Comparative study of the effects of increasing heat transfer area within compression and expansion chambers in combination with modified pistons in Stirling engines. A simulation approach based on CFD and a numerical thermodynamic model

Abstract

The aim of this research is to ascertain the potential benefits of increasing heat transfer area in the expansion and compression spaces of Stirling engines. This is achieved, as proposed in an accepted patent, by simultaneously attaching pins or fins to the chamber walls and modifying the geometry of the displacer. Different configurations involving pinned and finned surfaces were proposed for comparison with a hollow cylindrical chamber. In order to compute the heat transfer rate as well as the convective heat transfer coefficient for each suggested geometry, a CFD (Computational Fluid Dynamics) model was run under different temperatures, displacer speeds and pressures. For a selected geometry based on longitudinal cylindrical pins, the computed heat transfer coefficients were correlated with input parameters in order to obtain a simple equation. The equation was subsequently introduced into a three-camera second-order thermodynamic model of a gamma Stirling engine with crankshaft mechanism. Five different cases were evaluated. The models were run for different operational conditions (speed and charge pressure), for a hot wall temperature of 493 K and a cold wall temperature of 293 K. Pressure was varied from 1 to 2.5 atmospheres and the engine rotational speed ranged between 50 and 1000 rpm. Results showed an increase in engine power between 1.3 and 2.5 times when compared to a conventional engine without heat exchangers.