This animation shows a 4-6 screw compressor with SRM profiles; the rotor length is 168 mm, wrap angle of main rotor 300°, outer diameter of main rotor 102 mm, of gate rotor 101.2 mm, axis distance is 80 mm. The sizes of the radial gaps are 50 µm between rotors and casing, and 100 µm between both rotors. Axial gap sizes are 100 µm on suction and pressure side. The pressure port is designed for a built-in volume ratio of 2.2.
The meshes for the fluid region around the rotors were pre-generated for each 2° rotation angle of the main rotor with TwinMesh. We used the outerfix approach, i.e. the mesh nodes are fixed on the casing surface and move on the rotor surfaces. We used 222 elements in circumferential direction on the main rotor, 293 on the gate rotor, 15 elements in radial direction, and 150 elements in axial direction. The axial gaps were also meshed in TwinMesh for each rotation angle; TwinMesh 2023 allows to mesh them structured with hexahedrons with conformal nodes to the outer fluid regions; furthermore, the meshes around both rotors and those for the axial gaps can be merged into one single deforming mesh of hexahedrons without any mesh interfaces with approx. 1.4 million nodes. Meshes for the static parts, i.e. suction and pressure side and for the oil injection pipes, were generated with Ansys Meshing and consist of approx. 300,000 nodes (tetrahedrons, pyramids, wedges, and hexahedrons).
For the setup, we use air described as calorically perfect ideal gas, and incompressible oil with density 800 kg/m³. Boundary conditions are 1 bar and 20°C at air inlet and 3 bar at outlet with non-reflecting boundary conditions; the four oil inlets are at 2.5 bar total pressure and 50°C. The main rotor rotates at 6000 rpm so that the theoretical volume flow rate is 86.5 l/s, theoretical mass flow rate 0.103 kg/s. The multi-phase simulation starts from the almost periodic single-phase solution with oil inlets defined as walls; this single-phase solution has a mass flow rate of 0.063 kg/s, i.e. a volumetric efficiency of 61%.
Time step size is 55.5 µs (2° rotation angle at 6000 rpm), 7 coefficient loops are used at each time step with high resolution scheme for advection and first order backward Euler scheme in time. Simulation was running for 1800 time steps, i.e. 10 revolutions of the main rotor, on 32 cores local parallel with Ansys CFX and needed approx. 34 hours of simulation time, i.e. 3.4 hours for one main rotor revolution. Finally, the air mass flow increases to 0.081 kg/s, i.e. 79% volumetric efficiency. 1.3 kg/s oil enters through the oil inlets in average, with a maximum of 0.43 kg/s through the horizontal inlet at main rotor and a minimum of 0.24 kg/s through the vertical inlet at gate rotor. Mechanical power at the rotors was 11.2 kW in single- and multi-phase case.
The animation shows pressure distribution as contour plot on the rotor surfaces and on a cut plane on the left hand side; additionally, velocity at inlets and near the outlet is shown by streamlines in purple with a length of 2.5 ms. The pressure rise in the screw compressor from suction towards pressure side can be seen, and the high pressure peaks when an oil jet hits a rotor tooth. On the right hand side, volume fraction of oil is shown as coloured plot on the rotor surfaces and the cut plane, and additionally the isosurface for a volume fraction of 50% is added in purple.