The ongoing transition towards carbon-neutral aviation has triggered extensive research and development efforts into alternative fuels, with hydrogen emerging as a promising candidate due to the absence of any direct carbon emissions. However, 100% hydrogen combustion poses several operational challenges, even in ground-based gas turbine applications, including flashback risk, thermoacoustic instabilities, and bulky storage requirements for long-haul aircraft. In response, multi-fuel combustion strategies, simultaneously burning liquid kerosene and gaseous hydrogen, are gaining traction in both industrial initiatives and European R&D projects. This approach aims at mitigating the drawbacks associated with pure hydrogen combustion while enhancing the performance of kerosene combustion through improved flame stability at reduced equivalence ratios, thereby reducing carbon dioxide, NOx, and soot production in comparison to pure kerosene combustion. This study, carried out in the context of the HOPE Horizon Europe project, presents the development and preliminary application of a novel extension of the Thickened Flame Model (TFM), tailored for simulations of multi-fuel kerosene-hydrogen combustion within a scale-resolving framework. First, the standard TFM setup available in ANSYS Fluent is validated against two 100% hydrogen turbulent swirled flames - the HYLON and HOPE experimental burners - demonstrating good predictive capabilities in pure hydrogen regimes. Subsequently, the model is modified to operate in a multi-fuel context and preliminarily assessed against experimental data from the HOPE test rig, characterized by simultaneous kerosene-hydrogen injection and combustion. The results highlight the model’s ability to capture some of the key characteristics of the studied multi-fuel system, thereby providing a step towards the CFD-based design and optimization of next-generation, low-emission aeronautical multi-fuel combustors.

Hydrogen combustion is gaining attention for its potential to enable low-emission energy conversion in gas turbines. Since hydrogen is carbon-free, it produces no carbon-based pollutants and primarily forms water vapor. However, due to its higher adiabatic flame temperature with respect to hydrocarbons, hydrogen combustion is more prone to increased nitrogen oxides (NOx[jls-end-space/]) formation. Accurately predicting NOx formation remains a major challenge, particularly when scaling from laboratory experiments to industrial applications. While scaling laws are widely used in fluid dynamics, their application to NOx emissions is challenging due to the complex nature of NOx formation. This study develops a semi-empirical, physics-based correlation to estimate NOx emissions in swirl-stabilized, partially premixed lean hydrogen–air burners. The proposed correlation expresses the Emission Index of NOx (EINOx[jls-end-space/]) as a function of key operating and design parameters, including fuel mass flow rate, pressure, adiabatic flame temperature, equivalence ratio, residence time and swirl number. It builds upon Westenberg’s NOx formation rate equation, incorporating composition-dependent effects via equivalence ratio. The slow formation nature of NOx is accounted for via the combustor mean residence time. Additionally, the influence of swirl on the mixing process is modeled through a swirl-modified effective equivalence ratio, acknowledging that while partial premixing is a design choice, swirl intensity can either enhance or disrupt the degree of premixing before combustion. The model’s parameters were calibrated using experimental data from published literature. By providing a predictive tool for NOx scaling across different operating conditions, this model supports the development and design of experiments and devices for hydrogen–air combustion.