CAFS: The Combined Adiabatic–Finite Speed thermal model for simulation and optimization of Stirling engines

Highlights: • A novel thermal model was developed for simulation of Stirling engines. • Pressure drops, due to piston speed, friction and throttling, was considered. • Shaft power, thermal efficiency and power loss were evaluated using thermal model. • The thermal model was successively validated and used for optimization. • The GPU-3 Stirling engine was optimized in five scenarios. - Abstract: A novel thermal model based on the combination of Adiabatic analysis and finite speed thermodynamics called CAFS (Combined Adiabatic–Finite Speed) analysis was developed, which considered effect of finite speed of piston, pressure throttling in heat exchangers and regenerator and piston's mechanical friction in the basic Simple model. Pressure drops in the regenerator and heat exchangers were determined and the number of transfer unit (NTU) was corrected and a new correlation was used to calculate the regenerator's effectiveness. In addition, effect of finite speed of pistons and mechanical friction as sources of internal irreversibilities that resulted in reduction of the output power was included in the developed model. The objective functions were obtained based on the developed CAFS model. It was shown that the developed thermal model simulated the engine with +69.7%, +33.6% and +14.9% errors in estimating brake power, indicated power and thermal efficiency, respectively. It was shown that in the best optimization scenario among five optimization scenarios, with little change in operational and geometrical parameters of the engine, power and thermal efficiency increased by 2.33 times and 12.0% (as difference), respectively. Besides, in the optimized engine, 13.2% reduction was obtained in power loss