Introduction
The transition to more sustainable aviation is pushing the industry to explore new technological frontiers. Among these, hydrogen is emerging as one of the most promising candidates for future propulsion. Hydrogen fuel cells represent a revolutionary solution, combining energy efficiency with zero emissions. However, implementing such technology in the highly specialized and safety-critical aviation sector requires careful analysis to mitigate risk and prevent accidents.
The SERENA project (Sviluppo di architetture propulsive ad Emissioni zeRo per l’aviazione gENerAle) is the result of a collaboration between Distretto Tecnologico Aerospaziale (DTA), EnginSoft, Novotech, and the University of Salento – Department of Innovation Engineering. The project focuses on developing zero-emission propulsion systems for general aviation, combining battery and hydrogen fuel cell technologies. Its prototype, the Seagullaircraft—classified under recreational/sport flight—uses compressed hydrogen stored at 350 bar in a cylinder located behind the pilot seats.
Given hydrogen’s wide explosive concentration range (4%–75%) and low ignition energy, a careful risk assessment is essential. Reference data from databases like HIAD 2.1 were used to model accident scenarios involving hydrogen use.
CFD Analysis of Hydrogen Leakage
To assess risks in case of a valve rupture, a computational fluid dynamics (CFD) study was conducted. The simulation focused on understanding how hydrogen would behave if released into the cabin during flight. The objective was to ensure that the average hydrogen mole fraction would remain below 1%, and that the combustion danger zone (above 4%) would remain confined to a limited area.
Key modelling features included:
-
Transient analysis to track the time evolution of gas concentration.
-
Multicomponent simulation (air and hydrogen) to monitor gas distribution.
-
Gravitational effects to account for differing densities between air and hydrogen.
-
Turbulence modelling using the k-ω SST model, a robust URANS method.
-
Isothermal assumption (constant temperature at 15°C) with adiabatic surfaces.
A poly-hexcore mesh with 4,634,968 cells was used to ensure high accuracy, particularly around the complex fuselage and leak areas.
Key Findings
-
The hydrogen jet, initially at 1,103 m/s, loses momentum rapidly, dropping to 100 m/s within 40 cm.
-
In natural ventilation conditions, hydrogen concentration exceeds 1% in just over 5 seconds and reaches 4.8% at 30 seconds, putting up to 70% of the cabin at combustion risk.
From Natural to Forced Ventilation
Natural ventilation proved inadequate. The team then tested forced ventilation, adjusting the inlet/outlet geometry and positions (160 mm and 240 mm diameters; front vs. rear placement).
The best configuration included:
-
Rear-mounted 240 mm vents
-
Internal baffles
-
Ventilation flow rate of 1,800 m³/h
This setup delayed the 1% hydrogen threshold to 79 seconds and kept levels under 1.2% even after 140 seconds.
Sensor-Activated Ventilation
To improve safety further, hydrogen sensors were introduced. Four placement configurations were tested. The most effective was Position B, which activated ventilation just 3.5 seconds after leak detection. Other placements, like Position D, proved ineffective, with delays leading to unsafe hydrogen concentrations exceeding 9%.
Sensitivity Analysis
A final analysis tested leak sizes both smaller and larger than the 2.5 mm baseline. The chosen 1,800 m³/h flow rateproved sufficient to maintain safe hydrogen levels across all scenarios.
Conclusion
The CFD simulations revealed that natural ventilation is insufficient to prevent hazardous hydrogen accumulation. Through geometric optimization, effective sensor placement, and forced ventilation at 1,800 m³/h, it is possible to keep hydrogen concentration below the 1% safety threshold, even in worst-case leak scenarios.
by Marco Corti (EnginSoft), Leonardo Lecce (Novotech)
