Motor and thermodynamic losses in linear cryocooler compressors

J. Reed (1), P.B. Bailey (1), M.W. Dadd (1), T. Davis (2)

  1. Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK.
  2. AFRL, Albuquerque, NM, USA

Stirling cycle and pulse tube cryocoolers can be described by ideal thermodynamic cycles with discrete losses. These are relatively easy to analyse but experiments show a systematic underestimate of power consumed. It has been suspected that this extra "compression loss" is related to imperfect heat transfer in the compression space and this has been further implied by parametric studies relating it to the area of the cylinder PV loop. Existing correlations have been unable to explain its magnitude, but it can account for up to 50% of the power delivered to the gas. It is clear that even a modest decrease could significantly improve the efficiency of future machines. A problem inherent in studying this effect is the complex geometry of cryocoolers. Experiments have been performed on an existing moving coil compressor (of a relatively old design), with the simplest compression space geometry, a flat cylinder head. Measurements were made of the intrinsic motor losses (including windage), and the power delivered to the gas. The results show that for this machine the motor losses are greater than previously thought, accounting for up to 30% of the compression loss in the original cryocooler configuration. Measurements of the heat transfer losses are also presented.

Sponsored by the Air Force Office of Scientific Research, USAF.