Real-fluid effects on laminar premixed hydrogen flames under cryogenic and high-pressure conditions

The combustion of hydrogen (H₂) under cryogenic and high-pressure conditions has the potential to increase the volume-based energy density of H₂ and combustion efficiency. Predictive modeling of cryogenic H₂ premixed flames at high pressures requires a clear understanding of real-fluid effects. While substantial effort has been made to study the real-fluid effects in nonpremixed flames, comparatively fewer investigations have been performed to explore real-fluid effects on premixed flames, especially at cryogenic and high-pressure conditions. This work aims to fill a part of this gap by conducting a series of laminar premixed H₂ flame simulations at cryogenic and high-pressure conditions (T_u=50–350 K and p=10,20,40 MPa). Four cases are considered to examine the role of corrections of the equation of state (EOS), thermodynamic properties, and transport properties in predicting flame structure and properties. It is found that the real-fluid effects mainly occur in the fresh/preheat regions and that the correction of EOS plays a critical role in the prediction of flame structure and the laminar flame speed (S_L), while the correction of transport properties is critical for predicting flame thickness. Each correction could contribute to the predictions of the mass burning rate and the flame thickness but hardly affect the flame temperature (less than 1% relative difference). A scaling law of S_L^Real/S_L^Ideal=Z_u (Z_u is the compressibility factor of the unburned mixture) is proposed, which can be used to readily predict S_L of real fluid from the speed evaluated by the ideal gas model. In addition, the Monte Carlo sampling method is used to perform uncertainty quantification of S_L due to the uncertainty of the real-fluid model parameters. The results show that the critical properties of H₂ play a role in the uncertainty of S_L, while the overall relative deviation is less than 2%, and the uncertainty decreases significantly with the increase of unburned mixture temperature. Novelty and significance statement This study comprehensively evaluates real-fluid effects on laminar premixed H₂/O₂ flames under cryogenic and high-pressure conditions by considering different levels of real-fluid correction. We highlight the significance of the correction of transport properties for predicting flame thickness. A scaling law for real-fluid laminar flame speed is proposed for the first time. This work serves as a foundational step towards enhancing our understanding of real-fluid effects on cryogenic H₂ combustion.