Development of models for the calculation of viscosity and thermal conductivity of oil-refrigerant mixtures

• Applicant: Dr.-Ing. Monika Thol, Ph.D.
• Subject Area: Technical Thermodynamics ; Hydraulic and Turbo Engines and Piston Engines ; Fluid Mechanics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510921053

Oil-injected rotary displacement compressors are mainly used for the compression of refrigerants in heating, ventilation and air conditioning systems. The efficiency of such machines is largely determined by the inevitable two-phase surge and gap flows in these types of compressors. Due to the complexity of simulating the two-phase flow in the narrow gaps with moving boundaries as well as the lack of accurate models for the thermophysical properties of the highly asymmetric fluid mixtures of oil and refrigerant, the calculation of such flows is still a major challenge. The Research Unit FOR 5595 consisting of seven interdisciplinary project partners offers the unique opportunity to close this knowledge gap. The three subprojects of group A deal with the development of new analytical and computational methods for two-phase gap flows, while the four subprojects of group B deal with the measurement and modeling of the thermophysical properties of oil-refrigerant mixtures. Within this consortium, subproject B4 aims at developing models for viscosity in the Newtonian range and for thermal conductivity to characterize strongly asymmetric oil-refrigerant mixtures. For this purpose, existing literature models are first reviewed for their applicability and accuracy. Using these results, two development paths are then pursued: (1) The physically based entropy-scaling approach will be further developed for complex mixtures such as those addressed in the research group. Due to its physically based nature, this approach can subsequently be applied to other oil-refrigerant systems in a straightforward way. (2) The second development path foresees the development of purely empirical mixture models. While these will not be universally applicable to any mixture, they will provide significantly more accurate results than the entropy-scaling approach for selected systems. In order to achieve this goal, comparatively simple systems which have been extensively studied experimentally, such as mixtures of carbon dioxide with alkanes or other suitable homologous series, are first systematically investigated. In this context, the consideration of long-chain alkanes already leads to mixtures whose asymmetry and complexity are comparable to those of refrigerant/oil mixtures. The systematic results obtained from these studies are then applied to less well-measured systems such as highly asymmetric oil-refrigerant mixtures. At the same time, the knowledge gained in both development paths forms the basis for a subsequent development of suitable models for the thermal conductivity of strongly asymmetric mixtures.

• 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
• International Connection: USA
• Cooperation Partners: Ian H. Bell, Ph.D.; Dr. Eric Lemmon