Numerical investigation of ejector transient characteristics for a 130-kW PEMFC system

Improvement of fuel utilization is an important issue for proton exchange membrane fuel cell (PEMFC) system. As a promising anode recirculation method, ejector has attracted great attention because it does not require additional power consumption. However, some transient processes such as the suck, diffusion, and mix of fluids are still not thoroughly revealed, which significantly influence ejector performances. In this study, a dynamic three-dimensional (3D) multicomponent ejector model for a 130-kW PEMFC system is developed. The model is validated against experimental data, including the entrainment ratio and mass flow rates. The effects of operating conditions (eg, pressure, water vapor, and nitrogen mass fraction) are investigated. The results show that the fuel supply can be controlled by the primary flow pressure. When the pressure difference between the primary and secondary flow is less than 10 kPa, the secondary flow cannot be sucked into the ejector. The transient response of ejector during stack power variations can be classified into two periods: the primary flow impact period and the mixed flow impact period. Under normal fuel cell system operating conditions, when the inlet relative humidity of the secondary flow is higher than 85%, the water vapor condensation is possible to happen at the ejector outlet region, leading to fuel supply instability. Besides, the hydrogen entrainment ratio decreases with the increase of nitrogen mass fraction. The effects of geometric parameters (eg, nozzle convergence angle, secondary flow tube diameter, mixing tube length, and diffuser angle) on ejector performances are also studied. It is found that the relatively short tube leads to pressure fluctuations in the vacuum region. Increasing the tube length is beneficial to creating a stable vacuum region. However, excessive tube length can increase the friction loss. Increasing the secondary flow inlet tube diameter is beneficial to the entrainment ratio. However, further enlarging the diameter contributes negligibly to the increase of entrainment ratio once the secondary flow mass rate depends on pressure.