An Off-Design SAGE Thermodynamic Model of a Scaled Stirling Engine

Abstract

We present the development of an “off-design” Sage model of a free piston Stirling engine to simulate changes in piston and displacer motions and engine performance, in response to changes in engine operating conditions and design geometry. A scaled version of the Stirling Technology Company’s Technology Demonstrator Convertor (TDC) was developed at NASA Glenn Research Center (GRC) for use by Cleveland State University to avoid export control restrictions. The scaled convertor is called the “CSUmod.” NASA uses the term “convertor” to cover the engine plus the linear alternator. A Sage model of the CSUmod convertor is used for this modeling effort which employs steady-periodic spring-mass-damper models for the convertor’s power piston and displacer. This study is motivated by the need to know how, for example, degradation of the heater head temperature over the life of a multi-year, deep space engine mission would impact the power piston/displacer motions and engine thermodynamics. This study, however, models the behavior of an engine that does not have a mission qualified controller. The unconstrained Sage model predictions of the piston and displacer motions, engine powers, total heat input rate and overall efficiency agree with the constrained Sage model at the design point to within 0.03%. Results of engine sensitivities to changes in the heater head temperature, charge pressure and appendix gap are in line with qualitative expectations. These results clearly calibrate the offdesign model for the CSUmod reference operating conditions and demonstrate the feasibility of modeling an “off-design” free piston Stirling engine using spring-mass-damper models of the piston and displacer.