2 nd . Law Analysis of Sage and CFD-ACE+ Models Of MIT Gas Spring and "Two-Space" Test Rigs

The results of the numerical study of heat transfer effects under conditions of oscillating pressure and oscillating helium gas flow in two Massachusetts Institute of Technology (MIT) test rigs – piston-cylinder and piston-cylinderheat exchanger – are presented. Heat transfer effects in the presence of oscillating pressure and flow conditions are known to be important but poorly understood in a wide array of technically important mechanical power systems, such as gas compressors, automotive engines and Stirling engines to mention a few. Until recently, heat transfer estimates used in these equipment have been based on ordinary, steady-state convective models which contain no term to account for the oscillatory effects of pressure and fluid flow. Two solution domains, the gas spring (“singlespace”) and the gas spring + heat exchanger (“two-space”) in the MIT test rigs are of interest in this study. Sage and CFD-ACE+ commercial numerical codes are used to obtain 1-D and 2-D computer models, respectively, of each of the two solution domains and to simulate the oscillating gas flow and heat transfer effects in these domains. The 2 nd . Law analysis is also used to characterize the various thermodynamic losses inside the two test rigs. Temperature, pressure and surface heat transfer variations, pressure-volume diagrams and energy conservation and thermodynamic losses are compared between Sage and CFD-ACE+ results and with results obtained from the literature. In general, the results are qualitatively similar with some disagreement in magnitudes. The ultimate goal of this research effort is the improvement of numerical codes for more accurate predictions of heat transfer and thermodynamic losses in Stirling engines.