Screw, Expander, Displacement, Machine, Simulation, TwinMesh
The numerical simulation of rotary positive displacement (PD) machines has proven to be a very challenging task characterized by complex geometries with a time dependent change of the chamber volume along with complex flow physics. In addition, the leakage flow caused by housing clearances, front gaps and the intermesh clearance is of particular interest. The resulting losses have a severe impact on the overall machine efficiency. This paper shows workflow and results for Computational Fluid Dynamics (CFD) simulations of the Screw Expander SE-51 of TU Dortmund University. The simulation is performed using ANSYS CFX, where the computational grids for the rotary parts are derived from the grid generator TwinMesh. The SE-51 is a gearless, dry running twin screw expander that converts thermal into mechanical energy by decompressing air with pressure-ratios up to 6:1 by a rotational speed range up to 20,000 rev/min. The volume ratio of the expander is 2.5 at a shaft distance of 51 mm with a displaced volume per male rotor revolution of 285 cmÂ³. The Numerical model of this machine includes fully resolved clearances along with the meshing of the rotor solid end faces whereas a contact point between the rotors is neglected. For several rotational speeds, CFD results are compared to measurements at 4 bar at the high pressure side, expanding air down to 1 bar. The validation is based on integral values, e.g. mass flow, torque and power, all averaged over a certain time period for several operating points. Moreover, time- and space-resolved pressure measurements for distinct positions are compared to simulation results. Besides direct evaluation against experimental data, tendencies are analyzed by varying the numerical model with respect to mesh resolution or changing the size of clearances. Apart from local changes in the flow field, it is shown that also global values like torque or mass flow are clearly affected. In addition, flow regimes not captured by measurements, like leakage flows within clearances between rotors and housing, are evaluated and illustrated based on CFD results. While the simulation allows reliable analysis of dry running screw machines and their complex flow characteristics, oil- or water-injected machines are also commonly used in the industry, allowing higher pressure-ratios. The liquid fluid acts as a sealing for clearances and can also be used as lubricant. From the simulation point of view, multiphase calculations with corresponding multiphase-effects are required in order to cope with injected screw machines. Further investigation also involves heat transfer and resulting deformations of rotary and stationary parts. Clearances can vary due to deformations, eventually resulting in different machine characteristics and efficiency. Thus, fluid-structure interaction is of high demand. For both of these issues, a prospect is given to extend the scope of application, using CFD for screw machine analysis.