Preliminary Results from Simulations of Temperature Fluctuations in Stirling Engine Regenerator Matrices

The objective of this study has been to create a model for studying effects of temperature fluctuations in regenerator matrices on Stirling engine performance. A one-dimensional model with axial discretisation of engine components has been formulated using a fixed Eulerian grid. The model contains a system of ordinary differential equations (ODEs) describing the mass and energy balances for the working gas in all control volumes and energy balances for the matrix material in the control volumes of the regenerator. Interpolation methods with filtering properties are used for state variables at control volume interfaces in order to reduce numerical diffusion and/or non-physical oscillations. Known loss mechanisms are coupled directly into the governing equations instead of applying the losses as post processing. Steady state periodic solutions for the model are found using a shooting method that adjusts solutions so that they satisfy the necessary cyclic boundary conditions as well as integral conditions for cyclic heat transfer for walls in the engine and for the mean cycle pressure. It has been found that it is possible to accurately solve the stiff ODE system that describes the coupled thermodynamics of the gas and the regenerator matrix and to reliably find true steady state solutions to the model. Preliminary simulation results indicate that the variations in the regenerator matrix temperatures do have significant impact on the regenerator loss, the cycle power output, and the cycle efficiency and thus deserve further study.