Predictive capabilities of flamelet combustion models in hydrogen engines across a broad range of operating conditions

• CC-TFC predicts physically consistent flame speed trends for H2 mixtures. • ECFM-3Z underestimates flame speed under low turbulence for all equivalence ratios. • ECFM-3Z overpredicts flame speed under high turbulence compared to CC-TFC. • ECFM-3Z needs case-by-case calibration to match experimental trend of different engines. • CC-TFC agrees with experiments in different engines with minimal calibration. Hydrogen-fuelled internal combustion engines (H 2 ICEs) can reduce the CO 2 emissions of the transport sector. In literature, several 3D Computational Fluid Dynamics (CFD) methodologies to simulate H 2 ICE combustion are used. However, the wide range of conditions under which H 2 ICEs operate results in a large variety of numerical setups. In this work, Three-Zones Extended Coherent Flamelet Model (ECFM-3Z) and Complex Chemistry with Turbulent Flame Speed Closure model (CC-TFC) are analysed, and a robust methodology is proposed. Flame speed analyses on a combustion plane show that with low turbulence, CC-TFC flame speed tends to the correspondent laminar value, while the ECFM-3Z one tends to zero. As turbulence increases, the ECFM-3Z flame speed becomes higher than CC-TFC one. This behaviour is confirmed by in-cylinder simulations performed on a port fuel injection (PFI) lean H 2 ICE and on a stoichiometric hydrogen direct injection (HDI) engine, operated at part and full load. With ECFM-3Z, the MFB10-50 is 22 % shorter than experiments with low turbulence and 56 % faster with high turbulence, requiring significant calibration. As for CC-TFC, with a single setup, the MFB10-50 is less than 3 % longer than experiments in both PFI engine conditions and 15 % faster in the HDI engine at part load, with a minor tuning needed only for the full load condition. The different models behaviour with low turbulence affects also the wall-flame interaction, as with ECFM-3Z the flame is artificially quenched when approaching the walls, which leads to a 30 % less heat transferred to the walls compared to CC-TFC.