Quantifying helium substitution for hydrogen leakage in fuel cell vehicle low-pressure systems: A visual experimental study

Fuel cell vehicle (FCEV) onboard hydrogen system low-pressure leaks are among the main safety risks in real-world operation. Due to hydrogen’s flammable and explosive properties, helium is often used as a safe surrogate gas in leakage simulation studies; however, its validity for FCEV-related low-pressure, low-flow leaks has not been quantitatively established. Using a representative vertical free-jet configuration, this study experimentally investigates low-pressure, small-orifice releases relevant to FCEV systems. Schlieren visualization was performed with hydrogen across flowrates of 2–10 standard liters per minute (SLM) and concentrations of 20–100%, and corresponding helium cases were used for substitution analysis. A displacement-field analysis method is proposed to evaluate the substitutability of helium for hydrogen based on the optical information in the schlieren images. The study reveals that the laminar-turbulent transition height of the jet varies with the leakage conditions and is closely correlated with a Richardson-number-based scaling. Three commonly used equivalence strategies are assessed, and the results show that, within the tested conditions, an equal-buoyancy substitution method provides satisfactory accuracy. Furthermore, an empirical substitution formula based on concentration and flowrate is proposed, further reducing the average relative deviation by 2–3 percentage points. This work provides a quantitative framework for the design of helium-based surrogate leakage experiments in hydrogen systems.