Transient flow field and performance analysis of a claw pump for FCVs

The claw-type hydrogen pump has been applied in fuel cell vehicles (FCVs) because of its compact structure, high reliability, and oil-free quality. In this study, a three-dimensional transient computational fluid dynamics (CFD) modelling of a claw-type hydrogen pump used in FCVs was established. Hexahedral structured grids were generated and updated at an increment of 3° in rotating angle to ensure the mesh quality of the whole solving process. The leakage of radial clearance (RC) and axial clearance (AC) was considered. The presented modelling and simulation methods were validated by operating a claw pump at different pressure ratios. The pressure and velocity vector fields in both AC and middle plane, along with the mechanism of the fluid field distribution were analyzed in detail. The in-depth relationship amongst the fluctuation of discharge pressure, outlet mass flow rate and discharge area during the whole working process was revealed. P-θ and V-θ diagram of the whole operating cycle were analyzed. The influence of AC and RCs respectively on the volumetric efficiency of a claw pump was compared and evaluated. It is concluded that back flow in suction pipe happened near 360° as part of the discharge chamber was cut off from the exhaust port and high pressure gas from carryover flowed back into the inlet pipe. The pressure increase during the displacement process, theoretically zero, is actually significant and even comparable to the pressure increase during the compression and discharge process. In addition, volumetric efficiency is most sensitive to axial clearance, followed by radial clearance between rotor and casing, while radial clearance between the rotors has the least influence.