The high-pressure gaseous hydrogen (HPGH2) storage method is widely used owing to the low density of hydrogen gas at ambient temperature and atmospheric pressure. Therefore, rigorous safety analysis of the filling and discharging of compressed gas in a hydrogen tank is required to achieve reliable operational solutions for the safe storage of hydrogen. In this study, the behavior of compressed hydrogen gas in a hydrogen tank was investigated for its discharge. Numerical models for the adaptation of gas and turbulence models were examined. Gas model effects were examined to account for hydrogen gas behavior at the discharge temperature and pressure conditions. Turbulence model effects were analyzed to consider the accuracy of each model: the assessment of the turbulence models was compared in terms of the turbulence intensity. From the study of gas model effect, the Redlich–Kwong equation was found to be one of the realistic gas models of the discharging gas flow. Among the turbulence models, the shear stress transport model and Reynolds stress model predicted the compressed gas behavior more accurately, showing a lower turbulence intensity than those of the realizable and renormalization group models.
Microwave curing technology, which has seen increased commercialization recently due to its ability to cut the curing time and ensure high quality, requires an understanding of the curing characteristics of composite materials of varying thickness. Therefore, this study aimed to perform cure monitoring to evaluate the effects of variations in thickness on the quality of microwave curing. For this study, a fiber Bragg grating sensor was used to measure temperature changes in specimens during the curing cycle for cure monitoring which is generally used for optimization of the curing cycle; then, the time taken for temperature increase and overshoot of the specimen, and the times at which the specimen thickness varied, were quantitatively evaluated. Testing confirmed that microwave curing reduced the curing time in the sections in which the temperature rose; also, the specimen thickness caused overshoot of up to approximately 40 °C at the side, which can affect the curing quality of the composite materials. Furthermore, voids were observed on the side of all specimens. The results indicated that, in order to improve the quality of microwave curing of composite materials, the curing cycle should be optimized by considering the characteristics of the microwave curing equipment.
Due to the low density of hydrogen gas under ambient temperature and atmospheric pressure conditions, the high-pressure gaseous hydrogen storage method is widely employed. With high-pressure characteristics of hydrogen storage, rigorous safety precautions are required, such as filling of compressed gas in a hydrogen tank to achieve reliable operational solutions. Especially for the large-sized tanks (above 150 L), safety operation of hydrogen storage should be considered. In the present study, the compressed hydrogen gas behavior in a large hydrogen tank of 175 L is investigated for its filling. To validate the numerical approach used in this study, numerical models for the adaptation of the gas and turbulence models are examined. Numerical parametric studies on hydrogen filling for the large hydrogen tank of 175 L are conducted to estimate the hydrogen gas behavior in the hydrogen tank under various conditions of state of charge of pressure and ambient temperature. From the parametric studies, the relationship between the initial SOC pressure condition and the maximum temperature rise of hydrogen gas was shown. That is, the maximum temperature rise increases as the ambient temperature decreases, and the rise increases as the SOC decreases.
Selected laser melting (SLM) is a representative process of powder-bed type 3D printing technology that is used to manipulate metals and it generally results in various structural properties according to the process conditions. In this study, a thermal conductivity test was conducted on AlSi10Mg specimens that were manufactured using the SLM process to investigate the influence of various process conditions on the thermophysical characteristics and cellular microstructure of the samples. The building direction of the specimen, laser scan speed, and hatch spacing were considered as process variables, and the thermal conductivity was analyzed for each process variable. In the test results, as the polar angle of the specimen increased from 0° to 90°, the thermal conductivity increased. Furthermore, the thermal conductivity gradually decreased as the scan speed and hatch spacing increased. The differences in thermal characteristics are discussed in connection with the microstructure of the cells constituting the melt pool. The solidified melt pool that formed by the laser beam is composed of cells consisting of an Al matrix and a surrounding Si-rich area. The differences in thermal conductivity of the overall specimens are explained based on the variations in thermal conductivity and cell shape and size for each chemical component.
The majority of catastrophic wheelset failures are caused by surface opening fatigue cracks in either the wheel tread or wheel inner. Since failures in railway wheelsets can cause disasters, regular inspections to check for defects in wheels and axles are mandatory. Currently, ultrasonic testing, acoustic emissions, and the eddy current testing method are regularly used to check railway wheelsets in service. Yet, in many cases, despite surface and subsurface defects of the railroad wheels developing, the defects are not clearly detected by the conventional non-destructive inspection system. In the present study, a new technique was applied to the detection of surface and subsurface defects in railway wheel material. The results indicate that the technique can detect surface and subsurface defects of railway wheel specimens using the distribution of the alternating current (AC) electromagnetic field. In the wheelset cases presented, surface cracks with depths of 0.5 mm could be detected using this method.
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