Aspects of Two-Phase Flow Screw Compressor Modelling Part I: Leakage Flow and Rotor Tip Friction

A one-dimensional leakage flow model for two-phase ammonia-water twin screw compressors is presented. The model takes into account viscous and acceleration forces and is based on three conservation equations and on the equation of state for homogenous NHiH20 mixture. The governing equations are solved with a finite difference method. Numerical shooting is used to find such inlet flow velocity and position of the shock that the outlet boundary condition is fulfilled. Results of the solution are used for calculation of the shear stress and friction between rotors and housing. Comparison of the results of the proposed leakage flow model with the results of the isentropic converging nozzle model shows that the latter predicts up to two times higher mass flow rate. Calculations of the rotor tip friction force indicate that tip friction losses in the two-phase NHiH20 compressor are negligibly small.