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Hydrogen has a wide range of energy applications, but hydrogen energy systems can suffer from high-concentration leaks that pose security risks, therefore making the measurement of high-concentration hydrogen very important. Traditional ultrasonic gas-detection methods are based mainly on ultrasonic time-of-flight measurements and can be divided into threshold-detection and phase-difference techniques. Threshold detection suffers from low resolution and a complex structure in gas detection, while the phase-difference technique has high resolution and a simple structure but can only measure the time of flight within one period of the ultrasonic signal. In this study, a dual-frequency phase-difference technique is proposed that solves the problem of multi-period phase detection with the phase-difference technique and can be used to detect high-concentration hydrogen. Simulation analysis and an experiment show that the proposed technique can measure the multi-period phase difference accurately. The maximum hydrogen concentration can reach 50% with an uncertainty of less than 5%, which meets commercial requirements.

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Controlling the microstructure in the final product is significant to metal hot forming at either macroscale or microscale. The microstructure evolution during the hot forming must be fundamentally understood in order to control the microstructure. Elevated temperature contributes to minimize the size-effects of microforming processes and also enlarge the forming limits by microforming. Recrystallization and grain growth during microextrusion at elevated temperature followed by air-cooling were studied using conventional recrystallization theory models and finite element methods. The results showed the microstructure distribution in the extruded billet can be rationally predicted based on the conventional recrystallization theory models. The simulation of the grain size distribution agreed with the experiment. A larger extrusion ratio, a faster extrusion speed and a refiner initial grain all help to obtain a smaller grain size and a more homogeneous grain distribution in the extruded billet.

Methane is the main cause of sewer explosion accidents, and the available monitoring methods mainly use portable gas sensors for on-site detection. However, these methods cannot grasp the dynamic changes in methane in a sewer in real time or make better predictions of methane explosion accidents. Here, we present a low-power online monitoring system for sewer methane in order to ensure the safety of urban sewers. A low-power ultrasonic gas sensor was used to monitor the methane content in biogas. Due to the impacts of changes in CO2, temperature, and humidity in the sewer on the ultrasonic methane measurement results, a corresponding compensation method was proposed. The effectiveness of the compensation method was verified through experiments, which measured the methane concentration under different influences, and the methane values measured before and after compensation were compared. The power consumption was only 1.28 mW if the system worked in intermittent operation mode, and it could operate for about 1.5 years by selecting a lithium battery with a capacity of 5 A · H. The experimental results showed that the gas detection accuracy could meet the commercial requirements and had a high compensation for the impacts of carbon dioxide, temperature, and humidity.