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.
Numerical analysis of the transient leakage flow in axial clearance of a scroll refrigeration compressor
The transient flow model of a scroll refrigeration compressor which included axial clearance was established in the paper using PUMPLINX based on the computational fluid dynamics and the dynamic grid methodology. According to the actual size in the physical compressor, the flank and the axial clearances were set equal to 0.030 mm and 0.015 mm, respectively. As the fluid domain of the axial clearance was too small to generate enough layers of grids in PUMPLINX, it was first magnified 100 times to generate grids of high quality, and then the fluid domain and grids were recompressed to the original size. The simulation results were verified experimentally. The influence of the radial leakage on the downstream flow field was investigated. Moreover, the characteristics of radial and tangential leakage flows were analyzed. It was concluded that the predictions of the compressor performance, except the discharge temperature, are improved after the axial clearance was included in the model. The radial leakage is dominant while a portion of involute leakage flow exists near the flank clearance. The influence of radial leakage on the downstream temperature is lower than that of the tangential leakage in flank clearance. The tangential leakage flow is turbulent basically while the radial leakage flow is laminar. The tangential leakage is severer at the beginning of the discharge process, and the radial leakage is more remarkable at the end of the compression process. When the pressure ratio across the leakage path is higher than the critical pressure ratio, the tangential leakage flow becomes supersonic for the converging–diverging clearance structure, while the radial leakage flow remains subsonic due to the larger flow losses. The results obtained through this research could be applied to modify the current leakage models and optimize the clearances of scroll compressors.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.