Summary

A detailed knowledge of lubrication process in reciprocating compressors, particularly the piston assembly lubrication, has a fundamental role in the design of more efficient refrigeration systems. A radial clearance between the piston and the cylinder walls allows piston oscillatory radial movements that affect both the lubricant film thickness and the lubrication regime. Furthermore, cavitation and restoration of the lubricant film considering the oil/refrigerant interaction are not totally understood and these issues deserve additional studies for future improvements in the design of compressors. With this in mind, this work proposes a theoretical/experimental study of the ow of oil and refrigerant through the piston-cylinder clearance of reciprocating compressors used in small capacity refrigeration systems. Firstly, the project of a experimental apparatus is proposed to study ow characteristics in the piston-cylinder geometry with the piston steady. The apparatus consists of a test section in which the piston is placed inside a translucent cylinder equipped with thermocouples and pressure sensors at defined positions. The cylinder is mounted upon a set of micrometerguided stages that allows pressure measurements at several positions for different piston-cylinder misalignment degrees. Theoretical analysis includes three different models. The first one is an equilibrium model to describe the two-phase ow of the mixture through the clearance in which the piston is steady. The Reynolds and the energy conservation equations were solved numerically considering the variation of the physical properties in both phases in order to calculate the film pressure and temperature. The numerical results were compared with the experimental ones in order to validate the model. The second model extends the previous methodology to describe the piston dynamic lubrication in which piston movements and the heat transfer effect in the film were considered. The performance parameters related to piston movement, such as the power consumption and refrigerant leakage, were obtained and compared with those ones calculated by isothermal models. Finally, the third model regards characterization of growth process of individual bubbles in oil-refrigerant mixtures under isothermal depressurization, in which assumptions of mechanical and thermodynamic equilibrium were suppressed. This model is introduced as a potential tool to predict bubble growth that may occur as a result of cavitation in film, and the foamming process in the oil sump during compressor start-up.

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