysical properties of supercritical hydrogen were accurately considered, and the effects of buoyancy force and flow acceleration were also taken into account. Thereafter, the influences of tube diameter (6–8 mm), heat flux (1500–3000 kW/m²), fluid pressure (5–90 MPa), and mass flow rate (0.062–0.14 kg/s) on the heat transfer coefficient were analyzed. The results showed that increasing the heat flux, tube diameter, and fluid pressure, or reducing the fluid mass flow rate, was beneficial to increasing the wall–fluid heat transfer coefficient. Furthermore, a heat transfer correlation applicable to supercritical hydrogen flow in vertical tubes within the high-pressure range was obtained, with absolute errors below 10% when applied to previous studies. These results clarify the heat transfer characteristics of supercritical hydrogen flow in vertical tubes, providing a theoretical basis for the design of a supercritical hydrogen heat exchanger in practical scenarios. Full article

A database containing flow boiling heat transfer characteristics of various refrigerants inside micro-fin tubes with different structures under wide-range working conditions was built. Then the influencing mechanisms of refrigerant thermo-physical properties, fin structure and working conditions on nucleate boiling and forced-convection heat transfer characteristics were analyzed qualitatively. To reveal the actual heat transfer mechanism of refrigerant inside the micro-fin tube, some existing correlations were selected for evaluating the experimental data within the database. The comparison results indicate that there is no correlation achieving high-precision prediction for all experimental data and the prediction accuracy of correlation is influenced significantly by working conditions, particularly mass flux and heat flux. Finally, to acquire a general theoretical model, a new correlation was proposed based on the fitting mechanism of the Hamilton et al. correlation as it exhibits the most concentrated prediction deviation, which means the number of variables affecting correlation prediction effect is the least. After verification, it can be discovered that the average prediction deviation of the new correlation for all experimental data is less than ±30% when the two-phase fluid Reynolds number is less than 3500, which is enough to validate the application value of the theoretical model. Full article

We investigated how a magnetic topological defect affects the vacuum polarization of a charged massive scalar field in a flat $(3+1)$-dimensional space-time. The defect was modeled as an impenetrable-to-matter-field, finite-thickness tube with magnetic flux inside. We implemented the most general form of the Robin boundary condition on the surface of the magnetic tube, which enables a fully general analysis of the problem. We found that in flat space-time, the total vacuum energy generated by a magnetic topological defect depends on the curvature ($\xi$), except for special cases corresponding to the Dirichlet and Neumann boundary conditions. By contrast, when Robin’s general boundary conditions are imposed, the induced vacuum energy acquires an explicit dependence on the curvature coupling ($\xi$), which is significant even in flat space-time. A detailed study of the dependence of the effect on the boundary-condition parameter was carried out. The obtained results highlight the nontrivial role played by boundary conditions in vacuum polarization phenomena. Full article

This study investigates the flow and heat transfer mechanisms of high-temperature air flowing across staggered lined fine tubes in a SABRE-type precooler. Large-Eddy Simulation (LES) was employed to model three-dimensional unsteady flow under constant-property and variable-property air models at inlet temperatures of 400–800 K. The results show that increasing temperature substantially enhances vorticity, turbulent kinetic energy, heat flux, and Nusselt number, while flow separation and pressure drop are intensified. However, when temperature-dependent air properties are incorporated, the wake width increases and the separated shear layers become thicker, while the turbulence/unsteadiness intensity decreases. Consequently, the near-wall shear is reduced and the heat transfer coefficients are lower. Compared with variable-property predictions, constant-property models overestimate the average Nusselt number by 20–40% and the local pressure drop by 40–65%, and they underestimate the air-side temperature drop along the tube rows. These findings demonstrate that real-gas effects significantly alter both aerodynamic resistance and thermal performance. Overall, accurate representation of temperature-dependent air properties is essential for the reliable design, evaluation, and optimization of micro-tube precoolers. Full article

