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• 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]

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]

Currently, energy conversion methods in nanochannels have been developed for both fundamental sciences and technological applications. Graphene oxide membranes with excellent properties possess potential applications in numerous fields. Due to the confinement of nanocapillaries within graphene oxide membranes and the mobility difference of ions, an electricity generation process has been achieved using graphene oxide membranes. In this study, for the first time, the influence of source concentration, applied pressure and membrane thickness was investigated on electricity generation in aqueous environment. The results revealed that these three factors could successfully control the electricity generation with graphene oxide membranes. Electrical potential difference would become larger with increasing the source concentration and membrane thickness. In contrast, the applied pressure was not well-suited to capture energy from sources. This provided new insights into the design and utilization of graphene oxide films for highly efficient energy harvesting. [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]

The analysis of time-series concentrations (C TS) is of great importance when investigating the biogeochemical processes of nutrients in aquatic environments. However, obtaining C TS of nutrients remains a challenge using current sampling techniques. In this study, a novel in situ sampler was constructed using reverse osmosis membrane (ROM) osmotic pumps (OP) (ROM-OP sampler), and was used to obtain the C TS of nutrients in surface waters. The sampler consisted of a sampling OP, sample storing coil, filter, bubble injection module, and preservative adding module. When deployed, the sampling OP continuously draws ambient water through the filter into the sample storing coil, while simultaneously the preservative adding module continuously delivers preservative (H 2 SO 4 solution) into the water flow. The bubble injection module periodically injects air bubbles into the sample storing coil, to segment the sample and create time stamp indicators that allow the sample age to be defined. Upon retrieval, the sample segments in the coil are sequentially pumped out of the sample storing coil and transferred into different vials for further analysis. The sampler was applied to measure the C TS of various nutrients, including dissolved total nitrogen, dissolved total phosphorus, dissolved reactive phosphorus, and nitrate in a river over a 20 day period and in municipal sewage treatment plant effluent for a 36 h period. Results showed that the ROM-OP sampler successfully obtained C TS of nutrients, capturing nutrient variations at a high temporal resolution. This sampler is relatively low-cost (~USD 300), small in size, lightweight, robust and does not require an external power source, showing high promise as an effective and efficient tool for monitoring nutrient C TS in aquatic environments. Unlabelled Image • Time-series sampler developed using osmotic pumps with reverse osmosis membrane. • Air bubbles were used to segment the water sample, with H 2 SO 4 as a preservative. • Obtained a high temporal resolution time-series of nutrient concentrations. • Excellent agreement was achieved between sampler-collected samples and spot samples. • The sampler is cheap, small size, robust and requires no external power source. [ABSTRACT FROM AUTHOR]

• A new stable and cost-efficient solar absorber for solar-based applications. • Reveals the influencing factors of photothermal conversion efficiency of nanofluid. • The highest relative efficiency reaches 91.6% at 3 suns. For high-temperatures surfaces, surface absorbers possess a higher evaporation rate while volumetric receivers have a higher solar vapor generation efficiency by localizing high temperatures to the interior of the receiver and thus reduce surface losses. In this work, the solar vapor generation rates and efficiencies of several direct absorption solar collectors were compared by using reduced graphene oxide/graphene oxide and silver nanofluids as evaporators, two of which deserve to be noticed. One is the hybrid nanofluids containing reduced graphene oxide decorated with silver nanoparticles in volumetric solar absorbers and the other one is hybrid nanofluids containing reduced graphene oxide with silver floating on the surface, combining surface absorbers and volumetric ones. The results show that relative efficiency monotonously increases with the light density (e.g. correspondingly changes within 61.5% at 1 sun, 62% at 2 suns, 64% at 3 suns and 69% at 4 suns for 0.45 mg/ml reduced graphene oxide) and nanofluid concentration. The further experiments reveal that nanofluid-assisted vapor generation efficiencies of hybrid nanofluids containing reduced graphene oxide with silver floating on the surface were higher, reaching 91.6% at 3 suns, than the others due to its higher absorbance and plasmonic effect of the nanoparticles and high thermal conductivity of graphene nanosheets. This demonstration of hybrid nanofluids containing reduced graphene oxide with silver floating on the surface holds the promise of significantly expanding the potential applications in desalination, water treatment and power generation. [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]