Lubrication in refrigeration systems: Numerical model for piston dynamics considering oil-refrigerant interaction

Abstract

Piston dynamics plays a fundamental role in several processes related to the operation of hermetic reciprocating compressors used in refrigeration. For example, the refrigerant leakage through the radial clearance between piston and cylinder, which reduces compressor pumping efficiency, and also the viscous friction associated with the lubricant film in the radial clearance, which is related to energy consumption. It is important to optimize such variables, ensuring at the same time smooth operation of the piston in its reciprocating motion, minimizing wear and increasing lifetime. In this context, numerical models studying piston dynamics provide a useful tool for engineering design. These models usually consider an oil film filling the piston-cylinder clearance and operating in the hydrodynamic regime. Determining cavitation conditions occurring along the ringless piston represents an additional difficulty in modelling. As refrigerant is present in the compressor environment, it inevitably interacts with the oil, changing lubricant characteristics. The refrigerant can dissolve in the oil at higher pressures, reducing viscosity, and can be released at lower pressures, leading to a two-phase flow. This work explores how the interaction of oil and refrigerant affects piston dynamics, using a numerical model that considers as the lubricant a mixture of oil and refrigerant with variable properties. Comparing the results with simulations where pure oil is considered as the lubricant and a cavitation criterion is adopted, significant differences were observed in predicting piston trajectory and power consumption along the cycle.