This conference paper was submitted for presentation at the NAFEMS World Congress 2025, held in Salzburg, Austria from May 19–22, 2025.

Abstract

Positive displacement machines may be used as compressors (e.g. scroll, roots, screw, vane) or pumps (e.g. lobe, internal and external gear, screw, eccentric screw, gerotor). A typical positive displacement machine has a minimum of two rotors enclosed within a solid casing. The geometry of the rotor sections is a specialized gear profile with lobes of the first rotor interlocking into the flutes of the second rotor. To displace a fluid the rotors are rotated, which cause pockets of the fluid to travel around the outside of the rotors, so that the fluid is transferred from the inlet end to the discharge end of the machine. As the rotors in the machine are moving dynamically, often at high RPM, it is essential that these chambers are separated by small gaps (radial, axial, and interlobe gaps). Clearly, the shape and the size of the chambers and the gaps play a crucial role in the performance of the machine. Poor rotor geometry design with excessively large gaps causes an inefficient machine and unnecessary leakage, gaps that are too small risk contact damage and ultimately machine failure. Therefore, efficient positive displacement rotor design is important. The design of the rotor tooth shapes is a complex process. This process is constantly evolving with new design possibilities such as variable helix twist offering a step change in rotor efficiency. Rotor Design Solutions Ltd (RDS) offers a software package that designs the rotor profiles for multiple positive displacement machine types. The main design criterion for the rotor tooth profile is leak minimization rather than power transmission associated with standard gear design. The RDS software allows a rotor tooth to be built up in small sections of geometrical functions such as radii, parabolas, hyperbolas, trochoids and ellipses. These sections connect together to produce a continuous smooth rotor geometry. This geometry may then be analysed to measure and compare the size of the leakage paths so an optimized shape may be developed. Finally, precise clearance gaps may be added to the rotor profile to avoid unwanted contact. However, manufacturing and experimentally testing these new rotor geometries is time-consuming and expensive, and the optimum clearances required for efficient and reliable operation are difficult to predict. Therefore, 3D CFD simulation is often used to calculate the performance of new rotor designs and examine the flow behaviour, the rotor dynamic deformation and the thermal deformation in detail. This allows the clearance pattern between the rotors to be varied and tested with respect to build-up of pressure, leakage paths, and overall machine efficiency. This presentation shows how the rotor profiles are designed in RDS software, exported to TwinMesh which creates the meshes and simulation setup, and how the CFD simulation with Siemens Simcenter STAR-CCM+ is used for the design evaluation. The overall process is simplified by an automated data exchange between RDS software and TwinMesh, and by an automated pre-processing and post-processing for the CFD solver delivered by TwinMesh. This semi-automated process confirms the best design by comparing the CFD simulations. The manufacture of these CFD-proven rotors may then be progressed to experimental validation with confidence minimizing development cost and time.