High-pressure hydrogen at a maximum of 100 MPa in a 1-L-volume vessel is discharged through 0.1-mm- and 0.2-mm-diameter orifices that imitate cracks, and the transient temperature and pressure behavior of the hydrogen in the vessel is presented. The hydrogen at the initial pressure of 100 MPa during its discharge through the ϕ 0.2-mm orifice reaches half of the initial pressure after 16 s, while it takes approximately nine times longer for the ϕ 0.1-mm orifice to reach half of the initial pressure. We theoretically calculate the transient temperature and pressure according to the fundamental equations based on the mass and energy conservations using an accurate equation of state for hydrogen. The actual flow rate through an orifice is generally smaller than the theoretically calculated flow rate because of the contraction flow. Therefore, in this study, we adopt the effective diameters of the orifices instead of the actual diameters, and from comparisons with the experimental pressure, we estimate them to be 0.6 and 0.9 of the actual diameters for the ϕ 0.1-mm and ϕ 0.2-mm orifices, respectively. We use a functional form with a time constant for the heat transfer coefficient to represent the transient temperature behavior, and we describe the time dependence of the heat transfer coefficient. The results show that the calculated temperature and pressure are in good agreement with the experimental values obtained.
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