Performance analysis of the small scale alpha-type Stirling engine using CFD tools

The Stirling engine was invented and patented in 1816 by Scottish minister Robert Stirling. Basically, Stirling engine can be considered as an external combustion engine, but actually it can be also powered by waste heat or solar energy. The theoretical cycle of Stirling engine consists of two isotherms and two isochores, hence theoretically Stirling cycle can reach the same energetic efficiency as the Carnot cycle. However, it is difficult to achieve theoretical maximum efficiency due to finite rate of heat transfer between working medium and heat sources and finite rate of heat transfer in regenerator. Practically, this results in huge and complicated heat exchangers what was one of the main reasons for giving up the idea of Stirling engine for almost one century. Nevertheless, Stirling engine has a few important advantages over internal combustion engines. The combustion process in Stirling engine can be run continuously as a consequence it is simpler to control. Moreover construction of the engine is far simpler when comparing to internal combustion engines. Finally, the engine has potential to reach Carnot cycle efficiency. The paper presents development of the Computational Fluid Dynamics (CFD) model of small scale a -type Stirling. Developed mathematical model comprises of Unsteady Reynolds Averaged Navier-Stokes (URANS) set of equations i.e. continuity, momentum and energy equations, turbulence was modelled using standard k - w model and was built in the framework of commercial CFD software ANSYS Fluent. Pistons walls movements were modelled using dynamic mesh capability in ANSYS Fluent, their movement kinematics was described based on the geometry of the crankshaft geometry and it was implemented in the model using user defined functions (UDF) written in C programming language and compiled with core of the ANSYS Fluent software. Based on the numerical results, performance of the Stirling engine in the framework of the First and the Second Law of Thermodynamics. Using developed CFD tool, a set of analyses for different heating strategies, different revolution seeds and different working mediums were performed.