## 3 Result and Discussions
### Analysis of Pressure Coefficient (\(C_{p}\)) and Velocity Results in L-Shaped Bend of Check Valve  
In order to analyze the flow characteristics at a localized location of the L-shaped curved tube inside the check valve, the velocity_u results and the pressure coefficient (C\({}_{p}\)  
Figure 4: Pressure and velocity results of check valve. (**a**) Pressure-drop result of check valve at flow-in, core and discharging part; (**b**) Pressure and velocity results at x-axis measurement positions. results in the Y-axis direction were analyzed at X-axis line measurement positions at 1 mm intervals starting from the X = 90.7 mm point, as shown in Figure 1b. Figure 5 shows a graph of Velocity_u results as a function of line measurement position in an L-shaped curved tube for all differential pressure (\(\Delta\)P) and inlet temperature (T\({}_{\rm in}\)) conditions. In the L-shaped curved tube area of the core part, when the flow changes from the (\(-\))Y-axis to the X-axis, the flow-separation phenomenon occurs due to wall viscosity. At this time, due to the reverse pressure gradient, a reverse flow is formed within the flow stagnation point and flow-separation zone, and the flow velocity is very low and the reverse flow direction is produced. At a differential pressure (\(\Delta\)P) of 10 MPa and a hydrogen-inlet temperature (T\({}_{\rm in}\)) of 363 K, the maximum flow velocity of 303 m/s is obtained at the X = 90.7 mm point, but the flow velocity decreases to about 60% at the top of the valve at Y = 2.3 mm. This is considered to be due to the effect of flow separation, which is characterized by a reverse pressure gradient in the direction of flow, as the flow velocity decreases and the pressure increases due to wall viscosity, forming a separation point. Also, as the measurement position changes from X = 91.7-95.7 mm, the Velocity_u result at the top of the valve becomes negative, indicating a change in flow direction. The X = 91.7 mm point shows a negative characteristic at Y = 2 mm, while the X = 95.7 mm point is derived as negative at Y = 0.55 mm, indicating that the flow-separation region tends to extend towards the bottom of the valve. In this case, the flow velocity results in the lower region of the valve after the Y = 0 mm point show an increase to a maximum of approximately 550 m/s at the X = 93.7 mm point. This means that wake and reverse flows occur in the flow-separation area, and a recirculation area is formed to deflect the flow towards the bottom of the valve. As the flow-separation area increases, the flow resistance increases, and flow deflection occurs, which reduces the flow area of the main flow stream, which is considered to be the main influencing factor in the occurrence of the vena contract phenomenon and increases the flow velocity [26,27]. In addition, due to the decrease in the area of the main flow path area caused by the occurrence of the vena contract phenomenon, the flow of hydrogen tends to decrease in pressure and increase in flow velocity due to the Venturi effect. At the same time, the area of the flow path towards the rear end of the valve increases, resulting in a decrease in flow velocity and an increase in static pressure, which is believed to be due to the loss of kinetic energy, preventing the total kinetic energy from being completely converted into pressure energy [28].