Veröffentlichungen

Lubricant, as a key component in compressors, plays an important role in heat pump (HP) systems. Reliable knowledge of thermophysical properties of lubricants is essential, for example, to investigate the impact on the thermo-economic performance of a HP system when working fluids (typically refrigerants) are mixed with lubricants. Modelling all the required thermophysical properties (mainly: density, phase behavior, heat capacity, entropy, enthalpy, viscosity and thermal conductivity) of a lubricant and the lubricant + refrigerant mixtures in a HP system remains a key challenge. To tackle this challenge, we propose a novel approach based on treating the lubricant as a quasi-pure fluid, setting up a model set for all the required properties of the lubricant, and developing a parameter fitting procedure using the least amount of experiments. This model set includes the Patel-Teja-Valderrama equation of state, a simple expression for the ideal gas isobaric heat capacity as a linear function of temperature, and residual entropy scaling relations for viscosity and thermal conductivity. For parameter fittings, two extra models are required: Raoult's law of boiling point elevation and the modified Rackett equation. As a result, less than 20 experimental points are needed to fit all the parameters of a lubricant, and one experimental point is required to enable predictions for a binary lubricant + refrigerant mixture. For quasi-pure lubricants, in the liquid phase and not in the vicinity of the critical point, this modelling approach has an estimated uncertainty of 7 % for viscosity and 3 % for all other properties. Similar results are obtained for less asymmetric binary systems. For asymmetric binary systems, except for viscosity, the modelling approach still yield good prediction, typically within 10 %. With the developed modelling approach, the impact on the thermo-economic performance of a HP system when working fluids are mixed with lubricants are investigated.

Schlagwörter: Lubricant, Refrigerant, Thermophysical Property Model, Heat Pump