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This paper presents an empirical study of feature extraction methods for the application of low-speed slew bearing condition monitoring. The aim of the study is to find the proper features that represent the degradation condition of slew bearing rotating at very low speed (≈ 1 r/min) with naturally defect. The literature study of existing research, related to feature extraction methods or algorithms in a wide range of applications such as vibration analysis, time series analysis and bio-medical signal processing, is discussed. Some features are applied in vibration slew bearing data acquired from laboratory tests. The selected features such as impulse factor, margin factor, approximate entropy and largest Lyapunov exponent (LLE) show obvious changes in bearing condition from normal condition to final failure.

The wind energy in cities cannot be exploited effectively because natural wind is unstable and complex. Therefore, a triboelectric-electromagnetic hybrid generator with swing-blade structures (SBS-TEHG) was designed to effectively harvest intermittent and continuous wind energy in an urban environment. First, the spring structure and base were considered to realize the maximum output performance of triboelectric nanogenerators. Then, the computational fluid dynamics method was applied to optimize the structure of the SBS-TEHG to improve its aerodynamic performance. The starting wind speed of the SBS-TEHG was 2 m/s, and its energy conversion efficiency was 9.04%, 159% higher than that of the SBS-TEHG without guide plates at 4 m/s. The results demonstrated that the SBS-TEHG lit 105 light-emitting diodes (LEDs) under the intermittent-wind harvesting mode at a wind frequency of 1 Hz when the single swing blade operated, while a wireless PM 2.5 & PM 10 sensor was powered by the SBS-TEHG after a period of operation under the continuous-wind harvesting mode. The findings of this study provide a novel solution for low-speed wind energy harvesting in cities and demonstrate the potential of SBS-TEHG as a distributed energy source.

HighlightsA novel windmill-like hybrid nanogenerator with contact-separation structure was proposed for harvesting breeze energy at low wind speed.A spring steel sheet was creatively used both as an electrode of triboelectric nanogenerator and a booster for contact-separation activity.A magnetic acting as a bifunctional element supplies magnetic flux variation in electromagnetic generator and overcomes electrostatic adsorption between tribolayers simultaneously.Wind energy is one of the most promising and renewable energy sources; however, owing to the limitations of device structures, collecting low-speed wind energy by triboelectric nanogenerators (TENGs) is still a huge challenge. To solve this problem, an ultra-durable and highly efficient windmill-like hybrid nanogenerator (W-HNG) is developed. Herein, the W-HNG composes coupled TENG and electromagnetic generator (EMG) and adopts a rotational contact-separation mode. This unique design efficiently avoids the wear of friction materials and ensures a prolonged service life. Moreover, the generator group is separated from the wind-driven part, which successfully prevents rotation resistance induced by the friction between rotor and stator in the conventional structures, and realizes low-speed wind energy harvesting. Additionally, the output characteristics of TENG can be complementary to the different performance advantages of EMG to achieve a satisfactory power production. The device is successfully driven when the wind speed is 1.8 m s−1, and the output power of TENG and EMG can achieve 0.95 and 3.7 mW, respectively. After power management, the W-HNG has been successfully applied as a power source for electronic devices. This work provides a simple, reliable, and durable device for improved performance toward large-scale low-speed breeze energy harvesting.

In recent years, the International Maritime Organization (IMO), Europe, and the United States and other countries have set up different emission control areas (ECA) for ship exhaust pollutants to enforce more stringent pollutant emission regulations. In order to meet the current IMO Tier III emission regulations, an after-treatment device must be installed in the exhaust system of the ship power plant to reduce the ship NOx emissions. At present, selective catalytic reduction technology (SCR) is one of the main technical routes to resolve excess NOx emissions of marine diesel engines, and is the only NOx emission reduction technology recognized by the IMO that can be used for various ship engines. Compared with the conventional low-pressure SCR system, the high-pressure SCR system can be applied to low-speed marine diesel engines that burn inferior fuels, but its working conditions are relatively harsh, and it can be susceptible to operational problems such as sulfuric acid corrosion, salt blockage, and switching delay during the actual ship tests and ship applications. Therefore, it is necessary to improve the design method and matching strategy of the high-pressure SCR system to achieve a more efficient and reliable operation. This article summarizes the technical characteristics and application problems of marine diesel engine SCR systems in detail, tracks the development trend of the catalytic reaction mechanism, engine tuning, and control strategy under high sulfur exhaust gas conditions. Results showed that low temperature is an important reason for the formation of ammonium nitrate, ammonium sulfate, and other deposits. Additionally, the formed deposits will directly affect the working performance of the SCR systems. The development of SCR technology for marine low-speed engines should be the compromise solution under the requirements of high sulfur fuel, high thermal efficiency, and low pollution emissions. Under the dual restrictions of high sulfur fuel and low exhaust temperature, the low-speed diesel engine SCR systems will inevitably sacrifice part of the engine economy to obtain higher denitrification efficiency and operational reliability.

In order to study the sequential turbocharging switching process of low-speed diesel engine, this paper uses GT-power software to establish a simulation model of sequential turbocharging low-speed diesel engine. It is enhanced and improved into a model of a sequential supercharged diesel engine. Modules such as load, governor, valve control, and surge margin are set up in the simulation model. Then parameters such as switching speed, valve opening time and response time are set in the model. Through the above preparations, the thesis has carried out the simulation calculation of the sequential supercharging transient switching process and verified the rationality of the setting parameters and the accuracy of the module setting. This provides a certain reference for the valve control strategy of the sequential supercharging switching system of low-speed diesel engines.

