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The rapid development of wearable electronics highlights the urgence to develop the portable energy harvester with excellent output performance, comfortability, and sustainability. This work designs an electromagnetic walking energy harvester based on water turbine that can be embedded in shoes with good comfortability. Its working principle is that the walking generated pressure energy drives a miniature hydraulic turbine to output electricity. Experimental results show that an average power of 300 and 180 mW can be produced at heel and toe, respectively, when a man of 80 kg walks at a speed of 1.8 m s−1. This power output exceeds the piezoelectric, triboelectric, and electromagnetic walking energy harvesters reported in the past. Additionally, the simpler structure endows it better comfortability as compared with the electrostatic capacitances. Computational fluid dynamics simulations provide a further insight that the efficiency of turbine can reach 13.5% by optimizing parameters of blade number and outlet flow ratio. Finally, user real-time positioning and trajectory recording are successfully demonstrated via a wearable GPS means Global Positioning System module powered by the harvester. Due to the combination of high output performance, simple structure and low discomfort, the water turbine based walking energy harvester will provide a wide application potential in wearable devices. [ABSTRACT FROM AUTHOR]

• The erosion of runner is contributed to the inter-blade vortices. • Erosion distributions are highly consistent with the actual site observations. • Maximum average erosion rate occurs when particle diameter is 0.2 mm. Sediment erosion is a prevalent and significant challenge to hydro turbines in mountainous rivers. In order to gain an insight into the erosion characteristics of Francis turbines, this study considers the influence of actual sediment gradation and emphasizes the erosion mechanism from the perspective of flow structure. Numerical results reveal a significant correlation between erosion distributions and the location of inter-blade vortices, which is dependent on operating conditions. Specifically, at small or optimal openings, inter-blade vortices predominantly form on the suction side of blades, which coincidentally experiences the most severe sediment erosion. Conversely, at a large guide vane opening, sediment erosion and vortices are primarily distributed at outlet of pressure side, aligning closely with actual site observations at a hydropower station. With increase of operating heads, the average erosion rate of suction side decreases at all guide vane openings, while that of pressure side elevates significantly. In addition, the effect of particle size on sediment erosion was discussed and the maximum erosion rate is demonstrated to be proportional to sediment diameter. These findings would provide important engineering insights for operation optimization to reduce sediment erosion. [ABSTRACT FROM AUTHOR]

[Display omitted] • An effective strategy to prevent Cassie-wetting failure of structured hydrophobic surface was proposed by simply coating a single-layer graphene. • Droplets exhibit a stable Cassie-state wetting on graphene-coated surface due to the wetting transparency and water impermeability of graphene. • The modified surface showed a 40–100% increase in condensation rate in 5 h at a subcooling of 40 °C. Structured hydrophobic surfaces often suffer from Cassie-wetting failure due to trapped water in structure gaps for a long-term operation. Sustainable Cassie-wetting on such surface could be achieved by coating an atom-thick and moisture-impermeable graphene on it. Water contact angles were measured to clarify the effect of graphene on wetting, and water impermeability was verified by moisture deposition and evaporation. Sliding angle measurements and vapor condensation were carried out to demonstrate the stable Cassie-state wetting and application. Interestingly we found the graphene does not significantly disrupt the wetting behavior of the structured hydrophobic surface, showing a wettability transparency. Moreover, the impermeability of graphene keeps moisture away from the structure gaps. Owning to the combination of these two properties, droplets on the graphene-coated structured surface exhibit a stable Cassie-state hydrophobic wetting, even under the situation of moisture deposition and evaporation. Using the modified surface, we also found a 40–100% increase in condensation efficiency for a 5-hour vapor condensation at a subcooling of 40 °C. These results suggest an effective strategy to prevent Cassie-wetting failure of structured hydrophobic surface and are expected to promote its further application in more complex conditions. [ABSTRACT FROM AUTHOR]

Low-grade heat energy is enormous and widely distributed around the world, but it cannot be effectively converted to electricity owing to the small temperature difference and fluctuating heat source. As a promising technology that can convert low-grade heat energy into electricity while obtaining additional freshwater, thermo-osmotic energy conversion has attracted attentions of numerous researchers. However, traditional thermo-osmotic energy conversion technologies lack the utilization of the accompanying variation in salinity gradients throughout the process. In this study, a hybrid system was presented to achieve simultaneous freshwater production and electricity generation by combining thermo-osmosis system with a salinity gradient power recovery module. Continuous operation of the thermo-osmosis system will naturally produce a range of salinity differences, which will then be converted into electricity by the salinity gradient power recovery module. Our results show that this hybrid system can optimally increase the electricity output by ∼0.99 W m−2 over the conventional thermo-osmotic energy conversion system. Overall, our research demonstrates a promising device to harvest low-grade waste heat for co-generation of electricity and freshwater. This innovative approach has the potential to broaden the application possibilities of thermo-osmosis technology. • Salinity gradient recovery combined thermal-osmosis hybrid system was proposed. • A theoretical model was developed to predict electricity output of the system. • Comparing to traditional TOEC, increasing electricity output by ∼0.99 W m−2. • Co-generation of freshwater and electricity was achieved under real seawater. [ABSTRACT FROM AUTHOR]

