Effects of hydraulic oil viscosity on the operational performance of ultra-high-pressure hydrogen diaphragm compressors

Diaphragm compressors are progressing toward higher operational pressures. The viscosity of hydraulic oil increases drastically under ultra-high-pressure (UHP) conditions. It's necessary to investigate the impact of hydraulic oil viscosity on the hydraulic oil flow characteristics and the operational performance of diaphragm compressors. This study establishes a three-dimensional (3D) model of the hydraulic oil flow domain and conducts computational fluid dynamics (CFD) simulations. Results show that with increasing oil discharge pressure, the pressure distribution across the flow field becomes more non-uniform, and the oil discharge time is prolonged. Furthermore, a zero-dimensional (0D) fluid-structure interaction (FSI) model is proposed for thermodynamic calculations of the UHP diaphragm compressor, with its accuracy confirmed through experimental validation. The impacts of viscosity variations on the operational performance of the compressor are analyzed. Under UHP conditions, the increased viscosity of hydraulic oil leads to prolonged oil discharge time and shortened exhaust time in diaphragm compressors, which consequently reduces volumetric efficiency. Furthermore, this results in an elevation of the required flywheel torque and heightened stress concentration on the diaphragm. When the oil filling temperature is reduced from 50 °C to 10 °C, the aforementioned trends are further exacerbated, the volumetric efficiency falls from 65.12 % to 39.48 %, the flywheel moment rises by 25.84 %, and the radial stress at the diaphragm's suction orifice increases by 43.71 MPa. This study can provide references for fast simulation calculations and the control of hydraulic oil temperature of UHP diaphragm compressors.