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NASA Glenn Research Center and the Department of Energy (DOE) are developing a Stirling converter for an advanced radioisotope power system to provide spacecraft on-board electric power for NASA deep space missions. NASA Glenn is addressing key technology issues through the use of two NASA Phase II SBIRs with Stirling Technology Company (STC) of Kennewick, WA. Under the first SBIR, STC demonstrated a parallel connection of two thermodynamically independent free-piston Stirling converters and a 40 to 50 fold reduction in vibrations compared to an unbalanced converter. The second SBIR is for the development of an Adaptive Vibration Reduction System (AVRS) that will practically eliminate vibrations over an entire mission lifetime, even with one failed converter. This paper discusses the status and results for these two SBIR projects and also presents results for characterizing the friction factor of high-porosity random fiber regenerators that were evaluated for this application.

Results of visualization experiments are presented for the entry flow to circular tubes under oscillatory flow conditions. Geometries and conditions have been chosen to simulate the flow in a Stirling engine with straight heat exchanger tubes. Since oscillating flow in Stirling engines is unavoidably strongly influenced by the entry conditions, such documentation is useful when engine designs are being considered and is needed when test results are being interpreted. Two entry geometries are explored, one with unrestricted entry to a squared-edged tube and another with entry from one side. The visualization technique is by illumination of neutrally-buoyant, helium-filled soap bubbles with laser light, capturing with still photography. Each picture is an ensemble of exposures from 150 cycles. Each entry to the ensemble is taken at the same range in crank position, typically five degrees. Thus, one picture may visualize the flow from 75 to 80 degrees of crank rotation, for instance.