High-resolution simulations of gas-liquid Couette-type sealing gap flows

Abstract The two-phase flow in sealing gaps of twin-screw compressors is investigated using highly resolved coupled multiphase simulations. Oil-injected rotary-type positive displacement compressors (RPDC) such as twin-screw machines are used for the compression of refrigerants in HVAC (Heating, Ventilation, and Air Conditioning) systems. The efficiency of such machines is largely determined by the inevitable two-phase surge and gap flows. The simulation of flows in such compressors is still a challenge due to the complexity of the two-phase flow in the narrow gaps (<0.3mm) with organic working fluids and moving boundaries. Due to the rotation of the screws relative to the housing, gaps are present between the rotors and the end plates of the housing and between the tips of the rotors and the cylindrical housing walls. Oil injection can increase the efficiency of screw compressors or expanders by more than 10%, since oil enhances the sealing of the gaps where leakage occurs. On the other hand, additional momentum losses are caused by the two-phase gap flows. The oil injection, however, increases the hydraulic losses which depend on the geometry of the gap, the liquid, and the kinematics of the established multiphase flow in the gap. In this work, a highly efficient, high-resolution numerical method is used for the simulation of the gas-liquid multiphase flow in the screw compressor gap between the rotor tip and the stationary housing. The Lattice-Boltzmann method (LBM) defines the basis of the method used in this work, forming an in-house developed framework in which each fluid phase is solved by a separate solution algorithm. To capture the motion of the liquid-gas phase boundary, a level-set method is used. In the present work, a generic gap flow is considered. A Couette-type flow configuration, where the sealant flow in the gap is driven by the moving inner wall, is investigated. The Reynolds number based on the height of the channel is Re = 20000. Finally, we discuss the impact of bubble-laden sealing gaps on the momentum losses of the system.