CFD Simulation of a Dry Scroll Vacuum Pump Including Leakage Flows
One challenge for the numerical simulation of a dry scroll vacuum pump is the discretization of the chamber volume, which changes with time. In addition, the flow characteristics are very complex including the leakage flow caused by radial and axial gaps between the rotors and the housing. For engineers and designers it is imperative to understand the influence of such leakage flows on the efficiency of the dry scroll vacuum pump. This paper describes the workflow for Computational Fluid Dynamics (CFD) of a dry scroll vacuum pump and shows some preliminary results. The computational grids for the time dependent flow volume are generated by the grid generator TwinMesh. The meshing software generates and optimises all necessary grids for each time step prior to the actual simulation and calculates the mesh quality to assure high quality numerical results. Furthermore, the grid generation accurately considers axial gaps. The transient numerical simulations are performed by the commercial CFD software code ANSYS CFX, which is able to handle complex flow characteristics. The simulations consider compressibility, turbulence, and heat transfer effects using ideal gas properties for the fluid. The pressure ratio between inlet and outlet is varied as well as the gap size. The results are compared with analytical methods. This paper shows time dependent information such as pressure, velocity, and temperature for specific locations in the chamber volume and also integral values such as power, torque, and mass flow. Finally, cross-sectional views are presented for different positions and time steps.
Three-dimensional CFD Simulation of a Roots Blower for the Hydrogen Circulating Pump
Fuel cell vehicle (FCV) is more advantageous compared to a gasoline-powered internal combustion engine based vehicle and is a new and popular way to solve the environmental problem. Hydrogen circulating pump is an essential assembly unit for the fuel cell, which is used in FCV. Roots blower is employed in this field. Hydrogen has a low molecular weight, which means it is easy to leak through the clearances and hard to preserve. It increases the difficulty of design and operation. In this paper, the performance parameters of a three-lobe roots blower were studied. A multi-scale model was used since the clearance, compared with the rotor diameter, is quite small and easy to induce gas leakage. The long pipes and the throttle values were included in this model. To improve the accuracy and the mesh quality, the working chambers and clearances were represented by highly deforming dynamic structured grids and the grids of the clearances had a high density. The suction and discharge ports were meshed by static unstructured grids. The interfaces were used between the two rotors and between the rotors and the suction and discharge ports. The performances were compared at three different rotational speeds of 4500 rev/min, 6000 rev/min and 8000 rev/min using hydrogen as the working fluid in this numerical model with almost the same inlet and outlet pressures. Meanwhile, the effect of the leakage was also analyzed using two different clearances, which had an important influence for the designing and using of the roots blower.
Experimental verification of theory for the pumping mechanism of a dry-scroll vacuum pump
Abstract The ultimate pressure predicted by the theory that the authors have proposed agrees closely with the values from an actual machine at the rated speed, but shows smaller values than the actual machine for lower orbiting speeds as compared to the rated speed. In order to clarify this contradiction and examine the validity of the theory, the following three experiments were performed: (1) separation of the direct effect of orbiting speed on the ultimate pressure from the effect of clearance change with temperature change accompanying a change in orbiting speed, (2) measurement of the pressure inside the pump and (3) measurement of the pumping speed throughout the working pressure range. The validity of the theory was confirmed by a comparison of the theoretical predictions with the measurements.