Numerical and experimental investigation of a combustor-coupled free-piston Stirling electric generator

In this paper, a novel gas combustor with multiple nozzles is proposed to couple with a free-piston Stirling electric generator (FPSG) for distributed power application. Due to the simple layout of the combustor and intrinsic external combustion of the FPSG, the system is highly adaptive to gas fuels, such as butane, natural gas and liquefied petroleum (LPG). In order to unravel the heat transfer characteristics of external combustion with internal oscillating flow, a rigorous numerical simulation was conducted, and a series of experiments were carried out to verify the calculations. Initially, an in-depth analysis was performed in terms of the flow field, temperature distribution and heat flux density within the combustor. The effects of the combustor inlet flow rate, nozzle structure, and preheating temperature on the flow heat transfer characteristics were explored. Numerical calculations reveal that, at a fuel flow rate of 0.55 Nm3/h and an air-fuel ratio of 24, the high-temperature heat exchanger (HHX) absorbs 6.41 kW heating power, with convective heat occupying approximately 73.5 %. Moreover, the simulations unveil the significant influence of optimizing the air-fuel ratio, nozzle structure, and preheating temperature on enhancing the overall heat transfer performance and achieving fuel and air con-sumption reductions. Subsequently, we meticulously conducted experimental investigations to comprehensively evaluate the performance of the novel combustor-coupled FPSG. The experimental results demonstrate that, under a fuel flow rate of 0.55 Nm3/h, the system achieves an output electric power of 1531 W, corresponding to a net thermal-to-electric efficiency and fuel-to-electric efficiency of 30.9 % and 9.6 %, respectively.