In this paper, a numerical computation method combining a high-order compact difference scheme and immersed boundary method is used to simulate the flow around a hyperbolic cylinder with different aspect ratios and eccentricity. It is found that compared to the circular cylinder, the vortex-separation point in the wake of the hyperbolic cylinder moves backward, and the corresponding drag coefficient decreases by 27.6%. More interestingly, the time-averaged drag coefficient of hyperbolic cylinder behaviors parabolic relationship with cross-section aspect ratio and hyperboloid eccentricity, respectively. As the aspect ratio decreases from 1 to 0.6, the flow drag parallel to the long-axis reduces dramatically to 42.9%. Especially, there is no obviously low pressure region behind the hyperbolic cylinder when r = 0.4, which leads to the drag coefficient is only 0.0249 and close to zero. Besides, the average drag coefficient diminishes in a parabolic trend with the eccentricity that has a monotonously positive correlation with two parameters, and an increase in eccentricity e by varying a or c will also reduce the drag, but the effect of parameter a is more obvious than parameter c. The results presented here are expected to suggest a strategy for drag reduction in flow around the cylinder and provide theoretical guidance for the design and optimization of hyperbolic structures in flowing fluid. •Flow over a hyperbolic cylinder with different aspect ratio and eccentricity is investigated •Drag coefficient of hyperbolic cylinder decreases by 27.6% compared to circular cylinder; •Time-averaged drag has a parabolic relationship with the aspect ratio and eccentricity [ABSTRACT FROM AUTHOR]

Passive cooling for thermal comfort improvement has received extensive attention for its low energy consumption. However, most of the existing passive cooling technologies require a complex system design and supporting equipment, since they cool the ambient air. Herein, we propose a hybrid evaporative and radiative cooling membrane with a hygroscopic hydrogel sandwiched by two layers of a porous polyethylene aerogel (PEA). The hydrogel implements evaporative cooling. Combining the high solar reflection of PEA and the high infrared emissivity of hydrogel, this hybrid membrane also possesses radiative cooling. In addition, the high infrared transmittance and low thermal conductivity of PEA allow direct heat transfer between the hydrogel and human body, instead of the ambient air. Through comparative experiments and theoretical calculations, it is indicated that the net cooling power delivered by the hybrid membrane to the human body is up to 78.45 W m−2, which is much higher than that of conventional radiative cooling materials. Outdoor demonstration shows that emission below the hybrid membrane can achieve an average sub-ambient temperature drop of 6 °C, with a maximum of 14 °C, showing great potential for passive building cooling and human personal cooling. [ABSTRACT FROM AUTHOR]

A micro hydrokinetic turbine is a device for extracting energy from rivers or tides, and this device represents one of the frontiers for renewable energy technology. A detailed analysis of the acoustic characteristics of these turbines is of great significance for their cleaner production, further development, and use. Based on the Ffowcs Williams and Hawkings integration method, the acoustic characteristics of the horizontal axis of a micro hydrokinetic turbine are numerically studied in this work, with emphasis on the acoustic distribution, the acoustic spectrum in the time and frequency domains, and the acoustic directivity in different regions. It is found that the noise of a horizontal axis micro hydrokinetic turbine mainly occurs at the leading edge and tip of the blade due to the appearance and separation of vortices at these locations. The noise signal is mainly caused by the blade passing frequency, especially the first-order frequency, and with the features of a low frequency and broad band. In the progress of downstream propagation, rapid energy dissipation leads to the gradual decrease of the sound pressure level. Furthermore, the noise radiation of the blade has obvious dipole characteristics in the flow direction. The results presented here will contribute to a fundamental understanding of the acoustic characteristics of a turbine. • High intensity noise sources are near the leading edge and tip of the blade. • Acoustic pressure show obvious periodicity. • Peak value of SPL is mainly located at the first order BPF and its harmonic frequency. • Noise radiation of blade has an obvious dipole characteristic. [ABSTRACT FROM AUTHOR]