Measuring and modeling of two-phase gap flow for oil-refrigerant mixtures in rotary positive displacement machines

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Oil-injected rotary positive displacement (RPDC) compressors such as twin-screw machines are widely used to compress refrigerants in HVAC (heating, ventilation, and air conditioning) systems. The efficiency of these fluid energy machines is determined to a significant extent by the unavoidable surge flows in front of a rotor lobe and the subsequent two-phase oil-refrigerant gap flow between the rotor and the housing. Within subproject A1, models for the one-dimensional calculation of the two-phase gap flows are being developed in the 1st funding period of the Research Unit FOR 5595. The governing effects, such as the outgassing of the refrigerant from the oil due to the pressure gradient in the gap or the moving gap boundary, are taken into account. At the same time, the local dissipation due to wall friction are resolved. Generic experiments of a rotating contour in a gas cylinder form the basis for this model development. The ratio of the frictional forces acting in the direction of flow to the inertial forces is systematically varied. Accordingly, the experiments are started with a rotating circular cylinder as contour (Couette-flow experiments) and varied towards a rotating rotor lobe contour (Couette-Poiseuille flow experiments with variable gap cross section). In the Couette-flow experiments, the driving torque is measured as a measure of the acting wall shear stress. In the Couette-Poiseuille experiments, the two-phase gap mass flow rate is measured together with the pressure distribution in the gap flow direction. The two-phase mixtures studied are first water with nitrogen (low solubility), then water with carbon dioxide (moderate solubility), and then n-decane with carbon dioxide (strong solubility). The complex thermophysical properties for these increasingly asymmetric fluid mixtures will be developed by the B group subprojects. Based on high-speed videos, the flow patterns in the gap are analyzed together with subproject A2 (Kriegseis) and transferred into flow pattern maps. Subsequently, approaches for the calculation of the inertial forces and wall friction forces acting in the flow direction will be established. The necessary models, e.g. for the slip between the gaseous and liquid phases, are developed together with subproject A2 and the high-resolution numerical flow simulation by subproject A3 (Schröder). Finally, with the validated approaches for the inertial and frictional forces, the sought two-phase oil-refrigerant gap mass flow rate and the gap dissipation for an imposed pressure drop across the gap can be calculated by numerically integrating the one-dimensional conservation equations per phase (separated two-phase flow model).