Effects of cryogenic hydrogen on jet-flame combustion characteristics for aircraft propulsion applications

The development of future aircraft propulsion systems increasingly considers sustainable aviation fuels and hydrogen (H ₂ ) as alternative energy carriers. Due to its low density at ambient conditions, H ₂ must be either highly compressed or stored in liquid form at cryogenic temperatures to achieve higher densities and therefore, smaller system volume. While cryogenic hydrogen (LH ₂ ) offers higher density at low pressure, it introduces challenges related to flow behavior, thermal management, and safety, including icing phenomena. This study investigates the influence of low-temperature H ₂ injection on jet-flame characteristics and combustion stability. Experimental results are compared with literature correlations, with particular focus on the coupling between injection temperature, fuel density, jet velocity, mixing behavior, and flame temperature. The results show that cryogenic injection significantly alters combustion behavior. Increased density leads to reduced exit velocities at constant volumetric flow rates, while higher Reynolds numbers affect turbulence and mixing. Lower injection temperatures result in elongated flames, reduced local flame temperatures, and delayed heat release. Flame lift-off is strongly influenced by these effects and serves as a key stability indicator. In addition, nozzle icing is identified as a critical safety concern, particularly under lift-off conditions due to reduced thermal feedback, and is qualitatively discussed.