Due to financial and environmental benefits, the interest in high-temperature heat pumps, producing heat sinks above 100 °C, is rising. Research is intensifying, and technology is developing fast. However, of all the components of the heat pump cycle, the oil management system seems to be the least investigated, even though 12% of the faults in large-scale heat pumps are related to oil management. This study aims to develop a method that predicts the oil-refrigerant separation performance in high-temperature heat pumps.
Oil injection is necessary for high-temperature heat pumps to run without failures. The oil lubricates, seals and cools the compressor, increasing its performance and lifetime. However, if the oil is not separated correctly, it causes problems in the other components; The oil increases the pressure drop and decreases the heat transfer of the heat exchanger. Additionally, the refrigerant
partially dissolves in the oil, altering its thermophysical properties and increasing the risk of improper lubrication. Finally, the transport of the refrigerant deteriorates.
The oil management of a heat pump includes several components; an oil injector, oil separator, oil pump, oil sump/reservoir, oil temperature regulator and an oil filter. The core of oil management is the oil separator; three types of separation mechanisms are distinguished: impingement, centrifugal and coalescence. A centrifugal-type separator is most applied for industrial applications since it provides a wide operating range, allows high flow rates, and is simple and, thus, cheap to manufacture.
A data set containing the flow conditions and thermophysical properties is developed to use as input for the oil-refrigerant separation process modelling. A Daniel plot and developed correlations determine the properties of the oil-refrigerant mixture. The Tatterson equation is used to predict droplet size, and
the droplet distribution is described using the Rosin-Rammler distribution. A test case is designed to validate the developed method.
Finally, the oil separation due to a cyclone oil separator is modelled in Ansys fluent. A transient simulation based on the Reynolds stress modelling approach is conducted for the continuous phase. The discrete phase method simulates the oil-refrigerant droplets. The developed method serves as a strong foundation for future research. For the future, including models for droplet behaviour and wall-film models is strongly recommended to provide scientifically relevant outcomes.