Increase Of Spring Stiffness Research Articles

Dynamic characteristics analysis and valve core optimization for second stage hydrogen pressure reducer of hydrogen decompression valve

As an important component of pressure management system, hydrogen decompression valve (HDV) is used in hydrogen fuel cell vehicles, hydrogen refueling stations, etc. The high-pressure difference causes severe pressure fluctuation. Thus, the HDV is composed of multi-stage pressure reducers. Specially, the outlet pressure from the HDV highly affects the pressure of fuel cell. The service life and operation condition of fuel cell is affected significantly by the HDV. This paper conducted the research of the second stage hydrogen pressure reducer (SHPR) of on-boarding hydrogen decompression valve. The force and dynamic characteristics of the valve core are obtained and the flow characteristic is analyzed through the steady and transient flow model. The results are shown that the fluid force and combined force on the valve core exhibit linear increase with inlet pressure and throttling area. The flow characteristics in the throttling gap exhibit similar distribution law under different throttling lengths and inlet pressures. The pressure-drop at entrance and exit of the throttling gap is drastic. Besides, the spring stiffness has a significant impact on the dynamic characteristics of the valve core. With the increase of spring stiffness, the dynamic characteristics change from unstable-state to stable-state under the sudden increase of inlet pressure. The larger spring stiffness initially causes displacement fluctuation of the valve core and then the valve core maintains a stable opening under the stable state. Finally, the optimized structure of valve core is proposed and the better stability of SHPR has been achieved. Meanwhile, there is no significant difference in flow performance but the dynamic characteristics are improved.

Flutter Suppression Effects of Movable Vertical Stabilizers on Suspension Bridges With Steel Box Girders

As the combination of springs and vertical stabilizers, the movable downward vertical central stabilizer (MDVCS) is proposed to further control the nonlinear flutter of super long‐span suspension bridges in this paper. A series of flutter suppression tests of closed‐box girders and twin‐box steel girders with various MDVCSs are conducted. Based on the coupled flutter theoretical method, the sensitivity analysis of two important parameters including the height and stiffness of MDVCS are carried out to compare their nonlinear flutter control mechanism. The results show that the flutter critical wind speed (Ucr) of the closed‐box girder continued to increase with the decrease of the height of the DVCS and the increase of spring stiffness, whereas the Ucr of the twin‐box girder increased at first and then decreased. The cubic polynomial function and quadratic Holliday function are suitable to modify the correction coefficients of Ucr for the closed‐box girder with various stiffnesses and heights of MDVCS, while the Lorentz peak‐value function and cubic polynomial function are suitable to modify the Ucr of the twin‐box girder. Furthermore, the MDVCS significantly changes the rules of two positive and negative aerodynamic damping ratios for the closed‐box girder and two negative aerodynamic damping ratios for the twin‐box girder. Besides, the peak vertical displacement amplitudes of the box girder are about half of the MDVCS, since both the height and stiffness of MDVCS alter the elliptical radius of vertical phase planes to affect the limit cycle oscillation of soft flutter, especially for the leeward MDVCS for the twin‐box girder.

Investigation of frequency-up conversion effect on the performance improvement of stack-based piezoelectric generators

This paper originally investigates the influence of frequency-up conversion effect on piezoelectric stack generators for high-performance energy harvesting. A compressive-mode piezoelectric generator is proposed comprised of a piezostack and a spring-mass system for electromechanical transduction and mechanical excitation, respectively. The frequency-up conversion effect is induced by amplitude truncations of the mass under external excitation. A control group, i.e. without truncation, is set for performance comparisons. Experimental results indicate that, with the frequency-up conversion, instantaneous power and average power are enhanced by over 1000 and 177 times, respectively. Each voltage response cycle consists of two stages: high-frequency high-amplitude oscillation and the rest of low-frequency low-amplitude harmonic vibration. The frequency of the former stage determines impedance matching. Due to the frequency converted up by 69 times, and intrinsic high capacitance of the stack, the matched resistance is significantly reduced from over 5 kΩ to 73.10 Ω, resulting much higher power response than that without conversion. Additionally, increase of spring stiffness causes the decrease of voltage responses and less effectiveness of frequency conversion effect. In this work we conclusively report that an instantaneous peak power output of 0.32 W can be generated from a millimeter-size piezoelectric generator, which can foster development of high-viability self-powered applications.