Fluid flow through male rotor housing clearances of dry running screw machines using dimensionless numbers

The efficiency of positive displacement machines is strongly influenced by clearance flows. The prediction of the mass flow rate is a challenging task since it depends on thermodynamic and operating boundary conditions as well as the shape of the clearance path. When simulating positive displacement machines using chamber models, clearance flow is commonly estimated by using simplified approaches, such as equations for isentropic nozzle flow combined with a flow coefficient. Knowledge of these flow coefficients is important for an accurate simulation of the machine. For this reason one of the goals of the European Union funded project "MOTOR" (Multi-ObjecTive design Optimization of fluid eneRgy machines) is the determination of the flow coefficient for different clearance shapes. This paper deals with the simulation of the two-dimensional fluid flow in the housing clearance of twin-screw machines using the finite volume method for air as ideal gas. Dimensionless numbers are varied to show their individual influence on the mass flow. Results are compared with experimental data from reports in the literature and show good accordance. The primary factors affecting the results are Reynolds number and the moving boundary in the low Reynolds number region.