Approaching Reality: CFD Simulations of Positive Displacement Compressors With More Details in Geometry and Physics

Abstract

The numerical simulation of positive displacement compressors (or expanders), like screw or scroll compressors, is more complex compared to dynamic pumps, turbines or fans. The gas transport in size-changing chambers with very small clearances between the rotors, and between rotors and casing, demands complex meshes that change with each time step. Furthermore, the physical effects are complex, too: high-speed leakage flows occur, heat is generated by compression and friction and heats up rotors and casing, causing thermal deformation that changes clearance sizes, and often liquid phases due to oil injection (as lubricant, coolant and sealant) or condensation (in steam expanders or refrigerant compressors) are important. Most CFD simulations so far focus on gas transport and compression in non-deforming geometries only. We present simulation strategy and results with the commercial programs TwinMesh (for mesh generation and pre-processing) and ANSYS CFD (for CFD and CHT simulations) that allow the accurate prediction of compressor performance on fine structured meshes including multi-phases, CHT, and thermal and structural deformations in reasonable times. The results allow the time-resolved calculation of global quantities like forces, torque, power consumption, and local quantities like pressure, velocity, temperature, phase distribution, or heat fluxes. Losses due to throttling effects or leakages can be detected and minimized, effects of changes in rotor profile, inlet and outlet ports, or injection strategies can be visualized and compared. Furthermore, the interaction of surrounding system components like pipes, valves, vessels with the compressor can be examined through co-simulations with the 1D CFD solver Flownex. The presentation gives an overview on pre-processing and solution steps and shows examples for several compressor simulations: For a two-sided screw compressor, pressure and thermal loads on the rotors are computed from CFD and used for deformation calculation; a two-stage compressor in a full vapour-compression refrigeration cycle with R22 in gaseous and liquid state is shown; and a 3D screw compressor is simulated in a 1D CFD network of a pressurized air supply station to describe storage vessel loading.