Heat flows in piston compressors

Piston compressors are widely used in today's engineering applications. Among the most important applications is however the compression of thermal carrier gas in Rankine and Stirling refrigeration cycles. Fluids used in these cycles are commonly Ammonia and Helium. In order to improve the design and reliability of these compressor designs, transient thermal and fluid flow phenomena and thermo-mechanical processes in an unlubricated piston-cylinder gas spring are investigated. Conventional Nu-Re correlations, as derived for steady-state developed boundary-layer flows, are not applicable for the transient heat transfer between the gas undertaking rapid compression and expansion and the surrounding walls. In this paper results are presented of CFD simulations of laminar and turbulent flows inside the compressed volume and corresponding heat flows. Parameters varied in the present work are the compression ratio and operating frequency. The CFD results are compared with heat flux data post processed from pressure recordings performed at a test rig at MIT. In the post processing essential thermodynamic assumptions were made. It is shown that at low RPM and hence laminar flow, CFD results on heat flux compare well with these measurements. At more practical high RPM and turbulent fluid flow the CFD result differ significantly from the post-processed data. This can be explained by the break down of the thermodynamic assumptions in the measurement data processing. In the new and highly advanced test rig built at the University of Twente, heat fluxes will be measured directly.