CFD simulation of oil injection in screw compressor with free surface model

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 100 µm between rotors and casing, and 200 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 two oil inlets are located at the sides of the rotors and 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 an initialization with 1 bar, zero velocity, 20°C and gas everywhere.
For the multi-phase simulation, we use a homogeneous model, i.e. gas and oil phase share a common velocity and turbulence (SST model is used) field; two separate energy equations are solved, but coupled by transfer terms to ensure a common temperature field. Phase distribution is solved via the volume-of-fluid method; the free surface model with interface compression level 2 allows the separation of mixed phases by a gradient sharpening.
Time step size is 55.5 µs (2° rotation angle at 6000 rpm), 5 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 4 cores local parallel with Ansys CFX and needed approx. 6 days of simulation time, i.e. 7.2 hours for one main rotor revolution. Finally, the average air mass flow is 0.076 kg/s, i.e. 74% volumetric efficiency. 0.82 kg/s oil enter through both oil inlets in average. Mechanical power at the rotors was 10.9 kW.
The animation shows pressure distribution as contour plot on the rotor and stator surfaces on the left hand side. 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, regions on rotor and stator surfaces for volume fraction of oil greater than 50% and the isosurface for a volume fraction of 50% are shown in purple.