In nature, lakes and water channels offer abundant underwater energy sources. However, effectively harnessing these green and sustainable underwater energy sources is challenging due to their low flow velocities. Here, we propose an underwater energy-harvesting system based on a cylindrical bluff body and a cantilever beam composed of a macro fiber composite (MFC), taking advantage of the MFC’s low-frequency, lightweight, and high piezoelectric properties to achieve energy harvesting in low-frequency and low-speed water flows. When a water flow impacts the cylindrical bluff body, it generates vibration-enhanced and low-frequency vortices behind the bluff body. The optimized diameter of the bluff body and the distance between the bluff body and the MFC were determined using finite element analysis software, specifically COMSOL. According to the simulation results, an energy-harvesting system based on an MFC cantilever beam applied in a low-frequency and low-speed water flow was designed and prepared. When the diameter of the bluff body was 25 mm, and the distance between the bluff body and MFC was 10 mm and the maximum output voltage was 22.73 V; the power density could reach 0.55 mW/cm2 after matching the appropriate load. The simulation results and experimental findings of this study provide valuable references for designing and investigating energy-harvesting systems applied in low-frequency and low-speed water flows.

Microelectrode array has significant and diverse applications in contemporary technology. The most effective method for the precise manufacture of micro-electrode array with specific dimensions is low-speed wire electrical discharge machining (LS-WEDM) combined with workpiece rotation. Workpiece rotation results in the transformation of the machined part from a continuum to a thin-walled array of discontinuities. The change in workpiece structure influences the machining performance. This paper investigates the impact of workpiece thickness and the thin-walled array of workpieces on the discharge gap and burr of LS-WEDM for different materials (copper, copper-tungsten alloy, and pure tungsten). The results show that the discharge gap of thin-walled array depends on the distance between the top and bottom of the workpiece and the number of arrays, while the burr width of thin-walled array depends on the thickness of the individual thin-walled units. In addition, a formula for the discharge gap of a thin-walled array workpiece is given. Based on this, microelectrode array with a side length of 300 μm and a size error of less than 3 μm are prepared on three materials by compensating the machining gaps corresponding to each position, and the surface quality of microelectrode array is evaluated. This study has crucial implications for improving the dimensional accuracy of microelectrode array produced by LS-WEDM.

Objective To study the early phases of osseointegration at implants installed in sites prepared with either high rotational speed with irrigation or low rotational speed without irrigation. Material and methods After 3 months from tooth extraction, two implants were installed in one side of the mandible of twelve dogs. The osteotomies were prepared either at 60 rpm without irrigation or at 750 rpm with refrigeration. Biopsies were obtained after 4 and 8 weeks of healing, six animals each period for histological analyses. Results After 4 weeks of healing, new bone percentage in contact with the implant surface (BIC%) was 46.6 ± 7.3% and 43.1 ± 6.8% at the low- and high-speed sites, respectively ( p  = 0.345). After 8 weeks of healing, the fractions increased to 60.0 ± 11.1% and 60.2 ± 6.2%, respectively ( p  = 0.753). Conclusions Implants installed in sites prepared using either low-rotational drilling without irrigation or high speed with irrigation presented similar amounts of osseointegration.

Low-speed hoist bearings are characterized by fault features that are weak and difficult to extract. Multipoint optimal minimum entropy deconvolution adjusted (MOMEDA) is an effective method for extracting periodic pulses in a signal. However, the decomposition effect of MOMEDA largely depends on the selected pulse period and filter length. To address these drawbacks of MOMEDA and accurately extract features from the vibration signal of a hoist bearing, an adaptive feature extraction method is proposed based on iterative autocorrelation (IAC) and MOMEDA. To automatically identify the pulse period, a new evaluation index named autocorrelation kurtosis entropy (AKE) was constructed to select the optimal IAC. To eliminate the influence of the filter length on the decomposition effect, an iterative MOMEDA strategy was designed to gradually enhance signal impulse features. The Case Western Reserve University bearing dataset and bearing data from a self-made hoisting test setup were used to verify the effectiveness of IAC-MOMEDA in extracting weak features. Moreover, the capability of IAC-MOMEDA for features extraction of normal bearing vibration signal was further confirmed by field test data.

Resin matrix composites are commonly utilized in water-lubricated stern tube bearings for warship propulsion systems. Low-speed and high-load conditions are significant factors influencing the tribological properties of stern tube bearings. The wear characteristics of resin-based laminated composites (RLCs), resin-based winding composites (RWCs), and resin-based homogeneous polymer (RHP) blocks were investigated under simulated environmental conditions using a ring-on-block wear tester. Simulated seawater was prepared by combining sodium chloride with distilled water. The wetting angle, coefficient of friction (COF), and mass loss were measured and compared. Additionally, their surface morphologies were examined. The results indicate a significant increase in the COFs for the three materials with an increased speed or load under dry conditions. The COF of the RLCs is the lowest, indicating that it has superior self-lubricating properties. In wet conditions, the COFs of the three materials decrease with an increasing speed or load, exhibiting a pronounced hydrodynamic effect. The COF and mass loss of RWCs are the highest, while RLCs and RHP exhibit lower COFs and mass loss values. The reticulated texture and flocculent fibers on the surface of RLC enhance the heat diffusion and improve the material wettability and water storage capacity. The surface of RWC is dense, and the friction area under dry conditions is melted and brightened. The surface of RHP is smooth, while the worn material forms an agglomerate and exhibits susceptibility to burning and blackening under dry conditions. The laminated formation method demonstrates superior tribological performance throughout the wear evolution process.