Thin liquid film evaporation leverages latent heat and low thermal resistance to achieve superior heat transfer capabilities, making it pivotal for next-generation high-heat-flux thermal management systems. This paper presents a systematic review of the fundamental mechanisms, interfacial transport behaviors, and experimental techniques associated with static thin films and falling liquid films. This work elucidates the complex coupling of Marangoni convection, van der Waals disjoining pressure, and contact line dynamics. These mechanisms collectively govern film stability and the intensity of non-equilibrium phase change in the micro-region. The influence of surface wettability and dynamic contact angle hysteresis on hydraulic replenishment and dry spot formation is critically analyzed, offering insights into optimizing surface engineering strategies. In addition, the review categorizes advanced non-intrusive diagnostics, including optical interferometry, laser-induced fluorescence (LIF), and infrared thermography, evaluating their capacity to resolve spatiotemporal variations in film thickness (ranging from 10 nm to several μm) and temperature under complex boundary conditions. Special attention is directed toward falling film evaporation over horizontal tubes, addressing flow regime transitions and the impact of interfacial shear from external airflow. The work concludes by identifying key challenges in multi-physics coupling and proposing future directions for synchronized diagnostics and adaptive surface design. Full article

The heat transfer and flow characteristics of TiO₂/R123 nano-organic working fluid are investigated and compared with that of R123. A three-dimensional numerical model of the smooth circular tube with a diameter of 10 mm and a length of 1 m is established, and the thermodynamic properties of the nano-organic working fluids are rectified with the volume of fluid model. The grid independence validation is conducted, and the simulation results from three models (the k-ε model, the realizable k-ε model, and the Reynolds Stress Model) are evaluated against experimental data. When using the TiO₂/R123 nano-organic working fluid, the error between the simulation and experimental results is 6.1%. The flow field distribution is examined, and the effect of mass flux on heat transfer coefficient and pressure drop is discussed. Results demonstrated that the inclusion of TiO₂ nanoparticles significantly enhances heat transfer performance. At a 0.1 wt% nanoparticle concentration, the heat transfer coefficient increases by 23.2%, reaching a range of 1430.11 to 2647.25 W/(m²·K), compared to pure R123. However, this improvement in heat transfer performance is accompanied by an increase in flow resistance, with the flow resistance coefficient rising from 0.0353 to 0.0571. Additionally, pressure drops increase by up to 18.7%. Full article

This study reports on heat transfer augmentation by knitted wire coil turbulators in a fully developed turbulent regime. Four knitted wire coil turbulators with different wire loop number densities (N = 6, 8, 10, and 12 loops per pitch, with 1.0 pitch = 6.8 mm) were tested. Each was made by winding a 0.7 mm copper wire around a 1.0 mm core rod. Experiments were conducted under a constant 600 W/m² wall heat flux. The flow behaviors observed through a dye injection technique revealed that the wire coil induced secondary flows and developed shear layers, contributing to enhanced heat transfer. Heat transfer improved with increasing wire loop number density. Application of knitted wire coil turbulators increased the Nusselt number (Nu) by 86, 95.4, 103.2, and 109.3% for N = 6, 8, 10, and 12, respectively. This corresponded to increased friction factors (f) by 1.77, 1.97, 2.15, and 2.31 times, respectively. The tube with coils having N = 12 yielded the highest thermal performance index (TPI), 1.4, at a Reynolds number of 5000. The empirical correlations for Nu, f, and TPI showed deviations within ±2.1, ±0.68, and ±2.28%, respectively. Full article

This study addresses the optimized design of falling-film heat exchanger tubes, aiming to enhance heat transfer efficiency and reduce thermal losses, thereby offering potential pathways for efficient green energy utilization. Ten tube models were established and analyzed using computational fluid dynamics (CFD) under constant heat flux conditions. The study investigated the effects of the position, number, and ellipticity (e) of external protrusions on the flow characteristics and heat transfer performance of oily wastewater. The simulation revealed that different protrusion configurations significantly influence hydrodynamic behavior and heat transfer mechanisms. It was found that introducing flow disturbances at an early developmental stage enhances the overall heat transfer performance of the external fluid. Specifically, for a tube with e = 0.5, the heat transfer coefficients (HTC) initially increases and then decreases with increasing Reynolds numbers (Re). This behavior is attributed to the reduction in flow stability caused by the protrusions at higher Re values, which promotes vortex shedding and leads to more complex flow patterns, thereby impairing heat transfer efficiency. Furthermore, as the number of protrusions increases, the overall HTC of the enhanced elliptical tube also follows a trend of an initial increase and then decrease. These results suggest the existence of an optimal protrusion density that enhances turbulence without incurring excessive resistance that would degrade thermal performance. Full article