CFX Berlin-Video: CFD simulation of a 5-stage roots vacuum pump скачать в хорошем качестве

CFX Berlin-Video: CFD simulation of a 5-stage roots vacuum pump

The animation shows simulation results for a vacuum pump consisting of 5 roots pumps connected in series. Nitrogen is compressed from 50 Pa at vacuum side (left top) to 1 bar at pressure side (lower right). Pressure is shown at surface of the rotors and on the walls of the connecting channels; colour scale is logarithmic. All roots blowers are identically shaped and on two common shafts, but different in axial extensions. Rotational speed is 5400 rpm. Solid components as the rotors and the casing are not included in the simulation; here, the flow was assumed isothermal at 20°C to model ideal cooling. Radial gaps between rotors and casing (45 µm) and between both rotors (55 µm) are included, axial gaps and gaps between shaft and casing are neglected here. The meshes for the rotor chambers were created with TwinMesh; each rotor chamber consists of 21,480 to 88,605 hexahedrons (15 in radial, 179 in circumferential, 8 to 33 in axial direction, depending on axial length) , in total 515,520 hexahedrons for all rotor chambers. For each 2° rotational increment, the meshes were pre-generated with the outerfix approach, exported, and are read into the CFD solver at run-time to define the mesh deformation. The meshes for inlet region, connecting regions and outlet region were created with Ansys Meshing and consist of 1,259,137 elements in total, mainly tetrahedra (745,274) and prisms (510,752). The CFD simulation is done with Ansys CFX for nitrogen modelled as real gas via Peng-Robinson equation of state. Ansys CFX solves the equations for conservation of mass and momentum (Navier-Stokes equations), and two additional transport equations for Reynolds-averaged turbulence effects (shear stress transport / SST model). All walls are set to no slip (with respect to their motion), energy model is isothermal at 20°C. Time step size is 61.7 µs, corresponding to 2° rotation angle for the roots pumps at 5400 rpm. The animation shows 720 time steps, i.e. 4 revolutions of the roots pumps. The (almost) periodic state is shown, not the evolution from the initialization towards this periodic state. Pressure is shown as contour plot on the surfaces of the rotors and on the walls of the connecting channels and inlet and outlet pipe. Averaged pressure at suction side (inlet) is also given in the animation. The magenta vectors show velocity on inlet and outlet plane. For 50 Pa vacuum pressure, the volume flow rate of the pump is strongly decreased compared to the theoretical volume rate derived from chamber volume and rotational speed. The leakage flow through the radial gaps almost equals the nitrogen transport due to the rotationg chambers, i.e. a good resolution of the leakage flows is important. Axial gaps between rotors and casing and the radial gaps between the shafts and the casing can be included in the model to better capture leakage flows. The simulation can be further improved by adding the solid components and use an energy transport model with conjugate heat transfer to account for the gas heating (due to compression and visous heating) and cooling (due to cooling cycle). And finally, the application of a continuum solver as Ansys CFX based on Navier-Stokes equations is critical in low pressure regions. At 50 Pa, free mean path length of nitrogen is in the range of 0.1 mm. For continuum approach, Knudsen number (ratio of free mean path length and characteristic length, e.g. gap height) should be below 0.01, i.e. characteristic lengths should be above 10 mm here. This is clearly not the case for the radial gaps, so here at least a slip model (valid for Knudsen number less than 0.1) should be included, otherwise special solvers for molecular flows are necessary with dynamic coupling to continuum solvers for the parts at higher pressures. For more information go to https://www.twinmesh.com or contact us via e-mail: info@twinmesh.com