Modelling thermodiffusive instabilities in hydrogen flames and their impact on the combustion process in a direct-injection hydrogen engine

Hydrogen-fueled Internal Combustion Engines (H2-ICEs) are typically operated with lean mixtures to minimize NOx emissions and reduce the risk of abnormal combustion events. Due to hydrogen’s low Lewis number, premixed hydrogen-air flames in lean conditions exhibit strong thermodiffusive instabilities, which make the numerical simulation of the combustion process particularly challenging. Indeed, the intensity of these instabilities is significantly influenced by thermodynamic parameters – such as mixture temperature, pressure, and dilution rate – resulting in substantial variations in combustion behaviour across different operating conditions. Therefore, they have to be properly considered not only to ensure model robustness, but also to improve model accuracy over a wider range of operations. In this study, the combustion process in a Direct Injection H2-ICE was analyzed using 3D-CFD simulations, relying on a flamelet-based combustion model. Two sets of lookup flame speed maps were defined: laminar flame speed (SL) maps derived from standard 1D-CFD simulations in homogeneous reactor, and freely propagating flame speed (SM) maps which account for the effects of thermodiffusive instabilities. The model that uses SL maps required the recalibration of some combustion model parameters when changing the dilution rate to ensure consistency with experimental data. Instead, the model relying on SM maps featured a noticeable accuracy across different air-to-fuel ratios without the need for recalibration any combustion model parameter, highlighting the key role of thermodiffusive flame instabilities on the combustion process. Based on these findings, the impact of such instabilities was evaluated throughout the entire combustion process from both global and local perspectives. The relevance of thermodiffusive instabilities was observed to increase with the air-to-fuel ratio, thereby enhancing combustion speed in leaner mixtures. Additionally, the implementation of thermodiffusive instabilities was found to affect also preferred direction of flame propagation, as stronger instabilities were identified in the leanest and low-temperature portions of the flame front.