J. R. S. Fernandes,
Alvaro Toubes Prata
Date of publication
Piston dynamics plays a fundamental role in two critical processes related to ﬂuid ﬂow in reciprocating compressors. The ﬁrst is the gas leakage through the radial clearance, which may cause considerable loss in the pumping efﬁciency of the compressor. The second process is the viscous friction associated with the lubricant ﬁlm in the radial clearance. In the present contribution a numerical simulation is performed for a ringless piston inside the cylinder of a reciprocating compressor, including both the axial and the radial piston motion. The compressor considered here is a small hermetic compressor employed in domestic refrigerators, with the radial clearance between piston and cylinder ﬁlled with lubricant oil. In operation, the piston moves up and down along the axis of the cylinder, but the radial oscillatory motion in the cylinder bore, despite being usually small, plays a very important role on the compressor performance and reliability. The compromise between oil leakage through the piston-cylinder clearance and the friction losses requires a detailed analysis of the oscillatory motion for a good design. All corre-sponding forces and moments are included in the problem formulation of the piston dynamics in order to determine the piston trajectory, velocity and acceleration at each time step. The hydrodynamic force is obtained from the integration of the pressure dis-tribution on the piston skirt, which, in turn, is determined from a ﬁnite volume solution of the time dependent equation that governs the oil ﬂow. A Newton-Raphson procedure was employed in solving the equations of the piston dynamics. The results explored the effects of some design parameters and operating conditions on the stability of the piston, the oil leakage, and friction losses. Emphasis was placed on investigating the inﬂuence of the pin location, radial clearance and oil viscosity on the piston dynamics.
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