Experimental investigation of viscosity, interfacial tension, and thermal conductivity of oil-refrigerant mixtures by light scattering and conventional techniques

• Applicant: Professor Dr.-Ing. Andreas Paul Fröba
• Subject Area: Technical Thermodynamics ; Hydraulic and Turbo Engines and Piston Engines ; Fluid Mechanics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510921053

The efficiency of oil-injected rotary positive displacement compressors depends strongly on the inevitable two-phase surge and gap flow in the wall region, which to date is neither well understood nor calculable. One reason for this is the lack of accurate experimental data and calculation models for the required thermophysical properties of the very asymmetric mixtures of refrigerants and oils often processed in such machines. Taking up this situation, advanced experimental methods and partly also molecular dynamics simulation techniques for the determination of viscosity, interfacial tension, and thermal conductivity of such systems shall be further developed and applied here as a contribution to the DFG Research Unit FOR 5595. This will be done in state ranges that are relevant both for the field of application of the compressors mentioned and for the development of the required property models and of process simulation methods. From the data obtained first for systematically selected model systems and later for realistic refrigerant-oil mixtures, structure-property relationships will be derived, which will contribute in particular to model development. The focus is on future-oriented refrigerants such as CO2 and propane, which are combined with oil surrogates of increasing molecular size to approach the asymmetry of technical mixtures. The measurement of interfacial tension and viscosity in vapor-liquid equilibrium is mainly performed by surface light scattering (SLS). For viscosity measurements of the mixtures of interest in the compressed liquid phase, the vibrating-wire method will be used, while a further development of dynamic light scattering (DLS) applied to particle dispersions is additionally aspired. This includes the identification of dopant particles suitable in terms of dispersion stability for such systems. The aforementioned techniques will be combined with Raman spectroscopy, for which calibration approaches to accurately determine fluid composition will be investigated. Non-equilibrium molecular dynamics simulations will be used to access the shear-rate dependence of the viscosity of the relevant systems, which is hardly detectable experimentally. While the determination of the thermal conductivity of the mixtures of interest is a focus of a targeted second funding period, this property is already measured here for the oil components using a guarded parallel-plate instrument. First conclusions on the corresponding mixture behavior are drawn from data for the thermal diffusivity, which is accessed by DLS applied to the liquid bulk in parallel to the measurements by SLS.

• DFG Programme: Research Units
• Subproject of: FOR 5595: Oil-refrigerant multiphase flows in gaps with moving boundaries – Novel microscopic and macroscopic approaches for experiment, modeling, and simulation