The transition to hydrogen as a primary fuel in aviation requires innovative strategies to mitigate challenges associated with flashback in combustion systems. Due to its low volumetric energy density, hydrogen is preferably stored at cryogenic temperatures (T ≤ 100 K). Since stoichiometric hydrogen/air laminar premixed flames have been observed to sustain combustion even at temperatures as low as 100 K, low-temperature injection can be used as a strategy to control flashback and stabilize the flame. This study investigates the performance of low-temperature rich premixed hydrogen/air combustion within a model Rich-Quench-Lean (RQL) combustor configuration equipped with a Trapped Vortex Cavity (TVC). Large Eddy Simulations (LES) with Eulerian Stochastic Field (ESF) approach are conducted for this purpose. The LES are first validated against a recent experimental campaign involving high-speed chemiluminescence diagnostics. Subsequently, parametric studies explore the operating limits and flashback resistance of rich hydrogen flames under varying low-temperature conditions. Results indicate a strong sensitivity of the flame position within the TVC to the injection temperature. The strong reduction in laminar flame speed observed in experiments, however, is counteracted by the increase in turbulent/laminar flame speed ratio, adding to the complexity of the problem. Insights on how to control the flame dynamics in the cavity are provided within this study with the purpose of optimizing RQL-TVC designs for robust, low-emission, and flashback-resistant operation in hydrogen-based propulsion systems.

Due to climate change concerns, hydrogen is being considered for future aviation, but its commercial availability is limited, storage is bulky and its combustion with 100% concentration still poses numerous technical challenges. This leads to a certain interest in multi-fuel systems using both hydrogen and kerosene to facilitate the transition without completely redesigning the existing engines. Within the HOPE project, the present study focuses on an innovative multi-fuel combustion concept for aircraft propulsion, considering a laboratory-scale combustor hosted at TU Delft. Such a device, originally fueled with hydrogen and methane, is schematically composed of an axial swirler, four ducts for gaseous fuel injection, a mixing tube and a cylindrical combustion chamber. To avoid flashback, also an axial air injection duct is present that bypasses the swirler and directly reaches the air-fuel mixture in the mixing tube.
In this work, reactive CFD simulations are used to explore different spray injection configurations and assess the impact of kerosene on the flow field, the flame shape and the NO emissions of the modified system. In particular, three different injection positions are studied, featuring injection points on the backplane of the combustion chamber, inside the fuel/air mixing tube or on the axis of the burner. It is found that the most suitable position for kerosene injection is on the axis of the burner, so that the spray is surrounded by the swirling flow and undergoes a rapid mixing with the oxidising stream, limiting the maximum temperature reached by the mixture. Moreover, in this case, the addition of hydrogen leads to reduced NO emissions since it decreases the size of the hot spots generated by the combustion of kerosene.