Conjugate heat transfer simulation of the linear range extender: Thermal design considerations for cooling strategy

Thermal management in free piston engine generator (FPEG) presents unique challenges due to their distinctive free and linear dynamic characteristics, yet this critical aspect remains underexplored in current research. This study conducts a comprehensive parametric investigation into the efficacy of single-phase convection and boiling-based cooling strategies, examining their performance under varying heat source and heat sink conditions. Through conjugate heat transfer simulations based on a 10 kW-level experimental prototype, we demonstrate that boiling-based cooling significantly enhances thermal uniformity in the engine dome compared to the liner, particularly under both normal and abnormal combustion conditions. The implementation of boiling-based cooling reduces the dome's temperature nonuniformity factor by more than 53 % relative to single-phase cooling. Furthermore, the disparity in temperature nonuniformity between the dome and liner exceeds 67 % with boiling-based cooling, in contrast to less than 31 % observed with single-phase cooling. Crucially, the advantages of boiling-based cooling are contingent upon the minimization of dead zones, a critical consideration given the nature of subcooled flow boiling. The study also proposes an optimized geometric modification that reduces the area below critical velocity by approximately 16 %. Finally, a novel temperature correlation for the dome and liner is issued.