Evaluation of Pressure Oscillator Losses

Abstract

Efficiencies of regenerative cryocoolers are influenced by losses within the pressure oscillator (Stirling-type compressor). An evaluation of these losses is important when searching for ways to improve the overall cryocooler efficiency. Typically, compressor efficiency is taken as the ratio of PV power of the piston(s) to electrical input power. This definition ignores blowby, irreversible heat transfer, and flow losses within the compressor. The actual PV power delivered to the cold head is less than that measured at the face of the piston(s). We discuss a simple set of measurements for evaluating the total loss within the compressor. One measurement is the measurement of the electrical and PV required for a blanked off compressor to provide a given pressure ratio. The second is the measurement of electrical and PV power for a given stroke with the compressor connected to a large reservoir. The sum of these two mechanical losses is subtracted from the PV power measured at the piston face to give the estimated PV power delivered to an attached load. We compare such estimates with actual system measurements that use hot wire anemometry at the compressor outlet to determine the PV power delivered by the compressor to the load. Measurements for mean pressures from 1.5 to 2.5 MPa, for pressure ratios from 1.0 to 1.3, and for the corresponding mass flows are presented. The compressor used here had a swept volume of 4.3 cm 3 . The ratio of delivered PV power to piston PV power was about 0.73 + 0.05, estimated from the simple measurements, compared with 0.60 + 0.05 measured directly with the aid of the hot-wire anemometer.

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