In an internally geared positive displacement machine, an externally lobed inner rotor rotates inside an internally lobed outer rotor. The outer rotor has one lobe more than the inner one, the rotation axes are displaced. For these simulations, the rotor contours were constructed as combinations of epicycloids and hypocycloids with a normalized rotor profile parameter “lambda”, being zero for pure hypocycloids and 1 for pure epicycloids [M. Read, I. Smith, N. Stosic, Port flow losses in internally geared positive displacement machines, 24th International Compressor Engineering Conference, Purdue 2018]. The rotor profiles were generated with a Perl script as a csv file, meshing for straight chamber volumes and axial gaps was done with TwinMesh. Inlet (right side) and outlet port (left side) were simply constructed and meshed with Ansys ICEM CFD and the same for all cases, inlet and outlet are at 1 bar, fluid is air as ideal gas, the transient CFD simulations at 200 rpm were done with Ansys CFX.

The animation shows pressure contours for cases with three lobes on inner rotor and “lambda=0.2” (top left), “lambda=0.5” (top right), and “lambda=0.8” (bottom left), and for a case with four lobes on inner rotor and “lambda=0.5” (bottom right).